Supramolecular Spring-Like Fe(II) Spin-Crossover Complexes Experiencing Giant and Anisotropic Thermal Expansion...

Area of Science:

• Supramolecular Chemistry
• Materials Science
• Coordination Chemistry

Background:

• Dynamic molecules responding to stimuli are key for smart materials.
• Supramolecular springs and spin-crossover (SCO) complexes show macroscopic changes with external triggers.
• Integrating multiple dynamic functionalities in a single molecule is a significant challenge.

Purpose of the Study:

• To synthesize and characterize a unique trinuclear Fe(II)-SCO complex with tunable properties.
• To investigate the synergistic effects of SCO, ligand rotation, and metal-metal interactions on molecular behavior.
• To explore the potential for creating multifunctional dynamic materials.

Main Methods:

• Synthesis of a chiral trinuclear Fe(II)-SCO complex, [(R-L)Fe^II{Au(CN)₂}₂] (R 1), featuring rotatable benzyl rings.
• Structural analysis of the 2₁-helical supramolecular chains formed via Au-Au contacts.
• Magnetic and thermal expansion measurements to study transitions and dynamic responses.

Main Results:

• The complex exhibits two hysteretic magnetic transitions: a non-spin transition (360–380 K) and an SCO transition (160–280 K) with symmetry breaking (P2₁2₁2₁↔P2₁).
• Benzyl ring rotation (inward/outward) accompanies both magnetic transitions.
• Reversible, giant, and anisotropic spring-like motion of helical chains is observed, with helical pitch varying significantly with temperature.

Conclusions:

• The study successfully integrates spin-crossover behavior with supramolecular spring-like motion in a single Fe(II) complex.
• The observed giant and anisotropic thermal expansion ('breathing') highlights the synergistic effect of multiple dynamic factors.
• This work provides a foundation for designing advanced multifunctional dynamic materials for future technological applications.