Structural and Magnetic Properties of a Pyridyl(vinyl)benzoate-Based Metal-Organic Framework with Iron(II) as Spin Carrier

The synthesis and structural, magnetic, and hyperfine properties of a new metal-organic framework are presented. Its chemical formula is [Fe-6(pvb)(4)(HCOO)(6)(OH)(2)](n)2n(dmf)2n(H2O) (1), where pvb(-) = bis{4-[2-(4-pyridyl)vinyl]} benzoate. Compound 1 was obtained under hydro(solvo)thermal conditions, presenting a hexanuclear iron(II) motif, which extends through HCOO- and pvb(-) bridges, resulting in a 2D framework linked covalently to adjacent analogous layers by the pvb(-) ligands to form a 3D coordination polymer. 300 and 80 K Fe-57 M & ouml;ssbauer data suggested two paramagnetic iron(II) species in the extended structure of compound 1, presenting a distorted octahedral high-spin state. Alternating current magnetization data recorded in zero and in-field showed two well-defined ordering temperatures at ca. 13 and 2.5 K for the two iron(II) species in less and highly distorted octahedral symmetry, respectively. The system undergoes magnetic transition from a paramagnetic state at high temperatures to a magnetically ordered state at 13 K. At 2.5 K, the ordered spins present a canted-like structure. These magnetic states are field dependent, showing a metamagnetic phase transition for fields higher than 5 kOe. A remnant magnetization of 1.91 N beta and a coercive field, H-C, of 10 kOe are obtained at 2 K, indicating a hard magnetic state often found when AF and FM exchange interactions are present.