A Novel Bicyclophosphite Ligand Directly Yields Rhodium Hydride Complexes

Complexation of Rh(I) with o, o′-dimethylene-(tris-p-cresyl)-bicyclophosphite (BCP, 1) has been investigated in solution by NMR, semi-empirical quantum mechanical, and molecular mechanics calculations. 1H and 31P NMR spectroscopic data show that when the BCP/Rh ratio exceeds 2, Rh hydride complexes of the composition RhH(BCP)3 and RhH(BCP)4 are formed. The source of the hydride ion is the ligand itself; most probably, H− originates from the bridging CH2 groups of BCP. The chemical shifts of these protons are sensitive to complexation due to the considerable electron density of HOMO and LUMO at one of the bridging CH2 moieties. Molecular mechanics simulations of the molecular structure of these complexes show that two cavities are formed in [Rh(BCP)3]+ by the aromatic rings of the ligands. These cavities may alternatively open and close, thus providing for a flexibly shielded catalytic site which explains the unusual catalytic behaviour of Rh complexes with BCP in hydrogenation and hydroformylation reactions.