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

• Published in: Inorganic chemistry 2009-03, Vol.48 (5), p.2028-2042
• Main Authors: Zheng, Yan-Zhen, Xue, Wei, Zhang, Wei-Xiong, Tong, Ming-Liang, Chen, Xiao-Ming, Grandjean, Fernande, Long, Gary J, Ng, Seik-Weng, Panissod, Pierre, Drillon, Marc
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic8019155

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 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 [FeII 2(μ4-O)FeIII 2(μ4-O)]6+ mixed-spin-chain structure with no change in the P1̅ 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 Mössbauer 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 [FeII 2(μ4-O)FeIII 2(μ4-O)]6+ chains. Below 32 K the Mössbauer 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 str