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

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 C 2 v and D 6 h 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 (R G = 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, r a, are ca. 1.44, 1.72, and 1.78 Å, respectively. The C6 hexagon, with r a(C−C) = ca. 1.394 Å, either eclipses the C(1)−C(2) vector (overall C s symmetry) or more or less eclipses the C(1)−B(4) cluster bond (overall C 1 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 C 1 symmetry. The r e 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.