Ligand Effects on the Redox Chemistry of (Oxo)(Phosphine)Ruthenium(IV) Complexes: Oxidation of Alcohols

Oxoruthenium complexes which contain para-substituted triphenylphosphine ligands cleanly oxidize para-substituted benzyl alcohols to the corresponding benzaldehydes. Linear free energy correlations between the logarithms of the second-order rate constants of benzyl alcohol oxidation and either the potential values of the ruthenium(IV)/ruthenium(III) redox couples of the oxoruthenium complexes or the Hammett a values of the substituents of the para-substituted triphenylphosphine ligands were observed. However, there were no good correlations for plots of log (kx/ku) versus the Hammett substituent constants σ, σ⁺, or σ^- for the oxidation of the para-substituted benzyl alcohols by [Ru(bpy)₂(O)PPh₃](CiO₄)₂. Notably, the plot of log (kx/kn) -σ⁺ versus σ^-, where σ⁺ is the free radical stabilization constant, gave an excellent correlation with p = -0.57 and R² = 0.99 for the oxidation of the para-substituted benzyl alcohols by [Ru(bpy)₂(O)PPh₃](CIO₄)2 in methylene chloride. From these and other results, we propose a reaction pathway for the oxidation of benzyl alcohol by oxoruthenium(IV) complexes which involves a partial hydrogen atom abstraction from the benzylic carbon in the rate-determining step.