MAGNETIC STUDIES OF THE HIGH-POTENTIAL PROTEIN MODEL [FE4S4(S-2,4,6-(I-PR)3C6H2)4]- IN THE [FE4S4]3+ OXIDIZED STATE

The magnetic susceptibility of the title compound has been examined over the 5–320 K temperature range. The results have been analyzed by use of a spin-system Hamiltonian that includes two Heisenberg AF coupling parameters, J12= J + ΔJ12 and J34= J + ΔJ34, for the ferric (β) pair and the mixed-valence (α) pair, respectively, and one resonance delocalization parameter B for the α pair. The [Fe4S4]3+ core follows the Curie-Weiss law xmol= 0.416/(T + 0.82) in the 5–15 K range. The 30– K range was well fitted with parameters J = 571 cm−1, B = 598 cm−1, and ΔJ12= 144 cm−1(Δ J34 = 0), and 〈g〉 = 2.00. The entire temperature range could be successfully fitted only by allowing the g values for the five lowest energy spin states to vary individually. Best fit parameters settled at J = 652 cm−1, B = 592 cm−1, and ΔJ/12= 145 cm−1, and the two lowest lying energy levels |9/21/2(4)〉 and |7/21/2(3)| are nearly degenerate (ΔE = 11 cm−1). The coupling constants J(Fe3+−Fe3+) = 797 cm−1 and J(Fe3+−Fe2+) = 652 cm−1 are higher than those previously observed for Fe-Fe interactions in related clusters. Possible factors for this increase are the general compression of the [Fe4S4]3+ core as compared with the [Fe4S4]2+ core and also the distortion of the core. Further, fits involving both J and B parameters are conceptually distinctive from those using Heisenberg parameters alone. © 1990, American Chemical Society. All rights reserved.