Thermodynamics of homogeneous hydrogenation: Part VI. Thermodynamics of the homogeneous hydrogenation of cyclohexene catalyzed by Rh(I) and Ir(I) complexes of polydentate phosphine ligands

Abstract

The oxidative addition of H 2 to the mixed ligand Rh(I) complex [RhCl(PPh 3)(diphos)] 1 and the Rh(I) and Ir(I) complexes of the terdentate ligands NP 3, [N(CH 2CH 2PPl 2) 3] and triphos, [PhP(CH 2CH 2PPh 2] 2 MLC1 2 – 5 (where M = Rh(I), Ir(I); L = NP 3, triphos) proceeds at 30 °C and 1 atm of Ha in ethanol-benzene with the formation of the thermodynamically stable cis dihydrides [RhCl(PPh 3)(diphos)(H) 2] 1a, 1b; [RhCl(NP 3)(H) 2] 2a, 2b; [RhCl(triphos)(H)2] 3a, 3b; [IrCl(NP3)(H)2] 4a and [IrCl(triphos)-(H) 2] 5a, 5b characterized by their hydride NMR in situ. The hydrides of iridium 4a, 4b, 5a, 5b are more stable in solution than their corresponding rhodium analogues. The homogeneous hydrogenation of cyclohexene catalyzed by complexes 1 – 5 was investigated in the temperature range 10 – 40 °C at 0.6 – 1 atm of h 2 partial pressure. Thermodynamic parameters for the formation of the dihydrido complexes of 1 – 5 and the monoolefin complexes of Rh(I) and 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 Δ H o for the formation of hydrido and olefin complexes are more exothermic for the Ir(I) complexes compared to the Rh(I) complexes. The catalytic activity of the complexes decreases in the order 2 > 1 > 3 > 4 ≈ 5. The catalytic activity of the chelates 2 and 4 of the potential terdentate ligand NP 3 with a large chelate ring is much higher than complexes 3 and 5 of the rigid terdentate ligand triphos. The catalytic activity of the Rh(I) and Ir(I) complexes of triphos is more than those of the NP 2 [HN(CH 2CH 2PPh 2) 2] complexes. An inverse relationship exists between Δ H o, the enthalpy of formation of the hydrido complexes and Δ H ‡, the enthalpy of activation of the hydrogenation reaction of cyclohexene by a variety of polydentate complexes of Rh(I) and Ir(I) with P or N, P donors.