Hysteretic Spin Crossover with High Transition Temperatures in Two Cobalt(II) Complexes

Area of Science:

• Coordination Chemistry
• Materials Science
• Magnetism

Background:

• Spin crossover (SCO) phenomena in transition metal complexes are crucial for developing molecular switches and memory devices.
• Hysteretic SCO transitions, particularly at high temperatures, are rare and highly sought after for practical applications.
• Cobalt(II) complexes are known to exhibit SCO, but achieving cooperative transitions with significant thermal hysteresis remains challenging.

Purpose of the Study:

• To synthesize and characterize novel mononuclear cobalt(II) complexes with potential for hysteretic spin crossover.
• To investigate the magnetic properties and identify the structural origins of spin crossover transitions in these complexes.
• To explore the effect of ligand modification on the SCO behavior and thermal hysteresis.

Main Methods:

• Synthesis of two mononuclear cobalt(II) complexes, [Co(terpy-CH2OH)2]·X2 (X = SCN- (1) and SeCN- (2)).
• Structural characterization using X-ray diffraction.
• Magnetic measurements (temperature-dependent magnetic susceptibility) to identify spin crossover transitions and hysteresis.
• Analysis of crystallographic phase transitions related to ligand ordering.

Main Results:

• Both complexes displayed hysteretic spin crossover transitions at high temperatures (320–400 K).
• Complex 1 showed a one-step transition upon heating and a two-step transition upon cooling.
• Complex 2 exhibited a distinct hysteresis loop with a 5 K width.
• Single crystal X-ray diffraction revealed that hysteretic SCO transitions are linked to crystallographic phase transitions involving the ordering of -CH2OH groups.

Conclusions:

• The modification of the terpyridine ligand significantly influences the magnetic properties and SCO behavior of cobalt(II) complexes.
• The observed hysteretic SCO transitions are attributed to thermally induced order-disorder transitions of the pendant hydroxymethyl groups on the ligand.
• These findings provide a pathway for designing cobalt(II) SCO materials with tunable hysteresis for high-temperature applications.