Two‐Dimensional Infrared Spectroscopy Reveals the Presence of a Bridging CO Ligand in Two Catalytic Intermediates of [FeFe] Hydrogenase

[FeFe] hydrogenases are highly active, reversible enzymes for the interconversion of hydrogen with protons and electrons. Their active site H-cluster consists of a canonical [4Fe-4S] cluster covalently linked to a unique [2Fe]H centre. Their catalytic mechanism has been studied extensively, but several details remain disputed, and two rival models exist in the literature. One crucial difference between these models is the structure and catalytic relevance of two states named HredH+ and HsredH+. In the first model, these states are catalytic intermediates containing a reduced [Fe(I)Fe(I)]H centre and a bridging CO ligand (µCO), while in the second model they are inactive states containing an oxidised [Fe(II)Fe(II)]H site and a bridging hydride ligand (µH−). The second proposal was initially based on the lack of a prominent absorption peak attributed to a µCO ligand in the infrared (IR) spectra of both states. Here, we provide evidence for the presence of a µCO ligand in the HredH+ and HsredH+ states using two-dimensional (2D) IR spectroscopy, firmly establishing the structure of these states as [Fe(I)Fe(I)]H with a µCO ligand. The results suggest that these states are catalytically relevant intermediates with crucial implications for understanding hydrogen conversion in nature and designing new synthetic catalysts.