Synthesis, reactivity and structural studies of carboranyl thioethers and disulfides

Abstract

The equimolar reaction of 1-SH-2-R-1,2-closo-C₂B₁₀H₁₀ (R = Me, H, Ph) with KOH in ethanol produces the thiolate species [1-S-2-R-1,2-closo-C₂B₁₀H₁₀]⁻. These react with iodine to give the disulfide bridged dicluster (1-S-2-R-1,2-closo-C₂B₁₀H₁₀)₂ (R = H, Me, Ph) compounds as analytically pure, white and air-stable solids in high yield. Synthesis of monothioether bridged species is synthetically more difficult. In fact three procedures have been tested to obtain the thioether bridged dicluster compounds (2-R-1,2-closo-C₂B₁₀H₁₀)₂S (R = Me, H, Ph) but only (2-Me-1,2-closo-C₂B₁₀H₁₀)₂S was successfully synthesized and characterized. Attempts to produce mixed compounds (1-R-1,2-closo-C₂B₁₀H₁₀)S(1-R′-1,2-closo-C₂B₁₀H₁₀), R ≠ R′, were unsuccessful. Deboronation reaction of this dicarboranylthioether lead, depending on the reaction conditions, to monoanionic [(2-Me-1,2-closo-C₂B₁₀H₁₀)S(8-Me-7,8-nido-C₂B₉H₁₀)]⁻ or dianionic [(8-Me-7,8-nido-C₂B₉H₁₀)₂S]²⁻ sulfur bridge anions. Deboronation of carboranyl disulfides gave the corresponding dianionic [(7-S-8-R-7,8-nido-C₂B₉H₁₀)₂]²⁻ (R = H, Me, Ph) species. This reaction was very dependent, however, on the reaction conditions. With slight variation of the reaction conditions, splitting of the S–S bond leading to the thiolate species with retention of the closo cluster was also found. Carboranyl disulfides (1-S-2-R-1,2-closo-C₂B₁₀H₁₀)₂ (R = H, Me, Ph) do not lead to thiosulfinates R–S(O)–S–R′ by oxidation with H₂O₂ or I₂ as organic disulfides do. This behaviour is attributed to the presence of the sulfur atom directly bonded to the carbon cluster that produces electronic transfer from the filled orbitals on the sulfur atom into the cage LUMO (largely located on the cage C_c–C_c bond). This causes a depletion of electron density on the sulfur, thence impairing sulfur oxidation, and facilitating S–S breaking. Crystal structures of monothioethers (2-Me-1,2-closo-C₂B₁₀H₁₀)₂S, [NMe₄][(2-Me-1,2-closo-C₂B₁₀H₁₀)S(8-Me-7,8-nido-C₂B₉H₁₀)] (the first example reported in the literature of a two cluster compound incorporating the closo C₂B₁₀ and the nido [C₂B₉]⁻ moieties linked by a one member spacer) and disulfides (1-S-1,2-closo-C₂B₁₀H₁₁)₂, (1-S-2-Me-1,2-closo-C₂B₁₀H₁₀)₂, (1-S-2-Ph-1,2-closo-C₂B₁₀H₁₀)₂ are reported which support the behaviour of these species.