Synthesis and characterisation of labelled diphenylcarboranes

Article abstract of DOI:10.1016/S0277-5387(03)00103-7

The synthesis, by either acid-catalysed electrophilic substitution or recapitation, and characterisation of four new B-labelled diphenyl carboranes is described. Two of these, 1,2-Ph₂-9,12-I₂-1,2-closo-C₂ B₁₀H

Full text of DOI:10.1016/S0277-5387(03)00103-7

Polyhedron 22 (2003) 1293ꢂ1301
/
www.elsevier.com/locate/poly
Synthesis and characterisation of labelled diphenylcarboranes
Susan Robertson, David Ellis, Thomas D. McGrath, Georgina M. Rosair,
Alan J. Welch *
Department of Chemistry, Heriot-Watt University, Edinburgh, EH14 4AS, UK
Received 27 November 2002; accepted 21 January 2003
Abstract
The synthesis, by either acid-catalysed electrophilic substitution or recapitation, and characterisation of four new B-labelled
diphenyl carboranes is described. Two of these, 1,2-Ph2-9,12-I2-1,2-closo-C2B10H8 (1) and 1,2-Ph2-3-Me-1,2-closo-C2B10H9 (2) were
subjected to crystallographic study, whilst the 3-Et (3) and 3-F (4) analogues of 2 were studied spectroscopically. Decapitation of the
mono-iodo analogue of
[C6H5CH2NMe3]ꢁ (5c) salts, the last of which was subjected to crystallographic analysis. Decapitation of 3 and 4 selectively
removes B6, yielding the 3-labelled nido-carboranes [3-Et-7,8-Ph2-7,8-nido-C2B9H9]ꢀ and [3-F-7,8-Ph2-7,8-nido-C2B9H9]ꢀ
1 ,
affords [5-I-7,8-Ph2-7,8-nido-C2B9H9]ꢀ isolated as [HNEt3]ꢁ (5a) [HNMe3]ꢁ (5b) and
,
respectively, both isolated as [HNMe3]ꢁ salts (6a and 7). The ethyl species was also prepared as the [HNEt3]ꢁ salt (6b), and
was structurally characterised. Salts 5b, 6a and 7 are all afforded in good yield and the anions are ideal candidates for subsequent
deprotonation and metallation, which should result in low-temperature isomerisation of the transient 3,1,2-MC2B9 species thereby
produced, and thereby yield important, robust, mechanistic information on the rearrangement process.
# 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Carborane; Isomerisation; Synthesis; Spectroscopy; Crystallographic study; Vertex labelling
1. Introduction
identical to that predicted [7] in an ab initio computa-
tional study of the isomerisation of 1,2-closo-C2B10H12,
thus providing an important signpost for the isomerisa-
tion mechanism.
The mechanism of isomerisation of carboranes has
been of interest for many years [1ꢂ3]. However, the high
/
These findings have renewed interest in the feasibility
of constructing a complete experimental mapping of the
mechanism(s) of (hetero)carborane isomerisation from
the sum of the results of a series of individual labelling
experiments. Such experiments require access to deriva-
tives of [7,8-Ph2-7,8-nido-C2B9H9]2ꢀ carrying robust
labels at each and every symmetry-independent boron
vertex (Fig. 1). This paper describes some recent
synthetic and structural results achieved in pursuit of
that goal.
temperatures usually involved have precluded experi-
mental study of the mechanism via labelled vertices since
the integrity of the vertex-label bond cannot be guaran-
teed under such conditions [4].
Some years ago we showed that metallation of [7,8-
Ph2-7,8-nido-C2B9H9]2ꢀ with a suitably bulky metal-
ligand fragment could generate a 2,1,8-MC2B9 species at
or near room temperature [5]. Assuming that the initial
product of such metallation is a transient 3,1,2-MC2B9
species, this effectively represents low-temperature
1,20/1,7 C atom isomerisation of an icosahedral
(hetero)carborane (Scheme 1). Moreover, we were for-
tunate additionally to be able to isolate reaction
intermediates in some cases [6], with gross architectures
2. Experimental
2.1. Synthetic and spectroscopic studies
* Corresponding author. Tel.: ꢁ
451-3180.
E-mail address: a.j.welch@hw.ac.uk (A.J. Welch).
/
44-131-451-3217; fax: ꢁ44-131-
/
Experiments were performed under dry, oxygen-free,
N2 using standard Schlenk techniques, with some
0277-5387/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0277-5387(03)00103-7

1294
S. Robertson et al. / Polyhedron 22 (2003) 1293ꢂ1301
/
Scheme 1. Metallation of [7,8-Ph2-7,8-nido-C2B9H9]2ꢀ with a bulky metal-ligand fragment MLn generating a C-atom isomerised 2,1,8-MC2B9
species at or near room temperature via a notional 3,1,2-MC2B9 intermediate.
C6H5). 11B{1H} NMR d (ppm): ꢀ
(4B), ꢀ7.58 (2B), ꢀ10.03 (2B, B9, B12).
/
3.99 (2B), ꢀ5.46
/
/
/
2.1.2. Synthesis of 1,2-Ph2-3-Me-1,2-closo-C2B10H9 (2)
To a stirring solution of [HNMe3][7,8-Ph2-7,8-nido-
C2B9H10] (0.233 g, 0.67 mmol) in Et2O (15 ml) at 0 8C
was added n-BuLi in hexanes (0.54 ml of 2.5 M solution
Fig. 1. Boron atom numbering in [7,8-Ph2-7,8-nido-C2B9H9]2ꢀ
.
ꢀ1.35 mmol). The mixture was allowed to warm to
/
room temperature then heated to reflux for 2 h. After
cooling, Et2O was removed in vacuo and the residue
subsequent manipulation in the open laboratory. Sol-
vents were freshly distilled over CaH2 (CH2Cl2) or Na
suspended in 40ꢂ
/
60 petroleum ether and frozen to
wire (THF, Et2O, 40ꢂ60 petroleum ether) or stored over
/
ꢀ
/
196 8C. MeBBr2 (0.15 ml, 0.225 g, 1.21 mmol) was
˚
4 A molecular sieves (EtOH, CDCl3). NMR spectra at
200.13 (1H), 400.13 (1H), 128.38 (11B) or 161.98 MHz
(19F) were recorded on Bruker AC 200 or DPX 400
spectrometers from CDCl3 solutions at ambient tem-
perature, chemical shifts being recorded relative to
added, and the mixture allowed to warm to room
temperature with stirring. Volatiles were removed in
vacuo, 40ꢂ60 petroleum ether (20 ml) added, and the
/
mixture filtered. The filtrate was evaporated to yield a
white solid, which was recrystallised from 40 to 60
petroleum ether at 3 8C to afford 1,2-Ph2-3-Me-1,2-
closo-C2B10H9 (2) as a colourless solid. Yield 0.029 g,
14.0%. Anal. Found: C, 54.0; H, 7.63. Calc. for
C15H22B10: C, 53.0; H, 7.14%. IR n (cmꢀ1): 2575 (br).
SiMe4 (1H), BF3×
/
OEt2 (11B) or CFCl3 (19F). IR spectra
were recorded from CH2Cl2 solutions on a Perkinꢂ
/
Elmer Spectrum RX FTIR spectrophotometer. EI
mass spectra were recorded using a Kratos Concept
mass spectrometer. Elemental analyses were determined
by the departmental service. The starting materials 1,2-
1H NMR d (ppm): 6.9ꢂ
CH3). 11B{1H} NMR d (ppm): 2.03 (1B, B3), ꢀ
(2B), ꢀ5.21 (3B), ꢀ6.62 (3B), ꢀ8.63 (1B).
/
7.4 (m, 10H, C6H5), 0.73 (s, 3H,
0.11
/
Ph2-1,2-closo-C2B10H10
[8],
1,2-Ph2-9-I-1,2-closo-
/
/
/
C2B10H9 [9], [HNMe3][7,8-Ph2-7,8-nido-C2B9H10] [10],
MeBBr2 and EtBBr2 [11,12] were prepared by literature
methods or slight variants thereof. All other reagents
were used as supplied.
2.1.3. Synthesis of 1,2-Ph2-3-Et-1,2-closo-C2B10H9 (3)
Similarly, from [HNMe3][7,8-Ph2-7,8-nido-C2B9H10]
(2.598 g, 7.51 mmol) in Et2O (70 ml), n-BuLi in hexanes
(6.00 ml of 2.5 M solution ꢀ15.0 mmol) and EtBBr2
/
(2.50 ml, 3.755 g, 18.78 mmol) was synthesised 1,2-Ph2-
3-Et-1,2-closo-C2B10H9 (3) as a colourless solid. Yield
0.820 g, 33.6%. Anal. Found: C, 57.8; H, 7.62. Calc. for
C16H24B10: C, 59.2; H, 7.46%. IR n (cmꢀ1): 2576 (br).
2.1.1. Synthesis of 1,2-Ph2-9,12-I2-1,2-closo-C2B10H8
(1)
1,2-Ph2-1,2-closo-C2B10H10 (0.553 g, 1.85 mmol) and
I2 (2.366 g, 9.32 mmol) were heated to 50 8C in glacial
acetic acid (7.5 ml). Whilst using a blast screen, a 1:1
mixture of conc. HNO3 and conc. H2SO4 (4 ml) was
added dropwise over 30 min and the mixture stirred for
a further 2 h. The cooled solution was poured into H2O
1H NMR d (ppm): 7.0ꢂ
CH3), 1.04 (q, 2H, CH2). 11B{1H} NMR d (ppm): 3.77
(1B, B3), ꢀ0.26 (2B), ꢀ6.51 (6B), ꢀ8.45 (1B). MS m/z:
324 (Mꢁ).
/7.3 (m, 10H, C6H5), 1.18 (t, 3H,
/
/
/
(50 ml), filtered, and the residue washed with H2O (3ꢃ
10 ml). The purple solid was dissolved in Et2O (:150
ml) and dried over MgSO4. Recrystallisation from the
minimum amount of 40ꢂ60 petroleum ether at ꢀ30 8C
/
/
2.1.4. Synthesis of 1,2-Ph2-3-F-1,2-closo-C2B10H9 (4)
Similarly, from [HNMe3][7,8-Ph2-7,8-nido-C2B9H10]
(0.430 g, 1.24 mmol) in Et2O (20 ml), n-BuLi in hexanes
/
/
afforded 1,2-Ph2-9,12-I2-1,2-closo-C2B10H8 (1) as a
colourless solid. Yield 0.490 g, 48.2%. Anal. Found: C,
(1.00 ml of 2.5 M solution ꢀ
/
2.50 mmol) and BF3×
/
OEt2
(0.08 ml, 0.62 mmol) was synthesised 1,2-Ph2-3-F-1,2-
closo-C2B10H9 (4) as a white solid. Yield 0.126 g, 32.1%.
Anal. Found: C, 53.4; H, 6.31. Calc. for C14H19B10F: C,
30.4; H, 3.55. Calc. for C14H18B10I2: C, 30.7; H, 3.31%.
IR n (cmꢀ1): 2613 (br). H NMR d (ppm): 7.1ꢂ
1
/
7.5 (m,

S. Robertson et al. / Polyhedron 22 (2003) 1293ꢂ
/
1301
1295
1
53.5; H, 6.09%. IR n (cmꢀ1): 2582 (br). H NMR d
(ppm): 7.0ꢂ
7.6 (m, C6H5). 11B{1H} NMR d (ppm): 4.36
43 Hz, 1B, B3), ꢀ2.64 (2B), ꢀ7.65 (2B),
9.22 (4B), ꢀ 185
14.44 (1B). 19F NMR d (ppm): ꢀ
(cmꢀ1): 2524 (br). 1H NMR d (ppm): 6.8ꢂ
/
7.2 (m,
10H, C6H5), 2.71 (s, 9H, CH3). 11B{1H} NMR d (ppm):
/
1
1
3.17 (d, JBF
(d, JBF
ꢄ
/
/
/
ꢄ
/
55 Hz, 1B, B3), ꢀ
/
7.58 (2B), ꢀ
/
16.97
37.40 (1B). 19F NMR d (ppm):
ꢀ
/
/
/
(4B), ꢀ34.48 (1B), ꢀ
/
/
1
1
198 (1:1:1:1 quar., JBF
(1:1:1:1 quar., JBF
ꢄ
/
55 Hz). MS m/z: 314 (Mꢁ ).
ꢀ
/
ꢄ56 Hz).
/
2.1.5. Synthesis of [HNEt3][5-I-7,8-Ph2-7,8-nido-
C2B9H9] (5a)
2.2. Crystallographic studies
1,2-Ph2-9-I-1,2-closo-C2B10H9 (1.472 g, 3.47 mmol)
and KOH (0.488 g, 8.71 mmol) were heated to reflux in
EtOH (50 ml) for 72 h. After removal of excess KOH by
treatment with CO2 gas, the volatiles were removed in
vacuo leaving a colourless oil. This was dissolved in H2O
(20 ml), filtered, and treated with a slight excess of
[HNEt3]Cl (0.508 g, 3.69 mmol) in H2O (10 ml). The
solid thus precipitated was extracted into CH2Cl2 (30
Single, diffraction-quality, crystals of 1, 2, 5c and 6b
were grown by diffusion of a CH2Cl2 solution of the
compound and a fivefold excess of 40ꢂ60 petroleum
/
ether at room temperature. Diffraction data were
measured at 160(2) K on a Bruker AXS P4 diffract-
ometer equipped with an Oxford Cryosystems Cryo-
stream cooler. One asymmetric fraction of intensity data
was collected [13] to umax 258 with graphite-monochro-
˚
mlꢁ
/
3ꢃ
/
10 ml), and dried over MgSO4. Evaporation
mated Mo Ka radiation (lꢄ
/
0.71069 A) using v scans.
yielded [HNEt3][5-I-7,8-Ph2-7,8-nido-C2B9H9] (5a) as a
white solid. Yield 1.073 g, 60.2%. Anal. Found: C, 46.8;
H, 6.95; N, 2.70. Calc. for C20H35B9IN: C, 46.8; H, 6.87;
Standard reflections were re-measured every 100 data
and any crystal decay corrected. Data for 1, 5c and 6b
were corrected for absorption by f scans. All structures
were solved [14] by direct and difference Fourier
methods and refined by full-matrix least-squares against
F2, with non-hydrogen atoms assigned anisotropic
displacement parameters. The anion of 5c is disordered
with 50% occupancy of B3H and B12H fragments, this
rendering the anion non-crystallographically-imposed
Cs symmetry. A consequence of this disorder is that it
proved impossible to locate the H atom associated with
the open face. The crystal of 6b contains 0.5 molecule of
CH2Cl2 of solvation per ion pair, slightly disordered
1
N, 2.73%. IR n (cmꢀ1): 2543 (br). H NMR d (ppm):
6.8ꢂ
CH3). 11B{1H} NMR d (ppm): ꢀ
11.29 (1B), ꢀ13.30 (1B), ꢀ15.46 (2B), ꢀ
B5), ꢀ27.65 (1B), ꢀ30.68 (1B).
/
7.2 (m, 10H, C6H5), 3.28 (q, 6H, CH2), 2.34 (t, 9H,
4.35 (1B), ꢀ5.38 (1B),
20.88 (1B,
/
/
ꢀ
/
/
/
/
/
/
The salts [HNMe3][5-I-7,8-Ph2-7,8-nido-C2B9H9] (5b)
and [C6H5CH2NMe3][5-I-7,8-Ph2-7,8-nido-C2B9H9] (5c)
were prepared similarly in comparable yields.
2.1.6. Synthesis of [HNMe3][3-Et-7,8-Ph2-7,8-nido-
C2B9H9] (6a)
about a centre of symmetry, and Cꢀ
/
Cl was restrained to
˚
1.70(4) A in the refinement. Phenyl, methylene and
Similarly, from compound 3 (0.383 g, 1.18 mmol) and
KOH (0.152 g, 2.71 mmol) in EtOH (20 ml), followed by
treatment with [HNMe3]Cl (0.163 g, 1.19 mmol) in H2O
(5 ml), was synthesised [HNMe3][3-Et-7,8-Ph2-7,8-nido-
C2B9H9] (6a) as a white solid. Yield 0.311 g, 51.3%.
Anal. Found: C, 61.4; H, 9.37; N, 3.79. Calc. for
methyl H atom positions were calculated and treated as
˚
H distances 0.95, 0.99 and 0.98 A,
riding models (Cꢀ
/
respectively), with displacement parameters calculated
as 1.2, 1.2 and 1.5 times the bound carbon atom Ueq,
respectively. Cage H atoms were either treated as riding
C19H34B9N: C, 61.1; H, 9.17; N, 3.75%. IR n (cmꢀ1):
on the appropriate B atom (Bꢀ
/
Hꢄ
/
1.1 A, 2 and 5c) or
˚
1
˚
H distance of 1.10(2) A (1 and
2523 (br). H NMR d (ppm): 6.8ꢂ
/
7.1 (m, 10H, C6H5),
were restrained to a Bꢀ
/
2.77 (t, 9H, CH3), 1.15 (t, 3H, BCH2CH3), 0.82 (app. t,
2H, BCH2CH3). 11B{1H} NMR d (ppm): ꢀ
1.31 (1B,
B3), ꢀ6.22 (2B), ꢀ14.74 (4B), ꢀ31.63 (2B).
The salt [HNEt3][3-Et-7,8-Ph2-7,8-nido-C2B9H9] (6b)
was prepared in similar yield in an entirely analogous
manner.
6b) with UH set at 1.2 times UB in all cases. The only
exception was in 6b where H(10B) which was allowed
positional refinement and the U values of the B-bound
H atoms were freely refined. Table 1 lists details of unit
cell data, intensity data collection and structure refine-
ment.
/
/
/
/
2.1.7. Synthesis of [HNMe3][3-F-7,8-Ph2-7,8-nido-
C2B9H9] (7)
3. Results and discussion
Similarly, from compound 4 (0.295 g, 0.93 mmol) and
KOH (0.157 g, 2.80 mmol) in EtOH (20 ml), followed by
treatment with [HNMe3]Cl (0.112 g, 1.21 mmol) in H2O
(2 ml) and recrystallisation from CH2Cl2 was synthe-
sised [HNMe3][3-F-7,8-Ph2-7,8-nido-C2B9H9] (7) as a
colourless crystalline material. Yield 0.325 g, 96.1%.
Anal. Found: C, 56.7; H, 8.37; N, 3.76. Calc. for
C17H29B9FN: C, 56.1; H, 8.04; N, 3.85%. IR n
3.1. Labelled closo-carboranes
This paper describes new, labelled, derivatives of 1,2-
Ph2-1,2-closo-C2B10H10 prepared by two methods, elec-
trophilic substitution and recapitation.
Electrophilic substitution has for many years been an
established procedure by which carboranes may be

1296
S. Robertson et al. / Polyhedron 22 (2003) 1293ꢂ1301
/
Table 1
Crystallographic dataa for compounds 1, 2, 5c and 6b
1
2
5c
6b
Formula
Mr
C
548.18
14H18B10I2
C15H22B10
310.43
colourless
needle
monoclinic
C2/c
C24H35B9IN
560.71
C22H40B9N×
/0.5CH2Cl2
458.30
Colour
Habit
Colourless
Block
colourless
needle
colourless
block
triclinic
Crystal system
Space group
Orthorhombic
Pnma
monoclinic
P21/n
¯
P1
/
˚
a (A)
15.356(3)
15.601(2)
8.5730(10)
90
16.876(2)
7.835(4)
27.063(4)
90
11.673(3)
19.663(3)
13.427(2)
90
10.4960(10)
10.5080(10)
14.544(2)
74.940(10)
72.290(10)
63.300(10)
1350.8(3)
2
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
90
90
107.553(9)
90
113.620(17)
90
3
˚
U (A )
Z
2053.8(5)
4
1.773
3411.8(16)
8
1.209
2823.9(10)
4
1.319
Dcalc (Mg mꢀ3
)
1.127
0.154
490
m(Mo Ka) (mmꢀ1
F(0 0 0)
)
3.057
1032
0.059
1296
1.148
1132
Crystal size (mm)
0.28ꢃ
/
0.32ꢃ
/
0.36
0.14ꢃ/0.24ꢃ/0.76
0.08ꢃ/0.12ꢃ/0.42
0.16ꢃ
/
0.36ꢃ
/
0.42
Transmission factors
Data collected
Ind. data, n
0.75ꢂ0.83
2472
1867
/
ꢂ
/
0.75ꢂ0.99
6117
4971
/
0.875ꢂ
/
0.902
3846
2981
0.0314
227
5591
4742
0.0445
346
Rint
0.0321
138
0.0672
329
No. variables, p
R, wR2, (all data)
S (all data)
0.0519, 0.0886
1.023
0.0791, 0.1859
1.115
0.1462, 0.1469
1.012
0.0935, 0.1751
1.052
ꢀ3
˚
Emax, Emin (e A
)
0.863, ꢀ
/
0.499
0.369, ꢀ
/0.258
0.834, ꢀ
/
0.550
0.499, ꢀ
/1.055
a
Rꢄ
/
ajjFojꢀ
/
jFcj/ajFojj, wR2ꢄ
/
[a[w(Fo2ꢀ
/
Fc2)2]/aw(Fo2)2]1/2], (where
w
ꢀ1ꢄ
/
[s2c(Fo)2ꢁ(aP)2ꢁ
/
/
bP] and Pꢄ
/
[0.333 (Fo)2ꢁ
/
0.667(Fc)2]), Sꢄ
/
[a[w(Fo2ꢀ
/
Fc2)2(nꢀ
/
p)]1/2, (where n is the number of data and p the number of parameters).
halogenated. The Lewis acid-catalysed iodination of 1,2-
closo-1,2-C2B10H12 with elemental I2 affords, succes-
spectrum, identifying it as arising from the labelled
vertices B9 and B12.
A crystallographic study of 1 confirmed its identity.
Fig. 2 shows a perspective view of a single molecule and
Table 2 lists selected interatomic distances and inter-
sively, 9-I-1,2-closo-C2B10H11 [15ꢂ17] and 9,12-I2-1,2-
/
closo-C2B10H10 [17,18], whilst an analogous reaction
using the more strongly iodinating agent ICl also yields
the di-iodo derivative in high yield but more quickly
[18].
Iodination of 1,2-Ph2-1,2-closo-C2B10H10 is more
difficult to achieve because of the electron withdrawing
influence of the phenyl groups. We [9] achieved a 70%
yield of 1,2-Ph2-9-I-1,2-closo-C2B10H9 by iodination
(using 0.5 equiv. of I2) catalysed by strong mineral
acid, a method first reported by Vasil’eva and Khalfina
[19]. The same procedure, but with 2.5 equiv. of I2, gave
only relatively small amounts of the di-iodo compound
1,2-Ph2-9,12-I2-1,2-closo-C2B10H8, 1, (as assessed by 11
B
NMR spectroscopy) but using 5 equiv. of I2 afforded 1
in nearly 50% yield.
Compound 1 was characterised by microanalysis and
IR and NMR spectroscopies. Consistent with the
expected C2v molecular symmetry, there are only four
resonances in the 11B{1H} NMR spectrum, with relative
integrals 1:2:1:1 from high frequency to low frequency.
Of these, only the lowest frequency resonance, at
Fig. 2. Perspective view of compound 1 nearly perpendicular to the
crystallographically-imposed mirror plane. Thermal ellipsoids are
drawn at the 50% probability level, except for H atoms.
approximately ꢀ
10 ppm, remains a singlet in the 11B
/

S. Robertson et al. / Polyhedron 22 (2003) 1293ꢂ
/
1301
1297
Table 2
carborane to [7,8-nido-C2B9]2ꢀ carborane followed by
reaction with RBX2 (Scheme 2) whereupon the incom-
ing B atoms is pre-labelled with group R. Both
decapitation [24] and recapitation [25] were first de-
scribed by Hawthorne et al. and have become standard
procedures. We [26] have recently used the approach to
˚
Selected interatomic distances (A) and interbond angles (8) for 1
Distances
C(1)ꢀ
C(1)ꢀ
C(1)ꢀ
B(3)ꢀ
B(4)ꢀ
B(5)ꢀ
B(5)ꢀ
B(8)ꢀ
B(9)ꢀ
B(9)ꢀ
/
C(1A)
1.733(8)
1.713(6)
1.750(7)
C(1)ꢀ
C(1)ꢀ
B(3)ꢀ
/
B(3)
B(5)
1.746(7)
1.708(7)
1.783(7)
1.780(7)
1.787(7)
1.778(7)
1.752(11)
1.751(10)
1.499(6)
/B(4)
/
/B(6)
/B(4)
/
B(8)
B(8)
B(6)
B(10)
B(9)
B(10)
I(9)
1.786(10) B(4)ꢀ
/B(5)
prepare 1-R’-3-R-1,2-closo-C2B10H10 species (R’ꢄ
/Me,
/
1.793(6)
1.773(7)
1.773(7)
1.783(9)
1.797(9)
2.175(5)
B(4)ꢀ
B(5)ꢀ
B(6)ꢀ
B(9)ꢀ
C(1)ꢀ
/
B(9)
Ph; RꢄBr, I) and subsequently shown [27] that these
/
/
/
B(9)
compounds can be decapitated with selective loss of the
B6 vertex, as required to produce a 3-labelled nido
species (vide infra).
Recapitation of [7,8-Ph2-7,8-nido-C2B9H9]2ꢀ (pro-
duced in situ from heating to reflux [HNMe3][7,8-Ph2-
7,8-nido-C2B9H10] and 2 equiv. of n-BuLi in Et2O) with
{BMe2ꢁ}, {BEt2ꢁ} and {BF2ꢁ} fragments afforded by
/
/
B(10)
/
/
B(9A)
/
/
C(11)
/
Angles
C(11)ꢀ
C(11)ꢀ
C(11)ꢀ
I(9)ꢀB(9)ꢀ
I(9)ꢀB(9)ꢀ
/
C(1)ꢀ
C(1)ꢀ
C(1)ꢀ
/
C(1A)
117.8(2)
122.8(4)
117.0(4)
122.4(3)
118.2(3)
C(11)ꢀ
C(11)ꢀ
I(9)ꢀB(9)ꢀ
I(9)ꢀB(9)ꢀ
I(9)ꢀB(9)ꢀ
/
C(1)ꢀ
C(1)ꢀ
/
B(3)
B(5)
118.3(4)
122.3(4)
125.54(14)
118.2(3)
122.4(4)
/
/B(4)
/
/
/
/B(6)
/
/
B(9A)
BMeBr2, BEtBr2 and BF3×/Et2O, respectively, gave the
/
/
B(8)
/
/
B(4)
new 3-labelled diphenylcarboranes 1,2-Ph2-3-Me-1,2-
closo-C2B10H9, 2, 1,2-Ph2-3-Et-1,2-closo-C2B10H9, 3,
and 1,2-Ph2-3-F-1,2-closo-C2B10H9, 4, as colourless
solids. Yields of 3 and 4 are reasonable, but that of 2
is relatively poor, which we attribute largely to the
difficulty of handling BMeBr2 relative to the other
borane reagents.
/
/
B(5)
/
/
B(10)
bond angles. Compound 1 resides on a crystallographic
mirror plane through atoms B(3), B(6), B(8) and B(10).
˚
C(1A) distance in 1 is 1.733(8) A, marginally
The C(1)ꢀ
/
longer than that [C(1)ꢀ
/
C(2)] in 1,2-Ph2-1,2-closo-
˚
C2B10H10 [20] [1.727(6) A average for two crystal-
lographically-independent molecules] and 1,2-Ph2-9-I-
Compounds 2ꢂ4 were characterised by microanalysis,
/
IR and NMR spectroscopy, and, for 3, mass spectro-
1
metry. The H NMR spectra of all three compounds
˚
1,2-closo-C2B10H9 [9] [1.724(4) A]. The conformation of
the phenyl groups in 1,2-Ph2-1,2-closo-C2B10 species is
conveniently described by uPh, defined [21] as the
modulus of the average Ccage-Ccage-CPh-CPh torsion
angles. In 1 uPh is low, 4.18, with the crystallographic
mirror plane requiring that the rings are (slightly)
twisted from 08 in a disrotatory fashion. Ab initio
molecular orbital (MO) calculations on 1-Ph-1,2-closo-
C2B10H11 [22] have shown that conformations with low
values of uPh are very similar in energy, whilst semi-
empirical MO calculations on 1,2-Ph2-1,2-closo-
C2B10H10 [20], for which the average measured uPh is
only 5.58, suggest that conrotation of the phenyl rings by
up to 408 can be tolerated without significant destabi-
show the expected resonances arising from aromatic
and, for 2 and 3, alkyl protons. For 3 the signal arising
from the ꢀ
/
CH2ꢀ
/
group is a clear quartet at 400 MHz
but an apparent broad triplet at 200 MHz as a result of
the adjacency of the boron cage. The 11B{1H} spectra at
128 MHz show evidence of signal overlap, with only five
(1:2:3:3:1, high frequency to low), four (1:2:6:1) and five
(1:2:2:4:1) resonances, respectively, visible between ꢁ
/
5
and ꢀ10 ppm (ꢁ5 and ꢀ15 for 4). However, in all three
/
/
/
cases, the highest frequency signal is assigned to B3 as it
alone does not show additional coupling in the 11B
spectrum. For 2 and 3 the highest frequency 11B{1H}
signal is a singlet, but for 4 it is a broad doublet due to
1
lisation from Ph
/
ꢀ ꢀ ꢀ
/
Ph crowding.
Bꢀ
/
F coupling with JBF measured as 43 Hz. The 19F
˚
The BꢀI distance in 1 is 2.175(5) A, in excellent
/
NMR spectrum of 4 shows the expected 1:1:1:1 quartet
with 1JBF measured as 55 Hz, the apparent difference in
coupling constants being ascribed to poor resolution
arising from signal broadening.
˚
agreement with that [2.178(4) A] in 1,2-Ph2-9-I-1,2-
closo-C2B10H9 although longer than that [average
˚
2.153(14) A] in 9,10-I2-1,7-closo-C2B10H10 [18] where
the I-labelled B atoms are not antipodal to C. In both
1,2-closo-C2B10 compounds the B-I vector is not ideally
radial to the polyhedron but inclined somewhat towards
A diffraction study of a single crystal of 2 (Fig. 3 and
Table 3) confirmed its identity. The methyl-labelled
boron atom has recapitated the nido-carborane and
occupies vertex 3 (closo numbering scheme) adjacent to
both cage C atoms. The molecule has effective Cs
symmetry about a plane passing through B(3), B(6),
B(8) and B(10), but this is not crystallographically-
imposed. The steric influence of the methyl label has
caused the Ph substituents to re-orient in a disrotatory
manner relative to their conformation in 1,2-Ph2-1,2-
closo-C2B10H10 [20], as they now subtend uPh angles of
21.68 [ring on C(1)] and 21.78 [ring on C(2)]. This
the B(4)ꢀ
/
B(5) connectivity, as evidenced by inspection
of the IꢀBꢀ
/
/X angles (Table 2 for 1).
Whilst electrophilic substitution is an ideal way of
labelling a 1,2-closo-C2B10 carborane at positions 9 and
12, reflecting the relatively high negative charge at these
sites [23], an alternative strategy must be adopted for
labelling positions 3 and 6, the most positive boron
atoms. The approach here is one of recapitation, i.e.,
selective degradation (decapitation) of 1,2-closo-C2B10

1298
S. Robertson et al. / Polyhedron 22 (2003) 1293ꢂ1301
/
Scheme 2. Generalised decapitation/recapitation scheme affording 3-labelled closo-carboranes. Subsequent decapitation of the labelled carborane
occurs preferentially at position 6.
apparently has no influence on the C(1)ꢀ
/
C(2) distance
˚
in 2, 1.734(3) A, identical to that in 1,2-Ph2-1,2-closo-
C2B10H10.
3.2. Labelled nido-carboranes
The utility of labelled diphenylcarboranes such as 1ꢂ4
/
in helping to establish an experimental mapping of the
mechanism of (hetero)carborane isomerisation depends
on retention of the labelled vertex when the closo-
carborane is decapitated to the 7,8-Ph2-7,8-nido-C2B9
anion prior to metallation. It is well established [24] that
nucleophilic attack of 1,2-closo-C2B10 carboranes by
heating to reflux with ethanolic hydroxide results in
Fig. 3. Perspective view of compound 2. Thermal ellipsoids are drawn
at the 50% probability level, except for H atoms.
decapitation of the B3 [ꢁ
1 and its analogue 1,2-Ph2-9-I-1,2-closo-C2B10H9, the
only potential problem is that the BꢀI bond could be
broken under such conditions. For compounds 2ꢂ4 a
/
B6] vertex. Thus in compound
/
/
further problem could be that the substituted vertex is
that which is lost, although recent results obtained in
collaboration with Teixidor and co-workers [27] show
Table 3
˚
Selected interatomic distances (A) and interbond angles (8) for 2
that compounds 1-R’-3-R-1,2-closo-C2B10H10 (R’ꢄ
Me, Ph; RꢄBr, I) selectively decapitate at vertex 6,
leaving the labelled boron atom intact.
/
Distances
C(1)ꢀ
C(1)ꢀ
C(1)ꢀ
C(2)ꢀ
C(2)ꢀ
B(3)ꢀ
B(4)ꢀ
B(4)ꢀ
B(5)ꢀ
B(6)ꢀ
B(7)ꢀ
B(7)ꢀ
B(8)ꢀ
B(9)ꢀ
/
C(2)
B(4)
B(6)
B(11)
B(3)
1.734(3)
1.713(3)
1.736(3)
1.719(3)
1.770(3)
1.781(4)
1.776(4)
1.779(3)
1.781(4)
1.766(4)
1.775(3)
1.774(4)
1.783(4)
1.787(4)
1.799(4)
1.509(3)
1.583(3)
C(1)ꢀ
C(1)ꢀ
C(2)ꢀ
C(2)ꢀ
B(3)ꢀ
B(3)ꢀ
B(4)ꢀ
B(5)ꢀ
B(5)ꢀ
B(6)ꢀ
B(7)ꢀ
B(8)ꢀ
B(9)ꢀ
/
B(3)
B(5)
B(6)
B(7)
1.770(3)
1.717(3)
1.743(3)
1.714(3)
1.804(4)
1.796(4)
1.775(4)
1.774(4)
1.778(4)
1.776(3)
1.775(4)
1.791(4)
1.794(4)
1.786(4)
1.782(4)
1.510(3)
/
/
/
/
/
Because it is singly labelled, the mono-iodo diphenyl-
carborane 1,2-Ph2-9-I-1,2-closo-C2B10H9 [9] is poten-
tially more useful than 1 in mechanistic studies. Heating
to reflux an ethanolic solution of 1,2-Ph2-9-I-1,2-closo-
C2B10H9 with 2.5 equiv. of OHꢀ converted the closo-
carborane to [5-I-7,8-Ph2-7,8-nido-C2B9H9]ꢀ, isolated
as its [HNEt3]ꢁ, 5a, [HNMe3]ꢁ, 5b, and
[C6H5CH2NMe3]ꢁ, 5c, salts. The salt 5 was charac-
/
/
/
/B(7)
/
B(8)
/B(4)
/
B(8)
/B(9)
/
B(5)
/B(6)
/
B(9)
/
B(10)
B(11)
B(12)
B(9)
/
B(10)
B(11)
B(8)
/
/
/
/
/
1
/
B(12)
B(12)
/B(10)
terised by microanalysis, IR spectroscopy, and H and
11B NMR spectroscopy. The 11B{1H} spectrum of 5a
reveals eight resonances (one accidental co-incidence)
/
B(10)ꢀ
B(11)ꢀ
C(2)ꢀ
/B(11)
B(10)ꢀ
C(1)ꢀC(11)
B(3)ꢀC(3)
/
B(12)
/B(12)
/
/
C(21)
between ꢀ
species [28]. The third lowest frequency resonance
(ꢀ
20.9 ppm) remains a singlet in the 11B spectrum
/
4 and ꢀ31 ppm, typical for a 7,8-nido-C2B9
/
/
Angles
C(11)ꢀ
C(11)ꢀ
C(11)ꢀ
C(21)ꢀ
C(21)ꢀ
C(3)ꢀB(3)ꢀ
C(3)ꢀB(3)ꢀ
C(3)ꢀB(3)ꢀ
/
/
C(1)ꢀ
C(1)ꢀ
C(1)ꢀ
C(2)ꢀ
C(2)ꢀ
/
C(2)
B(4)
B(6)
B(3)
B(11)
121.38(18) C(11)ꢀ
119.76(18) C(11)ꢀ
119.44(17) C(21)ꢀ
118.21(18) C(21)ꢀ
120.06(17) C(21)ꢀ
123.30(19) C(3)ꢀB(3)ꢀ
B(3)ꢀ
/
C(1)ꢀ
C(1)ꢀ
C(2)ꢀ
C(2)ꢀ
C(2)ꢀ
/
B(3)
B(5)
C(1)
B(7)
B(6)
117.86(18)
119.74(18)
121.11(17)
120.36(18)
119.08(17)
123.01(19)
125.47(18)
and is thus identified as arising from the I-labelled
boron atom.
/
/
/
/
/
/
/
/
/
/
/
/
Salt 5c gave crystals suitable for crystallographic
study, the results of which are shown in Fig. 4 (view
of the anion only) and listed in Table 4. In the anion
there is limited disorder, with vertices 3 and 12 each 50%
occupied by {BH} to render the anion non-imposed Cs
/
/
/
/
/
/
C(1)
B(7)
B(4)
/
/
C(2)
/
/
123.6(2)
123.9(2)
C(3)ꢀ
/
/
B(8)
/
/

S. Robertson et al. / Polyhedron 22 (2003) 1293ꢂ
/
1301
1299
ꢀ
˚
analogue [7,8-Ph2-7,8-nido-C2B9H10] , 1.590(5) A in
the [HNEt3]ꢁ salt and 1.602(3)
in the
˚
A
[C6H5CH2NMe3]ꢁ salt [10]; the measured uPh values,
1.18 [ring on C(7)] and 5.28 [ring on C(8)], again are fully
comparable with those in [7,8-Ph2-7,8-nido-C2B9H10]ꢀ
(average of 7.88 and 19.08, respectively).
Analogous decapitation reactions were also per-
formed on the 3-labelled closo-diphenylcarboranes 3
and 4. In both cases a single product was formed in
good to excellent yield, isolated in the former case as
both [HNMe3]ꢁ salt, 6a, and [HNEt3]ꢁ salt, 6b, and in
the latter case as the [HNMe3]ꢁ salt, 7, and charac-
terised by spectroscopic and (for 6b) crystallographic
studies.
The 1H NMR spectra of 6a and 6b at 200 MHz shows
the expected resonances for Ph and (cation) Me or Et
groups with the correct relative integrals. As with
compound 3 the signal for the methylene protons of
the (anion) Et group appear as a broad triplet. The
highest frequency resonance in the 11B{1H} NMR
spectrum remains a singlet in the 11B spectrum and is
thus assigned to the Et-labelled boron atom. Salt 6b was
also studied crystallographically. Fig. 5 shows a per-
spective view of the anion, and Table 5 hosts selected
molecular dimensions. The key result is that the closo
precursor 3 has been selectively decapitated at position
Fig. 4. Perspective view of the anion of 5c. The partially-occupied 12th
BH vertex is omitted for clarity, and, because of this partial disorder,
the H atom associated with the open face was not located. Except for
H atoms, thermal ellipsoids are drawn at the 50% probability level.
Table 4
˚
Selected interatomic distances (A) and interbond angles (8) for 5c
Distances
B(1)ꢀ
B(1)ꢀ
B(1)ꢀ
B(2)ꢀ
B(2)ꢀ
B(3)ꢀ
B(4)ꢀ
B(4)ꢀ
B(5)ꢀ
B(6)ꢀ
C(7)ꢀ
C(8)ꢀ
/
B(2)
B(4)
B(6)
B(6)
B(11)
C(7)
B(5)
B(9)
B(9)
B(10)
1.781(12) B(1)ꢀ
1.803(13) B(1)ꢀ
1.806(13) B(2)ꢀ
1.760(13) B(2)ꢀ
1.816(14) B(3)ꢀ
1.791(19) B(3)ꢀ
1.747(13) B(4)ꢀ
1.809(13) B(5)ꢀ
1.757(13) B(5)ꢀ
1.773(14) B(6)ꢀ
1.609(10) C(7)ꢀ
1.665(11) B(9)ꢀ
1.787(12) B(6)ꢀ
1.806(18) C(8)ꢀ
/
B(3)
B(5)
B(3)
C(7)
B(4)
C(8)
C(8)
B(6)
B(10)
B(11)
1.68(2)
1.773(12)
1.71(2)
/
/
/
/
/
/
1.676(12)
1.761(17)
1.849(19)
1.674(11)
1.808(13)
1.776(13)
1.769(13)
1.680(11)
1.806(13)
1.773(14)
1.81(2)
/
/
6, leaving the labelled boron 3 intact. The C(7)ꢀ
/C(8)
/
/
˚
connectivity is 1.602(3) A, essentially identical to that in
5c and [7,8-Ph2-7,8-nido-C2B9H10]ꢀ [10]. Relative to
this latter unlabelled analogue, the Ph rings in 6b are
twisted in a disrotatory fashion, subtending uPh values
of 12.98 [ring on C(7)] and 24.18 [ring on C(8)], this
asymmetry presumably a steric consequence of the
orientation of the Et substituent. In spite of partially
disordered solvent in the lattice, the crystallographic
/
/
/
/
/
/
/
/
/
C(8)
/
B(11)
B(10)
B(10)
/
B(9)
/
B(10)ꢀ
C(7)ꢀB(12)
B(9)ꢀB(12)
B(11)ꢀB(12)
/
B(11)
/
/
/
B(12)
/
1.77(2)
B(10)ꢀB(12)
/
1.746(19)
/
1.71(2)
N(1)ꢀ
N(1)ꢀ
C(93)ꢀ
C(95)ꢀ
C(97)ꢀ
/
C(90)
1.495(10) N(1)ꢀ
1.511(10) C(92)ꢀ
1.394(11) C(94)ꢀ
1.367(14) C(96)ꢀ
1.387(11) C(98)ꢀ
/C(91)
1.488(10)
1.501(10)
1.401(12)
1.350(13)
1.408(12)
/
C(92)
/
C(93)
C(95)
C(97)
C(93)
/
C(94)
C(96)
C(98)
/
/
/
/
/
Angles
I(5)ꢀB(5)ꢀ
I(5)ꢀB(5)ꢀ
I(5)ꢀB(5)ꢀ
C(71)ꢀ
C(71)ꢀ
C(71)ꢀ
C(81)ꢀ
C(81)ꢀ
/
/
B(1)
118.0(5)
122.3(5)
120.4(5)
117.4(6)
120.4(6)
115.4(8)
117.8(8)
120.4(6)
I(5)ꢀ
I(5)ꢀ
/
B(5)ꢀ
B(5)ꢀ
/
B(4)
121.0(6)
119.7(5)
/
/B(9)
/
/B(10)
/
/B(6)
/
C(7)ꢀ
C(7)ꢀ
C(7)ꢀ
C(8)ꢀ
C(8)ꢀ
/
C(8)
B(2)
B(12)
B(3)
B(9)
C(71)ꢀ
C(71)ꢀ
C(81)ꢀ
C(81)ꢀ
C(81)ꢀ
/
C(7)ꢀ
C(7)ꢀ
C(8)ꢀ
C(8)ꢀ
C(8)ꢀ
/B(3)
117.0(8)
119.7(6)
116.4(6)
121.3(6)
115.2(8)
/
/
/
/
B(11)
C(7)
B(4)
/
/
/
/
/
/
/
/
/
/
/
/B(12)
symmetry about the plane defined by C(7), C(8), B(5)
and B(6). Such disorder is not uncommon in nido-7,8-
C2B9 species and does not affect the major conclusions
of the crystallographic study; the C(7)ꢀ
/
C(8) distance,
˚
1.609(10) A, is very similar to that in the unlabelled
Fig. 5. Perspective view of the anion of 6b. Except for H atoms,
thermal ellipsoids are drawn at the 50% probability level.

1300
S. Robertson et al. / Polyhedron 22 (2003) 1293ꢂ1301
/
Table 5
3-labelled nido species [5-I-7,8-Ph2-7,8-nido-C2B9H9]ꢀ,
[3-Et-7,8-Ph2-7,8-nido-C2B9H9]ꢀ, and [3-F-7,8-Ph2-7,8-
nido-C2B9H9]ꢀ, all conveniently available as
[HNMe3]ꢁ salts. Deprotonation of these, followed by
metallation with a bulky metal-ligand fragment, should
give rise to labelled C atom isomerised metallacarbor-
anes, structural study of which will provide important
information on the mechanism of rearrangement of
(hetero)carboranes. The results of these studies will be
the subject of subsequent publications [29].
˚
Selected interatomic distances (A) and interbond angles (8) for 6b
Distances
B(1)ꢀ
B(1)ꢀ
B(1)ꢀ
B(2)ꢀ
B(2)ꢀ
B(3)ꢀ
B(4)ꢀ
B(4)ꢀ
B(5)ꢀ
B(6)ꢀ
C(7)ꢀ
C(8)ꢀ
/
B(2)
B(4)
B(6)
B(6)
B(11)
C(7)
B(5)
B(9)
B(9)
B(10)
1.773(4)
1.750(4)
1.798(4)
1.760(4)
1.784(4)
1.768(4)
1.769(4)
1.798(4)
1.774(4)
1.781(4)
1.602(3)
1.642(3)
1.815(4)
1.41(3)
B(1)ꢀ
B(1)ꢀ
B(2)ꢀ
B(2)ꢀ
B(3)ꢀ
B(3)ꢀ
B(4)ꢀ
B(5)ꢀ
B(5)ꢀ
B(6)ꢀ
C(7)ꢀ
B(9)ꢀ
B(6)ꢀ
B(10)ꢀ
N(1)ꢀC(13)
C(11)ꢀ
C(15)ꢀ
/
B(3)
B(5)
B(3)
C(7)
B(4)
C(8)
C(8)
B(6)
B(10)
B(11)
1.797(4)
1.809(4)
1.785(4)
1.724(4)
1.773(4)
1.745(4)
1.716(4)
1.816(4)
1.774(4)
1.754(4)
1.625(3)
1.837(4)
1.781(4)
1.18(3)
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
C(8)
/
B(11)
B(10)
B(10)
5. Supplementary material
/
B(9)
/
B(10)ꢀ
B(9)ꢀH(10B)
N(1)ꢀ
N(1)ꢀ
/
B(11)
/
/
/
H(10B)
Crystallographic data for the structural analyses have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC No. 197923 (1), 197924 (2), 197925
(5c) and 197926 (6b). Copies of this information may be
obtained free of charge from The Director, CCDC, 12
/
C(11)
C(15)
1.500(4)
1.498(4)
1.507(6)
/
1.520(4)
1.504(4)
1.525(5)
/
/
C(12)
C(16)
C(13)ꢀ
Angles
C(31)ꢀ
C(31)ꢀ
C(31)ꢀ
C(71)ꢀ
C(71)ꢀ
C(81)ꢀ
C(81)ꢀ
C(11)ꢀ
C(13)ꢀ
N(1)ꢀC(13)ꢀ
/C(14)
/
/
B(3)ꢀ
/
B(1)
C(7)
B(4)
125.0(2)
127.4(2)
122.0(2)
C(31)ꢀ
C(31)ꢀ
B(3)ꢀC(31)ꢀ
C(7)ꢀ
C(7)ꢀ
C(8)ꢀ
C(8)ꢀ
N(1)ꢀ
C(11)ꢀ
C(15)ꢀ
/
B(3)ꢀ
B(3)ꢀ
/B(2)
125.0(2)
124.5(2)
114.3(2)
Union Road, Cambridge CB2 1EZ, UK (fax: ꢁ44-
/
/
B(3)ꢀ
/
/
/C(8)
1233-336033; e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).
/
B(3)ꢀ
/
/
/C(32)
/
C(7)ꢀ
/
C(8)
B(2)
C(7)
B(4)
121.30(19) C(71)ꢀ
118.80(19) C(71)ꢀ
121.16(19) C(81)ꢀ
/
/
B(3)
B(11)
B(3)
B(9)
119.03(19)
115.05(19)
120.35(19)
114.85(19)
113.5(3)
/
C(7)ꢀ
/
/
/
/
C(8)ꢀ
/
/
/
/
C(8)ꢀ
/
121.2(2)
111.8(2)
112.6(3)
114.4(3)
C(81)ꢀ
C(11)ꢀ
N(1)ꢀ
N(1)ꢀ
/
/
Acknowledgements
/
N(1)ꢀ
/
C(13)
C(15)
C(14)
/
/
C(15)
C(12)
C(16)
/
N(1)ꢀ
/
/
/
113.4(3)
114.2(3)
/
/
/
/
We thank the Leverhulme Trust (D.E.) and Heriot-
Watt University (S.R., T.D.McG.) for support, Mr G.
Evans and Mrs C. Graham for microanalysis, Dr A.S.F.
Boyd for NMR spectra and Dr R. Ferguson for mass
spectra. A.J.W. acknowledges the receipt of a Royal
Society Leverhulme Trust Senior Research Fellowship,
determination of 6b is relatively precise*
atom, H(10B), was successfully located and refined, and
/
the facial H
found to bridge the B(9)ꢀ
/
B(10) edge asymmetrically,
˚
H 1.18(3) A.
˚
B(9)ꢀ
/
H 1.41(3) A, B(10)ꢀ
/
2002ꢂ2003, and the hospitality of Prof. A. Laguna and
/
Salt 7 was characterised by microanalysis, IR spectro-
1
colleagues at the Universidad de Zaragoza, Spain, where
this manuscript was written.
scopy, and H, 11B and 19F NMR spectroscopy. In the
11B{1H} spectrum the highest frequency resonance is a
1
doublet, JBF
ꢄ55 Hz, clearly arising from the labelled
/
boron atom. This coupling is mirrored in the 19F
References
1
spectrum, which reveals a 1:1:1:1 quartet, JBF
ꢄ56
/
Hz, at d ꢀ198 in CDCl3, 13 ppm to low frequency of
/
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