title: Slow magnetic relaxation and electron delocalization in an S = 9/2 iron(II∕III) complex with two crystallographically inequivalent iron sites.
abstract: The magnetic, electronic, and Mössbauer spectral properties of [Fe(2)L(μ-OAc)(2)]ClO(4), 1, where L is the dianion of the tetraimino-diphenolate macrocyclic ligand, H(2)L, 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 [Fe(2)](V) cationic configuration with a S(t) = 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 D(9∕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, U(eff), of 9.8 cm(-1). At 5.25 K the Mössbauer 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 |D(9∕2)| = 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).
authors.affiliation: Department of Chemistry, University of Calcutta, Kolkata, India.
authors.forename: Susanta
authors.initials: S
authors.lastname: Hazra
authors.forename: Sujit
authors.initials: S
authors.lastname: Sasmal
authors.forename: Michel
authors.initials: M
authors.lastname: Fleck
authors.forename: Fernande
authors.initials: F
authors.lastname: Grandjean
authors.forename: Moulay T
authors.initials: MT
authors.lastname: Sougrati
authors.forename: Meenakshi
authors.initials: M
authors.lastname: Ghosh
authors.forename: T David
authors.initials: TD
authors.lastname: Harris
authors.forename: Pierre
authors.initials: P
authors.lastname: Bonville
authors.forename: Gary J
authors.initials: GJ
authors.lastname: Long
authors.forename: Sasankasekhar
authors.initials: S
authors.lastname: Mohanta
mesh_terms: D018360:Crystallography, X-Ray; D004583:Electrons; D005290:Ferric Compounds / Q000737:chemistry*; D005296:Ferrous Compounds / Q000737:chemistry*; D008280:Magnetics; D008958:Models, Molecular; D015204:Spectroscopy, Mossbauer
chemical_list: D005290:Ferric Compounds; D005296:Ferrous Compounds