Thermodynamics of the hydrogenation of olefins catalyzed by Rh(I) and Ir(I) complexes

Abstract

The homogeneous hydrogenation of cyclohexene catalyzed by Rh(I) and Ir(I) complexes of the terdentate ligands (L) HN(CH 2CH 2Aφ 2) 2 (A = P, As) was investigated in the temperature range 20 - 50°C. Thermodynamic parameters corresponding to the formation of the dihydrido complexes ML(H) 2Cl (M = Ir(I), Rh(I)) and the olefin complexes MLCl(olefin) were computed. The activation parameters corresponding to the rate constant were also calculated. An inverse relationship is found between the enthalpy of formation Δ H 0 of the dihydrido complexes and the enthalpy of activation Δ H ≠ of the hydrogenation step. This relationship establishes the involvement of the dihydrido complexes as the active intermediates in olefin coordination and hydrogen transfer. The stereochemistry of the terdentate complexes in dihydride formation is discussed. It is concluded that the enthalpy of formation Δ H 0 of the dihydrido complexes of terdentate ligands is very favourable, as there is no change in the configuration of the ligand in oxidative addition reaction. The significance of the steric factors in the hydrogenation step is discussed.