Synthesis and Characterization of New Iridium Photosensitizers for Catalytic Hydrogen Generation from Water

Novel phenylazole ligands were applied successfully in the synthesis of cyclometalated iridium(III) complexes of the general formula [Ir(phenylazole)2(bpy)]PF6 (bpy=2,2′‐bipyridine). All complexes were fully characterized by NMR, IR, and MS spectroscopic studies as well as by cyclic voltammetry. Three crystal structures obtained by X‐ray analysis complemented the spectroscopic investigations. The excited‐state lifetimes of the iridium complexes were determined and showed to be in the range of several hundred ns to multiple µs. All obtained iridium complexes were active as photosensitizers in catalytic hydrogen evolution from water in the presence of triethylamine as a sacrificial reducing agent. Applying an in situ formed iron‐based water reduction catalyst derived from [HNEt3]+[HFe3(CO)11]− and tris[3,5‐tris‐(trifluoromethyl)‐phenyl]phosphine as the ligand, [Ir(2‐phenylbenz‐oxazole)2‐(bpy)]PF6 proved to be the most efficient complex giving a quantum yield of 16 % at 440 nm light irradiation. Six new iridium complexes of the general formula [Ir(phenylazole)2(bpy)]PF6 (bpy=2,2′‐bipyridine) were tested as photosensitizers in hydrogen evolution from water. The complex with 2‐phenyl benzoxazole as a cyclometalating ligand (PS4) showed the best performance with a quantum yield of 16 % (see figure).