Metal-ligand cooperation in H2 production and H2O decomposition on a Ru(II) PNN complex: the role of ligand dearomatization-aromatization.

The molecular mechanism for H(2) production and H(2)O decomposition on an aromatic Ru(II) PNN complex developed by Milstein and co-workers has been elucidated by detailed density functional theory calculations. The rate-determining step is heterolytic coupling of the hydride with a proton transferred from the PNN ligand, which leads to the formation of H(2). The metal center and the PNN ligand, which can be dearomatized and aromatized again, play active and synergistic roles in H(2) production and the succeeding H(2)O decomposition. Formation of the cis-dihydroxo complex as the main final product is a result of thermodynamic control.