Sulfur, tin and gold derivatives of 1-(2′-pyridyl)-ortho-carborane, 1-R-2-X-1,2-C2B10H10 (R = 2′-pyridyl, X = SH, SnMe3 or AuPPh3)

Article abstract of DOI:10.1039/B411099D

Reaction of the lithium salt of 1-(2′-pyridyl)-ortho-carborane, Li[1-R-1,2-C₂B₁₀H₁₀] (R = 2′-NC ₅H₄), with sulfur, followed by hydrolysis, gave the mercapto-o-carborane, 1-R-2-SH-1,2-C₂B₁₀H₁₀ which forms chiral crystals containing helical chains of molecules linked by intermolecular S-H ... N hydrogen bonds. The cage C(1)-C(2) and exo C(2)-S bond lengths (1.730(3) and 1.775(2) A, respectively) are indicative of exo S=C π bonding. The tin derivative 1-R-2-SnMe₃-1,2-C ₂B₁₀H₁₀, prepared from Li[1-R-1,2-C ₂B₁₀H₁₀] and Me₃SnCl, crystallises with no significant intermolecular interactions. The pyridyl group lies in the C(1)-C(2)-Sn plane, oriented to minimise the N ... Sn distance (2.861(3) A). The tin environment is distorted trigonal bipyramidal with axial N and Me. The gold derivative 1-R-2-AuPPh₃-1,2-C₂B ₁₀H₁₀, prepared from Li[1-R-1,2-C₂B ₁₀H₁₀] and AuCl(PPh₃), reveals no N ... Au interaction in its crystal structure.

Full text of DOI:10.1039/B411099D

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F U L L P A P E R
Sulfur, tin and gold derivatives of 1-(2ꢀ-pyridyl)-ortho-carborane,
1-R-2-X-1,2-C2B10H10 (R = 2ꢀ-pyridyl, X = SH, SnMe3 or AuPPh3)
Andrei S. Batsanov,a Mark A. Fox,*a Thomas G. Hibbert,a Judith A. K. Howard,a
Raikko Kiveka¨s,b Anna Laromaine,†c Reijo Sillanpa¨a¨,d Clara Vin˜as*c and Kenneth Wadea
a Department of Chemistry, University of Durham, South Road, Durham, UK, DH1 3LE.
E-mail: m.a.fox@durham.ac.uk
b Department of Chemistry, University of Helsinki, P.O. Box 55, FIN-00014, Finland
c Institut de Cie`ncia de Materials de Barcelona (CSIC), Campus de la U.A.B., E-08193,
Bellaterra, Spain. E-mail: clara@icmab.es; Fax: + 34 93 5805729; Tel: + 34 93 5805729
d Department of Chemistry, University of Jyva¨skyla¨, FIN-40351, Finland
Received 20th July 2004, Accepted 28th September 2004
First published as an Advance Article on the web 15th October 2004
Reaction of the lithium salt of 1-(2ꢀ-pyridyl)-ortho-carborane, Li[1-R-1,2-C2B10H10] (R = 2ꢀ-NC5H4), with sulfur,
followed by hydrolysis, gave the mercapto-o-carborane, 1-R-2-SH-1,2-C2B10H10 which forms chiral crystals
containing helical chains of molecules linked by intermolecular S–H · · · N hydrogen bonds. The cage C(1)–C(2) and
˚
=
exo C(2)–S bond lengths (1.730(3) and 1.775(2) A, respectively) are indicative of exo S C p bonding. The tin
derivative 1-R-2-SnMe3-1,2-C2B10H10, prepared from Li[1-R-1,2-C2B10H10] and Me3SnCl, crystallises with no
significant intermolecular interactions. The pyridyl group lies in the C(1)–C(2)–Sn plane, oriented to minimise the
˚
N · · · Sn distance (2.861(3) A). The tin environment is distorted trigonal bipyramidal with axial N and Me. The gold
derivative 1-R-2-AuPPh3-1,2-C2B10H10, prepared from Li[1-R-1,2-C2B10H10] and AuCl(PPh3), reveals no N · · · Au
interaction in its crystal structure.
Derivatives of ortho-carborane of formula 1-R-1,2-C2B10H11,
bearing a nitrogen-containing substituent R on one cage carbon
atom, have attracted much recent attention as potential medic-
inal agents for use in boron neutron capture therapy (BNCT)1,2
and as potentially-chelating carboranyl ligand precursors.3–8
For example many metal complexes 1-R-2-MLn-1,2-C2B10H10
(with an exo-cluster C–M bond) or 1-R-2-X-3-MLn-1,2-C2B9H9
(where MLn replaces one boron vertex) have been prepared
from dialkylaminomethyl-ortho-carborane (1, R = Me or Et)4–6
(Fig. 1) or from 1-(2ꢀ-picolyl)-ortho-carborane (2).7 When a
metal atom is linked to amino or picolyl nitrogen atoms in these
derivatised carboranes, five-membered C3NM rings are formed
if a closo-C2B10 cage is retained; four-membered C2NM rings are
found in MC2B9 systems in which the MLn unit occupies a cage
site.
properties. Firstly, it contains a strong intramolecular hydrogen
bond between the pyridyl nitrogen atom and the acidic hydrogen
atom bonded to the cage carbon (C2), significantly stronger than
the comparable intramolecular hydrogen bonds in compounds
1 and 2.9 Secondly, substitution on the cage carbon (C2) of
compound 3, to form 1-(2ꢀ-pyridyl)-2-aryl-ortho-carboranes,
with aryl bromides or iodides in the presence of a copper
catalyst is facile.10 By contrast, attempted reaction of 1-phenyl-
ortho-carborane with aryl bromides or iodides in presence of
a copper catalyst did not afford 1,2-diaryl-ortho-carboranes.
The formation of 1-(2ꢀpyridyl)-2-aryl-ortho-carboranes from
3 was believed to proceed through a copper intermediate 4
(M = Cu) involving an intramolecular Cu · · · N interaction.
A copper derivative 1-R-2-Cu-1,2-C2B10H10 synthesised from
the lithio derivative of 1 (R = Et) with copper(I) chloride was
found to be air-sensitive and believed to contain a Cu · · · N
interaction.4 With 1 (R = Me) instead of 1 (R = Et), a stable
compound 2,2ꢀ-(1-R-1,2-C2B10H10)2Cu was obtained with two
Cu · · · N interactions. Copper carboranes are apparently more
stable when two or more cage C–Cu bonds are present.11,12
The only crystal structures of ortho-carboranes with cage C–
Cu single bonds are found in salts of [{(C2B10H10)2}2Cu]− and
We have previously reported the syntheses of 1-(2ꢀ-pyridyl)-
ortho-carborane 3 which resembles 1 in the proximity of its
nitrogen atom to the cage.9,10 Compound 3 has two notable
[{(C2B10H10)2}2Cu]2− where the copper atom is bonded to four
12
˚
cage carbons with C–Cu bond lengths between 2.01 and 2.07 A.
Here, we report the synthesis, spectroscopic and structural
characterisation of three new derivatives 1-(2ꢀ-pyridyl)-ortho-
carborane 3. One of these is the 2-mercapto derivative (5),
which we believe to be the first derivative of ortho-carborane
containing a mercapto group –SH to be structurally charac-
terised. It was expected (rightly, in the event) to be of interest
in connection with its hydrogen bonding, and can also be
seen as a potential precursor for chelated metal complexes
containing six-membered C3NMS rings (cf . the related thiol of
1 (R = Me) which has been previously studied8). The second
derivative we describe here is compound 6, the first ortho-
carborane with a trimethylstannyl (as opposed to an organotin
halide6) substituent to be structurally characterised. It was seen
as a possible model for the copper intermediate 4 (M = Cu)
though it was appreciated that the very weak Lewis acidity and
relative bulk of the SnMe3 residue might discourage chelation
Fig. 1 Compounds 1–7 discussed in this study. Each naked cluster
vertex represents BH.
† Enrolled in the UAB Ph. D. Program.
3 8 2 2
D a l t o n T r a n s . , 2 0 0 4 , 3 8 2 2 – 3 8 2 8
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
©

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and force the pyridyl ring plane out of the C–C(1)–C(2) plane.
The third derivative is the gold compound 7, also structurally
characterised. We assess the effect of the pyridyl group on
the cage geometry by comparing with several known gold
carborane complexes of formula 1-R-2-AuL-1,2-C2B10H10 (L =
PPh3, AsPh3). Compound 7 is also a possible model for the
copper intermediate 4 (M = Cu) though it is appreciated that
gold is a weaker acceptor than copper towards pyridyl nitrogen.
in vacuo giving a tacky yellow solid. This solid was triturated
with pentane (2 × 20 cm3) and recrystallised from Et2O. Yield:
285 mg, 74%. Mp 195–196 ◦C. C10H23B10NSn requires: C, 31.4;
H, 6.1; N 3.7. Found: C, 31.4; H, 6.0; N, 3.0%. IR: m/cm−1 2986w,
2916w, 2876w (pyridyl/CH3 str.) 2597vs, 2558sh (BH) 1636m,
1591m, 1471m, 1432vs (pyridyl skel.) 1079m, 1061m, 1003m,
11
825w, 765s, br (BH wag). 1H{ B} NMR (CDCl3): d 8.15 (d, 1H,
3JHH = 5, H6ꢀ), 7.65 (d, 1H, 3JHH = 8, H4ꢀ), 7.51 (d, 1H, 3JHH = 8,
H3ꢀ), 7.25 (dd, 1H, 3JHH = 8, 3JHH = 5, H5ꢀ), 2.43 (br s, 2H, BH),
2.31 (br s, 4H, BH), 2.11 (br s, 2H, BH), 1.94 (br s, 2H, BH), 0.20
Experimental
1
(br s + d, 9H, JSnH = 54, SnCH3). 11B{ H} NMR (CDCl3): d −1.3
(d, 1JBH = 145, 1B), −1.7 (d, 1JBH = 155, 1B), −5.9 (d, 1JBH = 149,
All manipulations were carried out under dry, oxygen-free
N2. Commercial grade acetonitrile, pentane, n-butyllithium
in hexanes and resublimed sulfur were used without further
purification. Dry Et2O and THF were obtained by reflux and
distillation over Na wire. Demineralised water was used in the
aqueous stages of syntheses. Compound 3 was prepared by the
literature methods.7,9
1
1
2B), −8.9 (d, 4B), −10.1 (d, JBH = 156, 2B); 13C{ H} NMR
(C6D6): d 151.8 (C2ꢀ), 145.9 (C6ꢀ), 137.5 (C4ꢀ), 123.9 (C5ꢀ), 122.5
(C3ꢀ), 77.3 (C1), 67.5 (C2), −3.2 (CH3); 119Sn NMR (CDCl3): d
21.9.
Preparation of 1-(2ꢀ-pyridyl)-2-(AuPPh3)-ortho-carborane (7)
Infrared spectra were recorded as KBr discs using a Perkin
Elmer 1720X FTIR spectrometer. Elemental analyses were
performed using Exeter Analytical CE-440 apparatus. 1H,
11B, 31P, 13C and 119Sn NMR spectra were recorded as room
temperature solutions on Varian Unity 300 MHz spectrometer
equipped with the appropriate decoupling accessories. Chemical
shift values for 11B NMR spectra were referenced to external
To a stirring solution of [1-(2ꢀ-pyridyl)-1,2-C2B10H11] (15.5 mg,
0.07 mmol) in 4 ml of dry diethyl ether at 0 ◦C, was added drop-
wise, 0.044 ml of a 1.6 M of n-BuLi (0.07 mmol). After addition
the reaction mixture was stirred at 0 ◦C for 30 min then at
ambient temperature for 30 min, then 35 mg of AuCl(PPh3)
(0.07 mmol) was added. After stirring for 30 min, the solid
was filtered off and recrystallized from chloroform and hexane,
to obtain an orange solid in 85% yield (40 mg, 0.059 mmol).
C25H29B10NPAu requires C, 44.2, H, 4.3, N, 2.1. Found: C, 44.0;
1
1
11
1
BF3·OEt2, those for H, H{ B} and 13C{ H} NMR spectra
were referenced to SiMe4, and for the 119Sn NMR spectrum
1
was referenced to SnMe4. H NMR spectra were referenced to
11
H, 4.3, N, 2.1%. H{ B} NMR (CDCl3): 8.06–7.10 (m, 19H,
residual protio impurity in the solvent (CDCl3, 7.26 ppm). 13C
NMR spectra were referenced to the solvent resonance (CDCl3,
77.0 ppm; (CD3)2CO, 30.0 ppm; C6D6, 128.0 ppm). 31P NMR
spectrum was referenced to external 85% H3PO4. Chemical shifts
are reported in units of parts per million downfield from the
reference, and all coupling constants are reported in Hertz.
PPh3, C5H4N), 2.83 (br s, 2H, B–H), 2.37 (br s, 8H, B–H).
1
11B NMR (CDCl3): d −3.5 (d, JBH = 119, 2B), −9.3 (m, 8B).
1
13C{ H} NMR (CDCl3): d 147.8 (s, C2ꢀ), 136.3 (s, C6ꢀ), 134.1 (d,
3
2J(C, P) = 12, CPPh ), 131.8 (d, J(C, P) = 29, CPPh3 ), 130.0 (s,
3
C4ꢀ), 129.1 (d, 4J(C, P) = 8, CPPh3 ), 123.5 (s, C5ꢀ), 123.3 (s, C3ꢀ).
31P NMR (CDCl3): d 36.4 (s, PPh3).
Preparation of 1-(2ꢀ-pyridyl)-2-mercapto-ortho-carborane (5)
X-Ray crystallography
To a solution of 3 (117 mg, 0.53 mmol) in dry Et2O (20 cm3) at
0 ◦C was added 1.6 M n-BuLi in hexanes (0.35 cm3, 0.56 mmol).
After stirring for ca. 20 min resublimed sulfur (51 mg, 1.59 mmol,
200% excess) was added and the reaction mixture was stirred at
room temperature for 24 h. Water (20 cm3) was then added and
stirring continued for 1 h. Unreacted sulfur was then removed
by filtration and the Et2O solution was sequentially extracted
with 1 M HCl solution (3 × 40 cm3) and water (3 × 40 cm3). The
combined aqueous fractions were extracted with Et2O (40 cm3)
and the recombined Et2O fractions were then dried over MgSO4.
Filtration, followed by removal of the Et2O in vacuo gave 5 as a
white powder, recrystallised from acetone. Yield: 114 mg, 85%.
C7H15B10NS requires: N, 5.5; C, 33.2; H, 6.0; S, 12.7. Found: N,
5.3; C, 33.4; H, 6.2; S, 12.4%. IR: m/cm−1 2935 (Caryl–H), 2644
Single crystals of 5 and 7 (colourless) were grown from acetone,
those of 6 (pale yellow) from Et2O, at room temperature. X-
ray experiments for 5–7 were carried out at low temperatures,
using Cryostream (Oxford Cryosystems) open-flow N2 gas
cryostats. Compound 5 was studied also at room temperature;
this structure (5a) is practically identical with the (more precise)
low-temperature one (5), which is referred to in the discussion.
Diffraction data were collected on an Enraf Nonius Kappa
diffractometer (for 5 and 7) or Bruker SMART three-circle
diffractometer (for 5a and 6) equipped with CCD area detectors.
¯
˚
Graphite-monochromated Mo-Ka radiation (k = 0.71073 A)
was used.
Reflection intensities were corrected for absorption by nu-
merical integration (based on crystal face-indexing) for 6 and
by the empirical method for 7. All structures were solved by
direct methods; carbon and boron atoms of the carborane
cage could be reliably distinguished by the bond distances and
electron density concentration. All non-hydrogen atoms were
refined with anisotropic displacement parameters and H atoms
‘riding’ in idealised positions (except the H atom bonded to S in
5 and 5a, which was refined in isotropic approximation), by full-
matrix least squares against F2 of all reflections, using SHELXL
programs.13 The absolute configuration of 5 was determined by
refinement of Flack x parameter14 converging at 0.02(8). The
crystal data and experimental details are listed in Table 1.
CCDC reference numbers 245430 (5), 239447 (5a), 239448 (6)
and 245431 (7).
=
(S–H); 2596, 2584, 2572 (B–H); 1585 (C N); 1463, 1433 (C–N);
885 (S–H); 740, 725 (Caryl–H). H{ B} NMR (CDCl3): d 8.68
1
11
(d, 1H, 3JHH = 4.4, H6ꢀ), 7.80 (m, 2H, H3ꢀ, H4ꢀ), 7.43 (dd, 1H,
3JHH = 4.4, JHH = 1.4, H5ꢀ), 3.77 (s, SH, 1H), 2.95 (br s, BH,
4
2H), 2.61 (br s, BH, 2H), 2.52 (br s, BH, 3H), 2.29 (br s, BH,
1H), 2.21 (br s, BH, 2H). 11B NMR (CDCl3): d −2.0 (d, 1JBH
=
1
1
164, 1B), −4.3 (d, JBH = 134, 1B), −7.8 (d, JBH = 177, 2B),
−9.5 (d, JBH = 187, 4B), −10.0 (d, JBH = 142, 2B). 13C{ H}
NMR (d6-acetone): d 149.5 (C2ꢀ), 148.6 (C6ꢀ), 137.7 (C4ꢀ), 126.1
(C3ꢀ), 125.5 (C5ꢀ), 85.8 (C1), 77.6 (C2).
1
1
1
Preparation of 1-(2ꢀ-pyridyl)-2-(trimethylstannyl)-ortho-
carborane (6)
See http://www.rsc.org/suppdata/dt/b4/b411099d/ for cry-
stallographic data in CIF or other electronic format.
To a solution of 1 (221 mg, 1.00 mmol) in dry THF (30 cm3)
◦
at 0 C was added 2.42 M n-BuLi in hexanes (0.42 cm3,
1.02 mmol). After stirring at 0 ◦C for 30 min Me3SnCl (200 mg,
1.00 mmol) was added and the reaction mixture was stirred for
a further hour, and allowed to warm to room temperature. The
nascent LiCl was removed by filtration and the THF removed
Computational section
The ab initio computations were carried out with the Gaussian 98
package.15 The two minima of 5 discussed here were optimised at
D a l t o n T r a n s . , 2 0 0 4 , 3 8 2 2 – 3 8 2 8
3 8 2 3

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Table 1 Crystal data
Compound
5
6
7
Formula
M
T/K
C7H15B10NS
253.36
173
C10H23B10NSn
384.08
150
C25H29B10NPAu
679.53
173
Crystal system
Space group
Monoclinic
P21 (no. 4)
8.3207(2)
8.1789(2)
10.5083(2)
109.596(1)
673.71(3)
2
Monoclinic
P21/c (no. 14)
10.009(1)
13.544(1)
13.516(1)
107.28(1)
1749.6(3)
4
Monoclinic
P21/c (no. 14)
12.6732(3)
15.8053(6)
14.3786(5)
103.159(2)
2804.46(16)
4
˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
U/A
Z
Dc/g cm−3
1.249
0.21
1.458
1.45
1.609
5.32
l/mm−1
Reflections measured
Unique reflections
Rint
2547
2547
—
0.036
8408
3149
0.043
0.031
14188
4914
0.036
0.047
R [F2 ≥ 2r(F2)]
wR (F2), all data
0.087
0.074
0.049
the HF/6–31G* level with no symmetry constraints. Frequency
calculations were computed on these optimised geometries at
the HF/6–31G* level and revealed no imaginary frequencies.
Optimisation of these geometries were then carried out at the
MP2/6–31G* level. Selected parameters for minimum (total
energy at −974.79590 au) of similar geometry to experimen-
with those reported for 1-phenyl-2-(triphenylphosphine)gold-
ortho-carborane19 indicates little Au · · · N interaction is present
in solutions of 7. The 31P chemical shifts are 36.4 ppm and
38.6 ppm respectively.
Structural aspects
◦
˚
tal: C(1)–C(2) 1.692 A, N–C(12)–C(1)–C(2) −70.7 , H(2)–S–
Compound 5 crystallises in a chiral space group P21, hence
it deserves checking for non-linear optical (NLO) properties,
for which a non-centrosymmetric structure is a prerequisite.20
Previous studies of carborane derivatives for NLO purposes
have been reported.21 The crystal structure of 5 shows no
intramolecular S–H · · · N interactions; the torsion angle C(2)–
C(1)–C(12)–N is 96.4(2)◦. Instead, the molecules (Fig. 2) are
linked by S–H · · · N hydrogen bonds into helices, spiralling
around a 21 screw axis (Fig. 3).
◦
˚
˚
C(2)–C(1) 81.2 , N · · · H 3.056 A, for less stable minimum
◦
(−974.79491 au) C(1)–C(2) 1.697 A, N–C(12)–C(1)–C(2) 77.2 ,
H(2)–S–C(2)–C(1) 106.6◦. The shorter C(1)–C(2) bond lengths
in these optimised geometries compared to experimental are
partly due to the different orientations of the SH and pyridyl
groups.26,28
Results and discussion
Synthetic and spectroscopic aspects
1-(2ꢀ-Pyridyl)-2-mercapto-ortho-carborane (5) was prepared
from compound 3: the second cage carbon atom of the parent
pyridyl carborane was lithiated using butyllithium, sulfur was
inserted into the carbon–lithium bond by reaction with elemen-
tal sulfur, and the thiol liberated by working up the product
with aqueous acid. The IR spectrum of 5 as a KBr disc has
an absorption at 2644 cm−1, consistent with the presence of a
hydrogen bonded S–H moiety.16
Compound 6, 1-(2ꢀ-pyridyl)-2-(trimethylstannyl)-ortho-
carborane, was also prepared from compound 3 via the lithio
derivative. With trimethyltin chloride, this lithio derivative
afforded 6 and lithium chloride. The tin atom in compound
6 has a tetrahedral arrangement in solution, indicated by the
2
119Sn chemical shift of 21.9 and the J(119Sn–C1H3) coupling
constant of 54 Hz in chloroform.6,17 A related compound, 1-
phenyl-2-(trimethylstannyl)-ortho-carborane (i.e. with a phenyl
2
group in place of the pyridyl group in 6), has a J(119Sn–C1H3)
coupling constant of 58 Hz in chloroform.18 A low-temperature
◦
1
(−60 C in CD2Cl2 solution) H NMR study of 6 was carried
out to explore whether the Sn · · · N interaction might be
strong enough to lock the trimethyltin residue in a particular
orientation at lower temperatures, so rendering the tin-attached
methyl groups inequivalent. However, these methyl groups
remained equivalent, consistent with free rotation of the SnMe3
group about the exo-cluster C–Sn bond, implying that any
Sn · · · N interactions are very weak, as expected in view of the
low Lewis acidity of the SnMe3 residue.
Fig.
2
Molecular structure of 1-(2ꢀ-pyridyl)-2-mercapto-ortho-
◦
˚
carborane 5. Selected bond distances (A) and torsion angles ( ):
C(1)–C(2) 1.730(3), C(1)–C(12) 1.507(3), C(2)–S 1.775(2); N–C(12)–
C(1)–C(2) 96.4(2), H(2)–S–C(2)–C(1) −99(2).
Compound 7, 1-(2ꢀ-pyridyl)-2-(triphenylphosphine)gold-
ortho-carborane, was also prepared from the lithio derivative of
compound 3. With AuCl(PPh3), this lithio derivative afforded
7 and lithium chloride. Comparison of the NMR data for 7
Within the helices, the hydrogen-bonded S · · · N distance
◦
˚
of 3.445(2) A and the S–H · · · N angle of 152(2) lie within
◦
˚
the ranges of 3.4–3.7 A and 140–152 typical of S–H · · · N
3 8 2 4
D a l t o n T r a n s . , 2 0 0 4 , 3 8 2 2 – 3 8 2 8

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the overall energy of an aryl-carborane may vary only slightly
with the aryl group orientation, the latter does have a perceptible
influence on the C(1)–C(2) bond distance, which is greatest
when the aryl group is aligned perpendicular to the aryl C–
C(1)–C(2) plane. This is because this orientation optimises
transfer of electronic charge from the filled p orbitals of the
aryl group into a cage LUMO that is r-antibonding with respect
to the cage bond C(1)–C(2). Similar calculations on hydroxyl,
amino or thiolato derivatives of ortho-carborane show that such
derivatives also experience C(1)–C(2) bond elongation as the
substituent orientation changes from coplanar to perpendicular,
so increasing the capacity for dative p-bonding from what would
otherwise be a lone pair p orbital on the exo-atom.28 The sulfur
‘lone-pair’ orbital of the SH group has a similar influence on the
C(1)–C(2) bond to the p orbitals of the phenyl group. Molecular
orbital computations on the crystal structure of 1,2-(SPh)2-1,2-
C2B10H10 indicate the transfer of electron density from the lone
pairs at the sulfur atoms to the cage as mainly responsible for
˚
its long C(1)–C(2) bond length (1.798(3) A) perhaps reinforced
by the lone-pair repulsion between the two neighbouring sulfur
atoms.29,30
These exo-dative p-bonding effects are believed to be respon-
sible for the length of the cage C(1)–C(2) bond in compound 5,
˚
˚
which at 1.730(3) A is ca 0.1 A longer than its counterpart in
pyridyl-ortho-carborane 3. Even longer cage C(1)–C(2) bonds
have been found in anionic thiolate compounds [1-R-2-S-1,2-
C2B10H10]− (R = Ph31 or Me32) in which the absence of the proton
on sulfur allows even stronger S–C(2) exo dative p-bonding than
in 5. Relevant data are listed in Table 2, which shows how the
cage C(1)–C(2) bond lengthens as the exo C(2)–S bond shortens.
The data in Table 2 also show that the cage bond-lengthening
effect of a pyridyl or phenyl group or a thiolate residue at C(1)
is far less than that of a pyridyl or phenyl group at C(1) and
a thiolate residue at C(2).9,33–35 It has to be pointed out that,
unlike in the disubstituted carboranes, the groups at C(1) of
the monosubstituted derivatives are not orientated to maximise
exo-dative p-bonding.
Fig. 3 Intermolecular hydrogen bonds in the crystal of 5. Hydrogen
◦
˚
bond distances (A) and angles ( ), S–H 1.27(3), H · · · N 2.17(3), S · · · N
3.445(2), S–H · · · N 152(2).
systems. It is interesting that the S–H · · · N hydrogen bonding
in 5 is exclusively intermolecular, in marked contrast to the
exclusively intramolecular C–H · · · N hydrogen bonding in the
parent 1-(2ꢀ-pyridyl)-ortho-carborane 3. In crystalline 3, the
pyridine ring is locked in an orientation coplanar with the
C(1)–C(2)–H plane by intramolecular C(2)–H · · · N hydrogen
bonding, involving the protic hydrogen on C(2). A similar
orientation of the pyridine ring in 5 might have permitted S–
H · · · N hydrogen bonding within a six-membered C3SH · · · N
ring in place of the five-membered C3H · · · N ring in 3. However,
there is no intramolecular S–H · · · N hydrogen bonding in the
crystals of 5, in which the pyridyl and thiol substituents are
orientated in planes roughly perpendicular to the C(12)–C(1)–
C(2)–S plane, orientations incompatible with intramolecular S–
H · · · N interactions.
The crystal structure of compound 6 (see Fig. 4) differs
markedly from that of 5 in that there are no significant
Further discussions of hydrogen bonding interactions in
organonitrogen derivatives of ortho-carborane, including the
inter- and intra-molecular N · · · H–C interactions in 2 (which
contains dimeric 1-(2ꢀ-picolyl)-1,2-C2B10H11 units in the crys-
tal) and also in an isomer of 5 (again with dimeric 1-(2ꢀ-
pyridyl)-2-SH-1,2-C2B10H10 units in the crystal) are to be found
elsewhere.9,22,23 Calculations on 5 optimised at the MP2/6–
31G* level of theory have revealed two minima, one minimum
comparable to that found experimentally and a slightly less
stable minimum (ca 0.7 kcal mol−1). The latter minimum has
the hydrogen atom at sulfur pointing away from the pyridyl
nitrogen. The experimental and computed geometries of 5
suggest intramolecular H-bonding is not favourable in 5 due
to the lack of flexibility along the S–C–C–C–N link compared
to the C–C–C–C–N link in 2.
The p orbital overlap between phenyl group and tangential
cluster orbitals has been invoked to explain the cage C–C
lengthening in aryl-ortho-carborane derivatives.24,25 This was
further explored by ab initio RHF/6–31G* and MP2/6–31G*
calculational studies on 1-phenyl-ortho-carborane and other
aryl-carboranes in order to probe the orientational prefer-
ences of aryl groups attached to the carbon atoms of ortho-
carborane.26,27 These calculations have indicated that, although
Fig.
4
Molecular structure of 1-(2ꢀ-pyridyl)-2-trimethylstannyl-
˚
ortho-carborane 6. Selected bond distances (A) and torsion an-
gles (◦): C(1)–C(2) 1.668(5), C(1)–C(12) 1.512(5), C(2)–Sn 2.207(4),
N–C(12)–C(1)–C(2) −0.4(4). Methyl hydrogens are omitted for clarity.
D a l t o n T r a n s . , 2 0 0 4 , 3 8 2 2 – 3 8 2 8
3 8 2 5

View Article Online
˚
Table 2 Bond distances (A) in ortho carborane derivatives 1-R-2-X-1,2-C2B10H10
R
X
d(C(1)–C(2))
d(C(1)–R)
d(C(2)–X)
Ref’
H
H
H
1.620(3)
1.632(3)
1.643(3)
1.649(2)
1.689(3)
1.730(3)
1.792(5)
1.836(5)
33
9
22
34
2ꢀ-Pyridyl
H
1.513(3)
S-2ꢀ-pyridyl
1.778(2)
Ph
H
1.511(2)
1.505(4)
1.507(3)
1.510(5)
1.495(5)
2ꢀ-Pyridyl
2ꢀ-Pyridyl
Me
2ꢀ-Pyridyl
1.506(3)
1.775(2)
1.735(4)
1.729(4)
35
SH
This work
32
31
S−
Ph
S−
intermolecular interactions. The pyridyl ring orientation is that
which minimises the N · · · Sn distance: the C(2)–C(1)–C(12)–N
torsion angle is zero within experimental error, and the tin atom
Of more relevance to 6, perhaps, is the crystal structure of
a dimeric carborane assembly consisting of two 1-Me2NCH2-
2-SnMe2-1,2-C2B10H10 moieties linked by a Sn–Sn bond.6 This
was made from 1, BuLi and Me2SnBr2 followed by reduction
with sodium metal. In this structure the Sn · · · N distances are
˚
lies within 0.10 A from the pyridyl ring plane. The Sn · · · N
˚
distance, 2.861(3) A, though shorter than the sum of the van der
˚
˚
Waals radii of Sn and N (3.75 A), is understandably longer than
long, averaging 3.640(8) A, and the coordination environment
˚
the sum of their covalent radii (2.15 A), as expected in view of
of the tin atom is effectively tetrahedral.
the low Lewis acidity of species Me3SnR.36 The coordination
at tin is distorted from four-coordinate tetrahedral towards
five-coordinate trigonal bipyramidal (TBP) in which the axial
positions are occupied by the nitrogen atom and a methyl
carbon atom, C(19), the N–Sn–C(19) angle being 168.1(1)◦. The
two geometries can be distinguished by the difference between
the average equatorial and average apical angles, zero for a
tetrahedron, 30◦ for an ideal TBP.37 In 6 this difference (for C–
C–C angles only), is 8.8(2)◦, i.e. closer to tetrahedron than TBP.
The most pronounced distortion is the widening to 118.0(2)◦
of the C(17)–Sn–C(18) angle, into which the pyridyl group is
‘wedged’.
The gold carborane 7 crystallises in a form where no
intermolecular interactions are detected. Both the Au · · · N
˚
distance of 3.192(3) A and the orientation of the pyridyl group
imply weak attraction between the two atoms (Fig. 5). As there
are five crystal structures in the literature19,43–45 of the formula
1-R-2-AuL-1,2-C2B10H10 (L = PPh3, AsPh3), selected data are
listed in Table 3 for comparison with 7. On close inspection
of the table, it appears that the more electron-withdrawing the
R group is, the longer the Au–C(2) bond becomes. A second
trend is the more bulky the R group is, the cage C(1)–C(2) bond
Conversely, 6 may be compared with an extensively studied
series of systems R3SnNX with a hypervalent interaction
along the N · · · Sn · · · X axis,38,39 (the path for nucleophilic
substitution of X) with Sn · · · N distances ranging from 2.37
39
˚
to 2.65 A. These additional intramolecular interactions can
have important chemical effects, such as enhanced reactivity of
Sn–C bonds40 or stabilisation toward hydrolytic decomposition41
and may afford unique synthetic routes to particular organotin
compounds.42 Most such studies have involved systems in
which X is a relatively electronegative ligand, e.g. a halogen.
Tetraorganotin compounds are very weak Lewis acids and only
one case of additional N · · · SnR4 coordination seem to have
been studied structurally, a pyrazine–trimethyltin derivative with
˚
a long Sn · · · N distance of 3.101(5) A and only very slight
˚
elongation of the Sn–CH3 bond trans to the latter (2.171 A)
compared to the two cis (pseudo-equatorial) bonds, averaging
41
˚
2.134 A. In 6 the Sn · · · N distance is shorter; however, the
˚
pseudo-apical Sn–C(19) bond (2.153(4) A) is not significantly
longer than the pseudo-equatorial bonds Sn–C(17), 2.134(4)
˚
˚
A and Sn–C(18), 2.141(4) A. The pyridine group tilts slightly
toward the tin atom, apparently due to the Sn · · · N attraction,
but the tilt is small with the B(12)–C(1)–C(12) angle in 6 being
176.6(3)◦.
Fig. 5 Molecular structure of 1-(2ꢀ-pyridyl)-2-AuPPh3-ortho-carbo-
◦
˚
rane 7. Selected bond distances (A) and torsion angles ( ); C(1)–C(2)
1.684(5), C(1)–C(12) 1.518(5), C(2)–Au 2.069(3), Au–P 2.272(1),
N–C(12)–C(1)–C(2) −64.3(4). Phenyl hydrogens are omitted for clarity.
˚
Table 3 Bond distances (A) in ortho carborane derivatives 1-R-2-AuL-1,2-C2B10H10
R
L
d(C(2)–Au)
d(C(1)–C(2))
Ref.
a
H
PPh3
AsPh3
PPh3
PPh3
PPh3
PPh3
2.039(8)
2.039(8)
2.044(15)b
2.050(4)
2.069(3)
2.11(3)b
—
43
19
44
45
MeOCH2
AuPPh3
1.667(11)
1.71(2)
t
SiMe2 Bu
1.706(6)
1.684(5)
2ꢀ-Pyridyl
This work
44
c
CB10H10CAu(PPh3)
—
a Cage disorder present in crystal. b Averaged. c Values of 1.595 and 1.655 for the two C(1)–C(2) bonds suggest poor quality data. The parent
28
˚
bis(carborane) has a C(1)–C(2) bond length of 1.625 A.
3 8 2 6
D a l t o n T r a n s . , 2 0 0 4 , 3 8 2 2 – 3 8 2 8

View Article Online
length increases. These trends thus may reflect electronic and
steric effects respectively.
3 Y. Zhu, K. Vyakaranam, J. A. Maguire, W. Quintana, F. Teixidor,
C. Vin˜as and N. S. Hosmane, Inorg. Chem. Commun., 2001, 4, 486;
Y. J. Lee, J. D. Lee, J. Ko, S. H. Kim and S. O. Kang, Chem. Commun.,
2003, 4, 1364.
4 L. I. Zakharkin and I. S. Savel’eva, Bull. Acad. Sci. USSR, Div. Chem.
Sci., 1979, 1294.
5 J. D. Lee, C. K. Baek, J. Ko, K. Park, S. Cho, S. K. Min and S. O.
Kang, Organometallics, 1999, 18, 2189; J. S. Park, D. H. Kim, J. Ko,
S. H. Kim, S. Cho, C. H. Lee and S. O. Kang, Organometallics, 2001,
20, 4632; D. H. Kim, J. H. Won, S. J. Kim, J. Ko, S. H. Kim, S. Cho
and S. O. Kang, Organometallics, 2001, 20, 4298; J. S. Park, D. H.
Kim, S. J. Kim, J. Ko, S. H. Kim, S. Cho, C. H. Lee and S. O. Kang,
Organometallics, 2001, 20, 4483.
6 J. D. Lee, S. J. Kim, D. Yoo, J. Ko, S. Cho and S. O. Kang,
Organometallics, 2000, 19, 1695.
7 A. R. Oki, O. Sokolova, R. Barnett and G. Pardhiva, Synth. React.
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8 J. Y. Bae, Y. J. Lee, S. J. Kim, J. Ko, S. Cho and S. O. Kang,
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883.
˚
˚
The C(1)–C(2) bond distance of 1.668(5) A in 6 is about 0.04 A
˚
longer than in 3 but 0.06 A shorter than in 5. The shortening of
the bond compared to 5 can be attributed in part to the parallel
orientation between the pyridyl ring and the C(1)–C(2) bond26
but more importantly to the absence of exo C(2)–S p bonding in
6. However, the lengthening of the cage C–C bond in 6 compared
to that in 3 is probably due to steric effects between the two bulky
substituents as found elsewhere.46 The C(1)–C(2) bond distance
˚
˚
of 1.684(5) A in 7 is only 0.01 A longer than in 6 (possibly due to
the different orientation of the pyridyl group). The similarities in
the [1-(2ꢀ-pyridyl)-1,2-C2B10H10] moiety for 6 and 7 suggest this
carborane geometry is present in the copper intermediate 4 (M =
Cu). Stronger metal · · · nitrogen intramolecular interaction in 4
(M = Cu) is likely as copper is a better ligand acceptor than
trimethyltin and gold moieties.
9 E. S. Alekseyeva, A. S. Batsanov, L. A. Boyd, M. A. Fox, T. G.
Hibbert, J. A. K. Howard, J. A. H. MacBride, A. Mackinnon and
K. Wade, Dalton Trans., 2003, 475.
10 R. Coult, M. A. Fox, W. R. Gill, P. L. Herbertson, J. A. H. MacBride
and K. Wade, J. Organomet. Chem., 1993, 462, 19.
11 L. I. Zakharkin and N. F. Shemyakin, Bull. Acad. Sci. USSR, Div.
Chem. Sci., 1984, 2572.
12 D. E. Harwell, J. McMillan, C. B. Knobler and M. F. Hawthorne,
Inorg. Chem., 1997, 36, 5951.
Conclusions and further work
Here we have described the syntheses and structural charac-
terisation of three compounds made from 1-(2ꢀ-pyridyl)-ortho-
carborane 3. The pyridyl group in these carboranes appears to
facilitate growth of suitable crystals for X-ray crystallography.
For 1-(2ꢀ-pyridyl)-2-mercapto-ortho-carborane 5, the pyridyl
nitrogen is involved in intermolecular hydrogen bonding—of a
type which may be a suitable candidate for NLO materials—
and gives the first structurally determined example of an ortho-
carborane with a thiol substituent.
13 G. M. Sheldrick, SHELX97. University of Go¨ttingen, Germany,
1997.
14 H. D. Flack, Acta Crystallogr., Sect. A, 1983, 39, 876.
15 Gaussian 98, Revision A.9, M. J. Frisch, G. W. Trucks, H. B. Schlegel,
G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A.
Montgomery, Jr., R. E. Stratmann, J. C. Burant, S. Dapprich, J. M.
Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas,
J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli,
C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala,
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Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson,
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E. S. Replogle, and J. A. Pople, Gaussian, Inc., Pittsburgh PA, 1998.
16 W. Kemp, Qualitative Organic Analysis: Spectrochemical Techniques,
McGraw-Hill, Maidenhead, UK, 2nd edn., 1986.
For 1-(2ꢀ-pyridyl)-2-(trimethylstannyl)-ortho-carborane 6, the
pyridyl nitrogen interacts weakly with the tin atom. This is the
first structurally determined example of an ortho-carborane with
a SnMe3 substituent.
Compounds 3 and 5 are potential chelating ligands in tran-
sition metal complexes. Thus, 1-(2ꢀ-pyridyl)-ortho-carborane 3
can chelate a metal atom through an exo-cluster bond C–M and
a (pyridyl)N→M bond whereas the thiol 5 can chelate a metal
atom through a S–M bond and a N→M bond. In fact, we have
recently found that the thiol 5 gives complexes of the type 1-
(C5H4N)-2-SMLn-1,2-C2B10H10 with the metal atom chelated by
the S and N atoms of 5.47 We are also looking at the possibility
of preparing 1-(2ꢀ-pyridyl)-2-X-3-MLn-1,2-C2B9H9 (where MLn
replaces a BH unit on one vertex) containing a (pyridyl)N→M
bond from 1-(2ꢀ-pyridyl)-ortho-carborane 3.
17 B. Wrackmeyer, Annu. Rep. NMR Spectrosc., 1985, 16, 73; J. Otera,
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18 L. A. Fedorov, V. N. Kalinin, K. G. Gasanov and L. I. Zakharkin,
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21 B. Gru¨ner, Z. Janousˇek, B. T. King, J. N. Woodford, C. H. Wang,
V. V sˇetecˇka and J. Michl, J. Am. Chem. Soc., 1999, 121, 3122;
M. Lamrani, R. Hamasaki, M. Mitsuishi, T. Miyashita and Y.
Yamamoto, Chem. Commun., 2000, 121, 1595; D. M. Murphy,
D. M. P. Mingos and J. M. Forward, J. Mater. Chem., 1993, 3,
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K. Wade, Polyhedron, 1993, 12, 2711.
25 R. Kiveka¨s, R. Sillanpa¨a¨, F. Teixidor, C. Vin˜as and R. Nun˜ez, Acta
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26 E. S. Alekseyeva, M. A. Fox, J. A. K. Howard, J. A. H. MacBride
and K. Wade, Appl. Organomet. Chem., 2003, 17, 499.
27 P. T. Brain, J. Cowie, D. J. Donohue, D. Hnyk, D. W. H. Rankin, D.
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T. G. Hibbert, J. A. K. Howard, A. Mackinnon, R. J. Peace and K.
Wade, Dalton Trans., 2004, 2786.
Acknowledgements
We thank EPSRC (M. A. F.), BNFL (T. G. H.) and MCyT
(MAT01-1575) (A. L., CV) for funding. Dr Andrei V. Churakov
(Durham University) is thanked for the structural determination
of compound 5 at 293 K.
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