Isomerisation of nido-C2B10H122– dianions: unprecedented rearrangements and new structural motifs in carborane cluster chemistry

摘要

Dianionic nido-[C2B10]^2– species are key intermediates in the polyhedral expansion from 12- to 13-vertex carboranes and metallacarboranes, and the isomer adopted by these nido intermediates dictates the isomeric form of the 13-vertex product. Upon reduction and metallation of para-carborane up to five MC2B10 metallacarboranes can be produced (Angew. Chem., Int. Ed., 2007, >46, 6706), the structures of which imply the intermediacy of >1,7-, >3,7-, >4,7-, >7,9- and >7,10-isomers of the nido-[C2B10]^2– species. In this paper we use density functional theory (DFT) calculations to characterise the reduction of closo-C2B10H12 carboranes and the subsequent isomerisations of the nido-[C2B10H12]^2– dianions. Upon reduction para-carborane initially opens to [>1,7-nido-C2B10H12]^2– (abbreviated to >1,7) and [>4,7-nido-C2B₁₀H₁₂]^2– (>4,7) and isomerisation pathways connecting >1,7 to >7,9, >4,7 to >7,10 and >1,7 to >3,7 have been characterised. For ortho- and meta-carborane the experimental reduction produces >7,9 in both cases and computed pathways for both processes are also defined; with ortho-carborane rearrangement occurs via >7,8, whereas with meta-carborane >7,9 is formed directly. The >7,9 isomer is the global minimum nido-structure. The characterisation of these isomerisation processes uncovers intermediates that adopt new structural motifs that we term basket and inverted nido. Basket intermediates feature a two-vertex basket handle bridging the remaining 10 vertices; inverted nido intermediates are related to known nido 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 nido species, which is attributed to their relation to the 13-vertex docosahedron through the removal of 5-connected vertices. Isomerisation pathways starting from nido geometries are most often initiated by destabilisation of the cluster through a DSD process causing the 3-connected C⁷ 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 nido species.