Ruthenium Complexes of 2,2′‐Bipyridine‐6,6′‐diphosphonate Ligands for Water Oxidation

Abstract

Abstract Two novel ruthenium complexes, [Ru(2,2′‐bipyridine‐6,6′‐diphosphonato)(pic) 2 ] ( 2 ) (pic=4‐picoline) and [Ru(6,6′‐diisopropyl‐2,2′‐bipyridine‐6,6′‐diphosphonato)(pic) 2 ] ( 3 ) bearing phosphonate groups have been synthesized and characterized by NMR spectroscopy, elemental analyses, X‐ray crystallography, cyclic voltammetry, and UV/Vis spectroscopy. Both complexes show catalytic water oxidation activity by electrochemistry. At pH 7, the Ru II/III redox couple of 2 is observed at a lower potential than that of 3 , yet significantly, 3 oxidizes water at a lower onset potential. At pH 1 however, 2 and 3 have comparable catalytic reactivity using sacrificial oxidant Ce IV . We propose that water oxidation activities of 2 and 3 are influenced by overall charges. For example, in oxidizing from Ru II to Ru III at pH 7, 2 acquires a −1 overall charge whereas 3 acquires a +1 charge. Charge‐dictated electrostatic effects may govern binding of a water molecule to the metal site.