Photochemical dihydrogen production using an analogue of the active site of [NiFe] hydrogenase

Photoproduction of dihydrogen (H₂) by a low molecular weight analogue of the active site of [NiFe] hydrogenase has been investigated by reduction of the [NiFe₂] cluster, 1, by a photosensitier PS (PS = [ReCl(CO)₃(bpy)] or [Ru(bpy)₃][PF₆] ₂). Reductive quenching of the ³MLCT excited state of the photosensitizer by NEt₃ or N(CH₂CH₂OH) ₃ (TEOA) generates PS^•-, and subsequent intermolecular electron transfer to 1 produces the reduced anionic form of 1. Time-resolved infrared spectroscopy (TRIR) has been used to probe the intermediates throughout the reduction of 1 and subsequent photocatalytic H ₂ production from [HTEOA][BF₄], which was monitored by gas chromatography. Two structural isomers of the reduced form of 1 (1a ^•- and 1b^•-) were detected by Fourier transform infrared spectroscopy (FTIR) in both CH₃CN and DMF (dimethylformamide), while only 1a^•- was detected in CH ₂Cl₂. Structures for these intermediates are proposed from the results of density functional theory calculations and FTIR spectroscopy. 1a^•- is assigned to a similar structure to 1 with six terminal carbonyl ligands, while calculations suggest that in 1b^•- two of the carbonyl groups bridge the Fe centers, consistent with the peak observed at 1714 cm^-1 in the FTIR spectrum for 1b^•- in CH ₃CN, assigned to a v(CO) stretching vibration. Formation of 1a ^•- and 1b^•- and production of H₂ was studied in CH₃CN, DMF, and CH₂Cl₂. Although the more catalytically active species (1a^•- or 1b ^•-) could not be determined, photocatalysis was observed only in CH₃CN and DMF.