Synthesis and molecular structure of 1,12-dicarba-closo-dodecaborane(12)-1,12-dithiol, 1,12-(SH)2-1,12-C2B10H10, in the gaseous phase, determined by electron diffraction and ab initio calculations; geometrical consequences of three-dimensional aromaticity in carbaboranes 1,12-X2-1,12-C2B10H10

Abstract

The molecular structure of 1,12-dicarba-closo-dodecaborane(12)-1,12-dithiol, 1,12-(SH)₂-1,12-C₂B₁₀H₁₀, prepared by means of an improved synthesis, has been determined by gas-phase electron diffraction restrained by ab initio calculations. The carbaborane core, shown by calculations at the MP2(fc)/6-31G* level to be very close to D_5d symmetry, gave good agreement between theoretical and experimental ¹¹B NMR chemical shifts. A model of the entire molecule in overall C₂ symmetry led to an experimental geometry (R_G = 0.077), in good agreement with the theoretical findings. The substituents do not distort the cage significantly. The well determined parameters, the C–B, B(2)–B(3), and B(2)–B(7) distances, 170.6(4), 177.5(3) and 176.5(9) pm respectively (r_a), are consistent with the analogous parameters established experimentally for other 1,12-disubstituted 1,12-dicarbadodecaboranes. Whereas the C–B and B–B distances are found to be relatively constant in the MP2(fc)/6-31G* geometries of a series of carbaboranes 1,12-X₂-1,12-C₂B₁₀H₁₀ (in addition to SH, X = H, Li, BeH, BH₂, CH₃, SiH₃, NH₂, OH, F and Cl) the C(1)⋯C(12) distances and B(2)–C(1)–C(12) angles are appreciably sensitive to the nature of X, in a similar manner to the para-disubstituted benzene derivatives, 2 × B(2)–C(1)–C(12) being viewed as an analogue of the ipso angle in the latter.