Developing tetranuclear Fe(II) spin crossover cages for gas sensing

The occurrence of spin crossover (SCO) usually induces different outputs, one of which is the colour change, an essential parameter for the design of a colorimetric sensor. Tetrahedral FeII-based metal organic cages have gained increasing attention as versatile SCO systems, owing to their unique ability to combine magnetic bistability with host–guest chemistry. Recent advances in ligand design and self-assembly strategies have enabled the construction of both face-capped FeII4L4 and edge-bridged FeII4L6 architectures, which display diverse SCO behaviors under thermal, chemical, or photonic stimuli. These cages serve as valuable models for understanding how factors such as ligand field strength, intermolecular interactions and guest encapsulation influence spin-state switching and distribution. Through a combination of magnetic susceptibility, 57Fe Mössbauer spectroscopy, and crystallographic analysis, structure–function relationships have been systematically established. These studies demonstrate the potential of Fe(ii) SCO cages not only for probing fundamental structure–property relationships, but also for developing functional materials that integrate magnetic, optical, and host-guest responsiveness in the solid state.1-4 Herein, by symmetric modification of the ligand architecture, two complexes: a FeII(L1)2 mononuclear high-spin (HS) complex (1) and a FeII4(L2)6 tetranuclear spin crossover cage (2) were constructed as colorimetric NH3(g) sensors, operating in the solid state. The sensing process is accompanied by a remarkable colour change from reddish brown (1) or light purple (2) to dark grey at room temperature. The cage presents a shorter response time (90 s) to NH3(g) compared to the complex (8 min) due to its empty cage structure, as revealed by single crystals X-ray diffraction, as well as by the large specific surface area increasing the adsorption rate of NH3(g). 57Fe Mössbauer spectroscopy was employed to investigate the sensing mechanism around the metal centre. A conversion of 33% FeII ions to the low-spin (LS) state was observed in 1@NH3, after the substitution of NH3(g) molecules, leading to FeN6 sites. The sensing mechanism of 2 also involves a HS to LS transition of FeII ions induced with a new FeN6 centre, but non-coordinated BF4− anions were also found to react with NH4+ to form NH4BF4. These findings provide a solid foundation for exploring FeII-based coordination complexes as potential NH3 gas sensors towards high nuclearity as well as tuneable porosity.5