Synthesis, Structure, and Magnetic Properties of a Tris[3-(2-pyridyl)-1,2,4-triazole]iron(II) Spin-Crossover Complex

Abstract

The synthesis and characterization of tris[3-(pyridin-2-yl)-1,2,4-triazole]iron(II) bis(tetrafluoroborate), obtained from the reaction of 3-(pyridin-2-yl)-1,2,4-triazole (Hpt) and hexaaquairon(II) tetrafluoroborate, [Fe(H2O)6](BF4)2, is described, together with its crystal structures at two temperatures. X-ray crystallographic parameters are as follows: [Fe(Hpt)3](BF4)2·nH2O (n ≈ 2) at 250 K: orthorhombic space group Pbam, a = 15.8068(18) A, b = 17.2800(14) A, c = 21.215(2) A, V = 5794.7(10) A3, and Z = 8. [Fe(Hpt)3](BF4)2·nH2O (n ≈ 2) at 95 K: orthorhombic space group Pbam, a = 15.7080(12) A, b = 17.1023(16) A, c = 21.006(2) A, V = 5643.1(9) A3, and Z = 8. The FeII ions are (at both temperatures) octahedrally surrounded in a mer-configuration. The complex has been subjected to both 57Fe Mossbauer spectroscopy and magnetic susceptibility measurements. The 57Fe Mossbauer spectrum shows the presence of two different iron(II) sites. The high-spin fraction vs. T curve of the crossover, obtained by SQUID measurements, is gradual and without hysteresis, with T1/2 = 135 K.