Spin-Crossover Transitions of Solid Solutions of Fe^II Complexes with Fluorine- and Chlorine-Substituted Schiff-Base Ligands, Hqsal^5F and Hqsal^5Cl

Spin-crossover (SCO) transitions of solid solutions of Fe^II complexes coordinated with fluorine- and chlorine-substituted Schiff-base ligands, [Fe^II(qsal^5F)<i>_<i>x</i></i>(qsal^5Cl)_2-<i>x</i>] (<i>x</i> = 0.0-2.0, qsal^5X = 5-halogeno-<i>N</i>-(8'-quinolyl)-2-hydroxy-1-salicylaldimine), were investigated by magnetic, single-crystal X-ray diffraction, and heat-capacity measurements. The SCO transition temperature decreases from 310.5 to 148.8 K as <i>x</i> increases from 0.0 to 2.0. At 0.0 ≤ <i>x</i> ≤ 1.0, both Δ_trs<i>H</i> and Δ_trs<i>S</i> decrease with increasing <i>x</i>, whereas Δ_trs<i>H</i> slightly decreases but Δ_trs<i>S</i> is almost constant at 1.5 ≤ <i>x</i> ≤ 2.0, showing that the <i>x</i>-dependence of <i>T</i>_trs is mainly caused by variation in Δ_trs<i>H</i>. The different trend at 0.0 ≤ <i>x</i> ≤ 1.0 and 1.5 ≤ <i>x</i> ≤ 2.0 is because of different crystal structures of the HS state (monoclinic at 0.0 ≤ <i>x</i> ≤ 1.0 and orthorhombic at 1.5 ≤ <i>x</i> ≤ 2.0). Density functional theory calculations reveal that the single-point energy in the HS state decreases with an increase in <i>x</i>, indicating that the decrease in Δ_trs<i>H</i> is partly caused by a molecular distortion induced by the partial replacement of halogen species. The molar heat capacity of the HS state decreases as <i>x</i> increases, showing that changes in vibrational motions also contribute to variation of Δ_trs<i>H</i>.