Coupling between two Ru(bda) catalysts bridged by a trans -dicyano complex.

We have prepared two trimetallic complexes [{Ru(bda)(DMSO)(μ-CN)} 2 Ru(L) 4 ] (with bda = 2,2'-bipyridine-6,6'-dicarboxylate) where two {Ru(bda)} centers are bridged by a cyanide complex of the trans -Ru(L) 4 CN 2 family (with L = pyridine and 4- tert -butylpyridine). The complex [{Ru(bda)(DMSO)(μ-CN)} 2 Ru(py) 4 ] is fully soluble in aqueous solution and is a catalyst for the oxidation of water both chemically, using Ce(IV) at pH = 1 as the terminal oxidant, and electrochemically. Both reactions are first order in the complex and the resting state of the catalyst is the [RuVRuIII(py)4RuIV]2+ redox state. Electrochemical and spectroelectrochemical studies together with (TD)DFT calculations show that the coupling between the Ru(bda) fragments for the [RuIIIRuII(py)4RuIII]2+ and [RuIVRuII(py)4RuIV]2+ redox states is very weak, but significant for the [RuVRuII(py)4RuIV]2+ ion due to the orientation of the orbitals involved. This coupling affects the reactivity of the [RuVRuII(py)4RuIV]2+ redox state, making it a much slower catalyst towards the water oxidation reaction than [RuVRuIII(py)4RuIV]2+.