Photoelectron spectroscopy of dithiolatodiironhexacarbonyl models for the active site of [Fe-Fe] hydrogenases: Insight into the reorganization energy of the rotated structure in the enzyme

Synthetic analogs, mu-(RS)(2)Fe-2(CO)(6), of the active site of [Fe-Fe] hydrogenases do not have the semi-bridged CO and rotated structure found in the enzyme. However, recent studies have shown that cations of dithiolatodiiron complexes adopt this rotated structure. This paper reports the use of photoelectron spectroscopy in combination with density functional theory calculations to show that two previously reported complexes: mu-(1,2-benzenedithiolato)Fe-2(CO)(6) and mu-(1,3-propanedithiolato)Fe-2(CO)(6) and two new complexes: mu-(2,3-pyridinodithiolato)Fe-2(CO)(6), and mu-(norbornane-2-exo,3-exo-dithiolato)Fe-2(CO)(6) favor the rotated structure in their corresponding cations. Furthermore, these methods provide a measure of the reorganization energy between the rotated and unrotated structures in the gas phase. The results provide insight on the entatic state of the dithiolatodiiron site in the enzyme, in which the protein controls the structure of the active site. This structure influences the redox energy and reorganization energy enabling fast electron transfer. (c) 2008 Published by Elsevier B.V.