Terminal ligand influence on the electronic structure and intrinsic redox properties of the [Fe4S4]2+ cubane clusters

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: Fe(4)S(4)Cl(4-x)(CN)(x)](2-), Fe(4)S(4)Cl(4-x)(SCN)(x)](2-), Fe(4)S(4)Cl(4-x)(OAc)(x)](2-), Fe(4)S(4)(SC(2)H(5))(4-x)(OPr)(x)](2-), and Fe(4)S(4)(SC(2)H(5))(4-x)Cl(x)](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 SC(2)H(5)(-) --> 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.