Thermodynamics of homogeneous hydrogenation: Part X. Thermodynamics of the homogeneous hydrogenation of cyclohexene catalysed by Ir(I) complexes of phosphine donor ligands

Abstract

The oxidative addition of H₂ to the iridium(I) complexes [IrCl(PPh₃)₂S] 1, [IrCl(diphos)S] 2 and [IrCl(diphos)PPh₃] 3 (diphos = 1,2-bis(diphenylphosphino)ethane, S = solvent) proceeds at 30°C and 1 atm of molecular hydrogen in ethanol-benzene with the formation of thermodynamically stable hydrides. The homogeneous hydrogenation of cyclohexene catalysed by complexes 1-3 was investigated in the temperature range 10-40°C and 0.6-1 atm of H₂ partial pressure. Thermodynamic parameters for the formation of the hydrido complexes of 1-3 and the mono-olefin complexes of Ir(I) were computed. The activation parameters corresponding to the rate constant k' for the homogeneous hydrogenation of cyclohexene were also calculated. The enthalpy of formation ΔH⁰ for the hydrido and olefin complexes are more exothermic for the Ir(I) complexes as compared to Rh(I) complexes.