Syntheses and reductions of C-dimesitylboryl-1,2-dicarba-closo-dodecaboranes

Article abstract of DOI:10.1039/c5dt00758e

Two C-dimesitylboryl-1,2-dicarba-closo-dodecaboranes, 1-(BMes₂)-2-R-1,2-C₂B₁₀H₁₀ (1, R = H, 2, R = Ph), were synthesised by lithiation of 1,2-dicarba-closo-dodecaborane and 1-phenyl-1,2-dicarba-closo-dodecaborane, respectively, with n-butyllithium and subsequent reaction with fluorodimesitylborane. These novel compounds were structurally characterised by X-ray crystallography. Compounds 1 and 2 are hydrolysed on prolonged exposure to air to give mesitylene and boronic acids 1-(B(OH)₂)-2-R-1,2-C₂B₁₀H₁₀ (3, R = H, 4, R = Ph respectively). Addition of fluoride anions to 1 and 2 resulted in boryl-carborane bond cleavage to give dimesitylborinic acid HOBMes₂. UV absorption bands at 318-333 nm were observed for 1 and 2 corresponding to local π-π-transitions within the dimesitylboryl groups while visible emissions at 541-664 nm with Stokes shifts of 11 920-16 170 cm^-1 were attributed to intramolecular charge transfer transitions between the mesityl and cluster groups. Compound 2 was shown by cyclic voltammetry to form a stable dianion on reduction. NMR spectra for the dianion [2]^2- were recorded from solutions generated by reductions of 2 with alkali metals and compared with NMR spectra from reductions of 1,2-diphenyl-ortho-carborane 5. On the basis of observed and computed ¹¹B NMR shifts, these nido-dianions contain bowl-shaped cluster geometries. The carborane is viewed as the electron-acceptor and the mesityl group is the electron-donor in C-dimesitylboryl-1,2-dicarba-closo-dodecaboranes.

Full text of DOI:10.1039/c5dt00758e

Dalton
Transactions
PAPER
Syntheses and reductions of C-dimesitylboryl-
1,2-dicarba-closo-dodecaboranes†‡
Cite this: Dalton Trans., 2015, 44,
9766
Jan Kahlert,a Lena Böhling,a Andreas Brockhinke,a Hans-Georg Stammler,a
Beate Neumann,a Louis M. Rendina,b Paul J. Low,c Lothar Weber*a and Mark A. Fox*d
Two C-dimesitylboryl-1,2-dicarba-closo-dodecaboranes, 1-(BMes2)-2-R-1,2-C2B10H10 (1, R = H, 2, R =
Ph), were synthesised by lithiation of 1,2-dicarba-closo-dodecaborane and 1-phenyl-1,2-dicarba-closo-
dodecaborane, respectively, with n-butyllithium and subsequent reaction with fluorodimesitylborane.
These novel compounds were structurally characterised by X-ray crystallography. Compounds 1 and 2 are
hydrolysed on prolonged exposure to air to give mesitylene and boronic acids 1-(B(OH)2)-2-R-1,2-
C2B10H10 (3, R = H, 4, R = Ph respectively). Addition of fluoride anions to 1 and 2 resulted in boryl-carbo-
rane bond cleavage to give dimesitylborinic acid HOBMes2. UV absorption bands at 318–333 nm were
observed for 1 and 2 corresponding to local π–π*-transitions within the dimesitylboryl groups while
visible emissions at 541–664 nm with Stokes shifts of 11 920–16 170 cm−1 were attributed to intra-
molecular charge transfer transitions between the mesityl and cluster groups. Compound 2 was shown
by cyclic voltammetry to form a stable dianion on reduction. NMR spectra for the dianion [2]2− were
recorded from solutions generated by reductions of 2 with alkali metals and compared with NMR spectra
from reductions of 1,2-diphenyl-ortho-carborane 5. On the basis of observed and computed 11B NMR
shifts, these nido-dianions contain bowl-shaped cluster geometries. The carborane is viewed as the elec-
tron-acceptor and the mesityl group is the electron-donor in C-dimesitylboryl-1,2-dicarba-closo-
dodecaboranes.
Received 20th February 2015,
Accepted 24th April 2015
DOI: 10.1039/c5dt00758e
www.rsc.org/dalton
which leads to a narrowing of the HOMO–LUMO-gap (HLG) by
lowering the LUMO energy. Indeed, the π-acceptor strength of
Introduction
Tri-coordinate boron compounds have been intensely investi- the BMes2 group is similar to those of cyano-2 and nitro-3
gated in the past two decades in view of potential applications groups. These electronic characteristics have led to organic
as functional materials.1 The most widely employed functional materials containing BMes2 units finding application as elec-
moiety containing a tri-coordinate boron atom is the dimesityl- tron-transporting materials in opto-electronic devices.4,5 Com-
boryl group (BMes2; Mes = 2,4,6-Me3C6H2) in which the unsa- pounds containing BMes2 can be strongly fluorescent and thus
turated boron centre is kinetically stabilised by steric shielding have been used in organic light emitting diodes (OLEDs).5,6
of the mesityl groups. The empty pz-orbital at the boron atom Moreover, the ability of the boron atom to form selectively
can interact with the π-system of attached organic skeletons covalent adducts with small anions has led to applications of
these compounds as colorimetric and luminescent sensors for
fluoride7–9 and cyanide.10,11
aFakultät für Chemie der Universität Bielefeld, 33615 Bielefeld, Germany.
Other boron-containing compounds that have gained con-
E-mail: lothar.weber@uni-bielefeld.de
bSchool of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia
cSchool of Chemistry and Biochemistry, University of Western Australia,
35 Stirling Highway, Crawley, Perth 6009, Australia
siderable interest in the last seven years in the field of opto-
electronic materials are derivatives of the dicarba-closo-dodeca-
borane isomers (1,2-, 1,7- and 1,12-C2B10H12 which are ortho-,
meta- and para-carborane, respectively).12,13 Due to their delo-
calised σ-electron systems (‘3D aromaticity’), these clusters
possess high thermal and chemical stabilities.14 The ortho-
carborane is a unique electron-acceptor when connected to a
donor at one or both cluster carbon atoms (at C1 and/or C2 in
1,2-C2B10H12) due to the elasticity of the cluster C1–C2
bond.15–17 The ortho-carborane unit thus can play an active
role as the acceptor in donor–acceptor molecules (dyads).
dDepartment of Chemistry, Durham University, Durham DH1 3LE, UK.
E-mail: m.a.fox@durham.ac.uk
†In memory of Ken Wade, a brilliant chemist and mentor.
‡Electronic supplementary information (ESI) available: Absorption spectra for 1
and 2, detailed CV data for 1 and 2, TD-DFT data for 1 and 2, spectroelectro-
chemical data for 2, computed GIAO-NMR data for 1–4, NMR spectra for 1–4 and
dianions [2]2− and [5]2−, crystallographic data for 1 and 2 and Cartesian coordi-
nates for eleven optimised geometries. CCDC 1048027–1048028. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c5dt00758e
9766 | Dalton Trans., 2015, 44, 9766–9781
This journal is © The Royal Society of Chemistry 2015

Dalton Transactions
Paper
yields by crystallisation from n-hexane/dichloromethane mix-
tures. The elemental analytical result for 1 was significantly
lower (0.8% for carbon) than the calculated value. Similar dis-
crepancies have been noted elsewhere for related com-
pounds.33 It is possible that the formation of boron carbide
during the combustion analysis may adversely affect the values
obtained.
Fig. 1 Syntheses of the novel C-dimesitylboryl-ortho-carboranes 1 and 2.
Signals in the 11B{1H} NMR spectra of 1 and 2 between 3.7
and −12.9 ppm confirm the presence of the ortho-carborane
clusters. The 11B peaks at 78.9 ppm (1) and 80.4 ppm (2) are
assigned to the BMes2 groups and are very broad compared to
the peaks corresponding to the cluster. The 11B chemical
shifts of the Mes2B groups in 1 and 2 are virtually identical to
trimesitylborane (79.2 ppm) and phenyldimesitylborane
(79.3 ppm).37 Thus, the ortho-carboranyl groups influence the
chemical shift of the tri-coordinate boron atom of the BMes2
unit in the same manner as a phenyl or mesityl substituent.
Photoexcitation of such dyads induces a charge transfer from
an organic scaffold to the carborane cluster which either led to
luminescence quenching18 or to charge transfer (CT) emis-
sions or both depending on the solvents used19–21 and
whether the materials were investigated as solids.22–28 Com-
pounds with both BMes2 and ortho-carboranyl groups are
known with para-phenylene bridges linking both units.29,30 In
these systems, the ortho-carboranyl group served as a strongly
inductive electron-withdrawing group as the cluster increased
the Lewis acidity of the triarylborane as found by fluoride ion
titrations compared to the triarylborane without the cluster
attached.29 Another compound containing both BMes2 and
ortho-carboranyl group was reported with an ethylene bridge
linking both units.31
Of the few ortho-carboranes with tri-coordinate boron
substituents at their carbon atoms reported,24–26,32–36 only
C-benzodiazaborolyl-ortho-carboranes have been investigated
regarding their photophysical, electrochemical and spectro-
electrochemical properties.25 However, the benzodiazaborolyl
group generally acts as a π-donor and is therefore electronically
quite distinct from the BMes2 moiety. In order to better under-
stand the photophysical and electrochemical properties of
ortho-carboranes with boryl groups, studies with compounds
containing a tri-coordinate boron π-acceptor would be appeal-
ing and allow one to determine more precisely the electronic
interplay between the carborane and tri-coordinate boron elec-
tron-withdrawing units. Therefore we present herein, the
syntheses and crystal structures of two ortho-carborane deriva-
tives 1 and 2 with a BMes2 group at one of the cage carbon
atoms and their photophysical and electrochemical properties
(Fig. 1). The geometry of the reduced species of 2 was also
determined by a combination of 11B NMR spectroscopy and
GIAO-NMR DFT computations.
Hydrolyses of 1 and 2
Since aqueous work-ups were used in the preparation of 1 and
2 these C-boryl-ortho-carboranes are considered to be water-
stable – unlike many reported C-boryl-ortho-carboranes that
could not be isolated pure due to facile hydrolysis.32 The 1H
and 11B NMR spectra for 1 and 2 in deuterated chloroform
solutions containing excess water also showed no changes.
The sterics of the mesityl and the carboranyl groups appear to
prevent facile hydrolysis of the boron atom in 1 and 2.
Solids of 1 and 2 do, however, hydrolyse on prolonged
exposure to air (complete conversion after three weeks for 1
and eighteen months for 2) to give mesitylene and the new carbo-
ranylboronic acids, 3 and 4 (Fig. 2). While these acids could
not be obtained pure, they were identified by multinuclear
NMR spectroscopy and mass spectrometry. The observed
cleavage of the B–C(mesityl) bond in the process is not
without precedent. It has been shown elsewhere that mesity-
lene is formed from the reaction of dimesitylborinic acid,
Mes2BOH, with trimethylaluminium.38 The initial steps in
these air-induced hydrolyses of 1 and 2 probably involve clea-
vage of the B–C(mesityl) bonds by oxygen (as in the B–C
Results and discussion
Syntheses and characterisation of 1 and 2
Compounds 1 and 2 were synthesised by reaction of fluorodi-
mesitylborane with the corresponding C-lithiocarborane, gene-
rated in situ by metallation of ortho-carborane and 1-phenyl-
ortho-carborane, in boiling toluene (Fig. 1). The elevated tem-
peratures proved to be essential as no conversion was observed
at ambient temperature. Purification was achieved by aqueous
Fig. 2 Hydrolysed products from reactions of 1 and 2 with air and
work-up and the target compounds were isolated in moderate fluoride ions.
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(phenyl) bond cleavage reaction of triphenylboron by oxygen39)
followed by hydrolysis with traces of water present in air.
As many organic dimesitylboranes have been explored as
fluoride sensors,7,29 the reactivity of 1 and 2 towards fluoride
ions was of interest. Chloroform solutions of 1 and 2 were
treated with an excess of tetra-n-butylammonium fluoride
hydrate (TBAFH) while acetonitrile solutions of 1 and 2 were
added with potassium fluoride (KF) and 18-crown-6 to obtain
the desired fluoride adducts [1·F]− and [2·F]−, respectively,
where the fluoride ion is bound to the boryl boron atom.
Hydrolysis took place instead in all cases to give Mes2BOH,
and the corresponding unsubstituted carborane, 1,2-C2B10H12
1
or 1-Ph-1,2-C2B10H11, as detected by H, 11B, 13C and 19F NMR
spectroscopies on the reaction mixtures (Fig. 2). These reac-
tions were complicated by fluoride-ion deboronation processes
on the unsubstituted carboranes to give 11B and 19F NMR
peaks corresponding to fluoroborates of the boron atom
initially removed from the cluster.40
Fig. 4 Molecular structure of 2 with hydrogen atoms omitted. Thermal
ellipsoids are plotted at 50% probability.
It is possible that fluorodimesitylborane, Mes2BF, is initially
formed in the reaction as Mes2BF is known to be easily hydro-
lysed to Mes2BOH.39,41 However, careful 19F NMR monitoring
of the reaction mixtures from 1 and 2 with TBAFH in the first Table 1 Selected bond lengths and angles for 1 and 2
few minutes did not reveal any evidence of an intermediate
1
2
such as the fluoride adducts [1·F]−, [2·F]− or Mes2BF. The reac-
tions of potassium hydroxide (KOH) and 18-crown-6 with 1
and 2 in acetonitrile gave Mes2BOH and the corresponding
unsubstituted carborane. Deboronation products were also
present in the latter reactions as the combination of KOH and
18-crown-6 is a strong deboronating agent.42
Exp.
Calcd.a Exp.
Calcd.a
Bond lengths [Å]
C1–C2
C1–B1
C1–B3
C1–B6
B1–C3
B1–C12
1.677(3)
1.635(3)
1.728(3)
1.756(3)
1.595(3)
1.581(3)
1.671
1.641
1.723
1.757
1.601
1.593
1.761(2)
1.791
1.648
1.724
1.751
1.597
1.602
1.629(2)
1.727(2)
1.748(2)
1.590(2)
1.602(2)
X-ray crystallography
Bond angles [°]
C1–B1–C3
C1–B1–C12
C3–B1–C12
Single crystals of 1 and 2 were grown from n-hexane/dichloro-
methane mixtures and their molecular structures were deter-
mined by X-ray diffraction (Fig. 3 and 4, Table 1). The BMes2
115.7(2)
121.0(2)
123.3(2)
116.9
121.0
122.2
118.2(1)
119.4(1)
122.4(1)
120.7
118.3
120.9
Torsion angles [°]
C2–C1–B1–C3
C2–C1–B1–C12
B3–C1–B1–C3
B3–C1–B1–C12
36.0(2)
34.9
22.5(2)
37.1
−144.5(2) −145.1 −157.8(2) −145.8
102.7(2)
−77.8(2)
102.1
−77.8
95.8(2)
−84.5(2)
110.6
−72.3
Interplanar angles [°]
(C1,B1,C3,C12)(C3–C8)
(C1,B1,C3,C12)(C12–C17) 54.1(1)
77.9(1)
74.2
58.3
78.6(1)
65.5(1)
69.8
64.0
a Calculated values from optimised geometries of 1 and 2 at B3LYP/6-31G*.
groups adopt orientations with C2–C1–B1–C3 torsion angles at
36.0(2)° and 22.5(2)° and C2–C1–B1–C12 at −144.5(2)° and
−157.8(2)° in 1 and 2 respectively (Table 1). Thus the empty pz-
orbitals are approximately in plane with the C1–B3 bonds in
the clusters and these C1–B3 bonds are shorter than the C1–
B6 bonds by 0.02–0.03 Å in both compounds. However, all B–B
and B–C bonds lengths in the clusters are within the usual
range.24
The C1–C2 distance of 1.677(3) Å in 1 agrees within 3 esd
with C1–C2 bond lengths of 1.667(1)–1.673(1) Å found in other
C-monoboryl-ortho-carboranes.24,33,36 By contrast, the C1–C2
Fig. 3 Molecular structure of
clarity. Thermal ellipsoids are drawn at 50% probability.
1 with hydrogen atoms omitted for
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bond of 1.761(2) Å in 2 is significantly longer than in its 1,3- Photophysics
diethyl-1,3,2-benzodiazaborol-2-yl analogue (1.701(2)–1.730(2)
Å),24 in 1,2-diphenyl-ortho-carborane (1.720(4)–1.733(4) Å)43 Photophysical data for 1 and 2 are listed in Table 2. The
and 1,2-diboryl-ortho-carboranes (1.695(1)–1.725(2) Å).26,36 The absorption maxima of both C-dimesitylboryl-ortho-carboranes
longer C1–C2 bond in 2 is explained by the different steric (Fig. S1‡) in solvents of different polarity do not display any
interactions between the BMes2 group at C1 and the phenyl significant solvatochromism. The lack of solvatochromism
ring at C2 in 2.
points to very similar dipole moments in the electronic ground
The BMes2 groups are linked to the cage carbon atoms by state and the initial excited state indicating that local tran-
B–C single bonds (C1–B1) with lengths of 1.635(3) Å in 1 and sitions within the dimesitylboryl unit give rise to the absorp-
1.629(2) Å in 2, which is at the upper edge of the range deter- tion bands observed. The presence of the phenyl ring at C2 in
mined for other C-boryl-ortho-carboranes (1.607(4)–1.649(12) 2 appears to lower the HOMO–LUMO energy gap as the
Å).24–26,33,36 The B–C bond lengths between the mesityl rings absorption maxima of 2 (330 nm–333 nm) are bathochromi-
and the boryl-boron atoms (B1–C3/C12 1.581(3)–1.602(2) Å) cally shifted by approximately 12–14 nm compared to 1
are typical for dimesitylboranes. As a consequence of the (318–319 nm) in all solvents used. The energy difference in the
three-dimensional shape of the cluster in both structures, the absorption maxima between 1 (329 nm) and 2 (332 nm) in the
interplanar angle enclosed by the mesityl ring pointing solid state is smaller.
towards the second cage carbon atom and the plane defined
Emission maxima of both compounds in cyclohexane are
by the boryl-boron atoms and the three neighbouring carbon virtually identical at 541 nm for 1 and 544 nm for 2 with large
atoms (77.9(1)° (1), 78.6(1)° (2)) is larger than in most reported Stokes shifts of 12 300 cm−1 for 1 and 11 920 cm−1 for 2. The
structures of BMes2 compounds.44 A virtually perpendicular luminescence spectra (Fig. 5) reveal positive solvatochromism
orientation of the phenyl substituent in 2 with respect to the with emission maxima in the more polar solvent dichloro-
C1–C2 axis (torsion angles = C1–C2–C21–C22 94.3(2)°, C1– methane in the red emission region at 664 nm for 1 and
C2–C21–C26 −91.5(2)°) corresponds to the situation in other 643 nm for 2. By using the Lippert–Mataga method46,47 with
disubstituted phenyl-ortho-carboranes and is preferred due to an Onsager radius of 4.00 Å estimated from the molecular
sterics.43,45
structures, the calculated transition dipole moments are 10.4 D (1)
Table 2 Photophysical data for 1 and 2. CyH = cyclohexane, THF = tetrahydrofuran, DCM = dichloromethanea
Solid
CyH
CHCl3
THF
DCM
Absorption λmax [nm] (ε)a
1
2
1
2
1
2
329
332
567
550
12 310
9210
318 (8300)
332 (6750)
400, 541 (0.15 : 1)
544
5500, 12 300
11 920
319 (7350)
331 (7330)
396, 641 (0.14 : 1)
620
5780, 15 610
14 310
319 (7060)
333 (5710)
408, 653 (0.91 : 1)
640
6240, 15 600
14 660
318 (7920)
330 (7760)
406, 664 (0.48 : 1)
643
6600, 16 170
14 800
Emission λmax [nm] (relative height)
Stokes shift [cm−1
]
a In L mol−1 cm−1
.
Fig. 5 Emission spectra of 1 and 2 in the solid state and in various solvents.
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Fig. 6 Crystals of 1 and 2. Left column: Without UV irradiation. Right
column: Under UV irradiation at 350 nm.
Fig. 7 Cyclic voltammograms of 1 and 2 with a glassy carbon working
electrode in acetonitrile and internal ferrocenium/ferrocene couples at
0.0 V.
and 9.2 D (2) – similar to transition dipole moments of
C-benzodiazaborolyl-ortho-carboranes (6.9–10.9 D).24–26 The
results show that the carborane cluster plays a part in the
emission process acting as the electron acceptor of the charge
transfer process after excitation. The solid-state emissions
occur at longer wavelengths than in cyclohexane (Fig. 6). This
bathochromic shift is more pronounced for 1 (567 nm) than
for 2 (550 nm) and is presumably caused by fluorophore–
fluorophore interactions. The quantum yields (Φ) are very low
in all solvents (<1%) with higher values of 2% (1) and 7% (2)
found in the solid state.
In addition to these low-energy emissions, compound 1 dis-
plays a weaker emission band at the violet edge of the visible
spectrum in polar solvents. The high-energy emissions with
smaller Stokes shifts of 5500–6600 cm−1 probably originate
from local transitions at the BMes2 group.48 Similar dual emis-
sions originating from both local and CT transitions have been
reported for some ortho-carboranes with substituents at one or
both cluster carbon atoms.20,24–27
between the cathodic and anodic peaks of the wave can be
several hundred millivolts due to the structural rearrangement
of the dianion on the CV timescale. Decomposition and
proton-coupled electron transfer (PCET) processes can also
complicate the electrochemical response.49,55
The CV of 1 in acetonitrile with a glassy carbon working
electrode shows a two-electron cathodic wave at −1.86 V and
two anodic waves at −1.43 V and −1.29 V with values refer-
enced to the ferrocenium/ferrocene redox couple at 0 V
(Fig. 7). The cathodic wave value of −1.80 V for 1 means that 1
is more easily reduced than C-monophenyl-ortho-carborane at
−2.25 V 50 reflecting the substantial electron-withdrawing
effect of the BMes2 group. Similar CV traces are observed for 1
with a platinum working electrode and with DCM as solvent
(Fig. S2 and Table S1‡). The non-equivalent current intensities
between the forward and reverse waves for 1 suggest that the
dianion [1]2− is not stable and would be difficult to isolate.
A CV trace for ortho-carborane with aryl substituents at both
cluster carbon atoms generally shows a reversible wave (or two)
Electrochemistry
The electrochemical properties of both C-dimesitylboryl-ortho-
carboranes 1 and 2 were investigated by cyclic voltammetry
(CV, Fig. 7). The peak potentials measured in acetonitrile and
dichloromethane solutions, with platinum and glassy carbon
working electrodes are listed in Table S1.‡ The traces resemble
on reduction.20,25–27,49,51–54 In several cases,
a stepwise
reduction involving two separated one-electron reduction steps
has been found, with the initial reduction process giving rise
to a radical anion with an unusual 2n + 3 skeletal electron (SE)
count. One example is diphenyl-ortho-carborane 5 where the
radical anion has been shown to be stable enough to be
observed spectroscopically at ambient temperature in solution
(Fig. 8).51 The first one-electron reduction process on the CV
reported
CV
data
on
reductions
of
carboranes
elsewhere20,21,25–27,49–54 and, by inference, reductions take
place at the carborane clusters in 1 and 2. CV traces for
reductions of the BMes2 group would involve a simple one-
electron reversible wave. In contrast, the one-electron
reduction wave associated with PhBMes2 occurs at −2.30 V (vs.
the ferrocenium/ferrocene couple at 0.0 V)2 and hence the
BMes2 localised reduction in 1 and 2 likely falls outside the
electrochemical window examined here.
A CV trace for ortho-carborane or a C-monosubstituted-
ortho-carborane usually shows a two-electron cathodic wave
and an anodic wave that is not of the same current intensity as
the cathodic wave.25,49,50 Often the peak–peak separation Fig. 8 Two one-electron reductions for 1,2-diphenyl-ortho-carborane 5.
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timescale is usually slow due to the rearrangement of the
cluster and is often immediately followed by a second one-elec-
tron process giving an apparently two-electron cathodic wave.
The latter wave is usually evident in DCM for these carbo-
ranes.26 However, on back oxidation two separate anodic waves
(with the combined current intensities similar to the current
intensity of the cathodic wave) are evident corresponding to
two one-electron processes.25,26
The CV of 2 in acetonitrile here shows two very closely
spaced and barely resolved one-electron waves (Fig. 7). These
waves are separated by less than 80 mV based on gaussian ana-
lyses of square-wave potential traces (Fig. S3‡). The two half-
wave potentials are estimated to be −1.31 and −1.39 V which
indicate that 2 is reduced more readily than diphenyl-ortho-
carborane 3 at −1.57 and −1.72 V under the same CV con-
ditions. Thus, compound 2 does form a radical anion with a
2n + 3 skeletal electron count. However, the very low compro-
portionation constant (Kc) associated with the intermediate
[2]•− means that spectroscopic observation is challenging, and
the monoanion could not be isolated in any appreciable con-
centration in the comproportionated mixture obtained follow-
ing one-electron reduction (Fig. S4‡). The CV of 2 in DCM
shows the expected CV pattern with a one two-electron catho-
dic wave and two one-electron anodic waves (Fig. S2 and
Table S1‡).
Fig. 9 Frontier molecular orbitals of 1 (left) and 2 (right). The percen-
tage values are the cluster contributions to the molecular orbitals.
agrees well with the observed bathochromic shift of the
absorption maximum of 2 compared to 1.
In order to elucidate the origin of the visible CT emission
of 1 and 2, their geometries were optimised at the first excited
singlet state (S1). In both cases “open” cluster geometries were
found with C1–C2 distances expanded to 2.384 Å (1) and
2.440 Å (2), respectively. The frontier orbitals of the S1 geome-
tries are depicted in Fig. 10. These orbitals were calculated at
the ground state, S0 thus HOMO and LUMO correspond to the
highest and second highest singly occupied orbitals in the S1
state. The HOMO is a π(Mes) orbital but the LUMO, in con-
trast, is an antibonding cluster orbital (cage*) with small con-
tributions from the exopolyhedral boron atom (1: 15%, 2: 8%).
Thus, the HOMO–LUMO transition corresponds to a charge
transfer between the cluster and the mesityl groups from the
excited state and the two compounds can be regarded as
donor–acceptor systems. TD-DFT calculations predicted low-
Computations
Calculated bond lengths and angles from geometries of 1 and
2 optimised at B3LYP/6-31G* are in good agreement with the
experimental values (Table 1). The lengthening of the C1–B6
bonds compared to the C1–B3 bonds is attributed to steric
repulsion between an ortho-methyl group of one of the mesityl
rings and the B6–H unit. Computed energy barriers for the
rotation around the C1–B1 bonds are 6.1 kcal mol−1 in 1 and
18.6 kcal mol−1 in 2. Comparison between computed
GIAO-NMR and experimental 11B NMR chemical shifts for 1–4
are in very good agreement (Table S2‡).
The HOMO is a combination of π-orbitals at the mesityl
groups (π(Mes)) and the LUMO consists mainly of the empty
p-orbital of the boryl boron atom (p(B)) (Fig. 9). Antibonding
orbitals with significant cluster contributions have much
higher energies (>−0.16 eV) and thus the influence of the clus-
ters on the absorption process is merely inductive in both
cases. According to TD-DFT calculations π(Mes)–p(B) tran-
sitions with oscillator strengths ( f ) of 0.0065 to 0.0698 can be
assigned to the absorption bands of both compounds (Tables
S3 and S4‡). Therefore, the electron density in the initially
formed excited state is shifted within the BMes2-unit only
which is not expected to entail strong changes in the overall
dipole moment. This is in agreement with the lack of solvato-
chromism in the absorption spectra. Weak transitions between
the π-orbitals of the phenyl group of 2 and the p(B) orbital as
well as π–π* transitions within the phenyl ring occur at con-
siderably higher energy far in the UV region. The HOMO–
Fig. 10 Frontier orbitals of the S1 geometries of 1 (left) and 2 (right).
The percentage values are the cluster contributions to the molecular
LUMO gap energy of 2 is 0.10 eV smaller than that in 1 which orbitals.
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Table 3 Calculated fluoride ion affinities in kcal mol−1 for 1, 2 and
related XBMes2 compounds in order of decreasing strengths
Fluoride
ion affinity
X
Reference
1-(1,2-C2B10H11)– (1)
1-(2-Ph-1,2-C2B10H10)– (2)
4-(1′-(2′-Me-1′,2′-C2B10H10))C6H4–
4-(1′-(2′-Ph-1′,2′-C2B10H10))C6H4–
4-Mes2BC6H4–
This work
This work
29
29
9
132.9
127.3
123.9
122.6
119.2
113.2
Ph–
11
energy emissions with low oscillator strengths at 859 nm (1; f =
0.0016) and 783 nm (2; f = 0.0066) for these transitions and
gave the same trend as experimentally observed with 1 emit-
ting at longer wavelength than 2.
Fig. 11 Geometries of nido-dicarbadodecaboranes determined by
X-ray crystallography.
The geometries of the fluoride adducts [1·F]− and [2·F]−
were optimised in order to examine why fluoride adducts were
not observed experimentally. The fluoride ion affinities of 1
and 2 are 132.9 and 127.3 kcal mol−1, respectively, which are
higher than the fluoride ion affinities of many aryldimesityl-
boranes (Table 3).8 Although the strong carborane electron-
withdrawing character increases the affinities for the fluoride
ion in 1 and 2 with respect to the affinities of the aryldimesityl-
borane analogues, the optimised geometries [1·F]− and [2·F]−
contain unusually long B–C(carboranyl) bonds of 1.786 and
1.823 Å respectively. These bond lengths suggest that they
would be easily cleaved either before the B–F bond is formed
or by hydrolysis as observed experimentally.
Geometries of the dianions [2]2−and [5]2−
Fig. 12 Geometries A–D of nido-dicarbadodecaborane dianions with
atom numbering.
While closo-dicarbadodecaboranes all adopt the pseudo-
icosahedral geometry, several different geometries of
nido-dicarbadodecaborane dianions have been determined
crystallographically (Fig. 11).56 Dianions with almost planar
C2B4 open faces like [6]2− and [7]2− are observed in Group have a geometry65 with a C2B4 open face A or a geometry54,66
1 metallacarboranes.57–60 Bowl-shaped geometries have been with a C2B2 open face B (Fig. 12). Thus, the geometry of the
observed in carborane dianions like [8]2− and [9]2− with tethers dianion [5]2− has not been established even though this
at both cage carbons.59,61 The bowl geometry in [9]2− differs from dianion has been known for many decades.67
that in [8]2− where [9]2− has a notably smaller open face.61 These
Chemical reductions of 5 were carried out with alkali
bowl-shaped geometries are similar to geometries determined metals (Li, Na, K) in THF solutions. The reactions were moni-
for neutral 12-vertex tetracarbadodecaboranes by X-ray crystallo- tored by 11B NMR spectroscopy until all the peaks corres-
graphy.62 The neutral compound 10 may also be regarded as a ponding to the starting carborane and the red colours of the
genuine 12-vertex dicarbadodecaborane dianion [R2C2B10H10]2− solutions due to the radical monoanions disappeared. The 11B
with two positively charged phosphonium groups.63
{1H} spectra of the dianions [5]2− in clear yellow solutions
While there are several structural studies published on show 2 : 2 : 4 : 2 patterns which are not significantly influenced
nido-dicarbadodecaborane dianions, the geometries of nido- by the different alkali metal cations (Fig. 13). This pattern
dicarbadodecaborane dianions in solutions have not been suggests a geometry of high symmetry or two mirror-image
determined. The successful method64 of comparing observed geometries that are fluctional in solution for [5]2− with the
and computed 11B NMR data to determine carborane cluster metal cations not strongly interacting in solution.
geometries is applied here for dianion [2]2−. Before discussing
Geometry optimisations on [5]2− reveal that the bowl-
the dianion [2]2− made by chemical reductions on 2, the shaped geometry C is more stable than A and B by 5.5 and
dianion [5]2− generated from diphenyl-ortho-carborane 5 is 11.0 kcal mol−1 respectively. More importantly, the computed
described here to demonstrate the use of the combined experi- GIAO 11B NMR chemical shifts of the bowl-shaped geometry fit
mental and calculated 11B NMR method in determining the well with observed shifts when fluctionality between the two
geometry of its dianion. Dianion [5]2− has been proposed to mirror-image geometries of C takes place in solution (Table 4).
9772 | Dalton Trans., 2015, 44, 9766–9781
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Paper
Fig. 13 11B{1H} NMR spectra of M2[5]2− and M2[2]2− (M = Li, Na or K) in THF. The broad peaks at 65–70 ppm corresponding to the boryl boron in
[2]2− are not shown here.
The geometry D found in [9]2− could not be located for [5]2− the disappearance of the peaks corresponding to the starting
where the initial geometry D rearranged to C on optimisation.
material, the clear solutions containing the dianions [2]2−
Reduction of 2 with sodium metal in 1,2-dimethoxyethane were dark red. 11B NMR spectra of the dianions [2]2− show
(DME) yielded a dark red solid identified as [Na(DME)n]2[2] by either a 2 : 1 : 1 : 2 : 2 : 2 (Li salt) or a 2 : 2 : 2 : 2 : 2 (Na, K) peak
1H, 11B and 13C NMR spectroscopy. Purification of this extre- pattern (Fig. 13) and are similar to peaks found for the diphe-
mely air-sensitive salt by crystallization was not successful. nylcarborane dianions [5]2− when taking into account the
Salts of dicarbadodecaborane dianions are known to be extre- lower symmetry in [2]2−
mely air- and moisture-sensitive.57 The 11B{1H} NMR spectrum The similarities in the 11B NMR peaks observed for [2]2−
.
recorded in CD3CN revealed a 2 : 1 : 1 : 2 : 2 : 2 pattern for the and [5]2− suggest that both have bowl-shaped geometries. Geo-
cluster atoms and a very broad signal at 67.5 ppm corres- metry optimisations on the BMes2 species [2]2− reveal that the
ponding to the boryl boron atom (Fig. 14). The latter peak is starting geometry B was rearranged to the bowl-shaped geome-
considerably shifted to higher field by about 13 ppm compared try D. The most stable geometry for [2]2− is C with D only
to the neutral starting material.
0.8 kcal mol−1 higher in energy. However, computed 11B NMR
Chemical reductions of the Mes2B compound 2 in THF shifts from geometry D fit better with observed 11B NMR shifts
with alkali metals were carried out as for 5. After observation than geometry C for [2]2− assuming fluctionalities between
of the purple colours corresponding to the radical species and mirror-image geometries occur in solution (Fig. 15).
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Table 4 Computed and observed 11B{1H} NMR data of [2]2− and [5]2− in ppm and relative energies of the optimised geometries in kcal mol−1
Geometry
B3,6
B12
B9
B4,5
B7,11
B8,10
BMes2
Rel. E
[2]2−
Aa
C
D
14.0
0.0
0.8
13.3
−4.1
0.1
−1.2
−10.5
−9.2
−8.4
−11.8
−10.1
−17.4
−15.4
−14.0
−21.3
−19.5
−17.8
−23.7
−29.2
−27.2
60.2
56.0
67.5
Exp.b
[5]2−
Ac
B
C
11.0
5.5
0.0
−23.5
12.2
7.0
−20.0
−7.6
−9.3
−20.0
−7.6
−9.3
−18.6
−20.7
−17.3
−18.6
−20.7
−17.3
−55.7
−25.5
−27.5
Exp.d
a All borons are non-equivalent in geometry A of [2]2−, values are calculated assuming same fluctionality process as in geometry A of [5]2−; 64.6
(BMes2), 14.1 (B12), 12.1 (B10), 5.7 (B11), −0.7 (B4), −1.7 (B6), −7.3 (B8), −18.2 (B5), −20.3 (B1), −23.9 (B3), −24.8 (B2). b Experimental data for
sodium salt in CD3CN, Fig. 14. c Calculated values are averaged assuming fluctionality between two mirror-image geometries; 10.0 (B10,12), −0.8
(B11), −6.0 (B8), −9.3 (B5,6), −18.8 (B3), −20.6 (B2,4), −25.6 (B1). d Experimental data for sodium salt in d8-THF.
nions of C,C′-diphenyl-carborane [5]2− and C-dimesitylboryl-C′-
phenyl-carborane [2]2− respectively. The calculated geometries
for [2]2− and [5]2− are similar to the experimental cluster geo-
metries61 of [9]2− and [8]2− respectively as shown from com-
parison of distances involving the cluster carbon atoms, C1
and C2, in Table 5. The combined experimental and calculated
11B NMR method is shown to be useful in determining nido-
Table 5 Comparison of selected distances in Å for the nido-dianions,
[2]2−, [5]2−, [8]2− and [9]2−
Fig. 14 11B{1H} NMR spectrum of [2]2− in CD3CN with peak assign-
ments based on GIAO-NMR data.
[2]2− calc.
[9]2− obs.
[5]2− calc.
[8]2− obs.
C1⋯C2
C1⋯B3
C1⋯B6
C2⋯B3
C2⋯B6
2.666
2.528
1.624
1.586
1.878
2.687(6)
2.302(9)
1.617(9)
1.534(8)
2.083(8)
2.915
2.606
1.546
1.546
2.606
2.87(1)
2.59(1)
1.51(1)
1.51(1)
2.55(1)
It is concluded here that bowl-shaped geometries are
present in solutions of 12-vertex nido-dicarbadodecaborane
dianions with fluctional cluster geometries of C and D in dia-
Fig. 15 Optimised geometries of [2]2− (top) and [5]2− (bottom) and the fluctional processes.
9774 | Dalton Trans., 2015, 44, 9766–9781
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Paper
12-vertex geometries in solutions and will aid further progress on sodium chloride solution (10 mL) subsequently. The com-
the intriguing range of nido-12-vertex geometries in the future.
bined aqueous layers were extracted with toluene (10 mL) and
the combined organic phases were dried over sodium sulfate
and freed from volatiles in vacuo. The crude product was
recrystallised from a mixture of n-hexane (30 mL) and dichloro-
methane (2 mL) to afford pure 1 as colourless crystals. Yield:
Conclusions
Two C-dimesitylboryl-1,2-dicarba-closo-dodecaboranes were 0.85 g (61%). Found: C, 60.42; H, 8.51; C20H33B11 requires C,
synthesised from fluorodimesitylborane and the corres- 61.22; H, 8.48; 1H-NMR (CDCl3): δ [ppm] = 1.4–3.2 (m, br,
ponding lithio-carboranes and structurally characterised by 10 H, BH), 2.24 (s, 6 H, para-CH3), 2.40 (s, 12 H, ortho-CH3),
X-ray crystallography. Photophysical studies and TD-DFT calcu- 3.85 (s, br, 1 H, B10H10C2H), 6.80 (s, 4 H, CHMes); 13C{1H}-NMR
lations showed that the absorptions correspond to local tran- (CDCl3): δ [ppm] = 20.9 (s, para-CH3), 25.9 (s, ortho-CH3), 61.6
sitions within the BMes2 groups whereas visible emissions (s, CB10H10CH), 75.4 (s, CB10H10CBMes2), 129.7 (s, CHMes),
with Stokes shifts up to 16 170 and 14 800 cm−1 in dichloro- 138.4 (s, Cipso), 139.4 (s, Cortho), 139.8 (s, Cpara); 11B{1H}-NMR
methane originate from intramolecular CT transitions (CDCl3): δ [ppm] = −12.9 (s), −9.1 (s), −6.9 (s), −2.3 (s), 1.9 (s)
between the mesityl rings and the cluster. Compound 2 with a (skeletal boron atoms), 78.9 (s, br, exopolyhedral boron atom)
phenyl substituent at the second cage carbon atom can be see Fig. S5–S7‡ for NMR spectra of 1; MS (EI): m/z = 392.4
easily reduced to a stable dianion [2]2− by cyclic voltammetry (M+, 3%), 272.3 (M+ − HMes, 51%), 249.2 (BMes2 , 46%), 120.1
+
and chemical reductions with alkali metals. Based on experi- (Mes+, 100%).
mental and calculated 11B NMR data, a dynamic bowl-shaped
1-Dimesitylboryl-2-phenyl-1,2-dicarba-closo-dodecaborane
nido-cage geometry is determined for the dianion. These find- (2). A solution of n-butyllithium (1.6 M in n-hexane, 3.10 mL,
ings indicate that the ortho-carboranyl group is a stronger elec- 4.96 mmol) was added to 1-phenyl-1,2-dicarba-closo-dodeca-
tron acceptor than the BMes2 group and C-dimesitylboryl-1,2- borane (0.97 g, 4.40 mL) in toluene (40 mL). After stirring for
dicarba-closo-dodecaboranes are dyads with the mesityl group 16 h at ambient temperature a solution of fluorodimesitylbor-
as the donor and the carborane as the acceptor.
ane (1.30 g, 4.85 mmol) in toluene (12 mL) was added and the
mixture was heated at reflux temperature for 5 h. Subsequently
it was washed with water (2 × 15 mL) and saturated sodium
chloride solution (15 mL). The combined organic phases were
dried over sodium sulfate and the solvent was removed
in vacuo. Impurities were sublimed from the residue at 80 °C
Experimental section
General
The reactions were performed under an atmosphere of dry in vacuo and the remaining solid was recrystallised from a
oxygen-free argon using Schlenk techniques unless otherwise mixture of n-hexane (80 mL) and dichloromethane (5 mL). The
stated. All solvents were dried by standard methods and product 2 was obtained as colourless crystals. Yield: 0.97 g
freshly distilled prior to use. Fluorodimesitylborane68 and (51%). Found: C, 66.38; H, 7.97; C26H37B11 requires C, 66.66;
1-phenyl-1,2-dicarba-closo-dodecaborane69 were prepared as H, 7.96; 1H{11B} NMR (CDCl3): δ [ppm] = 2.16 (s, 6 H, para-
described in the literature. 1,2-Dicarba-closo-dodecaborane was CH3), 2.26 (s, 14 H, BH + ortho-CH3), 2.33 (3H, BH), 2.41 (2H,
purchased commercially (KatChem). NMR spectra were BH), 2.70 (1H, BH), 3.47 (2H, BH), 6.56 (s, 4 H, CHMes), 6.87
recorded from solutions at room temperature on a Bruker AM (dd, 3JHH = 7.4 Hz, 3JHH = 7.6 Hz, 2 H, Hmeta), 7.13 (t, 3JHH = 7.4
3
Avance DRX500 (1H, 11B, 13C), a Bruker Avance III 500 and a Hz, 1 H, Hpara), 7.18 (d, JHH = 7.6 Hz, 2 H, Hortho); 13C{1H}
Bruker Avance 400 Spectrometer (1H{11B}, 19F) with SiMe4 (1H, NMR (CDCl3): δ [ppm] = 20.8 (s, para-CH3), 26.8 (s, ortho-CH3),
1
13C), BF3·OEt2 (11B) and CFCl3 (19F) as external standards. H- 86.5 (s, CB10H10CPh), 87.3 (s, CB10H10CBMes2), 127.7 (s, Cmeta,
and 13C{1H} NMR spectra were calibrated on the solvent signal Ph), 129.3 (s, Cpara, Ph), 129.4 (s, CHMes), 130.3 (s, Cortho, Ph),
[CDCl3: 7.24 (1H), 77.16 (13C); CD3CN: 1.94 (1H), 118.25, 1.32 131.7 (s, Cipso, Ph), 138.8 (s, Cipso, Mes), 139.1 (s, Cpara, Mes), 139.4
(13C); d8-THF: 3.58, 1.73 (1H), 67.57, 25.46 (13C)]. The 13C NMR (s, Cortho, Mes); 11B{1H} NMR (CDCl3): δ [ppm] = −9.9 (s), −8.0
peaks were assigned with the aid of observed 13C DEPT spectra (s), −2.8 (s), 3.7 (s) (skeletal boron atoms), 80.4 (s, br, exopoly-
and computed 13C NMR shifts. Electron Ionisation (EI) and hedral boron atom) see Fig. S8–S10‡ for NMR spectra of 2;
Atmospheric pressure Solids Analysis Probe (ASAP) mass MS (EI): m/z = 468.4 (M+, 4%), 453.4 (M+ − Me, 2%), 348.3
spectra were recorded with a VG Autospec sector field (Micro- (M+ − HMes, 100%), 332.3 (M+ − Me − Mes, 13%), 249.2
+
mass) and Xevo QTOF (Waters) mass spectrometers respectively. (BMes2 , 85%).
1-Dimesitylboryl-1,2-dicarba-closo-dodecaborane (1). A solu-
tion of n-butyllithium (1.6 M in n-hexane, 2.37 mL, 3.79 mmol)
Hydrolyses of 1 and 2
was added to 1,2-dicarba-closo-dodecaborane (0.52 g, A drop of water (excess) was mixed with a solution of 1 or 2 in
3.61 mmol) in toluene (35 mL) at 0 °C. After 16 h stirring at deuterated chloroform in an NMR tube and the mixture was
ambient temperature a solution of fluorodimesitylborane analysed by NMR spectroscopy. No change in the spectra was
(0.96 g, 3.58 mmol) in toluene (6 mL) was added to the result- observed after a week.
ing suspension. The mixture was heated at reflux temperature
Hydrolysis by air exposure. Solids of 1 (0.2 g, 0.51 mmol)
for 5 h and washed with water (2 × 10 mL) and saturated and 2 (0.3 g, 0.64 mmol) were left exposed to air in the
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Dalton Transactions
laboratory and checked periodically by NMR spectroscopy. dark red remainder was taken up in d3-acetonitrile and ana-
1
After three weeks, compound 1 was converted to mesitylene lysed by NMR spectroscopy. Na2[2] H NMR (CD3CN): δ [ppm]
and carboranylboronic acid 3 as determined by multinuclear = −0.4–3.0 (m, br, 10 H, BH), 2.15 (s, 6 H, para-CH3), 2.32 (s,
NMR spectroscopy. After eighteen months, mesitylene and 12 H, ortho-CH3), 6.52 (s, br, 4 H, CHMes), 6.76 (s, br, 1 H,
borinic acid 4 were identified as products of 2.
H
para), 6.92 (s, br, 2 H, HPh), 7.43 (s, br, 2 H, HPh); 13C{1H}
1
Mesitylene: H-NMR (CDCl3): δ [ppm] = 6.81, 2.25; 13C{1H}- NMR (CD3CN): δ [ppm] = 20.8 (s, para-CH3), 25.1 (s, ortho-
NMR (CDCl3): δ [ppm] = 137.9, 127.0, 21.3.
CH3), 70.5 (s, CB10H10CPh), 106.7 (s, CB10H10CBMes2), 122.0
(1,2-Dicarba-closo-dodecaboranyl)-1-borinic acid (3): 1H (s, CHPh para), 127.6 (br s, CHMes, CHPh meta), 128.7 (s, CHPh
{11B}-NMR (CDCl3): δ [ppm] = 2.05 (2H, BH), 2.17 (2H, BH), ortho), 133.6 (s, s, CMes-para), 140.9 (s, CMes-ortho), 149.4 (s,
2.26 (2H, BH), 2.33 (4H, BH), 3.67 (1H, B10H10C2H), 4.91 (2H,
C
Mes-ipso), 154.2 (s, CPh ipso); 11B{1H} NMR (CD3CN): δ [ppm]
OH); 13C{1H}-NMR (CDCl3): δ [ppm] = 57.4 (s, CB10H10CH), the = −27.2 (s), −17.8 (s), −14.0 (s), −10.1 (s), −9.2 (s), 0.1 (s) (skel-
13C peak corresponding to CB(OH)2 is not observed; 11B{1H}- etal boron atoms), 67.5 (s, br, exopolyhedral boron atom) see
NMR (CDCl3): δ [ppm] = −12.6 (s), −7.8 (s), −2.1 (s), −1.0 (s) Fig. 14; UV-Vis for [2]2− in CH3CN, [nm (ε)] = 344 (4600), 405
(skeletal boron atoms), 26.5 (s, exopolyhedral boron atom) (1700), 430 (1400), 515 (1300) (Fig. S4‡).
see Fig. S11‡ for 11B NMR spectra of 3; MS (ASAP, M =
Method 2. Finely-cut alkali metal pieces were added to a
solution of 2 (0.07 g, 0.14 mmol) in tetrahydrofuran (0.5 mL).
C2H13B11O2): m/z = 188.0 (M+, 21%), 171.2 (M+ − OH, 100%).
(2-Phenyl-1,2-dicarba-closo-dodecaboranyl)-1-borinic
acid A purple colour occurred immediately at the metal surface fol-
1
(4): H NMR (CDCl3): δ [ppm] = 2.32 (2H, BH), 2.42 (3H, BH), lowed by a clear dark red solution after 2 h. The reaction
2.48 (2H, BH), 2.61 (1H, BH), 2.92 (2H, BH), 4.46 (2H, OH), mixture was then analysed by 11B NMR spectroscopy and in
3
3
3
7.34 (dd, JHH = 7.4 Hz, JHH = 7.6 Hz, 2 H, Hmeta), 7.41 (t, JHH many experiments the desired dianion was present as the car-
3
1
= 7.4 Hz, 1 H, Hpara), 7.64 (d, JHH = 7.6 Hz, 2 H, Hortho); 13C borane compound (Table S5‡). H and 13C NMR spectra were
{1H} NMR (CDCl3): δ [ppm] = 20.8 (s, para-CH3), 26.8 (s, ortho- also obtained for Na2[2] when deuterated THF was used in
CH3), 86.5 (s, CB10H10CPh), 87.3 (s, CB10H10CBMes2), 127.7 (s, place of THF. Na2[2]. 1H{11B} NMR (d8-THF): δ [ppm] = 0.26 (s,
Cmeta, Ph), 129.3 (s, Cpara, Ph), 129.4 (s, CHMes), 130.3 (s, Cortho, 2 H, BH), 1.16 (s, 2 H, BH), 1.59 (s, 2 H, BH), 2.09 (s, 6 H, para-
Ph), 131.7 (s, Cipso, Ph), 138.8 (s, Cipso, Mes), 139.1 (s, Cpara, Mes), CH3), 2.19 (s, 1 H, BH), 2.35 (s, 12 H, ortho-CH3), 6.46 (s, 4 H,
139.4 (s, Cortho, Mes); 11B{1H} NMR (CDCl3): δ [ppm] = −11.7 (s), CHMes), 6.68 (t, 7.5 Hz, 1 H, Hpara), 6.82 (apparent triplet, ∼8
−10.6 (s), −8.3 (s), −3.1 (s), 0.9 (s) (skeletal boron atoms), 26.5 Hz, 2 H, HPh meta), 7.37 (d, 8 Hz, 2 H, HPh ortho); 13C{1H}
(s, exopolyhedral boron atom) see Fig. S12‡ for 11B NMR NMR (d8-THF): 21.3 (s, para-CH3), 122.3 (s, CHPh para), 126.7
spectra of 4; MS (ASAP, M = C8H17B11O2): m/z = 263.2 (M − 1+, (s, CHPh meta), 127.7 (s, CHMes), 128.9 (s, CHPh ortho), 133.1 (s,
44%), 247.2 (M+ − OH, 100%).
CMes-para), 140.8 (s, CMes-ortho), 148.7 (s, CMes-ipso), 153.5 (s,
Hydrolysis by fluoride ion. 1-Dimesitylboryl-1,2-dicarba- CPh ipso); the peak corresponding to ortho-CH3 groups is
closo-dodecaborane (1) (0.009 g, 0.023 mmol) was dissolved in hidden within the d8-THF peak and the peaks for the cage
a
solution of tetra-n-butylammonium fluoride trihydrate carbons were not detected above the noise levels, see Fig. S14
(0.014 g, 0.044 mmol) in deuterated chloroform (0.8 mL) and and S15‡ for H{11B} and 13C{1H} NMR spectra. Exposing the
the mixture was subject to NMR spectroscopy. Likewise, dark red solution containing Na2[2] slowly to air gave a light
1-dimesitylboryl-2-phenyl-1,2-dicarba-closo-dodecaborane (2) yellow solution identified by NMR spectroscopy to contain a
(0.026 g, 0.055 mmol) was dissolved in a deuterated chloro- mixture of the starting material 2 and 1-phenyl-ortho-carbor-
form solution of tetra-n-butylammonium fluoride trihydrate ane in a 9 : 1 ratio. Method 2 was also used in the reductions
(0.018 g, 0.057 mmol). The products identified by multinuclear of 5 with alkali metals (Li, Na, K) but with deuterated THF in
NMR spectroscopy were dimesitylborinic acid,37 Mes2BOH, all cases and NMR data of M2[5] (M = Li, Na, K) are listed in
and 1,2-dicarba-closo-dodecaborane70 from 1 and Mes2BOH Table S6.‡
1
and 2-phenyl-1,2-dicarba-closo-dodecaborane70 from 2. Deboro-
Photophysical measurements
nated products were also identified in the reaction mixtures
from their NMR data (see Fig. S13‡ for 19F NMR spectra).40 For all solution state measurements, samples were contained
1
Dimesitylborinic acid: H-NMR (CDCl3): δ [ppm] = 2.30 (12H), in quartz cuvettes of 10 × 10 mm (Hellma type 111-QS, supra-
2.32 (6H), 5.97 (1H, OH), 6.86 (4H); 13C{1H}-NMR (CDCl3): sil, optical precision). Cyclohexane was used as received from
δ [ppm] = 21.3, 22.6, 128.5, 137.0, 139.5, 141.2; 11B-NMR commercial sources (p.a. quality), the other solvents were
(CDCl3): δ [ppm] = 50.1.
dried by standard methods prior to use. Concentrations varied
from 20 to 100 µM in order to get analysable emission spectra
due to the low quantum yields. Effects of the concentration on
Reductions of 2 and 5
Method 1. A piece of excess sodium metal was added to a the shape of the observed emission spectra were excluded in
solution of 1-dimesitylboryl-2-phenyl-1,2-dicarba-closo-dodeca- this concentration range. Solid samples were prepared by
borane 2 (0.07 g, 0.14 mmol) in 1,2-dimethoxyethane (0.5 mL). vacuum sublimation on quartz plates (35 × 10 × 1 mm) using
A dark red colour occurred immediately on the surface of the standard Schlenk equipment and conditions. Each plate was
metal. The mixture was sonicated for 1 h and filtered sub- laid in a 100 mL round bottom flask and a crystal of the
sequently. The filtrate was freed from volatiles in vacuo and the sample substance placed below it was sublimed. Absorption
9776 | Dalton Trans., 2015, 44, 9766–9781
This journal is © The Royal Society of Chemistry 2015

Dalton Transactions
Paper
was measured with a UV/VIS double-beam spectrometer (Shi-
madzu UV-2550), using the solvent as a reference.
The spectroelectrochemical (SEC) experiment on 2 was
performed at room temperature in an airtight optically
The output of a continuous Xe-lamp (75 W, LOT Oriel) was transparent thin-layer electrochemical (OTTLE) cell72
wavelength-separated by a first monochromator (Spectra Pro equipped with Pt minigrid working and counter electrodes
ARC-175, 1800 l mm−1 grating, Blaze 250 nm) and then used (32 wires cm−1), Ag wire pseudo-reference electrode and CaF2
to irradiate a sample. The fluorescence was collected by mirror windows for a 200 μm path-length solvent compartment.
optics at right angles and imaged on the entrance slit of a Acetonitrile containing 0.1 M [Bu4N][PF6] electrolyte was
second spectrometer while compensating astigmatism at the used in the cell which was fitted into the sample compart-
same time. The signal was detected by a back-thinned CCD ment of a Cary 5000 (UV-Vis-NIR) spectrophotometer. Bulk
camera (RoperScientific, 1024\256 pixels) in the exit plane of electrolysis was carried out using an Autolab PG-STAT 30
the spectrometer. The resulting images were spatially and potentiostat.
spectrally resolved. As the next step, one averaged fluorescence
spectrum was calculated from the raw images and stored in
the computer. This process was repeated for different exci-
tation wavelengths. The result is a two-dimensional fluo-
rescence pattern with the y-axis corresponding to the
excitation, and the x-axis to the emission wavelength. The
wavelength range is λex = 230–430 nm (in 1 nm increments) for
the UV light and λem = 305–894 nm for the detector. The time
to acquire a complete EES is typically less than 15 min. Post-
processing of the EES includes subtraction of the dark current
background, conversion of pixel to wavelength scales, and
multiplication with a reference file to take the varying lamp
intensity as well as grating and detection efficiency into
account. Stokes shifts were calculated from excitation and
emission maxima, which were extracted from spectra that were
converted from wavelength to wavenumbers beforehand. The
quantum yields in solution were determined against POPOP
Crystallographic studies
Single crystals were coated with a layer of hydrocarbon oil and
attached to a glass fiber. Crystallographic data were collected
with a Bruker AXS X8 Prospector Ultra APEX II diffractometer
with Cu Kα radiation (graphite monochromator, λ = 1.54178 Å)
at 100 K. Crystallographic programs used for structure solution
and refinement were from SHELX-97.73 The structures were
solved by direct methods and were refined by using full-matrix
least squares on F2 of all unique reflections with anisotropic
thermal parameters for all non-hydrogen atoms. The hydrogen
atoms bonded to the carborane units were refined isotropi-
cally, all other hydrogen atoms were refined using a riding
model with U(H) = 1.5Ueq for CH3 groups and U(H) = 1.2Ueq for
all others. Crystallographic data for the compounds are listed
in Table S7.‡ CCDC 1048027 (1) and CCDC 1048028 (2)
contain the supplementary crystallographic data for this
paper.
(p-bis-5-phenyl-oxazolyl(2)-benzene) (ΦF
=
0.93) as the
standard.
The solid-state fluorescence was measured by addition of
an integrating sphere (Labsphere, coated with Spectralon,
Ø 12.5 cm) to the existing experimental setup. At the exit slit
of the first monochromator the exciting light was transferred
into a quartz fibre (LOT Oriel, LLB592). It passed a condenser
lens and illuminated a 1 cm2 area on the sample in the centre
of the sphere. The emission and exciting light was imaged by
a second quartz fibre on the entrance slit of the detection
monochromator. Post-processing of the spectra was done
as described above. The measurement and calculation of
quantum yields was performed according to the method
described by Mello.71
Computational details
All computations were carried out with the Gaussian 09
package.74 The model geometries were fully optimised with
the B3LYP functional75 with no symmetry constraints using
the 6-31G* basis set76 for all atoms. Frequency calculations on
all optimised geometries revealed no imaginary frequencies.
Computed absorption data were obtained from TD-DFT77 cal-
culations on S0 geometries whereas computed emission data
were from the S1 geometries. The MO diagrams and MO com-
positions were generated with the Molekel78 and GaussSum79
packages, respectively. Calculated 11B and 13C NMR chemical
shifts obtained at the GIAO80-B3LYP/6-31G*//B3LYP/6-31G*
level on the optimised geometries were referenced to BF3·OEt2
for 11B: δ(11B) = 111.7–σ(11B) and referenced to TMS for 13C:
δ(13C) = 189.4–σ(13C). Computed NMR values reported here
were averaged where possible.
Electrochemistry
Cyclic voltammetric measurements were carried out using an
EcoChemie Autolab PG-STAT 30 potentiostat at 298 K with a
platinum or glassy carbon working electrode and platinum
wires as counter and reference electrodes in a nitrogen-con-
taining glove box with 0.1 M tetra-n-butylammonium hexa-
fluorophosphate [Bu4N][PF6] in DCM or acetonitrile. Scan rates
Acknowledgements
of 100 mV s−1 and analyte concentrations of 10−3 M were used. We thank the Deutsche Forschungsgemeinschaft (DFG) and
The ferrocene/ferrocenium FcH/FcH+ couple served as internal the Engineering and Physical Sciences Research Council
reference at 0.0 V for potential measurements and peak–peak (EPSRC) for financial support. P.J.L. gratefully acknowledges
separations of this couple were generally in the region of an EPSRC Leadership Fellowship, and currently holds an ARC
90–110 mV.
Future Fellowship (FT120100073).
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 9766–9781 | 9777

Paper
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