Synthesis, crystal structure, and magnetic properties of the coordination polymer $[Fe(NCS)_{2}$(1,2-bis(4-pyridyl)-ethylene)]$_{n}$ showing a two step metamagnetic transition.

Reaction of iron(II) thiocyanate with an excess of trans-1,2-bis(4-pyridyl)-ethylene (bpe) in acetonitrile at room temperature leads to the formation of $[Fe(NCS)_{2}(bpe)_{2} \cdot (bpe)]$ (1), which is isotypic to its Co(II) analogue. Using slightly different reaction conditions the literature known compound $[Fe(NCS)_{2}(bpe)_{2}(H_{2}O)_{2}]$ (2) was obtained as a phase pure material. Simultaneous differential thermoanalysis and thermogravimetry prove that the hydrate 2 transforms into the anhydrate $[Fe(NCS)_{2}(bpe)_{2}]$ (3), that decomposes on further heating into the new ligand-deficient 1:1 compound of composition $[Fe(NCS)_{2}(bpe)]_{N}$ (4), which can also be obtained directly by thermal decomposition of 1. Further investigations reveal that 4 can also be prepared under solvothermal conditions, and single crystal structure analysis shows that the iron(II) cations are linked via $\mu$ -1,3 bridging thiocyanato anions into chains, that are further connected into layers by the bpe ligands. Magnetic measurements, performed on powder samples, prove that 1 and 2 show only Curie–Weiss behavior, whereas in 4 antiferromagnetic ordering with a Néel temperature of 5.0 K is observed. At T < 4.0 K a two-step metamagnetic transition occurs at applied magnetic fields of 1300 and 1775 Oe. The magnetic properties are discussed and compared with those of related compounds.