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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

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Abstract

Water-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.

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