Kinetic, spectroscopic, and X-ray crystallographic evidence for the cooperative mechanism of the hydration of Nitriles catalyzed by a tetranuclear ruthenium-μ-oxo-μ-hydroxo complex

The tetranuclear ruthenium-oxo-hydroxo-hydride complex {[(PCy3)(CO)RuH](4)(mu(4)-O)(mu(3)-OH)(mu(2)-OH)} (1) was found to be a highly cooperative catalyst for the nitrile hydration reaction. The cooperative mechanism of the hydration of benzonitrile was established by Hill inhibition kinetics. The treatment of a nitrile substrate with complex 1 led to the catalytically relevant nitrile-coordinated tetraruthenium complex 3. The X-ray structure of the nitrile-coordinated complex 3 showed a considerably "relaxed" tetrameric core structure compared to that of 1. The hydration of para-substituted berizoniffiles P-X-C6H4CN with an electron-withdrawing group (X = Cl, Br, CO2H, CF3) exhibited cooperative kinetics, as indicated by the sigmoidal saturation kinetics, while the hydration of nitriles with an electron-donating group (X = OH, OMe, t-Bu, CH3) obeyed Michaelis-Menten saturation kinetics. The formation of a ruthenium hydride species was observed during the hydration of methacrylonitrile, and its monomeric nature was established by using DOSY NMR techniques.