Investigation of the Reduced High-Potential Iron−Sulfur Protein from <i>Chromatium vinosum</i> and Relevant Model Compounds:  A Unified Picture of the Electronic Structure of [Fe<sub>4</sub>S<sub>4</sub>]<sup>2+</sup> Systems through Magnetic and Optical Studies (2003)

Magnetization measurements and variable temperature optical spectroscopy have been used to investigate, within the 4-300 K temperature range, the electronic structure of the reduced high-potential iron protein (HiPIP) from Chromatium vinosum and the model compounds (Cat)(2)[Fe(4)S(4)(SR)(4)], where RS(-) = 2,4,6-triisopropylphenylthiolate (1), 2,6-diphenylphenylthiolate (2), diphenylmethylthiolate (3), 2,4,6-triisopropylbenzylthiolate (4, 4'), 2,4,6-triphenylbenzylthiolate (5, 5'), 2,4,6-tri-tert-butylbenzylthiolate (6), and Cat(+) = (+)NEt(4) (1, 2, 3, 4', 5', 6), (+)PPh(4) (4, 5). The newly synthesized 2(2)(-), 3(2)(-), 5(2)(-), and 6(2)(-) complexes are, as 1(2)(-) and 4(2)(-), excellent models of the reduced HiPIPs: they exhibit the [Fe(4)S(4)](3+/2+) redox couple, because of the presence of bulky ligands which stabilize the [Fe(4)S(4)](3+) oxidized core. Moreover, the presence of SCH(2) groups in 4(2)(-), 5(2)(-), and 6(2)(-), as in the [Fe(4)S(4)] protein cores, makes them good biomimetic models of the HiPIPs. The X-ray structure of 2 is reported: it crystallizes in the orthorhombic space group Pcca with no imposed symmetry and a D(2)(d)()-distorted geometry of the [Fe(4)S(4)](2+) core. Fit of the magnetization data of the reduced HiPIP and of the 1, 2, 3, 4, 5, and 6 compounds within the exchange and double exchange theoretical framework leads to exchange coupling parameters J = 261-397 cm(-)(1). A firm determination of the double exchange parameters B or, equivalently, the transfer integrals beta = 5B could not be achieved that way. The obtained |B| values remain however high, attesting thus to the strength of the spin-dependent electronic delocalization which is responsible for lowest lying electronic states being characterized by delocalized mixed-valence pairs of maximum spin (9)/(2). Electronic properties of these systems are then accounted for by the population of a diamagnetic ground level and excited paramagnetic triplet and quintet levels, which are respectively J and 3J above the ground level. Optical studies of 1, 2, 4', 5', and 6 but also of (NEt(4))(2)[Fe(4)S(4)(SCH(2)C(6)H(5))(4)] and the isomorph (NEt(4))(2)[Fe(4)S(4)(S-t-Bu)(4)] and (NEt(4))(2)[Fe(4)Se(4)(S-t-Bu)(4)] compounds reveal two absorption bands in the near infrared region, at 705-760 nm and 1270-1430 nm, which appear to be characteristic of valence-delocalized and ferromagnetically coupled [Fe(2)X(2)](+) (X = S, Se) units. The |B| and |beta| values can be directly determined from the location at 10|B| of the low-energy band, and are respectively of 699-787 and 3497-3937 cm(-)(1). Both absorption bands are also present in the 77 K spectrum of the reduced HiPIP, at 700 and 1040 nm (Cerdonio, M.; Wang, R.-H.; Rawlings, J.; Gray, H. B. J. Am. Chem. Soc. 1974, 96, 6534-6535). The blue shift of the low-energy band is attributed to the inequivalent environments of the Fe sites in the protein, rather than to an increase of |beta| when going from the models to the HiPIP. The small differences observed in known geometries of [Fe(4)S(4)](2+) clusters, especially in the Fe-Fe distances, cannot probably lead to drastic changes in the direct Fe-Fe interactions (parameter beta) responsible for the delocalization phenomenon. These differences are however magnetostructurally significant as shown by the 261-397 cm(-)(1) range spanned by J. The cluster's geometry, hence the efficiency of the Femicro(3)-S-Fe superexchange pathways, is proposed to be controlled by the more or less tight fit of the cluster within the cavity provided by its environment.