Modulation of the spin crossover behaviour in four manganese(III) complexes through cation-anion interactions

Area of Science:

• Coordination Chemistry
• Materials Science
• Magnetochemistry

Background:

• Manganese(III) complexes are of interest due to their diverse magnetic properties.
• Schiff base ligands offer versatile coordination environments for metal ions.
• Anion choice significantly impacts the solid-state structure and properties of metal complexes.

Purpose of the Study:

• To synthesize and characterize a series of novel manganese(III) monomers with a specific Schiff base ligand.
• To investigate the influence of different counter-anions (PF₆^-, BF₄^-, I^-, Br^-/NO₃^-) on the structural and magnetic behavior.
• To explore the occurrence and characteristics of spin crossover (SCO) phenomena in these manganese(III) complexes.

Main Methods:

• Synthesis of four manganese(III) monomeric compounds using a Schiff base ligand derived from eugenol and diamine.
• Single-crystal X-ray diffraction for structural elucidation of the cation-anion interactions and packing.
• Variable temperature magnetic susceptibility measurements to probe magnetic transitions and spin states.
• Isothermal magnetization measurements to confirm ground spin states at low temperatures.

Main Results:

• Four [Mn(L)]X compounds were successfully synthesized, featuring the [Mn^III(L)]⁺ cation with different anions.
• Compound 1 exhibits a high spin (S=2) state across all temperatures, while compounds 2-4 display incomplete spin crossover (SCO) between 200-400 K.
• Structural analysis reveals that anions dictate cation-anion packing and influence the SCO behavior, with increasing structural distortion observed with temperature in SCO compounds.

Conclusions:

• The counter-anion plays a critical role in modulating both the crystal structure and the magnetic properties, including SCO, of manganese(III) complexes.
• Compounds 2-4 demonstrate temperature-induced incomplete spin crossover, transitioning between low spin (S=1) and high spin (S=2) states.
• The observed SCO behavior is well-described by monomeric spin models, with minor contributions from zero-field splitting and weak intermolecular interactions.