Disruption on adsorption

J. Am. Chem. Soc. 135, 15986–15989 (2013)

Credit: © 2013 ACS

Stimuli-responsive materials are attractive for the development of devices such as sensors or for memory storage, and with this in mind, molecules that display spin-crossover phenomena have received much interest. Spin-crossover arises when metal complexes can exist either in low-spin or high-spin electronic ground states; it involves a transition from one to the other. This causes changes in the magnetic properties of the compound and can be induced by an external stimulus — typically a temperature variation. Guillermo Mínguez Espallargas from the University of Valencia, Spain, and co-workers have now observed that the spin-crossover transition of an iron(II) coordination polymer is affected by the physisorption of CO₂ gas molecules.

Single-crystal X-ray characterization shows that both the low-spin and high-spin states, at 120 K and 240 K, respectively, adopt the same structure. The material consists of one-dimensional coordination polymer chains, [Fe(II)(btzx)₃]²⁺ (where btzx is a flexible ligand comprising two tetrazole units grafted on a phenyl core), closely packed in a parallel manner and separated by counteranions (ClO₄⁻). An interesting feature of the material is that it exhibits empty internal cavities between two iron centres, but no permanent channels. Nevertheless, the flexibility of the btzx ligands enables gas molecules to find their way to these cavities, presumably through rotation of the central phenyl ring. Measurements show that the material selectively adsorbs CO₂ over N₂, most likely owing to the smaller kinetic diameter of CO₂ and its stronger affinity for the cationic polymer chains.

The researchers observed that this CO₂ adsorption affected the material's spin-crossover transition temperature — it increased to 209 K for the CO₂-loaded material from 200 K for the unloaded material — and that this effect is reversible. Characterization of the CO₂-loaded material by X-ray powder diffraction crystallography showed that the material retained the same structure, and under ambient conditions (1 bar, 273 K) accommodated one CO₂ molecule per cavity through O=C=O···π interactions with the ligands' tetrazole units. This increases the framework's electron density, stabilizing the low-spin state, which in turn increases the spin transition temperature.