CO Affinity and Bonding Properties of [FeFe] Hydrogenase Active Site Models. A DFT Study

In this work a density functional theory study of the CO addition reaction to FeIFeI and FeIFeII models of the active site of [FeFe] hydrogenases is presented. A series of model complexes, ranging from simple diiron model complexes of the binuclear [2Fe]H subcluster to the full H-cluster, have been investigated. For each system, the thermodynamic parameters for the CO adduct formation, a reaction that mimics the enzyme CO inhibition, were computed. Parallel to the investigation of the CO addition reaction, the structural features of the various FeIFeI and FeIFeII species have been evaluated, with particular attention to the issue of the ligand arrangement as a function of the redox state. CO affinity depends on the redox state of the model and the chemical nature of its ligands. FeIFeII species are more favorable to form the CO adducts than the reduced FeIFeI species. According to the computed free energies and enthalpies for the CO adduct formation from Fe2(pdt)(CO)5L models, the CO affinity follows the ligand sequence L = SCH3- > CN- > PPh3 > CO (FeIFeI) and L = CO > CN- > PPh3 > SCH3- (FeIFeII). As the models become more similar to the H-cluster, the CO affinity increases, although the FeIFeI CO -inhibited H-cluster is not stable. The bonding properties of the models considered have been investigated by means of the quantum theory of atoms in molecules approach. Upon CO addiction, the new Fe-C bond is formed to the detriment of the Fe-Fe bonds and, to a lesser extent, the Fe-S bonds. Regarding the FeIFeII systems investigated, the spin density is initially localized on the rotated Fe atom, and the formation of the CO adducts results in a delocalization of the spin density. Consequently, the FeIFeII CO-inhibited forms are better described as (Fe+1.5)2.