Crystalline hydrogen bonded complexes of o-carborane

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Abstract

The crystalline complexes (1,2-dicarbadodecaborane(12))(1,10-phenanthroline)_1.5 and (1,2-dicarbadodecaborane(12))₂(1,2-dimethoxybenzene) grow from toluene solutions. Both complexes feature bifurcated hydrogen bonds between the acidic carborane C–H and the 1,2-arrangement of acceptor atoms of the organic fragment.

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Introduction

Carboranes such as o-, m-, and p-C₂B₁₀H₁₂ [1,2-, 1,7-, 1,12-dicarbadodecaborane(12)] (hereafter o-, m-, and p-carborane) are twelve-vertex icosahedral cages with applications in a number of diverse fields such as boron neutron capture therapy, boron carbide-type ceramics, as complexing agents for extraction of metal ions, in nonlinear optics, conducting polymers and more.¹ These neutral and related anionic carboranes show C–H acidity² and are consequently able to act as hydrogen bond donors. A handful of supramolecular complexes of carboranes exploiting such weak intermolecular interactions is known,^3–9 the first reported example being that of 1∶1 complexes between o-, m- or p-carborane and hexamethylphosphoramide, where all adopt different structures in the solid state commensurate with their differing arrangement of C–H sites.³ Carboranes covalently substituted with hydrogen bond acceptor groups can self-associate in the solid state.⁵o-Carborane 1 is present as a guest molecule in inclusion or host–guest complexes with cyclodextrins,¹⁰ a curved nickel macrocycle⁶ and arene-based bowl-shaped host molecules such as calixarenes⁷ or cyclotriveratrylene (CTV).^8,9 The use of arene-based hosts often results in the formation of nonclassical C–H⋯π hydrogen bonds between the polarised C–H bond of the carborane and the basic π-electrons of an aromatic ring.^7,8 Calix[5]arene forms 1∶1, 2∶1 and 1∶2 complexes with o-carborane, all of which feature ball-and-socket [(o-carborane)∩(calix[5]arene)] assemblies showing C–H⋯π hydrogen bonding.⁷ CTV forms 2∶1 and 1∶2 CTV∶o-carborane complexes; however, only the former displays C–H⋯π hydrogen bonding.⁸ In the latter example, one carborane acts as a guest molecule while the other forms bifurcated C–H⋯(OMe)₂ hydrogen bonds to the dimethoxy moieties of the CTV to create an infinite network.⁹ A similar hydrogen bonding interaction creates an infinite helical arrangement of o-carboranes and CTV molecules in the complex (o-carborane)(fullerene-C₇₀)(CTV)(1,2-dichlorobenzene).⁹ The interaction is calculated to be energetically favoured by 5.48 kcal mol⁻¹ for the model compound (o-carborane)·(1,2-dimethoxybenzene),⁹ approximately double the calculated stability of the carborane C–H⋯π interaction.⁸

Given the current interest in the formation of infinite hydrogen bonded networks in the field of crystal engineering the generality of this bifurcated hydrogen bond from o-carborane was investigated using chemically simpler hydrogen bond acceptors of the type illustrated in Fig. 1. 1,2-Dimethoxybenzene (veratrole) 2 is essentially a substructure of cyclotriveratrylene, and is known to form a bifurcated hydrogen bond with (R)arylglycyl(R)phenylglycine,¹¹ whereas 1,10-phenanthroline 3 has a physically similar arrangement of hydrogen bond acceptor groups with different chemical functionality, and is also reported to act as a bifurcated hydrogen bond acceptor.^12,13 Both form crystalline hydrogen bonded complexes with o-carborane, the structures of which were determined by X-ray crystallographic studies.

Fig. 1 Bifurcated interaction between an acidic C–H group of o-carborane and an (X)₂ acceptor in a rigid ortho arrangement.

Results and discussion

Needles of composition (o-carborane)₂(1,2-dimethoxybenzene) 4 were grown by slow evaporation of a toluene solution of o-carborane and two equivalents of 2.† Single crystals were not obtained from mixtures with other stoichiometries. The crystals are not stable and slowly sublime under ambient conditions, completely disappearing within a week. Sublimation of 4 gave solid 1. The complex crystallises in a monoclinic cell and the structure was solved in space group P2₁/n, and details of data collection and structure solution are given in Table 1. There are two crystallographically distinct carborane molecules and a 1,2-dimethoxybenzene molecule in the asymmetric unit. Both carboranes are involved in close interactions to the 1,2-dimethoxybenzene molecule.

Table 1 Details of data collection and structure refinement for (o-carborane)₂(1,2-dimethoxybenzene) 4 and (o-carborane)(1,10-phenanthroline)_1.55^a,b

Property	4	5
a X-Ray data were collected on an Enraf-Nonius KappaCCD diffractometer with Mo-Kα radiation (λ = 0.71073 Å). Data were corrected for Lorentzian and polarisation effects, but not absorption. Structures were solved by direct methods (SHELXS-97)^14 and refined by full-matrix, least squares on F^2 using SHELXL-97.^15 All non hydrogen atoms were refined anisotropically, and hydrogens were included as invariants for 4 and fully refined for 5. Carbon positions within the carborane cage were assigned by an analysis of displacement parameters and bond lengths. For 4, one carborane is disordered with three icosahedron vertex positions modelled as having 1/3 B and 2/3 C character. A number of crystallising conditions were tried for 4 and the crystals were somewhat unstable, with partial solubility in liquid 2 and a tendency to sublime. The reported structure is the best of several data collections on different crystals; however, the high mosaicity (ca. 1°) and Rint indicate that the data are of very poor quality, reflected by the high final R-factors obtained. Other indicators of the reliability of the structure – such as bond lengths and anisotropic displacement parameters – are reasonable. b Click b107198j.txt for full crystallographic data (CCDC 168664 and 168665).
Formula	C12H34B20O2	C20H24B10N3
Crystal size/mm	0.20 × 0.13 × 0.08	0.40 × 0.38 × 0.25
M	426.59	414.52
Crystal system	Monoclinic	Monoclinic
Space group	P21/n	C2/c
a/Å	6.9432(1)	32.9490(9)
b/Å	30.8896(5)	7.0142(2)
c/Å	11.7330(2)	24.8736(6)
β/°	90.564(1)	130.05(3)
U/Å^3	2516.29(7)	4400.3(2)
Z	4	8
T/K	150(1)	123(1)
D c/g cm^−1	1.126	1.251
μ/mm^−1	0.057	0.067
θ min, max/°	3.01, 24.99	3.01, 27.97
Data collected	14 608	29 636
Unique data, Rint	4410, 0.1585	4646, 0.076
Observed data [I > 2σ(I)]	2669	3123
Parameters	309	394
R 1 (observed data)	0.1174	0.0496
wR 2 (all data)	0.3299	0.1400
S	1.080	1.027

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One type of carborane, shown with B atoms in purple in Fig. 2, forms the anticipated bifurcated hydrogen bond to the dimethoxy groups of 2 through one of its acidic C–H groups. The interaction is out-of-plane and nearly symmetrical at C–H⋯(OMe)₂ distances 2.31 and 2.30 Å (corresponding C⋯B separations of 3.21 and 3.18 Å, respectively). This is a slightly shorter interaction that those found in analogous C–H⋯(OMe)₂ interactions between 1 and CTV.⁹ The ortho C–H group does not interact with dimethoxy groups, and is instead directed over the arene ring of an adjacent 1,2-dimethoxybenzene molecule, Fig. 2(a). The C–H⋯arene centroid distance is 2.89 Å, somewhat longer than other reported examples of carborane C–H⋯arene interactions,^7–9 and can be regarded as a weak C–H⋯π interaction. The previously reported optimised structure of a 1∶1 model compound⁹ has a longer and less symmetrical in-plane bifurcated hydrogen bond with C–H⋯O distances at 2.718 and 2.420 Å, while the other C–H group shows a long interaction (C–H⋯O 2.819 Å) to an O centre common with the former.

Fig. 2 Crystal structure of (o-carborane)₂(1,2-dimethoxybenzene) 4. (a) Close interactions between the components: the carborane (in purple) hydrogen bonds to two adjacent 1,2-dimethoxybenzene molecules through bifurcated C–H⋯(OMe)₂ and C–H⋯π interactions, while two of the three carborane positions with C character of the carborane (in pink) show close contacts to a single methoxy oxygen, with the third directed over the arene centre. Ellipsoids are shown at the 50% probability level. (b) Packing diagram viewed down the b axis, showing a layered structure. Click image or 2.htm to access a 3D representation.

The second type of carborane, shown in pink in Fig. 2, was modelled as disordered with three of the carborane vertex positions within one triangular face having 1/3 B and 2/3 C character. Each of these B/C–H groups shows close contact to a single 1,2-dimethoxybenzene molecule. Two B/C–H groups interact with a single O atom of the dimethoxybenzene at C–H⋯O distances of 2.49 and 2.79 Å (corresponding C⋯O distances 3.51, 3.59 Å, respectively). Although these separations are consistent with hydrogen bond formation there is an acute angle at the O atom; hence, the C–H is not directed towards the O lone pair, indicating that this may not constitute a genuine hydrogen bond. It should be noted that hydrogen bond acceptors have soft directionality, especially when coupled with weak donors.¹⁶ The third B/C–H group is directed over the centre of the dimethoxybenzene arene ring at the C/B–H⋯centroid distance 2.61 Å.

The crystal packing is shown in Fig. 2(b). Hydrogen bonded carborane–dimethoxybenzene chains run in the b direction and such chains with inverted orientations alternate along the c direction at closest contact B–H⋯H–B 2.75 Å. The overall packing has a layered structure with two layers of chains as described above alternating with layers of the second type of carborane [again shown in pink in Fig. 2(b)] along the long a axis.

Crystals of composition (o-carborane)(1,10-phenanthroline)_1.55 were grown by slow evaporation of a toluene solution of o-carborane and 1,10-phenanthroline.† The complex crystallises in a monoclinic cell and the structure was solved in space group C2/c; details of data collection and structure solution are given in Table 1. The asymmetric unit consists of one carborane and one and a half phenanthroline molecules, with one phenanthroline lying about a two-fold axis. Two major structural motifs are found throughout the structure. One is a hydrogen bonded carborane–phenanthroline assembly, the other a flat ribbon of phenanthrolines.

Carborane forms a bifurcated C–H⋯(N)₂ hydrogen bond with the aromatic nitrogens of one 1,10-phenanthroline at C–H⋯N distances 2.28 and 2.50 Å (corresponding C⋯N separations 3.09 and 3.23 Å, respectively). Unlike the case of 4 above the interaction is in-plane with the centre of the phenanthroline N atoms, and the hydrogen bonding carborane C–H forms an angle of 167.6° at the hydrogen. The ortho C–H group is rotated slightly away from the phenanthroline, at a C–H⋯O separation of 2.89 Å, so the interaction between the components is very similar to that calculated for a 1∶1 (o-carborane)(1,2-dimethoxybenzene) species.⁹ The o-carborane–1,10-phenanthroline dimers stack in the b direction in alternating orientations through face-to-face π stacking of the phenanthrolines, Fig. 3. The central rings of the phenanthrolines show a slipped but coplanar π–π interaction at the centroid separation of 3.54 Å. Interestingly, this assembly and its mode of stacking is virtually identical to that of the previously reported 1∶1 2,2′-biphenol∶1,10-phenanthroline hydrogen bonding complex.¹³ In that complex the one unsymmetrical bifurcated hydrogen bond is closer at O–H⋯N distances of 1.95 and 2.55 Å, while the π stacking mode and distances are comparable.

Fig. 3 Crystal structure of (o-carborane)(1,10-phenanthroline)_1.55. Hydrogen bonding (indicated by dashed lines) with face-to-face π stacking between phenanthroline molecules (in blue) is evident, and phenanthroline molecules (in green) form a novel, flat C–H⋯N hydrogen bonded ribbon. Click image or 3.htm to access a 3D representation.

The overall crystal lattice forms a layered structure with layers of the carborane–phenanthroline assemblies separated in the a direction by flat, one-dimensional ribbons of 1,10-phenanthroline molecules. Phenanthroline molecules within each of these ribbons are exactly coplanar with the bc plane (Fig. 3) and show close interactions between the 5,6 hydrogens of one molecule and the 1,10 nitrogens of the adjacent phenanthroline at distances consistent with weak C–H⋯N hydrogen bonds (C–H⋯N 2.52 Å). A similar structural motif occurs in the solid state structure of 3; however, the phenanthroline molecules within that ribbon are well separated (C–H⋯N distance ca. 3 Å) precluding the formation of hydrogen bonds.¹⁷ The two types of phenanthroline molecules stack together in an offset edge-to-face arrangement, at a closest C–H⋯C contact of 2.82 Å and an angle of 80.8° between adjacent molecules.

Conclusions

The chemically diverse hydrogen bond acceptors 1,2-dimethoxybenzene and 1,10-phenanthroline both feature a rigid ortho arrangement of acceptor atoms, and both form bifurcated hydrogen bonding interactions with o-carborane. This interaction has been previously reported for o-carborane–CTV complexes, indicating that it may be a general synthon able to be incorporated into crystal engineering studies. It is notable that o-carborane has failed to form a complex with nitrogen heterocycles such as pyrazine and pyridine where a non-bifurcated C–H⋯N interaction is the only likely hydrogen bonding interaction. It also fails to form a complex with 2,2′-bipyridyl which, although a chelating ligand, shows considerable rotation about the central C–C bond in the absence of a metal ion,¹⁸ ensuring that the acceptor atoms are not in a rigid ortho arrangement.

Acknowledgements

We are grateful to the Australian Research Council for support of aspects of this work.

References

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Footnote

† Synthesis of complex4: o-carborane (11.7 mg, 0.081 mmol) and 1,2-dimethoxybenzene (21.9 mg, 0.159 mmol) were mixed in toluene and allowed to stand to give needles of 4 contaminated with an excess of dimethoxybenzene after a few days. For complex 5: o-carborane (3 mg, 0.021 mmol) and 1,10-phenanthroline (4.2 mg, 0.021 mmol) were dissolved separately in toluene, mixed and allowed to stand. Crystals of 5 appeared on complete evaporation of the solvent.

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