Hydroxy-bridged resting states of a [NiFe]-hydrogenase unraveled by cryogenic vibrational spectroscopy and DFT computations.

The catalytic mechanism of [NiFe]-hydrogenases is a subject of extensive research. Apart from at least four reaction intermediates of H/H cycling, there are also a number of resting states, which are formed under oxidizing conditions. Although not directly involved in the catalytic cycle, the knowledge of their molecular structures and reactivity is important, because these states usually accumulate in the course of hydrogenase purification and may also play a role during hydrogenase maturation.

Vibrational characterization of a diiron bridging hydride complex - a model for hydrogen catalysis.

A diiron complex containing a bridging hydride and a protonated terminal thiolate of the form [(μ,κ-bdtH)(μ-PPh)(μ-H)Fe(CO)] has been investigated through Fe nuclear resonance vibrational spectroscopy (NRVS) and interpreted using density functional theory (DFT) calculations. We report the Fe-μH-Fe wagging mode, and indications for Fe-μD stretching vibrations in the D-isotopologue, observed by Fe-NRVS. Our combined approach demonstrates an asymmetric sharing of the hydride between the two iron sites that yields two nondegenerate Fe-μH/D stretching vibrations.