Paramagnetic States of Four Iron−Four Sulfur Clusters. 1. EPR Single-Crystal Study of 3+ and 1+ Clusters of an Asymmetrical Model Compound and General Model for the Interpretation of the g-Tensors of These Two Redox States

Abstract

This article contains (i) an experimental EPR study on [4Fe−4S]3+ and [4Fe−4S]+ centers in single crystals of a model compound having an asymmetrical cubane-type cluster and (ii) a theoretical model for the interpretation of g-tensors of such states found in synthetic compounds and protein active sites. In the first part, an extensive EPR study of a large number of paramagnetic species with S = 1/2 in γ-irradiated single crystals of the (Et4N)2[Fe4S4(SC6H4-o-OH)4] compound (a model complex of active sites of [4Fe−4S] proteins) is reported. This compound represents a simple case of asymmetric 4Fe−4S cluster being well characterized in the crystalline solid state: three iron atoms have the usual tetrahedral coordination with four sulfur atoms, while the fourth one has an extra coordination to the oxygen of the phenol group of its thiolate ligand. Thus, with respect to the local symmetry of its cubane cluster, this compound constitutes formally a representative model of asymmetric active sites, here introduced through the extra pentacoordination. The g-tensors of the different paramagnetic species created in (Et4N)2[Fe4S4(SC6H4-o-OH)4] single crystals could be deduced from the angular dependences of the EPR lines in three perpendicular planes. Three different [4Fe−4S]3+ centers (with gav > 2.0023 and relatively axial g-tensors) as well as eight [4Fe−4S]+ centers (with gav ≤ 2.0023 andmost oftenrhombic g-tensors) were identified. These different species exhibit a large variety of principal values. In the second part, a general model is proposed for the interpretation of the principal values and the principal directions of the g-tensors of [4Fe−4S]3+ and [4Fe−4S]+ centers. It is based on simple qualitative arguments, which are, at first, developed for symmetric compounds of C2v electronic structure. This model allows us to rationalize most of our previous results obtained for single crystals of the (Et4N)2[Fe4S4(SCH2Ph)4] compound as well as most of those presented here. This explains the relation existing between the location of the mixed-valence pair in the cubane cluster and the principal direction V1 corresponding to the largest principal value, g1, of the g-tensor. It is also shown that the two other eigenvalues of the g-tensor, g2 and g3, are expected to have their corresponding principal directions V2 and V3 aligned with the two pairs of identical iron ions. Moreover, in the [4Fe−4S]+ state, the existence of two nearly degenerate orbital configurations, called OC1 and OC2, has to be taken into account. Thereafter follows a qualitative discussion of the effects on the g-tensors, in the different cases, produced by the introduction of some asymmetry. Finally, this analysis allows us to understand the reasons leading to the number and the diversity of paramagnetic centers observed here and to propose a plausible set of localizations for almost all of them. It also confirms, for the case of the [4Fe−4S]3+ states, that taking into account the principal direction V1 is a valuable tool, a useful “rule of thumb” for the determination of the position of the mixed-valence pair, despite the fact that we deal here with asymmetric compounds. This conclusion is to be contrasted to what can be deduced for the [4Fe−4S]+ states because of the greater sensitivity of the ferrous ions, the main source of g-anisotropy, to their immediate surrounding. In this last case, we think that such a correlation still makes good sense for a (nearly) symmetric arrangement in which the two ferrous ions are (nearly) equivalent. However, breaking significantly this mirror-type symmetry within the ferrous pair can lead to unpredictable g-tensor axes.