Solvatomorphism-Induced 45 K Hysteresis Width in a Spin-Crossover Mononuclear Compound

Area of Science:

• Coordination chemistry
• Materials science
• Solid-state physics

Background:

• Spin-transition compounds exhibit bistability between high-spin and low-spin states.
• Thermal hysteresis width depends on intermolecular interactions and solvent presence.
• Solvent molecules can significantly influence the spin-transition properties of coordination complexes.

Purpose of the Study:

• To synthesize and characterize a new spin-transition compound, [Fe(3-bpp)2][Au(CN)2]2 (1).
• To compare the spin-transition properties of the new compound (1) with its solvated form (1.H2O).
• To investigate the relationship between structural transitions, thermal hysteresis, and photomagnetic behavior.

Main Methods:

• Synthesis of [Fe(3-bpp)2][Au(CN)2]2 and its solvated form.
• Variable-temperature magnetic susceptibility measurements to determine spin-transition behavior.
• Single-crystal X-ray diffraction and photocrystallography to study structural changes.
• Photomagnetic measurements to assess photoswitching efficiency.

Main Results:

• Compound 1 displays a two-step thermal hysteresis of 45 K, absent in the solvated 1.H2O.
• The hysteretic transition in 1 is associated with a reversible reconstructive structural transition and twinning.
• Stepped behavior is observed in photomagnetic properties, though photoswitching efficiency is low.
• Photocrystallography confirms low conversion, attributed to high energy costs for the high-spin phase formation.

Conclusions:

• The desolvation of [Fe(3-bpp)2][Au(CN)2]2 significantly induces a pronounced two-step thermal hysteresis.
• The observed hysteresis is coupled with reconstructive phase transitions, highlighting the importance of crystal packing.
• The study provides insights into the design of spin-transition materials with tunable hysteresis for potential applications.