Two-dimensional square-grid iron(ii) coordination polymers showing anion-dependent spin crossover behavior

Two Fe(ii)-based coordination polymers [Fe(tpmd)(2)(NCS)(2)]·5.5H(2)O (1) and [Fe(tpmd)(2)(NCSe)(2)]·7H(2)O (2) with the framework of square-grid type have been assembled from FeSO(4)·7H(2)O, N,N,N′,N′-tetrakis(pyridin-4-yl)methanediamine (tpmd), and KNCS/KNCSe in methanol and characterized. By utilizing two pyridine groups of a tpmd ligand, 1 and 2 are formed in two-dimensional layered structures through coordination of octahedral iron(ii) ions with the tpmd to NCS(−)/NCSe(−) ligands in which they have a supramolecular isomorphous conformation. 1 shows a paramagnetic behavior between 2 and 300 K, while 2 exhibits two-step spin crossover (ca. 145 and 50 K) in the temperature range due to the coordination of NCSe(−) ligands. At 300 K 2 is fully high-spin state. However, at 100 K 2 becomes ca. 50% high spin and 50% low spin iron(ii) ions, which is verified by magnetic moments. In the structural analysis of 2 at 100 K, two different layers are observed with different bond distances around iron(ii) ions in which the layers are stacked alternately.