How Do Proximal Hydroxy or Methoxy Groups on the Bidentate Ligand Affect [(2,2′;6′,2"‐Terpyridine)Ru(N,N)X] Water‐Oxidation Catalysts? Synthesis, Characterization, and Reactivity at Acidic and Near‐Neutral pH

Abstract

AbstractWater‐oxidation catalysts (WOCs) can potentially be improved by installing pendant electron‐donor groups that may also be proton donors or acceptors. We have modified one of the most well‐studied WOCs with alkoxy or hydroxy substituents on the bidentate bipyridine ligand (N,N), thereby forming [(terpy)RuII(N,N)X] (X = Cl, H2O; terpy = 2,2′;6′,2"‐terpyridine). A combination of NMR spectroscopy (particularly 15N chemical‐shift data), UV/Vis spectroscopy, X‐ray diffraction, and oxygen evolution data point to interesting and beneficial effects of an oxygenated group proximal to X. A methoxy group on the 2,2′‐bipyridyl (bipy) ring cis to X = Cl is shown to facilitate ionization of the chloride ligand in aqueous acetone, perhaps by acceptance of a hydrogen bond from the aquo ligand. Hydrogen‐bond donation of a proximal hydroxy group to a bound aquo ligand is shown by X‐ray diffraction. Distinct differences in pKa values for the 4,4′‐ and 6,6′‐dihydroxy bipy complexes are seen. In water oxidation driven by ceric ammonium nitrate, the 6,6′‐dimethoxy species is somewhat faster and longer‐lived than the analogue that lacks the oxygenated groups [a turnover number (TON) of 215 instead of 138 in 10 h, and a turnover frequency (TOF) of 0.36 min–1 instead of 0.23 over the same time period]. Taken together, oxygenated groups near the WOC active site are promising electron or proton donors and/or hydrogen‐bond acceptors, and are the subject of further scrutiny.