Selecting the spin crossover profile with controlled crystallization of mononuclear Fe(iii) polymorphs

Area of Science:

• Coordination Chemistry
• Materials Science
• Solid-State Chemistry

Background:

• Polymorphism in metal complexes can significantly influence their physical properties.
• Spin crossover (SCO) materials are of interest for molecular switches and sensors.
• Controlling crystallization conditions is key to isolating desired polymorphs.

Purpose of the Study:

• To investigate the effect of solvent evaporation rate on the polymorphic forms of [Fe(5-Br-salEen)2]ClO4.
• To characterize the spin crossover properties of the newly obtained polymorph.
• To elucidate the structural factors governing the spin transition behavior.

Main Methods:

• Controlled solvent evaporation crystallization.
• 57Fe Mössbauer spectroscopy.
• Differential scanning calorimetry (DSC).
• X-ray diffraction (XRD) studies.

Main Results:

• Two distinct polymorphs (1a and 1b) were selectively obtained by varying evaporation rates (slow for 1a, fast for 1b).
• Polymorph 1a displays an abrupt spin crossover transition at 172 K with a narrow 10 K range and 1 K hysteresis.
• Structural analysis reveals stronger hydrogen bonding and a cooperative network of weak interactions (hydrogen bonds, halogen bonds, π-π stacking) in polymorph 1a, absent in 1b.

Conclusions:

• The solvent evaporation rate is a critical parameter for controlling the polymorphism of [Fe(5-Br-salEen)2]ClO4.
• Polymorph 1a exhibits unique spin crossover behavior attributed to its specific crystal packing and intermolecular interactions.
• This study highlights the importance of crystal engineering in tuning the SCO properties of iron(III) complexes.