Photoinduced Terminal Hydride of [FeFe]-Hydrogenase Biomimetic Complexes

The active site of the [FeFe]-hydrogenase ([FeFe]-H₂ase) has a bridging carbonyl ligand and a terminal hydride in the key H-cluster intermediate H_hyd. However, nearly all of the synthetic mimics reported, so far, prefer a hydride bridging the two irons, and only few mimics with a terminal hydride were achieved by tuning the steric effects of bulky diphosphine ligands. Moreover, although intermediates with either a terminal hydride or a protonated bridging thiolate ligand were proposed to exist during protonation processes or hydrogen exchange in the [FeFe]-H₂ase mimic, [Fe₂(μ-pdt)(μ-H)(CO)₄₍PMe₃)₂]⁺ (1H), only bridging hydrides were observed by time-resolved IR spectroscopy. In this report, FTIR spectroscopy of 1H, under CO with longer irradiation time, revealed several new photoinduced species. In addition to the CO loss species, many of the photoinduced products can be assigned to 1H with a terminal hydride by comparison of their CO vibrational frequencies with density functional theory calculations.