Carbonyl-Amplified Catalyst Performance: Balancing Stability against Activity for Five-Coordinate Ruthenium Hydride and Hydridocarbonyl Catalysts

The activity of RuHCl(H-2)(PCy3)(L) 2a/b and RuHCl(CO)(PCy3)(L) 3a/b (a: L = PCy3; b: L = IMes; IMes = 1,3-dimesitylimidazol-2-ylidene) was assessed in hydrogenation of a range of molecular and polymeric olefins and in hydrogenation and isomerization of allylbenzene. Elevated temperatures and/or high H-2 pressures are required for efficient hydrogenation. Under these conditions, 3a/b outperform their dihydrogen analogues in both total productivity and turnover frequencies, despite theoretical and experimental evidence that the pi-acid CO ligand should be deactivating. A thermolysis study reveals that the CO complexes are much less susceptible to deactivation under conditions relevant to catalysis (also, the IMes derivative 2b is shorter-lived than 2a). We attribute the superior performance of 3a/b to their greater stability, which maintains higher concentrations of active catalyst than that possible for their H-2 analogues over the time scale of hydrogenation.