Homogeneous Catalytic Hydrogen Formation Using Dinuclear Multiply Bonded Complexes of Molybdenum (III) and Tungsten (III) and LowValency Metal Ions, M=Cr(II), V(II), in Aqueous Acidic Solutions

In the last twenty years intensive research has been done in the important field of multiply bonded dinuclear transition metal complexes. The chemistry of the (M-M)n+ containing compounds is extraordinarily rich and complex [1], and the factors governing the reactivity of metal-metal multiple bonds are now better understood. Additionally, some of the commonly observed reactions of these dimers are not found in mononuclear chemistry and point out to the importance of the (M-M)n+ unit as an inorganic functional group. In fact, the whole field of multiple bonds between metal atoms provides a distinct departure from classical coordination chemistry [2]. These species posses several attractive properties for their application as multielectron reagents or photoreagents. Firstly, their electronic structure is well defined, and often, the lowest energy excited states of many (M-M)n+ compounds are sufficiently long lived to permit their subsequent chemical reaction. Secondly, the binuclear metal core is an electron source or sink in oxidation-reduction processes. Addition of electrons to, or removal of, electrons from the metal-metal bond is accompanied by facile interconversion between (M-M)n+ dimers of different bond orders. Finally, substartes readily add to the coordinatively unsaturated metal-metal core. The ability to coordinate substrate molecules to redoxactive binuclear metal centers has important implications in the ultimate application of (M-M)n+ dimers as multi-electron catalysts or photocatalysts. Proton oxidative addition to mononuclear or cluster transition metal complexes leading to hydride formation [3] is a key step in many catalytic processes including chemical [4], photochemical [5], or biochemical [6], reactions in which dihydrogen is evolved.