Rhodium cyclooctadiene complexes of the weakly co-ordinating carborane anion [closo-CB11H12]−. Isolation and crystal structures of [(COD)Rh(η2-CB11H12)] and [(COD)Rh(THF)2][CB11H12]

Reaction of Ag[CB11H12] with [Rh(COD)Cl]2 (COD=1,4-cyclooctadiene) affords the complex [Rh(COD)(η2-CB11H12)] (2), which has been characterised by NMR spectroscopy and X-ray crystallography. The solid-state structure shows that the carborane is co-ordinated to the rhodium by two 3c–2e Rh–H–B bonds. The solution fluxional behaviour of the {Rh(COD)} fragment over the surface of the cage is discussed. The carborane ligand in 2 is displaced by THF to give the crystallographically characterised complex [(COD)Rh(THF)2][CB11H12] (3). Complex 3 is a structurally characterised model for the active species in [Rh(L2)(S)2]+ (L=bidentate ligand, S=weakly bound solvent) Lewis-acid catalysed hydrogenation and hydroacylation reactions. It is suggested that the low nucleophility of [CB11H12]− is an important factor in the isolation of 3.