[((H)L)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 ((H)L)(2)Fe(6) (1) is obtained from reaction of 3 equiv of Fe(2)(Mes)(4) (Mes = 2,4,6-Me(3)C(6)H(2)) with 2 equiv of the ligand ((H)L)H(6). Compound 1 was characterized by X-ray diffraction analysis, (57)Fe Mössbauer, SQUID magnetometry, mass spectrometry, and combustion analysis, providing evidence for an S=6 ground state and delocalized electronic structure. The cyclic voltammogram of [((H)L)(2)Fe(6)](n+) in acetonitrile reveals a rich redox chemistry, featuring five fully reversible redox events that span six oxidation states ([((H)L)(2)Fe(6)](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 [((H)L)(2)Fe(6)(NCMe)(m)][PF(6)](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, (57)Fe 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 [((H)L)(2)Fe(6)(NCMe)(m)](6+), which is characterized by NMR and (57)Fe Mössbauer spectroscopy. Importantly, several stepwise systematic metrical changes accompany oxidation state changes to the [Fe(6)] 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.