The missing intermediate in the catalytic cycle of [FeFe]-hydrogenases: Diiron site reduced state featuring a bridging CO ligand observed at room temperature

The metalloenzymes hydrogenases are natures redox catalyst for molecular hydrogen (H2) uptake and production. In [FeFe]-hydrogenases catalysis is facilitated at a unique diiron site. The two iron ions are connected by an azadithiolate bridging ligand (−SCH2NHCH2S−, ADT) and coordinated by a CO and CN− ligand each. Upon reduction of this diiron site the ligand arrangement of a third CO ligand is heavily debated. Two main geometries that differ by the origin of a bridging ligand in between the iron ions and the occupation of a free binding site are discussed. In a first scenario a hydride is bound in the bridging position and the third CO ligand occupies the open coordination site resulting in a geometry not favourable for catalysis. Experimental data indicative of the latter case has been collected at room temperature. In a second scenario the third CO ligand occupies a position in between the two iron ions resulting in a free coordination site favoured for H2 catalysis. Here up to date, experimental data indicative of this geometry, in particular the infrared band of the bridging CO (µCO) ligand, has only been observed at cryogenic temperatures questioning its catalytic relevance. In this study, we characterise the diiron site reduced intermediate that features the µCO geometry and preserves the open coordination site via FTIR spectroscopy at room temperature. The observation of this proposed diiron site reduced reaction intermediate with µCO geometry at room temperature makes its involvement in H2 catalysis more likely.