6533b7d1fe1ef96bd125ca86

RESEARCH PRODUCT

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

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Abstract The thermal expansion of the spin crossover system [Fe(pybzim) 3 ](ClO 4 ) 2  · H 2 O (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) A. The unit cell parameters of the triclinic crystal system were a =12.091 A, b =12.225 A, c =14.083 A, α =77.70°, β =80.35°, γ =74.35°, and V =1944.9 A 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 HS ( T ). The application of the Ising-like (two-level) model of the spin crossover led to the thermodynamic data Δ H =2.6 kJ mol −1 and Δ S =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.