Magnetic properties of dodecanuclear mixed valence iron clusters

Slow addition of dioxygen to a basic methanol solution of iron(II) chloride afforded the mixed valent polyiron oxo cluster [Fe-12(O)(2)(O2CCH2Cl)(5.3)Cl-0.7 (CH3O)(18)(CH3OH)(4)] (1). The structure of 1 was determined in a single crystal X-ray diffraction study; crystallographic data at 202 K: space group P1, a = 10.606(2) Angstrom, b = 12.173(2) Angstrom, c = 13.199(1) Angstrom, alpha = 100.48(1)degrees, beta = 96.63(1)degrees, gamma = 91.43(1)degrees, V = 1662.5(4) Angstrom(3), Z = 1, M(r) = 1907.3, rho(calc) = 1.91 g cm(-3). For 3499 unique observed reflections with F-2 > 3 sigma(F-2), R = 0.060, R(W) = 0.063. The core of the cluster contains a face-centered cubic array of oxygen atoms with eight iron(II) ions and four iron(III) ions occupying octahedral sites, Two mu(6)-OXO Ligands link 10 of the 12 iron atoms. The mixed valent nature of this air-sensitive compound was established by analyzing the Fe-O distances, by charge considerations, and by electronic and Mossbauer spectroscopy. Three iron subsites are resolvable in the Mossbauer spectrum, corresponding to localized Fe(III) and Fe(II) ions and consistent with the crystallographic data. The magnetic properties of 1 were satisfactorily reproduced by a simple model which assumes that exchange coupling interactions mediated by mu(2)-methoxo ligands are responsible for the observed magnetic behavior in the temperature range 20-300 K, whereas at lower temperature the effects of the mu(3)-methoxo, mu(2)-carboxylato and mu(6)-oxo ligands become more important. This behavior is reflected by the appearance of magnetic features in the Mossbauer spectra with decreasing temperature. 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 constant through mu(2)-methoxo bridges within the a subcluster, and the spins of subcluster b remain largely uncoupled.