1-Phenyl-1,2-dicarba-closo-dodecaborane, 1-Ph-1,2-closo-C(2)B(10)H(11). 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-C(6)H(5)-1,2-closo-C(2)B(10)H(11) (1), has been synthesized and characterized by a complete assignment of its (11)B NMR spectrum via (11)B{(1)H}/(11)B{(1)H} (COSY), (1)H{(11)B(selective)} and (1)H{(11)B}/(1)H{(11)B} (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 ofC(2)(v)() and D(6)(h)() for the 1,2-C(2)B(10) and C(6) 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 ?, respectively. The C(6) hexagon, with r(a)(C-C) = ca. 1.394 ?, 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 [C(8)H(16)B(10), monoclinic P2(1)/a: a = 12.047(3) ?, b = 18.627(4) ?, c = 12.332(5) ?, beta = 110.09(4) degrees, Z = 8]. The C-B distances span the range 1.681(6)-1.743(5) ?, and B-B lengths lie between 1.756(6) and 1.795(6) ?. 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 (11)B chemical shifts, in addition to SCF single-point energies of the GED structures, further support these observations.