A Novel Bicyclophosphite Ligand Directly Yields Rhodium Hydride Complexes

Summary.

 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.