From a mononuclear FeIIL2 complex to a spin crossover FeII4L6 cage by symmetric ligand architecture modification: insights into the ammonia gas sensing mechanism

Abstract

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^II₄(L2)₆ tetranuclear spin crossover cage (2) were constructed as colorimetric NH_3(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 NH_3(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 NH_3(g). ⁵⁷Fe Mössbauer 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 NH_3(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.