Iron and Manganese Alkoxide Cubes

Abstract

Four complexes with cubic {M4(OR)4}n+ cores, [Fe(OMe)(MeOH)(DPM)]4(1), [Fe(OMe)(MeOH)(DBM)]4 (2), [FeIIIFeII3(OMe)5(MeOH)3(OBz)4](3), and [Mn4(OEt)4(EtOH)2(DPM)4] (4), have been prepared and crystallographically characterized. The four metal ions and bridging alkoxide ligands are located at alternating vertices of a cube, with either alcohol or alkoxide and β-diketonate or benzoate ligands on the exterior of the core. Complexes 1,2, and 4 were synthesized in a single, high-yield step from MCl2, the β-diketone, and 2 equiv of the appropriate lithium alkoxide, whereas 3 was isolated upon the slow oxidation by dioxygen of FeCl2 in the presence of methoxide and benzoate. Complexes 1, 2, and 4 have all divalent metal ions, whereas cube 3 is a mixed-valent complex, with one iron(III) site. The terminal methanol coordinated to the ferric ion in 3 is deprotonated. The molecular symmetry ranges from S4 for 1 and 2 to C2 for 3 and C1 for 4. The iron atoms in 1–3 are octahedrally coordinated, whereas two of the four manganese ions in 4 have a distorted square pyramidal geometry. The cubes exhibit multiple electronic spectral features giving rise to the intense red, blue, green, and yellow colors of 1–4, respectively. These absorptions have been assigned to metal-ligand charge-transfer transitions, spin-allowed d-d transitions, and, for mixed-valent 3, an intervalence-charge-transfer band. The spectral studies in combination with 1H NMR and solution Mössbauer experiments suggest that the cubes remain intact in solvent mixtures containing alcohol. Complexes 1 and 2 display a single quadrupole doublet in their high-temperature Mössbauer spectra with parameters characteristic of high-spin iron(II). The ferrous ions in 3 also produce a single quadrupole doublet, and the lone ferric ion is valence-localized, as revealed by both the structural study at −85 °C and its Mössbauer spectra, which display no delocalization up to 250 K. At low temperatures, polycrystalline samples of 2 and 3 exhibit magnetic hyperfine interactions in the absence of an applied field, whereas complex 1 does not. The solution spectrum of 2 at 4.2 K consists of a single quadrupole doublet, suggesting that the slow relaxation is a solid-state effect. Solution studies of 3 are less definitive, and for both 2 and 3, the exact source of the increased relaxation times remains uncertain. The iron atoms in the {Fe4(OMe)4}4+ cube 1 are ferromagnetically exchange-coupled, with a coupling constant J = −1.88 cm−1 (where j > i), g = 2.29, and contributions from zero-field splitting. The predicted ST = 8 ground state for 1 is confirmed by its saturation moment, which at 1.2 K is 15.7 μB in a 19.7 T field. The spin multiplet has significant anisotropy, with D = 3 cm−1, E/D = 0.075, and g = 2.2. In contrast, the iron atoms of the mixed-valent {Fe4(OMe)4}5+ core of 3 are antiferromagnetically exchange-coupled. Coupling constants of 2.60 and 1.63 cm−1 with a composite g = 2.18 were determined for the Fe(II)-Fe(II) and Fe(II)-Fe(III) exchange interactions, respectively. As evident from these results, the iron and manganese alkoxide cubes display a remarkable variety of structural, physical, and electronic properties; and the facile synthesis of 1–4 affords a convenient route to the {M4(OR)4}n+ cubane architecture for iron and manganese. Show more