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

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) Angstrom. The unit cell parameters of the triclinic crystal system were a = 12.091 Angstrom b = 12.225 Angstrom, c = 14.083 Angstrom, alpha = 77.70degrees, beta = 80.35degrees, gamma = 74.35degrees, and V = 1944.9 Angstrom(3) 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, Mossbauer spectra, magnetic susceptibility, DSC) have been transformed to a common basis - the temperature dependence of the high-spin mole fraction x, is (T). The application of the Ising-like (two-level) model of the spin crossover led to the thermodynamic data DeltaH = 2.6 kJ mol(-1) and DeltaS = 19 J K-1 mol(-1) as well as to the cooperativeness J/k approximate to 110 K (subtracted from the susceptibility data) that characterizes the abruptness of the spin crossover in the solid state. (C) 2003 Elsevier B.V. All rights reserved.