Energy conversion from proton gradient to electricity based on characteristic redox behavior of an aqua ruthenium complex with a quinone ligand

A ruthenium aqua complex having a quinone ligand, [Ru(trpy)(dbq)(H₂O)](ClO₄)₂ (trpy = 2,2':6',2_-terpyridine, dbq = 3,5-di- t-butyl-1,2-benzoquinone) ([Ru(q)(H₂O)](ClO₄)₂) was prepared. Its electrochemical properties and electronic absorption spectra were measured in the presence of a base in acetone. The detailed analysis of those measurements revealed that the addition of a base caused not only the deprotonation but also the reduction of [Ru(q)(H₂O)]²⁺. The redox reactions coupled with acid-base reactions were demonstrated from the large difference in redox properties of aqua and hydroxo complexes. Taking advantage of unique redox reactions induced by the acid-base equilibrium between aqua andhydroxo complexes, we have succeeded in constructing the first energy transducer which converts the proton gradient to electricity. A similar ruthenium aqua complex with a bipyridine ligand, [Ru(trpy)(bpy)(H₂O)](ClO₄)₂, also reversibly dissociates a proton of aqua ligand. However, it has no ability to convert the proton gradient to electricity due to the lack of a suitable molecular orbital which can accommodate electrons on the electron-rich hydroxo ligand.