Recent advances in spin crossover Fe(ii) tetrahedral metal–organic cages and their solid-state applications

Abstract

Tetrahedral Fe(II)-based metal–organic cages represent a distinctive family of spin crossover materials that display tunable magnetic bistability in the solid state under thermal, chemical or photonic stimuli. This review summarizes the latest progress in the design, synthesis and characterization of such Fe(II) spin crossover cages, with particular emphasis on how ligand field strength, host–guest interactions, and intermolecular interactions influence their spin-state switching behavior and spin-state distribution. Representative cage architectures, including face-capped Fe(II)₄L₄ and edge-bridged Fe(II)₄L₆ systems, are discussed in detail, focusing on their structural features and spin crossover properties as elucidated by single-crystal X-ray diffraction, magnetic susceptibility measurements and ⁵⁷Fe Mössbauer spectroscopy. In addition, we highlight emerging solid-state applications of these spin crossover cages, such as gas sensing and guest-responsive adsorption. Finally, we outline main hurdles and future research directions for the development of tetrahedral Fe(II) based spin crossover cages as functional molecular materials.