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

Abstract

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 Fe(II)Fe(II) species characterized by OH or H(2)O groups terminally coordinated to the iron atom distal to the terminal sulfur ligand (Fe(d)) are less stable than corresponding mu-OH or mu-H(2)O species, suggesting that the latter are destabilized or kinetically inaccessible in the enzyme. In addition, results obtained investigating Fe(I)Fe(I) and Fe(II)Fe(I) 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 Fe(II)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, mu-H Fe(II)Fe(I) complexes are predicted to correspond to stable species and might be formed in the enzyme catalytic cycle.