Synthesis and characterization of N-heterocyclic carbene substituted phosphine and phosphite rhodium complexes and their catalytic properties in hydrogenation reactions

Mixed N-heterocyclic carbene-substituted phosphine and phosphite complexes of rhodium were prepared, starting from [Rh(COE)₂Cl]₂ (COE = cyclooctene) by addition of free N-heterocyclic carbenes (NHC) and PR ₃. All new complexes were characterized by spectroscopy. In addition, the structures of trans-chloro(1,3-dicyclohexylimidazol-2-ylidene)- bis(triphenylphosphite)rhodium(I) (5), chloro-trans-bis(1,3- dicyclohexylimidazol-2-ylidene) (triphenylphosphine)rhodium(I) (6), and chloro(η⁴-1,5-cyclooctadiene)(1,3-di-[(1R,2S,5R) -2-isopropyl-5-menthylcyclohex-1-yl]imidazol-2-ylidene)rhodium(I) (8) were determined by single crystal X-ray analyses. The hydrogenation of cyclohexene using molecular hydrogen has been optimized for some N-heterocyclic carbene-substituted phosphine and phosphite rhodium complexes by variation of the reaction conditions.