Slow magnetic relaxation and electron delocalization in an S = 9/2 iron(II/III) complex with two crystallographically inequivalent iron sites

The magnetic, electronic, and Mossbauer spectral properties of [Fe2L(μ-OAc)2]ClO4, 1, where L is the dianion of the tetraimino-diphenolate macrocyclic ligand, H2L, indicate that 1 is a class III mixed valence iron(II/III) complex with an electron that is fully delocalized between two crystallographically inequivalent iron sites to yield a [Fe2]V cationic configuration with a St = 9/2 ground state. Fits of the dc magnetic susceptibility between 2 and 300 K and of the isofield variable-temperature magnetization of 1 yield an isotropic magnetic exchange parameter, J, of −32(2) cm−1 for an electron transfer parameter, B, of 950 cm−1, a zero-field uniaxial D9/2 parameter of −0.9(1) cm−1, and g = 1.95(5). In agreement with the presence of uniaxial magnetic anisotropy, ac susceptibility measurements reveal that 1 is a single-molecule magnet at low temperature with a single molecule magnetic effective relaxation barrier, Ueff, of 9.8 cm−1. At 5.25 K the Mossbauer spectra of 1 exhibit two spectral components, assi...