Supramolecular self-assembly directed by carborane C-H···F interactions

Article abstract of DOI:10.1039/b007353i

The synthesis and crystallization of 9,12-bis-(4-fluorophenyl)-1,2-carborane 1 and 9,12-bis-(3,5-difluorophenyl)-1,2-carborane 2 provide the first examples of a supramolecular assemblage directed by C-H(carborane)···F interactions.

Full text of DOI:10.1039/b007353i

Supramolecular self-assembly directed by carborane C–H…F interactions
Hans Lee, Carolyn B. Knobler and M. Frederick Hawthorne*
Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, CA 90095, USA.
E-mail; mfh@chem.ucla.edu
Received (in Columbia, MO, USA) 21st August 2000, Accepted 31st October 2000
First published as an Advance Article on the web
The synthesis and crystallization of 9,12-bis-(4-fluoro-
phenyl)-1,2-carborane 1 and 9,12-bis-(3,5-difluorophenyl)-
1,2-carborane 2 provide the first examples of a supramo-
lecular
assemblage
directed
by
C–Hcarborane…F
interactions.
The potential utilization of the isomeric icosahedral carborane
(C2B10H12) cages as building blocks for macromolecules and
supramolecular assemblies has only recently been explored.1–3
The versatile chemistry available at the carborane C–H and B–
H vertices makes carborane derivatives attractive candidates for
crystal engineering.4 Intermolecular hydrogen bonding is an
important factor involved in supramolecular self-assembly
motifs. Consequently, the participation of the acidic carborane
C–H vertices in hydrogen bonding generates supramolecular
structures. The established intermolecular interactions involv-
ing carboranes are: classical C–H…O hydrogen bonds,5
bifurcated C–H…(O)2 hydrogen bonds,6 non-classical C–H…p
hydrogen bonds7 and B–H…H–N interactions.1,8 Thus far,
known C–H carborane interactions are limited to C–H…Y (YN
O, N, p) systems.1,9 Fluorocarbon sites (C–F) are poor
hydrogen bond components and cannot compete with O and N,
although weak C–H…F interactions contribute to crystal-
packing organization.10 We report here the novel ortho-
carborane derivatives: 9,12-bis-(4-fluorophenyl)-o-carborane
1, and 9,12-bis-(3,5-difluorophenyl)-o-carborane 2. Com-
pounds 1 and 2 provide the first examples of a supramolecular
assemblage directed by carborane C–H…F interactions.
Electron-rich boron vertices of ortho-carborane were easily
functionalized by electrophilic iodination followed by palla-
dium-catalyzed reactions of the regioselectively iodinated
carborane with the appropriate fluorophenyl or difluorophenyl
Grignard reagent.11 Colorless crystals of 1 and 2 were grown
from a methylene chloride–hexane solution and Et2O, re-
spectively,† by slow evaporation of the solvent. The structures
of compounds 1 and 2 were determined by single-crystal X-ray
diffraction analysis.
Crystal structures of both 1 and 2‡ show intermolecular
carborane C–H…F interactions. The unit cell of the solid-state
structure of 1 contains four crystallographically unrelated, yet
similar carboranes. Carborane C–H…F intermolecular dis-
tances lie in the range 2.49–3.05 Å and non-classical C–H…p
hydrogen bonds with carborane C–H…aromatic centroid sepa-
rations of 2.48 and 2.46 Å are observed (see caption to Fig. 1).
The observed carborane C–H…F distances are consistent
with phenyl C–H…F interactions reported for various fluor-
obenzene compounds (H…F 2.36–2.86 Å).10 Also, close
phenyl C–H…F distances (H…F 2.25–2.70 Å) are observed.
All carborane C–H vertices are in close proximity to either a
fluorine atom or p centroid, with the majority participating in
C–H…F interactions. The solid-state IR spectrum of 1 exhibits
two distinct stretching frequencies for carboranyl C–H at 3088
and 3060 cm21. These unassigned stretching frequencies
represent either a carborane C–H…(F or p) interaction. The C–
H stretching frequencies of the o-carborane C–H…p bond were
reported7 between 3066 and 3059 cm21 which suggests that the
lower energy stretching mode (3060 cm21) corresponds to those
carborane C–H bonds involved in C–H…p interactions.
Fig. 1 Crystal packing diagram of 1. (a) View of carborane C–H…F
interactions. Selected distances (Å) and angles (°): C(1C)…F(1D) 3.593(8),
H(1C)…F(1D) 2.91, C(2B)…F(2A) 3.416(7), H(2B)…F(2A) 2.49,
C(2C)…F(1B) 3.557(8), H(2C)…F(1B) 2.69; C(1C)–H(1C)–F(1D) 120,
C(2B)–H(2B)–F(2A) 146, C(2C)–H(2C)–F(1B) 136. (b) View of carborane
C–H…p interactions. Selected distances (Å) and angles (°): H(1D)…aro-
matic centroid(p) 2.48, H(1B)…p 2.46; C(1D)–H(1D)–p 24.3, C(1B)–
H(1B)–p 24.7. For clarity, the remaining phenyl and carborane H atoms are
not shown.
In the solid state structure, molecules of 2 form a one-
dimensional polymeric chain linked by two different carborane
interactions having characteristic C–H…F distances. Direction-
ality of both C–H vertices toward the fluorine atoms differ with
angles of C(1)–H(1)–F(3P) 160° and C(2)–H(2)–F(11P) 124°.
Intermolecular interactions of C(1)–H(1)…F(3P) 2.47 Å and
C(2)–H(2)…F(11P) 2.35 Å demonstrate that the hydrogen atom
bonded to C(2) interacts more strongly with fluorine than the
hydrogen atom bonded to C(1) (Fig. 2). Close phenyl C–H…F
interactions are not observed in 2. The solid-state IR spectrum
exhibits two different carborane C–H stretching frequencies of
equal intensity (3077 and 3098 cm21). This observation is
consistent with the presence of the two discrete sets of such
interactions observed in the X-ray diffraction study. The
position of the 3,5-difluorophenyl group in 2 allows the
DOI: 10.1039/b007353i
Chem. Commun., 2000, 2485–2486
This journal is © The Royal Society of Chemistry 2000
2485

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MHz, BF3·Et2O external standard, J/Hz) 8.4 (s, 2B), 28.3 (d, JBH 138, 2B),
213.0 (d, JBH 151, 4B), 215.6 (d, JBH 167, 2B); dF(376 MHz, CDCl3,
external Freon-113/C6D6 solution referenced at 268.0 ppm12 relative to
CFCl3) 2116.5; HR-EIMS: m/z: found 332.2376; calc. 332.2387.
Synthetic procedure for 2: the synthesis and separation procedure was
similar to that of 1. Crystallization from Et2O afforded 2 (79%).
Selected data for 2: mp 189–190 °C; n(KBr)/cm21 3077, 3098 (carborane
C–H); dH(400 MHz, CDCl3, J/Hz) 6.61 (br d, J 6.6, 4H), 6.52 (tt, J 9.1, 2.4,
2H), 3.74 (2H), 1.5–3.4 (BH); dC(100 MHz, CDCl3, J/Hz) 50.1, 102.7 (t,
JCF 25.2), 115.1 (dd, JCF 17.2), 162.2 (dd, JCF 251.5); dF(376 MHz, CDCl3,
external Freon-113/C6D6 solution referenced at 268.0 ppm12 relative to
CFCl3) 2112.4; dB(160 MHz, Et2O, BF3·Et2O external standard, J/Hz) 7.4
(s, 2B), 28.6 (d, JBH 140, 2B), 213.0 (d, JBH 154, 4B), 215.5 (d, JBH 168,
2B); HR-EIMS: m/z: found 368.2196; calc. 368.2191.
Fig. 2 Crystal packing diagram of 2. The three molecules are related by
translation. Selected distances (Å) and angles (°): C(1)…F(3P) 3.462(5),
H(1)…F(3P) 2.47, C(2)…F(11P) 3.107(5), H(2)…F(11P) 2.35; C(1)–H(1)–
F(3P) 160, C(2)–H(2)–F(11P) 124. For clarity, the remaining phenyl and
carborane H atoms are not shown.
‡ Single crystals of 9,12-bis-(4-C6H4F)2-1,2-C2B10H10 1 were crystallized
from methylene chloride–hexane, placed on a fiber and mounted on a
Syntex P-1 diffractometer.
orientation of both carborane C–H vertices to simultaneously be
in close proximity to fluorine atoms present on the neighboring
molecule. This orientation leads to formation of the observed
one-dimensional polymeric structure.
The crystal lattices of 1 and 2 are directed by a network of
weak carborane C–H…F and/or C–H…p contacts. Although
these C–H…F interactions are weak, their sum in lattice
networks provides stabilization. This study demonstrates an
additional motif for carboranes in supramolecular self-assembly
chemistry.
Crystal data for 1: C56B40H72F8, M = 1329.54, monoclinic, space group
P21/n, a = 17.504(8), b = 15.015(7), c = 27.57(1) Å, b = 91.16(1)°, V =
7244(6) Å3, T = 293 K, Z = 4, l(Cu-Ka) = 1.5418 Å, m = 0.592 mm21
,
7440 reflections measured, 7440 unique, which were used in all calcula-
tions. The final R1(F2) was 0.0746 [4642 reflections, I > 2s(I)].
Single crystals of 9,12-bis-(3,5-C6H3F2)2-1,2-C2B10H10 2 were crystal-
lized from Et2O, placed on a fiber and mounted on a Syntex P-1
diffractometer.
¯
Crystal data for 2: C14B10H18F2, M = 332.38, triclinic, space group P1,
a = 6.882(4), b = 9.710(5), c = 13.926(7) Å, a = 84.80(2), b = 81.41(2),
g = 78.50(2)°, V = 900 Å3, T = 298 K, Z = 2, l(Cu-Ka) = 1.5418 Å, m
= 0.793 mm21, 2468 reflections measured. The final R1(F2) was 0.0616
[1540 reflections, I > 2s(I)].
We are grateful to the National Science Foundation (Grant
No. CHE-9730006) for their support of this research.
CCDC 182/1844. See http://www.rsc.org/suppdata/cc/b0/b007353i/ for
crystallographic files in .cif format.
Notes and references
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† Synthetic procedure for 1: to a dry THF (300 ml) solution of 9,12-I2-
1,2-C2B10H10 (4.12 g, 10.4 mmol) was added 52 ml of 4-fluoro-
phenylmagnesium bromide (1 M, in THF) at 0 °C. At 25 °C, trans-
[PdCl2(PPh3)2] (120 mg, 0.17 mmol) was added to the solution. The mixture
was heated at the reflux temperature under nitrogen for 2 days, resulting in
a black solution. Aqueous 10% HCl was added cautiously to destroy
residual Grignard reagent, and all volatiles were removed in vacuo. The
residue was dissolved in 200 ml of diethyl ether and 10% HCl (aq). The
organic phase was separated and the aqueous layer was extracted with
diethyl ether (2 3 100 ml). The organic phases were combined, dried over
MgSO4, and filtered. The solvent was removed under reduced pressure to
yield a red solid. Purification by chromatography on basic aluminum oxide
(toluene) gave a white solid. Crystallization from a methylene chloride–
hexane solution afforded 1 (2.98 g, 8.95 mmol, 86%).
Selected data for 1: mp 132–133 °C; n(KBr)/cm21 3060, 3088 (carborane
C–H); dH(400 MHz, CDCl3, J/Hz) 1.5–3.4 (BH), 3.68 (br s, 2H), 6.80 (tt, J
9.0, 2.3, 4H), 7.13 (br dd, J 8, 6, 4H); dC(100 MHz, CDCl3, J/Hz) 162.8 (d,
JCF 245.5, CF), 134.6 (d, JCF 7.3, CH), 114.3 (d, JCF 19.9, CH), 49.4; dB(160
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