Towards biomimetic models of the reduced [FeFe]-hydrogenase that preserve the key structural features of the enzyme active site; A DFT investigation

[FeFe]-hydrogenases are the most efficient biological catalysts available for the H2 evolution reaction. Their active site-the H-cluster-features a diiron subsite which has the peculiar characteristic of bearing cyanide groups hydrogen-bonded to the apoprotein as well as carbonyl ligands. Notably, one of the CO ligands is disposed in bridging position between the metal centers. This allows one of the Fe ions to retain a square pyramidal coordination-which determines the assumption of the so-called "rotated structure"-with a vacant coordination site in trans to the μ-CO group, ready to bind protons when the active site is in the FeIFeI state. Many FeIFeI biomimetic models have been synthesized and characterized so far, but most of them fail to reproduce the orientation of the diatomic ligands that is observed in the enzyme active site. In the present contribution we carried out a density functional theory investigation, with the aim of evaluating whether the establishment of hydrogen bonding at the level of cyanides is sufficient to favor rotation of ligands around one of the Fe centers, in analogy with the reduced H-cluster. To this end, we carried out an investigation of the potential energy surface of an isolated Fe2S2 model bearing CN and CO groups, as well as of the supramolecular complex formed by the diiron model and a porphyrin derivative hydrogen-bonded to the former. As far as the isolated Fe2S2 species are concerned, the sole μ-CO models individuated in the course of our potential energy surface scans are the ones in which both cyanides are bound to same iron center, while no CO-bridged minima could be found in the case of models having one CN group bound to each of the metal ions. The latter represents a major difference with respect to the coordination geometry of the reduced diiron subcluster in the enzyme. However, perturbation of the diiron model by a porphyrin ring designed to donate hydrogen bonds to the cyanide groups-thus, at least partially, reproducing the network of H-bond between the H-cluster and the apoprotein in [FeFe]-hydrogenases-significantly changes the picture in this regard. Therefore, possible strategies to modulate the disposition of ligands around the metal centers of biomimetic [FeFe]-hydrogenases models are discussed in light of computed geometries and relative stabilities of the supramolecular complexes