Cooperative Spin Crossover Near Room Temperature in Fe(II) Complexes Based on Acylhydrazone Ligands

The magnetic bistability of spin-crossover (SCO) materials is very appealing for applications, such as molecular switches and information storage. However, examples of concerted spin transitions at or near room temperature are still uncommon. This paper reports intriguing SCO properties in a series of Fe­(II) complexes based on the acylhydrazone ligand. Complexes 1–3 have various 4-pyridyl and 4-benzyl substituents (NH2/Cl in 1; NH2/CH3 in 2; and H/CH3 in 3). 1 and 2 are structurally equivalent. The amino group promotes the formation of hydrogen bonds and the incorporation of solvent molecules. Single crystal data, DSC, and magnetic studies have demonstrated that their solvated structures are stabilized in a high spin (HS) state, while the desolvated forms are capable of showing cooperative SCO. They exhibit reproducible thermal hysteresis of 12 and 16 K wide, respectively, centered at 277 and 161 K at a temperature scan rate of 2 K min–1. 3, on the other hand, crystallizes in the absence of solvent molecules and has a dense packing connected by π···π, C–H···O, and C–H···N hydrogen bonding. It is stabilized in the LS state below room temperature and shows an onset of a very gradual SCO above 300 K. A thorough structural analysis of 1–3 and a literature review of related complexes were carried out in order to look into some crucial implications on SCO characteristics.