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

Abstract

A diiron complex containing a bridging hydride and a protonated terminal thiolate of the form [(μ,κ₂-bdtH)(μ-PPh₂)(μ-H)Fe₂(CO)₅]⁺ has been investigated through ⁵⁷Fe 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 ⁵⁷Fe-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. The studied complex provides an important model relevant to biological hydrogen catalysis intermediates. The complex mimics proposals for the binuclear metal sites in [FeFe] and [NiFe] hydrogenases. It is also an appealing prototype for the ‘Janus intermediate’ of nitrogenase, which has been proposed to contain two bridging Fe–H–Fe hydrides and two protonated sulfurs at the FeMo-cofactor. The significance of observing indirect effects of the bridging hydride, as well as obstacles in its direct observation, is discussed in the context of biological hydrogen intermediates.