Peculiar Spin-Crossover Behavior in the 2D Polymer K[Fe^III(5Cl-thsa)₂].

Area of Science:

• Coordination Chemistry
• Materials Science
• Solid-State Physics

Background:

• Spin-crossover (SCO) phenomena in iron(III) complexes are crucial for developing molecular switches and sensors.
• Understanding the interplay between spin states, structural changes, and polymorphs is key to controlling SCO behavior.
• Thiosemicarbazone ligands offer versatile coordination environments for transition metals, influencing their magnetic properties.

Purpose of the Study:

• To synthesize and characterize a novel potassium salt of an N2S2-coordination Fe(III) anion, K[Fe(5Cl-thsa)2] (1).
• To investigate the structural and magnetic properties of two polymorphs of salt 1 over a wide temperature range.
• To elucidate the mechanism of a complex, multistep spin-crossover transition in the dominant polymorph.

Main Methods:

• Synthesis and structural characterization of K[Fe(5Cl-thsa)2] (1).
• Polymorph identification and structural refinement (X-ray diffraction at various temperatures).
• Magnetic susceptibility measurements, 57Fe Mössbauer spectroscopy, electron spin resonance, and DFT calculations.

Main Results:

• Two polymorphs of salt 1 were identified, both with a 2D polymer structure and orthorhombic Pbcn space group.
• The dominant polymorph exhibits a three-step spin-crossover transition (2-50 K, 200-250 K, 250-440 K) with hysteresis.
• The spin-crossover transition is accompanied by unusual structural changes, including minimal bond length variations and a phase transition to monoclinic P21/n symmetry.

Conclusions:

• The studied Fe(III) complex displays a complex, multistep spin-crossover behavior influenced by structural transitions.
• The findings challenge conventional understanding of SCO transitions, highlighting the importance of subtle structural modifications.
• This work provides a new framework for designing and understanding SCO materials based on Fe(III) thiosemicarbazone complexes.