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, <b>1</b>, 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^{<b>•–</b>}, and subsequent intermolecular electron transfer to <b>1</b> produces the reduced anionic form of <b>1</b>. Time-resolved infrared spectroscopy (TRIR) has been used to probe the intermediates throughout the reduction of <b>1</b> and subsequent photocatalytic H₂ production from [HTEOA]­[BF₄], which was monitored by gas chromatography. Two structural isomers of the reduced form of <b>1</b> (<b>1a</b>^{<b>•–</b>} and <b>1b</b>^{<b>•–</b>}) were detected by Fourier transform infrared spectroscopy (FTIR) in both CH₃CN and DMF (dimethylformamide), while only <b>1a</b>^{<b>•–</b>} was detected in CH₂Cl₂. Structures for these intermediates are proposed from the results of density functional theory calculations and FTIR spectroscopy. <b>1a</b>^{<b>•–</b>} is assigned to a similar structure to <b>1</b> with six terminal carbonyl ligands, while calculations suggest that in <b>1b</b>^{<b>•–</b>} two of the carbonyl groups bridge the Fe centers, consistent with the peak observed at 1714 cm^–1 in the FTIR spectrum for <b>1b</b>^{<b>•–</b>} in CH₃CN, assigned to a ν­(CO) stretching vibration. Formation of <b>1a</b>^{<b>•–</b>} and <b>1b</b>^{<b>•–</b>} and production of H₂ was studied in CH₃CN, DMF, and CH₂Cl₂. Although the more catalytically active species (<b>1a</b>^{<b>•–</b>} or <b>1b</b>^{<b>•–</b>}) could not be determined, photocatalysis was observed only in CH₃CN and DMF