Decomposition of a Mixed-Valence [2Fe-2S] Cluster to Linear Tetra-Ferric and Ferrous Clusters.

Despite the ease of preparing di-ferric [2Fe-2S] clusters, preparing stable mixed-valence analogues remains a challenge, as these clusters have limited thermal stability. Herein we identify two decomposition products of the mixed-valence thiosalicylate-ligated [2Fe-2S] cluster, [Fe2S2(SArCOO)2]3− ((SArCOO)2− = thiosalicylate). Keywords: iron, sulfur, clusters 1. Introduction Iron-sulfur ([Fe-S]) clusters are ubiquitous electron-transfer (ET) co-factors, and are essential components of several metalloenzymes that facilitate multi-e−/multi-H+ transformations [1, 2]. Over the years, many synthetic analogues have been prepared that mimic the rich cluster composition and redox-states that are found in their natural counterparts [3]. For example, thiolate ligated synthetic clusters of the type [Fe4S4(SR)4]2−/3− (R = alkyl or aryl) that mimic the [Fe4S4(Cys)4]2−/3− redox states found in ferredoxins can be readily prepared. Use of bulky thiolate ligands allows for the isolation of synthetic analogues that mimic the [Fe4S4(Cys)4]−/2− redox states of high-potential proteins, as first shown by Michelle Millar in the [Fe4S4(S-2,4,6-iPr3C6H2)4]−/2− cluster system [4]. In the absence of steric bulk, the oxidized clusters undergo solvent-mediated degradation [5]. The smaller [2Fe-2S] cluster is also biologically relevant, cycling between the di-ferric and FeIII/FeII mixed-valence states [1, 3]. These enzymatic clusters range in composition from all Cys-ligated ferredoxins [1], to 3 Cys/1 His-ligated mitoNEET clusters [6], to 2 Cys/2 His-ligated Rieske clusters [1]. Stable synthetic [2Fe-2S] clusters can be accessed with a variety of auxiliary ligands in the di-ferric state [3, 7–15], but the mixed-valence analogues have limited stabilities. For instance, mixed-valence [Fe2S2X4]3− clusters (X = anionic ligand) readily re-arrange to the more thermodynamically stable [Fe4S4X4]2− clusters [3]. The use of chelating ligands hinders this decomposition pathway, but clusters of the type [Fe2S2(XX)2]3− (XX = dianionic chelating ligand) also prove to be unstable [7, 9, 12]. In some instances, the redox potentials of the di-ferric/mixed-valence and mixed-valence/di-ferrous couples are close, introducing the possibility of redox disproportionation [12]. In one case, attempts to crystalize a mixed-valence cluster resulted in isolation of the di-ferric species [15]. An exception to the observed instability of mixed-valence [2Fe-2S] clusters is bis-benzimidazolate ligated clusters, which are stable both in solution and as solids [8, 12–14]. As part of our studies on proton-coupled electron transfer reactions at [Fe-S] clusters [14], we have examined Gibson’s thiosalicylate-ligated [2Fe-2S] cluster system [7]. The mixed-valence congener of this system is amongst the most stable for non bis-benzimidazolate ligated [2Fe-2S] clusters. In his original report, the di-ferric cluster was characterized by optical, IR, and NMR spectroscopy, and the presence of the mixed-valence congener was indicated by electrochemistry and EPR spectroscopy. Herein we identify by X-ray crystallography (XRD) two of the decomposition products of the mixed-valence cluster. We also present additional characterization of both the di-ferric (XRD and NMR) and mixed-valence (NMR) clusters.