Thermosalience coupled to abrupt spin crossover with dynamic ligand motion in an iron(ii) molecular crystal

Abstract

Herein, we report an iron(II) molecular crystal that shows thermosalient effects (crystal jumping) coupled to cooperative high-spin (HS) to low-spin (LS) spin crossover (SCO). The new iron(II) complex [Fe(L^Ph,Et)₂(NCS)₂] (L^Ph,Et = 1-phenyl-1H-1,2,3-triazole-4-yl-methylideneaminoethyl) shows abrupt reversible spin transition at around 182 K with hysteresis (T_1/2↓ = 179 K and T_1/2↑ = 184 K) and thermosalience as well as thermochromism from red-orange to dark red in the same temperature region upon cooling. These effects were characterized by magnetic, calorimetric, variable-temperature single-crystal X-ray diffraction, and cold-stage microscopy studies. The intermolecular interaction energies between the HS and LS states of the supramolecular architecture of this molecular crystal were also compared quantitatively via energy framework analyses based on the measured crystal structures. The structural analyses reveal that the thermosalient effect is induced by an anisotropic lattice change with space group transformation in a single-crystal-to-single-crystal manner upon HS-to-LS SCO. This unusual thermosalience accompanied by symmetry change is caused by a 90° rotation of one of the two ethyl (Et) groups of each molecule around the C–N bond, in which this dynamic ligand motion of the molecules propagates directionally (along the crystal a-axis) and concertedly through the space in the three-dimensional supramolecular network keeping repulsive energies between paired Et groups, resulting in the anisotropic expansion of the a-axis and the contraction of the other two axes. This work presents the potential applicability of supramolecular SCO crystals bearing cooperativity and microscopic dynamic molecular motion to exhibit the desired macroscopic active movement of the crystals towards the development of new actuating materials.