Of the three known low-nuclearity iron−sulfur clusters in metallobiomolecules with the core units Fe2S2, Fe4S4, and Fe3S4, the last has not been obtained in stable form outside a protein environment. We describe a direct route to such clusters in the [Fe3S4]0 oxidation state, and demonstrate an effective stereochemical and electronic structural congruence with the native cluster. The synthesis is based on iron-site-differentiated clusters. Reaction of [Fe4S4(LS3)(SEt)]2- with (Et3NH)(OTf) affords [Fe4S4(LS3)(OTf)]2-, whose unique site is activated toward terminal ligand substitution. Treatment with 1 equiv of (Et4N)2(Meida) affords [Fe4S4(LS3)(Meida)]3-, which is readily converted to [Fe3S4(LS3)]3- with 1−2 equiv of additional reactant. The trinuclear cluster is formed by abstraction of Fe2+ from a precursor cubane-type [Fe4S4]2+ core and complexation as [Fe(Meida)2]2-. An analogous procedure starting with [Fe4Se4(LS3)(SEt)]2- yields [Fe3Se4(LS3)]3-. The compound (Et4N)3[Fe3S4(LS3)]·MeCN crystallizes in o...

The Cuboidal Fe3S4 Cluster: Synthesis, Stability, and Geometric and Electronic Structures in a Non-Protein Environment

Iron-sulfur polymetallic centers pose interesting questions on the oxidation state of each metal ion, on the possibility of electron delocalization over the metal ions versus the possibility of existence of equilibria among different localized-valence states, and on the factors determining if, and to what extent, electron delocalization is present. By interpreting the hyperfine coupling of unpaired electrons with 57Fe as well as with 1H, and by analyzing the EPR spectra, the valence distributions of Fe-S polymetallic centers in proteins and model systems are obtained. The systems analyzed here are: [Fe2S2]2+/+, [Fe3S4]+/0, [Fe4S4]3+/2+/+. The understanding of the parameters describing the exchange coupling within pairs of spin vectors, as well as the double exchange coupling, is attempted on the basis of the oxidation states of the various ions. Upon proper scaling of these parameters, we show that it is possible to transfer them from one polymetallic center to another. The electronic properties are tentatively related to the microscopic reduction potentials of each iron in the center.

The electronic structure of FeS centers in proteins and models a contribution to the understanding of their electron transfer properties

Conformational studies of chelated ethylenediamines by nuclear magnetic resonance. Paramagnetic nickel(II) complexes of N-alkylethylenediamines

Steric and electronic factors influencing the structure of nickel(II) complexes

Spectroscopic studies of the metal complexes of ethylenediaminetetra-acetic acid in aqueous solution.

The proton resonance spectra of some aminopolycarboxylate complexes of nickel have been measured. The assignments of the large contact shifts of the CH protons are discussed, and it is suggested that the two protons of the N—CH2 group in a chelate ring are often non-equivalent. This is ascribed to the puckering of the rings, and a consideration of the structures of the complexes indicates that the magnitude of the shift for an N—CH proton depends on the dihedral angle across the group Ni—N—C—H in a manner similar to that found for the vicinal proton-proton coupling in aliphatic groups in diamagnetic compounds. The ligands N-2-hydroxyethyl iminodiacetate and nitrilo β-propionate diacetate, which are tetradentate in the 1 : 1 complexes, are shown to be tridentate in the 2 : 1 complexes. With the 2 : 1 complexes of iminodiacetate and N-methyl iminodiacetate, spatial isomers appear to be present.

L. Pratt

Papers

Proton resonance spectra of some nickel complexes in aqueous solution