Slow magnetic relaxation and electron delocalization in an S ⁹₂ iron(IIIII) complex with two crystallographically inequivalent iron sites

The magnetic, electronic, and Mssbauer spectral properties of Fe ₂L(-OAc)₂ClO₄, 1, where L is the dianion of the tetraimino-diphenolate macrocyclic ligand, H₂L, indicate that 1 is a class III mixed valence iron(IIIII) complex with an electron that is fully delocalized between two crystallographically inequivalent iron sites to yield a Fe₂^V cationic configuration with a S_t 92 ground state. Fits of the dc magnetic susceptibility between 2 and 300K 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 D₉₂ 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, U_eff, of 9.8 cm^-1. At 5.25 K the Mssbauer spectra of 1 exhibit two spectral components, assigned to the two crystallographically inequivalent iron sites with a static effective hyperfine field; as the temperature increases from 7 to 310 K, the spectra exhibit increasingly rapid relaxation of the hyperfine field on the iron-57 Larmor precession time of 5 10^-8 s. A fit of the temperature dependence of the average effective hyperfine field yields |D₉₂| 0.9 cm^-1. An Arrhenius plot of the logarithm of the relaxation frequency between 5 and 85 K yields a relaxation barrier of 17 cm^-1.