Spin crossover complexes as building units of hydrogen-bonded nanoporous structures

Abstract

The paper reports on the synthesis, crystal structure, differential scanning calorimetry (DSC) and magnetic properties of a spin crossover salt of formula [Fe(bpp)₂](cis,cis-1,3,5-Hchtc)·5.5 H₂O (1) (bpp = 2,6-bis(pyrazol-3yl)pyridine; cis,cis-1,3,5-Hchtc = monoprotonated cyclohexanetricarboxylate dianion). The salt shows a porous structure with water molecules sitting in the channels. The electronic configuration of the Fe²⁺ ions is high-spin (HS). Desolvation of 1 yields a material exhibiting an abrupt spin crossover with a thermal hysteresis loop located near room temperature (T_1/2↑ = 319 K and T_1/2↓ = 309 K). Rehydration of this desolvated salt yields an essentially low-spin (LS) material. This is the first example of a series of hydrogen-bonded porous structures that combine a Fe(bpp)₂ spin crossover unit acting as a hydrogen-bond donor and polycarboxylate anions acting as hydrogen-bond acceptors with different connectivities.