Web Release Date: May 15,
Terminal Ligand Influence on the Electronic Structure and Intrinsic Redox Properties of the [Fe4S4]2+ Cubane Clusters
W. R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, and Department of Physics, Washington State University, 2710 University Drive, Richland, Washington 99352
Received April 11, 2004
Abstract:
We used photoelectron spectroscopy (PES) to study how the terminal ligands influence the electronic structure
and redox properties of the [4Fe-4S] cubane in several series of ligand-substituted analogue complexes:
[Fe4S4Cl4-x(CN)x]2-, [Fe4S4Cl4-x(SCN)x]2-, [Fe4S4Cl4-x(OAc)x]2-, [Fe4S4(SC2H5)4-x(OPr)x]2-, and [Fe4S4(SC2H5)4-xClx]2-
(x = 0-4). All the ligand-substituted complexes gave similar PES spectral features as the parents, suggesting that
the mixed-ligand coordination does not perturb the electronic structure of the cubane core significantly. The terminal
ligands, however, have profound effects on the electron binding energies of the cubane and induce significant
shifts of the PES spectra, increasing in the order SC2H5- 
Cl- OAc-/OPr- CN- SCN-. A linear
relationship between the electron binding energies and the substitution number x was observed for each series,
indicating that each ligand contributes independently and additively to the total binding energy. The electron binding
energies of the gaseous complexes represent their intrinsic oxidation energies; the observed linear dependence on
x is consistent with similar observations on the redox potentials of mixed-ligand cubane complexes in solution. The
current study reveals the electrostatic nature of the interaction between the [4Fe-4S] cubane core and its coordination
environment and provides further evidence for the electronic and structural stability of the cubane core and its
robustness as a structural and functional unit in Fe-S proteins.
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