Example of an intramolecular, non-classical C-H?π hydrogen bonding interaction in an Arylcarborane derivative

Article abstract of DOI:10.1016/j.molstruc.2005.10.006

The synthesis and X-ray diffraction study of 3-(2-biphenyl)-1,2- C ₂B₁₀H₁₁, 1, provides an example of an intramolecular non-classical C-H?π hydrogen bonding interaction in which the same aromatic π-electron-donating group

Full text of DOI:10.1016/j.molstruc.2005.10.006

Journal of Molecular Structure 785 (2006) 167–169
www.elsevier.com/locate/molstruc
Example of an intramolecular, non-classical C–H/p hydrogen
bonding interaction in an Arylcarborane derivative
*
Bhaskar M. Ramachandran, Carolyn B. Knobler, M. Frederick Hawthorne
Department of Chemistry and Biochemistry, University of California, Los Angeles 607 Charles E. Young Dr. East, Los Angeles, CA 90095, USA
Received 19 July 2005; received in revised form 5 October 2005; accepted 6 October 2005
Available online 11 November 2005
Abstract
The synthesis and X-ray diffraction study of 3-(2-biphenyl)-1,2- C2B10H11, 1, provides an example of an intramolecular non-classical C–H/p
hydrogen bonding interaction in which the same aromatic p-electron-donating group which interacts with the carborane C–H is also connected to
the carborane cage.
q 2005 Elsevier B.V. All rights reserved.
Keywords: Carborane; C–H–X interaction; Intramolecular; Crystal structure
1. Introduction
2. Experimental section
2.1. General considerations
The rigid geometry and three-dimensional nature of the
isomeric icosahedral carborane (C2B10H12) cages makes them
excellent candidates as building blocks for macromolecules
and supramolecular assemblies [1–5]. Because of the diverse
chemistry of their carborane C–H and B–H vertices, carborane
derivatives are an attractive choice for crystal engineering [6].
Also, the acidity of the carborane C–H vertices enables the
formation of hydrogen bonds [7], generating supramolecular
structures. The well-established intermolecular hydrogen
bonding interactions involving carboranes are: classical C–
H/O hydrogen bonds [2], bifurcated C–H/(O)2 hydrogen
bonds [8], non-classical C–H/p hydrogen bonds [9], and B–
H/H–N interactions [10]. The known carborane cage C–H
interactions are C–H/X (XZO, N, P, S, H, p, halogens) [7].
Reported instances of intramolecular C–H/X hydrogen
bonding interactions in 1,2-carboranes are limited [7]. Herein
is reported the synthesis and crystal structure of 3-(2-biphenyl)-
1,2-C2B10H11, 1, which was prepared as part of our ongoing
efforts to increase the scope of derivatization of the 3-vertex of
the 1,2-C2B10H12 carborane cage.
Standard Schlenk line and glove-box techniques were
employed for the manipulation of water- and air-sensitive
reagents and products. Solvents used were reagent-grade, dried
over an appropriate drying agent and distilled under nitrogen
flow. Tetrahydrofuran was distilled from a sodium-benzophe-
none still prior to use. 2-biphenylmagnesium bromide was
obtained from Aldrich Chemical Co. and used as received.
trans-Dichlorobis(triphenylphosphine)palladium (II) was
obtained from Strem Chemical Co. and used as received.
2.2. Physical measurements
1
The H, 13C and 11B NMR spectra were recorded on a
1
Brucker ARX 500 spectrometer. Chemical shifts for H and
13C NMR spectra were referenced to signals of residual 1H and
13C present in deuteriated solvents. Chemical shifts values for
11B NMR spectra were referenced relative to external
BF3$OEt2 (dZ0.0 ppm with negative values indicating an
upfield shift). Mass spectra was obtained using a VG Autospec
(EI) mass spectrometer.
3. Synthesis of 1, 3-(2-biphenyl)-1,2-C2B10H11
*
Corresponding author. Tel.: C1 310 825 7378; fax: C1 310 825 5490.
To a stirred mixture of 10 mmol (2.7 g) of 3-I-1,2-C2B10H11
and 0.2 mmol (140 mg) of trans-Dichlorobis(triphenylpho-
sphine)palladium (II) in 25 mL of anhydrous THF at 0 8C was
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2005.10.006

168
B.M. Ramachandran et al. / Journal of Molecular Structure 785 (2006) 167–169
added 40 mmol (80 mL of 0.5 M solution) of 2-biphenylmag-
nesium bromide in diethyl ether. After the addition was
completed, the reaction mixture was set to reflux for 36 h. The
reaction was checked by 11B NMR and was stopped when the
starting material was gone. The reaction mixture was then
cooled to room temperature. An additional 100 mL of ether
was added, and the excess Grignard reagent was quenched by
the dropwise addition of water. The water layer was washed
with ether (3!15 mL). The combined organic layers were
washed with 3!15 mL of 3% HCl solution and then water and
dried over MgSO4. The solvent was removed under vacuum
leaving a solid white residue. The crude product was purified
by recrystallization from hexane giving white crystals of pure
3-(2-phenylphenyl)-1,2-C2B10H11 1. YieldZ97% (2.9 g,
1
9.7 mmol). Mp: 91 8C H NMR (d, ppm, C6D6) 7.90–6.89
(m, 9H), 2.38 (br, s, 2H). 11B NMR (d, ppm, C6D6) K1.87 (d,
2B, JB–H 144), K4.3 (s, 1B), K7.3 (d, 1B, JB–H 144), K11.2
(d, 3B, JB–H 161), K12.8 (d, 2B, JB–H 176), K14.0 (d, 1B, JB–H
140). 13C NMR (d, ppm, C6D6) 146.4–127.3 (s, aromatic CH),
56.0 (s, carborane CH). HR-EIMS: m/z: calc. 298.2496; found
298.2505. n (KBr)/cmK1 3091, 3059 (carborane C–H).
CCDC 271931 contains the supplementary crystallographic
data for this communication.
Fig. 1. ORTEP of compound 1.
In previously [1] reported compounds where intermolecular
non-classical C–H/p hydrogen bonding interactions were
observed, the carborane C–H/p aromatic centroid separations
˚
were 2.48 and 2.46 A. C–H stretching frequencies have been
4. Results and discussion
reported [9] for the carborane C–H/p bond between 3066 and
3059 cmK1. The solid state IR of compound 1 exhibits two
distinct stretching frequencies at 3091 and 3059 cmK1 which
The crystal structure of 1 provides a new example of an
intramolecular non-classical C–H/p hydrogen bonding
interaction in which the same aromatic p-electron-donating
group which interacts with the carborane C–H is also
connected to the carborane cage. To our knowledge, only
two previous [11,12] examples of carborane compounds
exhibiting cage C–H/phenyl ring p intramolecular hydrogen
bonding interaction have been reported.
suggests that the lower energy stretching mode (3059 cmK1
)
1
corresponds to the C–H/p interaction. In the H NMR, the
two cage C–H protons appear as a singlet at 2.38 ppm. This
unusual up-field shift of the C–H protons of the carborane cage
can be attributed to their close proximity to the phenyl ring of
the biphenyl moiety there-by resulting in an increased
shielding effect.
Functionalization of the 1,2-carborane cage at the 3-vertex
was achieved by the palladium-catalyzed reaction of 3-I-1,2-
C2B10H11 with 2-biphenylmagnesium bromide [13–15]. Color-
less crystals of 1 were grown from a 1:1 hexane:ethyl acetate
solution by slow evaporation of the solvent. The structure of 1
was confirmed by X-ray crystal analysis, which revealed a
novel structural phenomenon (Fig. 1). In the biphenyl
substituent, the maximum deviation of C1L to C6L from the
Compound 1 is similar to 3-phenyl-1,2-C2B10H11 whose
crystal structure has been previously reported [16,17]. A
comparison of the bond angles at the ipso-carbon atoms of
the phenyl substituent for this compound and for compound
1 [117.71(3), 117.12(17) and 117.38(12)8, respectively]
indicates that there is very little difference in the electron
donation by the carborane cage. Examination of the
molecular geometry of 1 reveals that the carborane cage
has a near icosahedral structure, with the C–B distances in
˚
plane through these 6 atoms is 0.003(1) A and the maximum
deviation of C7L to C12L from the plane through these 6 atoms
˚
is 0.0138(9) A. An interesting feature of the structure is the
˚
the range 1.681(2)–1.7373(18) A, B–B distances in the
˚
range 1.752(2)–1.791(2) A and a C–C bond length of
˚
1.6173(17) A. The presence of a biphenyl substituent in 1
angle between the normals to these two phenyl planes,
88.99(4)8. The twisting of these rings can also be described
in accord with the following torsional angles: C5L–C6L–C7L–
C12LZ89.84(16)8, C1L–C6L–C7L–C12LZK89.29(15)8.
The twist of the 2-biphenyl substituent along the C6L–C7L
bond (Fig. 1) allows the least squares plane through C7L–C8L–
C9L–C10L–C11L–C12L to come in close proximity to the
hydrogen on the C1 cage carbon (C1–H/aromatic centroid
has a significant influence on the geometry of the carborane
cage resulting in the lengthening of the B3–B4, B3–B8 and
˚
B3–B7 distances by approximately 0.01 A with respect to
the B5–B6, B6–B10 and B10–B11 bonds, similar to the
effect of a phenyl [16] group in 3-phenyl-1,2-C2B10H11.
˚
Atoms C1 and C2 are 3.097(2) and 3.385(2) A from the
˚
plane through C7L to C12L, respectively.
This study demonstrates an additional structural presen-
tation for 1,2-carborane C–H/p interactions.
separation of 2.43 A). This arrangement is suitable for an
intramolecular non-classical C–H/p hydrogen bonding
interaction. The C–H/p centroid angle in 1 is 155.68.

B.M. Ramachandran et al. / Journal of Molecular Structure 785 (2006) 167–169
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We are grateful to the National Science Foundation (Grant
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(CHE-9974928) for their support of this research. We also
thank US Borax, Inc. for a generous grant-in aid.
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