Metal Ion Incorporation Reactions of the Cluster [Fe₃S₄(LS₃)]^3-, Containing the Cuboidal [Fe₃S₄]⁰ Core

Abstract

The frequent occurrence of the cuboidal cluster [Fe3S4(S·Cys)3] in a variety of proteins has prompted extensive investigation of its chemical and biological properties. The biological function remains in question, but the cluster is known to sustain two reactions: (i) electron transfer, and (ii) heterometal ion incorporation. The recent preparation of [Fe3S4(LS3)]3- (3) [Zhou, J.; Hu, Z.; Münck, E.; Holm, R. H. J. Am. Chem. Soc. 1996, 118, 1966] has permitted detailed structural, electronic, and reactivity characterization of the cuboidal [Fe3S4]0 oxidation state (LS3 = 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)benzene(3−). Redox properties (i) have been reported previously: here reaction type (ii), resulting in the formation of cubane-type MFe3S4 clusters, has been investigated. Reaction of 3 with [M(PPh3)4]1+ affords [(Ph3P)MFe3S4(LS3)]2- (M = Cu (6), Ag (8)) while [(NC)M(PPh3)3] leads to [(NC)MFe3S4(LS3)]3- (M = Cu (7), Ag (9)). Treatment of 3 with Tl(O3SCF3) yields [TlFe3S4(LS3)]2- (10). The fragment formalism {M1+ + [Fe3S4]0} applies to 6−10, which retain the S = 2 ground state of 3. Reaction of 3 with [M(PPh3)3Cl] yields [(Ph3P)MFe3S4(LS3)]2- (M = Co (12), Ni (14)) in inner-sphere redox reactions. Clusters 12 (S = 1) and 14 (S = 3/2) are formulated as {M2+ + [Fe3S4]1-}; antiferromagnetic coupling of fragment spins gives rise to the indicated spin ground states. The reactions (ii) are metal-ion incorporation processes, a new reaction type in Fe−S chemistry. Previously, all cubane-type MFe3S4 clusters had been synthesized by spontaneous self-assembly or reductive rearrangement reactions. Cluster 7 exhibits reversible oxidation and reduction reactions; it is the only cluster that forms a stable oxidized product containing the [Fe3S4]1+ fragment. All other clusters show a reversible reduction and an irreversible or quasireversible oxidation. Potentials of the synthetic clusters are considered intrinsic to the various core units, being less influenced by environmental factors than are those in proteins. At parity of cluster charge and terminal ligation, the potential order is M = Fe < Co < Ni and Co < Ni < Cu < Ag < Tl for the [MFe3S4]2+,1+ and [MFe3S4]1+,0 core redox reactions. These orders are compared with those determined in proteins.