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, [(FeFeIII)-Fe-II(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-2(II)(mu(4)-O)Fe-2(III)-(mu(4)-O)](6+) mixed-spin-chain structure with no change in the P (1) over bar space group but with subtle changes in the Fe-O and Fe center dot center dot center dot 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-2(II)(mu(4)-O)Fe-2(III)-(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.