Thermodynamics of homogeneous hydrogenation: Part VII. Thermodynamics of the homogeneous hydrogenation of cyclohexene catalyzed by some water-soluble ruthenium complexes containing π-acidic ligands

Abstract

The complexes K[Ru(EDTA-H)Cl]2H₂O 1, [Ru(EDTA-H)(PPh₃)] 2, K[Ru(EDTA-H)(CO)] 3 and K[Ru(EDTA-H)(SnCl₃⁻)] 4 activate molecular hydrogen at 30°C and 1 atm H₂ in 7:3 alcoholrwater mixture by heterolytic cleavage of the H-H bond to form the thermodynamically stable hydrido complexes [Ru(EDTA-H)(H)]²⁻5, [Ru(EDTA-H)(PPh₃)(H)]²⁻6, [Ru(EDTA-H)(CO)(H)]²⁻7 and [RutEDTA-H)(SnCl₃⁻(H)]³⁻8 in solution characterized by their proton NMR in situ. Complexes 6-8 form isomeric hydrides where H iscis or trans to the π-acidic group L (L = PPh₃, CO, SnCl₃⁻), the cis species being predominant in solution. The hydrido proton peaks in complexes 5-8 shift downfield in the order 4 > 3 > 2 > 1, in accord with the decreasing π-acidity of the coordinated group. The homogeneous hydrogenation of cyclohexene catalyzed by complexes 1-4 was investigated in the temperature range 10-40°C at 0.4-1 atm of H₂ partial pressure. Thermodynamic parameters corresponding to the formation of the monohydrido complexes 5-8 and the monoolefin complexes 1-5 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 ligand. The entropies of the hydride and olefin complex formation are large positive numbers, indicating either the dissociation of a Cl⁻ from the coordination sphere of the metal ion in 1, or removal of a coordinated carboxylate group from the coordination positions of EDTA to accommodate H⁻ or olefin. The catalytic activity of complexes 1-4 decreases in the order 4 > 3 > 2 > 1, in line with the decreasing π-acidity of the secondary group L coordinated to Ru(II). The thermodynamic parameters corresponding to the steps K₁ and K₂ show small exothermic values for ΔH‡ and large negative values for ΔS‡. The slow step K₁ is the predominant step for the catalytic transfer of hydride proton to the olefin. There is a correlation between ΔH‡ of path A (K₁) , ΔH0 of hydride formation and ΔS‡ and the hydride proton shift δ (ppm), indicating the involvement of the hydride in the rate-determining step.