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 various fields such as 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 unique trinuclear Fe(II)-SCO complex, [(R-L)Fe^II{Au(CN)₂}₂] (R 1), using a chiral chelating ligand decorated with rotatable benzyl rings. The [FeAu₂] trinuclear molecules form a 2₁-helical supramolecular chain via elastic Au <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mo>⋯</mml:mo> <mml:annotation>${\cdots }$</mml:annotation> </mml:semantics> </mml:math> Au contacts. Interestingly, the synergy between the multiple dynamic factors (SCO event, rotation of the rings, and flexibility in Au <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:semantics><mml:mo>⋯</mml:mo> <mml:annotation>${\cdots }$</mml:annotation> </mml:semantics> </mml:math> 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 R 1 undergoes two hysteretic magnetic transitions: a non-spin transition between 360 and 380 K and an unsymmetric SCO transition in the region of 160-280 K, associated with a symmetry-breaking event between the non-polar and polar space groups (P2₁2₁2₁↔P2₁). Both transitions are triggered/accompanied by the rotation (inward vs. outward) of the benzyl rings. Correspondingly, reversible spring-like motions of the helical chains with the helical pitches varying from 11.345^{140 K} to 12.509^{280 K} then back to 11.630^{380 K} Å 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.