Capping Arene Ligated Rhodium-Catalyzed Olefin Hydrogenation: A Model Study of the Ligand Influence on a Catalytic Process That Incorporates Oxidative Addition and Reductive Elimination

The ligand influence on olefin hydrogenation using four capping arene ligated Rh(I) catalyst precursors (FP)Rh(η²-C₂H₄)Cl {FP = capping arene ligands, including 6-FP (8,8′-(1,2-phenylene)diquinoline), 6-^NPFP (8,8′-(2,3-naphthalene)diquinoline), 5-FP (1,2-bis(N-7-azaindolyl)benzene) and 5-^NPFP [2,3-bis(N-7-azaindolyl)naphthalene]} has been studied. Our studies indicate that relative observed rates of catalytic olefin hydrogenation follow the trend (6-FP)Rh(η²-C₂H₄)Cl > (5-FP)Rh(η²-C₂H₄)Cl. Based on combined experimental and density functional theory modeling studies, we propose that the observed differences in the rate of (6-FP)Rh(η²-C₂H₄)Cl and (5-FP)Rh(η²-C₂H₄)Cl-catalyzed olefin hydrogenation are most likely attributed to the difference in the activation energies for the dihydrogen oxidative addition step. We are unable to directly compare the rates of olefin hydrogenation using (6-^NPFP)Rh(η²-C₂H₄)Cl and (5-^NPFP)Rh(η²-C₂H₄)Cl as the catalyst precursor since (5-^NPFP)Rh(η²-C₂H₄)Cl undergoes relatively rapid formation of an active catalyst that does not coordinate 5-^NPFP.