The synthesis and molecular and crystal structures of 1-methyl-2-carboxy-1, 2-dicarba-closo-dodecaborane(12), 1-phenyl-2-carboxy-1,2-dicarba-closo- dodecaborane(12) and 1-phenyl-2-benzoyl-1,2-dicarba-closo-dodecaborane(12)

Article abstract of DOI:10.1016/j.poly.2003.10.016

The structures of 1-Me-2-COOH-1,2-closo-C₂B₁₀H ₁₀ (1), 1-Ph-2-COOH-1,2-closo-C₂B₁₀H ₁₀ (2) and 1-Ph-2-COPh-1,2-closo-C₂B₁₀H ₁₀ (3) have been determined by X-ray crystallography. In 1 the orientation of the COOH group is defined by θ_COOH=65.0(2) _syn°, and this group H-bonds to that in a second molecule via a centrosymmetric eight-membered ring, R₂²(8). DFT calculations on the parent species 1-COOH-1,2-closo-C₂B ₁₀H₁₁ reveal that the structure with θ _COOH=90_syn°is preferred, with a barrier to COOH rotation of ≈17 kJmol^-1. Similar calculations on 1 yield a lower rotational barrier, ≈7 kJmol^-1, because internal H-bonding is now denied. In compound 2 the COOH group is twisted to lower θ values [5.0(3)°and 39.7(3)°] but both crystallographically independent molecules exist as dimers in the solid state by virtue of R₂ ²(8) rings. Compound 2 crystallises from wet solvent as a monohydrate, the structure of which reveals a non-planar centrosymmetric R ₄⁴(12) ring and near-orthogonal Ph and COOH substituents [θ_Ph=6.7(2) and θ_COOH=78.0(2) _syn°]. In compound 3 molecular association by H-bonding is blocked, so 3 serves as a useful comparative structure for 2.

Full text of DOI:10.1016/j.poly.2003.10.016

Polyhedron 23 (2004) 629–636
www.elsevier.com/locate/poly
The synthesis and molecular and crystal structures of
1-methyl-2-carboxy-1,2-dicarba-closo-dodecaborane(12),
1-phenyl-2-carboxy-1,2-dicarba-closo-dodecaborane(12)
and 1-phenyl-2-benzoyl-1,2-dicarba-closo-dodecaborane(12)
a
Ulaganathan Venkatasubramanian , David J. Donohoe , David Ellis , Barry T. Giles ,
a
a
a
a
a
a
Stuart A. Macgregor , Susan Robertson , Georgina M. Rosair , Alan J. Welch
a,
*
,
b
b
b
Andrei S. Batsanov , Lynn A. Boyd , Royston C.B. Copley , Mark A. Fox
b,
*
,
b
Judith A.K. Howard , Kenneth Wade
b
a
School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK
b
Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, UK
Received 29 July 2003; accepted 30 October 2003
Abstract
The structures of 1-Me-2-COOH-1,2-closo-C2B10H10 (1), 1-Ph-2-COOH-1,2-closo-C2B10H10 (2) and 1-Ph-2-COPh-1,2-closo-
C2B10H10 (3) have been determined by X-ray crystallography. In 1 the orientation of the COOH group is defined by
hCOOH ¼ 65:0ð2Þsyn°, and this group H-bonds to that in a second molecule via a centrosymmetric eight-membered ring, R22(8). DFT
calculations on the parent species 1-COOH-1,2-closo-C2B10H11 reveal that the structure with hCOOH ¼ 90syn° is preferred, with a
barrier to COOH rotation of ꢀ17 kJ molꢁ1. Similar calculations on 1 yield a lower rotational barrier, ꢀ7 kJ molꢁ1, because internal
H-bonding is now denied. In compound 2 the COOH group is twisted to lower h values [5.0(3)° and 39.7(3)°] but both crystallo-
graphically independent molecules exist as dimers in the solid state by virtue of R22(8) rings. Compound 2 crystallises from wet
solvent as a monohydrate, the structure of which reveals a non-planar centrosymmetric R44(12) ring and near-orthogonal Ph and
COOH substituents [hPh ¼ 6:7ð2Þ and hCOOH ¼ 78:0ð2Þsyn°]. In compound 3 molecular association by H-bonding is blocked, so 3
serves as a useful comparative structure for 2.
Ó 2003 Elsevier Ltd. All rights reserved.
Keywords: Carborane; Spectroscopy; DFT calculations; X-ray crystallography
1. Introduction
engineering has begun to be exploited only fairly re-
cently [2] the potential is vast. Heteroboranes are gen-
erally chemically and thermally stable species, existing
as polyhedra or fragments of polyhedra of differing size
and shape, but having architectures that are well-
understood [3]. Isomeric possibilities mean that C-substi-
tuted (hetero)carboranes have the potential to function
as supramolecular building blocks with differing direc-
tionalities in two-dimensions, which can further be ex-
panded into three-dimensions via additional B-atom
substitution [4]. Moreover, heteroboranes are usually
neutral or anionic, but can also be cationic [5], thus
further extending their potential applications as building
block units.
Over the last few years the field of crystal engineering,
part of which involves the development of rational
strategies for the design of new crystalline materials, has
emerged as an area of significant research activity [1].
Although the use of heteroboranes in studies of crystal
* Corresponding authors. Tel.: +44-131-451-3217; fax: +44-131-451-
3180 (A.J. Welch), Tel.: +44-191-334-2038; fax: +44-191-384-4737
(M.A. Fox).
E-mail addresses: a.j.welch@hw.ac.uk (A.J. Welch), m.a.fox@
durham.ac.uk (M.A. Fox).
0277-5387/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2003.10.016

630
U. Venkatasubramanian et al. / Polyhedron 23 (2004) 629–636
There are currently two main strategies for crystal
2.1.1. Synthesis of 1-Me-2-COOH-1,2-closo-C2B10H10
(1)
engineering, based on the use of either co-ordinate
bonds or weaker intermolecular interactions. Within the
latter area, H-bonding has attracted most interest due to
its relative strength and directionality [6]. Carboxylic
acid groups are frequently used in the field of crystal
engineering because they are capable of forming various
types of hydrogen bonds between themselves and/or
with other species such as solvents [7]. Accordingly, we
have begun a systematic study of the crystal structures
of carboxylic acid carboranes, their derivatives, and re-
lated species, in order further to increase our under-
standing of the effect of the carboxy group on the cage
and any other substituents. We have already described
1-COOH-1,2-closo-C2B10H11 [2m] (a dimer), 1,2-
(COOH)2-1,2-closo-C2B10H10 ꢂ 1/2C2H5OH [2p] and
1-CH2OMe-2-COOH-1,2-closo-C2B10H10 [2q] (both
tetramers). Herein we report the structures of the methyl
and phenyl derivatives 1-Me-2-COOH-1,2-closo-
C2B10H10 and 1-Ph-2-COOH-1,2-closo-C2B10H10, as
well as that of the related species 1-Ph-2-COPh-1,2-clo-
so-C2B10H10. Although the compounds described in this
paper do not themselves give rise to extended arrays via
H-bonding they do afford us a greater understanding of
substituent effects in carborane structures that will be
important if carboranes are to be used for crystal engi-
neering in the future.
To a stirred solution of 1-Me-1,2-closo-C2B10H11 (0.8
g, 5 mmol) in diethylether (25 ml) at 0 °C was added
dropwise 1.1 equivalents of MeLi (as an ether solution).
The solution was allowed to warm to room temperature,
at which point gaseous CO2 was bubbled through the
solution for about 20–30 min. The milky solution thus
formed was hydrolysed with 2 M HCl and transferred to
a separating funnel. The ethereal layer plus ethereal
washings of the aqueous layer were dried over MgSO4
overnight. Removal of solvent afforded 1-Me-2-COOH-
1,2-closo-C2B10H10 (1) as an off-white microcrystalline
solid. Yield: 0.69 g, 67%. Anal. Found: C, 23.7; H, 7.3.
Calc. for C4H14B10O2: C, 23.8; H, 7.0%. IR m(cmꢁ1):
2630–2560 (s, BH), 1724 (s, CO). 1H NMR, d(ppm):
2.22 (s, CH3). 11B–{11H} NMR, d(ppm): 2.7 (1B), )2.4
(1B), )6.3 (8B).
2.1.2. Synthesis of 1-Ph-2-COOH-1,2-closo-C2B10H10
(2)
Starting from 1-Ph-1,2-closo-C2B10H11 (2.2 g, 10
mmol) in toluene (40 ml) and using one equivalent of n-
butyllithium as deprotonating reagent, was similarly
prepared 1-Ph-2-COOH-1,2-closo-C2B10H10 (2) as a
white solid, recrystallised from 40–60° petroleum ether.
Yield: 1.48 g, 56%. Anal. Found: C, 40.6; H, 6.1; Mr (EI
mass spectrum) 264 with typical carborane isotope
pattern 258–268. Calc. for C9H16B10O2: C, 40.9; H,
6.1%; Mr 264. IR m(cmꢁ1): 3200–2800 (OH), 2651–2570
(s, BH), 1724 (s, CO). 1H NMR, d(ppm): 7.53 (d, 2H, o-
C6H5), 7.36 (t, 1H, p-C6H5), 7.25 (t, 2H, m-C6H5), 3.05
(2H, B3,6H), 2.58 (1H, B9H), 2.49 (2H), 2.43 (3H, inc.
B12H), 2.35 (2H). 11B–{11H} NMR, d(ppm): )0.2 (1B,
B9), )3.2 (1B, B12), )9.4 (4B), )10.3 (2B), )11.3 (2B,
B3,6). 13C NMR, d(ppm): 162.8 (CO), 130.7 (i, o-C6H5),
130.5 (p-C6H5), 128.6 (m-C6H5), 83.6 (C1), 75.3 (C2).
Compound 2 can be recrystallised pure from dry 40–60°
petroleum ether, or recrystallised from wet solvents as
the 1:1 water solvate, 2 ꢂ H2O. Anal. Found: C, 38.3; H,
6.6; Calc. for C9H16B10O2 ꢂ H2O: C, 38.3; H 6.4%.
2. Experimental
2.1. Synthetic and spectroscopic studies
Experiments were performed under dry, oxygen-free
N2 using standard Schlenk techniques, with some sub-
sequent manipulation in the open laboratory. Toluene,
diethylether and 40–60° petroleum ether were freshly
distilled over Na. IR spectra were recorded as a CH2Cl2
solution or a KBr disk on a Perkin–Elmer Spectrum RX
or 337 FTIR spectrophotometer. NMR spectra were
recorded on a Varian Unity 300 or Bruker DPX 400
spectrometer as CDCl3 solutions at ambient tempera-
ture, chemical shifts being recorded relative to SiMe4 or
CHCl3 (1H), BF3 ꢂ OEt2 (11B) and CDCl3 (13C). Some
11B NMR peak assignments were determined with the
2.1.3. Synthesis of 1-Ph-2-COPh-1,2-closo-C2B10H10 (3)
The compound 1-Ph-1,2-closo-C2B10H11 (1.2 g, 5
mmol) in toluene (40 ml) was treated with n-butyllithium
(2 ml of 2.5 M solution ¼ 5 mmol). After heating to
reflux for 30 min and cooling, PhCOCl (0.7 g, 5 mmol)
was added. After further heating to reflux for 2 h, sol-
vents were removed in vacuo and the residue dissolved in
Et2O and washed successively with NaHCO3 (3 ꢃ 20 ml)
and H2O (4 ꢃ 30 ml). The ethereal layer was separated
and dried over MgSO4 overnight. Removal of solvent
and recrystallisation from 40–60° petroleum ether affor-
ded 1 Ph-2-COPh-1,2-closo-C2B10H10 (3) as large white
crystals. Yield: 1.00 g, 62%. Anal. Found: C, 55.6; H, 6.0;
Mr (EI mass spectrum) 324, typical carborane isotope
1
aid of H–{11B} (selective) spectra. 13C NMR peak as-
signments corresponding to the aromatic carbons were
determined with the benefit of 13C coupled spectra.
Mass spectra were recorded on a VG Micromass 7070E
instrument operating in the EI mode at 70 eV. Elemental
analyses were determined by the departmental
services at Heriot–Watt and Durham. The starting ma-
terials 1-Me-1,2-closo-C2B10H11 [8] and 1-Ph-1,2-closo-
C2B10H11 [9] were prepared by literature methods or
slight variants thereof. All other reagents were used as
supplied.

U. Venkatasubramanian et al. / Polyhedron 23 (2004) 629–636
631
pattern 318–327. Calc. for C15H20B10O: C, 55.5; H, 6.2%;
Mr 324. IR m(cmꢁ1): 2621–2562 (s, BH), 1701 (s, CO). 1H
NMR, d(ppm): 7.48 (d, 2H, o-C6H5), 7.40 (t, 1H, p-
C6H5), 7.28–7.23 (m, 7H, C6H5), 3.15 (2H, B3,6H), 2.55
(3H, inc. B9H), 2.39 (3H, inc. B12H), 2.33 (2H). 11B–
{11H} NMR, d(ppm): 0.2 (1B, B9), )3.0 (1B, B12), )9.3
(4B), )10.5 (4B, inc. B3,6). 13C NMR, d(ppm): 185.5
(CO), 136.3 (i-COC6H5), 132.5 (p-COC6H5), 130.8 (o-
C6H5), 130.6 (i; p-C6H5), 128.5 (o-COC6H5, m-C6H5),
127.9 (m-COC6H5), 85.0 (C1), 82.6 (C2).
were allowed such refinement, all other H atoms being
treated as riding. For 2 ꢂ H2O and 3 non-phenyl H atoms
were refined independently and phenyl H atoms treated
as riding. Table 1 lists details of unit cell data, intensity
data collection and structure refinement.
2.3. Computational studies
DFT calculations on 1-COOH-1,2-closo-C2B10H11
and 1-Me-2-COOH-1,2-closo-C2B10H10 were performed
using the ADF1999 programme [11] with the BP86
functional [12] and incorporating the quasi-relativistic
corrections of Snijders and co-workers [13]. A double-f
plus polarisation STO basis set was employed for all
atoms and the frozen core approximation was applied
(C, B: 1s). The C2–C1–C@O torsion angle, s, was varied
in nine steps of 22.5° from 0° to 180°, with full optimi-
sation of the structure at every step. Within the COOH
group the H–O–C@O torsion angle was originally set at
0° and did not deviate significantly from this during
optimisation.
2.2. Crystallographic studies
Single crystals of compounds 1, 2, 2 ꢂ H2O and 3 were
grown from MeCN, dry 40–60° petroleum ether,
CH2Cl2/40–60° petroleum ether, and hexane, respec-
tively. Compounds 1 and 2 were studied at 160(2) K on
a Bruker P4 diffractometer (Heriot-Watt University)
with graphite-monochromated Mo Ka radiation
ꢀ
(k ¼ 0:71069 A) using x scans. Compounds 2 ꢂ H2O and
3 were studied at 293(2) and 150(2) K, respectively, on a
Rigaku AFC6S diffractometer (University of Durham)
with graphite-monochromated Cu Ka radiation
ꢀ
(k ¼ 1:54178 A) using x scans. All structures were
3. Results and discussion
solved by direct and difference Fourier methods and
refined by full-matrix least-squares against F 2, with non-
hydrogen atoms assigned anisotropic displacement
parameters [10]. For 1, all H atoms were refined posi-
tionally and thermally. For 2, only the hydroxy H atoms
3.1. Syntheses and spectroscopy
The carboxycarboranes 1-Me-2-COOH-1,2-closo-
C2B10H10 (1) and 1-Ph-2-COOH-1,2-closo-C2B10H10 (2)
Table 1
Crystallographic dataa
1
2
2ꢂH2O
3
Colour and habit
Crystal size (mm)
Formula
colourless plate
0.14 ꢃ 0.80 ꢃ 0.98
C4H14B10O2
202.25
colourless block
0.20 ꢃ 0.20 ꢃ 0.33
C9H16B10O2
264.32
colourless block
0.24 ꢃ 0.30 ꢃ 0.54
C9H16B10O2 ꢂ H2O
282.33
colourless block
0.15 ꢃ 0.30 ꢃ 0.30
C15H20B10
324.41
O
M
System
monoclinic
P21=c
160(2)
monoclinic
P21=n
293(2)
monoclinic
P21=c
160(2)
monoclinic
P21=n
150(2)
Space group
T (K)
ꢀ
a (A)
12.073(2)
7.0945(8)
13.4283(12)
102.235(12)
1124.0(3)
4
12.3493(13)
13.2722(12)
18.4075(16)
90.632(8)
3016.9(5)
8
10.330(2)
22.151(4)
7.0260(10)
106.240(10)
1543.5(5)
4
10.469(8)
15.154(7)
11.945(8)
106.57(5)
1816(2)
4
ꢀ
b (A)
ꢀ
c (A)
b (°)
U (A )
3
ꢀ
Z
Dcalc (Mg mꢁ3
)
1.195
0.066 (Mo KaÞ
416
1.164
0.489 (Cu Ka)
1088
1.215
0.072 (Mo Ka)
584
1.186
0.458 (Cu Ka)
672
l (mmꢁ1
F (0 0 0)
)
hdatacollection (°)
3.10–24.99
2663
4.11–60.00
7262
2.05–25.01
5279
4.84–75.09
5320
Data measured
Unique data, Rint
Data I > 2rðIÞ
R, wR2 , (all data)
S (all data)
1970, 0.0336
1775
3951, 0.0343
2479
2726, 0.0499
2000
3491, 0.0794
2481
0.0446, 0.1195
1.058
0.0923, 0.1390
1.021
0.0730, 0.1133
1.015
0.1068, 0.2462
0.880
a, b
Variables
0.0728, 0.2098
201
0.0770, 0.3147
387
0.0461, 0.4911
251
0.1860, 0
276
ꢁ3
ꢀ
q
max; qmin (e A
)
0.2610, )0.278
0.160, )0.156
0.223, )0.207
0.300, )0.275
P
P
P
P
2
2 1=2
2
2
2
2
a R ¼ jjFoj ꢁ jFcjj= jFoj, wR2 ¼ ½ ½wðFo2 ꢁ Fc2Þ ꢄ= wðFo2Þ ꢄ
,
(where wꢁ1 ¼ ½r2ðFoÞ þ ðaPÞ þ bPꢄ and P ¼ ½0:333ðFoÞ þ 0:667ðFcÞ ꢄÞ,
P
S ¼ ½ ½wðFo2 ꢁ Fc2Þ ꢄ=ðn ꢁ pÞꢄ1=2, (where n is the number of data and p the number of parameters).
2

632
U. Venkatasubramanian et al. / Polyhedron 23 (2004) 629–636
were prepared in good yields by the established [14,15]
method of deprotonation of the corresponding 1-R-1,2-
closo-C2B10H11 species with LiR0, addition of CO2, and
protonation during aqueous work-up. Although 1 and 2
are known species [14,15], spectroscopic characterisa-
tion has not previously been reported.
3.2. Structural and computational studies
We performed crystallographic analyses of com-
pounds 1 and 2 to investigate the influence (if any) of
adjacent Me and Ph substituents on the ability of the
COOH group to participate in intermolecular H-bond-
ing. A structural study of 3 was undertaken for com-
parison with that of 2, since in 3 H-bonding is blocked
by the OH unit of 2 having been replaced by Ph.
In broad terms, compound 1 forms a double H-
bonded dimer in the solid state (Fig. 1) via the mutual
association of two carboxylic acid groups in an eight-
membered ring about a crystallographic inversion
centre, denoted R22(8) in graph-set terminology [20].
Essentially the same situation is seen in both crystallo-
graphically independent molecules of 2, Fig. 2 showing
the dimer of molecule A. In the water solvate 2 ꢂ H2O
two molecules of H2O are incorporated into the H-
bonded ring to give a 12-membered ring, R44(12), shown
in Fig. 3.
Spectroscopically, both 1 and 2 are consistent with
time-averaged Cs molecular symmetry (or, of course,
simple coincidence), arising from C(O)OH/C(OH)O
exchange and/or rotation of the COOH group about the
C
cage–C bond. The 11B–{1H} NMR spectrum of 1 at 128
MHz is fairly uninformative, with only three resonances
of relative integrals 1:1:8 (high frequency to low fre-
quency), each appearing as doublets (1JBH ꢀ140 Hz) in
the proton-coupled spectrum. In contrast, the 11B–{1H}
spectrum of 2 is somewhat more informative. The pre-
sumed 2+2+2+2 low-frequency co-incidence of 1 (as-
suming time-averaged Cs symmetry) is partially resolved
(appearing as a 4[2+2]:2:2 set of three peaks). Moreover,
the highest and lowest frequency resonances have been
assigned. Given that we assign the highest-frequency
BHresonance in the 1H spectrum (3.05 ppm) of 2 to
B3,6Hon the basis of several precedents [16], then by
1H–{11B} (selective) spectroscopy the lowest-frequency
11B resonance (–11.3 ppm) is due to B3,6. It is well es-
tablished that B atoms antipodal to C in closo carbor-
anes occur at high frequency [17], and therefore the two
resonances at –0.2 and –3.2 ppm are due to B9 and B12.
Of these we assign the resonance at –0.2 ppm to B9 and
that at –3.2 ppm to B12 by analogy with the B9,12
chemical shifts in the disubstituted analogues 1,2-
(COOH)2-1,2-closo-C2B10H10 (0.3 ppm) [2p] and 1,2-
3.2.1. Intramolecular parameters in 1 and 2
Since we have previously reported crystallographic
studies of 1-COOH-1,2-closo-C2B10H11 (I) [2m] and 1,2-
(COOH)2-1,2-closo-C2B10H10 ꢂ 1/2C2H5OH (II) [2p] it is
instructive, on more detailed analysis, to consider first
the influence of the Me and Ph groups of 1 and 2 on
intramolecular parameters involving the COOH group.
Table 2 summarises C1–C2, C–COOH, and hCOOH (the
modulus of the average Ccage–Ccage–C–O torsion angle)
for 1, 2 (two independent molecules) and 2 ꢂ H2O, as well
as for the mono- and dicarboxycarboranes referred to
above. For completeness, hPh [21] (the modulus of the
average Ccage–Ccage–CPh–CPh torsion angle) is also given
for 2 and 2 ꢂ H2O. Our definition of hCOOH here [2p,2q]
1
Ph2-1,2-closo-C2B10H10 (–2.7 ppm), respectively. In
the 1H NMR spectra of 1 and 2 the OH peak shift varies
with concentration and the presence of water. The 13C
NMR spectrum of 2 is fully assigned, albeit with over-
lapping resonances at 130.7 ppm.
We have also prepared 1-Ph-2-COPh-1,2-closo-
C2B10H10 (3), a further known compound [18,19] not
1
previously characterised spectroscopically. Both the H
and 11B spectra are only partially assigned, but reveal
clear evidence for time-averaged Cs molecular symmetry
(which must involve rotation of the COPh group about
the Ccage–C bond that is rapid on the NMR timescale at
293 K). Thus H3 and H6 are equivalent (3.15 ppm) and
the 11B spectrum appears as a 1:1:2+2:2+2 pattern (high
to low frequency). Again the two highest-frequency 11B
resonances are due to the B atoms antipodal to C [17],
with that at higher frequency (d 0.2, B9) opposite the
carboxy substituent. The 13C NMR spectrum of 3 is
fully assigned, albeit with overlapping resonances at
130.6 and 128.5 ppm.
Fig. 1. Perspective view of the H-bonded dimer of compound 1.
Thermal ellipsoids are drawn at the 50% probability level, except for H
atoms.
1
128.38 MHz, CDCl3, d relative to neat BF3 ꢂ OEt2 at 0 ppm: –2.6
(2B, B9,12), –9.3 (4B, B4,5,7,11), –10.6 (2B, B8,10), –11.6 (2B, B3,6).

U. Venkatasubramanian et al. / Polyhedron 23 (2004) 629–636
633
Me and COOH substituents in 1. 2 Since such crowding
could have been relieved by re-orientation of the COOH
group, we were prompted to investigate the preferred
orientation of the carboxy group in carboxycarboranes I
and 1 by DFT calculations. Fig. 4 shows (upper plot) the
energy of I as a function of fixed Ccage–Ccage–C@O
torsion angle, s, varied in 22.5° steps from 0° to 180°,
with full structural optimisation at each stage; s ¼ 0°
corresponds to hCOOH ¼ 90syn°, s ¼ 90° corresponds to
hCOOH ¼ 0°, and s ¼ 180° corresponds to hCOOH
¼
90anti°. The orientation hCOOH ¼ 90syn° is preferred to
hCOOH ¼ 90anti° by ꢀ6 kJ molꢁ1, and the barrier to ro-
tation is ꢀ17 kJ molꢁ1. In I the observed [2m] orienta-
tion, 87.5(2)syn°, is close to the optimal one predicted.
The lower plot in Fig. 4 shows the result of a similar
DFT calculation on 1 drawn on the same scale as the
upper plot. This time the energy profile is flatter (barrier
to rotation ꢀ7 kJ molꢁ1) which we ascribe to the
Fig. 2. Perspective view of the H-bonded dimer of compound 2
(molecule A only). Thermal ellipsoids are drawn at the 30% probability
level, except for H atoms.
hCOOH ¼ 90anti° and (particularly) the hCOOH ¼ 90syn
°
orientations no longer being stabilised by intramolecular
H-bonding, as was the case with I. Such intramolecular
H-bonding, involving the relatively acidic proton on C2,
has recently been noted [22] in 1-(C5H4N-20)-1,2-closo-
C2B10H11 and its 40-Br analogue, both of which have a
C
cage–Ccage–C–N torsion angle close to 0°. In 1 the ob-
served 25° twist about the C–COOH bond
[hCOOH ¼ 65:0ð2Þsyn°], presumably due to the steric in-
fluence of the adjacent Me group, is accurately repro-
duced in the calculations, which show a minimum
energy when s is ꢀ30°.
The 1,2-dicarboxycarborane crystallises as the
ꢂ 1/2C2H5OH solvate, II [2p]. In II there are four crys-
tallographically independent carborane molecules ar-
ranged in two centrosymmetric tetrameric units held
together by H-bonding that incorporates the solvate.
Every carborane has one COOH in a low and one
COOH in a high h orientation (half the high h orienta-
tions are syn, half are anti). The experimental data in
Table 2 reveal a definite pattern – high h (whether syn or
anti) is associated with a C–COOH bond length of
Fig. 3. Perspective view of the H-bonded dimeric unit of compound
2ꢂH2O. Thermal ellipsoids are drawn at the 50% probability level,
except for H atoms.
ꢀ
ꢀ1.51 A and low h is associated with a measurably
ꢀ
differs from that originally used [2m] for 1-COOH-1,2-
closo-C2B10H11 and 1,2-(COOH)2-1,2-closo-C2B10H10
but is preferred as it is now calculated in an analogous
way to hPh. Complete listings of derived molecular pa-
rameters for 1, 2 and 2 ꢂ H2O have been deposited as
supplementary material.
longer C–COOH bond, ꢀ1.52–1.53 A. This pattern is
reproduced in the DFT calculations on I, which give C–
COOH 1.515 (h ¼ 90syn°), 1.514 (h ¼ 90anti°) and 1.525
ꢀ
A (h ¼ 0°).
Compound 2 crystallised from dry solvent has two
crystallographically independent molecules (A and B)
per asymmetric unit. In both A and B the orientations of
Ph and COOH substituents are characterised by low h
In compound 1 the orientation of the COOH group
with respect to C1 is specified by hCOOH ¼ 65:0ð2Þsyn°,
the subscript [necessary for large hCOOH ( P 45°) values]
identifying that the C@O moiety is roughly syn to C1.
2
ꢀ
Further evidence for intramolecular crowding in 1 is demonstrated
The C1–C2 distance, 1.6694(17) A and C2–C21 distance,
ꢀ
ꢀ
from fully optimised MP2/6-31G* geometries of 1-Me-1,2-closo-1,2-
C2B10H11 and 1 where the C–C–C(Me) angle widens from 118.4° to
120.6°, respectively. The crystallographic structure of the carborane 1-
Me-1,2-closo-1,2-C2B10H11 remains unreported (search of the Cam-
bridge Crystallographic Database, version 5.24, July 2003).
1.5172(16) A, are 0.04 and 0.01 A longer, and the
C1–C2–C21 angle, 118.75(10)°, is 4° wider, than the
corresponding parameters in I, collectively suggesting a
degree of intramolecular steric crowding between the

634
U. Venkatasubramanian et al. / Polyhedron 23 (2004) 629–636
Table 2
ꢀ
Summary of C1–C2 and C–COOH distances (A) and Ph and COOH orientations (°) for carboxycarboranes
a
b
Compound
X
C1–C2
C–COOH
hPh
hCOOH
Reference
I
H
COOH
1.631(2)
1.660(2)
1.507(2)
1.527(3)
1.515(3)
1.523(3)
1.510(3)
1.525(2)
1.510(3)
1.522(3)
1.514(3)
1.5172(16)
1.509(4)
1.504(4)
1.517(2)
–
–
87.5(2)syn
5.3(2)
[2m]
[2p]
II (A)c
74.5(2)anti
0.6(2)
II (B)
II (C)
II (D)
COOH
COOH
COOH
1.651(2)
1.652(2)
1.653(2)
–
–
–
[2p]
[2p]
[2p]
87.9(2)syn
4.9(2)
89.2(2)syn
14.1(2)
60.2(2)anti
65.0(2)syn
5.0(3)
1
Me
Ph
Ph
Ph
1.6694(17)
1.678(3)
1.691(3)
1.705(2)
–
this work
this work
this work
this work
2(A)d
2(B)
2 ꢂ H2O
26.6(3)
16.2(3)
6.7(2)
39.7(3)
78.0(2)syn
a hPh defined [21] as the modulus of the average Ccage–Ccage–CPh–CPh torsion angle.
b hCOOH defined as the modulus of the average Ccage–Ccage–C–O torsion angle. For high values of hCOOH it is instructive also to specify the
conformation of the C ¼ O moiety of the COOH group relative to the Ccage atom to which COOH is not bound as either syn or anti. These terms
correspond to s values of 0° and 180° respectively (Fig. 4).
c Four crystallographically independent molecules.
d Two crystallographically independent molecules.
20
When crystallised from wet solvent, 1-Ph-2-COOH-
1,2-closo-C2B10H10 crystallises as the monohydrate
2 ꢂ H2O and incorporates two molecules of H2O in an
extended (12-membered) H-bonded ring system. In
2 ꢂ H2O the COOH group stands effectively perpendic-
ular to the Ph ring, hCOOH ¼ 78:0ð2Þsyn°, reminiscent of
its relative orientation in 1. The C–COOH distance in
2 ꢂ H2O is identical to that in 1, but C1–C2 is the longest
of all the structures described herein.
Relative energy
(kJ mol–1
18
16
14
12
10
8
6
4
2
0
)
1-COOH-1,2-closo-C2B10H11
0
22.5 45 67.5 90 112.5 135 157.5 180
–
–
Ccage Ccage C=O torsion angle, τ (º)
3.2.2. Intermolecular parameters in 1 and 2
20
18
16
14
12
10
8
6
4
2
0
Relative energy
(kJ mol–1
)
As previously noted, both 1 and 2 form centrosym-
metric eight-membered H-bonded rings in the solid state
[R22(8)] whilst 2ꢂH2O forms a centrosymmetric 12-mem-
bered ring that incorporates water [R44(12)]. Parameters
involved in these H-bonding rings are summarised in
Table 3.
1-Me-2-COOH-1,2-closo-C2B10H10
According to current conventions [24], all the H-
bonds observed in this study are classified as ‘‘strong’’,
0
22.5 45 67.5 90 112.5 135 157.5 180
ꢀ
with Hꢂ ꢂ ꢂO distances ðdÞ < 2 A and O–Hꢂ ꢂ ꢂ O angles
–
–
Ccage Ccage C=O torsion angle, τ (º)
(dÞ close to 180°. The planar eight-membered cyclic H-
bonded homodimer of two COOH groups observed in 1
and 2 is ubiquitous [7] for this well-known supramo-
lecular synthon [1] and dimensions within it are quite
standard. The 12-membered H-bonded ring of 2ꢂH2O is
of approximate chair conformation. Within it,
Fig. 4. Plot of relative energy of 1-COOH-1,2-closo-C2B10H11 (I) and
1-Me-2-COOH-1,2-closo-C2B10H10 (1) as a function of the Ccage
Ccage–C@O torsion angle, s, by DFT calculation.
–
values (broadly similar to the situation observed in 1,2-
diaryl-1,2-closo-C2B10 species [23]), but in 2(A) the Ph
#
ꢀ
group is the more twisted [hPh ¼ 26:6ð3Þ°, hCOOH
¼
H3Aꢂ ꢂ ꢂO2 , d ¼ 1:87ð4Þ A, appears somewhat weaker
ꢀ
5:0ð3Þ°] whilst in 2(B) hCOOH is the greater [hPh
¼
than H1ꢂ ꢂ ꢂO3, d ¼ 1:65ð3Þ A, and the former is ac-
16:2ð3Þ°, hCOOH ¼ 39:7ð3Þ°]. In 2 C1–C2 distances are
marginally larger, and C–COOH distances marginally
shorter, than those in 1.
companied by an angle / at the COOH acceptor that is
close to 180° (c.f. / in 1 and 2 near 120°). A search of the
Cambridge Crystallographic Database (version 5.24,

U. Venkatasubramanian et al. / Polyhedron 23 (2004) 629–636
635
Table 3
Hydrogen bonding distances (A) and angles (°) in the carboxycarborane compounds 1 and 2
ꢀ
Compound
O1
H
O2
Y
r
d
d
/
1
O1
H1
O2a
O2Aa
O2Ba
O3
C21a
0.90(2)
1.11(6)
1.02(5)
0.90(3)
1.75(2)
1.55(6)
1.61(5)
1.65(3)
175(2)
171(4)
176(4)
173(3)
122(2)
128(4)
124(4)
120(3)
120(3)
174(3)
2(A)
2(B)
2 ꢂ H2O
O1A
O1B
O1
HOA
HOB
H1
C19Aa
C19Ba
H3A
H3B
O1
O3
H1
H3A
O3
O2a
2 ꢂ H2O
C21a
0.85(4)
1.87(4)
172(3)
a Atom generated by inversion in centre of H-bonding ring.
Table 4
July 2003) reveals 57 similar 12-membered rings in-
volving two COOH groups and two H2O molecules in
the current literature, of which 42 do not involve
charged species. In only 15 cases are all four Hꢂ ꢂ ꢂO
ꢀ
Selected interatomic distances (A) and interbond angles (°) for 3
Bond distances
C(1)–C(2)
C(1)–C(13)
C(2)–C(19)
1.695(3)
1.507(3)
1.547(3)
C(19)–O
C(19)–C(20)
1.210(3)
1.485(3)
ꢀ
distances <2 A, with the shortest distances [HCOOHꢂ ꢂ ꢂ
ꢀ
OH2O 1.43; HH2Oꢂ ꢂ ꢂOCOOH 1.61 A] recorded for 1,3,5-
Bond angles
cyclohexanetricarboxylic acid monohydrate [25]. The
H-bonded distances observed in 2ꢂH2O are most clearly
mirrored in 2,3-dihydroxysuccinic acid monohydrate
C(2)–C(19)–C(20)
C(2)–C(19)–O
124.2(9)
117.2(9)
O–C(19)–C(20)
93.48(9)
ꢀ
[26] [HCOOHꢂ ꢂ ꢂOH2O 1.64; HH2Oꢂ ꢂ ꢂOCOOH 1.90 A].
and similar to those in 2, is shorter than that in 1,2-Ph2-
ꢀ
1,2-closo-C2B10H10 [23a] [1.727(6) A, averaged over the
3.2.3. The structure of 3
two crystallographically independent molecules]. The
cage-bound Ph group is oriented with hPh ¼ 6:9ð2Þ°.
The primary benzoyl orientation hbenz (the modulus of
the average C1–C2–C19–O and C1–C2–C19–C20 tor-
sion angles) is 12.3(2)°, and there is a twist of 48.8(2)°
about the C19–C20 bond, as is clearly evident from
Fig. 5.
As a complement to the crystallographic study of 2
we have also determined the structure of the phen-
ylbenzoylcarborane 3. Fig. 5 shows a perspective view of
a single molecule and Table 4 hosts selected intramo-
lecular parameters.
Compound 3 crystallises as discrete molecules with no
significant intermolecular contacts – replacing the car-
boxy group of 2 with the benzoyl group prevents associ-
ation via H-bonding. As far as we are aware this
structural study is the first reported of a simple carborane
ketone, i.e. carborane-C(O)–R. The C1–C2 distance,
4. Conclusions
In the solid state the substituted carboxy carboranes
1-R-2-COOH-1,2-closo-C2B10H10 give rise to homodi-
mers with eight-membered H-bonded rings (R ¼ Me, 1;
R ¼ Ph, 2). The monohydrate 2ꢂH2O crystallises with a
12-membered H-bonded ring incorporating two car-
boxylic acid groups and one water molecule. In 1 the
orientation of the COOH group is close to that shown
computationally to be preferred and a similar orienta-
tion persists in 2ꢂH2O, whilst in 2 it is twisted to a much
lower hCOOH value. In the context of the relative orien-
tations of cage substituents, 2 is similar to 1-Ph-2-
COPh-1,2-closo-C2B10H10 (3), which crystallises as
discrete monomers.
ꢀ
1.695(3) A, identical to that in related acetato compound
[27]
[{1-Ph-2-COO(SnnBu2)-1,2-closo-C2B10H10}2O]2
5. Supplementary material
Crystallographic data for the structural analyses have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC Nos. 214908 (1), 214909 (2),
214910 (2ꢂH2O) and 214911 (3). Copies of this infor-
Fig. 5. Perspective view of compound 3. Thermal ellipsoids are drawn
at the 50% probability level, except for H atoms.

636
U. Venkatasubramanian et al. / Polyhedron 23 (2004) 629–636
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We thank the ORS Award Scheme, UK, (U.V.),
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