Oxygen dependent photocatalytic water reduction with a ruthenium(imidazolium) chromophore and a cobaloxime catalyst

Detailed investigations of a photocatalytic system capable of producing hydrogen under aerobic conditions are reported. These systems consist of the NHC-precursors chromophores [Ru(tbbpy)2(RR’ip)][PF6]3 Ru(RR’ip)[PF6]3 (tbbpy = 4,4’-di-tert-butyl-2,2’-bipyridine, RR’ip = 1,3-disubstituted-1H-imidazo[4,5-f][1,10]phenanthrolinium), the reduction catalyst Co(dmgH)2 (dmgH = dimethylglyoximato) and the electron donor ascorbic acid (AA). Screening studies with respect to solvent, cobaloxime catalysts, electron donors and corresponding pH-values and concentration of the single components yielded optimized photocatalytic conditions. The system show high activity according to Ru with 2000 turnover numbers under oxygen-free and aerobic conditions. The Ru complexes show high photostability and their first excited state is primarily located on the RR’ip ligand. X-ray crystallographic analysis of the rigid cyclophane-type ligand dd(ip)2(Br)2 (dd(ip)2 = 1,1',3,3'-Bis(2,3,5,6-tetramethyl-1,4-phenylene)bis(methylene)bis(1H-imidazo[4,5-f][1,10]phenanthrolinium)) and the corresponding catalytic activity of its Ru complex [{(tbbpy)2Ru}2(μ-dd(ip)2)][PF6]6 Ru2(dd(ip)2)[PF6]6 suggest an intermolecular catalytic cycle.