Thermodynamics of homogeneous hydrogenation: Part II. Thermodynamics of the homogeneous hydrogenation of 1-heptene catalysed by bis(diphenylphosphinoethyl)amine chlororhodium(I)

Abstract

The homogeneous hydrogenation of 1-heptene catalysed by bis(di- phenylphosphinoethyl)amine chlororhodium(I) (RhClHN(CH₂CH₂PPh₂)₂, or RhCl(DPPA)) over the temperature range 10-40°C is reported. The dependence of the rate of hydrogenation on factors such as temperature (10-40°C), catalyst, substrate and H₂ concentrations (partial pressure 0.6-1atm) was studied and reported. The order of the reaction with respect to [1-heptene] and [H₂] is fractional, and that to [catalyst] is first order. A mechanism for the homogeneous hydrogenation reaction is proposed whereby the coordination of the olefin to the dihydridorhodium(III) catalyst intermediate is rate-determining, before the rapid transfer of the hydrides to the olefin to form the alkane. The thermodynamic parameters corresponding to the equilibrium constants K₁ and K₂ for the formation of the dihydrido and olefin complexes, respectively, and the activation parameters corresponding to the rate constant k' (rate-determining step) were calculated. The thermodynamic parameters involving the formation of the complex olefin (1-heptene) (S) species [RhCl(DPPA)(S)] were compared with those for the corresponding cyclohexene complex formation. The activation parameters for the hydrogenation of 1-heptene and cyclohexene catalysed by RhCl(DPPA) are compared, and the variation of the rates with the nature of the alkane discussed.