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 l
ocated at alternating vertices of a cube, with either alcohol or alkox
ide and beta-diketonate or benzoate ligands on the exterior of the cor
e. 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 o
f 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 comp
lex, with one iron(III) site. The terminal methanol coordinated to the
ferric ion in 3 is deprotonated. The molecular symmetry ranges from S
4 for 1 and 2 to C2 for 3 and C1 for 4. The iron atoms in 1-3 are octa
hedrally coordinated, whereas two of the four manganese ions in 4 have
a distorted square pyramidal geometry. The cubes exhibit multiple ele
ctronic spectral features giving rise to the intense red, blue, green,
and yellow colors of 1-4, respectively. These absorptions have been a
ssigned 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 Mo
ssbauer experiments suggest that the cubes remain intact in solvent mi
xtures containing alcohol. Complexes 1 and 2 display a single quadrupo
le doublet in their high-temperature Mossbauer spectra with parameters
characteristic of high-spin iron(II). The ferrous ions in 3 also prod
uce a single quadrupole doublet, and the lone ferric ion is valence-lo
calized, as revealed by both the structural study at -85-degrees-C and
its Mossbauer spectra, which display no delocalization up to 250 K. A
t low temperatures, polycrystalline samples of 2 and 3 exhibit magneti
c hyperfine interactions in the absence of an applied field, whereas c
omplex 1 does not. The solution spectrum of 2 at 4.2 K consists of a s
ingle quadrupole doublet, suggesting that the slow relaxation is a sol
id-state effect. Solution studies of 3 are less definitive, and for bo
th 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 ferromagnet
ically 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 ze
ro-field splitting. The predicted S(T) = 8 ground state for 1 is confi
rmed 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-coup
led. Coupling constants of 2.60 and 1.63 cm-1 with a composite g = 2.1
8 were determined for the Fe(II)-Fe(II) and Fe(II)-Fe(III) exchange in
teractions, respectively. As evident from these results, the iron and
manganese alkoxide cubes display a remarkable variety of structural, p
hysical, and electronic properties; and the facile synthesis of 1-4 af
fords a convenient route to the {M4(OR)4}n+ cubane architecture for ir
on and manganese.