Spin crossover in iron(II) tris(2-(2′-pyridyl)benzimidazole) complex monitored by variable temperature methods: synchrotron powder diffraction, DSC, IR spectra, Mössbauer spectra, and magnetic susceptibility

Abstract

The thermal expansion of the spin crossover system [Fe(pybzim)3](ClO4)2 · H2O (pybzim=2-(2′-pyridyl)benzimidazole) has been determined from powder X-ray data between 50 and 250 K; the wavelength of the synchrotron source was 1.21888(1) Å. The unit cell parameters of the triclinic crystal system were a=12.091 Å, b=12.225 Å, c=14.083 Å, α=77.70°, β=80.35°, γ=74.35°, and V=1944.9 Å3 at 250 K. In addition to the linear thermal expansion of the unit cell volume, an extra expansion due to the low-spin (LS) to high-spin (HS) transition is observed. The V(T) function shows a sudden increase comparable with the step in the effective magnetic moment at the transition region (140 K). A similar behavior is obtained on the basis of the infrared spectra. The absorption bands corresponding to the metal–ligand stretching modes change their intensities upon heating: the bands corresponding to the low-spin molecules (at ca. 409, 430, 443, and 460 cm−1) disappear in the gain of the high-spin bands (at 259 and 285 cm−1). The variable-temperature data obtained by different techniques (powder diffraction, EXAFS, IR spectra, Mössbauer spectra, magnetic susceptibility, DSC) have been transformed to a common basis – the temperature dependence of the high-spin mole fraction xHS(T). The application of the Ising-like (two-level) model of the spin crossover led to the thermodynamic data ΔH=2.6 kJ mol−1 and ΔS=19 J K−1 mol−1 as well as to the cooperativeness J/k≈110 K (subtracted from the susceptibility data) that characterizes the abruptness of the spin crossover in the solid state.