SINGLE-CRYSTAL SPECTRAL STUDIES OF FE(S2,3,5,6-(ME)4C6H)4- - THE ELECTRONIC-STRUCTURE OF THE FERRIC TETRATHIOLATE ACTIVE-SITE

Single-crystal polarized absorption, magnetic circular dichroism (MCD), and EPR spectroscopies have been employed to elucidate the electronic structure of the oxidized rubredoxin model complex Fe(SR)4- [R = 2,3,5,6-(Me)4C6H], These studies haveled to a definitive assignment of the S4 symmetry-split components of the6A1→4T1a, 4T2a, and Ea spin-forbidden ligand field transitions, which are analyzed to give an experimental determination of the Fe d orbital axial splitting diagram. The transitions are correlated with the ground-state zero-field splitting (ZFS) to examine the origin of the large ZFS in ferric tetrathiolate complexes. As in our earlier studies37 on a D2d distorted Td Fe(Cl)4- complex, it is found that anisotropic covalency must be included to account for the observed ZFS. From comparisons to the data on D2d Fe(Cl)4- the orientation of the R group is found to determine the electronic structure of the iron tetrathiolatecomplex. A definitive assignment of the spin-allowed charge-transfer spectrum is presented. This isbased on the polarized absorption data, and the sign and magnitude of the excited-state spin-orbitsplittings as determined by single-crystal MCD spectroscopy. It is found that the charge-transfer spectrum is dominated by the thiolate S-pσ to Fe-dσ transitions (between 16000 and 28 000 cm-1 with e ≈ 5000-10000 M-1 cm-1), with the S-to Fe-dir set of transitions at 13000 cm-1 being weak (e ≈ 300 M-1 cm-1). This observation, along with the d orbital splitting pattern, and the lack of measurable in-state spin-orbit splitting of the Sπ → Feπ CT band indicate that the Sπ bonding to the iron is not significant. In contrast, the dominance of the thiolate to iron bonding interaction is suggested by the d → d and CT data. This interaction is very dependent on the orientation of the thiolate R group. This thiolate S bonding model is used to evaluate existing electronic structure calculations on ferric thiolates and is used to analyze the spectroscopic features of the oxidized rubredoxin active site. © 1990, American Chemical Society. All rights reserved.