Supramolecular Spring‐Like Fe(II) Spin‐Crossover Complexes Experiencing Giant and Anisotropic Thermal Expansion Across Two Distinct Temperature Regimes

Dynamic molecules with tunable chemical and physical properties in response to external stimuli hold great potential for applications in information storage, smart molecular machines, and biomimetics. Among them, supramolecular springs and spin‐crossover (SCO) complexes can both undergo visible macroscopic changes under heat or light stimulation. In this study, we synthesized a trinuclear Fe(II)‐SCO complex, [(R‐L)FeII{Au(CN)2}2] (R1), using a chiral chelating ligand decorated with rotatable benzyl rings. The [FeAu2] trinuclear molecules form a 21‐helical supramolecular chain via elastic Au∙∙∙Au contacts. Interestingly, the synergy between the multiple dynamic factors (SCO event, rotation of the rings, and flexibility in Au∙∙∙Au distance) endows the complex with multiple switchings in both magnetism and structure, as well as the most intriguing characteristic of giant and anisotropic “breathing” feature in thermal expansion within two distinct temperature regimes. Specifically, complex R1 undergoes two hysteretic magnetic transitions: a non‐spin transition and an unsymmetric SCO transition. Both transitions are triggered/accompanied by the rotation of the benzyl rings. Correspondingly, reversible spring‐like motions of the helical chains are observed in the two distinct temperature regimes. This work demonstrates a significant success in incorporating both SCO and spring‐like motion in one system, paving the way for designing multifunctional dynamic materials for future devices.