Homogeneous hydrogenation of cyclohexene catalyzed by Ru(II) and (III) and their trichloro-stannato complexes: proton NMR and rate studies

Abstract

The hydrogenation of the complexes [RuCl₃(DPA)] 1, [Ru(SnCl₃)-Cl₂(DPA)]2, [RuCl₂(DPA)₂]Cl 3, [Ru(SnCl₃)Cl(DPA)₂]Cl 4, [RuCl₂(PPh₃)-(DPA)] 5, [Ru(SnCl₃)Cl(PPh₃)(DPA)] 6, [RuCl(DMSO)₂(DPA)]Cl 7 and [Ru(SnCl₃)(DMSO)₂(DPA)]Cl 8, where DPA=HN(CH₂CH₂PPh₂)₂ show the formation of hydrides in benzene-ethanol and 1 atm of H₂ at 30°C in situ, as confirmed by ¹H NMR spectra. The homogeneous hydrogenation of cyclohexene catalysed by the above complexes was studied at 30°C and 1 atm of molecular hydrogen in benzene-ethanol mixture. A mechanism for the homogeneous hydrogenation reaction is proposed where the coordination of the olefin to the hydride complex is rate determining, followed by a rapid transfer of the hydrogen to the olefin to form the alkane. The ratio of the equilibrium constants K₂/K₁ for the formation of olefin/hydrido complex and the rate constant k (rate-determining step) were calculated and reported. The variation of the rates for catalytic hydro-genation of cyclohexene with the nature of the catalyst is discussed.