Water oxidation catalysis with ligand substituted Ru-bpp type complexes

A series of symmetric and non-symmetric dinuclear Ru complexes of general formula {[Ru(R²-trpy)(H₂O)][Ru(R³-trpy)(H₂O)](μ-R¹-bpp)}³⁺ where trpy is 2,2′:6′,2′′-terpyridine, bpp^- is 3,5-bis(2-pyridyl)-pyrazolate and R¹, R² and R³ are electron donating (ED) and electron withdrawing (EW) groups such as Me, MeO, NH₂ and NO₂ have been prepared using microwave assisted techniques. These complexes have been thoroughly characterized by means of analytical (elemental analysis), spectroscopic (UV-vis, NMR) and electrochemical (CV, SQWV, CPE) techniques. The single crystal X-ray structures for one acetate- and one chloro-bridged precursor have also been solved. Kinetic analysis monitored by UV-vis spectroscopy reveals the electronic effects exerted by the ED and EW groups on the substitution kinetics and stoichiometric water oxidation reaction. The catalytic water oxidation activity is evaluated by means of chemically (Ce^IV), electrochemically, and photochemically induced processes. It is found that, in general, ED groups do not strongly affect the catalytic rates whereas EW groups drastically reduce catalytic rates. Finally, DFT calculations provide a general and experimentally consistent view of the different water oxidation pathways that can operate in the water oxidation reactions catalyzed by these complexes.