Thermodynamics of homogeneous hydrogenation: Part V. Hydrogenation of cyclohexene catalyzed by Rh(I) complexes of the bidentate (NP) ligand, (2-(diphenylphosphino)ethyl)benzylamine

Abstract

The oxidative addition of H₂ to the Rh(I) complexes of the bidentate NP ligand, (2-(diphenylphosphino)ethyl)benzylamine, viz. [Rh(NP)(Cl)(CO)] 1, [Rh(NP)] 2, [Rh(NP)₂Cl] 3 proceeds at 30°C and l atm H₂ m 7:3 benzene-ethanol solvent with the formation of the thermodynamically stable cis dihydrides [Rh(NP)(Cl)(CO)(H)₂] 1a, 1b, [Rh(NP)] 2a, 2b and [Rh(NP)₂(H)₂Cl] 3a, 3b characterized by their hydride NMR in situ. The homogeneous hydrogenation of cyclohexene catalyzed by complexes 1-3 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 Rh(III) dihydrido complexes of 1-3 and the corresponding monoolefin complexes of Rh(I) were computed. The activation parameters corresponding to the rate constant k for the homogeneous hydrogenation of cyclohexene were also calculated. The catalytic activity of the complexes decreases in the order 2 > 3 » 1. Except complex 1, the catalytic activity of the bidentate complexes of Rh(I) 2 and 3 is higher than those of RhCl(diphos) 4 (diphos = Ph₂PCH₂CH₂PPh₂) and RhCl(NP₂) 5 (NP₂=H-N(CH₂CH₂-PPh₂)₂). This is because the stabilities of the hydrides of 2 and 3 are lower than those of 4 and 5.