Synthetic, spectroscopic, computational and structural studies of some 13-vertex ruthenacarboranes

Abstract

Reduction of 1,2-closo-C₂B₁₀H₁₂ followed by treatment with [RuCl₂(p-cymene)]₂ (p-cymene = C₆H₄MeⁱPr-1,4) affords the 13-vertex ruthenacarborane 4-(p-cymene)-4,1,6-closo-RuC₂B₁₀H₁₂, characterised both spectroscopically and, in two crystalline forms, crystallographically. Although asymmetric in the solid state, having a docosahedral cage architecture with cage C atoms at vertices 1 and 6, this species clearly has C_s symmetry on the NMR timescale at room temperature. However, the fluctional process in operation can be arrested at low temperature, and an activation energy of 43.1 kJ mol⁻¹ is estimated. A computational study of the related species 4-(η-C₆H₆)-4,1,6-closo-RuC₂B₁₀H₁₂ reveals that the fluctionality is due to a double diamond–square–diamond process, first suggested by Hawthorne et al for the analogous CpCo species. These calculations yield an activation energy of 40.4 kJ mol⁻¹, in excellent agreement with that derived from experiment. Reduction of 1,2-Ph₂-1,2-closo-C₂B₁₀H₁₀ followed by treatment with [RuCl₂(η-C₆H₆)]₂ or [RuCl₂(p-cymene)]₂ yields the analogous species 1,6-Ph₂-4-(η-C₆H₆)-4,1,6-closo-RuC₂B₁₀H₁₀ and 1,6-Ph₂-4-(p-cymene)-4,1,6-closo-RuC₂B₁₀H₁₀, respectively. These C,C-diphenyl compounds were again studied spectroscopically and crystallographically, the p-cymene species again showing two crystalline modifications. In contrast to their CpCo and Cp*Co analogues all three ruthenacarboranes do not undergo isomerisation in refluxing toluene.