Catalysis Today, Vol.290, 10-18, 2017
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]X-m [1 L-n = (H2O)(3), X-m(-) = (NO3-)(2); 2, L-n =N-dimethylimidazolin-2-ylidene and (Cl-)(2); 4, L-n =1,1'-dimethy1-3,3'-ethylene-diimidazol-2,2'- diylidene and Cl- X- =PF6-; 5, L-n = 2-phenylpyridine (ppy) and NO3-; 6, L-n = 2-benzoylpyridine (bzpy) and NO3-; 8, L-n = 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 mu M -100 mu M) in photocatalytic experiments exploiting [Ru(bpy)(3)]Cl-2 (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 mM(-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 H-1 NMR studies, using CH3COOH as molecular probe. (C) 2016 Elsevier B.V. All rights reserved.