Proton-Induced Reversible Spin-State Switching in Octanuclear Fe^III Spin-Crossover Metal-Organic Cages

Responsive spin-crossover (SCO) metal-organic cages (MOCs) are emerging dynamic platforms with potential for advanced applications in magnetic sensing and molecular switching. Among these, Fe^III-based MOCs are particularly noteworthy for their air stability, yet they remain largely unexplored. Herein, we report the synthesis of two novel Fe^III MOCs using a bis-bidentate ligand approach, which exhibit SCO activity above room temperature. These represent the first SCO-active Fe^III cages and feature an atypical {FeN₆}-type coordination sphere, uncommon for Fe^III SCO compounds. Our study reveals that these MOCs are sensitive to acid/base variations, enabling reversible magnetic switching in solution. The presence of multiple active proton sites within these SCO-MOCs facilitates multisite, multilevel proton-induced spin-state modulation. This behavior is observed at room temperature through ¹H NMR spectroscopy, capturing the subtle proton-induced spin-state transitions triggered by pH changes. Further insights from extended X-ray absorption fine structure (EXAFS) and theoretical analyses indicate that these magnetic alterations primarily result from the protonation and deprotonation processes at the NH active sites on the ligands. These processes induce changes in the secondary coordination sphere, thereby modulating the magnetic properties of the cages. The capability of these Fe^III MOCs to integrate magnetic responses with environmental stimuli underscores their potential as finely tunable magnetic sensors and highlights their versatility as molecular switches. This work paves the way for the development of SCO-active materials with tailored properties for applications in sensing and molecular switching.