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

Abstract

We have prepared two trimetallic complexes [{Ru(bda)(DMSO)(μ-CN)}₂Ru(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)₄CN₂ family (with L = pyridine and 4-tert-butylpyridine). The complex [{Ru(bda)(DMSO)(μ-CN)}₂Ru(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 [Ru^VRu^III(py)₄Ru^IV]²⁺ redox state. Electrochemical and spectroelectrochemical studies together with (TD)DFT calculations show that the coupling between the Ru(bda) fragments for the [Ru^IIIRu^II(py)₄Ru^III]²⁺ and [Ru^IVRu^II(py)₄Ru^IV]²⁺ redox states is very weak, but significant for the [Ru^VRu^II(py)₄Ru^IV]²⁺ ion due to the orientation of the orbitals involved. This coupling affects the reactivity of the [Ru^VRu^II(py)₄Ru^IV]²⁺ redox state, making it a much slower catalyst towards the water oxidation reaction than [Ru^VRu^III(py)₄Ru^IV]²⁺.