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.
S. Hazra and S. Sasmal and M. Fleck and F. Grandjean and M. T. Sougrati and M. Ghosh and T. D. Harris and P. Bonville and G. J. Long and S. Mohanta, "Slow Magnetic Relaxation and Electron Delocalization in an S 9/2 Iron(IIIII) Complex with Two Crystallographically Inequivalent Iron Sites," Journal of Chemical Physics, American Institute of Physics (AIP), Jan 2011.
The definitive version is available at http://dx.doi.org/10.1063/1.3581028
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