Isomerisation of -[CBH] dianions: unprecedented rearrangements and new structural motifs in carborane cluster chemistry.

Dianionic -[CB] species are key intermediates in the polyhedral expansion from 12- to 13-vertex carboranes and metallacarboranes, and the isomer adopted by these intermediates dictates the isomeric form of the 13-vertex product. Upon reduction and metallation of -carborane up to five MCB metallacarboranes can be produced (, 2007, , 6706), the structures of which imply the intermediacy of -, -, -, - and -isomers of the -[CB] species. In this paper we use density functional theory (DFT) calculations to characterise the reduction of -CBH carboranes and the subsequent isomerisations of the -[CBH] dianions. Upon reduction -carborane initially opens to [--CBH] (abbreviated to ) and [--CBH] () and isomerisation pathways connecting to , to and to have been characterised. For - and -carborane the experimental reduction produces in both cases and computed pathways for both processes are also defined; with -carborane rearrangement occurs , whereas with -carborane is formed directly. The isomer is the global minimum -structure. The characterisation of these isomerisation processes uncovers intermediates that adopt new structural motifs that we term and . intermediates feature a two-vertex basket handle bridging the remaining 10 vertices; intermediates are related to known species, in that they have 5- and 6-membered belts, but where the latter, rather than the former, is capped, leaving a 5-membered open face. These new intermediates exhibit similar stability to the species, which is attributed to their relation to the 13-vertex docosahedron through the removal of 5-connected vertices. Isomerisation pathways starting from geometries are most often initiated by destabilisation of the cluster through a DSD process causing the 3-connected C vertex to move into a 4-connected site and a neighbouring B vertex to become 3-connected. The ensuing rearrangement of the cluster involves processes such as the pivoting of a 4-vertex diamond about its long diagonal, the pivoting of two 3-vertex triangles about a shared vertex and DSD processes. These processes are all ultimately driven by the preference for carbon to occupy low-connected vertices on the open 6-membered face of the resulting species.