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, [Fe IIFe III-(μ4-O)(1,4-chdc) 1.5]∞, 1, has been synthesized hydrothermally by mixing iron powder and 1,4-chdcH 2 and investigated by X-ray diffraction, dc and ac magnetic susceptibili...
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| Main Authors: | , , , , , , , , , |
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| Format: | Article |
| Published: |
American Chemical Society
2009
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| Subjects: | |
| Online Access: | http://eprints.um.edu.my/25092/ https://doi.org/10.1021/ic8019155 |
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| Summary: | A three-dimensional mixed-valent iron(II, III) trans-1,4- cyclohexanedicarboxylate, 1,4-chdc, coordination polymer, [Fe IIFe III-(μ4-O)(1,4-chdc) 1.5]∞, 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 Mössbauer 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 ll 2(μ 4-O)Fe lll 2(μ 4-O)] 6+ mixed-spin-chain structure with no change in the P1̄ space group but with subtle changes in the Fe-0 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(ll) and iron(lll) 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 μ 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(μ 4-O)Fe lll 2-(μ 4-O)] 6+ chains. Below 32 K the Mössbauer spectra of 1 exhibit sharp sextets for both the iron(lll) and iron(ll) 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(ll) and iron(lll). 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(ll), iron(lll), and iron(ll/lll) 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. © 2009 American Chemical Society. |
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