Temperature-dependent interactions and disorder in the spin-transition compound [Fe^II(L)₂][ClO₄]₂·C₇H₈ through structural, calorimetric, magnetic, photomagnetic, and diffuse reflectance investigations

Abstract

The title compound [Fe^II(L)₂][ClO₄]₂·C₇H₈ (L = 2-[3-(2'-pyridyl)pyrazol-1-ylmethyl]pyridine) has been isolated while attempting to grow single crystals of the spin-transition (continuous-type) compound [Fe^II(L)₂][ClO₄]₂, published earlier (Dalton Trans. 2003, 3392-3397). Magnetic susceptibility measurements, as well as Mssbauer and calorimetric investigations on polycrystalline samples of [Fe(L)₂][ClO₄]₂·C₇H₈ revealed the occurrence of an abrupt HS (⁵T₂) ↔ LS (¹A₁) transition with steep and narrow (2 K) hysteresis at ~232 K. The photomagnetic properties exhibit features typical for a broad distribution of activation energies, with relaxation curves in the shape of stretched exponentials. We performed a crystal structure determination of the compound at 120, 240, and 270 K. A noteworthy temperature-dependent behavior of the structural parameters was observed, in terms of disorder of both the anions and solvent molecules, leading to a strong thermal dependence of the strength and dimensionality of the interaction network. Additional data were obtained by diffuse reflectance measurements. We model and discuss the antagonistic effects of interactions and disorder by using a two-level cooperative mean-field approach which includes a distribution of barrier energies at the microscopic scale.