"Normal" and "reverse" spin crossover induced by two different structural events in iron(ii) coordination polymer.

Area of Science:

• Coordination Chemistry
• Materials Science
• Physical Chemistry

Background:

• Spin crossover (SCO) materials exhibit temperature- or pressure-induced changes in electronic spin states.
• The SCO phenomenon is sensitive to molecular structure and intermolecular interactions.
• Understanding hysteresis in SCO is crucial for designing molecular switches and memory devices.

Purpose of the Study:

• To investigate the spin crossover behavior and associated hysteresis in a specific iron(II) complex, [Fe(ebbtr)2(CH3CN)2](CF3SO3)2·4CH3CN.
• To elucidate the molecular mechanisms responsible for the observed normal and reverse hysteresis loops.

Main Methods:

• Synthesis and characterization of the iron(II) complex.
• Variable-temperature magnetic susceptibility measurements to probe spin crossover transitions.
• Structural analysis to correlate molecular conformation with SCO behavior.

Main Results:

• The complex displays spin crossover with distinct "normal" and "reverse" hysteresis loops.
• A region of stable high-spin (HS) state is observed between the hysteresis loops.
• The hysteresis is attributed to trans-trans → gauche-trans conformational changes of the ebbtr ligand and anion reorientation.

Conclusions:

• The observed hysteresis phenomena are linked to specific conformational changes of the organic ligand and anion dynamics.
• The distinct pathways for molecular and anion reorientation during cooling and heating lead to the asymmetric hysteresis.
• This study provides insights into the complex interplay of molecular structure and SCO behavior, relevant for molecular device applications.