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

Addition of H-2 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)(2)Rh(closo-CB11H12)]. These have been characterised in solution by H-1- and B-11-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 (I 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 B-11-NMR spectrum and a small downfield shift of the CH vertex in the H-1-NMR spectrum are shown to the most reliable indicators of borane interaction in solution. (C) 2003 Elsevier Science B.V. All rights reserved.