Slow Magnetic Relaxation and Electron Delocalization in an S 9/2 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 St 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, Ueff, 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. © 2011 American Institute of Physics.



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