title: Spin-frustrated complex, [Fe(II)Fe(III)(trans-1,4-cyclohexanedicarboxylate)1.5]infinity: interplay between single-chain magnetic behavior and magnetic ordering.
abstract: A three-dimensional mixed-valent iron(II,III) trans-1,4-cyclohexanedicarboxylate, 1,4-chdc, coordination polymer, [Fe(II)Fe(III)(mu(4)-O)(1,4-chdc)(1.5)](infinity), 1, has been synthesized hydrothermally by mixing iron powder and 1,4-chdcH(2) and investigated by X-ray diffraction, dc and ac magnetic susceptibility, and iron-57 Mossbauer spectroscopy over a wide range of temperatures. Single-crystal X-ray diffraction studies of 1 at 90(2), 293(2), and 473(2) K reveal a tetrahedral [Fe(II)(2)(mu(4)-O)Fe(III)(2)(mu(4)-O)](6+) mixed-spin-chain structure with no change in the P1 space group but with subtle changes in the Fe-O and Fe...Fe distances with increasing temperature. These changes are associated with the electron delocalization observed by Mossbauer spectroscopy above 225 K. Magnetic studies reveal three different magnetic regimes in 1 between 2 and 320 K. Above 36 K 1 is a one-dimensional ferrimagnetic-like complex with frustration arising from competing exchange interactions between the iron(II) and iron(III) ions in the chains. Between 36 and 25 K the interchain interactions are non-negligible and 1 undergoes three-dimensional ordering at 32.16 K but with some residual fluctuations. Below 25 K the residual fluctuations slow and eventually freeze below 15 K; the small net moment of 0.22 mu(B) per mole of 1 observed below 15 K may be attributed to a non-collinear or canted spin structure of the spins of the four iron ions in the [Fe(II)(2)(mu(4)-O)Fe(III)(2)(mu(4)-O)](6+) chains. Below 32 K the Mossbauer spectra of 1 exhibit sharp sextets for both the iron(III) and iron(II) ions and are consistent with either a static long-range or a short-range magnetic ground state or a slow relaxation between two canted magnetic states that are indistinguishable at the observed spectral resolution. The 85 and 155 K spectra reveal no electron delocalization and correspond solely to fixed valence iron(II) and iron(III). Between 225 and 310 K the spectra reveal the onset of electron delocalization such that, at 295 to 310 K, 25, 25, and 50% of the iron in 1 is present as iron(II), iron(III), and iron(II/III) ions, respectively. The absence of any spectral line broadening associated with this electron delocalization and the coexistence of four doublets between 225 and 310 K indicate that the delocalization occurs through electron tunneling via vibronic coupling. The sudden increase in the tunneling rate beginning above about 260 K and the presence of a cusp in the magnetic susceptibility centered at about 275 K strongly suggest the existence of a charge order/disorder transition whose nature and order are discussed.
authors.affiliation: Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry & Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China.
authors.forename: Yan-Zhen
authors.initials: YZ
authors.lastname: Zheng
authors.forename: Wei
authors.initials: W
authors.lastname: Xue
authors.forename: Wei-Xiong
authors.initials: WX
authors.lastname: Zhang
authors.forename: Ming-Liang
authors.initials: ML
authors.lastname: Tong
authors.forename: Xiao-Ming
authors.initials: XM
authors.lastname: Chen
authors.forename: Fernande
authors.initials: F
authors.lastname: Grandjean
authors.forename: Gary J
authors.initials: GJ
authors.lastname: Long
authors.forename: Seik-Weng
authors.initials: SW
authors.lastname: Ng
authors.forename: Pierre
authors.initials: P
authors.lastname: Panissod
authors.forename: Marc
authors.initials: M
authors.lastname: Drillon
mesh_terms: D018360:Crystallography, X-Ray; D058085:Iron Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D008024:Ligands; D008280:Magnetics; D008968:Molecular Conformation; D015204:Spectroscopy, Mossbauer; D013237:Stereoisomerism; D013696:Temperature
chemical_list: D058085:Iron Compounds; D008024:Ligands