Fe(II) aminotriazole spin-crossover coordination polymers: from structure and reactivity to functional applications

Fe(II) aminotriazole coordination polymers constitute one of the most extensively investigated families of spin-crossover (SCO) materials owing to their robust switching behaviour, synthetic accessibility, and pronounced sensitivity to composition, particle size, morphology, and solid-state organization. As benchmark SCO materials, they have attracted considerable interest for applications ranging from thermometry and actuation to responsive composites and multifunctional materials. This review provides a comprehensive overview of the synthesis, structural diversity, spin-state switching, chemical reactivity, and applications of Fe(II) aminotriazole coordination polymers. Particular emphasis is placed on structure–property relationships, highlighting how synthetic parameters, counterion identity, crystal packing, particle size, and morphology govern cooperativity, transition characteristics, and material performance. Recent advances in post-synthetic modification (PSM) have considerably expanded the chemical space and functional scope of these materials while revealing the accessibility of coordination chains, the reactivity of coordinated ligands, and the interplay between chemical transformations, structure, and SCO behaviour. Despite their dense, apparently non-porous architectures, these coordination polymers remain accessible to external molecules, undergo diverse chemical transformations, and exhibit spin-state-dependent reactivity. These findings have stimulated studies in molecular recognition, sensing, catalysis, and biomedical applications. Although many remain at an exploratory stage, they demonstrate the importance of guest access to the coordination chains and provide new insights into host–guest interactions, molecular accessibility, cooperative phenomena, and spin-state-dependent reactivity. Collectively, these advances establish Fe(II) aminotriazole coordination polymers as versatile model systems for understanding the relationships between synthesis, structure, spin-state switching, chemical reactivity, and function, while opening new opportunities for responsive molecular materials.