[(HL)2Fe6(NCMe)m]n+ (m = 0, 2, 4, 6; n = −1, 0, 1, 2, 3, 4, 6): An Electron-Transfer Series Featuring Octahedral Fe6 Clusters Supported by a Hexaamide Ligand Platform

Abstract

Using a trinucleating hexaamide ligand platform, the all-ferrous hexanuclear cluster (HL)2Fe6 (1) is obtained from reaction of 3 equiv of Fe2(Mes)4 (Mes = 2,4,6-Me3C6H2) with 2 equiv of the ligand (HL)H6. Compound 1 was characterized by X-ray diffraction analysis, 57Fe Mössbauer, SQUID magnetometry, mass spectrometry, and combustion analysis, providing evidence for an S = 6 ground state and delocalized electronic structure. The cyclic voltammogram of [(HL)2Fe6]n+ in acetonitrile reveals a rich redox chemistry, featuring five fully reversible redox events that span six oxidation states ([(HL)2Fe6]n+, where n = −1 → 4) within a 1.3 V potential range. Accordingly, each of these species is readily accessed chemically to provide the electron-transfer series [(HL)2Fe6(NCMe)m][PF6]n (m = 0, n = −1 (2); m = 2, n = 1 (3); m = 4, n = 2 (4); m = 6, n = 3 (5); m = 6, n = 4 (6)). Compounds 2–6 were isolated and characterized by X-ray diffraction, 57Fe Mössbauer and multinuclear NMR spectroscopy, and combustion analysis. Two-electron oxidation of the tetracationic cluster in 6 by 2 equiv of [NO]+ generates the thermally unstable hexacationic cluster [(HL)2Fe6(NCMe)m]6+, which is characterized by NMR and 57Fe Mössbauer spectroscopy. Importantly, several stepwise systematic metrical changes accompany oxidation state changes to the [Fe6] core, namely trans ligation of solvent molecules and variation in Mössbauer spectra, spin ground state, and intracluster Fe–Fe separation. The observed metrical changes are rationalized by considering a qualitative, delocalized molecular orbital description, which provides a set of frontier orbitals populated by Fe 3d electrons.