| Abstract
| - 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 sitewith a variety of ligands including thiolates, phenolates, cyclictriamines and a trisulfide, imidazoles, and tertiaryphosphines. Reactions are readily followed because of the extremesensitivity of isotropically shifted resonancesto the nature of ligand L‘ in the product clusters[Fe4Q4(LS3)L‘]2-,-.Isotropic shifts and redox potentials arereported for over 40 clusters, including many cluster pairs differingonly in the core chalcogenide atom. In thisway, comparative properties of sulfide and selenide clusters can beelicited. It is shown that the larger isotropicshifts consistently observed for selenide clusters and dominantlycontact in nature arise from larger magneticsusceptibilities, which indicate a lesser extent of antiferromagneticcoupling. It is further demonstrated that,without exception, redox potentials of selenide clusters are morepositive than those of sulfide clusters, usuallyby 20−60 mV, at parity of ligation. The difference in potentialsof ca. 300 mV between the[Fe4S4]2+,+couplesof[Fe4S4(LS3)(SEt)]2-and[Fe4S4(LS3)(Im)]-(Im = imidazole) clusters is the best available estimate oftheintrinsic potential difference between protein-bound[Fe4S4(Cys-S)4]and[Fe4S4(Cys-S)3(His-N)]clusters. Theligand LS3 undergoes spontaneous transfer between[Fe4S4]2+,[Fe4Se4]2+, and[MoFe3S4]3+ cores inreactionswhose equilibrium constants are near the statistical value. Theligand binding affinity order PhS-> PhO->CN- ≫ Cl- for[Fe4Q4]2+ cores wasestablished. When taken together with earlier results from thislaboratory,a comprehensive picture of ligand binding to single iron sites in[Fe4Q4]2+ clustersemerges.
- The [1:3] site-differentiated[Fe4Q4]2- clusters (Q = S,Se) bind a variety of ligands including thiolates, phenolates, halides,cyclic triamines and trisulfides, imidazoles, and tertiary phosphines.Isotropic shifts and redox potentials are reported for over 40clusters. Shifts are larger and potentials are less negative forselenide clusters. The ligand binding affinity orderPhS-> PhO-> CN- ≫Cl- was established. Ligands, including thesemirigid tridentate cavitand ligand LS3, undergo exchangebetween [Fe4S4]2+,[Fe4Se4]2+, and[MoFe3S4]3+cores.
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