Untethered 4,1,2-MC2B10 supraicosahedral metallacarboranes, their C,C′-dimethyl 4,1,6-, 4,1,8- and 4,1,12-MC2B10 analogues, and DFT study of the (4,)1,2- to (4,)1,6-isomerisations of C2B11carboranes and MC2B10 metallacarboranes

Abstract

Reduction of the tethered carborane 1,2-μ-(CH₂SiMe₂CH₂)-1,2-closo-C₂B₁₀H₁₀ followed by metallation with {CpCo} or {(p-cymene)Ru} fragments affords both C,C′-dimethyl 4,1,2-MC₂B₁₀ and 4,1,6-MC₂B₁₀ species. DFT calculations indicate that the barriers to isomerisation of both 4-Cp-4,1,2-closo-CoC₂B₁₀H₁₂ and 4-(η-C₆H₆)-4,1,2-closo-RuC₂B₁₀H₁₂ to their respective 4,1,6-isomers are too high for this to be the origin of the unexpected formation of 4,1,6-MC₂B₁₀ products (in marked contrast to the related isomerisation of 1,2-closo-C₂B₁₁H₁₃ to 1,6-closo-C₂B₁₁H₁₃), and, indeed, the 4,1,2-species are recovered unchanged from refluxing toluene. Equally, the DFT-calculated profile for the isomerisation of [7,8-nido-C₂B₁₀H₁₂]²⁻ to [7,9-nido-C₂B₁₀H₁₂]²⁻ suggests that the unexpected formation of 4,1,6-metallacarboranes is unlikely to result from isomerisation of a reduced (nido) carborane following desilylation. Instead, the source of the 4,1,6-MC₂B₁₀ compounds is traced to desilylation of 1,2-μ-(CH₂SiMe₂CH₂)-1,2-closo-C₂B₁₀H₁₀ by Li or Na prior to reduction. The supraicosahedral metallacarboranes 1,8-Me₂-4-Cp-4,1,8-closo-CoC₂B₁₀H₁₀, 1,12-Me₂-4-Cp-4,1,12-closo-CoC₂B₁₀H₁₀ and 1,12-Me₂-4-(p-cymene)-4,1,12-closo-RuC₂B₁₀H₁₀ are also reported with all new species characterised both spectroscopically and crystallographically.