From a mononuclear (FeL2)-L-II complex to a spin crossover (Fe4L6)-L-II cage by symmetric ligand architecture modification: insights into the ammonia gas sensing mechanism

The occurrence of spin crossover usually induces different outputs, one of which is the colour change, an essential parameter for a colorimetric sensor. Herein, by symmetric modification of the ligand architecture, two complexes: a Fe-II(L1)(2) mononuclear high-spin (HS) complex (1) and a Fe-4(II)(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. 2 presents a shorter response time (90 s) to NH3(g) compared to 1 (8 min) due to its empty cage structure, as revealed by single crystals X-ray diffraction, and large specific surface area increasing the adsorption rate of NH3(g). Fe-57 Mossbauer spectroscopy was employed to investigate the sensing mechanism around the metal centre. A conversion of 33% Fe-II 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 Fe-II ions induced with a new FeN6 centre, but non-coordinated BF4- anions also react with NH4+ to form NH4BF4. These findings provide a foundation for exploring Fe-II-based coordination complexes as potential NH3 gas sensors towards high nuclearity as well as tuneable porosity.