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 H2 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 CH2Cl2. 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 CH3CN, assigned to a ν(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.