Investigation of the reduced high-potential iron-sulfur protein from Chromatium vinosum and relevant model compounds: A unified picture of the electronic structure of [Fe4S4](2+) systems through magnetic and optical studies

摘要

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)[Fe4S4(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(+) = +NEt4 (1, 2, 3, 4', 5', 6), +PPh4 (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 [Fe4S4](3+/2+) redox couple, because of the presence of bulky ligands which stabilize the [Fe4S4](3+) oxidized core. Moreover, the presence of SCH2 groups in 4(2-), 5(2-), and 6(2-), as in the [Fe4S4] protein cores, makes them good biornimetic 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-2d-distorted geometry of the [Fe4S4](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 (NEt4)(2)[Fe4S4-(SCH2C6H5)(4)] and the isomorph (NEt4)(2)[Fe4S4(S-t-Bu)(4)] and (NEt4)(2)[Fe4Se4(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 [Fe2X2](+) (X = S, Se) units. The B and eta values can be directly determined from the location at 10B 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 eta when going from the models to the HiPIP. The small differences observed in known geometries of [Fe4S4](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 Fe-mu(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. [References: 77]