Mapping the pathway of heteroborane isomerisation: Two parallel "1,2 -> 1,7" isomerisations of a crowded molybdacarborane and the isolation of isomerisation intermediates: Two parallel "1,2 → 1,7" isomerisations of a crowded molybdacarborane and the isolation of isomerisation intermediates

The reaction between [7,8-Ph-2-7,8-nido-C2B9H9](2-) and [(eta-C7H7)Mo(MeCN)(3)](+) affords five products. Four have been isolated and shown to be structural isomers of (eta-C7H7)MoPh2C2B9H9. Compound I has a pseudocloso structure. In solution it gives way to the non-icosahedral compound 2 which in turn rearranges into the "1,2 -> 1,7" C-atom isomerised compound 5 having a 2,1,8-MoC2B9 structure. A further "1.2 -> 1,7" C-atom isomerised species, compound 4, is also isolated but has a 1,2,4-MoC2B9 architecture. Compound 4 forms via an intermediate 3, which is too unstable to characterise. Structurally the sequence of compounds 1, 2 and 5 maps well onto the sequential diamond-square-diamond isomerisation mechanism of 1,2-closo-C2B10H12 into 1,7-closo-C2B10H12 proposed by Wales. An alternative pathway from the notional first product of the metallation, 1,2-Ph-2-3-(eta-C7H7)-3,1,2-closo-MoC2B9H9, is required to rationalise the intermediate compound 3 and, from it, compound 4. (c) 2006 Elsevier B.V. All rights reserved.