Thermodynamics of homogeneous hydrogenation: Part VIII. Effects of secondary ligand on the catalytic properties in the homogeneous hydrogenation of cyclohexene by some water soluble ruthenium complexes

Abstract

Homogeneous catalytic hydrogenation of cyclohexene catalyzed by Na[Ru(EDTA-H)N₃]·2H₂O 1 and [Ru(EDTA-H)NO]BF₄2 was investigated in the temperature range 283-313 K at 0.4 to 1 atm of hydrogen partial pressure in a 7:3 alcohol-water mixture. The dependence of the rate of hydrogenation on factors such as catalyst concentration, cyclohexene concentration, hydrogen partial pressure and temperature is reported. Kinetic investigations reveal a non-linear dependence of rate on cyclohexene and molecular hydrogen concentration, and a first-order dependence on the catalyst concentration. Based on the kinetic data, a mechanism for the homogeneous catalytic hydrogenation has been proposed. Thermodynamic parameters corresponding to the formation of the monohydrido and monoolefin complexes were computed. The activation parameters corresponding to the rate constants k₁ and k₂ for the homogeneous hydrogenation of cyclohexene were also calculated. The enthalpy of the formation of hydrido and olefin complexes decreases with an increase in the π-acidity of the coordinated secondary ligand. The large positive value of entropies for the hydride and olefin complex formations indicates that the reaction mechanism is dissociative, in accord with the removal of a coordinated carboxylate group of EDTA from the coordination sphere of the metal ion to make way for H⁻ or olefin. A ligand constant P_L for the secondary ligands in the complexes [Ru(EDTA-H)X]ⁿ⁻ was calculated from the redox potential values E½ for the Ru^IIIRu^II couple in the complexes. Taking the E½ value of [Ru(EDTA-H)(H₂O)] as standard, the P_L values for other π-acidic substituents were calculated by subtracting E½ (H₂O) from E½ (X). The values of P_L increase with an increase in the π-acidity of the ligand in the order N₃⁻ < H₂O < PPh₃ < NO < CO < SnCl₃⁻ < olefin. The catalytic activity of the complexes [Ru(EDTA-H)X]ⁿ⁻ increases with an increase in the π-acidity of the secondary ligand. The hydride proton NMR signals of the [Ru(EDTA-H₂)(H)(X)]ⁿ⁻ hydrides shift further downfield with the positive value of P^L.