Compensation effects and relation between the activation energy of spin transition and the hysteresis loop width for an iron(ii) complex?

Physical Chemistry Chemical Physics Pub Date: 2016-05-25 DOI: 10.1039/C6CP01892K

Abstract

The enthalpy–entropy compensation was observed for the cooperative 1A/LS → 1A/HS spin transition (the phase 1A is a mononuclear complex [FeL2](BF4)2, L is 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-(pyridin-2-yl)-6-methylpyrimidine). The physical origin of this effect is the fact that the 1A/LS → 1A/HS spin transition is the first order phase transition accompanied by noticeable variations in the Tonset↑, ΔH and ΔS values. Higher ΔH and ΔS values are correlated with higher Tonset↑ values. The higher the enthalpy and entropy of the spin transition, the wider the hysteresis loop. The kinetic compensation effect, i.e. a linear relationship between ln?A and Ea, was observed for the 1A/LS → 1A/HS spin transition. Moreover, an isokinetic relationship was detected in this system: the Arrhenius lines (ln?k vs. 1/T) obtained from magnetochemical data for different samples of the phase 1A undergoing the 1A/LS → 1A/HS transition show a common point of intersection (Tiso = 490 ± 2 K, ln?kiso = ?6.0 ± 0.2). The validity of this conclusion was confirmed by the Exner–Linert statistical method. This means that the isokinetic relationship and the kinetic compensation effect (ln?A vs. Ea) in this system are true ones. The existence of a true kinetic compensation effect is supported independently by the fact that the hysteresis loop width for the cooperative spin transition 1A/LS ? 1A/HS increases with increasing activation barrier height. Estimating the energy of excitations for the phase 1A/LS with Tiso ～ 490 K yields wavenumbers of ca. 340 cm?1 corresponding to the frequencies of the stretching vibrations of the FeLS–N bonds, i.e. the bonds directly involved in the mechanism of the spin transition. This is the first observation of the kinetic compensation effect (ln?A vs. Ea) and the isokinetic relationship for a cooperative spin crossover system showing thermal hysteresis. Our results provide the first experimental evidence that the higher the activation barrier for the spin transition, the wider the hysteresis loop for a series of related spin crossover systems.

Graphical abstract: Compensation effects and relation between the activation energy of spin transition and the hysteresis loop width for an iron(ii) complex
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