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

The reaction between [7,8-Ph₂-7,8-nido-C₂B₉H₉]^2- and [(η-C₇H₇)Mo(MeCN)₃]⁺ affords five products. Four have been isolated and shown to be structural isomers of (η-C₇H₇)MoPh₂C₂B₉H₉. Compound 1 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-Ph₂-3-(η-C₇H₇)-3,1,2-closo-MoC₂B₉H₉, is required to rationalise the intermediate compound 3 and, from it, compound 4.