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 H² to the mixed ligand Rh(I) complex [RhCl(PPh₃)(diphos)] 1 and the Rh(I) and Ir(I) complexes of the terdentate ligands NP₃, [N(CH₂CH₂PPl₂)₃] and triphos, [PhP(CH₂CH₂PPh₂]₂ MLC1 2-5 (where M=Rh(I), Ir(I); L=NP₃, triphos) proceeds at 30°C and 1 atm of Ha in ethanol-benzene with the formation of the thermodynamically stable cis dihydrides [RhCl(PPh₃)(diphos)(H)₂] 1a, 1b; [RhCl(NP₃)(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 h₂ 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 δH^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 NP₃ 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 NP₂ [HN(CH₂CH₂PPh₂)₂] complexes. An inverse relationship exists between δH^o, the enthalpy of formation of the hydrido complexes and δH^‡, 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.