Single-crystal spectral studies of Fe(SR)4- [R = 2,3,5,6,-(Me)4C6H]: the electronic structure of the ferric tetrathiolate active site

Abstract

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)i [R = 2,3,5,6-(Me),c6H], These studies have led to a definitive assignment of the S4 symmetry-split components of the 6Al -4Tla, 4T3, 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 studies3' on a Du distorted Td Fe(C1); complex, it is found that anisotropic covalency must be included to account for the observed ZFS. From comparisons to the data on DZd Fe(CI),-the orientation of the R group is found to determine the electronic structure of the iron tetrathiolate complex. A definitive assignment of the spin-allowed charge-transfer spectrum is presented. This is based on the polarized absorption data, and the sign and magnitude of the excited-state spin-orbit splittings as determined by single-crystal MCD spectroscopy. It is found that the charge-transfer spectrum is dominated by the thiolate S-pu to Fe-du transitions (between 16000 and 28000 cm-' with c = 5000-10000 M-I cm-I), with the S-ps to Fe-dn set of transitions at 13000 cm-' being weak ( t = 300 M-I cm-I). This observation, along with the d orbital splitting pattern, and the lack of measurable in-state spin-orbit splitting of the S, -Fe, CT band indicate that the S, bonding to the iron is not significant. In contrast, the dominance of the thiolate to iron u bonding interaction is suggested by the dd and CT data. This interaction is very dependent on the orientation of the thiolate R group. This thiolate S pu 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. (21) Ueyama, N.; Sugawara, T.; Tatsumi, K.; Nakamura, A. Inorg. Chem. 0002-7863/90/ 15 12-22 17$02.50/0 0 1990 American Chemical Society 2218 , \ h T E V \ \ \ \ \ / support of this research. M.S.G. and J.C.D. acknowledge NSF for graduate research fellowships.be small for the rubredoxin active site. In addition, in the oxidized site the d z and dXLy2 orbitals should be substantially split. If these orbitals were close to degenerate, reduction to give the ferrous 5E ground state would result in a Jahn-Teller distortion; however, this splitting of these orbitals removes need for this distortion.

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“…In fact, the calculation with the "field-swept" linewidth fits the methemoglobin experimental spectrum best. There are other examples, such as a ferric tetrathiolate model compound (Gebbhard et al, 1990), for which a "field-swept" linewidth might be relevant.…”

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“…For model complex [Fe(SPh) 4 ] -, using the observed and assigned d-d transitions a ligand field analysis gave a best fit with the ligand field parameters: C＝2222, B＝22 and Dq＝-480 cm -1 . 18 The electron repulsion parameter B was drastically reduced from the free ion value of B 0 ＝928 cm -1 , 19 indicating that an extreme covalent interaction is present in this complex.…”