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Investigation of the ultrafast dynamics occurring during unsensitized photocatalytic H2 evolution by an [FeFe]-hydrogenase subsite analogue

Frederix, Pim W. J. M. and Adamczyk, Katrin and Wright, Joseph A. and Tuttle, Tell and Ulijn, Rein V. and Pickett, Christopher J. and Hunt, Neil T. (2014) Investigation of the ultrafast dynamics occurring during unsensitized photocatalytic H2 evolution by an [FeFe]-hydrogenase subsite analogue. Organometallics, 33 (20). 5888–5896. ISSN 0276-7333

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Biomimetic compounds based upon the active subsite of the [FeFe]-hydrogenase enzyme system have been the focus of much attention as catalysts for hydrogen production: a clean energy vector. Until recently, use of hydrogenase subsite systems for light-driven hydrogen production has typically required the involvement of a photosensitizer, but the molecule [(μ-pdt)(μ-H)Fe2(CO)4(dppv)]+, (1; dppv = cis-1,2-C2H2(PPh2)2; pdt = 1,3-propanedithiolate) has been reported to catalyze the evolution of hydrogen gas under sensitizer-free conditions. Establishing the molecular mechanism that leads to photohydrogen production by 1 is thus an important step that may enable further development of this family of molecules as solar fuel platforms. Here, we report ultrafast UVpump–IRprobe spectroscopy of 1 at three different excitation wavelengths and in a range of solvents, including under the conditions required for H2 production. Combining spectroscopic measurements of the photochemistry and vibrational relaxation dynamics of 1 with ground-state density functional theory (DFT) calculations shows that, irrespective of experimental conditions, near-instantaneous carbonyl ligand loss is the main photochemical channel. No evidence for a long-lived excited electronic state was found. These results provide the first time-resolved data for the photochemistry of 1 and offer an alternative interpretation of the underlying mechanism of light-driven hydrogen generation.