Metal complexes containing boron based ligands

Owen, Gareth (Ed.)

October 2019

110

MDPI - Multidisciplinary Digital Publishing Institute

Boron-based compounds have been utilized as ligands within transition metal complexes for many decades. The diversity of such compounds in terms of varying functional groups is truly exceptional. Boron compounds are of high interest due to the great potential to modify the substituents around the boron center and to produce a broad range of structural motifs. The many different ways these compounds can coordinate or interact with transition metal centers is astonishing. Examples of transition metal complexes containing boron-based ligands include scorpionates, cluster-type borane- and carboranes, borates, and phosphine-stabilized borylene ligands. This Special Issue brings together a collection of articles focusing on recent developments in the aforementioned boron-based ligands. The articles reported in this book will provide the reader with an overview of the types of boron-based ligands which are currently being researched in groups around the world.

Science (General)
Chemistry (General)
Inorganic Chemistry

English

9783039215843; 9783039215850

10.3390/books978-3-03921-585-0

Metal Complexes
Containing Boron
Based Ligands
Edited by

Gareth Owen
Printed Edition of the Special Issue Published in Inorganics

www.mdpi.com/journal/inorganics

Metal Complexes Containing Boron
Based Ligands

Metal Complexes Containing Boron
Based Ligands

Special Issue Editor
Gareth Owen

MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade

Special Issue Editor
Gareth Owen
University of South Wales
UK

Editorial Office
MDPI
St. Alban-Anlage 66
4052 Basel, Switzerland

This is a reprint of articles from the Special Issue published online in the open access journal Inorganics
(ISSN 2304-6740) in 2019 (available at: https://www.mdpi.com/journal/inorganics/special issues/
Metal Boron Complexes)

For citation purposes, cite each article independently as indicated on the article page online and as
indicated below:
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Page Range.

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Contents
About the Special Issue Editor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Preface to ”Metal Complexes Containing Boron Based Ligands” . . . . . . . . . . . . . . . . . .

Koushik Saha, Urminder Kaur, Rosmita Borthakur and Sundargopal Ghosh
Synthesis of Trithia-Borinane Complexes Stabilized in Diruthenium Core:
[(Cp*Ru)2(η1-S)(η1-CS){(CH2)2S3BR}](R = H or SMe)
Reprinted from: Inorganics 2019, 7, 21, doi:10.3390/inorganics7020021 . . . . . . . . . . . . . . . .

ix

1

Michael Tüchler, Melanie Ramböck, Simon Glanzer, Klaus Zangger, Ferdinand Belaj and
Nadia C. Mösch-Zanetti
Mono- and Hexanuclear Zinc Halide Complexes with Soft Thiopyridazine Based
Scorpionate Ligands
Reprinted from: Inorganics 2019, 7, 24, doi:10.3390/inorganics7020024 . . . . . . . . . . . . . . . . 15
Phil Liebing, Nicole Harmgarth, Florian Zörner, Felix Engelhardt, Liane Hilfert,
Sabine Busse and Frank T. Edelmann
Synthesis and Structural Characterization of Two New Main Group Element
Carboranylamidinates
Reprinted from: Inorganics 2019, 7, 41, doi:10.3390/inorganics7030041 . . . . . . . . . . . . . . . . 29
Mohammed
A.
Altahan,
Michael
A.
Beckett,
Simon
J.
Coles
and
Peter
N.
Horton
Hexaborate(2−)
and
Dodecaborate(6−)
Anions
as
Ligands
to
Zinc(II)
Centres:
Self-Assembly
and

Single-Crystal
XRD
Characterization
of
[Zn{Ϩ3O-B6O7(OH)6}(Ϩ3N-dien)]·0.5H2O
(dien
=
NH(CH2–
CH2NH2)2),
(NH4)2[Zn{Ϩ2O-B6O7(OH)6}2
(H2O)2]·2H2O
and
(1,3-pnH2)3[(Ϩ1N-H3N{CH2}3NH2)

Zn{Ϩ3O-B12O18(OH)6}]2·14H2O
(1,3-pn
=
1,3-diaminopropane)
Reprinted from: Inorganics 2019, 7, 44, doi:10.3390/inorganics7040044 . . . . . . . . . . . . . . . . 37

Marina Yu. Stogniy, Svetlana A. Erokhina, Irina D. Kosenko, Andrey A. Semioshkin and
Igor B. Sivaev
Dimethyloxonium and Methoxy Derivatives of nido-Carborane and Metal Complexes Thereof
Reprinted from: Inorganics 2019, 7, 46, doi:10.3390/inorganics7040046 . . . . . . . . . . . . . . . .

49

Leon Maser, Christian Schneider, Lukas Alig, Robert Langer
Comparing the Acidity of (R3 P)2 BH-Based Donor Groups in Iridium Pincer Complexes
Reprinted from: Inorganics 2019, 7, 61, doi:10.3390/inorganics7050061 . . . . . . . . . . . . . . . .

62

Marta Gozzi, Benedikt Schwarze, Peter Coburger and Evamarie Hey-Hawkins
On the Aqueous Solution Behavior of C-Substituted 3,1,2-Ruthenadicarbadodecaboranes
Reprinted from: Inorganics 2019, 7, 91, doi:10.3390/inorganics7070091 . . . . . . . . . . . . . . . . 73
Joseph Goldsworthy, Simon D. Thomas, Graham J. Tizzard, Simon J. Coles and
Gareth R. Owen
Adding to the Family of Copper Complexes Featuring Borohydride Ligands Based on
2-Mercaptopyridyl Units
Reprinted from: Inorganics 2019, 7, 93, doi:10.3390/inorganics7080093 . . . . . . . . . . . . . . . . 87

v

About
the
Special
Issue
Editor
Gareth
Owen
(Associate
Professor
in
Inorganic
Chemistry)
received
his
Ph.D.
from
Imperial
College

London
in
2003.
He
subsequently
took
a
postdoctoral
post
in
the
research
group
of
Professor

John
A.
Gladysz
in
Germany.
During
this
time,
Dr.
Owen
was
awarded
an
Alexander
von
Humboldt

Research
Fellowship.
He
later
returned
to
the
UK
to
take
up
a
Centenary
Ramsay
Memorial
Research

Fellowship,
hosted
at
the
University
of
Bristol.
This
was
followed
by
a
Royal
Society
Dorothy

Hodgkin
Research
Fellowship,
again
at
Bristol.
Dr
Owen
is
currently
working
as
an
Associate

Professor
in
Inorganic
Chemistry
at
the
University
of
South
Wales.
His
main
research
interests

lie
in
the
chemistry
of
boron-based
ligands
which
act
as
reversible
hydrogen
atom
shuttles,
the

investigation
of
novel
modes
of
small-molecule
activation
and
their
application
to
the
construction

of
new
molecules.

vii

Preface to ”Metal Complexes Containing Boron Based
Ligands”
Boron-based compounds have been utilized as ligands for many decades, during which time
there has been a fascinating array of compounds reported. Boron is most notable for its potential
to be modified with an extraordinarily broad range of functional groups, and for the diverse way
in which these groups interact with metal centers. For this reason, they remain curiosities and
there is still much to understand. There have been plenty of ground-breaking developments along
the way. For example, an enduring interest in Trofimenko-type scorpionate ligands as well as in
cluster-type borane- and carborane-based ligands. In addition to interstitial boron atoms within
metal clusters, the coordination chemistry of boron-containing heterocycles has also been established.
There have recently been some very exciting developments which have further reinvigorated the
field. Pioneering works by outstanding leaders have led to the discovery of yet more ways in which
novel boron functional groups can interact with metal centers. Alongside this, there has been a
significant growth in the chemistry of metal-boryl, -borane, and borohydride compounds and their
interconversions via migrations of hydrogen and other groups between boron and metal centers.
These have found application within element–hydrogen bond activations and ligand cooperation
catalysis. The nature of the metal–boron interaction has also been of great interest. Boron-based
ligands have been shown to act as X- and Z-type ligands, and in some cases, even as L-type (acting
as a Lewis base). Furthermore, the way in which they influence other ligands within the complex has
also attracted significant attention.
This Special Issue brings together a collection of articles focusing on recent developments
in some of the aforementioned areas of the chemistry of boron ligands.

Ghosh and

co-workers report the synthesis of novel trithia-borinane clusters stabilized by two ruthenium
pentamethylcyclopentadienyl fragments.

Mösch-Zanetti and co-workers extend their work

on their hydrotris-(6-tert-butyl-3-thiopyridazinyl)borate ligand, providing a new series of zinc
complexes including some interesting hexanuclear structural motifs. Edelmann and co-workers
expand the research area of carborane complexes by providing two new main group element
carboranylamidinates. The Beckett research group report the construction of hexaborate(2− ) and
dodecaborate(6− ) anions at zinc(II) centers via a self-assembly approach. Sivaev and co-workers
outline the synthesis of the 9-methoxy and 10-methoxy derivatives of nido-carborane and their
subsequent coordination to iron and cobalt centers. The Langer research group outline the results
of their investigations comparing the acidity of phosphine-stabilized borylene ligands in iridium
pincer complexes with the related species protonated carbodiphosphorane and secondary amine
ligands. Hey-Hawkins and co-workers report the synthesis and characterization of C-substituted
3,1,2-ruthenadicarbadodecaboranes along with a comparison of their aqueous solution behavior.
Finally, my research group report on the synthesis and characterization of two copper complexes
containing a mono-substituted borohydride ligand containing a 2-mercaptopyridyl heterocyclic
supporting unit.

ix

These articles provide a flavor of the fascinating and continually expanding field in the area of
transition metal complexes containing boron-based ligands. This area is ripe for further development,
and given the nature of boron as a ligand, it is likely that there is going to be some intriguing new
transition metal–boron functional groups and structural motifs just around the corner. Watch out for
future developments in this area.
Gareth Owen
Special Issue Editor

x

inorganics
Article

Synthesis of Trithia-Borinane Complexes
Stabilized in Diruthenium Core:
[(Cp*Ru)2(η1-S)(η1-CS){(CH2)2S3BR}] (R = H or SMe)
Koushik Saha, Urminder Kaur, Rosmita Borthakur and Sundargopal Ghosh *
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, TN, India;
koushik.suri@gmail.com (K.S.); urminderkaur27@gmail.com (U.K.); roschem07@gmail.com (R.B.)
* Correspondence: sghosh@iitm.ac.in; Tel.: +91-44-22574230
Received: 12 December 2018; Accepted: 7 February 2019; Published: 13 February 2019

Abstract: The thermolysis of arachno-1 [(Cp*Ru)2 (B3 H8 )(CS2 H)] in the presence of tellurium powder
yielded a series of ruthenium trithia-borinane complexes: [(Cp*Ru)2 (η1 -S)(η1 -CS){(CH2 )2 S3 BH}]
2, [(Cp*Ru)2 (η1 -S)(η1 -CS){(CH2 )2 S3 B(SMe)}] 3, and [(Cp*Ru)2 (η1 -S)(η1 -CS){(CH2 )2 S3 BH}] 4.
Compounds 2–4 were considered as ruthenium trithia-borinane complexes, where the central
six-membered ring {C2 BS3 } adopted a boat conformation. Compounds 2–4 were similar to our recently
reported ruthenium diborinane complex [(Cp*Ru){(η2 -SCHS)CH2 S2 (BH2 )2 }]. Unlike diborinane,
where the central six-membered ring {CB2 S3 } adopted a chair conformation, compounds 2–4
adopted a boat conformation. In an attempt to convert arachno-1 into a closo or nido cluster, we
pyrolyzed it in toluene. Interestingly, the reaction led to the isolation of a capped butterfly cluster,
[(Cp*Ru)2 (B3 H5 )(CS2 H2 )] 5. All the compounds were characterized by 1 H, 11 B{1 H}, and 13 C{1 H}
NMR spectroscopy and mass spectrometry. The molecular structures of complexes 2, 3, and 5 were
also determined by single-crystal X-ray diffraction analysis.
Keywords: boron-containing heterocycles; thiolato ligand; borinane; metallaborane

1. Introduction
The mutually synergistic interactions between metals and organic ligands often generate
compounds of fundamental and practical importance [1–6]. The structure and reactivity of
metallaboranes, which features compounds with an M–B bond, is greatly influenced by transition
metals as well as organic ligands [7–25]. Previous studies have been carried out to understand the
ways in which metal and borane fragments can interact to generate novel geometries [1–4,16–25].
However, there is still little understanding of how a transition metal can be used to vary the
chemistry of metallaborane compounds. In this regard, our group was actively involved in the
synthesis of various electron-precise transition metal–boron complexes such as σ-borane [26–31],
boryl [32,33], triply-bridged trimetallic borylene [34–38], diborane [39], B-agostic [26,27,40–42], and
metallaboratrane [26,27,43,44] complexes using of different synthetic precursors. An important
aspect is the incorporation of transition metals into the chemistry of p-block elements other than
carbon [45–47]. The literature contains numerous examples for boron, but other elements illustrate the
possibilities as well [48,49]. The chemistry of transition-metal complexes with main group elements,
particularly with chalcogen ligands, are of substantial importance. The homo- and heterometallic
sulfido complexes with a wide range of substrates are well-documented in the literature [50–53].
In contrast, thioborates are not regularly seen in the coordination sphere of transition metals, mostly
due to the lack of synthetic routes. It is interesting to see how a change of metal or ligand plays an
important role in determining the nature of the molecules (Chart 1).

Inorganics 2019, 7, 21; doi:10.3390/inorganics7020021

1

www.mdpi.com/journal/inorganics

Inorganics 2019, 7, 21

Chart 1. Change in the coordination modes of the molecules with a change in metal or ligand. I–V:
borane, borate, and diborane; VI–X: borane, borate, and agostic; XI–XV: metallaboratrane; XVI–XX:
boryl and borylene complexes.

Several research groups have explored this idea, which has led to the isolation of unique molecules
with interesting bonding interactions [1,54–65]. Here, we have tried to provide a quick overview of
several such examples reported by us and others [26,54–65]. Hartwig in 1996 reported the first example
of a σ-borane metal complex, I, from the reaction of catecholborane and dimethyl titanocene [1].
Following this, several research groups were successful in isolating σ-borane/borate complexes [54–56].
Weller and colleagues synthesized a novel bis(σ-amine–borane) complex of rhodium through the
displacement of a labile fluoroarene ligand from [Rh(η6 -C6 H5 F){P(C5 H9 )2 (η2 -C5 H7 )}][BArF4 ] [54].
Inspired by this, our group recently reported a σ-borane complex of ruthenium from
the reaction of ruthenium bis(σ)borate and [Mn2 (CO)10 ] [26,27]. The first metalladiborane
[(η5 -C5 H8 )Fe(CO)2 (η2 -B2 H5 )], II, was structurally characterized by Shore in 1989 [57,58]. We recently
reported a ruthenium diborane, a derivative of diborane(6) from the reaction of [(Cp*Ru)2 B3 H9 ] (Cp*
= η 5 -C5 Me5 ) and 2-mercaptobenzothiazole [26]. Sabo-Etienne and colleagues have recently shown the
formation of a ruthenium agostic complex [RuH2 {η 2 -H-B(Ni Pr2 )-CH2 PPh2 }(PCy3 )2 ], VII, by treating
phosphinomethyl(amino)borane [Ph2 PCH2 BHNi Pr2 ] and [RuH2 (η 2 -H2 )2 (PCy3 )2 ] [59]. The reaction
of Na[(H2 B)mp2 ] (mp = 2-mercaptopyridyl) and [Re2 CO10 ] enabled us to isolate an agostic complex
of rhenium, [Re(CO)3 (μ-H)BH(C5 H4 NS)2 ], X [27]. Hill and colleagues established how scorpionate
ligands can be utilized for the formation of complexes that have a direct metal boron bond through
the isolation of the first metallaboratrane, [M(CO)(PPh3 ){B(mt)3 }](M→B) (mt = methimazolyl, M = Ru
and Os) in 1999 [60]. Following this, Bourissou and Parkin synthesized a RhI metallaboratrane [61],
XII, and a ferraboratrane [{k4 -B(mimtBu )3 }Fe(CO)2 ] (mimtBu = 2-mercapto-1-tert-butylimidazolyl) [62],
XIII, respectively. We successfully isolated a ruthenaboratrane by using [(η 6 -p-cymene)RuCl2 ]2 as a
2

Inorganics 2019, 7, 21

precursor XIV [43], whereas a rhoda/irida boratrane, [Cp*M(BHL2 )], (L = C5 H4 NS, M = Rh or Ir) [43],
XV, could be synthesized from the reaction of [Cp*MCl2 ]2 with Na[H2 B(mp)2 ]. Marder and colleagues
synthesized metal-bridged-boryl complexes by using catecholborane [63]. In 2005, Braunschweig
reported a heterometallic Fe–Pd bridged-boryl complex from the reaction of [Cp*Fe(CO)2 BCl2 ] and
[Pd(Cy3 )2 ] [64]. Later, our group successfully synthesized a homometallic ruthenium bridged-boryl
complex from the reaction of HBcat (catecholborane, cat = 1,2-O2 C6 H4 ) and [{Ru(CO)}2 B2 H6 ] [32].
Following this, we recently reported a bis(bridging-boryl) complex, [{Cp*Ru(μ,η2 -HBS2 CH2 )}2 ], from
the thermolysis of [Cp*Ru(μ-H)2 BH(S-CH=S)] with chalcogen powder [33]. Fehlner and colleagues
reported a homometallic bridging borylene complex XVIII [65] from the reaction of [CpCo(PPh3 )2 ] and
BH3 ·THF. Our group was successful in synthesizing heterometallic triply bridged borylene complexes
[(Cp*Co)2 (μ3 -BH)(μ-CO){M(CO)5 }] (M = W, Mo, Cr) from the reaction of [{Cp*CoCl}2 ] and LiBH4 ·THF
with [M(CO)3 (MeCN)3 ] [34–38].
Ligands such as COS, CS2 , and CO2 interact with transition metal complexes, showing
a wide range of chemical transformations, such as insertion, dimerization, disproportionation,
coupling, and catalytic reactions [66–68]. On the basis of the general concern of the electron
donating/accepting properties of CS2 and CO2 , various binding modes with one or more metal
atoms have been recognized [69]. However, reactivities of these ligands towards polyhedral
metallaborane clusters have been sparsely explored [70–74]. In this context, Fehlner and colleagues
described the reactivity of CS2 with an unsaturated chromaborane cluster [(Cp*Cr)2 B4 H8 ], which
underwent metal-assisted hydroboration and successively converted to a methanedithiolato ligand [71].
Following this, our group reported the reaction of CS2 with nido-[(Cp*Ru)2 (μ-H)2 B3 H7 ], which
subsequently transformed into [(Cp*Ru)2 (B3 H8 )(CS2 H)], 1, containing a dithioformato ligand
(CHS2 ) [69]. Recently, we reported for the first time a ruthenium trithia-diborinane complex,
1-thioformyl-2,6-tetrahydro-1,3,5-trithia-2,6-diborinane [(Cp*Ru){(η2 -SCHS)CH2 S2 (BH2 )2 }], from the
reaction of [{Cp*RuCl(μ-Cl)}2 ] and Na[BH3 (SCHS)] [33]. Encouraged by these results, we became
interested in exploring the reactivity of 1 under different reaction conditions, especially with heavier
chalcogen ligands. Thus, we performed the reaction of 1 in the presence of chalcogen powder.
As expected, the reaction enabled us to isolate some interesting ruthenium trithia-borinane complexes.
2. Results and Discussion
Synthesis of Ruthenium Borinane Complexes, 2–4
As shown in Scheme 1, the pyrolysis of 1 in the presence of tellurium powder in toluene yielded
compounds 2–4 along with compounds [{Cp*Ru(μ,η3 -SCHS)}2 ] and [Cp*Ru(μ-H)2 BH(SCHS)] [33].
The 11 B{1 H} NMR spectra at room temperature display single resonance at δ = −4.1, 7.4, and 4.9 ppm
for compounds 2, 3, and 4, respectively, indicating the presence of a single boron atom. While the
1 H NMR spectrum of compounds 2 and 4 shows the presence of a terminal B–H proton at δ = 3.75
and 2.58 ppm, respectively, compound 3 does not show any indication of a B–H terminal. Instead, it
shows a resonance at δ = 2.06 ppm, indicating the presence of a (SCH3 ) unit. Apart from that, both 2
and 3 display resonances in the region δ = 3.96–1.69 ppm, which may be attributed to the presence
of methylene protons. Both compounds display signals for two sets of Cp* protons around 1.79 and
1.72 ppm in a 1:1 ratio. The presence of the Cp* ligands, methylene, and SCH3 units are also supported
by 13 C{1 H} NMR spectroscopy. Apart from that, the 13 C{1 H} NMR spectra also show a resonance at δ
= 288.6 and 285.8 ppm, indicating the presence of a C=S group in the molecules of 2 and 3 respectively.
Furthermore, the mass spectra show molecular ion peaks (ESI+ ) at m/z = 686.9603, 732.9479, and
686.9604 for compounds 2, 3, and 4 respectively. Although we isolated the majority of Te powder after
workup, we are not in a position to comment on the exact role of chalcogen powder, in particular Te
powder, in the formation of complexes 2–4 from 1.

3

Inorganics 2019, 7, 21

Scheme 1. Reaction of [(Cp*Ru)2 (B3 H8 )(CS2 H)], 1, in the presence of tellurium powder.

The single-crystal X-ray diffraction study disclosed the core geometry (C2 S3 B ring) of compounds
2 and 3 to be very similar to each other (Figure 1a,b). The only difference between the two is the
position of the boron atom in the central six-membered ring {C2 S3 B}. Compounds 2 and 3 can be
called as 1,3,5-trithia-4-borinane and 1,3,5-trithia-2-borinane complexes of ruthenium, respectively,
which is similar to our recently reported diborinane [(Cp*Ru){(η2 -SCHS)CH2 S2 (BH2 )2 }] [33]. Unlike
diborinane, compounds 2 and 3 have only one boron atom in the six-membered ring {C2 S3 B} and are
the monoborane derivatives of [(Cp*Ru){(η2 -SCHS)CH2 S2 (BH2 )2 }]. While the central six-membered
ring adopts a chair conformation in diborinane [33], 2 and 3 adopt a boat conformation. A significant
difference between 2 and 3 is the presence of the {SMe} moiety instead of a terminal hydrogen attached
to the boron atom in compound 3. The B–S bond length (av. 1.921 Å) in 2 and 3 is within the B–S single
bond distance and is in accord with the ruthenium diborinane complex [33]. One of the interesting
features observed in these molecules is the presence of the thioformyl unit bonded to the ruthenium
atoms. While the diborinane has only one ruthenium atom, compounds 2 and 3 has two ruthenium
atoms bridged by one thiocarbonyl unit on one side and B–S on the other side. The C–S distance
in the thiocarbonyl unit (1.612(15) Å in 2 and 1.617(7) Å in 3) is found to be shorter than that of 1.
The Ru1–Ru2 distances of 2.759(6) Å in 2 and 2.759(6) Å in 3 are significantly shorter when compared
to 1, but are well within the reported Ru–Ru single bond distance [69]. The ruthenium atoms are
connected to two sulfur atoms S2 and S4 present in the (C2 S3 B) ring and the bridging sulfur is connected
to the ring boron atom B1. Although we failed to crystallize compound 4, it was characterized in
comparison to its spectroscopic data with 2 and 3. Based on the spectroscopic data, compound 4 is
expected to have a structure similar to that of compound 3 where instead of the SMe group, a terminal
H is attached to the B atom (Scheme 1).

Figure 1. Molecular structures and labelling diagrams of 2 (a) and 3 (b). Selected bond lengths (Å) and
angles (◦ ): 2: B1–S3 1.885(8), S1–Ru1 2.3436(14), Ru1–Ru2 2.7590(6), C21–S5 1.612(5), C23–S3 1.789(6),
C23–S2 1.826(5); Ru1–C21–Ru2 86.75(18), S3–C23–S2 118.4(3), S3–B1–S1 119.0(4), Ru1–S1–Ru2 72.36(4).
3: B1–S3 1.922(8), B1–S8 1.879(8), B1–S4 1.968(9), Ru2–S3 2.3364(17), Ru1–Ru2 2.7590(7), C21–S6 1.617(7),
C22–S8 1.803(7), C22–S5 1.802(7); Ru2–C21–Ru1 87.2(3), S8–B1–S3 119.7(4), S8–B1–S4 112.5(5), S3–B1–S4
98.9(4), Ru1–S3–Ru2 72.44(5).
4

Inorganics 2019, 7, 21

The six-membered ring containing a {C2 BS3 } moiety adopts a boat conformation, similar to
the reported diborinanes, such as bis(cAAC)-stabilized 3,6-dicyano-1,2,4,5-tetrasulfa-3,6-diborinane
reported by Braunschweig et al. where the central {B2 S4 } ring displayed a boat conformation and was
the first example of a structurally and NMR-spectroscopically characterized {B2 S4 }-heterocycle [75].
Meller et al. reported the synthesis and characterization of a diborinane-tungsten adduct,
[(BMe)2 (NH){N(SiMe3 )}2 (S){W(CO)5 }] [76]. In contrast, the structurally characterized dioxaborinane,
[CN(C6 H5 )(BO2 C3 H5 )(C6 H4 )(C4 H9 )], adopted the half-chair conformation [77].
Recently,
our group reported for the first time a trithia-diborinane stabilized ruthenium complex,
[(Cp*Ru){(η2 -SCHS)CH2 S2 (BH2 )2 }] [33]. Although some examples of trithia-diborinane compounds
have been reported, there are no examples of metal complexes of such trithia-diborinane species except
the one reported by us [33]. Compounds 2–4 are the monoborinane derivatives, and are a novel entry
to the class of transition metal borinane complexes. The few structurally characterized borinane and
diborinane derivatives are listed in Table 1.
In order to check whether arachno-[(Cp*Ru)2 (B3 H8 )(CS2 H)], 1, can be converted to a nido or closo
geometry with the release of hydrogen, we pyrolyzed 1. Interestingly, the reaction led to the formation
of 5 having a capped butterfly geometry, instead of a nido or closo geometry (Scheme 2). The mass
spectrometry of the new compound gives a molecular ion peak at m/z = 613.0588 that corresponds to
C21 H37 Ru2 B3 S2 Na. The room-temperature 11 B{1 H} NMR spectrum of 5 rationalizes the presence of
two boron environments, which appear at δ = 43.6 and −24.1 ppm. Besides the BH terminal protons,
one B–H–B and one Ru–H–B proton is observed in the 1 H NMR spectrum. Furthermore, the 1 H NMR
spectrum implies the presence of two equivalent Cp* ligands in 5.

+
+

5X

5X

%

%

+

+
5X

%
+
&

6


+
+ &

+

+
+

6
7ROXHQHƒ&K

%

+
%
+

+



+

<LHOG 
Scheme 2. Thermolysis of [(Cp*Ru)2 (B3 H8 )(CS2 H)], 1.

5

+

6

+

6

%

5X

Inorganics 2019, 7, 21

Table 1. Selected structural and spectroscopic data of borinane derivatives and complexes [33,75–78].
Entry

dav [B–E] b [Å]

Conformations c

8.3 d

1.352

half chair

−5.0 and −15.6

1.915

chair

f

1.414

planar

37.6

1.433

boat

−11.2 e

1.943

boat

−4.1

1.919

boat

7.3 (3)
4.9 (4)

1.923

Boat

11 B

NMR (ppm) a

f

f

a

NMR spectra were recorded in a CDCl3 solvent unless stated. b E = hetero atom in the central ring. c conformation
of the central six-membered ring. d In [D6 ]-acetone. e In CD2 Cl2 . f Data not available.

The identity of 5 is confirmed by its solid-state X-ray crystal analysis. The asymmetric unit
of 5 contains two independent molecules and the structural data presented here are from one of
the units (Figure 2). In one of the units, the B5–B4–B3–S4–C43–S3 moiety is disordered over two
positions with occupancy factors 0.602 and 0.398. As shown in Figure 2, the molecular structure of
5 can be viewed as a capped butterfly cluster, where one of the triangular faces (Ru1–B2–Ru2) is
capped by a BH fragment (B1 in Figure 2). The Ru1–Ru2 distance in 5 is shorter than that observed

6

Inorganics 2019, 7, 21

in 1 by 0.258 Å. While the Ru–B distances in both 1 and 5 is comparable, the B–B distances show
considerable variation. It is worth noting that the B1–B2 bond distance of 1.679(11) Å in 5 is shorter
than the normal B–B single bond, but it is comparable to that of a manganese hexahydridodiborate
complex [{(OC)4 Mn}(η6 -B2 H6 ){Mn(CO)3 }2 (μ-H)] [39]. The interatomic separation between B3 and
S1 (3.029 Å) is significantly longer for the formation of a direct B–S bond, and is bridged via the
{S-CH2 } unit. With seven-skeletal-electron-pairs (sep), compound 5 satisfies the electron count for a BH
capped arachno-butterfly structure. By the fused polyhedral model of Mingos [79–82], 5 should have 44
electrons [Ru2 B2 (butterfly); 42 + Ru2 B2 (tetrahedron); 40 – Ru2 B (face); 38], which is also supported by
the cve count of 44 electrons [2 (Cp*Ru) × 13 + 1 (μ2 -S) × 1 + 1 × (μ3 -S) × 3 + 3 (BH) × 4 + 2 (H) × 1].
Compound 5 thus obeys the Wade–Mingos rule for an arachno system [79–82].

Figure 2. Molecular structure and labelling diagram of 5: B1–B2 1.679(11), B1–Ru2 2.098(7), B1–Ru1
2.116(7), B2–Ru2 2.174(6), B2–Ru1 2.231(7), S1–Ru2 2.3017(15), S1–Ru1 2.3035(15), Ru1–Ru2 2.7157(6);
Ru2–B1–Ru1 80.2(2), Ru2–B2–Ru1 76.1(2), B1–Ru1–S1 103.2(2), B2–Ru1–S1 83.16(19), B1–Ru1–Ru2
49.59(19), B2–Ru1–Ru2 50.98(16).

3. Materials and Methods
3.1. General Procedures and Instrumentation
All manipulations were conducted under an Ar/N2 atmosphere using standard Schlenk
techniques or glove box techniques. The solvents were distilled prior to use under argon.
Compound arachno-1 was prepared according to the literature method [69], while other chemicals
were obtained commercially and used as received. The external reference [Bu4 N][B3 H8 ] for the
11 B NMR was synthesized with the literature method [83]. Preparative thin layer chromatography
was performed with Merck 105554 silica-gel TLC plates (Merck, Darmstadt, Germany). The NMR
spectra were recorded on a 400 or 500 MHz Bruker FT-NMR spectrometer (Bruker, Billerica, MA,
USA). Residual solvent protons were used as reference (δ, ppm CDCl3 , 7.26), while a sealed tube
containing [Bu4 N(B3 H8 )] in [d6 ]-benzene (δB , ppm, −30.07) was used as an external reference for the
11 B NMR. The FT-IR spectrum was recorded using a Jasco FT/IR-4100 spectrometer (JASCO, Easton,
MD, USA). The HRMS (ESI) spectra were obtained using a Bruker Micro TOF-II instrument (Bruker,
Billerica, MA, USA). Note that all the reported compounds were isolated by the preparative thin
layer chromatographic technique (TLC), using silica-gel-coated aluminum TLC plates. The impure
reaction mixture was slowly loaded on the TLC and eluted by using the hexane/CH2 Cl2 mixture in
inert atmosphere. Elution with the particular solvent mixture allowed us to separate the compounds
in pure form.

7

Inorganics 2019, 7, 21

3.2. Synthesis
3.2.1. Synthesis of Compounds 2, 3, and 4
In a flame-dried Schlenk tube, compound 1 (0.1 g, 0.169 mmol) was suspended in toluene (20 mL),
and Te powder (0.58 g, 0.97 mmol) was added. The reaction mixture was stirred for 24 h at 80 ◦ C.
The solvent was evaporated in vacuum, then the residue was extracted into hexane/CH2 Cl2 (60:40
v/v) and passed through Celite. After the removal of the solvent from the filtrate, the residue was
subjected to chromatographic workup using silica-gel TLC plates. Elution with hexane/CH2 Cl2 (60:40
v/v) yielded pink solid 2 (0.012 g, 10%), pink solid 3 (0.009 g, 7%), and pink solid 4 (0.008 g, 7%) along
with the compounds [{Cp*Ru(μ,η3 -SCHS)}2 ] (0.002 g, 2%) and [Cp*Ru(μ-H)2 BH(SCHS)] (0.003 g, 4%).
2: HR-MS (ESI+) calcd. for C23 H36 S5 BRu2 + [M + H]+ m/z 686.9601, found 686.9603; 11 B{1 H} NMR
(160 MHz, CDCl3 , 22 ◦ C): δ = −4.1 ppm (br, 1B); 1 H NMR (500 MHz, CDCl3 , 22 ◦ C): δ = 3.81, 2.94, 2.01,
1.70 (d, 4H, CH2 S2 ), 3.75 (br, 1H, BHt ,), 1.74, 1.72 (s, 30H, 2 × Cp*); 13 C{1 H} NMR (125 MHz, CDCl3 , 22
◦ C): δ = 288.6 (s, CS), 96.5, 96.4 (s, C Me ), 28.7, 11.8 (s, CH S ), 9.8, 9.4 ppm (s, C Me ); IR (CH Cl ): ν
5
5
2 2
5
5
2 2 
= 2494 (BHt ), 1089 cm−1 (μ-CS).
3: HR-MS (ESI+) calcd for C24 H38 BS6 Ru2 + [M + H]+ m/z 732.9478, found 732.9479; 11 B{1 H} NMR
(160 MHz, CDCl3 , 22 ◦ C): δ = 7.4 ppm (br, 1B); 1 H NMR (500 MHz, CDCl3 , 22 ◦ C): δ = 3.97, 3.17, 2.19,
1.82 (d, 4H, CH2 S2 ), 2.05 (s, 3H, SCH3 ), 1.79, 1.73 (s, 30H, 2 × Cp*); 13 C{1 H} NMR (125 MHz, CDCl3 , 22
◦ C): δ = 285.8 (s, CS), 97.3, 96.5 (s, C Me ), 35.3, 17.1 (s, CH S ), 12.7 (s, SCH ), 10.1, 9.4 ppm (s, C Me );
5
5
2 2
3
5
5
IR (CH2 Cl2 ): ν = 1085 cm−1 (μ-CS).
4: HR-MS (ESI+) calcd for C23 H36 BS5 Ru2 + [M + H]+ m/z 686.9601, found 686.9604; 11 B{1 H} NMR
(160 MHz, CDCl3 , 22 ◦ C): δ = 4.9 ppm (br, 1B); 1 H NMR (500 MHz, CDCl3 , 22 ◦ C): δ = 3.93, 3.17, 2.20,
1.76 (d, 4H, CH2 S2 ), 2.58 (br, 1H, BHt ), 1.80, 1.73 (s, 30H, 2 × Cp*); 13 C{1 H} NMR (125 MHz, CDCl3 ,
22 ◦ C): δ = 97.3, 96.5 (s, C5 Me5 ), 35.3, 17.1 (s, CH2 S2 ), 10.1, 9.4 ppm (s, C5 Me5 ); IR (CH2 Cl2 ): ν =
2383 cm−1 (BHt ), 1081 cm−1 (μ-CS).
3.2.2. Synthesis of Compound 5
In a flame-dried Schlenk tube, compound 1 (0.1 g, 0.169 mmol) was suspended in toluene (20 mL),
and was stirred at 80 ◦ C for 18 h. The solvent was evaporated in vacuum, and the residue was extracted
into hexane/CH2 Cl2 (70:30 v/v) and passed through Celite. After the removal of the solvent from the
filtrate, the residue was subjected to chromatographic workup using silica-gel TLC plates. Elution
with hexane/CH2 Cl2 (70:30 v/v) yielded orange 5 (0.030 g, 30%).
5: HR-MS (ESI+) calcd for C21 H37 B3 NaS2 Ru2 + [M + Na]+ m/z 613.0601, found 613.0588; 11 B{1 H}
NMR (160 MHz, CDCl3 , 22 ◦ C): δ = 43.6, −24.1 ppm (br, 2B); 1 H NMR (500 MHz, CDCl3 , 22 ◦ C): δ =
5.09 (br, 3H, BHt ) 3.89, 2.94 (d, 2H, CH2 S2 ), 1.86, 1.81 (s, 30H, 2 × Cp*), −2.08 (br, 1H, B–H–B), −13.41
(br, 1H, Ru–H–B); 13 C{1 H} NMR (125 MHz, CDCl3 , 22 ◦ C): δ = 95.8, 92.2 (s, C5 Me5 ), 41.1 (s, CH2 S2 ),
11.7, 11.1 ppm (s, C5 Me5 ); IR (CH2 Cl2 ): ν = 2450 (BHt ), 2046 (Ru–H–B).
3.3. X-ray Crystallography
The crystal data for compounds 2, 3, and 5 were collected and integrated using a Bruker
APEX II CCD diffractometer (Bruker, Billerica, MA, USA), with graphite monochromated Mo-Kα
(λ = 0.71073 Å) radiation at 296 K (2 and 3) and 293 K (5). The structures were solved by heavy atom
methods using SHELXS-97 [84] and refined using SHELXL-2013 for compound 2 and SHELXL-2014 [85]
for compound 3. The structure of compound 5 was solved by heavy atom method using SIR-92 [86]
and SHELXL-2014. The crystallographic data were deposited at the Cambridge Crystallographic Data
Centre as Supplementary Materials no. CCDC-1856640 (2), CCDC-1828322 (3), and CCDC-1407806
(5). These data can be obtained free-of-charge from the Cambridge Crystallographic Data Center via
www.ccdc.cam.ac.uk/data_request/cif.
Crystal data for compound (2): C23 H35 BRu2 S5 , Mr = 684.76, monoclinic, space group C2/c, a =
31.732(2) Å, b = 10.7145(7) Å, c = 17.6302(14) Å, β = 116.019(3), V = 5386.5(7) Å3 , Z = 8, ρcalcd = 1.689 g
8

Inorganics 2019, 7, 21

cm−3 , μ = 1.520 mm−1 , F(000) = 2768, R1 = 0.0409, wR2 = 0.0772, 3120 independent reflections [θ ≤
24.999◦ ] and 283 parameters.
Crystal data for compound (3): C24 H37 BRu2 S6 , Mr = 730.84, orthorhombic, space group Pbcn,
a = 34.1570(11) Å, b = 8.5558(3) Å, c = 19.8431(8) Å, V = 5799.0(4) Å3 , Z = 8, ρcalcd = 1.674 g cm−3 ,
μ = 1.487 mm−1 , F(000) = 2960, R1 = 0.0457, wR2 = 0.0884, 2850 independent reflections [θ ≤ 24.93◦ ]
and 309 parameters.
Crystal data for compound (5): C21 H37 B3 Ru2 S2 , Mr = 588.19, monoclinic, space group P21 /n, a =
8.5681(2) Å, b = 39.1432(9) Å, c = 15.1808(3) Å, β = 95.9220(10), V = 5064.21(19) Å3 , Z = 8, ρcalcd = 1.543
g cm−3 , μ = 1.363 mm−1 , F(000) = 2384, R1 = 0.0420, wR2 = 0.1018, 6613 independent reflections [θ ≤
23.02◦ ] and 580 parameters.
4. Conclusions
The present work describes the synthesis of various borinane complexes of a group-8 heavier
transition metal (i.e., ruthenium) from a dithioformato stabilized arachno-diruthenium pentaborane
cluster. The new molecules have similar structures, but they differ in terms of the boron atom’s
position in the central six-membered ring {C2 S3 B}. With a single boron atom in the six-membered
ring {C2 S3 B}, these mono-borinanes can be called 1,3,5-trithia-4-borinane and 1,3,5-trithia-2-borinane
complexes of ruthenium. In all the mono-borinane complexes, the six-membered ring {C2 BS3 }
adopt a boat confirmation, which is in contrast to our previously reported trithia-diborinane
complexes of ruthenium, [(Cp*Ru){(η2 -SCHS)CH2 S2 (BH2 )2 }], which adopt a chair conformation. The
method reported in this article describing the synthesis of trithia-borinane complexes is unique and
may be further utilized to introduce one or more boron atoms to the six-membered ring {C2 BS3 }.
The isolation of these complexes opens up a gateway for the synthesis of early and late transition metal
trithia-borinane complexes. Furthermore, in an attempt to convert arachno-[(Cp*Ru)2 (B3 H8 )(CS2 H)],
1, to a closo or nido geometry, we performed the pyrolysis of 1 that led to the formation of a capped
butterfly cluster. With seven-skeletal-electron-pairs (sep), it satisfies the electron count for a BH capped
arachno-butterfly structure. These results demonstrate that both the transition metal and the ligands
play an important role in the formation of these complexes. It is interesting to see that the properties
and reactivity of molecules can be largely controlled by a variation in the metal or ligand.
Supplementary Materials: The following are available online at http://www.mdpi.com/2304-6740/7/2/21/s1.
NMR and mass spectra of compounds 2–5; The CIF and the checkCIF output files of
compounds 2, 3 and 5.

1 H, 11 B{1 H}, 13 C{1 H}

Author Contributions: K.S. and U.K. conceived and designed the experiment; K.S. and U.K. performed the
synthesis and the spectroscopic analysis; results were discussed with R.B. and S.G.; R.B. prepared the manuscript
with feedback from S.G.; S.G. supervision, S.G. project administration.
Funding: This research was funded by Indo-French Centre for the Promotion of Advanced Research (CEFIPRA),
India, grant number 5905-1.
Acknowledgments: DST-FIST, India, is gratefully acknowledged for the HRMS facility. K.S. thank CSIR, India for
the research fellowship. We thank V. Ramkumar and P.K. Sudhadevi Antharjanam for X-ray data analysis. X-ray
support from Department of Chemistry, IIT Madras and SAIF, IIT Madras, are gratefully acknowledged.
Conflicts of Interest: The authors declare no conflict of interest.

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© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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inorganics
Article

Mono- and Hexanuclear Zinc Halide Complexes with
Soft Thiopyridazine Based Scorpionate Ligands
Michael Tüchler 1 , Melanie Ramböck 1 , Simon Glanzer 2 , Klaus Zangger 2 , Ferdinand Belaj 1 and
Nadia C. Mösch-Zanetti 1, *
1

2

*

Institute of Chemistry, Inorganic Chemistry, University of Graz, Schubertstrasse 1, 8010 Graz, Austria;
michael.tuechler@uni-graz.at (M.T.); melanie.ramboeck@edu.uni-graz.at (M.R.);
ferdinand.belaj@uni-graz.at (F.B.)
Institute of Chemistry, Organic and Bioorganic Chemistry, University of Graz, Heinrichstrasse 28, 8010 Graz,
Austria; simon.glanzer@uni-graz.at (S.G.); klaus.zangger@uni-graz.at (K.Z.)
Correspondence: nadia.moesch@uni-graz.at

Received: 20 December 2018; Accepted: 5 February 2019; Published: 19 February 2019

Abstract: Scorpionate ligands with three soft sulfur donor sites have become very important in
coordination chemistry. Despite its ability to form highly electrophilic species, electron-deficient
thiopyridazines have rarely been used, whereas the chemistry of electron-rich thioheterocycles
has been explored rather intensively. Here, the unusual chemical behavior of a thiopyridazine
(6-tert-butylpyridazine-3-thione, HtBu Pn) based scorpionate ligand towards zinc is reported. Thus,
the reaction of zinc halides with tris(6-tert-butyl-3-thiopyridazinyl)borate Na[TntBu ] leads to
the formation of discrete torus-shaped hexameric zinc complexes [TntBu ZnX]6 (X = Br, I) with
uncommonly long zinc halide bonds. In contrast, reaction of the sterically more demanding ligand
K[TnMe,tBu ] leads to decomposition, forming Zn(HPnMe,tBu )2 X2 (X = Br, I). The latter can be prepared
independently by reaction of the respective zinc halides and two equiv of HPnMe,tBu . The bromide
compound was used as precursor which further reacts with K[TnMe,tBu ] forming the mononuclear
complex [TnMe,tBu ]ZnBr(HPnMe,tBu ). The molecular structures of all compounds were elucidated by
single-crystal X-ray diffraction analysis. Characterization in solution was performed by means of 1 H,
13 C and DOSY NMR spectroscopy which revealed the hexameric constitution of [TntBu ZnBr] to be
6
predominant. In contrast, [TnMe,tBu ]ZnBr(HPnMe,tBu ) was found to be dynamic in solution.
Keywords: soft scorpionate; zinc; hexanuclear compounds

1. Introduction
The use of borate-based ligands in coordination chemistry has gained significant attention over
the last 50 years, when Trofimenko introduced the ligand class of scorpionates [1–3]. In particular,
substituted polypyrazolyl borates have been widely used for the biomimetic modelling of nitrogen-rich
active sites, as they enforce a facial coordination and thus allow mimicking of a tetrahedral
geometry [1,4,5]. In addition, sulfur donating scorpionates, in which the pyrazolyl moiety is replaced
by a thioheterocycle such as methimidazole [6], thiopyridine [7] or thiopyridazine [8], were developed.
Such ligands, first introduced by Reglinski and coworkers [9], exhibit soft coordination properties,
thereby significantly enlarging the scope of this chemistry.
Recently, we introduced a new electron-deficient thiopyridazine based soft scorpionate ligand and
investigated its coordination behavior towards cobalt, nickel [8] and copper [10,11]. We found that the
electron deficiency of this ligand class leads to new reactivity compared to more electron-rich analogues.
This is demonstrated by the high tendency to form boratrane compounds with a direct metal boron
interaction [8,10,11]. Furthermore, the pyridazine based scorpionate ligands exhibit photochemical
reactivity, as observed with potassium hydrotris(6-tert-butyl-3-thiopyridazinyl)borate K[TntBu ] which
Inorganics 2019, 7, 24; doi:10.3390/inorganics7020024

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Inorganics 2019, 7, 24

is, upon exposure to light, transformed into 2 equiv of 6-tert-butylpyridazine-3-thione and 1 equiv
of 4,5-dihydro-6-tert-butylpyridazine-3-thione [12]. The parent 6-tert-butylpyridazine-3-thione is
redox-active in presence of iron(II) under formation of di-organotrisulfide based iron complexes and
concomitant C–N-coupled, desulfurized pyridazinyl-thiopyridazines [13]. The iron compounds exhibit
unusually high redox potentials due to the electron-deficiency of the pyridazine heterocycle.
Inspired by the tris-histidine site of the active site of Carbonic Anhydrase, much effort has
been placed into the synthesis and structural characterization of zinc complexes that contain
trispyrazolyl borate ligands [4,14–17]. Since in several other zinc enzymes, the metal is—beside
histidine—coordinated by cysteine, a number of sulfur-based scorpionate zinc complexes have also
been reported [9,18–20]. The electron-deficient pyridazine heterocycle is expected to enhance the
Lewis acidity of the zinc center promoting interesting reactivity which prompted us to investigate
the coordination chemistry of thiopyridazine based scorpionate ligands towards zinc. With zinc, a
boratrane complex is not feasible, as boratrane complexes may be formed by reaction of a borate
ligand and a metal salt under reduction of the metal which is not an option with zinc. On the other
hand, tris(thiopyridazinyl) scorpionate ligands, in which the borate backbone is replaced by carbon,
allow the preparation of various mononuclear zinc complexes with a direct zinc carbon bond [21,22].
Furthermore, we previously have observed that the hybrid thiopyridazine-methimazole scorpionate
ligand forms a bridging, dinuclear species [23]. For these reasons, we were interested in whether
the borate scorpionate ligands Na[TntBu ] or Na[TnMe,tBu ] can coordinate to zinc in order to form
mononuclear complexes.
Here, the reactivity of electron-deficient hydrotris-(6-tert-butyl-3-thiopyridazinyl) borate (TntBu )
and hydrotris-(6-tert-butyl-4-methyl-3-thiopyridazinyl) borate (TnMe,tBu ) scorpionate ligands towards
zinc halides is reported with the former ligand forming a novel, neutral, three-dimensional hexameric
cage structure.
2. Results and Discussion
2.1. Complex Synthesis
Na[TntBu ] was prepared according to literature procedures [12] and was subjected to a metathesis
reaction with the respective zinc halides in dry dichloromethane to obtain complexes 1a and 1b as
shown in Scheme 1.

Scheme 1. Reaction of Na[TntBu ] with zinc halides to yield hexameric [TntBu ZnX]6 complexes (X = Br 1a, I 1b).

Because of the light sensitivity of the ligand [12], the syntheses of the complexes were conducted
under exclusion of light. An excess of zinc salt was used in order to complete conversion of the ligand
as otherwise unreacted Na[TntBu ] is difficult to remove. After reaction overnight and workup, the
products were obtained as yellow powders in good yield (72–83%). In contrast to Na[TntBu ], 1a and
1b are not photo-reactive and are found to be stable under ambient atmosphere.
Characterization of the products in solution by 1 H and 13 C NMR spectroscopy revealed three sets
of resonances for thiopyridazine substituents. Thus, the 1 H NMR spectrum of compound [TntBu ZnBr]6
(1a) in CDCl3 shows six doublets between 8.83 and 7.03 ppm for the six aromatic thiopyridazine protons

16

Inorganics 2019, 7, 24

(Figure 1) and three singlets at 1.10, 1.04 and 0.91 ppm for the three tert-butyl groups. This asymmetric
chemical surrounding within the scorpionate ligand is in contrast to a mononuclear [TntBu ZnBr]
complex with an expected C3 -symmetry, like in the case of the sodium salt of TntBu , where only one
set of resonance for all three thiopyridazine heterocycles is observed (Figure 1). Upon changing the
halide from bromide in 1a to iodide in 1b, very similar spectra are observed with only the protons
at C4 showing a slight downfield shift consistent with reduced electron density at zinc in the latter.
The B–H atom is apparent at 5.88 ppm as a broad resonance for both complexes.

1D>7Qt%X@

D

E

Figure 1. Aromatic region of the 1 H NMR spectra of Na[TntBu ] and the zinc complexes 1a and 1b in
CDCl3 .

In addition, we consistently noticed a broad singlet integrating for two protons at 2.73 ppm for
1a and 2.65 ppm for 1b, respectively. This finding points towards the presence of one molecule of
water in the products. The significant downfield shift compared to residual water in CDCl3 (1.56
ppm) [24], indicates some sort of interaction with the zinc complexes. This is further supported by
the observation that extensive drying for more than two days under reduced pressure (<0.05 mbar)
did not remove the water molecule (increasing the temperature to 50 ◦ C led to decomposition of the
complexes). The source of water is as yet unclear, since all reactions were performed under inert
atmosphere and in dry solvents. Possibly, our commercially available zinc halide starting materials
were not dry enough.
By performing the preparation of 1a and 1b in tetrahydrofuran instead of methylene chloride,
similar observations were made. The 1 H NMR spectra of the obtained solids revealed the same
resonances, however, instead of the signal for H2 O, resonances for molecules of THF between one
and two equiv were observed at 3.84 ppm and 1.89 ppm for 1a and 3.96 ppm and 1.99 ppm for
1b, respectively. Also in these complexes, extensive drying did not remove the THF molecules
(again heating led to decomposition). A thermogravimetric analysis of 1a showed a loss of mass of
approximately 10 wt % up to 90 ◦ C, in line with a loss of 2 equiv THF for this sample (see Supplementary
Materials, Figure S16).
After dissolving these THF or water containing complexes 1a and 2a in dry chloroform,
stirring for two days and subsequent solvent evaporation, powdery materials were obtained.
Their characterization by 1 H NMR spectroscopy in dry CDCl3 revealed again three sets of resonances
for an asymmetric scorpionate ligand but any additional solvent molecules seemed to be absent.
The resonances are slightly shifted to lower field compared to 1a (especially of the C4 thiopyridazine
protons: 8.96, 8.71 and 8.30 ppm vs. 8.83, 8.61 and 8.32 ppm in 1a). We therefore conclude that the
donor molecules H2 O or THF are displaced by the excess chloroform solvent molecules, which allows
their removal by evaporation. Upon re-addition of THF to a chloroform solution of 1a, 1 H NMR
spectra again show the presence of two coordinated THF molecules. Alternatively, pyridine—another

17

Inorganics 2019, 7, 24

excellent Lewis-basic donor molecule—can be added to solutions of 1a and 1b, also resulting in shifted
NMR peaks (vide infra).
Single crystals of 1a and 1b could be obtained via slow diffusion of pentane into saturated CHCl3
solutions. The molecular structure of 1a and 1b, as determined by single-crystal X-ray diffraction
analysis (vide infra), revealed hexanuclear, cyclic arrangements (see Section 2.2), explaining the observed
lack of symmetry in the recorded 1 H NMR spectra. We therefore reason that the hexanuclear structure
is also preserved in solution. This raises the question of whether molecules might be trapped in the
cavity. Such a situation could explain the observed shifted NMR signals of the donor molecules, but
an interaction with the outside of the torus is also possible.
This was further investigated by diffusion-ordered 1 H NMR spectroscopy (DOSY) [25] of the
crystalline compound [TntBu ZnBr]6 (1a). The DOSY experiment was performed with PPh3 as internal
standard, as PPh3 would have a similar hydrodynamic radius compared to the mononuclear complex
[TntBu ZnBr]. After determination of the diffusion coefficient, the hydrodynamic radius was calculated
according to the Stokes-Einstein equation (see Supplementary Materials, Figure S12, Equation 1) and
the results are displayed in Table 1.

1H

Table 1. Diffusion coefficient D and calculated hydrodynamic radius RH of 1a and PPh3 .
Compound

D (10−10 m2 /s)

RH (Å)

1a
PPh3

4.12
7.96

9.8
5.1

DOSY clearly reveals only one species in solution precluding a breaking of hexanuclear 1a into
lighter fragments. The smaller diffusion coefficient D found for 1a compared to PPh3 shows it to be
significantly larger than a hypothetic monomer. This is supported by the calculated hydrodynamic
radius for 1a which was found to be 9.8 Å and thus in good agreement to the dimensions of the
hexamer observed in the solid state (vide infra).
In order to gather information on the observed interaction with donor molecules, to a solution of
[TntBu ZnBr]6 in CDCl3 , 2 equiv of pyridine were added (Py(1a) ). In this case, cyclooctene (COE) was
used as internal standard, as there is a published value for the diffusion coefficient D available [26].
DOSY experiments of the mixture were performed and the diffusion coefficients were measured
and referenced to COE. Furthermore, the diffusion coefficient of free pyridine was determined in an
independent experiment (Figure 2).

>7Qt%X=Q%U@D

3\D
&2(
3\
Figure 2. Diffusion ordered 1 H NMR spectroscopy (DOSY NMR) data of 1a, the 1a+2pyridine mixture
(Py(1a) , blue), free pyridine (Py, red) and cyclooctene (COE) as internal standard.

The DOSY NMR spectra (Figure S13, Supplementary Materials) of the 1a+2pyridine mixture
revealed two different diffusion coefficients D for the hexamer 1a and the pyridine molecules, with
18

Inorganics 2019, 7, 24

the latter being higher. This provides evidence that the pyridine is not covalently bound to 1a as it
diffuses much faster. However, comparison of D of the pyridine in the mixture and of free pyridine
from an independent experiment reveals a slightly lower diffusion coefficient (D = 19.1 × 10−10 m2 /s
of the mixture 1a+2pyridine vs. D = 24.5 × 10−10 m2 /s of free Py; Table S1, Supplementary Materials).
The small difference, however, hints to only a weak interaction of pyridine with 1a. Calculation of the
diffusion partition coefficient (Equation 2 in Supplementary Materials) reveals that approximately 30%
of the total pyridine in the mixture is on average interacting in a dynamic fashion. Nevertheless, from
this data the assignment of the location (within or outside the cavity) cannot be determined.
While many coordination modes and applications for scorpionate complexes have been reported,
the self-assembly of polynuclear zinc-frameworks is rare [27–31]. With soft scorpionates, only one
tetranuclear [28] and one trinuclear complex [29] could be isolated, albeit in very low yield.
We wondered whether using a similar, but sterically more demanding, soft scorpionate ligand
based on 4-methyl-6-tert-butyl-substituted thiopyridazines K[TnMe,tBu ] will allow the isolation of
a mononuclear zinc complex. However, application of the same reaction conditions used for the
preparation of [TntBu ZnX]6 leads to decomposition of K[TnMe,tBu ] with the only isolable product
being Zn(HPnMe,tBu )2 X2 (X = Br, 2a; I, 2b; Scheme 2). For complex 2a, single crystals could be
obtained, and the solid-state structure could be solved by single-crystal X-ray diffraction analysis (see
Supplementary Materials).

Scheme 2. Formation of Zn(HPnMe,tBu )2 X2 (X = Br 2a, I 2b) upon reaction of K[TnMe,tBu ] with
zinc halides.

For unambiguous identification, 2a and 2b were synthesized independently by addition of 2 equiv
of 4-methyl-6-tert-butyl-3-thiopyridazine (HPnMe,tBu ) to a stirred solution of the respective zinc halide
allowing their isolation as light yellow powders in excellent yield (95–97%). The slightly reduced
electrophilic nature of 2a,b compared to the respective zinc halides led us to consider them as starting
materials for the preparation of TnMe,tBu complexes as decomposition of the latter might be suppressed.
To prove this, the example of 2a was used in the reaction with K[TnMe,tBu ] in methylene chloride under
exclusion of light to yield the mononuclear compound [TnMe,tBu ]Zn(HPnMe,tBu )Br (3) as shown in
Scheme 3.

Scheme 3. Reaction of Zn(HPnMe,tBu )2 Br2 (2a) with K[TnMe,tBu ] forming the mononuclear complex
[(TnMe,tBu )Zn(HPnMe,tBu )Br] (3) and one equiv of HPnMe,tBu .
19

Inorganics 2019, 7, 24

The molecular structure of 3, as determined by single-crystal X-ray diffraction analysis (vide
infra), revealed a mononuclear compound coordinated by an intact TnMe,tBu ligand, albeit only in
the κ2 -S,S mode. For this reason, one molecule of HPnMe,tBu remains coordinated to Zn in order
to conserve a tetrahedral geometry, while the second molecule of HPnMe,tBu of 2a is released into
solution. Although single crystals could be obtained, we were unable to isolate 3 in bulk, but in fact
the 1:1 mixture of 3 and HPnMe,tBu was isolated in good yield (83%). Any attempt to separate the
thiopyridazine from 3 by crystallization led to impure products. Furthermore, 3 shows limited stability
in solution and decomposes within 24 h, both under ambient and inert atmosphere. Nevertheless, the
isolated mixture 3/HPnMe,tBu was subjected to 1 H NMR spectroscopy. The spectrum in CDCl3 at room
temperature revealed an unexpected, highly symmetric species in solution (Figure S10). No signals
for free HPnMe/tBu were observed, indicating a fast, dynamic equilibrium between coordinated and
uncoordinated HPnMe/tBu . In the aliphatic region, only three broadened resonances for the five methyl
(2.47 ppm; green peak in the r.t. spectrum, Figure 3) and tBu-groups (1.22 and 0.99 ppm, blue and red
peak in the r.t. spectrum, Figure 3) were observed, further pointing towards a dynamic behavior in
solution. Indeed, by lowering the temperature to −50 ◦ C, de-coalescence of all signals was observed
(Figure S11). The signal at 0.99 ppm splits into three peaks of equal intensity, which is consistent with
the non-symmetric solid state structure of 3. In addition, signals for one equivalent of free HPnMe,tBu
(2.45 and 1.30 ppm) [11] and one coordinated HPnMe,tBu moiety also appear (Figure 3). The observed
dynamic behavior of 3 in solution at room temperature might explain its limited stability in solution.

UW
ƒ&






Figure 3. Aliphatic region of the 1 H NMR spectra of complex 3 at room temperature (top) and at −50
◦ C (bottom).

The observed different reactivity of TnMe,tBu compared to the TntBu ligand is fairly interesting.
While the additional methyl group is certainly exhibiting both electronic and steric effects, we assume
the former to be more pronounced. We have previously observed that the additional methyl group
has little structural effect in the respective copper boratrane complexes [11]. However, the methyl
substituted complexes are slightly better soluble and together with the increased donating properties,
ligand substitution at the TnMe,tBu zinc complexes might be facilitated, generating more dynamic and
thus more labile systems.
2.2. Molecular Structures
Single crystals suitable for X-ray diffraction analysis of the complexes were obtained by slow
diffusion of pentane (1a) or hexane (1b) into a chloroform solution or by slow evaporation of a

20

Inorganics 2019, 7, 24

chloroform solution (3). Compounds 1a and 1b were determined to be isostructural; however, the
quality of the X-ray data of 1a did not allow the discussion of structural details.
Compound 1b was found to be of hexameric nature with six zinc iodide units coordinated by six
scorpionate ligands (Figure 4). The complex forms a three-dimensional, cylindrical framework, where
each thiopyridazine coordinates to a different zinc atom. While two arms of the scorpionate coordinate
to two different zinc atoms in the same plane, the third thiopyridazine coordinates to a zinc atom on a
different level.

Figure 4. Molecular structure of 1b. Left: view along the x-axis; right: view along the y-axis. Hydrogen
atoms, except for those located at boron and disordered hexane solvent molecules, are omitted for
clarity. Atom code: Zn gray, S yellow, B green, H black, I brown.

Each zinc center is coordinated by three sulfur donors from three different thiopyridazine ligands
and by a halide atom leading to a distorted tetrahedral environment. This alternating coordination
leads to the general framework displayed in Scheme 1. The dimension of the hexagon is approx. 20 Å
in diameter and 12 Å in height, resulting in a volume of approximately 3800 Å3 . This is consistent with
the determined hydrodynamic radius of 9.6 Å found by 1 H DOSY measurements.
The zinc-sulfur bond lengths (2.334–2.350 Å) are within the expected range of other sulfur
coordinated zinc iodine scorpionate complexes (2.348–2.376 Å) [19,32–34]. In contrast, the zinc–iodine
bonds (2.591–2.616 Å) are significantly longer than in other sulfur coordinated zinc iodine complexes
(2.560 Å–2.580 Å) [19,32–34]. The only other example exhibiting similarly long Zn–I bonds represents
the previously reported zinc–iodide containing tinsulfide cluster (2.605–2.611 Å) [35].
The structure also reveals a cavity which is approximately 8 Å wide and 6 Å deep and with a
volume of approximately 300 Å3 shielded by the tert-butyl groups of the ligands (Figure 5). This is
very similar to the dimensions of cucurbit[6]uril (CB[6]), a macrocyclic cavitand comprising of six
glycoluril units forming a cavity which is 5.5 Å wide and 6 Å high [36,37]. Applications of CB[6] are
manifold including catalytic processes, molecular recognition with highly selective binding interactions,
waste-water remediation, or as artificial enzymes or molecular switches [38]. Thus, the observation of
the donor molecule interaction properties of complex 1a, as described above, are interesting as 1a and
1b might show potential for similar applications with the right choice of guest molecules.

21

Inorganics 2019, 7, 24

Figure 5. Space filling representation of 1b.

The solid-state structure is consistent with the asymmetric nature observed by 1 H and 13 C
NMR spectroscopy supporting the stability of the hexameric structure in solution. Thus, the C3 axis
running through the torus reveals three thiopyridazine rings that differ in their relative orientation:
two thiopyridazine rings in the plane, that are perpendicular to each other, and one ring which is
perpendicular to the plane (Figure 4). This results in three different thiopyridazines as observed by
NMR spectroscopy.
Details regarding the solid-state structure and data refinement of 2a can be found in the supporting
information (Figure S20, Table S4). The molecular structure of 3 is displayed in Figure 6. It reveals
a mononuclear zinc complex, coordinated by the TnMe,tBu ligand in a κ2 -S,S fashion, a bromine and
a sulfur atom from an additional thiopyridazine molecule. Furthermore, interaction between the
borohydride and the zinc center is evidenced by the relatively short Zn1–H1 distance of 2.45(5) Å,
the almost linear H1–Zn1–Br1 angle (175.2(12)◦ ) and the distortion from a tetrahedral to a distorted
trigonal bipyramidal coordination at zinc (Br1–Zn1–S1 102.51(8)◦ , Br1–Zn1–S2 95.75(7)◦ , Br1–Zn1–S4
105.47(8)◦ ). The HPnMe,tBu molecule is further stabilized by hydrogen bonding to the sulfur atom of
the non-coordinating arm of the scorpionate ligand (S3–H42 2.322(10) Å).

Figure 6. Molecular structure of [TnMe,tBu ]Zn(HPnMe,tBu )Br (3). Hydrogen atoms, except for those on
B1 and N42, as well as solvent molecules are omitted for clarity. Hydrogen bonding is depicted in
dashed lines.

Compared to zinc bromide complexes coordinated by various methimazolyl-based scorpionate
ligands, the Zn1–Br1 bond with a length of 2.4250(13) Å is significantly elongated (2.334 Å–2.372
Å) [9,39,40]. This might be due to the additional B–H–Zn interaction, because the Zn–Br bond lengths

22

Inorganics 2019, 7, 24

in 2a (2.41252(18) Å and 2.38838(18) Å) as well as in the hybrid methimazolyl-thiopyridazinyl based
dinuclear [(Pn Bm)ZnBr]2 zinc scorpionate complex (2.409 Å) are in the same range as in 3 [21].
3. Experimental Section
3.1. General Information
All reactions were carried out using standard Schlenk techniques. 6-tert-butyl-3-thiopyridazine
(HPntBu ), 4-methyl-6-tert-butyl-3-thiopyridazine (HPnMe,tBu ), Na[TntBu ] and K[TnMe,tBu ] were
synthesized according to literature procedures [11,12,41]. NMR spectra, except for the DOSY
experiments, were measured with a Bruker Avance III 300 MHz spectrometer (Bruker, Billerica,
MA, USA) at 25 ◦ C. DOSY experiments were carried out at 300 K on a 500 MHz Bruker Avance III
spectrometer, equipped with a 5 mm TXI probe with z-gradient. To measure the diffusion coefficients,
bipolar pulse pair longitudinal eddy current delay sequences (BPP-LED) [42] were used together
with an additional convection compensation sequence (double stimulated echo BPP-LED) [43,44].
The diffusion time Δ was 30 ms and the spoil gradient δ was 1 ms. High resolution mass spectrometry
was measured at the University of Technology of Graz, using a Waters GCT Premier Micromas MS
Technologies mass spectrometer (Waters, Milfird, MA, USA) with DI-EI and a TOF detector.
X-ray Structure Determinations were performed with a Bruker AXS SMART APEX 2 CCD
diffractometer (Bruker, Billerica, MA, USA) equipped with an Incoatec microfocus sealed tube and a
multilayer monochromator (Mo Kα, 0.71073 Å) at 100 K. The structures were solved by direct methods
(SHELXS-97) [45] and refined by full-matrix least-squares techniques against F2 (SHELXL-2014/6) [45].
The non-hydrogen atoms were refined with anisotropic displacement parameters without any
constraints. The H atoms bonded to the B atoms could be clearly identified in a difference Fourier
map and were refined with a common isotropic displacement parameter. H atoms bonded to N
atoms could be clearly identified in a difference Fourier map, the N–H distances were fixed to 0.88 Å
and refined without constraints to the bond angles. The H atoms of the pyridazine rings were put
at the external bisectors of the C–C–C angles at C–H distances of 0.95 Å and a common isotropic
displacement parameter was refined for the H atoms of the same ring. The H atoms of the tert-butyl
groups were included at calculated positions with their isotropic displacement parameter fixed to
1.1 times Ueq of the C atom they are bonded to and idealized geometries with tetrahedral angles,
staggered conformations, and C–H distances of 0.98 Å.
CCDC 1510468 (1b), 1850650 (2a) and 1850650 (3) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033;
E-mail: deposit@ccdc.cam.ac.uk).
3.2. Synthetic Procedures
[TntBu ZnBr]6 (1a). Under exclusion of light, 200 mg (0.37 mmol, 1.0 equiv) of Na[TntBu ] and 125
mg (0.56 mmol, 1.5 equiv) of ZnBr2 were suspended in 5 mL of methylene chloride and the beige
suspension was stirred for 16 h. Thereafter, the insoluble parts were removed by filtration and the
yellow solution was dried in vacuo. The crude product was washed with 2× 10 mL of pentane and
dried in vacuo to obtain 210 mg (83%) of 1a·H2 O as a light yellow powder. 1 H NMR (CDCl3 ) δ (ppm):
8.83 (d, J = 9.3 Hz, 1H, ArH), 8.61 (d, J = 9.0 Hz, 1H, ArH), 8.32 (d, J = 9.3 Hz, 1H, ArH), 7.38 (d, J =
9.3 Hz, 1H, ArH), 7.27 (d, J = 9.0 Hz, 1H, ArH), 7.03 (d, J = 9.3 Hz, 1H, ArH), 5.88 (bs, 1H, BH), 2.73
(bs, 2H, H2 O), 1.10 (s, 9H, tBu), 1.04 (s, 9H, tBu), 0.91 (s, 9H, tBu). 13 C NMR (CDCl3 ) δ (ppm): 175.71
(Ar-C), 174.76 (Ar-C), 173.48 (Ar-C), 163.31 (Ar-C), 162.80 (Ar-C), 162.53 (Ar-C), 140.58 (Ar-C), 139.83
(Ar-C), 138.31 (Ar-C), 125.11 (Ar-C), 124.38 (Ar-C), 123.94 (Ar-C), 36.69 (2× tBu-C), 36.63 (tBu-C), 29.06
(tBu-CH3 ), 29.03 (tBu-CH3 ), 28.90 (tBu-CH3 ). MALDI-HR-MS: [Zn2 Tn2 Br]+ calc: 1237.194 m/z, found:
1237.199 m/z, [Zn4 Tn4 I4 Na]+ calc: 2658.21 m/z, found: 2657.20 m/z; no peaks for the hexanuclear

23

Inorganics 2019, 7, 24

molecular ion could be detected. Crystals suitable for X-ray diffraction analysis were obtained by slow
diffusion of pentane into a chloroform solution.
A sample of 1a was dissolved in CDCl3 in a Young tube and stored for 2 days at room temperature.
After the yellow solution has turned slightly bluish, the solvent was removed under reduced pressure,
to obtain 1a without additional H2 O as a slightly bluish powder. Recrystallization from CDCl3 and
pentane yielded slightly blue plates. 1 H NMR (CDCl3 ) δ (ppm): 8.96 (d, J = 9.3 Hz, 1H, ArH), 8.71 (d,
J = 9.0 Hz, 1H, ArH), 8.30 (d, J = 9.3 Hz, 1H, ArH), 7.40 (d, J = 9.3 Hz, 1H, ArH), 7.26 (d, J = 9.0Hz,
1H, ArH), 7.04 (d, J = 9.3 Hz, 1H, ArH), 5.88 (bs, 1H, BH), 1.13 (s, 9H, tBu), 1.05 (s, 9H, tBu), 0.93 (s,
9H, tBu). 13 C NMR (CDCl3 ) δ (ppm): 175.71 (Ar-C), 174.76 (Ar-C), 173.48 (Ar-C), 163.31 (Ar-C), 162.80
(Ar-C), 162.53 (Ar-C), 140.58 (Ar-C), 139.83 (Ar-C), 138.31 (Ar-C), 125.11 (Ar-C), 124.38 (Ar-C), 123.94
(Ar-C), 36.69 (tBu-C), 36.63 (tBu-C), 29.06 (tBu-CH3 ), 29.03 (tBu-CH3 ), 28.90 (tBu-CH3 ).
[TntBu ZnI]6 (1b). Under inert atmosphere and light exclusion, 200 mg (1.0 equiv 0.37 mmol) of
Na[TntBu ] and 190 mg (1.5 equiv 0.56 mmol) ZnI2 were suspended in 5 mL of dry methylene chloride
and the beige suspension was stirred for 16 h. Thereafter, the insoluble salts were removed by filtration
and the yellow solution was dried in vacuo. The crude product was washed with 2× 10 mL of dry
pentane and dried in vacuo to obtain 195 mg (72%) of 1b·H2 O as a light yellow powder. 1 H NMR
(CDCl3 ) δ (ppm) 8.96 (d, J = 9.1 Hz, 1H, ArH), 8.70 (d, J = 9.1 Hz, 1H, ArH), 8.29 (d, J = 9.2 Hz, 1H, ArH),
7.40 (d, J = 9.2 Hz, 1H, ArH), 7.26 (bd, 1H, ArH), 7.04 (d, J = 9.1 Hz, 1H, ArH), 5.88 (bs, 1H, BH), 2.65 (bs,
2H, H2 O), 1.12 (s, 9H, tBu), 1.05 (s, 9H, tBu), 0.92 (s, 9H, tBu). 13 C NMR (CDCl3 ) δ (ppm): 175.73 (Ar-C),
174.82 (Ar-C), 173.23 (Ar-C), 163.26 (Ar-C), 162.99 (Ar-C), 162.59 (Ar-C), 140.92 (Ar-C), 138.89 (Ar-C),
137.53 (Ar-C), 124.98 (Ar-C), 124.18 (Ar-C), 123.98 (Ar-C), 36.85 (tBu-C), 36.67 (tBu-C), 36.64 (tBu-C),
29.16 (tBu-CH3 ), 29.08 (tBu-CH3 ), 28.94 (tBu-CH3 ). MALDI-HR-MS: [Zn2 Tn2 I]+ calc: 1285.180 m/z,
found: 1285.187 m/z, no peaks for the hexanuclear molecular ion could be detected. Crystals suitable
for X-ray diffraction analysis were obtained by slow diffusion of hexane into a chloroform solution.
A sample of 1b was dissolved in CDCl3 in a Young tube and stored for 2 days at room temperature.
After the yellow solution has turned slightly bluish, the solvent was removed under reduced pressure,
to obtain H2 O free 1b as a bluish powder. 1 H NMR (CDCl3 ) δ (ppm) 8.95 (d, J = 9.1 Hz, 1H, ArH), 8.70
(d, J = 9.1 Hz, 1H, ArH), 8.29 (d, J = 9.2 Hz, 1H, ArH), 7.40 (d, J = 9.2 Hz, 1H, ArH), 7.26 (d, J = 9.2 Hz,
1H, ArH), 7.03 (d, J = 9.1 Hz, 1H, ArH), 5.88 (bs, 1H, BH), 1.12 (s, 9H, tBu), 1.05 (s, 9H, tBu), 0.92 (s,
9H, tBu). 13 C NMR (CDCl3 ) δ (ppm): 175.73 (Ar-C), 174.82 (Ar-C), 173.23 (Ar-C), 163.26 (Ar-C), 162.99
(Ar-C), 162.59 (Ar-C), 140.92 (Ar-C), 138.89 (Ar-C), 137.53 (Ar-C), 124.98 (Ar-C), 124.18 (Ar-C), 123.98
(Ar-C), 36.85 (tBu-C), 36.67 (tBu-C), 36.64 (tBu-C), 29.16 (tBu-CH3 ), 29.08 (tBu-CH3 ), 28.94 (tBu-CH3 ).
Zn(HPnMe,tBu )2 Br2 (2a). ZnBr2 (50 mg, 0.222 mmol) and HPnMe,tBu (81 mg, 0.444 mmol) were
dissolved in 3 mL of dichloromethane and the resulting solution was stirred under inert conditions
and exclusion of light overnight. Subsequently, all volatiles were removed in vacuo, the crude product
was washed with 5 mL of pentane and dried to obtain a light yellow powder of 2a (127 mg, 97%). 1 H
NMR (CDCl3 ) δ 14.28 (bs, 2H, NH), 7.46 (d, 2H, ArH), 2.47 (d, 6H, Me), 1.35 (s, 18H, tBu); 13 C NMR
(CDCl3 ) δ 172.37 (Ar-C), 164.85 (Ar-C), 148.75 (Ar-C), 127.46 (Ar-C), 36.84 (tBu-CH3 ), 29.20 (tBu-C),
20.69 (Me-C). Anal. calcd. for C18 H28 Br2 N4 S2 Zn (589.76): C: 36.66, H: 4.79, N: 9.50, S: 10.87; found
C: 36.84, H: 4.78, N: 9.24, S: 10.41. Single crystals suitable for X-ray diffraction measurement were
obtained by slow evaporation of a CHCl3 solution.
Zn(HPnMe,tBu )2 I2 (2b). ZnI2 (44 mg, 0.137 mmol) and 2 equiv of HPnMe,tBu (50 mg, 0.274 mmol)
were dissolved in 3 mL of dichloromethane and the resulting solution was stirred under inert conditions
and exclusion of light overnight. Subsequently, all volatiles were removed in vacuo, the crude product
was washed with 5 mL of pentane and dried to obtain a light yellow powder of 2b (89 mg, 95%). 1 H
NMR (CDCl3 ) δ 13.48 (bs, 2H, NH), 7.43 (d, 2H, ArH), 2.46 (d, 6H, Me), 1.35 (s, 18H, tBu); 13 C NMR
(CDCl3 ) δ 164.52 (Ar-C), 149.13 (Ar-C), 127.05 (Ar-C), 36.86 (tBu-CH3 ), 29.22 (tBu-C), 20.80 (Me-C).
Anal. calcd. for C18 H28 I2 N4 S2 Zn (683.76): C: 31.62, H: 4.13, N: 8.19, S: 9.38; found C: 33.52, H: 4.36, N:
8.66, S: 9.83.

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Inorganics 2019, 7, 24

[TnMe,tBu ]Zn(HPnMe,tBu )Br (3). K[TnMe,tBu ] (326 mg, 0.549 mmol) was dissolved under exclusion
of light in 8 mL of dichloromethane. Subsequently, 2a (324 mg, 0.549 mmol) was added to the yellow
solution. The reaction mixture was stirred in the dark for 5 h after which the formed precipitate was
filtered off and the solvent evaporated. The crude material was washed with 5 mL of pentane and
dried in vacuo to obtain 480 mg (82%) of 3·HPnMe,tBu as a light yellow solid. 1 H NMR (CDCl3 ) δ 13.09
(bs, 2H, NH of HPnMe,tBu ), 7.28 (s, 3H, ArH of 3), 7.19 (s, 2H, ArH of HPnMe,tBu ), 6.91 (bs, 1H, B–H of
3), 2.47 (bs, 15H, Me), 1.26 (bs) and 0.99 (bs, 45H, tBu). Due to the dynamic behavior of the complex, no
13 C NMR data could be obtained. Anal. calc. of C H BBrN S Zn·C H N S: calc: C: 50.73, H: 6.43,
36 54
8 4
9 14 2
N: 13.15, S: 15.04; found C: 50.32, H: 6.28, N: 13.03, S: 14.77. Single crystals suitable for X-ray diffraction
measurement were obtained by slow evaporation of a CHCl3 solution.
4. Conclusions
Herein we present the high yield synthesis of neutral, three-dimensional, hexanuclear zinc
complexes that derive from hydrotris-(6-tert-butyl-3-thiopyridazinyl)borate. The complexes display
the first structurally characterized zinc dependent molecular cage with a scorpionate ligand. 1 H DOSY
NMR measurements confirmed only one species in solution and revealed a hydrodynamic radius of
9.8 Å, which is consistent with the dimensions observed in the solid state structure as determined by
single crystal X-ray diffraction analysis. The molecular structure reveals a torus with an 8 Å wide and
6 Å deep cavity that is surrounded by tert-butyl groups. Residual electron density in- and outside of
the hexameric structure points to large amounts of solvent molecules which could however not be
further resolved (also see Supplementary Materials). These solvent molecules can be exchanged by
polar molecules such as water, tetrahydrofuran or pyridine. Based on 1 H DOSY experiments they are
not covalently bound to the hexamer. Although only weakly bound—presumably by van-der-Waals
forces—they cannot be removed from the solid material by evaporation. This is also consistent with
the properties of the cucurbit[n]uril family (CB[n]) which act as host-guest materials [38]. The cavity of
the best-studied congener CB[6] has very similar dimensions to those of the hexameric zinc species
1b rendering the latter a potential host material. Although likely, with the data in hand we cannot
conclusively state whether the “guest” molecules are indeed inside the cavity in our hexamers.
Increased steric demand on the scorpionate ligand leads under the same reaction conditions
predominantly to decomposition of the ligand under formation of Zn(HPnMe,tBu )2 X2 . However, using
the latter (X = Br) as precursor allows for the isolation of a monomeric zinc scorpionate complex in
which the zinc center is coordinated by the scorpionate ligand in the κ2 -S,S mode and additionally by
a protonated thiopyridazine molecule and bromine, as confirmed by single-crystal X-ray diffraction
analysis. Furthermore, these data showcase a short Zn–H distance within an almost linear Zn–H–B
interaction. Low temperature 1 H NMR spectroscopy is consistent with the solid state structure, while
at room temperature dynamic behavior was observed, possibly explaining the limited stability the
methyl substituted system.
This research shows that the thiopyridazine based scorpionate ligands [TntBu ] and [TnMe,tBu ]
can coordinate to zinc centers, albeit they do not form mononuclear species of the formula [TnR ]ZnX.
Although the additional methyl group in [TnMe,tBu ] prevents formation of a polynuclear framework,
the resulting Lewis acidity of the zinc center leads to decomposition of the ligand, forming the less
acidic Zn(HPnMe,tBu )2 X2 . The usage of this precursor circumvents the problem of increased Lewis
acidity, but the formed product cannot be properly purified and decomposes after prolonged time
in solution.
Supplementary Materials: The following are available online at http://www.mdpi.com/2304-6740/7/2/24/s1:
NMR spectra of all compounds, Thermogravimetric analysis of 1a and crystallographic details.
Author Contributions: For research articles with several authors, a short paragraph specifying their individual
contributions must be provided. Conceptualization, N.C.M.-Z.; synthetic experiments, M.T. and M.R.; DOSY
experiments, S.G. and K.Z.; X-ray analysis, F.B.; writing—original draft preparation, M.T.; writing—review and
editing, contributions of all authors visualization; supervision, N.C.M.-Z.

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Acknowledgments: Support from NAWI Graz is gratefully acknowledged.
Conflicts of Interest: The authors declare no conflict of interest.

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© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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inorganics
Article

Synthesis and Structural Characterization of Two
New Main Group Element Carboranylamidinates
Phil Liebing, Nicole Harmgarth, Florian Zörner, Felix Engelhardt, Liane Hilfert, Sabine Busse
and Frank T. Edelmann *
Chemisches Institut der Otto-von-Guericke-Universität Magdeburg, 39106 Magdeburg, Germany;
phil.liebing@ovgu.de (P.L.); Nicole.Harmgarth@t-online.de (N.H.); fzoerner@outlook.de (F.Z.);
fengelh@gwdg.de (F.E.); liane.hilfert@ovgu.de (L.H.); sabine.busse@ovgu.de (S.B.)
* Correspondence: frank.edelmann@ovgu.de; Tel.: +49-391-67-58327; Fax: +49-391-67-42933
Received: 26 February 2019; Accepted: 11 March 2019; Published: 13 March 2019

Abstract: Two new main group element carboranylamidinates were synthesized using a bottom-up
approach starting from o-carborane, ortho-C2 B10 H12 (1, = 1,2-dicarba-closo-dodecaborane). The first
divalent germanium carboranylamidinate, GeCl[HLCy ] (3, [HLCy ]− = [o-C2 B10 H10 C(NCy)(NHCy)]− ,
Cy = cyclohexyl), was synthesized by treatment of GeCl2 (dioxane) with 1 equiv. of in situ-prepared
Li[HLCy ] (2a) in THF and isolated in 47% yield. In a similar manner, the first antimony(III)
carboranylamidinate, SbCl2 [HLiPr ] (4, [HLiPr ]− = [o-C2 B10 H10 C(Ni Pr)(NHi Pr)]− ), was obtained
from a reaction of SbCl3 with 1 equiv. of Li[HLiPr ] in THF (56% yield). The title compounds were
fully characterized by analytical and spectroscopic methods as well as single-crystal X-ray diffraction.
Both compounds 3 and 4 are monomeric species in the solid state, and the molecular geometries are
governed by a stereo-active lone pair at the metal centers.
Keywords: boron; carborane; carboranylamidinate; germanium; antimony; crystal structure

1. Introduction
Dodecahedral carborane cage compounds of the composition C2 B10 H12 [1] are of tremendous
scientific and technological interest due to a variety of practical applications, including the synthesis of
polymers and ceramics [2], catalysts [3–5], radiopharmaceuticals [6], and non-linear optical materials [7].
The novel chelating ligand type of ortho-carboranylamidinates was first synthesized in our laboratory
in 2010 by in-situ metalation of o-carborane, ortho-C2 B10 H12 (1, = 1,2-dicarba-closo-dodecaborane) with
n-butyllithium, followed by treatment with 1 equiv. of a 1,3-diorganocarbodiimide [8]. They combine
the carborane cage with the versatile chelating amidinate anions, [RC(NR )2 ]− [9–12] in one ligand
system. In the resulting lithium ortho-carboranylamidinates Li[(o-C2 B10 H10 )C(NR)(NHR)] (= Li[HL];
2a: R = i Pr, 2b: R = Cy (cyclohexyl)), a proton is formally shifted from a carboranyl carbon atom to
the amidinate unit, resulting in an amidine moiety acting as a monodentate N-donor functionality
(Scheme 1a). The lithium derivatives were further treated with various metal and non-metal chloride
precursors to yield carboranylamidinates of e.g., Sn(II) and Cr(II) [8], Rh(I) and Ir(I) [13–16], Fe(II) and
Fe(III) [17,18], Mo(II), Mn(II), Co(II), Ni(II), Cu(II) [18,19], Ti(IV), Zr(IV), Si(IV), Ge(IV), Sn(IV), Pb(IV),
and P [20–22]. In the case of reactions with Cp2 TiCl2 , Cp2 ZrCl2 , PhPCl2 , and various dichlorosilanes
R2 SiCl2 , formal dehydrochlorination led to complexes with dianionic [(o-C2 B10 H10 )C(NR)2 ]2− (= [L]2− )
ligands having a deprotonated amidine group [20,22]. In a recent study, we have shown that the formation
of this product class is preferred for highly Lewis-acidic centers, while “soft” metal centers form stable
complexes with the original [(o-C2 B10 H10 )C(NR)(NHR)]− (= [HL]− ) ligand [22]. In all cases (i.e., for both
[HL]− - and [L]2− -type ligands, and independent from the choice of the central atom), the ligand adopts
a characteristic κC,κN-chelating coordination mode instead of the “normal” κN,κN -chelating mode of

Inorganics 2019, 7, 41; doi:10.3390/inorganics7030041

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coordinated amidinate anions [23,24]. In this contribution, we report the synthesis and full characterization
of the first germanium(II) carboranylamidinate as well as the first antimony compound of this type.

Scheme 1. General schematic representation of carboranylamidinate complexes showing the characteristic
κC,κN-chelating coordination mode [23,24], (a) with a monoanionic [(o-C2 B10 H10 )C(NR)(NHR)]– (= [HL]– )
ligand, and (b) with a dianionic [(o-C2 B10 H10 )C(NR)2 ]2− (= [L]2− ) ligand.

2. Results and Discussion
2.1. Synthesis and Characterization of GeCl[HLCy ] (3) and SbCl2 [HLiPr ] (4)
The synthetic protocol leading to the title compounds is outlined in Scheme 2. In the first step,
the lithium carboranylamidinates 2a and 2b were prepared in a one-pot reaction from o-carborane
(1) and the corresponding carbodiimide. Subsequent reaction of 2a with 1 equiv. of the readily
accessible germanium(II) precursor GeCl2 (dioxane) [25] led to formation of GeCl[HLCy ] (3) as the
first carbonylamidinate of divalent germanium. Compound 3 was isolated in 47% yield as colorless,
block-like crystals after recrystallization from toluene. In a similar manner, the first antimony(III)
carboranylamidinate, SbCl2 [HLiPr ] (4) was prepared from SbCl3 and 1 equiv. of Li[HLiPr ] (2b) in
THF. After crystallization from toluene, compound 4 could be isolated in 56% yield as colorless,
needle-like crystals which, like 3, are significantly moisture-sensitive. In both cases, the complex
having a [HL]− -type ligand is the only identified product, and no evidence for the formation of
products with [L]2− ligands has been observed. Consequently, the divalent germanium precursor
turned out to react with Li[HL] in a similar manner as GeCl4 [22], while the reaction of SbCl3 took
a different course than that of PhPCl2 [20].
Both title compounds 3 and 4 were fully characterized through the usual set of elemental analyses
and spectroscopic methods. The 1 H- and 13 C-NMR data of 3 were in good agreement with the expected
composition. In the 1 H-NMR spectrum, a singlet at δ 8.06 ppm could be assigned to the uncoordinated
NH functionality of the amidine unit. High molecular mass peaks in the mass spectrum of 3 were
detected at m/z 457 (87% rel. int.) [M − H]+ and 422 (13% rel. int.) [M − Cl]+ . The absence of peaks at
higher molecular masses confirmed the monomeric nature of 3. In the IR spectrum of 3, typical bands
of the amidine moiety were observed at 3403 cm−1 (νN–H ), 1577 cm−1 (νC=N ), and 1260 cm−1 (νC–N ).
A medium strong band at 2584 cm−1 can be assigned to the carborane cage (νB–H ) [22]. The antimony
derivative 4 was fully characterized in the same manner. The 1 H-NMR spectrum of 4 displayed
a characteristic signal pattern of the two chemically inequivalent isopropyl groups (two doublets
and two septets). In this case, the NH resonance could not be observed. However, the presence of
a [HLiPr ]− ligand in 4 was confirmed by a sharp νN–H band at 3396 cm− 1 in the IR spectrum. Additional
characteristic bands of the amidine group were observed at 1605 cm− 1 (νC=N ) and 1251 cm− 1 (νC–N ),
and the carborane backbone gave rise to a series of strong bands around 2590 cm− 1 (νB–H ) [22]. In the
mass spectrum of 4, the highest molecular mass peak at m/z 426 (60% rel. int.) could be assigned to
the ion [M − Cl]+ .

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Scheme 2. Synthetic route to the title compounds 3 and 4.

2.2. Crystal and Molecular Structures
Both title compounds 3 and 4 crystallize from toluene in solvent-free form with one monomeric
molecule in the asymmetric unit. Crystal structure determinations confirmed the presence of one
monoanionic carboranylamidinate ligand attached to the metal center in a typical κC,κN-chelating
mode. The protonated NHR residue (3: R = Cy; 4: R = i Pr) is directed away from the metal center
and does not contribute to coordinative saturation thereof. Both 3 and 4 exist as the antirotamer in
the crystal (relating to the orientation of the NHR group relative to the carboranyl group). In both
compounds, the C–N bond to the metal-attached nitrogen (N1) is shorter than the C–N bond to the
protonated nitrogen (N2), which is in agreement with the presence of a formal double bond between
C1 and N1. The observed C–N distances resemble those observed in previously described complexes
with [HL]− ligands [21,22].
In the germanium(II) derivative 3, the stereo-active lone pair leads to a trigonal-pyramidal
coordination environment of the Ge center (Figure 1). At 204.0(5) and 229.4(2) pm, respectively,
the Ge–C and Ge–Cl bond lengths are expectedly longer than in the previously reported germanium(IV)
derivative GeCl3 [HLiPr ] (Ge–C 195.6(2) pm, Ge–Cl 226.4(1) pm) [22]. However, the Ge–N distances
are very similar in both compounds (3: 205.3(5) pm, GeCl3 [HLiPr ]: 204.8(2) pm). Rather untypical for
carboranylamidinates, the molecules in 3 are assembled through weak N–H· · · Cl hydrogen bonds
to infinite supramolecular chains (Figure 2). In the previously reported complexes with [HL]– -type
ligands, no hydrogen bonding with participation of the amidine NH moiety has been observed [21,22].
In the antimony(III) derivative 4, the central Sb atom displays a pseudo-trigonal-bipyramidal
coordination by the κCκN-chelating [HLiPr ]− ligand, two chlorido ligands, and a stereo-active lone
pair (Figure 3). The axial positions are occupied by the nitrogen donor (N1) and one of the chlorine
atoms (Cl2), with the N1–Sb1–Cl2 angle being 163.63(5)◦ . This assignment is in agreement with the
Sb1–Cl2 bond lengths of 249.7(1) pm, which is considerably longer than the equatorial Sb1–Cl1 bond
(234.8(1) pm). The Sb1–C3 bond is 218.6(2) pm and therefore slightly longer than the mean value for
tetra-coordinated Sb(III) compounds in the Cambridge Structural Database (214 pm for 664 entries
with R1 ≤ 0.075) [26]. The same is true for the Sb1–N1 bond, which is 237.0(2) pm (mean value for
167 CSD entries with R1 ≤ 0.075: 230 pm) [26]. The molecular structure of 4 is closely related to those
of the previously reported ECl3 [HL] compounds (E = Ge, Sn) [22], with one of the equatorial chlorido
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Inorganics 2019, 7, 41

ligands being formally replaced by a lone pair. Different from 3, the amidine NH moiety in 4 is not
involved in hydrogen bonding.

Figure 1. Molecular structure of 3 in the crystal. Displacement ellipsoids of the heavier atoms are drawn
with 50% probability. Selected bond lengths (pm) and angles (deg.): Ge1–C3 204.1(5), Ge1–N1 205.3(4),
Ge1–Cl1 229.4(2), C3–Ge1–N1 82.5(2), C3–Ge1–Cl1 95.1(2), N1–Ge1–Cl1 97.3(1), C1–N1 130.3(7), C1–N2
133.2(7), C1–C2 150.8(7), N1–C1–N2 128.8(5).

Figure 2. Hydrogen-bonded chain structure of 3 in the crystalline state. Hydrogen atoms attached to B
and C atoms omitted for clarity. N2· · · Cl1 488.7(5) pm, Cl1· · · H approximately 268 pm.

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Figure 3. Molecular structure of 4 in the crystal. Displacement ellipsoids of the heavier atoms are
drawn with 50% probability. Selected bond lengths (pm) and angles (deg.): Sb1–C3 218.7(3), Sb1–N1
237.0(2), Sb1–Cl1 234.8(1), Sb1–Cl2 249.7(1), C3–Sb1–N1 75.44(8), C3–Sb1–Cl1 97.25(7), C3–Sb1–Cl2
88.75(7), N1–Sb1–Cl1 88.11(5), N1–Sb1–Cl2 163.63(5), Cl1–Sb1–Cl2 89.74(3), C1–N1 128.9(3), C1–N2
134.1(3), C1–C2 151.3(3), N1–C1–N2 130.8(2).

3. Experimental Section
3.1. General Procedures and Instrumentation
All reactions were carried out in oven-dried or flame-dried glassware under an inert atmosphere
of dry argon employing standard Schlenk and glovebox techniques. The solvent THF was distilled
from sodium/benzophenone under nitrogen atmosphere prior to use. GeCl2 (dioxane) was prepared
according to a published procedure [25]. All other starting materials were purchased from commercial
sources and used without further purification. 1 H-NMR (400 MHz) and 13 C-NMR (100.6 MHz)
spectra were recorded in THF-d8 solution on a Bruker DPX 400 spectrometer (Bruker BioSpin,
Rheinstetten, Germany). IR spectra were measured with a Bruker Vertex 70V spectrometer (Bruker
Optics, Rheinstetten, Germany) equipped with a diamond ATR unit between 4000 cm−1 and
50 cm−1 . Microanalyses (C, H, N) were performed using a VARIO EL cube apparatus (Elementar
Analysensysteme, Langenselbold, Germany).
3.2. Synthesis of Compound 3
A solution of Li[HLCy ] was prepared as described previously [8] by treatment of 1 (0.95 g,
6.56 mmol) in THF (50 mL) with a 2.5 M solution of n BuLi in hexanes (2.7 mL, 6.56 mmol) followed
by addition of 1,3-dicyclohexylcarbodiimide (1.35 g, 6.56 mmol). After stirring for 2 h at r.t.,
GeCl2 (dioxane) (1.52 g, 6.56 mmol) was added as a solid and stirring was continued for 24 h.
The reaction mixture was evaporated to dryness, and the solid residue was extracted with toluene
(2 × 20 mL). The combined extracts were filtered and the clear, yellow filtrate was concentrated to
a total volume of ca. 10 mL. Crystallization at r.t. for a few days afforded 3 (1.39 g, 47%) as colorless,
block-like, moisture-sensitive crystals. M.p. 177 ◦ C (dec. ca. 220 ◦ C). Elemental analysis calculated
for C15 H33 B10 ClGeN2 (457.59 g·mol−1 ): C, 39.37; H, 7.27; N, 6.12; found C, 38.88; H, 7.20; N, 5.99. 1 H
NMR (400.1 MHz, THF-d8 , 23 ◦ C): δ 8.06 (s, NH), 3.30–3.22 (m, CH), 3.15–3.03 (m, CH), 1.85–0.67 (m,
Cy/BH) ppm. 13 C NMR (100.6 MHz, THF-d8 , 23 ◦ C): δ 157.5 (CN(NH)), 56.0 (CH), 53.8 (CH), 34.3 (Cy),

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26.2 (Cy) ppm. IR (ATR): ν 3403 w (νN–H ), 3305 w, 3066 w, 2929 m, 2854 m (νB–H ), 2634 w, 2582 s, 2113
w, 1661 w, 1577 s (νC=N ), 1531 s, 1464 w, 1449 m, 1366 w, 1348 w, 1332 m, 1300 w, 1260 w (νC–N ), 1243 w,
1229 w, 1192 w, 1146 w, 1078 m, 1059 m, 1042 m, 1022 m, 973 w, 940 w, 921 w, 907 w, 890 m, 868 w, 843
m, 820 m, 799 w, 790 w, 767 w, 729 m, 718 m, 693 m, 656 m, 593 w, 558 w, 541 w, 507 w, 480 w, 446 w,
410 w, 380 w, 361 w, 300 s, 266 s, 227 m, 197 m, 176 m, 158 m, 121 m, 113 m, 98 m, 75 m, 66 m cm−1 . MS
(EI): m/z (%) 457 (87) [M − H]+ , 422 (13) [M − Cl]+ , 367 (47) [M − Cy + H]+ , 351 (14) [M − GeCl]+ ,
339 (17) [M − Cy − Cl]+ , 295 (60) [M − 2Cy]+ , 269 (69) [M − GeCl − Cy]+ , 255 (100) [C4 H7 ]+ , 83 (83)
[Cy]+ , 187 (60) [M − GeCl − 2 Cy + 2H]+ , 98 (26) [NCy + H]+ , 58 (16) [M − Cl − 2Cy + H]+ .
3.3. Synthesis of Compound 4
In a similar manner as for 3, a solution of Li[HLiPr ] was prepared from 1 (0.95 g, 6.56 mmol) in THF
(50 mL), a 2.5 M solution of n BuLi in hexanes (2.7 mL, 6.56 mmol) and 1,3-diisopropylcarbodiimide
(0.83 g, 1 mL, 6.56 mmol) [8]. The addition of solid SbCl3 (1.50 g, 6.56 mmol) produced a yellow
solution and precipitation of a small amount of black solid (presumably Sb). Work-up as described
for 3 afforded compound 4 as colorless, needle-like, moisture-sensitive crystals in 56% isolated yield
(1.70 g). M.p. 141 ◦ C. Elemental analysis calculated for C9 H25 B10 Cl2 N2 Sb (462.07 g·mol−1 ): C, 23.39;
H, 5.45; N, 6.06; found C, 23.50; H, 5.47; N, 6.10. 1 H NMR (400.1 MHz, THF-d8 , 23 ◦ C): δ 3.26 (sept, 2 H,
CH, J = 6.4 Hz), 3.15 (sept, 2 H, CH, J = 6.4 Hz), 1.48–1.16 (br m, BH), 0.86 (d, 6 H, CH3 , J = 6.4 Hz), 0.55
(d, 6 H, CH3 , J = 6.4 Hz) ppm. 13 C NMR (100.6 MHz, THF-d8 , 23 ◦ C): δ 153.2 (CN(NH)), 50.3 (CH), 47.8
(CH), 23.1 (CH3 ), 23.0 (CH3 ) ppm. IR (ATR): ν 3396 w (νN–H ), 3375 w, 2970 w, 2930 w, 2873 w, 2599 m,
2590 m (νB–H ), 2568 w, 2113 w, 1999 w, 1738 w, 1605 m (νC=N ), 1530 m, 1459 w, 1390 w, 1370 w, 1333 w,
1289 w, 1251 w (νC–N ), 1159 w, 1122 m, 1067 m, 1038 w, 969 w, 947 w, 930 w, 899 w, 872 w, 856 w, 838 w,
815 w, 760 w, 735 w, 681 w, 665 w, 634 w, 621 w, 597 w, 575 w, 555 w, 539 w, 517 w, 480 w, 455 w, 412 w,
380 w, 341 m, 303 w, 249 s, 213 m, 193 s, 160 s, 141 s, 113 s, 78 s cm−1 . MS (EI): m/z (%) 426 (60) [M
− Cl]+ , 368 (31) [M − Cl − i Pr − CH3 ]+ , 326 (24) [Sb(C2 H10 B10 )CNH + H]+ , 270 (10) [M − SbCl2 ]+ ,
256 (20) [M − SbCl2 − CH3 + H]+ , 227 (97) [M − SbCl2 − i Pr]+ , 213 (18) [M − SbCl2 − i Pr − CH3 +
H]+ , 192 (54) [SbCl2 ]+ , 170 (25) [(C2 H10 B10 )CNH + H]+ , 120 (9) [Sb]+ , 462 (3) [M]+ , 69 (35) [CNi Pr]+ , 58
(100) [HNi Pr]+ .
3.4. X-ray Crystallography
Single crystal X-ray intensity data of 3 and 4 were collected on a STOE IPDS 2T diffractometer [27]
equipped with a 34 cm image plate detector, using graphite-monochromated Mo Kα radiation,
at T = 100(2) K. The structure was solved by dual-space methods (SHELXT-2014/5) [28] and refined
by full matrix least-squares methods on F2 using SHELXL-2017/1 [29]. Crystallographic data for the
compounds (see Supplementary Materials) have been deposited at the CCDC, 12 Union Road, Cambridge
CB21EZ, UK. Copies of the data can be obtained free of charge on quoting the depository numbers 1899321
(3) and 1899321 (4) (Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
4. Conclusions
To summarize the results reported here, two new carboranylamidinates of main group elements
in low oxidation states were prepared and structurally characterized. Compound 3 represents the
first carboranylamidinate species containing divalent germanium, while 4 is the first antimony
carboranylamidinate. Both compounds were formed in a straightforward manner from the
corresponding Li[HL] derivative, and no products containing dianionic [L]2− ligands were obtained.
This finding meets the expectation in view of the previously discussed influence of the “hardness”
of the central atom on the resulting product [22], as Ge(II) and Sb(II) are rather soft. In both
products, the molecular geometries are governed by a stereo-active lone pair at the metal centers.
Due to their chloro functions, both compounds should be promising starting materials for further
derivative chemistry.

34

Inorganics 2019, 7, 41

Supplementary Materials: The following are available online at http://www.mdpi.com/2304-6740/7/3/41/s1:
Cif and Checkcif files for 3 and 4.
Author Contributions: N.H. and F.Z. performed the experimental work. P.L. and F.E. carried out the crystal
structure determinations. L.H. measured the IR and NMR spectra, and S.B. measured the mass spectra and carried
out the elemental analyses. F.T.E. conceived and supervised the experiments. F.T.E. and P.L. wrote the paper.
Acknowledgments: This work was financially supported by the Otto-von-Guericke-Universität Magdeburg.
Conflicts of Interest: The authors declare no conflict of interest.

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inorganics
Article

Hexaborate(2−) and Dodecaborate(6−) Anions as
Ligands to Zinc(II) Centres: Self-Assembly and
Single-Crystal XRD Characterization of
[Zn{κ3O-B6O7(OH)6}(κ3N-dien)]·0.5H2O (dien =
NH(CH2–CH2NH2)2), (NH4)2[Zn{κ2O-B6O7(OH)6}2
(H2O)2]·2H2O and (1,3-pnH2)3[(κ1N-H3N{CH2}3NH2)
Zn{κ3O-B12O18(OH)6}]2·14H2O
(1,3-pn = 1,3-diaminopropane)
Mohammed A. Altahan 1,† , Michael A. Beckett 1, *, Simon J. Coles 2 and Peter N. Horton 2
1
2

*
†

School of Natural Sciences, Bangor University, Bangor LL57 2UW, UK; chs030@bangor.ac.uk
Chemistry, University of Southampton, Southampton SO17 1BJ, UK; S.J.Coles@soton.ac.uk (S.J.C.);
P.N.Horton@soton.ac.uk (P.N.H.)
Correspondence: m.a.beckett@bangor.ac.uk; Tel.: +44-1248-382-378
Current address: Chemistry Department, College of Science, University of Thi-Qar, Nasiriyah, Iraq.

Received: 27 February 2019; Accepted: 23 March 2019; Published: 27 March 2019

Abstract: Two zinc(II) hexaborate(2−) complexes, [Zn{κ3 O-B6 O7 (OH)6 }(κ3 N-dien)]·0.5H2 O
(dien = NH(CH2 CH2 NH2 )2 ) (1) and (NH4 )2 [Zn{κ2 O-B6 O7 (OH)6 }2 (H2 O)2 ]·2H2 O (2), and a zinc(II)
dodecaborate(6−) complex, (1,3-pnH2 )3 [(κ1 N-H3 N{CH2 }3 NH2 )Zn{κ3 O-B12 O18 (OH)6 }]2 ·14H2 O
(1,3-pn = 1,3-diaminopropane) (3), have been synthesized and characterized by single-crystal XRD
studies. The complexes crystallized through self-assembly processes, from aqueous solutions
containing 10:1 ratios of B(OH)3 and appropriate Zn(II) amine complex: [Zn(dien)2 ](OH)2 ,
[Zn(NH3 )4 ](OH)2 , and [Zn(pn)3 ](OH)2 . The hexaborate(2−) anions in 1 and 2 are coordinated
to octahedral Zn(II) centres as tridentate (1) or bidentate ligands (2) and the dodecaborate(6−) ligand
in 3 is tridentate to a tetrahedral Zn(II) centre.
Keywords: dodecaborate(6−); hexaborate(2−); oxidoborate; polyborate; self-assembly; X-ray structure;
zinc(II) complex

1. Introduction
There are more than two hundred known borate (polyborate) minerals, and many more known
synthetic polyborates [1–3]. Borates are generally comprised of cationic moieties partnered with
anionic units containing boron, oxygen, and in many cases hydroxyl hydrogen. Oxidoborates
(or hydroxyoxidoborates) are the more appropriate terms, but the term borate (or polyborate) has
been used for many years and will be used in this manuscript. Borates are a class of compounds
with rich structural diversity [4–7], and have been synthesized by solvothermal methods or from
aqueous solution by the addition of B(OH)3 to a solution containing the appropriate templating
cation [7]. Polyborate salts obtained from aqueous solution usually contain discrete, isolated or
insular hydroxyl anions, whilst polyborate salts prepared via solvothermal methods are often more
condensed and contain anionic polymeric 1-D chains, 2-D layers or 3-D networks with a variety of
framework building blocks [1,7]. Salts formed from aqueous solution often contain the pentaborate(1−)
[B5 O6 (OH)4 ]− anion since this anion is structurally well suited to forming crystalline supramolecular
Inorganics 2019, 7, 44; doi:10.3390/inorganics7040044

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Inorganics 2019, 7, 44

lattices, which are held together by strong H-bond interactions [8–11]. We have developed a
strategy to overcome pentaborate(1−) salt formation by utilizing more highly charged (> (+1)) metal
complex cations with ligands having the potential to form multiple H-bond interactions to template
crystallization from aqueous solution of polyborate salts of unusual structures. In this context we
have isolated two novel polyborate anions: [B7 O9 (OH)6 ]3− [12] and [B8 O10 (OH)6 ]2− [13]. We have
also recently started to investigate Zn(II)/polyborate chemistry and have been able to isolate an
insular bi-Zn(II) complex containing a rare dodecaborate(6−) anion [14] and two polymeric 1-D
coordination chains with hexaborate(2−) ligands bridging Zn(II) centres [15]. There are a number of
other structural reports on polyborate/Zn(II) chemistry [16–23], including the industrially important
Zn[B3 O4 (OH)3 ] [24].
In this manuscript we describe the synthesis and XRD structures of two new
Zn(II)/hexaborate(2−) complexes: [Zn{κ3O-B6O7(OH)6}(κ3N-dien)]·0.5H2O (dien = NH(CH2CH2NH2)2)
(1) and (NH4)2[Zn{κ2O- B6O7(OH)6}2(H2O)2]·2H2O (2). We also report a Zn(II)/dodecaborate(6−) complex
(1,3-pnH2)3[(κ1N-H3N{CH2}3NH2)Zn{κ3O-B12O18(OH)6}]2 ·14H2O (1,3-pn = 1,3-diaminopropane) (3). All
three complexes are insular and the hexaborate(2−) ligand is tridentate in 1, whereas in 2 it is bidentate
to octahedral Zn(II) centres. The dodecaborate(6−) ligand in 3 is tridentate to a tetrahedral Zn(II) centre.
The structures of these two anions are drawn schematically in Figure 1.

Figure 1. The (a) hexaborate(2−) anion, [B6 O7 (OH)6 ]2− , observed in 1 and 2; and (b) dodecaborate(6−)
anion, [B12 O18 (OH)6 ]6− , observed in 3. These diagrams show the location of formal Lewis charges.

2. Results and Discussion
2.1. Synthesis and Characterization
Compounds 1, 2 and 3 were prepared in moderate yield through crystallization from aqueous
solution initially containing B(OH)3 and [Zn(dien)2 ](OH)2 , [Zn(NH3 )4 ](OH)2 or [Zn(pn)3 ](OH)2 for 1,
2 and 3, respectively. The hydroxide salts were prepared in situ from the corresponding sulphate salts
by the addition of Ba(OH)2 and removal of precipitated BaSO4 (Scheme 1).

38

Inorganics 2019, 7, 44

ȱ
Scheme 1.
Synthesis of Zn(II) hexaborate(2−) and dodecaborate(6−) complexes (dien =
NH(CH2 CH2 NH2 )2 , pn = 1,3-diaminopropane).

Compounds 1, 2 and 3 are formed through self-assembly processes. B(OH)3 , when dissolved
in aqueous solution at moderate to high pH, exists not as boric acid but as a dynamic combinatorial
library (DCL) [25,26] of a variety of polyborate anions which are in rapid equilibria [27,28]. Likewise,
Zn(II) complexes are labile [29], and a DCL of Zn(II)/amine species are also present in the solution.
The products crystallize from solution maximizing energetically favourable solid-state interactions,
including coordination bonds, Coulombic attractions, H-bonding and steric effects [30,31].
Compounds 1, 2 and 3 were characterized spectroscopically (NMR and IR), by thermal
DSC/TGA analysis and by single-crystal XRD studies (Section 2.2). They all gave satisfactory bulk
elemental analysis.
The thermal TGA/DSC data obtained for 1–3 (see Supplementary Materials) were consistent with
the structures determined by single-crystal X-ray diffraction studies (see below) and can be interpreted
by multi-step decomposition processes. For 1 this involved loss of interstitial water (<190 ◦ C), further
loss of water with cross-condensation of hexaborate(2−) ligands (190–380 ◦ C) and finally oxidation
and/or evaporation of the organic dien ligand (380–650 ◦ C) to leave an anhydrous zinc borate ZnB6 O10
(= ZnO·3B2 O3 ) as a glassy residue. Glassy solids with masses consistent with ZnB12 O19 (= ZnO·6B2 O3 )
were obtained as the final residues for both 2 and 3 since the initial starting Zn/B ratio was 1:12.
The thermal decomposition of 3 followed a similar pattern to 1. Compound 2 had a TGA trace
consistent with loss of initial interstitial water (<110 ◦ C), loss of ammonia (110–250 ◦ C), and final
condensation of hexaborate(2−) anions (250–500 ◦ C). Similar thermal behaviour has been observed
in other metal polyborate species [12,13,24,32–35], including 1-D zinc hexaborate(1−) coordination
polymers [Zn(en){B6 O7 (OH)6 }·2H2 O and [Zn(pn){B6 O7 (OH)6 }]·1.5H2 O [15]. Magnetic susceptibility
χm data for 1–3 were ~ −200 × 10−6 cm3 ·mol−1 and typical for diamagnetic zinc(II) complexes.
IR spectra can be used to characterize polyborate species since characteristic B–O stretches are
generally strong and often diagnostic [36]. Hexaborate(2−) ions, which are never “isolated” and
usually found coordinated tridentate to metal centres, have been reported to show such bands at
~953(m) cm−1 and 808(s) cm−1 . Compound 1 displayed bands at 950(m), 861(m) and 806(s) whilst
2 showed bands at 953(m), 904 (s) and 857(m). Thus, the strong band usually observed at 808 cm−1
was absent in 2 and replaced by a strong band at 904 cm−1 . This may be a reflection on the unusual
centrosymmetric bidentate hexaborate(2−) coordination mode observed in 2. The IR spectrum of 3
showed peaks at 1047(s), 952(m), 902(s) and 855(m), and there were corresponding absorptions in
the reported spectrum of [(H3 NCH2 CH2 NH2 )Zn{B12 O18 (OH)6 }Zn(en)(NH2 CH2 CH2 NH3 )]·8H2 O [14],
which also contains a coordinated dodecaborate(6−) ion. Possible diagnostic absorption bands for this
anion have not been described before.
39

Inorganics 2019, 7, 44

Compounds 1–3 were all insoluble in organic solvents but “dissolved” with decomposition in
aqueous solution. 1 H, 11 B spectra of these solutions were obtained in D2 O, as were the 13 C spectra
of 1 and 3. The 1 H and 13 C spectra showed peaks consistent with the organics present and the 1 H
spectra additionally displayed at H2 O/exchangeable hydrogen peak (H2 O, NH, BOH) at ~4.8 ppm.
11 B spectra of 1–3 all showed a single signal at a + 17.4, +15.9 and +14.0 ppm, respectively. These
signals are all downfield of those calculated [10] (at infinite dilution) for the boron/charge ratio of three
(+13.8) for a hexaborate(2−) system, and two (+11.0) for the dodecaborate(6−) ions. This assumes fast
B(OH)3 /[B(OH)4 ]− exchange [27,28] and is also associated with the pH of the solution. The influence
of the zinc(II) ions may also be important here by reducing the effective charge at boron.
2.2. X-ray Diffraction Studies
The structures of 1, 2 and 3 were determined by single-crystal XRD methods. Crystal data are
given in the experimental section and all XRD data are available as Supplementary Materials.
Compounds 1 and 2 both contained the hexaborate(2−) anion coordinated to a Zn(II) centre and
the structures of 1 and 2, showing their atomic numbering schemes, are shown in Figures 2 and 3,
respectively. The anionic complex in 2 was centrosymmetric with the asymmetric unit comprising of
half the anion with the zinc(2+) ion on the inversion centre. Compound 1 was a neutral zinc(II) complex
with 0.5 waters of crystallization. The neutral Zn(II) complex, [Zn{B6 O7 (OH)6 }(dien)], contained
a tridentate (κ3 N) dien ligand and a tridentate (κ3 O) hexaborate(2−) ligand. Compound 1 was
disordered with two heavy atoms (O10, C4) of the ligand, and associated hydrogen atoms, split in
a 1:1 ratio. One position also had an associated water of crystallization (O21). Compound 2 was a
salt comprised of [NH4 ]+ cations, [Zn{B6 O7 (OH)6 }2 (H2 O)2 ]2− anions and interstitial H2 O molecules.
Both hexaborate(2−) ligands in 2 were bidentate (κ2 O) and the coordinated H2 O molecules were
trans. The Zn–O (hexaborate) distances in 2 {2.0692(9) Å (O11) and 2.1208(9) Å (O12)} were within
the range of distances observed for 1 {2.0612(11)–2.1864(10) Å} despite the change in coordination
mode of the hexaborate(2−) ligand. The Zn–O (H2 O) distance in 2 was 2.1292(9) Å (O21), and the
three Zn–N (dien) distances in 1 ranged from 2.1283(14)–2.1473(15) Å. The angles about the Zn(II)
centres were 82.56(5)–100.26(5)◦ and 166.45(5)–175.22(5)◦ for 1, and 87.90(3)–92.10(3)◦ and 180.00◦ for
2. These angles and distances were consistent with previous reported octahedral complexes of Zn(II)
with O and N donor ligands [37]. Bond lengths (B–O) and OBO and BOB bond angles associated
with the hexaborate(2−) ligands in both 1 and 2 were very similar. For example, bond lengths to the
central pyramidal O+ (1.5154(18)–1.5231(18) Å, 1; 1.5053(15)–1.5247(16) Å, 2) > other bond lengths to
four coordinate borons (1.4407(19)–1.4791(19)Å, 1; 1.4413(18)–1.4889(15) Å, 2) > bond-lengths to three
coordinate borons (1.362(2)–1.418(4) Å, 1; 1.3570(17)–1.3793(17) Å, 2) and consistent with distances
and angles previously reported specifically for hexaborate(2−) complexes [15,32,38,39] and related
polyborate systems [8–24,32–36,38–40].

40

Inorganics 2019, 7, 44

Figure 2. Molecular structure of [Zn{κ3 O-B6 O7 (OH)6 }(κ3 N-dien)]·0.5H2 O (dien = NH(CH2 CH2 NH2 )2 )
(1) showing atomic labelling.

Figure 3. Molecular structure of the asymmetric unit of (NH4 )2 [Zn{κ2 O-B6 O7 (OH)6 }2 (H2 O)2 ]·2H2 O
(2), showing atomic labelling.
41

Inorganics 2019, 7, 44

H-bonding interactions are commonly observed in most polyborate solid-state structures. They
were observed at many locations in the solid-state structures of 1 and 2 and must be partly responsible
for the self-assembly of these structures from their constituents. Compound 1 showed H-bond
interactions between the neutral complexes as well as these complexes and the water of crystallization.
Compound 2 showed H-bond cation/anion and anion/H2 O interactions. The energetically favourable
reciprocal R2 2 (8) (Etter [41] nomenclature) O8H8→O3*, O8*H8*→O3) linked hexaborate(2−) units
in 1. There were also unusual R2 2 (6) (O9H9→O12*H12*→O4) and R2 2 (8) (N2H2→O8* and
O13H13→O2*) arrangements between neighbouring hexaborate units in 1; the latter ring included
Zn(1). Compound 2 also had two energetically favourable reciprocal R2 2 (8) interactions between
neighbouring hexaborate(2−) units (O13H13→O6*, O13*H13*→O6 and O8H8→O3*,O8*H8*→O3).
There was also an unusual intramolecular H-bond in 2 between the coordinated H2 O molecule and
the hexaborate(2−) ligand (O21H21A→O13) as part of an intramolecular R1 1 (8) system incorporating
the Zn1 centre (Figure 4). The coordinated H2 O also H-bonded to a neighbouring hexaborate
O21H21B→O2*. O13 is the hexaborate hydroxyl oxygen atom that fulfilled the role as third
coordination donor atom in 1 and in other tridentate hexaborate complexes. In this particular local
environment of 2, the energetics of forming this H-bond and the H2 O–Zn coordination bond must
outweigh the energetics of a simple borate O–Zn coordinate bond. O13H13 also H-bonded to a
neighbouring hexaborate (O13H13→O6*). Full details of these H-bond interactions are given in the
Supplementary Materials.

Figure 4. The intramolecular O21H21A→O13 H-bond interaction in 2. [d(O21–H21) 0.87 Å, d(H21–O13)
1.79 Å; d(O21···O13) 2.6446(13) Å; angle O21H21O13, 169.7◦ ] which is part of two R1 1 (8) rings,
incorporating Zn–O coordinate bonds (symmetry i = 2 − x, 1 − y, 2 − z).

Compound 3 was an ionic compound comprised of [H3 N(CH2 )3 NH3 ]2+ cations and
[(H3 N(CH2 )3 NH2 )ZnB12 O18 (OH)6 ]3− anions, with the anions containing the dodecaborate(6−) ligand
coordinated κ3 O to a tetrahedral Zn(II) centre which also had a monoprotonated monodentate

42

Inorganics 2019, 7, 44

κ1 N-H3 N(CH2 )3 NH2 ligand. There were also seven waters of crystallization per Zn(II) centre.
A diagram of the structure is shown in Figure 5.

Figure 5. Diagram of (1,3-pnH2 )3 [(κ1 N-H3 N{CH2 }3 NH2 )Zn{κ3 O-B12 O18 (OH)6 }]2 ·14H2 O (1,3-pn =
1,3-diaminopropane) (3) showing atomic labelling.

The Zn–O (dodecaborate) distances in 3 {1.9592(18) Å (O3)–1.9717(18) Å (O1)} were
shorter than those observed for 1 or 2, reflecting tetrahedral vs.
octahedral coordination
geometries. The Zn1N1 distance was 2.006(2) Å, and internuclear angles about Zn1 ranged from
103.43(7)–117.86(9)◦ . These data are very similar to those of the closely related di-Zn(II) complex
[(NH3 CH2 CH2 NH2 )Zn{B12 O18 (OH)6 }Zn(en)(NH2 CH2 CH2 NH3 )] [14] that features 1,2-diaminoethane.
The dodecaborate(6−) anion (Figure 1b) is comprised of six boroxole rings fused so as to
produce a larger central {B6 O6 } ring, with each boron atom within this ring carrying a formal
negative charge due to their four-coordinate nature. This anion was first reported in 1990
in the structure of Ag6 [B12 O18 (OH)6 ]·3H2 O [42]. The dodecaborate(6−) anion in 3 is closely
related to the deprotonated structures found in Na8 [B12 O20 (OH)4 ] [43] and Zn6 [B12 O24 ] [44].
The central ring oxygen atoms alternate up and down on different sides of the central ring
and are ideally set-up to bind tridentate to a metal centre.
The dodecaborate(6−) anion
has been previously observed to coordinate in a tridentate mode in the following compounds:
[(NH3CH2CH2NH2)Zn{B12O18(OH)6}–Zn(en)(NH2CH2CH2NH3)] [14], Na2Cs4Ba2[B12O18(OH)6](OH)4 [45],
K7[(BO3)Mn{B121O18(OH)6}]·H2O [46] and K7[(BO3)Zn{B12O18(OH)6}]·H2O [47].
43

Inorganics 2019, 7, 44

The six four-coordinate boron atoms had B–O distances ranging between 1.441(3)–1.506(3)
Å and their O–B–O angles ranged from 106.3(2)–112.1(2)◦ . The remaining six borons of the
anion were three-coordinate and had significantly shorter B–O distances {1.351(3)–1.386(3) Å} and
larger O–B–O angles {115.4(2)–123.1(2)◦ }. These bond lengths are similar to those observed for
[(NH3 CH2 CH2 NH2 )Zn(B12 O18 (OH)6 }Zn(en)(NH2 CH2 CH2 NH3 )] [14], other similarly fused boroxole
systems [14,45–47] and the hexaborate(2−) complexes 1 and 2.
The hydroxyl hydrogen atom, the amino hydrogen atoms of the protonated 1,3-diaminopropane
ions and ligands and the waters of crystallization form numerous H-bond interactions and they
were presumably responsible—at least in part—for this remarkable self-assembly from mono-boron
species. There are numerous cation/anion H-bond interactions, and three of the six potential
dodecaborate hydroxyl interactions are R2 2 (8): O20H20→O15*, O23H23→O9* and O24H24→O17*,
with only the latter reciprocal. “Simple” inter-borate H-bonds originate from O19H19 and O22H22
whilst O21H21 has a non-borate interaction and H-bonds to an H20 (O31). This configuration
contrasts with that of [(NH3 CH2 CH2 NH2 )Zn{B12 O18 (OH)6 }Zn(en)(NH2 CH2 CH2 NH3 )], where all
six were involved in R2 2 (8) interactions. However, a structural motif that is similar to that
found in [(NH3 CH2 CH2 NH2 )Zn{B12 O18 (OH)6 }Zn(en)(NH2 CH2 CH2 NH3 )] is that amino hydrogen
atoms of the uncoordinated nitrogen (N2) of the H3 N(CH2 )3 NH2 ligand H-bond and link with
dodecaborate(6−) units of adjacent complexes. Full details of these H-bond interactions are in the
Supplementary Materials.
3. Experimental
3.1. General
All chemicals were obtained from commercial sources. Combustion analysis (CHN) were obtained
from OEA laboratories Ltd. in Callington, Cornwall, UK. NMR spectra were obtained on a Bruker
Avance spectrometer (Bruker, Coventry, UK) (in D2 O) operating at 400.1 MHz (1 H), 100.6 MHz (13 C)
or 128.4 MHz (11 B) with data reported as δ (ppm) with positive chemical shifts to a high frequency of
tetramethylsilane (TMS) (1 H, 13 C) and BF3 ·OEt2 (11 B). FTIR spectra were obtained on a PerkinElmer
100 FTIR spectrometer (PerkinElmer, Seer Green, UK) as KBr pellets. TGA/DSC analyses were
undertaken in air on an SDT Q600 V4.1 Build 59 instrument (New Castle, DE, USA), using Al2 O3
crucibles between 10–800 ◦ C with a ramp temperature rate of 10 ◦ C·min−1 .
3.2. Synthesis, Spectroscopic, Analytical and Crystallographic data for 1
A solution of NH(CH2 CH2 NH2 )2 (dien) (2.16 mL, 20 mmol) in H2 O (5 mL) was added to a
solution of ZnSO4 ·H2 O (1.79 g, 10 mmol) in H2 O (10 mL). The reaction mixture was stirred at room
temperature for 60 min before the addition of Ba(OH)2 ·8H2 O (3.15 g, 10 mmol) in H2 O (25 mL). This
mixture was rapidly stirred for a further 30 min. The white precipitate of BaSO4 was removed by
filtration and B(OH)3 (6.18 g, 10 mmol) dissolved in H2 O (50 mL) was added to the filtrate, which was
further stirred at room temperature for 3 h. The volume of this solution was reduced to 20 mL by
gentle evaporation in a warm water bath. The concentrated solution was left for 10 days in NMR
tubes for crystallization and yielded colourless crystals of [Zn(dien){B6 O7 (OH)6 }]·0.46H2 O (1) (1.9 g,
42%). Mp ≥ 300 ◦ C. Anal. Calc.: C = 10.5%, H = 4.4%, N = 9.2%. Found: C = 10.7%, H = 4.1%, N =
9.3%. NMR. 1 H/ppm: 2.5 (m, 8H), 4.8 (s, 37H, NH2 , H2 O, OH). 13 C/ppm: 38.10. 11 B/ppm: 17.4.
IR (KBr/cm−1 ): 3549(s), 3384(s), 1642(m), 1442(s), 1427(s), 1362(s), 1249(m), 1193(s) 1108(s), 1028(s),
951(m), 861(m), 808(m). TGA: 100–190 ◦ C, loss of 0.46 interstitial H2 O 2.5 (1.8 calc.); 190–380 ◦ C,
condensation of polyborate with loss of three further H2 O 15.2% (13.7% calc.); 380–650 ◦ C, oxidation
of dien 38.5% (36.3% calc.); residual ZnB6 O10 61.5% (63.4% calc.). Magnetic susceptibility: χm =
−210 × 10−6 cm3 ·mol−1 .
Crystal data: C4 H19.91 B6 N3 O13.5 Zn, Mr = 456.46, monoclinic, C2/c (No. 15), a = 26.0212(3) Å, b =
9.15620(10) Å, c = 13.6318(2) Å, β = 99.5800(10)◦ , α = γ = 90◦ , V = 3202.55(7) Å3 , T = 100(2) K, Z = 8,

44

Inorganics 2019, 7, 44

Z’ = 1, μ(Mo Kα) = 1.613 mm−1 , 18390 reflections measured, 3651 unique (Rint = 0.0241) which were
used in all calculations. The final wR2 was 0.0666 (all data) and R1 was 0.0240 (I > 2σ(I)).
3.3. Synthesis, Spectroscopic, Analytical and Crystallographic Data for 2
A solution of NH3 (35%, 2.4 mL, 36 mmol) was added dropwise to a solution of ZnSO4 ·H2 O
(1.08 g, 6 mmol) in H2 O (15 mL). The addition of Ba(OH)2 ·8H2 O (1.89 g, 6 mmol) in H2 O (35 mL)
followed by rapid stirring for 15 min resulted in a precipitate of BaSO4 which was removed by
filtration. B(OH)3 (3.71 g, 60 mmol) dissolved in H2 O (30 mL) was added to the filtrate which was
further stirred at room temperature for 30 min. The volume of this solution was reduced to 5 mL by
gentle evaporation on a warm water bath and the concentrated solution was left for 3 days in NMR
tubes for crystallization and yielded colourless crystals of [NH4 ]2 [Zn{B6 O7 (OH)6 }2 (H2 O)2 ]·2H2 O (2)
(2.1 g, 48%). Mp ≥ 300 ◦ C. Anal. Calc.: H = 3.8%, N = 3.8%. Found: H = 4.0%, N = 3.7%. NMR:
11 B/ppm: 15.9. IR (KBr/cm−1 ): 3212(s), 1400(s), 1357(s), 1048(s), 953(m), 904(m), 857(m). TGA: 100–110
◦ C, loss of 4 interstitial/coordinated H O 10.2% (9.9% calc.); 110–250 ◦ C, loss of 2 NH 15.5% (14.8%
2
3
calc.); 250–500 ◦ C, condensation of polyborate with loss of six further H2 O 31.1 (29.6 calc.); residual
ZnB6 O19 68.9% (68.2% calc.). Magnetic susceptibility: χm = −290 × 10−6 cm3 ·mol−1 .
Crystal data: B12 H28 N2 O30 Zn, Mr = 731.33, triclinic, P−1 (No. 2), a = 7.4831(2) Å, b = 7.8551(2) Å,
◦
c = 11.0111(3) Å, α = 108.065(2)◦ , β = 95.020(2) , γ = 90.118(2)◦ , V = 612.68(3) Å3 , T = 100(2) K, Z = 1,
Z’ = 0.5, μ(Mo Kα) = 1.138 mm−1 , 16475 reflections measured, 2799 unique (Rint = 0.0314) which were
used in all calculations. The final wR2 was 0.0559 (all data) and R1 was 0.0212 (I > 2σ(I)).
3.4. Synthesis, Spectroscopic, Analytical and Crystallographic Data for 3
A solution of NH2 CH2 CH2 CH2 NH2 (1,3-pn) (2.52 mL, 30 mmol) in H2 O (10 mL) was added to a
solution of ZnSO4 ·H2 O (1.79 g, 10 mmol) in H2 O (10 mL). The reaction mixture was stirred at room
temperature for 60 min before the addition of Ba(OH)2 ·8H2 O (3.15 g, 10 mmol) in H2 O (25 mL). This
mixture was rapidly stirred for a further 30 min. The white precipitate of BaSO4 was removed by
filtration and B(OH)3 (6.18 g, 10 mmol) dissolved in H2 O (50 mL) was added to the filtrate, which was
further stirred at room temperature for 30 min. The volume of this solution was reduced to 5 mL by
gentle evaporation in a warm water bath. The product was collected by filtration and carefully washed
with cold H2 O followed by CH3 COCH3, and then dried at 40 ◦ C for 1 h to yield colourless crystals
of [H3 N(CH2 )3 NH3 ]3 [(H3 N(CH2 )3 NH2 )ZnB12 O18 (OH)6 ]2 ·14H2 O (3) (4.1g, 46%). Mp ≥ 300 ◦ C. Anal.
Calc.: C = 10.0%, H = 5.9%, N = 7.8%. Found: C = 9.7%, H = 5.2%, N = 7.8%. NMR. 1 H/ppm: 1.93 (p,
10H, CH2 ), 3.01 (t, 20H, CH2 ) 4.8 (s, 68H, NH2 , H2 O, OH). 13 C/ppm: 26.9, 37.6. 11 B/ppm: 14.0. IR
(KBr/cm−1 ): 3405(s), 3263(s), 1644(m), 1532(m), 1352(s), 1151(m) 1047(s), 952(m), 902(s), 855(m). TGA:
100–190 ◦ C, loss of 14 interstitial H2 O 14.1% (13.9% calc.); 190–350 ◦ C, condensation of polyborate with
loss of six further H2 O 6.9 (6.0 calc.); 350–800 ◦ C, oxidation of organics 22.8% (22.0% calc.); residual
Zn2 B24 O38 56.6% (55.4% calc.). p-XRD: d-spacing (Å)/(% rel. int.): 9.98(36), 9.44 (100), 8.50 (54), 8.08
(35), 6.93 (43). Magnetic susceptibility: χm = −180 × 10−6 cm3 ·mol−1 .
Crystal data: C7.5 H49 B12 N5 O31 Zn, Mr = 900.60, triclinic, P−1 (No. 2), a = 9.3681(2) Å, b = 10.6910(2)
Å, c = 19.2746(4) Å, α = 82.954(2)◦ , β = 76.156(2)◦ , γ = 68.655(2)◦ , V = 1744.44(7) Å3 , T = 100(2) K, Z = 2,
Z’ = 1, μ(Mo Kα) = 0.821 mm−1 , 38,867 reflections measured, 7958 unique (Rint = 0.0389) which were
used in all calculations. The final wR2 was 0.1053 (all data) and R1 was 0.0425 (I > 2σ(I)).
3.5. X-ray Crystallography
Single-crystal X-ray crystallography was undertaken at the Engineering and Physical Sciences
Research Council (EPSRC) National Crystallography service at the University of Southampton,
(Southampton, UK). Suitable crystals of 1, 2 and 3 were selected and mounted on a MITIGEN holder
in perfluoroether oil on a Rigaku FRE+ equipped with HF Varimax confocal mirrors and an AFC12
goniometer and HG Saturn 724+ detector diffractometer. The crystals were kept at T = 100(2) K during
data collection. Using Olex2 [48], the structures were solved with the ShelXT [49] structure solution
45

Inorganics 2019, 7, 44

program using the Intrinsic Phasing solution method. The models were then refined with ShelXL [50]
using least squares minimisation. Cambridge Crystallographic Data Centre (CCDC) 1898912 (1),
1898913 (2), 1898914 (3) contain the supplementary crystallographic data for this paper. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retreiving.html (or from CCDC,
12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; email deposit@ccdc.ac.uk).
4. Conclusions
The strategy of using more highly charged cationic labile transition-metal complexes to template
self-assembly (by crystallization) of polyborate anions from alkaline aqueous solutions originally
containing B(OH)3 has resulted in the synthesis of three new zinc polyborate complexes in moderate
yields (40–50%). These complexes contain either hexaborate(2−) or dodecaborate(6−) ligands
and are stabilized by Zn–O coordinate bonds. The solid-state structures are further stabilized
by multiple intramolecular and/or intermolecular H-bond interactions which are prevalent in
polyborate structures.
Supplementary Materials: The following are available online at http://www.mdpi.com/2304-6740/7/4/44/s1.
TGA and single-crystal XRD data. Cif and checkcif files.
Author Contributions: M.A.B. conceived the experiments; M.A.A. synthesized and characterized the complexes
and grew the single crystals; P.N.H. and S.J.C. solved the crystal structures; M.A.B. wrote the paper with
contributions from all co-authors.
Funding: This research received no external funding.
Acknowledgments: We thank the EPSRC for the use of the X-ray Crystallographic Service (NCS, Southampton, UK).
Conflicts of Interest: The authors declare no conflict of interest.

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© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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inorganics
Article

Dimethyloxonium and Methoxy Derivatives of
nido-Carborane and Metal Complexes Thereof
Marina Yu. Stogniy 1, *, Svetlana A. Erokhina 1 , Irina D. Kosenko 1,2 , Andrey A. Semioshkin 1,2
and Igor B. Sivaev 1,3, *
1

2
3

*

A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov Str.,
119991 Moscow, Russia; hoborova.svetlana@yandex.ru (S.A.E.); kosenko@ineos.ac.ru (I.D.K.);
semi@ineos.ac.ru (A.A.S.)
Globalchempharm Company, Sadovo-Kurinskaya Str. 32-1, 123001 Moscow, Russia
Basic Department of Chemistry of Innovative Materials and Technologies,
G.V. Plekhanov Russian University of Economics, 36 Stremyannyi Line, 117997 Moscow, Russia
Correspondence: stogniymarina@rambler.ru (M.Y.S.); sivaev@ineos.ac.ru (I.B.S.);
Tel.: +7-(495)-135-92-42 (I.B.S.)

Received: 27 February 2019; Accepted: 22 March 2019; Published: 27 March 2019

Abstract: 9-Dimethyloxonium, 10-dimethyloxonium, 9-methoxy and 10-methoxy derivatives
of nido-carborane (9-Me2 O-7,8-C2 B9 H11 , 10-Me2 O-7,8-C2 B9 H11 , [9-MeO-7,8-C2 B9 H11 ]− , and
[10-MeO-7,8-C2 B9 H11 ]− , respectively) were prepared by the reaction of the parent nido-carborane
[7,8-C2 B9 H12 ]− with mercury(II) chloride in a mixture of benzene and dimethoxymethane.
Reactions of the 9 and 10-dimethyloxonium derivatives with triethylamine, pyridine,
and 3-methyl-6-nitro-1H-indazole result in their N-methylation with the formation of the
corresponding salts with 9 and 10-methoxy-nido-carborane anions. The reaction of the symmetrical
methoxy derivative [10-MeO-7,8-C2 B9 H11 ]− with anhydrous FeCl2 in tetrahydrofuran in the
presence of t-BuOK results in the corresponding paramagnetic iron bis(dicarbollide) complex
[8,8 -(MeO)2 -3,3 -Fe(1,2-C2 B9 H10 )2 ]− , whereas the similar reactions of the asymmetrical methoxy
derivative [9-MeO-7,8-C2 B9 H11 ]− with FeCl2 and CoCl2 presumably produce the 4,7 -isomers
[4,7 -(MeO)2 -3,3 -M(1,2-C2 B9 H10 )2 ]− (M = Fe, Co) rather than a mixture of rac-4,7 - and
meso-4,4 -isomers.
Keywords: nido-carborane; iron bis(dicarbollide); cobalt bis(dicarbollide); dimethyloxonium
derivatives; methoxy derivatives; synthesis; properties

1. Introduction
Cyclic oxonium derivatives of polyhedral boron hydrides are well studied due to their use
as convenient starting compounds for the preparation of various functional derivatives [1,2].
In particular, this approach was used for synthesis of various derivatives of nido-carborane, including
boron-containing biomolecules [3–5] and crown ethers [6,7]. At the same time, in the literature there
are only a few examples of acyclic oxonium derivatives of polyhedral boron hydrides [8–14], and to
the best of our knowledge, there are no examples of dimethyloxonium derivatives.
In this contribution we describe synthesis of dimethyloxonium derivatives of nido-carborane
[9-Me2 O-7,8-C2 B9 H11 ] and [10-Me2 O-7,8-C2 B9 H11 ], their demethylation reactions to the corresponding
methoxy derivatives [9-MeO-7,8-C2 B9 H11 ]− and [10-MeO-7,8-C2 B9 H11 ]− as well as the formation of
ferra- and cobaltacarborane complexes thereof.

Inorganics 2019, 7, 46; doi:10.3390/inorganics7040046

49

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Inorganics 2019, 7, 46

2. Results and Discussion
Electrophile-induced nucleophilic substitution (EINS) reactions of nido-carboranes with a various
nucleophiles are well known and widely used for their modification. Typical are HgCl2 -mediated
reactions of nido-carborane with nucleophilic solvents resulting in the [10-L-7,8-C2 B9 H11 ]
(L = 1,4-dioxane [15], tetrahydrofuran [15,16], tetrahydropyran [17], alkylnitriles [18], and pyridine [16])
derivatives. It is assumed that initially formed mercuric derivatives [19,20] decompose at elevated
temperatures to form quasi-borinium cations, which acts as the potent Lewis acids [21] react with
nucleophilic solvent molecules. The corresponding acyclic oxonium derivatives of polyhedral boron
hydrides are much less studied and limited mainly by diethoxy derivatives [8–14]. Since dimethyl ether
is gaseous under normal conditions, working with it at elevated temperatures is possible only with the
use of high-pressure vessels that is normally unacceptable in common laboratories.
The comparative analysis of 1 H NMR spectral data of a series of polyhedral boron hydride
derivatives BL (L = SMe2 , 1,4-dioxane) and the corresponding MX5 L complexes (M = Nb, Ta; X = F, Cl)
demonstrated their very close similarity that could be explained by comparable electronic effects of
the metal and boron moieties in these compounds [22]. It is known that NbCl5 is effective reagent for
removal of the methoxy methyl ether protecting group in organic synthesis [23]. More detailed study of
reactions of MX5 (M = Nb, Ta; X = F, Cl) with acetals/ketals (1,1-dialkoxyalkanes) or trimethylformate
revealed that the ethereal bonds can be broken by the MX5 Lewis acids and the rate of the process is
enhanced by the presence of the further vicinal ether function. The reaction pathway was found to
include formation of the MX5 (OMe2 ) complexes, which were identified by NMR spectroscopy [24,25].
It prompted us to study reaction of nido-carborane with dimethoxymethane MeOCH2 OMe in the
presence of HgCl2 .
We found that the reaction of potassium 7,8-dicarba-nido-undecaborate K[7,8-C2 B9 H12 ] with
mercury(II) chloride in a mixture of dimethoxymethane and benzene results in the formation of
mixture of symmetrically and asymmetrically substituted dimethyloxonium derivatives 1 and 2, as
well as the corresponding methoxy derivatives K[3] and K[4] (Scheme 1), that was separated by column
chromatography on silica.
+




+













.













0H2&+ 20H
+J&O
EHQ]HQHUHIOX[





+

20H




.





























.>@ 



+


20H




 





&+%+%




+




20H


 



.















20H


.>@ 

Scheme 1. Preparation of dimethyloxonium and methoxy derivatives of nido-carborane.

The 11 B{1 H} NMR spectrum of 1 displays characteristic 1:2:2:2:1:1 pattern with signals at −8.8,
−12.4, −16.9, −21.8, −22.3 and −39.5 ppm, respectively, that agree well with the planar symmetry
of B(10)-substituted nido-carborane cage. The signal corresponding to the B(10) atom is observed at
−8.8 ppm that is close to the corresponding signals in other oxonium derivatives of nido-carborane
[10-R2 O-7,8-C2 B9 H11 ] [11,15,17]. The 1 H NMR spectrum of 1 contains signal of the dimethyloxonium
group at 4.17 ppm, signal of the carborane CH groups at 1.94 ppm, broad signal of the BH groups
in the range 2.6–0.1 ppm and signal of the endo-BH hydrogen at −2.6 ppm. The 13 C NMR spectrum
of 1 contains signals of the dimethyloxonium group and the carborane CH groups at 73.4 ppm and
50

Inorganics 2019, 7, 46

43.1 ppm, respectively. Taking into account the strong electron-donating effect of the boron cage, the
signals of the dimethyloxonum group are very close to those of the trimethyloxonium cation Me3 O+
(4.68 and 78.8 ppm, respectively) [26].
The 11 B{1 H} NMR spectrum of 2 contains nine non-equivalent signals at 8.3, −12.9, −13.8, −19.1,
−21.9, −22.8, −25.3, −34.0, and −39.9 ppm, which is consistent with asymmetry of B(9)-substituted
nido-carborane cage. The signal corresponding to the B(9) is observed at 8.3 ppm, which is close to
the corresponding signal in the diethyloxonium derivative [9-Et2 O-7,8-C2 B9 H11 ] [11]. The 1 H NMR
spectrum of 2 contains signal of the dimethyloxonium group at 4.12 ppm, signals of the carborane CH
groups at 1.94 and 2.02 ppm, broad signal of the BH groups in the range 2.6–0.1 ppm and signal of the
bridging BHB hydrogen at −2.5 ppm. It is worth noting that, unlike the analogous dimethylsulfonium
derivative [9-Me2 S-7,8-C2 B9 H11 ] where the methyl groups are not equivalent [27] due to interaction
of a sulfur lone pair with the B9-B10 antibonding orbital of the nido-carborane cage [28], both methyl
groups in 2 are equivalent indicating free rotation around the B-O bond and low inversion barrier
at the oxygen atom. The 13 C NMR spectrum of 2 contains signals of the dimethyloxonium group at
72.0 ppm and the carborane CH groups at 41.5 and 34.4 ppm.
In the 1 H NMR spectra of K[3] and K[4] the signals of methoxy groups are shifted to high field
in comparison with 1 and 2 up to 3.22 and 3.17 ppm, respectively, and appear as 1:1:1:1 quartets due
to long-range B–H coupling (3 JB,H = 3.7–3.8 Hz). Such coupling has also been previously observed
for some organoboron compounds [29–32], methylsulfanyl derivatives of the closo-dodecaborate
anion [33,34] and B-methysulfanyl derivatives of cobalt bis(dicarbollide) anion [35].
The dimethyloxonium derivatives of nido-carborane can be easily demethylated to the
corresponding methoxy derivatives with triethylamine or pyridine within 30 min at ambient
temperature (Scheme 2). These results demonstrated that the dimethyloxonium derivatives 1 and 2 are
active methylating agents.
+

20H

+

20H

(W1
(W10H
0H&1
r.t.PLQ

(W10H>@ 


+

+
&+10H

20H

S\ULGLQH

20H

r.t.PLQ

0H3\>@ 

+
(W1
0H&1
r.t.PLQ

20H

(W10H

(W10H>@ 



Scheme 2. Demethylation of dimethyloxonium derivatives of nido-carborane.

This prompted us to study reactions of 1 and 2 with 3-methyl-6-nitro-1H-indazole. This compound is
a starting material for the manufacture of pazopanib hydrochloride (Figure 1). Pazopanib hydrochloride
is tyrosine kinase inhibitor and is used clinically as angiogenesis modulating and antineoplastic agent [36].
The first stage of its manufacture includes N-methylation of 3-methyl-6-nitro-1H-indazole. This process is
critical stage since desirable 2,3-dimethyl-6-nitro-2H-indazole (5) is always contaminated with isomeric
1,3-dimethyl-6-nitro-1H-indazole (6). Several papers have reported optional reagents and conditions for
preparation of 5 [37–39], however, laborious recrystallizations have been still required to purify 5 from
isomeric 6.

51

Inorganics 2019, 7, 46

621+

+&O
1
1
+

1
1
+

1

1

1

2 1

PHWK\ODWLRQ

1+
2 1

1+

1



1
1

2 1

1
1

2 1




Figure 1. Pazopanib hydrochloride and critical stage of its manufacture.

Indeed, the both dimethyloxonium derivatives of nido-carborane were found to N-methylate
3-methyl-6-nitro-1H-indazole, however, the results of these reactions were different (Scheme 3).
The reaction of 3-methyl-6-nitro-1H-indazole with 2 in acetonitrile at room temperature followed
by aqueous alkaline treatment led to a 1:1 mixture of 5 and 6 which were resolved by column
chromatography on silica. To our best knowledge, indazole 6 was not described previously.
Surprisingly, the reaction of 3-methyl-6-nitro-1H-indazole with 1 resulting in the regioselective
formation of desired compound 5 with almost a quantitative yield.

1

1+
2 1

1

1

2 1

 

0H&1
r.t.


1
1

2 1

 


2 1

1
1

 

Scheme 3. Methylation of 3-methyl-6-nitro-1H-indazole by 9-dimethyloxonium and 10-dimethyloxonium
derivatives of nido-carborane.

Transition metal complexes with carborane ligands, or metallacarboranes, found application
in a wide variety of fields including nuclear fuel reprocessing [40,41], catalysis [42], new material
design [43–46], medicine [4,5,47–52], etc. Therefore the obtained methoxy derivatives of nido-carborane
K[3] and K[4] were used for synthesis the corresponding iron and cobalt bis(dicarbollide) complexes.
Earlier we described the synthesis of symmetric 8,8 -dimethoxy derivative of cobalt bis(dicarbollide)
[8,8 -(MeO)2 -3,3 -Co(1,2-C2 B9 H10 )2 ]− by alkylation of the corresponding dihydroxy derivative [53].
In this contribution we report synthesis of analogous paramagnetic 8,8 -dimethoxy derivative of iron
bis(dicarbollide) K[8,8 -(MeO)2 -3,3 -Fe(1,2-C2 B9 H10 )2 ] (K[7]) by the reaction of K[3] with anhydrous
FeCl2 in tetrahydrofuran in the presence of potassium tert-butoxide (Scheme 4). The 11 B NMR spectrum
of [7]− contains signals at 114.6, 6.2, −8.0 and −69.1 ppm corresponding to boron atoms, which are

52

Inorganics 2019, 7, 46

the most distant from the metal atom, and the wide high-field signal at −443.2 ppm due to the boron
atoms, which are directly connected to the metal with a general relative integral ratio 2:4:4:2:6.










+







20H


)H&O





















.

.










)H




0H2

20H







t-%X2.7+)





















.>@



.>@ 
Scheme 4. Synthesis of

8,8 -dimethoxy

derivative of iron bis(dicarbollide).

Unlike the 9-methylsulfide derivative [9-MeS-7,8-C2 B9 H11 ]− , the reaction of asymmetric K[4] with
anhydrous FeCl2 unexpectedly gave a single isomer [8]− instead of mixture of rac- and meso-diastereomers
(Scheme 5). The 11 B NMR spectrum of [8]− contains signals at 109.5, 9.7, 7.5, 1.1, −21.8 and −40.7 ppm
corresponding to boron atoms which are the most distant from the metal atom, and the wide high-field
signals at −403.4, −431.7, and −461.1 ppm due to the boron atoms, which are directly connected to the
metal with general relative integral ratio 2:2:2:2:2:2:2:2:2. Based on the comparison of this spectrum
with the 11 B NMR spectra of the methylsulfide derivatives rac-[4,7 -(MeS)2 -3,3 -Fe(1,2-C2 B9 H10 )2 ]−
and meso-[4,4 -(MeS)2 -3,3 -Fe(1,2-C2 B9 H10 )2 ]− [54], we tentatively identified the compound obtained
as the 4,7 -isomer rac-[4,7 -(MeO)2 -3,3 -Fe(1,2-C2 B9 H10 )2 ]− . In a similar way, the reaction of K[4] with
anhydrous CoCl2 in tetrahydrofuran in the presence of potassium tert-butoxide gave diamagnetic
rac-[4,7 -(MeO)2 -3,3 -Co(1,2-C2 B9 H10 )2 ]− as the single isomer (Scheme 5). The 11 B NMR spectrum of [9]−
contains singlets at 13.9 ppm and doublets at 5.2, −0.8, −7.9, −9.0, −19.8, and −24.6 ppm with an integral
intensity ratio 2:2:2:4:2:4:2. The 1 H NMR spectrum of [9]− contains the 1:1:1:1 quartet of the methoxy
group at 3.23 ppm (3 JB,H = 3.9 Hz), signals of the carborane CH groups at 3.81 and 3.70 ppm and broad
signal of the BH groups in the range 2.6–0.5 ppm.







+









20H

.









0&O
t-%X2.7+)
















.>@







%X1 0H2

%X1%U+2









0








20H




















0 )H%X1>@ 
&R%X1>@ 
Scheme 5. Synthesis of 4,7 -dimethoxy derivatives of iron and cobalt bis(dicarbollides).

The reason for the formation of solely the 4,7 -isomers of the dimethoxy derivatives of iron
and cobalt bis(dicarbollides) is not very clear, but it probably caused by a lower stability of the
corresponding 4,4 -isomers.
53

Inorganics 2019, 7, 46

3. Materials and Methods
3.1. General Procedures and Instrumentation
The potassium salt of 7,8-dicarba-nido-caborane was prepared according to the literature
procedure [55]. Dimethoxymethane, tetrahydrofuran and iron(II) chloride were purchased
from Sigma-Aldrich and used without further purification. Triethylamine, pyridine,
3-Methyl-6-nitro-1H-indazole, ethyl acetate and benzene were commercially analytical grade
reagents and used without further treatment. Acetonitrile was dried by distillation over CaH2 using
the standard procedure [56]. Anhydrous CoCl2 was prepared by dehydration of CoCl2 . 6H2 O using
the standard procedure [57]. The reaction progress was monitored by a TLC (Merck F254 silica gel on
aluminum plates) and visualized using 0.5% PdCl2 in 1% HCl in aq. MeOH (1:10). Acros Organics
silica gel (0.060–0.200 mm) was used for column chromatography. The NMR spectra at 400.1 MHz
(1 H), 128.4 MHz (11 B) and 100.0 MHz (13 C) were recorded with a Bruker Avance-400 spectrometer
(Bruker, Zurich, Switzerland) (See Supplementary Materials). The residual signal of the NMR solvent
relative to tetramethylsilane was taken as the internal reference standard for 1 H and 13 C NMR spectra.
11 B NMR spectra were referenced using BF ·Et O as the external standard. Infrared spectra were
3
2
recorded on an IR Prestige-21 (SHIMADZU) instrument (Shimadzu Corporation, Duisburg, Germany).
High resolution mass spectra (HRMS) were measured on a Bruker micrOTOF II instrument (Bruker,
Bremen, Germany) using electrospray ionization (ESI). The measurements were done in a negative
ion mode (3200 V); mass range from m/z 50 to m/z 3000; external or internal calibration was done
with ESI Tuning Mix, Agilent (Santa Clara, CA, USA). A syringe injection was used for solutions in
acetonitrile (flow rate 3 mL/min). Nitrogen was applied as a dry gas; interface temperature was set at
180 ◦ C. The electron ionization mass spectra were obtained with a Kratos MS 890 instrument (Kratos
Analytical Ltd, Manchester, UK) operating in a mass range of m/z 50–800.
3.2. Synthesis
3.2.1. Preparation of 10-Me2 O-7,8-C2 B9 H11 (1), 9-Me2 O-7,8-C2 B9 H11 (2), K[10-MeO-7,8-C2 B9 H11 ]
(K[3]), and K[9-MeO-7,8-C2 B9 H11 ] (K[4])
The potassium salt of 7,8-dicarba-nido-undecaborate (1.00 g, 5.80 mmol) and mercury(II) chloride
(1.60 g, 5.80 mmol) in a mixture of benzene (20 mL) and dimethoxymethane (20 mL) was heated under
reflux for about 4 h. After cooling to room temperature, the solution was decanted, and the residue was
washed with benzene. The washings were combined with the solution and evaporated under reduced
pressure. The column chromatography on silica gel was used for the separation of the substances with
ethyl acetate as an eluent to give white crystalline products 1–4. The first fraction (TLC RF = 0.88)
contained 2, the second (TLC RF = 0.81) contained 1, the third (TLC RF = 0.62) was identified as 4, and
the fourth (TLC RF = 0.17) contained 3.
1. Yield 0.23 g (22%). 1 H NMR (CDCl3 , ppm): δ 4.17 (s, 6H, OCH3 ), 2.03 (s, 2H, CHcarb ), 2.9–0.1 (br s,
8H, BH), −2.6 (br s, 1H, BHB). 13 C NMR (CDCl3 , ppm): δ 73.4 (OCH3 ), 43.1 (CHcarb ). 11 B NMR (CDCl3 ,
ppm): δ −8.8 (s, 1B), −12.4 (d, J = 144 Hz, 2B), −16.9 (d, J = 137 Hz, 2B), −21.8 (d, J = 150 Hz, 2B),
−22.3 (d, J = 126 Hz, 1B), −39.5 (d, J = 145 Hz, 1B). IR (film, cm−1 ): 3035 (br, νC–H ), 2963 (br, νC–H ),
2918 (br, νC–H ), 2849 (br, νC–H ), 2545 (br, νB–H ), 1464, 1447, 1425, 1260. MS (EI) for C4 H17 B9 O: calcd.
m/z 178 [M]+ , obsd. m/z 178 [M]+ .
2. Yield 0.21 g (20%). 1 H NMR (CDCl3 , ppm): δ 4.12 (s, 6H, OCH3 ), 2.02 (s, 1H, CHcarb ), 1.94 (s,
1H, CHcarb ), 2.6–0.1 (br s, 8H, BH), −2.5 (br s, 1H, BHB). 13 C NMR (CDCl3 , ppm): δ 72.0 (OCH3 ),
41.5 (CHcarb ), 34.4 (CHcarb ). 11 B NMR (CDCl3 , ppm): δ 8.3 (s, 1B), −12.9 (d, J = 128 Hz, 1B), −13.8 (d,
J = 131 Hz, 1B), −19.1 (d, J = 166 Hz, 1B), −21.9 (d, J = 135 Hz, 1B), −22.8 (d, J = 126 Hz, 1B), −25.3 (d,
J = 151 Hz, 1B), −34.0 (dd, J = 137 Hz, J = 54 Hz, 1B), −39.9 (d, J = 144 Hz, 1B). IR (film, cm−1 ): 3031 (br,

54

Inorganics 2019, 7, 46

νC–H ), 2963 (br, νC–H ), 2925 (br, νC–H ), 2863 (br, νC–H ), 2524 (br, νB–H ), 1464, 1448, 1423, 1260. MS (EI)
for C4 H17 B9 O: calcd. m/z 178 [M]+ , obsd. m/z 178 [M]+ .
K[3]. Yield 0.33 g (28%). 1 H NMR (acetone-d6 , ppm): δ 3.22 (q (1:1:1:1), 3 JB,H = 3.7 Hz, 3H, OCH3 ),
1.47 (s, 2H, CHcarb ), 2.7–0.0 (br s, 8H, BH), −0.6 (br s, 1H, BHB). 13 C NMR (acetone-d6 , ppm): δ 56.8
(OCH3 ), 38.3 (CHcarb ). 11 B NMR (acetone-d6 , ppm): δ −8.7 (s, 1B), −12.4 (d, J = 137 Hz, 2B), −17.5 (d,
J = 136 Hz, 2B), −24.1 (d, J = 156 Hz, 2B), −25.4 (d, J = 167 Hz, 1B), −40.6 (d, J = 143 Hz, 1B). IR (film,
cm−1 ): 3031 (br, νC–H ), 2983 (br, νC–H ), 2931 (br, νC–H ), 2885 (br, νC–H ), 2526 (br, νB–H ), 1458, 1394, 1206.
ESI HRMS for C3 H14 B9 O− : calcd. m/z 164.1926, obsd. m/z 164.1926.
K[4]. Yield 0.18 g (15%). 1 H NMR (acetone-d6 , ppm): δ 3.17 (q (1:1:1:1), 3 JB,H = 3.8 Hz, 3H, OCH3 ),
1.53 (s, 1H, CHcarb ), 1.34 (s, 1H, CHcarb ), 2.5–0.0) (br s, 8H, BH), −3.0 (br s, 1H, BHB). 13 C NMR
(acetone-d6 , ppm): δ 55.1 (OCH3 ), 39.6 (CHcarb ), 25.8 (CHcarb ). 11 B NMR (acetone-d6 , ppm): δ 11.2 (s,
1B), −12.3 (d, J = 132 Hz, 1B), −16.2 (d, J = 136 Hz, 1B), −19.7 (d, J = 157 Hz, 1B), −21.7 (d, J = 151 Hz,
1B), −25.5 (d, J = 135 Hz, 2B), −31.3 (dd, J = 138 Hz, J = 55 Hz, 1B), −38.7 (d, J = 136 Hz, 1B). IR (film,
cm−1 ): 3035 (br, νC–H ), 2986 (br, νC–H ), 2948 (br, νC–H ), 2930 (br, νC–H ), 2525 (br, νB–H ), 1483, 1451, 1209.
ESI HRMS for C3 H14 B9 O− : calcd. m/z 164.1926, obsd. m/z 164.1927.
3.2.2. Reactions of 10-Me2 O-7,8-C2 B9 H11 and 9-Me2 O-7,8-C2 B9 H11 with Triethylamine
To a solution of 1 (0.10 g, 0.49 mmol) or 2 (0.10 g, 0.49 mmol) in acetonitrile (1 mL), trimethylamine
(0.68 mL, 4.90 mmol) was added. The mixture was stirred at room temperature for about 1 h and the
solution was evaporated under reduced pressure to give yellow crystalline products (Et3 NMe)[3] or
(Et3 NMe)[4], respectively.
(Et3 NMe)[3]. Yield 0.13 g (97%). 1 H NMR (acetone-d6 , ppm): δ 3.57 (q, J = 7.2 Hz, 6H, Et3 NMe+ ),
3.22 (q (1:1:1:1), 3 JB,H = 3.7 Hz, 3H, OCH3 ), 3.19 (s, 3H, Et3 NMe+ ), 1.45 (tt, J = 7.2 Hz, J = 1.9 Hz, 11H,
Et3 NMe+ + CHcarb ), 2.7–0.0 (br s, 8H, BH), −0.6 (br s, 1H, BHB). 13 C NMR (acetone-d6 , ppm): δ 56.2
(OCH3 ), 55.9 (t, Et3 NMe+ ), 46.4 (t, Et3 NMe+ ), 38.3 (CHcarb ), 7.2 (Et3 NMe+ ). 11 B NMR (acetone-d6 ,
ppm): δ −8.7 (s, 1B), −12.4 (d, J = 132 Hz, 2B), −17.5 (d, J = 135 Hz, 2B), −24.2 (d, J = 155 Hz, 2B),
−25.5 (d, J = 171 Hz, 1B), −40.5 (d, J = 140 Hz, 1B). IR (film, cm−1 ): 3030 (br, νC–H ), 2982 (br, νC–H ),
2929 (br, νC–H ), 2886 (br, νC–H ), 2819, 2524 (br, νB–H ), 1456, 1391, 1376, 1303, 1260, 1205. ESI HRMS for
C3 H14 B9 O− : calcd. m/z 164.1926, obsd. m/z 164.1925.
(Et3 NMe)[4]. Yield 0.14 g (98%). 1 H NMR (acetone-d6 , ppm): δ 3.55 (q, J = 7.2 Hz, 6H, Et3 NMe+ ), 3.17 (s,
6H, OCH3 + Et3 NMe+ ), 1.53 (s, 1H, CHcarb ), 1.44 (tt, J = 7.2 Hz, J = 1.9 Hz, 9H, Et3 NMe+ ), 1.34 (s, 1H,
CHcarb ), 2.5–0.0 (br s, 8H, BH), −2.9 (br s, 1H, BHB). 13 C NMR (acetone-d6 , ppm): δ 55.9 (t, Et3 NMe+ ),
55.2 (OCH3 ), 46.4 (t, Et3 NMe+ ), 39.3 (CHcarb ), 25.9 (CHcarb ), 7.2 (Et3 NMe+ ). 11 B NMR (acetone-d6 ,
ppm): δ 11.0 (s, 1B), −12.4 (d, J = 131 Hz, 1B), −16.2 (d, J = 137 Hz, 1B), −19.7 (d, J = 156 Hz, 1B),
−21.6 (d, J = 151 Hz, 1B), −25.5 (d, J = 139 Hz, 2B), −31.2 (dd, J = 139 Hz, J = 55 Hz, 1B), −38.7 (d,
J = 135 Hz, 1B). IR (film, cm−1 ): 3395, 3214, 3034 (br, νC–H ), 2987 (br, νC–H ), 2949 (br, νC–H ), 2931 (br,
νC–H ), 2821, 2520 (br, νB–H ), 1486, 1456, 1396 1208. ESI HRMS for C3 H14 B9 O− : calcd. m/z 164.1926,
obsd. m/z 164.1944.
3.2.3. Reaction of 9-Me2 O-7,8-C2 B9 H11 with Pyridine
Compound 2 (0.10 g, 0.49 mmol) and pyridine (4.90 mmol, 0.4 mL) were stirred at room
temperature for about 1 h and the solution was evaporated under reduced pressure to give yellow
crystalline product (N-MePy)[4]. Yield 0.12 g (98%). 1 H NMR (acetone-d6 , ppm): δ 9.16 (d, J = 5.9 Hz,
2H, o-HAr ), 8.75 (t, J = 7.8 Hz, 1H, p-HAr ), 8.29 (m, 2H, m-HAr ), 4.66 (s, 3H, NCH3 ), 3.16 (q (1:1:1:1),
3J
B,H = 3.8 Hz, 3H, OCH3 ), 1.53 (s, 1H, CHcarb ), 1.34 (s, 1H, CHcarb ), 2.5–0.0 (br s, 8H, BH), −3.0 (br s,
1H, BHB). 13 C NMR (acetone-d6 , ppm): δ 145.8 (t, o-CAr ), 145.5 (p-CAr ), 128.2 (m-CAr ), 55.0 (OCH3 ), 48.3
(t, NCH3 ), 39.6 (CHcarb ), 25.9 (CHcarb ). 11 B NMR (acetone-d6 , ppm): δ 11.2 (s, 1B), −12.3 (d, J = 131 Hz,
1B), −16.2 (d, J = 137 Hz, 1B), −19.7 (d, J = 158 Hz, 1B), −21.7 (d, J = 147 Hz, 1B), −25.5 (d, J = 136 Hz,

55

Inorganics 2019, 7, 46

2B), −31.1 (dd, J = 139 Hz, J = 55 Hz, 1B), −38.7 (d, J = 135 Hz, 1B). IR (film, cm−1 ): 3139, 3133, 3074,
2955 (br, νC–H ), 2930 (br, νC–H ), 2917 (br, νC–H ), 2890 (br, νC–H ), 2848, 2823, 2516 (br, νB–H ), 1636, 1498,
1490, 1287, 1259, 1207. ESI HRMS for C3 H14 B9 O− : calcd. m/z 164.1926, obsd. m/z 164.1943.
3.2.4. Reactions of 10-Me2 O-7,8-C2 B9 H11 and 9-Me2 O-7,8-C2 B9 H11 with 3-Methyl-6-nitro-1H-indazole
a. To a solution of 1 (30 mg, 0.17 mmol) in dried acetonitrile (1 mL) under an Ar atmosphere
3-methyl-6-nitro-1H-indazole (20 mg, 0.11 mmol) was added. The mixture was stirred at room
temperature for about 5 days and the solution was evaporated under reduced pressure. An aqueous
solution of 30% KOH (5 mL) was added. The solution was dropped off and the formed yellow residue
was washed with water and extracted with AcOEt. The residue was purified form the remained
nido-carborane by column chromatography with 1:3 n-hexane/AcOEt to give the only product 5 as a
yellow solid (20 mg, 98%). This product has been described previously and our obtained NMR data
perfectly matched with data represented in the literature [36–38].
b. The procedure was analogous to that described for 3.2.4(a) using 2 (30 mg, 0.17 mmol) and
3-methyl-6-nitro-1H-indazole (20 mg, 0.11 mmol) to give the mixture 1:1 of 5 and 6. Products were
separated by column chromatography with 1:3 n-hexane/AcOEt. The first band (TLC RF = 0.35)
contained 5 (10 mg, 49%), the second (TLC RF = 0.20) was identified as 6 (10 mg, 49%).
NMR data for 5. 1 H NMR (DMSO-d6 , ppm): δ 8.52 (d, J = 1.6 Hz, 1H, H-7), 7.94 (d, J = 9.1 Hz, 1H,
H-5), 7.74 (dd, J = 9.1 Hz, J = 1.9 Hz, 1H, H-6), 4.16 (s, 3H, 2-CH3 ), 2.68 (s, 3H, 3-CH3 ).
NMR data for 6. 1 H NMR (DMSO-d6 , ppm): δ 8.63 (d, J = 1.4 Hz, 1H, H-7), 7.95 (d, J = 8.8 Hz,
1H, H-5), 7.90 (dd, J = 8.8 Hz, J = 1.7 Hz, 1H, H-6), 4.10 (s, 3H, 2-CH3 ), 2.54 (s, 3H, 3-CH3 ). 13 C NMR
(DMSO-d6 , ppm): δ 146.2, 141.5, 139.4, 126.0, 121.8, 114.2, 107.0, 36.0, 11.8.
3.2.5. Synthesis of K[8,8 -(MeO)2 -3,3 -Fe(1,2-C2 B9 H10 )2 ] (K[7])
To a solution of K[3] (0.20 g, 0.98 mmol) in dried tetrahydrofuran under argon atmosphere
potassium tert-butoxide (0.55 g, 4.92 mmol) and anhydrous FeCl2 (0.62 g, 4.92 mmol) were added.
The reaction mixture was refluxed for 12 h and left overnight in the air. The solid was filtered off and
the filtrate was evaporated under reduced pressure. The residue was dissolved in acidified water (1 mL
of HCl in 30 mL of H2 O) and extracted by diethyl ether (2 × 30 mL). Organic fractions were collected
and evaporated under reduced pressure to give 0.15 g (73%) of dark red solid. 1 H NMR (acetone-d6 ,
ppm): δ 79.7 (br s, 4H, CHcarb /BH), 53.5 (br s, 4H, CHcarb /BH), 29.5 (br q, J = 129 Hz, 2H, BH), 2.7 (br
m, 4H, BH), −6.0 (s, 6H, OCH3 ), −10.1 (br q, J = 166 Hz, 4H, BH), −24.1 (br q, 2H, BH). 13 C NMR
(acetone-d6 , ppm): δ 70.2 (OCH3 ), −398.0 (CHcarb ), −408.0 (CHcarb ). 11 B NMR (acetone-d6 , ppm): δ
114.6 (d, 2B), −6.2 (d, 4B), −8.0 (d, 4B), −69.1 (d, 2B), −443.2 (br s, 6B). IR (film, cm−1 ): 3034 (br, νC–H ),
2952 (br, νC–H ), 2926 (br, νC–H ), 2856 (br, νC–H ), 2564 (br, νB–H ), 1696, 1488, 1458, 1377. ESI HRMS for
C6 H26 B18 FeO2 − : calcd. m/z 381.3077, obsd. m/z 381.3069.
3.2.6. Synthesis of (Bu4 N)[4,7 -(MeO)2 -3,3 -Fe(1,2-C2 B9 H10 )2 ] ((Bu4 N)[8])
To a solution of K[4] (0.20 g, 0.98 mmol) in dried tetrahydrofuran under argon atmosphere
potassium tert-butoxide (0.55 g, 4.92 mmol) and anhydrous FeCl2 (0.62 g, 4.92 mmol) were added.
The reaction mixture was refluxed for 12 h. and left overnight in the air. The solid was filtered off
and the filtrate was evaporated under reduced pressure. The residue was dissolved in acidified water
(1 mL of HCl in 30 mL of H2 O) and extracted by diethyl ether (2 × 30 mL). Organic fractions were
collected and evaporated under reduced pressure. The resedue was dissolved in water (10 mL) and
reprecipitated by tetrabutylammonium bromide (0.16 g, 0.5 mmol) in water (5 mL) to give 0.13 g (43%)
of dark red solid. 1 H NMR (acetone-d6 , ppm): δ 69.4 (br s, 2H, CHcarb /BH), 66.3 (br s, 2H, CHcarb /BH),
60.8 (br s, 2H, CHcarb /BH), 53.9 (br s, 2H, CHcarb /BH), 41.6 (br q, J = 135 Hz, 4H, BH), 28.6 (br m,
2H, BH), 3.0 (m, 8H, Bu4 N+ ), 2.9 (s, 6H, OCH3 ), 1.4 (m, 8H, Bu4 N+ ), 0.9 (m, 8H, Bu4 N+ ), 0.7 (m, 12H,
Bu4 N+ ), −2.8 (br q, J = 170 Hz, 2H, BH), −7.6 (br q, 4H, BH). 13 C NMR (acetone-d6 , ppm): δ 77.7
(OCH3 ), 58.1 (t, Bu4 N+ ), 23.1 (Bu4 N+ ), 19.1 (Bu4 N+ ), 12.7 (Bu4 N+ ), −475.2 (CHcarb ), −500.1 (CHcarb ).
56

Inorganics 2019, 7, 46

NMR (acetone-d6 , ppm): δ 109.5 (d, 2B), 9.7 (d, 2B), 7.5 (d, 2B), 1.1 (d, 2B), −21.8 (d, 2B), −40.7 (d,
2B), −403.4 (br s, 2B), −431.7 (br s, 2B), −461.1 (br s, 2B). IR (film, cm−1 ): 2963 (br, νC–H ), 2933 (br,
νC–H ), 2876 (br, νC–H ), 2824 (br, νC–H ), 2559 (br, νB–H ), 1482, 1462, 1381. ESI HRMS for C6 H26 B18 FeO2 − :
calcd. m/z 381.3077, obsd. m/z 381.3068.

11 B

3.2.7. Synthesis of (Bu4 N)[4,7 -(MeO)2 -3,3 -Co(1,2-C2 B9 H10 )2 ] ((Bu4 N)[9])
To a solution of K[4] (0.20 g, 0.98 mmol) in dried tetrahydrofuran under argon atmosphere
potassium tert-butoxide (1.10 g, 9.83 mmol) was added. The mixture was stirred at r.t. for 30 min
and the anhydrous CoCl2 (1.27 g, 9.83 mmol) was added. The reaction mixture was refluxed for
18 h. The solid was filtered off and the filtrate was evaporated under reduced pressure. The residue
was dissolved in water (30 mL) and extracted by diethyl ether (2 × 30 mL). Organic fractions were
collected and evaporated under reduced pressure. The residue was dissolved in water (10 mL) and
reprecipitated by tetrabutylammonium bromide (0.16 g, 0.5 mmol) in water (5 mL) to give 0.14 g (45%)
of orange solid. 1 H NMR (acetone-d6 ): δ 3.81 (s, 2H, CHcarb ), 3.70 (s, 2H, CHcarb ), 3.45 (m, 8H, Bu4 N+ ),
3.23 (q (1:1:1:1), 3 JB,H = 3.9 Hz, 6H, OCH3 ), 1.84 (m, 8H, Bu4 N+ ), 1.45 (m, 8H, Bu4 N+ ), 1.00 (t, 12H,
Bu4 N+ ), 2.6–0.5 (br s, 16H, BH). 13 C NMR (acetone-d6 ): δ 58.5 (t, Bu4 N+ ), 55.6 (OCH3 ), 44.9 (CHcarb ),
23.5 (Bu4 N+ ), 19.5 (Bu4 N+ ), 13.0 (Bu4 N+ ). 11 B NMR (acetone-d6 ): δ 13.9 (s, 2B), 5.2 (d, J = 139 Hz, 2B),
−0.8 (d, J = 137 Hz, 2B), −7.9 (d, J = 142 Hz, 4B), −9.0 (d, J = 142 Hz, 2B), −19.8 (d, J = 152 Hz, 4B),
−24.6 (d, J = 170 Hz, 2B). IR (film, cm−1 ): 3035 (br, νC–H ), 2961 (br, νC–H ), 2926 (br, νC–H ), 2874 (br,
νC–H ), 2853 (br, νC–H ), 2559 (br, νB–H ), 1712, 1478, 1459, 1379. ESI HRMS for C6 H26 B18 CoO2 – : calcd.
m/z 384.3059, obsd. m/z 384.3052.
4. Conclusions
The reaction of nido-carborane [7,8-C2 B9 H12 ]− with dimethoxymethane in the presence of
mercury(II) chloride lead to a mixture of four products that can be separated by column chromatography.
The first two products represent symmetrical and asymmetrical charge compensated dimethyloxonium
derivatives of nido-carborane 10-Me2 O-7,8-C2 B9 H11 and 9-Me2 O-7,8-C2 B9 H11 , whereas two other
products are the corresponding methoxy derivatives of nido-carborane [10-MeO-7,8-C2 B9 H11 ]− and
[9-MeO-7,8-C2 B9 H11 ]− . It was demonstrated, that dimethyloxonium derivatives of nido-carborane
can act as active methylating agents. The reaction of the symmetrical methoxy derivative
[10-MeO-7,8-C2 B9 H11 ]− with anhydrous FeCl2 in tetrahydrofuran in the presence of t-BuOK results in
the corresponding iron bis(dicarbollide) complex [8,8 -(MeO)2 -3,3 -Fe(1,2-C2 B9 H10 )2 ]− , whereas the
similar reactions of the asymmetrical methoxy derivative [9-MeO-7,8-C2 B9 H11 ]− with FeCl2 and CoCl2
give solely the 4,7 -isomers [4,7 -(MeO)2 -3,3 -M(1,2-C2 B9 H10 )2 ]− (M = Fe, Co) rather than a mixture of
rac-4,7 - and meso-4,4 -isomers.
Supplementary Materials: The following are available online at http://www.mdpi.com/2304-6740/7/4/46/s1,
NMR spectra of compounds 1–9.
Author Contributions: M.Y.S. designed the studies, performed synthesis of the nido-carborane and
metallacarborane derivatives, analyzed data and wrote the paper, S.A.E. performed synthesis of nido-carborane
derivatives and study of their stability; I.D.K. performed the NMR studies; A.A.S. performed experiments on
alkylation of 3-methyl-6-nitro-1H-indazole and wrote the paper; I.B.S. designed the studies, analyzed data and
wrote the paper.
Funding: This work was supported by the Russian Science Foundation (Grant No. 17-73-10321).
Acknowledgments: The NMR spectral data were obtained using equipment of Center for Molecular Structure
Studies at A. N. Nesmeyanov Institute of Organoelement Compounds. The basic physical and organizational
structures, facilities and power supplies needed for the operation of the institute are partially supported by
Ministry of Science and Higher Education of the Russian Federation.
Conflicts of Interest: The authors declare no conflict of interest.

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61

inorganics
Article

Comparing the Acidity of (R3P)2BH-Based Donor
Groups in Iridium Pincer Complexes
Leon Maser, Christian Schneider, Lukas Alig and Robert Langer *
Department of Chemistry, Philipps-Universität Marburg, Hans-Meerwein-Str. 4, 35032 Marburg, Germany;
leon.maser@chemie.uni-marburg.de (L.M.); c.schneider2013@gmail.com (C.S.);
lukas.alig@uni-goettingen.de (L.A.)
* Correspondence: robert.langer@chemie.uni-marburg.de; Tel.: +49-6421-282-5617
Received: 31 March 2019; Accepted: 29 April 2019; Published: 7 May 2019

Abstract: In the current manuscript, we describe the reactivity of a series of iridium(III) pincer
complexes with the general formulae [(PEP)IrCl(CO)(H)]n (n = +1, +2) towards base, where PEP is
a pincer-type ligand with different central donor groups, and E is the ligating atom of this group
(E = B, C, N). The donor groups encompass a secondary amine, a phosphine-stabilised borylene and
a protonated carbodiphosphorane. As all ligating atoms E exhibit an E–H bond, we addressed the
question of wether the coordinated donor group can be deprotonated in competition to the reductive
elimination of HCl from the iridium(III) centre. Based on experimental and quantum chemical
investigations, it is shown that the ability for deprotonation of the coordinated ligand decreases in the
order of (R3 P)2 CH+ > R2 NH > (R3 P)2 BH. The initial product of the reductive elimination of HCl from
[(PBP)IrCl(CO)(H)]n (1c), the square planar iridium(I) complex, [(PBP)Ir(CO)]+ (3c), was found to be
unstable and further reacts to [(PBP)Ir(CO)2 ]+ (5c). Comparing the C–O stretching vibrations of the
latter with those of related complexes, it is demonstrated that neutral ligands based on tricoordinate
boron are very strong donors.
Keywords: boron; iridium; pincer; carbodiphosphorane

1. Introduction
Tricoordinate boron compounds, BR3 , are typically Lewis acids and stabilise their electron
deficiency by π-donating substituents, hyperconjugation or dimerisation and formation of two-electron
three-centre bonds. In consequence, they can accept electron donation from electron rich metal centres
and serve as Z-type ligands [1,2]. More recently, several groups demonstrated that the introduction of
π-accepting substituents allows to stabilise an occupied pz -orbital and therewith of a trigonal planar
Lewis-base with the general formulae L2 BR (III) [3–9]. Consequently, such compounds are able to
serve as electron-donating or L-type ligands, but the coordination chemistry of such nucleophilic boron
compounds is rather unexplored [8–10].
In particular, the similarity to related carbon compounds of the type L2 CH+ (II) and secondary
amines (I) caught our attention. Pseudo-tetrahedral, secondary amines (I) can serve as cooperative
ligands in homogeneous catalysts (Figure 1), by providing a proton in concerted proton hydride
transfers or simply by pre-coordination of the substrate via hydrogen bridge bonds (e.g., in Figure 1,
cycle A) [11]. Protonated carbodiphosphoranes of the type (R3 P)2 CH+ (II) can be deprotonated by
strong bases and easily form their deprotonated analogues when coordinated to a metal centre [12].
For the corresponding boron compounds, (R3 P)2 BH (III), previous studies indicated that the
boron-bound hydrogen atom in such ligands is not hydridic [13,14]. Due to the π-accepting nature of
the cyanido substituents in compounds like [HB(CN)3 ]− , they can be deprotonated [15], which stands
in contrast to the reactivity of the majority of hydrogen-containing boron compounds.

Inorganics 2019, 7, 61; doi:10.3390/inorganics7050061

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Inorganics 2019, 7, 61



















































































































































Figure 1. (a) Secondary amines (I), protonated carbodiphosphoranes (II) and phosphine-stabilized
borylenes (III) in comparison; (b) Secondary amine ligands and their role in cooperative catalysis in
comparison to the analogous metal complexes with II and III as ligands.

Motivated by these observations, we began to study a series of isotypical iridium complexes in
their reactivity towards base. Herein, we demonstrate that among this series I–III the carbon-based
ligand II is the most acidic ligand, while for the other ligands a competitive reductive elimination is
observed. In case of the boron-based ligand, this leads to an unique iridium(I) complex. The comparison
with related iridium dicarbonyl complexes reveals strong electron donating properties of donor groups
akin to III.
2. Results and Discussion
As the starting point for our study, we choose the isotypical iridium(III) pincer complexes
1a–1c to investigate.
In this context, we compare the amine based pincer-type complex
[{(PPh2 CH2 CH2 )2 NH}IrCl(CO)(H)]+ Cl – (1a) with the formally carbon(0)- and boron(I)-based
complexes [{(dppm)2 CH}IrCl(CO)(H)]2+ 2 Cl – (1b) and [{(dppm)2 BH}IrCl(CO)(H)]+ Br – (1c) [16].
In principle, the deprotonation of 1a–1c can take place at several positions in the complex, but
commonly either the central donor group E is deprotonated or the hydrido ligand is abstracted in a
reductive elimination (Figure 2).

 

 






 














 























































 





















































































Figure 2. Cooperative ligand site vs. redox reactivity—principle reaction pathways of octahedral
iridium(III) complexes 1a–1c towards base (n = +, 2+). X− = Cl− (a,b), Br− (c)

2.1. Deprotonation vs. Reductive Elimination
The reaction of the cationic complex 1a with one equivalent of LiN(SiMe3 )2 results in the formation
of a new complex 2a (Figure 3), as judged by the observation of a single resonance at 55.5 ppm in the
31 P{1 H} NMR spectrum of the reaction mixture. The resonance at −16.12 ppm in the 1 H NMR spectrum,
63

Inorganics 2019, 7, 61

corresponding to the hydrido ligand in 1a disappears and the absence of a resonance in this region
(0 to −40 ppm) suggests that no hydrido ligand is present in the newly formed 2a (Supplementary
Materials). By comparison of NMR spectroscopic data with analogues isopropyl-substituted iridium
pincer complexes [17], we concluded that the reductive elimination of HCl is the preferred reaction
pathway. Addition of a second equivalent of LiN(SiMe3 )2 resulted in the formation of a mixture of
complexes and the 31 P{1 H} NMR spectrum displayed several new singlet resonances as well as a new
AB spin system (Supplementary Materials). The latter finding either indicates a conformational change
to a facially coordinated ligand with different ligands in trans-position, but this seems to be unlikely for
a square pyramidal iridium(I) complex that is already formed with the first equivalent of base. A second
possibility involves a β-hydride elimination from the amide ligands and subsequent tautomerisation,
as previously observed for different noble metal complexes with this type of ligand [18].






 



























































































Figure 3. Reactivity of 1a towards base.

The NMR spectra of the iridium(III) complex 1b at ambient temperature show the presence of
the cis- and the trans-isomers (ca. 1:1) as well as small quantities of 3b (ca. 1%) [16]. The 1 H NOESY
NMR spectrum of 1b at ambient temperature displays exchange correlations between the hydride
resonances of cis- and trans-1b, as well as between the resonances of trans-1b and 3b (Figure 4a).
These findings suggests the presence of an equilibrium between the two isomers of 1b (Figure 4b).
Furthermore one of the isomers (trans-1b) seems to be in an equilibrium with the deprotonated species
1b, even though no additional base is present in the mixture. A broad resonance at 3.51 ppm in the 1 H
NMR spectrum is assigned to HCl [19], which provides further support for reversible (de)protonation
equilibrium. To get further insights about the solution behaviour of 1b, we acquired 1 H and 1 H{31 P}
NMR spectra at different temperatures. The ratio of integrals for the hydride resonances enables to
estimate the equilibrium constant Kcis/trans at different temperatures. The corresponding Van’t Hoff
plot (Figure 4c) displays two regions of linearity between 300 and 270 K (R2 = 0.995) as well as between
260 and 230 K (R2 = 0.937), which might be related to the presence of a second equilibrium or solubility
issues at low temperatures. However, a reliable quantification of 3b turned out to be difficult, due to
the low concentration at ambient temperature, which decreases even further at lower temperatures.
The corresponding exchange rates were accessed by line-shape-analysis of the hydride resonances in
the 1 H{31 P} NMR spectra at different temperatures. An Eyring analysis (Figure 4d) revealed an Gibbs
|
=
enthalpy of activation ΔG298 = 69.23 kJ·mol−1 for the cis-/trans-isomerisation process.
In view of the primary question, these observations suggest that 1b gets selectively deprotonated
at the coordinated donor group. The iridium(III) complex 3b is indeed observed by NMR spectroscopy
in reactions with base. As complex 1b, in contrast to 1a and 1c, is dicationic, one would expect a higher
acidity of the coordinated donor group, but the acidity of hydrido ligands was previously demonstrated
to be increased by several orders of magnitude with an increasing charge of the complex [20].
Addition of an excess base (DBU) to 1b results in the formation of the iridium(I) complex
4b as major product according to the 31 P{1 H} NMR spectrum of the reaction mixture (Figure 5),
which displayed new triplet resonances at 23.4 ppm (2 JP,P = 48.5 Hz) and 38.3 ppm (2 JP,P = 49.3 Hz).
A broad multiplet resonance at 4.01–4.12 ppm with an integral of four in combination with multiplet
resonances between 6.9 and 7.8 ppm with an overall integral of 40 protons are observed in the 1 H NMR
spectrum (Supplementary Materials), while the absence of resonances corresponding to a hydrido
ligand or a protonated CDP moiety indicate that a deprotonated pincer ligand is coordinated in 4b.
The observation of one band at 1925 cm−1 for the C–O stretching vibration of a carbonyl ligand

64

Inorganics 2019, 7, 61

is in line with an electron-rich mono-carbonyl complex. The composition of the cationic complex
[{(dppm)2 C}Ir(CO)]+ Cl – in 4b was further confirmed by high resolution ESI-MS.

Figure 4. (a) Hydride region in the 1 H NOESY NMR spectrum at ambient temperature, showing
chemical exchange correlations; (b) Equilibrium of the complexes in solution; (c) Van’t Hoff plot for the
cis-/trans-isomerisation of 1b; (d) Eyring plot for the cis-/trans-isomerisation of 1b.


 

 




 















 


































































































































































Figure 5. Reactivity of 1b towards base.

A similar observation to the reaction of 1a is made for the boron-based iridium pincer
complex (1c). Treatment of complex 1c with one equivalent LiN(SiMe3 )2 leads to the formation of
two species according to the 31 P{1 H} NMR spectrum of the reaction mixture, broadened resonance
at −5.6 ppm, as well as a broad resonance at 24.9 and a multiplet at 2.9 ppm, both assignable to the
newly formed complex 5c (Figure 6). After removal of all volatiles and washing of the residue with
n-hexane, complex 5c is obtained in analytically pure form. The 1 H NMR spectrum of 5c shows a
complete set of resonances for the dppm arms of the coordinated ligand (Supplementary Materials),
65

Inorganics 2019, 7, 61

while resonances corresponding to a boron-bound hydrogen atom and potential hydrido ligands are
absent (Figure 7b). Upon 11 B-decoupling a triplet resonance at 3.20 ppm (2 JP,H = 23.2 Hz) is observed
in the 1 H{11 B} NMR spectrum, assignable to a boron-bound hydrogen atom, clearly indicating that a
reductive elimination is favoured over of the ligand deprotonation. The 11 B{1 H} NMR spectrum of 5c
gives rise to a broadened resonance at −35.4 ppm, which is in agreement with previously reported
boron-based donor ligands [8–10,13,21]. The identity of 5c was finally confirmed by single crystal
X-Ray diffraction experiments (Figure 7a), which revealed a cationic iridium(I) complex with a trigonal
bipyramidal environment (τ5 = 0.70) [22]. In addition to the facially coordinated PBP-ligand, two
carbonyl ligands are observed, one occupying an equatorial and one an axial coordination site. The
Ir–B bond in 5c is with 2.276 Å slightly shorter than in the octahedral iridium(III) complex 1c (d Ir− B =
2.285 Å) [16].
As the yield of the dicarbonyl complex 5c was below 50% and no other potential source of carbon
monoxide was present in the reaction mixture, we assumed that the formation of 5c proceeds via a
square planar iridium(I) intermediate 2c that subsequently reacts in carbonyl transfer step to 5c and
unidentified decomposition products (Figure 6). This hypothesis is further verified by an increased
yield of 59% in the deprotonation reaction in the presence of carbon monoxide.


 














 



 












































































 






















































Figure 6. Reactivity of 1c towards base.
a)

b)
1

11

H NMR

B{1H} NMR

P
P

-30

-40

B
P
P

1

Ir

C
C

H{11B} NMR

31

-50
δB / ppm

-60

-70

P{1H} NMR

O
3.50

3.40

3.20

3.30

δH / ppm

3.10

3.00

2.90

30

20

10
δP / ppm

0

O

Figure 7. (a) Molecular structure of the cationic complex in 5c in the solid state (ellipsoids are drawn at
50% probability level; carbon atoms of the phenyl rings, carbon-bound hydrogen atoms, co-crystallized
solvent molecules and counter ion are omitted for clarity); (b) Selected NMR spectra of complex 5c.

2.2. Proton Affinities and Deprotonation Pathways
Quantum chemical investigations using density functional theory (DFT) were performed to
get further insights about the reactivity of the reported iridium complexes towards bases. First we
confirmed that deprotonation of the coordinated donor group results in an energetic minimum (3a–3c)
according to the frequency calculation (no imaginary modes) and calculated the proton affinities (PAs)
for 3a–3c (Table 1 and Figure 8). In agreement with the experimental results, complex 3b exhibits the
lowest proton affinity (PA, represents the energy difference between complexes calculated without
solvation and counter ions; the energy of free proton is not considered) with 864 kJ·mol−1 , while
the PAs of the neutral complexes 3a (1129 kJ·mol−1 ) and 3c (1257 kJ·mol−1 ) are significantly higher.

66

Inorganics 2019, 7, 61

The low PA of the CDP-group in the coordinated pincer-type ligand indicates that it might be less
efficient as internal base in a potential catalyst, but in turn it suggests that protonated CDPs might be
potential cooperative groups that facilitate an efficient proton-hydride-transfer from or to the catalyst.
In comparison, the value of 1257 kJ·mol−1 is too high to expect metal-ligand-cooperativity
via proton-hydride-transfer, but it clearly suggests that deprotonation of coordinated (R3 P)2 BH
groups should be facile with strong bases in the absence of more acidic sites, which would yield
an unprecedented phosphine-stabilized boride.
Table 1. Calculated Proton affinities of complexes 3a–3c and 6a–6c (G16, B97D/def2-TZVPP).
Donor in 1

Reactivity

PA(3)/kJ·mol−1

Reactivity

PA(6)/kJ·mol−1

ΔPA/kJ·mol−1

R2 NH
(Ph2 RP)2 CH
(Ph2 RP)2 BH

1a→3a
1b→3b
1c→3c

1129
864
1257

1a→6a
1b→6b
1c→6c

1126
900
1175

3
−36
82

To elucidate the reductive elimination pathway, we removed a proton from the metal-coordinated
hydrido ligand in 1a–1c in a gedankenexperiment and performed geometry optimisations. The resulting
complexes (6a–6c) exhibit elongated iridium chloride distances (Figure 8), but were confirmed as
energetic minima by frequency calculations. Although the Ir–Cl distances in 6a–6c are in range
between a weak bond (2.737 Å) and non-bonding (4.181 Å), the resulting proton affinities may be used
as estimate in comparison to 3a–3c.
H
E

[Ir]

- H+

H

H
E

[Ir]

- H+

H
E

[Ir]

1257
3c

PA / kJ mol-1

1175
1129

6c

1126

3a
900

6a

864

1a-1c
0.0
3b

6b

Figure 8. Proton affinities and DFT-optimized structures of 3a–3c and 6a–6c (G16, B97D/def2-TZVPP).

It becomes evident that in case of the amine-based ligand product of ligand- (3a) and
metal-deprotonation (6a) exhibit very similar proton affinities (ΔPA = 3 kJ·mol−1 ), which suggests that
67

Inorganics 2019, 7, 61

both pathways are in principle favourable. The experimentally observed selectivity for the reductive
elimination might be kinetically favoured. In case of the protonated CDP-based ligand in 1b the
ligand deprotonation is favoured 36 kJ·mol−1 over the deprotonation at the metal site, which again is
in line with the experimental observations. Notably, both PAs, of 3b and 6b, are rather low. For the
boron-based pincer-type ligand in 1c the deprotonation at the metal centre is clearly favoured.
2.3. Comparison with Related Iridium(I) Dicarbonyl Complexes
In comparison to related trigonal bipyramidal iridium(I) dicarbonyl complexes, 5c exhibits
very similar structural features (Table 2). All complexes with two Ph2 RP-groups and one carbonyl
ligand in the equatorial plane differ in the ligand or donor group in the apical position, trans to
the second carbonyl ligand [23–25]. With τ5 -parameters between 0.58 and 0.75, four of the
five complexes are best described as trigonal bipyramidal complexes. In the IR spectrum, two bands
for the C–O-stretching frequency are observed for each complex, which in principle allow to
estimate the net electron donor ability of the specified donor group in comparison. Like for other
dicarbonyl-based ligand parameters [26–28], averaging of cis- and trans-influences on symmetric
and asymmetric C–O-stretching modes can provide a rough picture of the net donor strength.
For the neutral complexes, all values, respectively, indicate an increasing donor ability in the order
R3 SiCH2 – > Cl – > Br – . The cationic complex with a Ph2 RP-group in the apical position gives rise
to an increased value of ν̃CO (av) = 1996 cm−1 , confirming that anionic ligands exhibit stronger
donor abilities. An unexpected finding in this context is the low value measured for complex 5c
(ν̃CO (av) = 1958 cm−1 ), which is significantly lower than those of the anionic donor groups. Despite the
fact that donor groups based on (Ph3 P)2 BH are overall neutral, this observation suggests that they are
stronger donors than alkyl-ligands, which are known as one of the strongest donors in coordination
and organometallic chemistry.
Table 2. Comparison of Iridium(I) dicarbonyl complexes from literature with the new complex 5c.
Complex

Donor

τ5

ν̃CO /cm−1

ν̃CO (av)/cm−1

Ref.

Ph2 RP

0.42

2047, 1944

1996

[23]

Br−

0.70

2023, 1950

1987

[24]

Cl−

0.58

2017, 1944

1981

[24]

R3 SiCH2−

0.75

2001, 1927

1964

[25]

(R3 P)2 BH

0.70

2000, 1916

1958

this work





































  

 





















3. Materials and Methods
All experiments were carried out under an atmosphere of purified argon or nitrogen in the
MBraun glove boxes LABmaster 130 and UNIlab or using standard Schlenk techniques. THF and
diethyl ether were dried over Na/K alloy, n-hexane was dried over LiAlH4 , toluene was dried over

68

Inorganics 2019, 7, 61

sodium, dichloromethane was dried over CaH2 , methanol was dried over magnesium and ethyl
acetate was dried over potassium carbonate. After drying, solvents were stored over appropriate
molecular sieves. Deuterated solvents were degassed with freeze-pump-thaw cycles and stored over
appropriate molecular sieves under argon atmosphere. Complexes 1a–1c synthesised according to
previously reported procedures [16].
1 H, 13 C, 11 B and 31 P NMR spectra were recorded using Bruker BioSpin GmbH (Rheinstetten,
Germany) Avance HD 250, 300 A, DRX 400, DRX 500 and Avance 500 NMR spectrometers at 300 K.
1 H and 13 C{1 H}, 13 C-APT (attached proton test) NMR chemical shifts are reported in ppm downfield
from tetramethylsilane. The resonance of the residual protons in the deuterated solvent was used
as internal standard for 1 H NMR spectra. The solvent peak of the deuterated solvent was used as
internal standard for 13 C NMR spectra. The assignment of resonances in 1 H and 13 C NMR spectra was
further supported by 1 H COSY, 1 H NOESY, 1 H,13 C HMQC and 1 H,13 C HMBC NMR spectra. 11 B NMR
chemical shifts are reported in ppm downfield from BF3 · Et2 O and referenced to an external solution
of BF3 · Et2 O in CDCl3 . 31 P NMR chemical shifts are reported in ppm downfield from H3 PO4 and
referenced to an external 85 % solution of phosphoric acid in D2 O. The following abbreviations are
used for the description of NMR data: br (broad), s (singlet), d (doublet), t (triplet), q (quartet), quin
(quintet), m (multiplet). FT-IR spectra were recorded by attenuated total reflection of the solid samples
on a Bruker Tensor IF37 spectrometer. The intensity of the absorption band is indicated as w (weak),
m (medium), s (strong), vs (very strong) and br (broad). HR-ESI mass spectra were acquired with a
LTQ-FT mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The resolution was set
to 100,000.
Reactivity
of
[({Ph2 PCH2 CH2 }2 NH)IrCl(CO)(H)]Cl
(1a)
towards
base
20
mg
[({Ph2 PCH2 CH2 }2 NH)IrCl(CO)(H)]Cl (1a, 27.3 μmol, 1.0 eq.) and 4.6 mg LiHMDS (27.5 μmol,
1.0 eq.) were suspended in 0.6 mL THF-d8 . After stirring for 16 h, the resulting light orange
suspension was filtered and, after addition of 0.2 ml THF-d8 , the first NMR spectra were recorded.
[({Ph2 PCH2 CH2 }2 NH)Ir(CO)]Cl (2a) was identified as the main product, while small amounts of 1a
remained unreacted. Further 4.7 mg of LiHMDS (27.5 μmol, 1.0 eq.) were added, upon which the color
changed to a dark orange, and the second set of NMR spectra were recorded.
NMR spectra after addition of 1.0 eq. LiHMDS: 1 H NMR (300 MHz, THF-d8 , 300 K): δ = 2.61–2.86
(m, 4H, CH2 ), 3.08–3.46 (m, 4H, CH2 ), 7.03–7.24 (m, 4H, Harom ), 7.25–7.51 (m, 12H, Harom ), 7.73–8.03
(m, 4H, Harom ) ppm. Neither N–H nor Ir–H resonances could be identified. 31 P{1 H} NMR (122 MHz,
THF-d8 , 300 K) δ = 31.7 (s, 1a), 55.5 (br s, 2a) ppm.
NMR spectra after addition of 2.0 eq. LiHMDS: 31 P{1 H} NMR (122 MHz, THF-d8 , 300 K)
δ = −3.8 (s), −0.9 (s), 25.0 (s), 31.9 (s, 1a), 36.1 (s), 39.8 (d, JP,P = 291.7 Hz), 52.6 (d, JP,P = 292.3 Hz),
56.1 (br s, 2a) ppm. 1 H NMR (300 MHz, THF-d8 , 300 K): Due to the multiple decomposition products
visible in the 31 P{1 H} NMR spectrum, no analysis was performed.
Formation of [({dppm}2 C)Ir(CO)]Cl (4b) 57 mg [({dppm}2 CH)IrCl(CO)(H)]Cl2 (1b, 51.4 μmol,
1.0 eq.) were dissolved in 2 mL deuterated dichloromethane. After addition of 15.3 μL DBU (103 μmol,
2.0 eq.), the solution changed color from colorless to yellow. After removal of the solvent in vacuo,
a yellow solid remained, containing [({dppm}2 C)Ir(CO)]Cl (4b). 1 H NMR (300 MHz, CD2 Cl2 ,
300 K): δ = 4.01–4.12 (m, 4H, CH2 ), 7.06–7.18 (m, 8H, Harom. ), 7.25–7.46 (m, 24H, Harom. ), 7.59–7.78
(m, 8H, Harom. ) ppm. 13 C APT NMR (75 MHz, CD2 Cl2 , 300 K): δ = 129.0–129.3 (m, Carom. ), 131.4
(br s, Carom. ), 132.7 (br s, Carom. ), 132.9 (t, JC,P = 5.1 Hz, Carom. ), 133.4 (t, JC,P = 7.2 Hz, Carom. ) ppm.
Neither the carbonyl nor the CH2 resonances were observed. 31 P{1 H} NMR (122 MHz, CD2 Cl2 , 300 K)
δ = 23.4 (t, 2 JP,P = 48.5 Hz), 38.3 (t, 2 JP,P = 49.3 Hz) ppm. FT-IR/cm−1 : 3050 (w), 2962 (w), 2932 (m), 2925
(m), 2858 (m), 2855 (w), 2013 (w), 1979 (w), 1925 (s, CO), 1646 (s), 1612 (s), 1586 (s), 1481 (m), 1434 (s),
1323 (s), 1207 (w), 1119 (m), 1103 (m), 1097 (s), 1070 (s), 824 (m), 740 (s), 721 (m), 691 (s), 543 (m),
527 (m), 503 (s), 481 (s). HRMS: (ESI+, MeCN/CH2 Cl2 ): 1001.1966 [({dppm}2 C)Ir(CO)]+ measured,
1001.1972 calculated, Δ = 0.60 ppm.

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Inorganics 2019, 7, 61

Synthesis of [({dppm}2 BH)Ir(CO)2 ]Br (5c) Complex 1c was generated in situ by the reaction of
90.0 mg [IrCl(CO)(PPh3 )2 ] (116 μmol) with 100.0 mg of [(dppm)2 BH2 ]Br (116 μmol, 1.0 eq.) in 5 mL
THF. The resulting solution of 1c was cooled to −74 ◦ C and 20.0 mg LiN(SiMe3 )2 (116 μmol, 1.0 eq.)
dissolved in 2 mL THF were added drop-wise. The reaction mixture was allowed to warm to ambient
temperature, the argon atmosphere was replaced by carbon monoxide and the mixture was stirred
for further two hours at ambient temperature. All volatiles were removed in vacuo, the residue was
washed with 5 mL toluene and dried under high vacuum to yield 74.0 mg of a colorless solid, containing
[({dppm}2 BH)Ir(CO)2 ]Br (4c, 68 μmol, 59 %). 31 P{1 H} NMR (101.3 MHz, CD2 Cl2 , 300 K): δ = 25.4
(br, 2P, P–B–P), 3.5-2.2 (m, 2P, P–Ir–P) ppm. 11 B{1 H} NMR (96.3 MHz, CD2 Cl2 , 300 K): δ = −35.4 (br, 1B,
BH) ppm. Only resonances that are change upon 11 B-decoupling are reported in the 1 H{11 B} NMR
spectrum. 1 H NMR (300 MHz, CD2 Cl2 , 300 K): δ = 7.51–7.66 (m, 4H, Harom. ), 7.40–7.49 (m, 8H, Harom. ),
7.08–7.31 (m, 8H, Harom. ), 6.82–7.10 (m, 20H, Harom. ), 5.42–5.61 (m, 2H, CH2 ), 4.07–4.16 (m, 2H, CH2 )
ppm. 1 H{11 B} NMR (300 MHz, CD2 Cl2 , 300 K) δ = 3.20 (t, 2 J HP = 23.2 Hz, 1H, BH) ppm. 13 C{1 H} NMR
(121.5 MHz, CD2 Cl2 , 300 K) δ = 134.7 (vt, 4C, Carom. ), 133.5 (s, 4C, Carom. ), 133.0 (s, 4C, Carom. ), 132.0
(s, 4C, Carom. ), 131.3 (s, 4C, Carom. ), 130.9 (vt, 4C, Carom. ), 130.2 (s, 4C, Carom. ), 129.2 (s, 4C, Carom. ), 129.2
(s, 4C, Carom. ), 128.9 (s, 4C, Carom. ), 128.7 (s, 4C, Carom. ), 128.3 (s, 4C, Carom. ), 33.5 (vt, 1C, CH2 ), 30.3
(vt, 1C, CH2 ) ppm. FT-IR: ν̃/cm−1 = 3050 (w), 3017 (w), 2962 (w), 2823 (w), 2724 (w), 2000 (s, CO), 1916
(s, CO), 1586 (w), 1574 (w), 1483 (m), 1434 (s), 1379 (w), 1333 (w), 1306 (w), 1260 (m), 1094 (s), 1024
(s), 869 (w), 797 (s), 778 (s), 731 (vs), 685 (vs), 616 (w), 554 (m), 523 (s), 480 (s). HRMS (ESI+, MeOH)
m/z = 969.1884 [({dppm}2 BH)Ir(CO)2 ]+ , calc. 969.1887 (Δ = 0.31 ppm).
4. Conclusions
In the current manuscript, we reported the first iridium(I) complex formally containing
phosphine-stabilised borylene as a donor group. The comparison to related iridium(I) dicarbonyl
complexes suggests strong donor properties of this type of nucleophilic boron compounds.
In an internal competition with a hydrido-ligand, the reactivity towards base reveals that analogous
carbon compounds and protonated CDPs are easy to deprotonate, while only strong bases contribute
to deprotonate phosphine-stabilized borylenes in the coordination sphere of a central metal atom.
Supplementary Materials: The following are available online at http://www.mdpi.com/2304-6740/7/5/61/s1,
Figures S1–S12: NMR and IR spectra of compounds 2a, 4b and 5c; Table S1: crystallographic data for compound 5c;
xyz-coordinates.
Author Contributions: L.M., C.S. and L.A. performed the experiments. All calculations were made by L.M., R.L.
and L.M. wrote the manuscript. R.L. designed and directed the project.
Funding: This work was supported by the Deutsche Forschungsgemeinschaft (LA 2830/3-2, 2830/5-1 and 2830/6-1).
Acknowledgments: R.L. is grateful to S. Dehnen for her continuous support.
Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations
The following abbreviations are used in this manuscript:
CDP
DBU
dppm
DFT
ESI
HMDS
NMR
HRMS
THF

carbodiphosphorane
1,8-Diazabicyclo[5.4.0]undec-7-ene
1,1-bis(diphenylphosphino)methane
density functional theory
electro spray ionisation
hexamethyldisilazane
nuclear magnetic resonance
high resolution mass spectrometry
tetrahydrofurane

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Inorganics 2019, 7, 61

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inorganics
Article

On the Aqueous Solution Behavior of C-Substituted
3,1,2-Ruthenadicarbadodecaboranes
Marta Gozzi, Benedikt Schwarze, Peter Coburger and Evamarie Hey-Hawkins *
Institute of Inorganic Chemistry, Leipzig University, Johannisallee 29, 04103 Leipzig, Germany
* Correspondence: hey@uni-leipzig.de; Tel.: +49-341-9736151
Received: 26 June 2019; Accepted: 16 July 2019; Published: 22 July 2019

Abstract: 3,1,2-Ruthenadicarbadodecaborane complexes bearing the [C2 B9 H11 ]2− (dicarbollide) ligand
are robust scaffolds, with exceptional thermal and chemical stability. Our previous work has shown
that these complexes possess promising anti-tumor activities in vitro, and tend to form aggregates (or
self-assemblies) in aqueous solutions. Here, we report on the synthesis and characterization of four
ruthenium(II) complexes of the type [3-(η6 -arene)-1,2-R2 -3,1,2-RuC2 B9 H9 ], bearing either non-polar
(R = Me (2–4)) or polar (R = CO2 Me (7)) substituents at the cluster carbon atoms. The behavior in
aqueous solution of complexes 2, 7 and the parent unsubstituted [3-(η6 -p-cymene)-3,1,2-RuC2 B9 H11 ]
(8) was investigated via UV-Vis spectroscopy, mass spectrometry and nanoparticle tracking analysis
(NTA). All complexes showed spontaneous formation of self-assemblies (108 –109 particles mL−1 ),
at low micromolar concentration, with high polydispersity. For perspective applications in medicine,
there is thus a strong need for further characterization of the spontaneous self-assembly behavior in
aqueous solutions for the class of neutral metallacarboranes, with the ultimate scope of finding the
optimal conditions for exploiting this self-assembling behavior for improved biological performance.
Keywords: metallacarborane; ruthenium; aggregation; UV-Vis spectroscopy; NTA

1. Introduction
Metallacarborane complexes of the icosahedral type can be roughly divided into two categories:
those which feature an exo-polyhedral bond to a metal ion, and those where the metal is coordinated
by an approximately planar open face of the carborane cluster, e.g., the C2 B3 open face of
nido-[C2 B9 H11 ]2− , commonly known as “dicarbollide” (see Appendix A for cluster nomenclature) [1].
Complexes belonging to the latter typically show closo structures, formally derived from the parent
C2 B10 H12 clusters by replacement of a BH unit with an isolobal metal complex fragment (Figure 1),
which therefore contributes three orbitals to the cluster bonding [2].

Figure 1.
General structure of 1,2-dicarba-closo-dodecaborane(12) (left) and
3,1,2-closo-metallacarboranes(11) (right). Only one isomer per each structure is shown. For cluster
nomenclature see Appendix A.

Inorganics 2019, 7, 91; doi:10.3390/inorganics7070091

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Inorganics 2019, 7, 91

One main motivation that pushes investigations on the chemistry and physico-chemical properties
of metallacarboranes is the long-known isolobal analogy between the cyclopentadienyl (C5 H5 − ,
Cp− ) ligand and the dicarbollide C2 B9 H11 2− cluster [3]. This is, in turn, reflected in the types of
application which have been investigated for metallacarborane complexes, ranging from catalysis [4],
to medicine [5] and materials science [6] where often the performance of the metallacarborane is
evaluated in comparison to analogous Cp-based complexes (see, for example, Grishin et al. in Pol. Sci.
(2015) [7], and Louie et al. in J. Med. Chem. (2011) [8]).
Recently, we have focused on mixed-sandwich ruthenacarborane complexes of the type
closo-[3-(η6 -arene)-3,1,2-RuC2 B9 H11 ] (with arene = p-cymene, biphenyl, 1-Me-4-CO2 Et-C6 H4 ), and on
half-sandwich molybdacarboranes of the type [3-{L-κ2 N,N}-3-(CO)2 -closo-3,1,2-MoC2 B9 H11 ] (with
L = N,N-chelating ligand) for potential applications in medicine, specifically as anti-tumor
agents [9,10]. In our previous investigations, we showed that the ruthenacarboranes are chemically
exceptionally stable compounds under biologically relevant conditions and possess moderate
anti-proliferative activities in vitro against human colorectal carcinoma and breast adenocarcinoma
cell lines, and a 10× higher selectivity towards cancer cell lines than to healthy cells (primary
fetal fibroblasts and macrophages). Moreover, spectrophotometric studies on aqueous solutions of
closo-[3-(η6 -biphenyl)-3,1,2-RuC2 B9 H11 ] strongly suggested a tendency to form aggregates, at low
micromolar concentrations of the complex [9]. The dynamics of aggregation for the anionic
metallacarboranes of type [commo-3,3’-Co(1,2-C2 B9 H11 )2 ]− (COSAN) are broadly studied in the
literature [11–13], and these complexes are generally described as non-classical amphiphiles which
spontaneously self-assemble into nano- or microstructures [14]. On the other hand, no studies are found
on the aggregation properties of neutral closo-metallacarboranes. Moreover, for potential application in
medicine, characterization of the aggregation behavior of a drug candidate is of primary importance,
for ensuring validity and reproducibility of the biological tests, as already discussed for aggregate-based
organic inhibitors [15]. Here, we report a small series of 3,1,2-ruthenadicarbadodecaborane(11)
complexes, bearing either polar (R = CO2 Me) or non-polar (R = Me) groups at the carbon atoms of
the dicarbollide ligand. The complexes were fully characterized, and the formation of aggregates in
aqueous solutions was investigated via UV-Vis spectroscopy, mass spectrometry, and nanoparticle
tracking analysis (NTA).
2. Results and Discussion
2.1. Synthesis and Characterization of Complexes 2–4 and 7
Complex 2, which bears a p-cymene ligand, is a known compound and was synthesized according
to the literature [16]. Complexes 3 and 4 (Figure 2) were synthesized in moderate yields (45% for
3, 32% for 4), in an analogous way as previously reported [9], from Tl[3-Tl-1,2-Me2 -3,1,2-C2 B9 H9 ]
(1) and the respective ruthenium(II)–arene dimer [{(η6 -arene)RuCl(μ-Cl)}2 ] (arene = biphenyl or
1-Me-4-CO2 Et-C6 H4 ). The spectroscopic data for complexes 2 to 4 are in accordance with those reported
for mixed-sandwich closo-ruthenacarboranes, which also incorporate an arene ligand [9,17–19].

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Figure 2. Structure of complexes 2 to 4.

Complex 7 was synthesized in three steps from 1,2-(CO2 Me)2 -closo-1,2-C2 B10 H10 (5) (Scheme 1).
5 was deboronated under mild conditions (MeCN/H2 O (2:1) (v/v) at room temperature) [20], to avoid
cleavage of the Ccluster –CO2 Me exo-skeletal bonds. For the deprotonation of 6, thallium(I) ethanolate
was used as base at low temperature (−30 ◦ C), instead of the KOH/thallium(I) acetate couple at 0 ◦ C,
used by Safronov et al. for the deprotonation of unsubstituted [nido-7,8-C2 B9 H12 ]− [21], to avoid
base-promoted cleavage of the methoxy ester.

Scheme 1. Synthesis of 7 from 1,2-(CO2 Me)2 -closo-1,2-C2 B10 H10 (5).

The weighted average (see definition in Appendix B) of the 11 B NMR signals of 7 is +3.5 ppm,
which is in accordance to previously reported values for pseudocloso-ruthenacarborane structures [16,22]
that are formally derived from a closo structure via breaking of the Ccluster –Ccluster bond. In comparison,
the weighted average of the 11 B signals for 2, 3, and 4 is −13.6, −12.8, and −11.7 ppm, respectively,
which indicates closo structures. X-ray diffraction analysis of single crystals of 4 and 7 confirmed the
closo and pseudocloso structures (Figure 3), with C(1)· · · C(2) distances of 1.680(5) Å and 2.243(2) Å,
respectively. It is not unexpected that complex 7 presents a pseudocloso structure, since closo-to-pseudocloso
cluster deformation is a commonly encountered phenomenon in ruthenacarborane complexes, when
carbon-bound substituents introduce additional electron density into the Ccluster –Ccluster bond, as in
the case of phenyl substituents reported by Brain et al. and Bould et al. [16,22]. The structural
75

Inorganics 2019, 7, 91

distortions in 7 are generally in accordance with those reported by Welch and co-workers for
pseudocloso-[3-(η6 -arene)-1,2-Ph2 -3,1,2-RuC2 B9 H9 ] [22]. For example, the Ru–B(6) distance in 7 is 2.979(2)
Å, which is 0.5 Å shorter than in the corresponding undistorted closo-[3-(η6 -p-cymene)-3,1,2-RuC2 B9 H11 ]
(8) (Table 1) [9], and the B(6)–B(10) and the C(1)–B(4) bonds are 1.885(2) Å (vs. 1.759(1) Å in 8) and
1.636(2) Å (vs. 1.718(1) Å in 8), respectively. The B(4)–B(5) bond is, however, 0.04 Å longer in the
pseudocloso structure 7, compared to the closo one (8), in contrast to what was observed by Welch for
diphenyl-substituted pseudocloso-[3-(η6 -arene)-1,2-Ph2 -3,1,2-RuC2 B9 H9 ] complexes, with respect to the
corresponding closo-1,2-Ph2 -C2 B10 H10 [22].

Figure 3. Molecular structures of 4 (left) and 7 (right). Thermal ellipsoids are shown at the 50%
probability level. Hydrogen atoms are omitted for clarity. Numbering of selected boron and carbon
positions is given.
Table 1. Selected bond lengths, distances (Å) and angles (◦ ) in 4 and 7, and the respective unsubstituted
ruthenacarboranes 8 and 9.

Ru–Ctd1 b
Ru–Ctd2 b
Ru–B(C2 B3 face) c
Ru–C(C2 B3 face) c
Ru–C(arene) c
C–C(cluster)
B–B d
B–C(cluster) c
C(cluster)–C(exo) c
Ru–B(6)
B(6)–B(10)
B(4)–B(5)
C(1)–B(4)
C(1)–B(5)
Deviation from coplanarity e
Ru–C(1)–B(6)
C(1)–B(6)–C(2)
B(6)–C(2)–Ru
C(2)–Ru–C(1)

[3-(η6 -p-cymene)-3,1,2-RuC2 B9 H11 ]
(8) a

7

1.714(4)
1.619(4)
2.203(3)
2.171(2)
2.224(3)
1.627(4)
1.774(7)
1.720(5)
–
3.494(1)
1.759(1)
1.797(1)
1.718(1)
1.696(1)
5.11(9)
126.79(3)
55.99(2)
126.49(5)
44.02(4)

1.768(1)
1.485(1)
2.216(2)
2.127(2)
2.265(2)
2.243(2)
1.799(3)
1.662(3)
1.497(1)
2.979(2)
1.885(2)
1.838(3)
1.636(2)
1.614(2)
2.5(1)
100.12(9)
88.7(1)
100.14(9)
69.75(6)

[3-{η6 -(4-Me-1-COOEt-C6 H4 )}-3,1,2-RuC2 B9 H11 ]
(9) a

4

1.708(2)
1.623(2)
2.205(8)
2.166(5)
2.217(7)
1.623(3)
1.778(7)
1.719(3)
–
–
–
–
–

1.738(1)
1.598(1)
2.195(5)
2.171(3)
2.237(3)
1.680(5)
1.772(7)
1.722(6)
1.517(5)
–
–
–
–

2.3(5)
–
–
–
–

6.3(1)
–
–
–
–

From [9]. b Ctd1 = centroid of the C6 ring of the arene ligand. Ctd2 = centroid of the C2 B3 face of the dicarbollide
ligand. c Average value. d Average B–B value. For 7, the B(6)–B(10) bond length is not included. e Deviation from
coplanarity of the arene and dicarbollide ligands was measured between the least-squares plane formed by the
C6 H4 ring of the arene ligand, and the least-squares plane formed by the lower boron belt (B5 H5 ) of the cluster,
as reported previously [9].
a

2.2.

11 B

NMR Spectra of Complex 3

Complexes 2–4 and 7 show moderate to good solubility in chloroform and dichloromethane,
and good solubility in dimethylsulfoxide (DMSO). No displacement of either the arene or the
(substituted) dicarbollide ligands occurred in wet DMSO-d6 , at room temperature for over a month,
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Inorganics 2019, 7, 91

in all complexes, as evidenced by 1 H and 11 B NMR spectroscopic analysis (Figures S1 and S2 in
Supplementary Materials). This is in analogy to what was previously observed for unsubstituted
closo-[3-(η6 -arene)-3,1,2-RuC2 B9 H11 ] complexes [9], supporting the use of ruthenacarboranes as stable
organometallic scaffolds for applications in medicine.
The 11 B NMR spectra of complex 3 deserve special attention. In addition to the four (in DMSO-d6 )
or five (in CD2 Cl2 ) doublets for the nine boron atoms of the [η5 -(7,8-Me2 -nido-7,8-C2 B9 H9 )]2- ligand,
additional low-intensity 11 B signals are present in the region 0 to −20 ppm (Figure 4), which are unlikely
due to impurities from the sample, as confirmed by elemental analysis. These low-intensity signals are
instead most likely due to solvent effects on the dicarbollide cluster, which are already described in the
literature for decaborane in terms of solvent polarizability that can give rise to additional peaks or
shoulders in the 11 B NMR spectra [23]. Particularly noteworthy is the small broad signal at +19.8 ppm
(Figure 4, bottom), which is present in DMSO-d6 solution, but not in CD2 Cl2 . The small peak is present
already in freshly dissolved samples of 3 in wet DMSO-d6 and remains stable in shift and intensity
over one month.

Figure 4. 11 B NMR spectra (at 128.83 MHz) of 3 freshly dissolved in CD2 Cl2 (top) and wet DMSO-d6
(bottom). Signals for monomeric 3 and the signal for self-assemblies of 3 are observed in DMSO-d6 ,
as suggested by Deore et al. and Crociani et al. [24,25]. * marks the low-intensity additional 11 B signals,
probably due to solvent effects.

This cannot be attributed to the protonated uncoordinated nido-carborane(−1) ligand. Deore et al.
and Crociani et al. showed that the chemical shift of the 11 B NMR signals is sensitive to changes in
coordination geometry at the boron atom (trigonal at 20 to 30 ppm vs. tetrahedral at 5 to 10 ppm),
and that such shifts could be used to distinguish between nano-sized polymeric structures and
monomeric forms in solution [24,25]. The signal at +19.8 ppm in the 11 B NMR spectrum of 3 could,
therefore, be due to the presence of self-assembled nano-structures of 3 in solution, which rapidly

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interchange with monomers of 3, which are, under the conditions of the NMR experiment, still the
dominant species in solution.
The interpretation of the 11 B NMR data of potentially aggregating carborane-containing
compounds is, however, not trivial and remains somewhat confusing and elusive in the literature.
Just to give an example, Bonechi et al. investigated the solution behavior of sugar-substituted
closo-ortho-carboranes via 1 H and 11 B NMR spectroscopy in parallel under aggregating (D2 O) and
“non-aggregating” conditions (C2 D5 OD) [26]. In the 11 B{1 H} NMR spectra in both D2 O and C2 D5 OD,
the presence of down-field shifted small peaks (ca. +20 ppm), analogous to that for complex 3 in
DMSO-d6 , is evident, but no rational behind this was proposed. It was simply concluded by the
authors that there is no difference in the NMR spectra between aggregating and “non-aggregating”
conditions, although it is not clear why an ethanolic solution should represent “non-aggregating”
conditions, since closo-carborane derivatives are also known to form nano-structures in ethanol [27].
2.3. UV-Vis Spectroscopy, Mass Spectrometry and Nanoparticle Tracking Analysis (NTA)
The behavior of 2, 7 and the parent unsubstituted [3-(η6 -p-cymene)-3,1,2-RuC2 B9 H11 ] (8) in
aqueous solution was investigated, via UV-Vis spectroscopy, mass spectrometry and nanoparticle
tracking analysis (NTA). The three ruthenacarborane complexes bear the same arene ligand (p-cymene)
and differ only in the type of substituents at the cluster carbon atoms (methyl (2), CO2 Me (7), and H (8)).
UV-Vis spectra of 3, which bears a biphenyl ligand, were also measured, to support the 11 B NMR data.
UV-Vis spectroscopy is a useful technique for studying both absorption and scattering phenomena,
since the UV-Vis spectrum (ελ ) is the result of two components, namely absorption and scattering [28].
The two phenomena can be distinguished, and sometimes separated, based on their different
dependency on the wavelength (λ), ε ∝ λ for absorption, and ε ∝ λ−4 for Rayleigh scattering,
respectively. The UV-Vis spectra of 2, 7, and 8 in phosphate-buffered saline (PBS)/DMSO mixtures
do not show a clear absorption maximum in the range of 250 to 550 nm, whereas complex 3 has an
absorption maximum at 290 nm (Figure 5).

Figure 5. UV-Vis spectra of 2, 3, 7, and 8 in PBS/DMSO mixtures. Content of DMSO is 1 vol % for
all samples. [ruthenacarborane] = 20 μM. Spectra are corrected via subtraction of the blank (PBS +
1 vol % DMSO).

The absorbance shows, however, for all four complexes, an exponential increase towards the blue
region of the spectrum, which approximates the case limit of pure Rayleigh scattering. Increasing the
concentration of the ruthenacarboranes up to 50 μM only increased the intensity of the exponential

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decay of the spectrum, and no absorption maxima were visible. Scattering is thus the major component
of the absorbance spectra of 2, 3, 7, and 8, although the scattering intensity of 7 and 8 is much lower
than for 2 and 3. This suggests the presence of self-assemblies of the ruthenacarborane complexes
in PBS/DMSO mixtures, albeit, possibly, in different concentrations. Complex 3 shows the highest
scattering intensity of the series, i.e., the highest concentration of aggregates in solution, which is likely
the reason why aggregation could also be observed in its 11 B NMR spectrum in DMSO-d6 (see above),
but not in the spectra of 2 and 7, nor in the previously reported 11 B NMR spectra of 8 [9].
ESI mass spectra of 2, 7, and 8 in MeCN/H2 O (98:2, v/v) mixtures show a rather complicated
fragmentation, with many, partially overlapping, isotopic patterns of carborane-containing species
(Figure 6 (2) and Figure S3 (7,8) in Supplementary Materials). In the case of 2, for example, both the
monomer ([M + Na]+ ), the dimer ([2M + Na]+ ), and the trimer ([3M + NH4 ]+ ) were found in the ESI(+)
mass spectrum, together with many other peaks, which could not be unequivocally assigned (see the
peaks marked with * in Figure 6). Moreover, reproducibility of the MS fragmentation patterns was
very poor for all three complexes under the same experimental conditions, which suggests a random
and uncontrolled spontaneous self-assembly in solution. From the analysis of the mass spectra alone,
one might thus infer that the compound is not pure. Fortunately, the other analytical techniques used
to characterize compounds 2, 7, and 8, i.e., NMR and IR spectroscopy, X-ray diffraction, and elemental
analysis, clearly indicate that the complexes are analytically pure and void of any kind of impurities.

Figure 6. ESI(+) mass spectrum of 2 (M = 397.22), measured in MeCN/H2 O (98:2, v/v). The peaks
which could not be unequivocally assigned are indicated by *. The inset shows a section of the region
m/z = 950 to 1400.

Samples of 2, 7, and 8 in PBS/DMSO mixtures were also measured via nanoparticle tracking
analysis (NTA) to estimate the relative concentration, size, and size distribution of self-assemblies in
solution observed by ESI mass spectrometry and UV-Vis spectroscopy. Nanoparticle tracking analysis
(NTA) is a fairly new technique for the measurement of colloidal and nano-sized suspensions, which
was first commercialized in 2006 by NanoSight Ltd, Salisbury, UK [29]. It has been used for the
study of different kinds of samples, ranging from atmospheric [30], to food [31] and to biological

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samples [32]. The analysis principles and instrument set-up have been extensively discussed in the
literature [33]. NTA is a light-scattering technique, in which particle tracking is based on the Brownian
motion description of suspended particles in a fluid, captured simultaneously but individually by
a charge-coupled device (CCD) camera. The software calculates size (hydrodynamic radius), size
distribution, and concentration of the particles. NTA has the advantage over dynamic light scattering
(DLS) methods in that it does not suffer from the known bias in size and size distribution of the
latter. However, the applicability of NTA is limited to a narrow range of concentrations (106 –109
particles mL−1 ), and the calculated values of size and concentration are highly sensitive to capture
and processing parameters, as discussed recently [34]. Samples of 2, 7, and 8 were therefore measured
using the same capture and processing parameters, for direct comparison.
All three metallacarboranes form self-assemblies of nanometer size in PBS/DMSO mixtures at
25 ◦ C, albeit in different concentrations, namely 108 for 7 and 8, vs. 109 particles mL−1 for 2 (Figure 7
and Table S2 in Supplementary Materials). 2 shows a bimodal distribution of particle sizes, centered at
115 and 155 nm, respectively, but also presents a smaller fraction of particles with sizes up to 400 nm.
Samples of 7 and 8 show broad size distributions of the particles, in the range of 95 to 300 nm (7) or
145 to 400 nm (8). Thus, all three complexes form fairly polydisperse self-assemblies in PBS/DMSO
mixtures at room temperature, that is, under conditions, which approximate those of biological tests
in vitro.

Figure 7. Size distribution of 2, 7, and 8 in PBS/DMSO mixtures, from nanoparticle tracking analysis
(NTA). [2] = [7] = [8] = 20 μM. The dilution factor is the same for all samples. Content of DMSO
was 1 vol % in all samples. Average data from five independent captures are shown. T = 25 ◦ C.
Particle concentrations and size values, with relative standard deviations, are given in Table S2
(Supplementary Materials).

As already mentioned before, aqueous self-assembly of neutral (metalla)carboranes has been
so far poorly investigated, and is limited to a few examples of C-substituted closo-carboranes [26,27].
No studies on the effect of spontaneous aggregation on the biological activity profile or stability in
the biological medium are found in the literature. Therefore, comprehensive multi-spectroscopic
bioanalytical investigations are now underway.

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3. Materials and Methods
3.1. General Procedures and Instrumentation
Chemicals were used as purchased.
Phosphate-buffered saline (PBS) was purchased
from Sigma Aldrich (Taufkirchen, Germany).
Tl[3-Tl-1,2-Me2 -3,1,2-C2 B9 H9 ] (1) [35–37],
closo-[3-(η6 -p-cymene)-1,2-Me2 -3,1,2-RuC2 B9 H9 ] (2) [16] and closo-[3-(η6 -p-cymene)-3,1,2-RuC2 B9 H11 ]
(8) [9] were synthesized as previously reported. Synthesis and characterization of 5 and 6 (precursor
compounds) are given in the Supplementary Materials. All manipulations were carried out in a dry
and oxygen-free nitrogen atmosphere using standard Schlenk techniques, unless otherwise stated.
Thallium(I) ethanolate (Alfa Aesar© , Kandel, Germany) was stored under argon at −20 ◦ C, protected
from light. All manipulations involving thallium(I) compounds were performed wearing personal
protective equipment as prescribed in the material safety data sheet (MSDS), and thallium(I)-containing
waste was disposed of according to official regulations. Dried and degassed dichloromethane
(CH2 Cl2 ) and n-hexane were obtained from an MBRAUN solvent purification system (MB SPS-800, M.
Braun Inertgas-Systeme GmbH, Garching, Germany) and stored under a nitrogen atmosphere over
molecular sieves (4 Å). Tetrahydrofuran (THF) was dried over Na/benzophenone, freshly distilled
prior to use and stored under nitrogen atmosphere over molecular sieves (4 Å). Acetonitrile (MeCN)
was degassed, freshly distilled prior to use and stored under nitrogen. DMSO was dried over CaH2 ,
freshly distilled prior to use and stored under nitrogen over molecular sieves (4 Å).
Thin-layer chromatography (TLC) was carried out on precoated glass plates (Merck Silica Gel
60 F254 ). Visualization of the compounds on TLC plates was achieved by means of an iodine chamber,
or by treatment with a solution of PdCl2 (1 wt % in MeOH). Column chromatography was carried out
with silica gel (0.035–0.070 mm, 60 Å). NMR spectra were acquired at room temperature with a Bruker
AVANCE III HD 400 MHz spectrometer (Bremen, Germany). 1 H (400.13 MHz) and 13 C{1 H} (100.16 MHz)
NMR spectra were referenced to tetramethylsilane (TMS) as internal standard. 11 B (128.38 MHz) NMR
spectra were referenced to the unified Ξ scale [38]. Mass spectrometry measurements were carried out
with an ESI-MS Bruker ESQUIRE 3000 (Benchtop LC Iontrap, Bremen, Germany) spectrometer. FT-IR
spectra were obtained with a PerkinElmer system 2000 FTIR spectrometer (Baesweiler, Germany),
scanning between 400 and 4000 cm−1 . Elemental analyses were performed with a Heraeus VARIO
EL oven (Lagenselbold, Germany). X-ray data were collected with a GEMINI CCD diffractometer
(Rigaku Inc., Neu-Isenburg, Germany), using Mo-Kα radiation (λ = 0.71073 Å), T = 130(2) K and
ω-scan rotation. Data collection and refinement data are given in Table S1 (Supplementary Materials).
Absorption corrections were performed with SCALE3 ABSPACK [39]. The structures were solved by
direct methods with SHELXS [40]. Structure refinement was done with SHELXL-2016 [41] by using
full-matrix least-square routines against F2 . All non-hydrogen atoms were refined with anisotropic
thermal parameters, and the HFIX command was used to locate all hydrogen atoms for non-disordered
regions of the structure. Crystals of 4 and 7 contain no solvent molecules. The C2 unit of the carborane
cluster was located with bond length analysis. The pictures were generated with the program Diamond
(version 3.2) [42]. CCDC 1915985 (4) and 1915986 (7) contain the supplementary crystallographic data
for this paper. UV-Vis absorption spectra were measured with a PerkinElmer UV/VIS/NIR Lambda
900 spectrometer (Baesweiler, Germany), equipped with a xenon arc lamp, using quartz cuvettes
(V = 3 cm3 ). Spectra were recorded at 25 ◦ C, in the range of 250 to 550 nm at 1.0 nm resolution.
All measurements were corrected by subtracting the blank (PBS + 1 vol % DMSO). Nanoparticle tracking
analysis (NTA) measurements were performed using the NanoSight LM10 instrument from Malvern
Instruments Ltd. (Worcestershire, UK), containing a sample chamber of about 0.25 mL, and equipped
with a 532 nm laser, a microscope LM14B, and a camera sCMOS. All measurements were performed at
25 ± 0.1 ◦ C. Each sample was measured in five independent captures. The time of each capture was set
to 60 s. The NTA 3.0 analytical software (NanoSight Ltd., Salisbury, UK) was used for both capture
and processing.

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3.2. Syntheses
3.2.1. closo-[3-(η6 -Biphenyl)-1,2-Me2 -3,1,2-RuC2 B9 H9 ] (3)
Following Bould et al. [16], [{(η6 -biphenyl)RuCl(μ-Cl)}2 ] (0.20 g, 0.31 mmol, 1.0 eq.) was dissolved
in dry THF (15 mL) and cooled to 0 ◦ C. 1 (0.52 g, 0.92 mmol, 3.0 eq.) was added in one portion, and the
mixture was stirred at room temperature for 17 h. Silica (0.5 g) was then added to the brown-orange
mixture and the solvent was evaporated in vacuo. The residue was purified via filtration through
a short pad of silica gel (length = 5 cm, diameter = 2.5 cm) using CH2 Cl2 as eluent, which yielded
a single yellow band (Rf = 0.88 in CH2 Cl2 ). The latter was collected and evaporated to dryness,
yielding pure 3 as pale yellow, air-stable solid. 3 is soluble in CH2 Cl2 and DMSO, and moderately
soluble in CHCl3 . Yield: 35.0 mg (45%). 1 H NMR (CD2 Cl2 ): δ (ppm) = 0.55–3.88 (br, B–H), 2.05 (6H, s,


Ccage –CH3 ), 6.08–6.21 (3H, m, H1 , H2 and H2 ), 6.46 (2H, d, 3 JHH = 5.7 Hz, H3 and H3 ), 7.51 (3H, m, H7 ,


H7 , and H8 ), 7.74 (2H, dd, 3 JHH = 8.3, 1.6 Hz, H6 and H6 ). 11 B NMR (CD2 Cl2 ): δ (ppm) = 2.4 (1B, d,
1J
1
1
1
BH = 129 Hz), 0.5 (1B, d, J BH = 126 Hz), −2.9 (2B, d, J BH = 147 Hz), −9.4 (2B, d, J BH = 140 Hz), -14.1
1
13
1
(3B, d, JBH = 158 Hz). C{ H} NMR (CD2 Cl2 ): δ (ppm) = 32.2 (s, Ccage –CH3 ), 75.9 (s, Ccage ), 88.9 (s, C3




and C3 ), 90.7 (s, C1 ), 91.1 (s, C2 and C2 ), 106.0 (s, C4 ), 128.1 (s, C6 and C6 ), 129.2 (s, C7 and C7 ), 129.8 (s,
C8 ), 133.5 (s, C5 ). IR (KBr; selected vibrations): 
ν (cm−1 ) = 3079 (m, νCHarom ), 2929 (m, νCHcage ), 2561 (s,
νBH ), 2515 (s, νBH ), 1455 (s, νC=C ), 1405 (m, νC=C ), 1387 (m), 1015 (s, νCC ), 835 (m) 764 (s, νBB ), 694 (s,
νBB ). ESI-MS(−): m/z = 865.2356 (100%, [2M + Cl]− ). Anal. calcd for C16 H25 B9 Ru (415.74): C, 46.23; H,
6.06. Found C, 46.70; H, 6.20.
3.2.2. closo-[3-(η6 -(1-Me-4-COOEt-C6 H4 ))-1,2-Me2 -3,1,2-RuC2 B9 H9 ] (4)
from
4 was synthesized in an analogous manner as described for 3,
[{(η6 -(1-Me-4-COOEt-C6 H4 ))RuCl(μ-Cl)}2 ] (0.20 g, 0.30 mmol, 1.0 eq.) and 1 (0.51 g, 0.90 mmol, 3.0 eq.).
The crude product was recrystallized from CH2 Cl2 /acetone (10:1, v/v) at room temperature to yield
yellow plates of pure 4, suitable for single crystal X-ray diffraction analysis. 4 is an air-stable pale
yellow solid, soluble in CH2 Cl2 , CHCl3 , and DMSO. Yield: 25.3 mg (32%). 1 H NMR (CD2 Cl2 ): δ
(ppm) = 0.56–3.96 (br, B–H), 1.39 (3H, t, 3 JHH = 7.1 Hz, H8 ), 2.12 (6H, s, Ccluster –CH3 ), 2.42 (3H, s, H5 ),

4.41 (2H, q, 3 JHH = 7.1 Hz, H7 ), 6.02 (2H, d, 3 JHH = 6.4 Hz, H3 and H3 ), 6.55 (2H, d, 3 JHH = 6.4 Hz,

H2 and H2 ). 11 B NMR (CD2 Cl2 ): δ (ppm) = 2.7 (1B, br s), 1.6 (1B, br s) (the two doublets centered
at 2.7 and 1.6 ppm in the 11 B NMR spectrum are very broad, and it is therefore not possible to give
accurate values of 1 JBH coupling constants), −2.3 (2B, d, 1 JBH = 147 Hz), −8.9 (2B, d, 1 JBH = 140 Hz),
−13.5 (3B, d, 1 JBH = 160 Hz). 13 C{1 H} NMR (CD2 Cl2 ): δ (ppm) = 14.0 (s, C8 ), 19.0 (s, C5 ), 31.7 (s,


Ccluster –CH3 ), 62.7 (s, C7 ), 76.2 (s, Ccluster ), 91.0 (s, C2 and C2 ), 91.9 (s, C3 and C3 ), 93.1 (s, C1 ), 105.0 (s,
4
6
−1
C ), 164.9 (s, C ). IR (KBr; selected vibrations): 
ν (cm ) = 3067 (w, νCHarom ), 2982 (w, νCHcluster ), 2931
(w, νCHcluster ), 2563 (s, νBH ), 2520 (s, νBH ), 1720 (s, νC=O ), 1379 (s, νCO ), 1369 (m, νCO ), 1294 (s, νCO ),
1015 (s, νCC ), 881 (m), 776 (m, νBB ). ESI-MS (−): m/z = 483.1953 (100%, [M + CO2 Me]− ). Anal. calcd for
C14 H27 B9 O2 Ru (425.73): C, 39.50; H, 6.39. Found C, 39.67; H, 6.50.
3.2.3. pseudocloso-[3-(η6 -p-Cymene)-1,2-(CO2 Me)2 -3,1,2-RuC2 B9 H9 ] (7)
Deprotonation of the nido-carborane(−1) precursor. 6 (0.106 g, 0.39 mmol, 1.0 eq.) was dissolved
in dry THF (6 mL) and cooled to −30 ◦ C, protected from light. Thallium(I) ethanolate (0.243 g,
0.07 mL, 0.97 mmol, 2.5 eq.) was then added in one portion, causing immediate formation of a yellow
precipitate. The mixture was allowed to warm to room temperature over one hour. Stirring was
stopped and the mixture was left standing overnight. The supernatant solution was carefully removed
via filtration, and the precipitate was washed with n-hexane (6 mL), THF (8 mL), and ethanol (3 mL).
The yellow residue (Tl[Tl6]) was further dried in vacuo (10−3 mbar) (the thallium salt Tl[Tl6] was
dried in vacuo without heating, because heating of a carborane dithallium salt promotes reprotonation
to the nido-carborane(−1) species, as reported [43]) and used directly, without further purification.

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Complexation reaction. [{(η6 -p-cymene)RuCl(μ-Cl)}2 ] (86 mg, 0.14 mmol, 1.0 eq.) and Tl[Tl6] were
placed in a Schlenk flask, thoroughly mixed and cooled to −65 ◦ C. Degassed CH2 Cl2 (10 mL) was then
added, and the reaction mixture was left stirring for 1.5 h at −65 ◦ C, then slowly warmed to room
temperature, over one hour. The dark red-brown mixture was filtered, and the solution concentrated
in vacuo to a 2 mL volume. Degassed silica was then added, and all volatiles were removed in vacuo.
The residue was then purified via filtration through a silica gel pad (length = 10 cm, diameter 2.5 cm),
under nitrogen atmosphere, using CH2 Cl2 as eluent, which yielded a single orange band. The latter
was collected and evaporated to dryness. The crude product was recrystallized from CH2 Cl2 /n-hexane
(1.5:1, v/v) at −20 ◦ C, to yield orange prisms of pure 7, suitable for single crystal X-ray diffraction
analysis. 7 is air-stable, soluble in CHCl3 , CH2 Cl2 , acetone, and DMSO. Yield: 54.0 mg (39%). 1 H

NMR (CDCl3 ): δ (ppm) = 0.53–3.38 (br, B–H), 1.33 (3H, d, 3 JHH = 6.9 Hz, H7 and H7 ), 2.32 (3H, s, H5 ),


3
6
3
2.89 (1H, hept, JHH = 6.9 Hz, H ), 3.78 (6H, s, OCH3 ), 5.83 (2H, d, JHH = 6.3 Hz, H2/2 or H3/3 ), 5.88


(2H, d, 3 JHH = 6.3 Hz, H2/2 or H3/3 ). 11 B NMR (CDCl3 ): δ (ppm) = 27.7 (1B, d, 1 JBH = 122 Hz), 11.1
(1B, d, 1 JBH = 149 Hz), 8.7 (1B, d, 1 JBH = 115 Hz), 0.11 (2B, d) (the 1 JBH coupling constant could not
be determined, due to overlap with the peak at -1.6 ppm), −1.6 (3B, d, 1 JBH = 142 Hz), -21.8 (1B, d,
1J
ν (cm−1 ) = 3076 (w, νCHarom ), 2950 (w, νCHcluster ), 2548 (s,
BH = 172 Hz). IR (KBr; selected vibrations): 
νBH ), 1716 (s, νC=O ), 1482 (w, νC=C ), 1458 (w, νC=C ), 1431 (m, νC=C ), 1261 (s, νCO ), 1110 (m, νCC ), 1020
(m, νCC ), 860 (w), 765 (w, νBB ). ESI-MS(+): m/z = 483.1948 (100%, [M + H]+ ), 519.1705 (6%, [M + K]+ ).
Anal. calcd for C16 H29 B9 O4 Ru (483.76): C, 39.73; H, 6.04. Found C, 39.78; H, 5.92.
3.3. Preparation of 2, 7, and 8 for UV-Vis Spectroscopy, Mass Spectrometry, and NTA Measurements
Stock solutions of 2, 3, 7, and 8 in DMSO (1.0 mM) were freshly prepared before measurements.
An aliquot of the DMSO stock solution of 2, 3, 7 or 8 was added to a PBS solution (3 mL) so that
the final concentration of metallacarborane was 20 μM. DMSO content was 1 vol % in all samples.
The samples were measured via UV-Vis spectroscopy and nano tracking analysis (NTA) 30 min to
one hour after preparation. Samples of 3 were only measured by UV-Vis spectroscopy. Capture and
processing parameters for the NTA measurements were the same for all samples for direct comparison.
Samples were measured undiluted.
Compounds 2, 7, and 8 (ca. 1.0 mg) were dissolved in a minimum amount of MeCN (a few
μL) and brought to a final volume of 500 μL with MeCN/H2 O (98:2, v/v). The final concentration of
ruthenacarborane was ca. 100 μM. Samples were measured via ESI mass spectrometry (positive and
negative mode) within 5 h from preparation.
4. Conclusions
A small series of neutral 3,1,2-ruthenadicarbaborane(11) complexes bearing either non-polar
(methyl, 2–4) or polar (CO2 Me, 7) groups at the cluster carbon atoms were synthesized and fully
characterized. The complexes possess a closo (2–4) or pseudocloso (7) structure in analogy to other
C-substituted ruthenacarboranes in the literature. 11 B NMR spectra of 3 in DMSO-d6 suggested the
presence of aggregates of the complex in solution, confirmed by spectrophotometric analysis of 3
in PBS/DMSO mixtures at 20 μM. Moreover, spontaneous self-assembly in aqueous solutions was
observed for all tested complexes in PBS/DMSO and MeCN/H2 O mixtures, regardless of the specific
type of substitution at the Ccluster vertices. They form particles with diameters on the nanometer scale,
with high polydispersity and concentrations ranging from 108 (7 and 8) to 109 (2) particles mL−1 .
This study thus suggests that for perspective applications in medicine there is a strong need
for further characterization of the spontaneous self-assembly in aqueous solutions of this class of
ruthenacarboranes, as well as other neutral metallacarboranes, with the ultimate scope of finding
the optimal conditions for modulating the aqueous behavior of the complexes. These studies are
currently underway.

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Inorganics 2019, 7, 91

Supplementary Materials: The following are available online at http://www.mdpi.com/2304-6740/7/7/91/s1,
Synthesis and characterization of compounds 5 and 6; Table S1: Crystal data for 4 and 7; Figure S1: 1 H NMR
spectra (400.13 MHz) of complexes 2–4 in wet DMSO-d6 in air at room temperature, after one month; Figure S2:
11 B NMR spectra (128.83 MHz) of complexes 2–4 and 7 in wet DMSO-d in air at room temperature, after one
6
month; Figure S3: ESI(+) mass spectra of 7 (top) and 8 (bottom) measured in MeCN/H2 O (98:2, v/v); Table S2:
Mean size and concentration of particles for PBS/DMSO solutions of 2, 7 and 8.
Author Contributions: M.G. designed the studies, performed the syntheses, analyzed data and wrote the paper;
M.G. and B.S. performed the UV-Vis and the NTA experiments and analyzed the data; P.C. performed the
single-crystal XRD measurements and solved the structures; E.H.-H. designed the studies and wrote the paper.
Funding: This work was supported by the Saxon State Ministry for Sciences and Arts (SMWK, doctoral grant for
M.G.) [grant No. LAU-R-N-11-2-0615], the German chemical industry association (VCI, doctoral grant for B.S.)
[grant No. 197021], the Studienstiftung des deutschen Volkes (doctoral grant for P.C.) and the Graduate School
“Leipzig School of Natural Sciences—Building with Molecules and Nano-objects” (BuildMoNa).
Acknowledgments: We thank C. Zilberfain and I. Estrela-Lopis (Institute of Medicinal Physics and Biophysics,
Leipzig University) for access to the NTA equipment and fruitful discussions on the NTA data and D.
Maksimović-Ivanić and S. Mihatović (Institute for Biological Research “Siniša Stanković”, University of Belgrade)
for fruitful discussion on aggregating compounds for application in medicine.
Conflicts of Interest: The authors declare no conflict of interest.

Appendix A
Nomenclature adopted for carborane clusters (according to IUPAC convention): closo = 12-vertex
icosahedral cluster, with (n − 1) skeletal electron pairs (n = total number of vertices); nido = 11-vertex
open-face cluster, with (n − 2) skeletal electron pairs (n = total number of vertices); ortho-, meta-,
para- = 1,2-, 1,7-, 1,12-dicarba-closo-dodecaborane(12), respectively. For numbering of the carborane
clusters refer to the IUPAC project 2012-045-1-800 by Beckett et al., Nomenclature for boranes and related
species, Chemistry International 2018, 40, 33.
Appendix B
The weighted average was calculated multiplying the chemical shift value of each 11 B signal by
its relative intensity, and then dividing by the total number of 11 B signals of the spectrum.
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inorganics
Article

Adding to the Family of Copper Complexes Featuring
Borohydride Ligands Based on
2-Mercaptopyridyl Units
Joseph Goldsworthy 1 , Simon D. Thomas 1 , Graham J. Tizzard 2 , Simon J. Coles 2 and
Gareth R. Owen 1, *
1
2

*

School of Applied Sciences, University of South Wales, Pontypridd CF37 4AT, UK
UK National Crystallography Service, University of Southampton, Highfield, Southampton SO17 1BJ, UK
Correspondence: gareth.owen@southwales.ac.uk; Tel.: +44-1443-65-4527

Received: 13 June 2019; Accepted: 19 July 2019; Published: 24 July 2019

Abstract: Borohydride ligands featuring multiple pendant donor functionalities have been prevalent
in the chemical literature for many decades now. More recent times has seen their development
into new families of so-called soft scorpionates, for example, those featuring sulfur based donors.
Despite all of these developments, those ligands containing just one pendant group are rare.
This article explores one ligand family based on the 2-mercaptopyridine heterocycle. The coordination
chemistry of the monosubstituted ligand, [H3 B(mp)]− (mp = 2-mercaptopyridyl), has been explored.
Reaction of Na[BH3 (mp)] with one equivalent of Cu(I) Cl in the presence of either triphenylphosphine
or tricyclohexylphosphine co-ligands leads to the formation of [Cu{H3 B(mp)}(PR3 )] (R = Ph, 1;
Cy, 2), respectively. Structural characterization confirms a κ3 -S,H,H coordination mode for the
borohydride-based ligand within 1 and 2, involving a dihydroborate bridging interaction (BH2 Cu)
with the copper centers.
Keywords: scorpionate; copper; borohydride; ligand; sulfur

1. Introduction
The coordination chemistry of borohydride and substituted borohydride units with transition
metals has been a major focus of research over many decades now [1–9]. One particular focus
of research has been on substituted borohydride units attached to other donor functional groups.
This gave rise to a research area known as “scorpionate chemistry”, where the borohydride moiety
is typically substituted by two or more pyrazolyl rings, thus forming multidentate ligand systems.
This area of research has provided an expansive and fascinating array of compounds with wide
ranging applications. These have been explored in homogeneous catalysis and bioinorganic chemistry,
for example [10–16]. In many examples, the borohydride unit is positioned away from the metal center,
playing a spectator role within the complex. The polyprazolylborates, for example, are known as
“octahedral enforcers”, furnishing highly rigid stable complexes.
More recent developments, have led to new generations of ligand systems, where the borohydride
unit is positioned in direct contact with the metal center. In some cases these can undergo direct
transformations at the boron center (Figure 1) [17–22]. This occurs when an additional atom is
incorporated between the boron and donor atom. The publication of this new generation of
more “flexible scorpionates” opened up a new area of research with respect to the formation
of Z-type ligands [17–22]. This revolutionized the field and altered the perspective on the
coordination chemistry of such ligands. The first of the more flexible scorpionate ligands was
[Tm]− [hydrotris(methylimidazolyl)borate] (Figure 1; middle) [23]. This new ligand had two major
differences when compared to Trofimenko’s original scorpionates. The ligand was based on soft
Inorganics 2019, 7, 93; doi:10.3390/inorganics7080093

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Inorganics 2019, 7, 93

sulfur donor atoms [16], and perhaps more significantly, greater flexibility had been incorporated
into the ligand by addition of the extra atom between the boron and the donor atom. It was this
greater flexibility within the ligand structure that opened up the potential for activation at the boron
bridgehead and formation of metal-borane (metallaboratrane) complexes [17–20,24–27], giving rise to
reactivity not observed in the analogous polypyrazolylborate ligands [10–16].

Figure 1. Selected examples of ligands in which the borohydride unit is positioned away from the
metal center (left), directed towards the metal center (middle), and a system in which a B-H activation
has occurred (right). The additional atom between boron and the donor atoms is typically necessary
for metal-boron bond formation.

Over the following twenty years since the first report of hydride migration from the boron
center of a scorpionate ligand, a number of research groups have focused on new, more flexible
borohydride ligands containing a range of supporting units based on nitrogen [28–31] and other
sulfur heterocycles [32–46]. As part of our research, we have focused on providing new derivative
ligand systems. In 2009, we introduced a new family of flexible scorpionate ligands derived from
the 2-mercaptopyridine heterocycle [36]. This original report provided a borohydride-based ligand
substituted by two and three of these heterocycles. Last year, we extended this family to include
the monosubstitued ligand, [H3 B(mp)]− (where mp = 2-mercaptopyridyl; Figure 2) [37]. Herein,
we report the synthesis and characterization of the first copper complexes containing this new ligand.
The complexes have been structurally characterized and compared to related complexes.

Figure 2. The monosubstituted borohydride salt, Na[H3 B(mp)].

2. Results and Discussion
2.1. Synthesis and Characterization of Copper Complexes
The coordination chemistry of [H3 B(mp)]− is limited to one example to date. The complex
[Rh{κ3 -B,H,H-H3 B(mp)}(NBD)] (where NBD = 2,5-norbornadiene), was reported by us in 2018 [37].
Accordingly, we set out to prepare some further examples of complexes containing this ligand.
We have previously synthesized a series of copper(I) complexes containing the bis- and tris-substituted
derivatives [36]. A similar synthetic protocol was, therefore, undertaken to prepare the complexes,
[Cu{H3 B(mp)}(PR3 )] (R = Ph, 1; Cy, 2), as shown in Scheme 1. These complexes were readily prepared
by reaction of one equivalent of Na[H3 B(mp)] with one equivalent of CuCl in the presence of a
stoichiometric amount of the corresponding phosphine co-ligand. The reactions were performed in
methanol solvent, from which the products precipitated out as yellow solids.

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Inorganics 2019, 7, 93

Scheme 1. Synthesis of [Cu{κ3 -S,H,H-H3 B(mp)}(PR3 )] (R = Ph, 1; Cy, 2).

The air stable products were obtained in good yields and were fully characterized by NMR and IR
spectroscopy, mass spectrometry, and elemental analysis. Selected characterization data for complexes
1 and 2 are presented in Table 1, along with data for the corresponding copper complexes containing
the bis- and tris-substituted ligands, [H2 B(mp)2 ]− and [HB(mp)3 ]− , for comparison. The 11 B NMR
spectra of complexes 1 and 2, in CDCl3 , revealed single broad resonances at −13.9 ppm and −13.4 ppm,
respectively (see Figures S3 and S10 in the Supplementary Materials). Both signals presented as
poorly unresolved quartets with 1 JBH coupling constants of 75 Hz for 1 and 82 Hz for 2. Both were
found to be singlet resonances in the corresponding 11 B{1 H} NMR spectra (with half height widths
113 Hz and 90 Hz, respectively), confirming that three hydrogen substituents remain at the boron
center. The change in chemical shift from the starting material to the complexes was insignificant
(c.f. −14.1 ppm in CD3 CN), particularly when taking into account the different solvent. There does
seem to be a small reduction in the 1 JBH coupling constant upon coordination of [H3 B(mp)]− to the
copper center. In Na[H3 B(mp)], this value is 93 Hz. From these data, it appears that the BH3 unit of
the ligand is not strongly interacting with the copper metal center. Similar observations have been
reported for neutral borane adducts, of the type H3 BNR3 , with copper complexes [47,48]. This is in
contrast to those observations for [Rh{κ3 -B,H,H-H3 B(mp)}(NBD)], in which the boron chemical shift in
complexes was found to be −7.8 ppm. As highlighted in Table 1, the change in boron chemical shift
upon complexation was a little more pronounced for the copper complexes bearing the [H2 B(mp)2 ]−
and [HB(mp)3 ]− ligands.
Table 1. Selected NMR (ppm) and IR (cm−1 ) spectroscopic data for [Hn B(mp)4−n ] pro-ligands and their
corresponding copper complexes.
Compound 1
Na[H3 B(mp)] 5
[Cu{H3 B(mp)}(PPh3 )] (1)
[Cu{H3 B(mp)}(PCy3 )] (2)
Na[H2 B(mp)2 ] 6
[Cu{H2 B(mp)2 }(PPh3 )] (3)
[Cu{H2 B(mp)2 }(PCy3 )]
K[HB(mp)3 ] 6
[Cu{HB(mp)3 }(PPh3 )]
[Cu{HB(mp)3 }(PCy3 )]

11 B{1 H}

NMR 2

−14.1 (44)
−13.9 (113)
−13.4 (90)
−3.7 (211)
0.7 (265)
−0.7 (248)
4.4 (560)
−0.1 (412)
−0.5 (331)

31 P{1 H}

NMR

4.8
27.2
1.7
19.0
−2.4
17.4

13 C{1 H}
NMR C=S

1 H{11 B}
NMR 3 BHn

IR B–H 4

181.3
175.9
176.1
182.6
n.o. 7
178.2
182.5
178.3
181.0

2.11
2.64
2.42
3.64
4.12
3.99
4.83
n.o. 7
5.86

2307
2439 (t)/2078 (κ2 )
2448 (t)/2085 (κ2 )
2438, 2370
2425
2374
2468
2458
n.o. 7

Note: 1 The NMR spectroscopic data for all complexes were recorded in CDCl3 ; 2 the values in brackets are the
half-height widths of the measurement of the signal; 3 this signal corresponds to chemical environments of hydrogen
substituents at the boron center. In all cases, only one single chemical environment was observed for the BHn
units; 4 recorded as a powder film, where clear the terminal (t) and BH2 Cu bridging modes (κ2 ) are highlighted in
brackets; 5 in CD3 CN NMR solvent; 6 in DMSO-d6 NMR solvent; 7 this chemical environment or B–H stretch was
not observed in this spectrum.

Further information on these complexes was obtained from their 31 P{1 H} NMR spectra. The 31 P{1 H}
NMR spectra of 1 and 2 revealed single broad resonances at 4.8 ppm and 27.2 ppm, respectively (Figures
S6 and S13). These both represent downfield chemical shifts with respect to the free phosphines,
confirming their coordination to the metal centers. These changes in chemical shift with respect to the
free phosphines are more significant than the corresponding bis- and tris-complexes, suggesting that
89

Inorganics 2019, 7, 93

the phosphines are more strongly bound in the lower coordination complexes, as might be expected.
As indicated above, the 11 B NMR data did not unambiguously confirm coordination of H3 B(mp) unit.
The 1 H NMR data, on the other hand, were a little more convincing, exhibiting a new set of signals
for the mercaptopyridyl protons with clear shifts from the starting material. The 1 H NMR spectrum
for 1 (Figure S1) showed an integration ratio of 3H:16H:1H:1H:1H corresponding to the BH3 group,
15 aromatic protons on the triphenylphosphine ligand, plus one overlapping proton environment on
the mercaptopyridine unit. The three remaining signals corresponded to the other proton environments
on the mercaptopyridyl unit. A similar situation was found for complex 2, confirming the presence of
the BH3 unit, the mercaptopyridyl heterocycle, and the PCy3 ligand within the complex (Figure S8).
For both complexes, the BH3 protons were located at significantly broad signals at 2.64 ppm for 1 and
2.42 ppm for 2, in their 1 H{11 B} NMR spectra. These were shifted downfield with respect to [H3 B(mp)]− ,
which were observed at 2.11 ppm. Again, the corresponding shifts for [Rh{κ3 -B,H,H-H3 B(mp)}(NBD)]
were −2.72 ppm (integrating for 2 H) and 2.89 ppm (integrating for 1 H) for the bridging and terminal
hydrogen substituents on boron. This, of course, represents a static BH2 bridging interaction with the
rhodium center, whereas a fluxional interaction must be present in complexes 1 and 2, since all three
hydrogens at boron are in the same chemical environment. A series of 13 C{1 H} and two-dimensional
correlation NMR experiments were carried out to fully assign all hydrogen and carbon chemical
environments within the two complexes (see Experimental section). Further evidence of coordination
of [H3 B(mp)]− to the metal center was found in the infrared spectrum. Powder film samples gave
characteristic bands at 2439 cm−1 for 1 and 2448 cm−1 for 2, corresponding to the terminal B-H stretch.
These compared to the 2307 cm−1 value found for Na[H3 B(mp)] [37]. Two additional bands were also
located in the IR spectra for 1 and 2 at 2078 cm−1 and 2085 cm−1 , respectively. These correspond to
the BH2 Cu interactions, where two of the three B-H bonds in the BH3 unit interact with the metal
center [1–4]. The crystal structure previously reported for [Cu{H2 B(mp)2 }PPh3 ] contains a κ3 -S,S,H
coordination mode for the scorpionate ligand, involving the interaction of one of the B-H bonds with
the copper center [36]. This is presumably due to the preference for coordination of an additional
sulfur donor to the metal center over the BH2 Cu mode and the restriction against a κ4 -S,S,H,H
coordination mode. The compounds were also analyzed by mass spectrometry. The molecular ion
peak was found for 2 by mass spectrometry. For complex 1, only the fragment [Cu(mpH)(PPh3 )]
was observed. Finally, confirmation for the formation of the targeted products was confirmed by
satisfactory elemental analysis.
2.2. Structural Characterization of Copper Complexes
Single crystals of complexes 1 and 2, suitable for X-ray crystallography, were obtained from slow
evaporation of the solvent from diethyl ether—methanol (1:1) mixtures. The molecular structures
of these complexes are shown in Figure 3. Selected bond distances and angles for these complexes
are shown in Table 2, along with those for [Cu{H2 B(mp)2 }PPh3 ] (3) for comparison. Crystallographic
parameters are provided in the supporting information. The two new structures contained disorder in
the position of the [H3 B(mp)]− ligand in ratios 56:44 for complex 1 and 79:21 for complex 2. The lack
of strong H-bond donor/acceptors in either complex results in simple close-packed crystal structures
driven by dispersion forces. The structures of both 1 and 2 confirmed the coordination of one phosphine
ligand and one [H3 B(mp)]− ligand to the metal center. The solid state structures confirmed that the
BH3 unit was bound to the copper center via a BH2 Cu bridging mode. This is, therefore, consistent
with the IR spectroscopic data. The BH2 Cu mode can either be considered as two separate B-H agostic
type interactions (η2 ,η2 ) or as a three-centered dihydroborate interaction [1–4]. This coordination
mode in the mono-substituted ligand allows for a different morphology about the copper center in
comparison to that found in complex 3 [36]. If the hydrogen substituents are ignored and the boron
center of the BH3 unit is considered as the site of coordination at the copper, then the geometries
around the metal center are highly distorted trigonal planar structures. In both cases, if a plane is
defined by the atoms P(1), B(1), and S(1), then the copper center sits in a position that is very close to

90

Inorganics 2019, 7, 93

this plane. The distance of the copper center from these planes is 0.062(7) Å for 1 and 0.019(6) Å for
2. The sums of the aforementioned angles are very close to the idealized 360◦ . The ligand forms a
six-membered ring where it links to the copper via the sulfur donor and the hydrogen substituents
at boron. Whilst the BH2 Cu interaction does not appear to be strong in solution, it appears that the
BH3 unit is held in close proximity to the metal center via the mercaptopyridine supporting unit.
In the case of [Cu{κ3 -S,S,H-H2 B(mp)2 }PPh3 ], a distorted geometry between tetrahedral and trigonal
pyramidal is observed as demonstrated by the sum of the same angles, which is 350.4◦ . In this complex,
two six-membered rings are formed as a result of the κ3 -S,S,H coordination mode. In the absence of
the BHCu interaction, this would have led to formation of one eight membered ring.

Figure 3. Molecular structures of [Cu{κ3 -S,H,H-H3 B(mp)}(PR3 )] (R = Ph, 1; Cy, 2). Thermal ellipsoids
drawn at 50% level. Hydrogen atoms, with the exception of those attached to the boron centers,
have been omitted for clarity. Both structures contain disorder in the position of the [H3 B(mp)]− ligand.
Only the major component is shown (see text for details).
Table 2. Selected Bond Distances (Å) and Angles (◦ ) for 1–3.

Cu(1)–P(1)
Cu(1)–B(1)
Cu(1)–S(1)
C(1)–S(1)
B(1)–N(1)
N(1)–C(1)
B(1)–H(1AA)
B(1)–H(1AB)
B(1)–H(1AC)
Cu(1)–H(1AA)
Cu(1)–H(1AB)
S(1)–Cu(1)–P(1)
S(1)–Cu(1)–B(1)
P(1)–Cu(1)–B(1)
Σangles around Cu 4
C(1)–S(1)–Cu(1)
N(1)–B(1)–Cu(1)

[Cu{H3 B(mp)}PPh3 ] 1

[Cu{H3 B(mp)}PCy3 ] 2

[Cu{H2 B(mp)2 }PPh3 ] 3

2.1789(4)
2.113(17)/2.229(14)
2.205(2)/2.221(4)
1.7515(17)/1.722(2)
1.551(8)/1.465(10)
1.3506(19)/1.3506(19)
1.17(2)/1.18(2)
1.16(2)/1.18(2)
1.17(2)/1.17(2)
1.75(3)/1.81(4)
1.81(3)/1.85(4)
129.93(3)/134.69(5)
89.2(2)/87.3(2)
140.5(2)/137.5(3)
359.63/359.49
99.53(9)/99.14(16)
110.0(8)/108.7(7)

2.1876(4)
2.153(16)/2.10(3)
2.2523(12)/2.296(12)
1.7244(17)/1.751(13)
1.602(16)/1.61(2)
1.3550(19)/1.3550(19)
1.16(2)/1.16(2)
1.17(2)/1.15(2)
1.14(2)/1.15(2)
1.75(2)/1.68(8)
1.81(2)/1.82(8)
129.93(3)/135.9(3)
89.7(4)/90.2(5)
140.3(4)/133.9(6)
359.93/360.0
99.53(8)/96.2(5)
107.0(8)/110.3(13)

2.216(3)
2.7479(15)
2.255(4) and 2.248(4)
1.707(14) and 1.708(14)
1.592(2) and 1.583(18)
1.3649(17) and 1.3648(19)
1.090(18) (terminal)
1.150(17) (bridging)
1.832(17)
111.88(15) and 124.56(14)
82.29(3) and 80.27(3)
135.64(3)
350.4
106.49(5) and 109.83(5)
95.36 and 99.09

Note: 1 the [H3 B(mp)]− ligand is disordered over two positions (with an approximate ratio 56:44). Where a second
value is provided in the table, it represents the value corresponding to the minor occupancy component; 2 the
[H3 B(mp)]− ligand is disordered over two positions (with an approximate ratio 79:21). Where a second values is
provided in the table, it represents the value corresponding to the minor occupancy component; 3 data obtained
from reference [36], the two values here result from the fact that there are two mp units within the complex; 4 the
value quoted involves the sum of all angles around the copper center involving all non-hydrogen atoms.

91

Inorganics 2019, 7, 93

The Cu(1)–B(1) distances in complexes 1 are 2.113(17) Å (major component in disorder) and
2.229(14) Å (minor component). The corresponding distances in 2 are 2.153(16) Å and 2.10(3) Å,
respectively. These distances are consistent with similar copper complexes featuring a neutral H3 BN
moiety bound to the metal center with a dihydroborate mode [46,47]. Again, the difference in the
coordination mode from κ3 -S,H,H in 1 and 2 to κ3 -S,S,H in 3 is significant. In complex 3, the Cu-B
distance is 2.7479(15) Å, since this represents a Cu-H-B bridging interaction. The Cu(1)–S(1) distances
for complex 1 are 2.205(2) Å (major) and 2.221(4) Å (minor). For complex 2, the corresponding
Cu(1)–S(1) distances are 2.2523(12) Å and 2.296(12) Å. This indicates that the interaction of the thione
unit with the metal center in 2 is weaker than in 1, as might be expected, since complex 2 contains the
more electron-rich phosphine ligand.
The B-N and C-S distances within the complexes are of interest in order to explore the extent of
different resonance forms within the [H3 B(mp)]− ligand. The ligand can be described as a thiopyridone
species forming a borohydride entity (Figure 4, left), or as a pyridine-2-thiolate forming a borane adduct
(Figure 4, right). As can be observed in Table 2, the B-N and C-S distances vary significantly within
the disordered components of the complexes. For example, in complex 1 the C(1)–S(1) distances are
1.7515(17) Å (for the major component of disorder) and 1.722(2) Å (for the minor). The former represents
a significant difference in bond order between single and double bond character. It is interesting to note
that the corresponding distances in the previously reported complex, [Cu{H2 B(mp)2 }PPh3 ], are shorter,
suggesting a more double-bonded character in the bis-substituted ligand.

Figure 4. Two bonding descriptions for the [H3 B(mp)]− .

3. Materials and Methods
3.1. General Remarks
The syntheses of the complexes were carried out using standard Schlenk techniques. Solvents
were sources as extra dry from “Acros Organics” (Morris Plains, NJ, USA) and were stored over
either 4 Å or 3 Å molecular sieves. The NMR solvent, CDCl3 , was stored in Young’s ampule over
4 Å molecular sieves, under a N2 atmosphere and was degassed through freeze–thaw cycles prior
to use. Reagents were used as purchased from commercial sources. The ligand Na[H3 B(mp)] [36]
was synthesized according to standard literature procedures. NMR spectroscopy experiments were
conducted on a Bruker 400 MHz AscendTM 400 spectrometer (Billerica, MA, USA). All spectra were
referenced internally, to the residual protic solvent (1 H) or the signals of the solvent (13 C). Proton (1 H)
and carbon (13 C) assignments were further supported by heteronuclear single-quantum correlation
spectroscopy (HSQC), heteronuclear multiple-bond correlation spectroscopy (HMBC), and correlation
spectroscopy (COSY) two-dimensional correlation NMR experiments. The symbol “τ” is used to
represent an apparent triplet, where the resonance is expected to be a “dd”. In these cases, the apparent
coupling constant has been provided. Infrared spectra were recorded on a PerkinElmer Spectrum Two
Attenuated total reflectance infra-red (ATR FT-IR) spectrometer as powder films (Foster City, CA, USA).
Elemental analysis was performed at London Metropolitan University by their elemental analysis
service. Mass spectra were recorded by the EPSRC National Mass Spectrometry Facility (NMSF) at
Swansea University. The numbering scheme used for NMR assignments is highlighted in Figure 5.

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Inorganics 2019, 7, 93

+


%
1

+
+






3  




6





Figure 5. Numbering Scheme used for [H3 B(mp)]− and PCy3 .

3.2. Synthesis of [Cu{H3 B(mp)}(PPh3 )]
To a Schlenk flask containing CuCl (24 mg, 0.24 mmol), PPh3 (117 mg, 0.45 mmol), and Na[H3 B(mp)]
(33 mg, 0.22 mmol) was added methanol (5 mL). The stirred solution gradually turned yellow and a
precipitate formed. The reaction was left stirring for 36 h, after which the flask was cooled to −40 ◦ C
and left overnight to further precipitate the product out of solution. The filtrate was removed via
cannula filtration and the resultant solid dried under vacuum to give [Cu{H3 B(mp)} (PPh3 )] as a pale
yellow powder (68 mg, 0.14 mmol, 68%).
1 H NMR (δ, CDCl ): 6.76 (1H, τ, J
mp CH-(4)), 7.17–7.44 (16H, m, P(C H ) +
3
HH = 6.5 Hz,
6 5 3
mp CH-(5) [49]), 7.80 (1H, d, 3 J
mp CH-(6)), 8.51 (1H, d, 3 J
mp CH-(3)). 1 H{11 B} (δ,
=
8.5
Hz,
HH
HH = 5.8 Hz
CDCl3 ): 2.64 (3H, s br, BH3 ). 13 C{1 H} (δ, CDCl3 ): 115.6 (mp CH-(4)), 128.6 (d, 2 JCP = 9.6 Hz, Portho (C6 H5 )3 ),
130.0 (d, 4 JCP = 1.5 Hz, Ppara (C6 H5 )3 ), 131.5 (mp CH-(6)), 132.9 (d, 1 JCP = 32 Hz, Pipso (C6 H5 )3 ), 133.8 (d,
3J
meta (C H ) ), 135.0 (mp CH-(5)), 146.5 (mp CH-(3)), 175.9 (mp C=S-(2)). 31 P{1 H} NMR (δ,
CP = 16 Hz, P
6 5 3
CDCl3 ): 4.8 (s, h.h.w. = 392 Hz). 11 B NMR (δ, CDCl3 ): −13.9 (q, 1 JBH = 75 Hz, BH3 ). 11 B{1 H} NMR
(δ, CDCl3 ): −13.9 (s, h.h.w. = 113 Hz). MS APCI (ASAP+) m/z = 436.03 [M – BH3 + H]+ . IR (cm−1 ,
powder film) 2439 w (B–H), 2078 w (BH2 Cu), 1614 s, 1568 s. Elemental analysis (%): Calculated for
CuSNPC23 H22 B: C 61.41 H 4.93 N 3.11 Found: C 61.56 H 4.80 N 3.15.
3.3. Synthesis of [Cu{H3 B(mp)}(PCy3 )]
To a Schlenk flask containing CuCl (22 mg, 0.22 mmol), PCy3 (123 mg, 0.44 mmol), and Na[H3 B(mp)]
(30 mg, 0.20 mmol) was added methanol (5 mL). The stirred solution gradually turned yellow and a
precipitate formed. The reaction was left stirring for 36 h, after which the flask was cooled to −40 ◦ C
and left overnight to further precipitate the product out of solution. The filtrate was removed via
cannula filtration and the resultant solid dried under vacuum to give [Cu{H3 B(mp)} (PCy3 )] as an off
white powder (62 mg, 0.14 mmol, 59%).
1 H NMR (δ, CDCl ): 1.19–1.35 (21H, m, PCy ), 1.64–1.87 (23H, m, PCy ), 2.42 (3H, d vb, 1 J
3
3
3
BH =
106 Hz, BH3 ), 6.71 (1H, τ, JHH = 6.6 Hz, mp CH-(3)), 7.29 (1H, τ, JHH = 7.6 Hz, mp CH-(4)), 7.75 (1H, d,
J = 8.3 Hz, mp CH-(5)), 8.48 (1H, d, J = 6.3 Hz, mp CH-(6)). 1 H{11 B} NMR (δ, CDCl3 ): 2.42 (3H, s br, BH3 ).
13 C{1 H} (δ, CDCl ): 26.2 (PCy -(4)), 27.4 (d, 3 J
2
3
3
CP = 11 Hz, PCy3 -(3)), 30.6 (d, J CP = 4 Hz, PCy3 -(2)),
1
mp
mp
mp
31.8 (d, JCP = 18 Hz, PCy3 -(1)), 115.3 ( CH-(4)), 131.4 ( CH-(6)), 134.8 ( CH-(5)), 146.3 (mp CH-(3)),
176.1 (mp C=S-(2)). 31 P{1 H} NMR (δ, CDCl3 ): 27.2 (s br, h.h.w. = 111 Hz). 11 B NMR (δ, CDCl3 ): −13.4 (q,
1J
11 B{1 H} NMR (δ, CDCl ) −13.4 (s, h.h.w. = 90 Hz). IR (cm−1 , powder film) 2448 w
BH = 82 Hz, BH3 ).
3
(B–H), 2085 w (BH2 Cu), 1606 s, 1540 s. MS APCI (ASAP+) m/z = 467.2 [M]+ . Elemental analysis (%):
Calculated for C23 H40 BCuSNP: C 59.03 H 8.62 N 2.99, Found: C 59.21 H 8.48 N 2.90.
3.4. Crystallography
Single-crystal X-ray diffraction studies of complexes 1 and 2 were undertaken at the U.K. National
Crystallography Service (NCS) at the University of Southampton [50]. Single crystals of each of
the complexes were obtained by allowing a 1:1 mixture of methanol and diethyl ether to slowly
evaporate at room temperature. For each sample, single crystal was mounted on a MITIGEN holder
in perfluoroether oil on a Rigaku FRE+ equipped with HF Varimax confocal mirrors and an AFC11
goniometer and HyPix 6000 detector. The data for the crystals was collected at T = 100(2) K. Data were
collected and processed via standard protocols. Empirical absorption corrections were carried out
93

Inorganics 2019, 7, 93

using CrysAlisPro [51]. The structures were solved by Intrinsic Phasing using the ShelXT structure
solution program [52] and refined on Fo 2 by full-matrix least squares refinement with version 2018/3
of ShelXL [53], as implemented in Olex2 [54]. All hydrogen atom positions, with the exception of
those at boron, were calculated geometrically and refined using the riding model. Crystal Data for
1. C23 H22 BCuNPS, Mr = 449.79, monoclinic, C2/c (No. 15), a = 11.90994(6) Å, b = 13.21619(7) Å, c
= 26.83905(13) Å, β = 97.6274(4)◦ , α = γ = 90◦ , V = 4187.20(4) Å3 , T = 100(2) K, Z = 8, Z’ = 1, μ(Cu
Kα) = 3.175 mm−1 , 38,239 reflections measured, 3963 unique (Rint = 0.0259), which were used in
all calculations. The final wR2 was 0.0664 (all data) and R1 was 0.0244 (I > 2(I)). Crystal Data for 2.
C23 H40 BNPSCu, Mr = 467.94, triclinic, P-1 (No. 2), a = 8.16720(10) Å, b = 9.38370(10) Å, c = 17.2612(2)
Å, α = 96.9390(10)◦ , β = 95.6170(10)◦ , γ = 112.3730(10)◦ , V = 1199.33(3) Å3 , T = 100(2) K, Z = 2, Z’ = 1,
μ(Cu Kα) = 2.773 mm−1 , 30,937 reflections measured, 4471 unique (Rint = 0.0278), which were used in
all calculations. The final wR2 was 0.0628 (all data) and R1 was 0.0236 (I > 2(I)). A summary of the
crystallographic data collection parameters and refinement details for the complexes are presented
in the supplementary information. Anisotropic parameters, bond lengths, and (torsion) angles for
these structures are available from the CIF files, which have been deposited with the Cambridge
Crystallographic Data Centre and given the following deposition numbers, 1922838 (1) and 1922839
(2). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
4. Conclusions
The synthesis and characterization of the first examples of copper complexes containing the
mono-substituted borohydride ligand, [H3 B(mp)]− , have been reported. These add to the family of
ligands in which the bis- and tri-substituted versions have previously been reported. Mono-substituted
soft borohydride derivatives are a rare class of compound and these examples are an interesting
addition to the family. The new complexes were also structurally characterized by X-ray crystallography,
which confirmed the κ3 -S,H,H coordination mode where the BH3 unit coordinated via a BH2 Cu bridging
mode. The spectroscopic data appears to suggest the coordination of this unit to the metal center
is weak in the case of copper. This is in contrast to a much stronger interaction that was found in
the previously reported complex, [Rh{κ3 -H,H,S-H3 B(mp)}(NBD)]. The additional knowledge on the
coordination chemistry of mono-substituted ligand systems, particularly the nature of the BH2 Cu
bridging mode, is of value.
Supplementary Materials: The following are available online at http://www.mdpi.com/2304-6740/7/8/93/s1.
Table S1—crystallographic parameters for 1 and 2; Figures S1–S13—NMR spectra for complexes 1 and 2; CIF file
and checkCIF file—crystallographic data for 1 and 2.
Author Contributions: J.G. and S.D.T. performed the experiments. G.J.T. and S.J.C. carried out the crystallography
work. G.R.O. wrote the manuscript and directed the project.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflict of interest.

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© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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