IRON AND MANGANESE ALKOXIDE CUBES

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 beta-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 beta-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 H-1 NMR and solution Mossbauer 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 Mossbauer 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-degrees-C and its Mossbauer 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 (H = JSIGMA(i)j S(i).S(j) where j > i), g = 2.29, and contributions from zero-field splitting. The predicted S(T) = 8 ground state for 1 is confirmed by its saturation moment, which at 1.2 K is 15.7 mu(B) 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.