Revealing the P-B coupling event in the rhodium catalysed dehydrocoupling of phosphine-boranes H3B center dot PR2H (R = Bu-t, Ph)

We demonstrate that [Rh(eta(6)-FC6H5)(Ph2PCH2CH2CH2PPh2)][BAr4F] is a competent catalyst for the dehydrocoupling of H3B center dot PR2H to give H3B center dot PR2BH2 center dot PR2H (R = Ph, Bu-t). The isolation of intermediates (R = Bu-t, Ph) and kinetic/isotopic labelling experiments (for R = Ph) point to a mechanism, for R = Ph, in which the rate-limiting process for dehydrocoupling involves B-H activation, while the turnover limiting process for catalysis involves the substitution of the oligomeric product at the metal, which forms a strong chelate complex with a Rh(III) centre, by H3B center dot PPh2H. When R = Bu-t no chelate product complex forms, and it is speculated that the dehydrocoupling process now becomes turnover limiting. These observations provide, for the first time we believe, data on the P-B bond forming events that are central to the dehydrocoupling and dehydropolymerisation of phosphine-boranes.