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 C2B11 carboranes and MC2B10 metallacarboranes

Reduction of the tethered carborane 1,2-mu-(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-(eta-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-mu-(CH2SiMe2CH2)-1,2-closo-C2B10H10 by Li or Na prior to reduction. The supraicosahedral metallacarboranes 1,8-Me-2-4-Cp-4,1,8-closo-CoC2B10H10, 1,12-Me-2-4-Cp-4,1,12-closo-CoC2B10H10 and 1,12-Me-2-4-(p-cymene)-4,1,12-closo-RuC2B10H10 are also reported with all new species characterised both spectroscopically and crystallographically.