Controlling Spin States in Metallosupramolecular Iron(II) Grid Architectures through Light, Temperature, and...

Stimuli-responsive molecular materials capable of reversibly modulating their electronic states are of considerable interest for molecular electronics, information storage, and quantum technologies. In this context, we report a series of metallosupramolecular iron(II) [2 × 2] grid complexes constructed from bis-hydrazone ligands that incorporate proton-responsive N-H functionalities within the coordination framework. Single-crystal X-ray diffraction reveals well-defined grid architectures in which the ligand design enables precise tuning of the ligand field through reversible protonation-deprotonation processes. This chemical modulation provides an effective pathway for controlling the electronic configuration of the iron centers. The resulting complexes display spin-state switching behavior triggered by thermal, light, and protonation-deprotonation effects. Spin-state modulation is experimentally demonstrated both in the solid state through variable-temperature magnetic susceptibility measurements and in solution using paramagnetic ¹H NMR spectroscopy. Diamagnetic Zn(II) analogues are also prepared as reference systems to elucidate the electronic behavior of the iron(II) grids. These results establish a family of metallosupramolecular iron(II) grid architectures exhibiting multistimuli-responsive spin-state switching and highlight how proton-coupled ligand-field modulation within a supramolecular framework can regulate bistable magnetic behavior. This work provides new insight into the design of chemically addressable spin-state switchable supramolecular systems for responsive molecular materials.