Structural distortions of the [Fe 4S 4] 2+ core of [Fe 4S 4(S-t-C 4H 9) 4] 2− in different crystalline environments and detection and instability of oxidized ([Fe 4S 4] 3+) clusters

Abstract

The structural and redox chemistry of the clusters [Fe 4S 4(SR) 4] 2− with R = t-alkyl have been investigated for the purpose of determining the structures of the same cluster in different environments and the stability of the oxidized species [Fe 4S 4(SR) 4] 1−. (Me 3NCH 2Ph) 2[Fe 4S 4(S-t-Bu) 4] crystallizes in the monoclinic space group P2 1/c with no imposed symmetry. (Et 4N) 2[Fe 4S 4(S-t-Bu) 4] crystallizes in the tetragonal space group I 4 2m with D 2d symmetry imposed on the cluster. The [Fe 4S 4] 2+ cluster cores in both compounds exhibit compressed tetragonal structures with different extents of distortion from T d symmetry. These structures are compared to those of other [Fe 4S 4] 2+ clusters by means of core shape parameters. Clusters with R = t-alkyl (t-Bu, C(CH 3) 2CH 2OH, C(CH 3) 2CH 2NHPh) in DMF exhibit, in addition to the usual 2−/3− and 3−/4− redox reactions common to all [Fe 4S 4(SR) 4] 2− species, discrete one-electron oxidations near −0.1 V vs. s.c.e. Cyclic voltammetry of [Fe 4S 4(S-t-Bu) 4] 2− reveals an essentially reversible 1−/2− couple with E 1 2 = −0.12 V, supporting the authenticity of clusters containing an oxidized ([Fe 4S 4] 3+) core. This couple cannot be electrochemically resolved from multi-electron oxidation in the case of clusters in DMF with other types of R substituents, a behavior apparently due to cathodic potential shifts by t-alkyl groups. Stability of oxidized clusters is low, and [Fe 4S 4(S-t-Bu) 4] 1− could not be generated in appreciable concentrations at longer times by coulometric or chemical oxidation. The relative stabilities of analogue and protein [Fe 4S 4] 3+ clusters are discussed. Preparations of four new [Fe 4S 4(SR) 4] 2− cluster salts are described including water-soluble (Et 4N) 2[Fe 4S 4(SC(CH 3) 2CH 2OH) 4].