Magnetic Properties of Dodecanuclear Mixed Valence Iron Clusters

Slow addn. of dioxygen to a basic MeOH soln. of Fe(II) chloride afforded the mixed valent polyiron oxo cluster [Fe12(O)2(O2CCH2Cl)5.3Cl0.7(CH3O)18(MeOH)4] (1). The structure of 1 was detd. in a single crystal x-ray diffraction study; crystallog. data at 202 K: space group P-1, a = 10.606(2), b = 12.173(2), c = 13.199(1) Å, α = 100.48(1), β = 96.63(1), γ = 91.43(1)°, Z = 1, Mr = 1907.3, ρcalc = 1.91 g cm-3. For 3499 unique obsd. reflections with F2 > 3σ(F2), R = 0.060, Rw = 0.063. The core of the cluster contains a fcc. array of O atoms with eight Fe(II) ions and four Fe(III) ions occupying octahedral sites. Two μ6-oxo ligands link 10 of the 12 Fe atoms. The mixed valent nature of this air-sensitive compd. was established by analyzing the Fe-O distances, by charge considerations, and by electronic and Moessbauer spectroscopy. Three Fe subsites are resolvable in the Moessbauer spectrum, corresponding to localized Fe(III) and Fe(II) ions and consistent with the crystallog. data. The magnetic properties of 1 were satisfactorily reproduced by a simple model which assumes that exchange coupling interactions mediated by μ2-methoxo ligands are responsible for the obsd. magnetic behavior in the temp. range 20-300 K, whereas at lower temp. the effects of the μ3-methoxo, μ2-carboxylato and μ6-oxo ligands become more important. This behavior is reflected by the appearance of magnetic features in the Moessbauer spectra with decreasing temp. The magnetic properties of 1 may be understood as arising from two subclusters, a and b, for which the following two sets of parameters are proposed and discussed: (i) g(a) = 2.319(3), g(b) = 1.831(4), J = 15.9(1) cm-1 and (ii) g(a) = 2.23(3), g(b) = 2.1(1), J = 19(2) cm-1, where J is the exchange coupling const. through μ2-methoxo bridges within the a subcluster, and the spins of subcluster b remain largely uncoupled.