New family of Hofmann-like coordination polymers constructed with imidazole ligands and associated with spin crossover and anisotropic thermal expansions

Abstract

Instead of the conventionally utilized pyridine derivatives ligands for the Hofmann-like spin crossover (SCO) structure, here we used five-membered-aromatic-ring ligands to afford a new series of Hofmann-like compounds Fe(L)₂[Ni(CN)₄] (L = imidazole (1), 1-methylimidazole (2)). The synthesis, crystal structures, magnetic properties and thermal variation of the lattice constants for 1 and 2 were studied. The structures of the currently synthesized compounds were, as expected, found to be similar to that of traditional Hofmann-like 2D layer structure. Compound 1 showed a high spin state at all temperatures, while compound 2 displayed strong cooperative SCO centered at 144 K accompanied by a 35.2 K-wide hysteresis loop. Interestingly, compound 2 clearly showed anisotropic thermal expansion behaviour.