Chemistry on a metallathiaborane cluster Part 4: reactions of 11-vertex rhodathiaboranes with bidentate phosphines and their subsequent rearrangements

The phosphine ligands on [8,8-(PPh3)2-nido-8,7-RhSB9H10] (I) may be replaced by the bidentate phosphines, (CH2)n(PPh2)2, to form species of the type [8,8-{η2-(CH2)n(PPh2)2}-8-{η1-(CH2)n(PPh2)2}-nido-8,7-RhSB9H10] (II), in the case of dppm (n=1) and of the type [8,8-{η2-(CH2)n(PPh2)2}-nido-8,7-RhSB9H10] (III), in the case of dppe (n=2) and dppp (n=3). It was originally thought that the chemistry followed by dppm differed substantially but predictably from that of dppe and dppp. In fact the reactions are more complex and mixed-ligand systems are accessible which provide insight into these reactions and rearrangement mechanisms. Reactions involving the transfer of a metal bonded bidentate phosphine to the cage are observed for dppe and dppp but only for dppm can a species of the type [1-PPh3-{1,3-(μ-dppm)}-closo-1,2-RhSB9H8] (IV), which may be the key to the identification of the intermediate in these reactions, be observed. We also observed the ultimate product in this series of reactions which appears to be a mixture of isomers, [3-(η1-dppm)-1,1-(η2-dppm)-closo-1,2-RhSB9H8] (V) and [1-(η1-dppm)-1,3-(μ-dppm)-closo-1,2-RhSB9H8] (VI). Additional ligand exchange reactions can be effected so that such species as [3-(η1-dppm)-1,1-(η2-dppe)-closo-1,2-RhSB9H8] (VII) may be isolated from IV. From these results, suggestions for possible reaction pathways are presented.