Synthesis and Electronic and Redox Properties of "Double-Cubane" Cluster Complexes Containing MoFe3S4 and WFe3S4 Cores

Article abstract of DOI:10.1021/ja00534a023

The reaction system MS4^2-/3-3.5FeCl3/9-12NaSR (M = Mo,W) in methanol or ethanol affords as principal products four "double-cuban" cluster anions, ^3- (1), ^3- (2), ^3- (3),and ^4- (4), which are isolable as appropriate quaternary ammonium salts.Synthesis of cluster types 1 (M = W), 3, and 4 are described together with spectroscopic and voltammetric properties of the four cluster types.All clusters exhibit isotropically shifted 1H NMR spectra which serve as criteria for adequate purity, reveal from shift patterns contact and dipolar mechanisms at terminal and bridge substituents, respectively, and support the existence of single geometrical isomers in solution.Cluster types 1 and 2 form three-membered electron-transfer series in wich individual MFe3S4(SR)3 clusters are reduced in weakly coupled steps.Cluster type 3 affords a four-membered series in which the initial reduction is Fe^III -> Fe^II in the Fe(SR)6 bridge unit and subsequent reductions occur at individual clusters.Potential separations for the latter two steps (ca. 0.10 V) more closely approach the statistical value of 36 mV than do those of types 1 and 2 (ca. 0.19 V), owing to larger intercluster separations.The pressence of Fe^III and Fe^II in the bridge units of type 3 and 4 clusters, respectively, is demonstrated by Moessbauer spectroscopy.From observations of narrow intervals of 57Fe isomer shifts, hyperfine magnetic fields at Fe sites, terminal methylene proton contact shifts, and redox potentials of 1 and 2 it is concluded that the Fe3 portions of MFe3S4 core units are electronically delocalized and, within and among all cluster types 1-4, are virtually equivalent electronically.Core Fe isomer shifts are considered to accord best with the mean oxidation state Fe^2.67+.This conclusion, together with prior observations of a pronounced structural core similarity in all clusters, leads to the core formal electronic descriptions 4+Fe³⁺2Fe²⁺S4>⁴⁺ + 3+Fe³⁺2Fe²⁺S4>³⁺ (type 1) and 23+Fe³⁺2Fe²⁺S4>³⁺ (types 2-4), thereby difining total oxidation levels of the cores in each cluster type.Full tabulations of Moessbauer spectral parameters and 1H NMR isotropic shifts are presented together with representative Moessbauer and NMR spectra and cyclic voltammograms.