1-Phenyl-1,2-dicarba-closo-dodecaborane, 1-Ph-1,2-closo-C2B10H11. Synthesis, Characterization, and Structure As Determined in the Gas Phase by Electron Diffraction, in the Crystalline Phase at 199 K by X-ray Diffraction, and by ab Initio Computations

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Abstract
The compound 1-phenyl-1,2-dicarba-closo-dodecaborane(12), 1-C6H5-1,2-closo-C2B10H11 (1), has been synthesized and characterized by a complete assignment of its 11B NMR spectrum via 11B{1H}/11B{1H} (COSY), 1H{11Bselective} and 1H{11B}/1H{11B} (COSY) spectroscopy. An electron- and X-ray diffraction investigation of 1, complemented by ab initio calculations, has been undertaken. The gas-phase electron-diffraction (GED) data can be fitted by several models describing conformations which differ in the position of the phenyl ring with respect to the carborane cage. Local symmetries of C2v and D6h for the 1,2-C2B10 and C6 moieties, respectively, were adopted in the GED model in order to simplify the problem. In addition, constraints among the close-lying C−C and B−B bonds were employed. However, even though such simplifications led to satisfactory refinements (RG = 0.069−0.071), a unique, definitive solution could not be gained. The (C−C)mean, (C−B)mean and (B−B)mean bond lengths, ra, are ca. 1.44, 1.72, and 1.78 Å, respectively. The C6 hexagon, with ra(C−C) = ca. 1.394 Å, either eclipses the C(1)−C(2) vector (overall Cs symmetry) or more or less eclipses the C(1)−B(4) cluster bond (overall C1 symmetry). In contrast, in the solid at 199 K, the ring lies at a position intermediate between the two GED positions, as determined by X-ray crystallography [C8H16B10, monoclinic P21/a: a = 12.047(3) Å, b = 18.627(4) Å, c = 12.332(5) Å, β = 110.09(4)°, Z = 8]. The C−B distances span the range 1.681(6)−1.743(5) Å, and B−B lengths lie between 1.756(6) and 1.795(6) Å. A similar conformation was found for the theoretical (RHF/6-31G* level) structure which was fully optimized in C1 symmetry. The re distances are consistent with the dimensions derived in the experimental studies. IGLO calculations of the 11B chemical shifts, in addition to SCF single-point energies of the GED structures, further support these observations.

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