The [1:3] site-differentiated cubane-type clusters [Fe4Q4(LS3)Cl]2- (Q = S, Se; LS3 = 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)benzenate(3−)) undergo substitution reactions at the unique iron site with a variety of ligands including thiolates, phenolates, cyclic triamines and a trisulfide, imidazoles, and tertiary phosphines. Reactions are readily followed because of the extreme sensitivity of isotropically shifted resonances to the nature of ligand L‘ in the product clusters [Fe4Q4(LS3)L‘]2-,-. Isotropic shifts and redox potentials are reported for over 40 clusters, including many cluster pairs differing only in the core chalcogenide atom. In this way, comparative properties of sulfide and selenide clusters can be elicited. It is shown that the larger isotropic shifts consistently observed for selenide clusters and dominantly contact in nature arise from larger magnetic susceptibilities, which indicate a lesser extent of antiferromagnetic coupling. It is further demonstrated that, without exception, redox potentials of selenide clusters are more positive than those of sulfide clusters, usually by 20−60 mV, at parity of ligation. The difference in potentials of ca. 300 mV between the [Fe4S4]2+,+ couples of [Fe4S4(LS3)(SEt)]2- and [Fe4S4(LS3)(Im)]- (Im = imidazole) clusters is the best available estimate of the intrinsic potential difference between protein-bound [Fe4S4(Cys-S)4] and [Fe4S4(Cys-S)3(His-N)] clusters. The ligand LS3 undergoes spontaneous transfer between [Fe4S4]2+, [Fe4Se4]2+, and [MoFe3S4]3+ cores in reactions whose equilibrium constants are near the statistical value. The ligand binding affinity order PhS- > PhO- > CN- ≫ Cl- for [Fe4Q4]2+ cores was established. When taken together with earlier results from this laboratory, a comprehensive picture of ligand binding to single iron sites in [Fe4Q4]2+ clusters emerges.

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The catalytic site of sulfite reductase consists of exchange-coupled cubane-type Fe 4 S 4 cluster and siroheme components bridged by a cysteinate sulfur atom in the Escherichia coli enzyme and, presumably, by sulfide in certain other bacterial enzymes. A synthetic analogue of the latter in the form of a sulfide-bridged assembly has been synthesized by the reaction of [Fe(OEiBC)Cl] and the site-differentiated cluster [Fe 4 S 4 (LS 3 )(SSiMe 3 )] 2-(4) in benzene/acetonitrile. Demonstration of the [Fe 4 S 4 (LS 3 )-S-Fe III (OEiBC)] 2-(5) formulation follows from spectroscopic evidence. The Mo ¨ssbauer spectrum proves the [Fe 4 S 4 ] 2+ core oxidation state. 1 H NMR spectra demonstrate a close juxtaposition of macrocycle and cluster owing to the appearance of doubled cluster resonances when two diastereomers of 5 are prepared from an isomeric mixture of [Fe(OEiBC)Cl]. Isotropic shifts of 5 are dominantly contact in origin and are 7-11 times larger than in 4, a typical [Fe 4 S 4 ] 2+ cluster, owing to spin delocalization involving Fe(III). These shifts exhibit a 1/T dependence whereas those of 4 are non-Curie. Spin delocalization requires the presence of a covalently bridged structure. Assembly 5 is a closer analogue of the native site than other bridged assemblies prepared in this laboratory because the OEiBC macrocycle, as that in siroheme, is at the isobacteriochlorin oxidation level. This work contributes to an experimental protocol for coupling cluster and heme components into bridged assemblies. (LS 3 ) trianion of 1,3,5-tris((4,6-dimethyl-3mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)benzene; OEiBC ) dianion of octaethylisobacteriochlorin.)