J. Phys. Chem. A, 109 (9), 1815 -1820, 2005. 10.1021/jp045177k S1089-5639(04)05177-1
Web Release Date: February 10, 2005

Copyright © 2005 American Chemical Society

Probing the Electronic Structure of [2Fe-2S] Clusters with Three Coordinate Iron Sites by Use of Photoelectron Spectroscopy

You-Jun Fu, Xin Yang, Xue-Bin Wang, and Lai-Sheng Wang*

Department of Physics, Washington State University, 2710 University Drive, Richland, Washington 99352, and W. R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352

Received: October 22, 2004

In Final Form: December 6, 2004

Abstract:

Five series of [2Fe-2S] complexes, [Fe2S2Cl2-x(CN)x]-, [Fe2S2(SEt)2-xClx]-, [Fe2S2(SEt)2-x(CN)x]-, [Fe2S2Cl2-x(OAc)x]- (OAc = acetate), and [Fe2S2(SEt)2-x(OPr)x]- (OPr = propionate) (x = 0-2), were produced by collision-induced dissociation of the corresponding [4Fe-4S] complexes, and their electronic structures were studied by photoelectron spectroscopy. All the [2Fe-2S] complexes contain a [Fe2S2]+ core similar to that in reduced [2Fe] ferredoxins but with different coordination geometries. For the first three series, which only involve tricoordinated Fe sites, a linear relationship between the measured binding energies and the substitution number (x) was observed, revealing the independent ligand contributions to the total electron binding energies. The effect of the ligand increases in the order SEt Cl CN, conforming to their electron-withdrawing ability in the same order. The carboxylate ligands in the [Fe2S2Cl2-x(OAc)x]- and [Fe2S2(SEt)2-x(OPr)x]- complexes were observed to act as bidentate ligands, giving rise to tetracoordinated iron sites. This is different from their monodentate coordination behavior in the [4Fe-4S] cubane complexes, reflecting the high reactivity of the unsatisfied three-coordinate iron site in the [2Fe-2S] complexes. The [2Fe-2S] complexes with tetracoordinated iron sites exhibit lower electron binding energies, that is, higher reductive activity than the all tricoordinate planar clusters. The electronic structures of all the [2Fe-2S] complexes were shown to conform to the "inverted energy level scheme".

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