A Two-dimensional Iron(II) Carboxylate Linear Chain Polymer That Exhibits a Metamagnetic Spin-canted Antiferromagnetic to Single-chain Magnetic Transition


A two-dimensional iron(II) carboxylate coordination polymer, [Fe(pyoa) 2]∞, where pyoa is 2-(pyridin-3-yloxy)acetate, has been prepared by hydrothermal synthesis. Its crystal structure reveals a single iron(II) site with an elongated octahedral coordination environment containing four equatorial carboxylate oxygens and two axial pyridyl nitrogens; the iron(II) sites are linked by syn-anti μ-carboxylates to form chains along the b axis that have an Fe⋯Fe separation of 4.910 Å. The shortest interchain and interlayer Fe⋯Fe distances are 6.453 and 11.125 Å, respectively. The 4.2-295 K Mössbauer spectra of [Fe(pyoa) 2]∞ consist of a single paramagnetic high-spin iron(II) quadrupole doublet. The axial Fe-N bond direction defines the Jahn-Teller axis at an iron(II) site and, consequently, the orientation of the single-ion magnetic anisotropy. Thus, along the b axis in a given chain, the spins are collinear and parallel to the Jahn-Teller axis. The Jahn-Teller axes of adjacent intralayer chains have different orientations with an angle of 79.2° between the axes in adjacent chains in a bc layer. [Fe(pyoa) 2]∞ exhibits field-induced metamagnetic behavior such that, in an applied field smaller than the critical field, the iron(II) spin-canted moments experience intrachain ferromagnetic interactions and weak interchain antiferromagnetic interactions; the spin canting yields weak ferromagnetism. In an applied field larger than the critical field, the weak antiferromagnetic interchain interactions are overwhelmed to yield superparamagnetic-like slow-magnetic relaxation with an energy barrier of 23(3) K. Single-crystal magnetic studies reveal a quasi-uniaxial magnetic anisotropy with the a axis as the easy-magnetic axis and the b axis as the hard-magnetic axis; the susceptibility measured along the easy a axis may be fit with the Glauber model to yield an effective intrachain exchange coupling constant of 2.06(8) K. A dynamic analysis of the susceptibility yields a 6.3(1) K energy barrier for intrachain domain wall creation. The observed field-assisted superparamagnet-like behavior is consistent with the dynamics of a single-chain magnet. Thus, [Fe(pyoa)2]∞ is best considered as a “metamagnetic-like” single-chain magnet. © 2008 American Chemical Society.



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