exo-closo-Rhodacarboranes: synthesis and characterisation of [{exo-(R3P)2Rh}(closo-CB11H12)] [R3P=P(OMe)3, PCy3, 1/2dppe]

Addition of H2 to CH2Cl2 solutions of [(diene)Rh(L)2][closo-CB11H12] (diene=norbornadiene, cyclooctadiene, L=PCy3, P(OMe)3, 1/2dppe) results in the formation of the exo-closo complexes [(PR3)2Rh(closo-CB11H12)]. These have been characterised in solution by 1H- and 11B-NMR spectroscopy, and for L=PCy3 by a single crystal X-ray diffraction study. This suggests that the metal fragment is bound with the cage through the 7,8- and not the 7,12-{BH} vertices. DFT calculations on a model system where L=PMe3 show that there is only a negligible energy difference between these two isomers (1 kcal mol−1), suggesting that both represent stable structures. The salient spectroscopic markers that indicate an interaction of [closo-CB11H12]− with a metal fragment are discussed and compared across a range of metal complexes. Large upfield shifts in the 11B-NMR spectrum and a small downfield shift of the CH vertex in the 1H-NMR spectrum are shown to the most reliable indicators of borane interaction in solution.