N-Heterocyclic Dicarbene Iridium(III) Catalysts Enabling Water Oxidation under Visible Light Irradiation

Novel iridium(III) di-NHC complexes have been synthesized by transmetalation of the dicarbene ligand from the corresponding dicarbene silver complex. The iridium(III) complex [IrClCp*(di-NHC)](PF6) (2; di-NHC = 1,1'-dimethyl-3,3'-ethylenediimidazole- 2,2'-diylidene) behaves as an efficient catalyst for the oxidation of water by using Ce-IV or NaIO4 as sacrificial oxidants, reaching a TOF of 0.20 s(-1), comparable with the most active catalysts reported in the literature. Complex 2 also displays remarkable catalytic activity in the photocatalyzed oxidation of water by using [Ru(bpy)(3)](2+) as sensitizer and persulfate as sacrificial electron acceptor. The fate of the catalyst under turnover conditions was investigated by combined spectroscopic, kinetic and GC-MS methods. In particular, EPR experiments performed under photoirradiation revealed the formation of a high-valent Ir-IV molecular intermediate involved in the primary photoinduced electron-transfer events.