Density functional theory investigation of the active site of [Fe]-hydrogenases: Effects of redox state and ligand characteristics on structural, electronic, and reactivity properties of complexes related to the [2Fe](H) subcluster

The effects of redox state and ligand characteristics on structural, electronic, and reactivity properties of complexes related to the [2Fe](H) subcluster of [Fe]-hydrogenases have been investigated by DFT calculations and compared with experimental and theoretical data obtained investigating both the enzyme and synthetic model complexes. Our results show that (FeFeII)-Fe-II species characterized by OH or H2O groups terminally coordinated to the iron atom distal to the terminal sulfur ligand (Fe-d) are less stable than corresponding mu-OH or mu-H2O species, suggesting that the latter are destabilized or kinetically inaccessible in the enzyme. In addition, results obtained investigating (FeFeI)-Fe-I and (FeFeI)-Fe-II complexes show that structure and relative stability of species characterized by a mu-CO group are significantly affected by the electronic properties of the ligands coordinated to the iron atoms. The investigation of reaction pathways for H-2 activation confirms and extends a previous hypothesis indicating that H-2 can be cleaved on (FeFeII)-Fe-II species. In particular, even though [Fe]-hydrogenases are proposed to bind and activate H-2 at a single iron center, the comparison of our data with experimental results obtained studying synthetic complexes (Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Mejia-Rodriguez, R.; Chiang, C.-Y.; Darensbourg, M. Y. Inorg. Chem. 2002, 41, 3917) suggests that activation paths involving both metal ions are also possible. Moreover, p-H (FeFeI)-Fe-II complexes are predicted to correspond to stable species and might be formed in the enzyme catalytic cycle