Photocatalytic water oxidation mediated by iridium complexes

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 (10 μM–100 μM) in photocatalytic experiments exploiting [Ru(bpy)3]Cl2 (1 mM) as photosensitizer and Na2S2O8 (2.5 mM–30 mM) 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 > 2.5 min−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.