Spin-frustrated Complex, [FeIIFeIII(trans-1,4-cyclohexanedicarboxylate)₁.₅]∞: Interplay between Single-chain Magnetic Behavior and Magnetic Ordering


A three-dimensional mixed-valent iron(II,III) trans-1,4-cyclohexanedicarboxylate, 1,4-chdc, coordination polymer, [FeIIFeIII(μ4-O)(1,4-chdc)1.5]∞, 1, has been synthesized hydrothermally by mixing iron powder and 1,4-chdcH2 and investigated by X-ray diffraction, dc and ac magnetic susceptibility, and iron-57 Mssbauer 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 [FeII2(μ4-O)FeIII2(μ4-O)]6+ mixed-spin-chain structure with no change in the P 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 Mssbauer 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 μB 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 [FeII2(μ4-O)FeIII2(μ4-O)]6+ chains. Below 32 K the Mssbauer 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.




National Fund for Scientific Research, Belgium
National Natural Science Foundation (China)
Guangdong Province (China). Science and Technology Department

Document Type

Article - Journal

Document Version


File Type





© 2009 American Chemical Society (ACS), All rights reserved.