Photocatalytic water oxidation mediated by iridium complexes

Abstract

The results of a systematic investigation on the photocatalytic water oxidation activity of eight Ir complexes are reported. In particular, [Cp*IrLn]Xm [1, Ln=(H2O)3, X−m=(NO3−)2; 2, Ln=N-dimethylimidazolin-2-ylidene and (Cl−)2; 4, Ln=1,1′-dimethyl-3,3′-ethylene-diimidazol-2,2′- diylidene and Cl− X−=PF6−; 5, Ln=2-phenylpyridine (ppy) and NO3−; 6, Ln=2-benzoylpyridine (bzpy) and NO3−; 8, Ln=2,2′-bipyridine (bpy) and Cl−], [Ir(HEDTA)Cl]Na (3) and [Ir(ppy)2(OH2)2]OTf (7), whose water oxidation activity driven by chemical oxidants was already proved, were tested as mediators (10μM–100μM) in photocatalytic experiments exploiting [Ru(bpy)3]Cl2 (1mM) as photosensitizer and Na2S2O8 (2.5mM–30mM) as electron acceptor, at pH 5.2 and pH 8 by Na2SiF6/NaHCO3 and Na2B4O7 buffers, respectively. All complexes showed to be competent catalysts for the photolytic water oxidation. Best performances were obtained with 2 at pH 5.2, where TOF>2.5min−1, TON>80 and quantum yield>0.1. H/D kinetic isotopic effect was evaluated for 1, 2 and 8 and related to their tendency to undergo oxidative transformation as deduced by 1H NMR studies, using CH3COOH as molecular probe.