Push and Pull Effect of Methoxy and Nitro Groups Modifies the Spin-State Switching Temperature in Fe(III) Complexes

Area of Science:

• Coordination Chemistry
• Materials Science
• Physical Chemistry

Background:

• Spin crossover (SCO) is a phenomenon in transition metal complexes where a change in spin state can be triggered by external stimuli.
• Salen-type ligands are versatile platforms for designing SCO complexes with tunable properties.

Purpose of the Study:

• To investigate the effect of electron-donating and electron-withdrawing substituents on the SalEen ligand on the spin crossover behavior of Fe(III) complexes.
• To elucidate the underlying electronic and structural factors governing these SCO properties.

Main Methods:

• Synthesis of [Fe(3-X-SalEen)2]·NCSe complexes with X = OMe, H, and NO2.
• Solid-state and solution-state magnetic susceptibility measurements.
• UV-Vis spectroscopy.
• Density Functional Theory (DFT) calculations.

Main Results:

• The spin transition temperature (T1/2) decreased with electron-donating OMe groups (219 K) and increased with electron-withdrawing NO2 groups (366 K) compared to the unsubstituted complex (251 K).
• UV-Vis spectra indicated increased π-delocalization in the ligand with the nitro substituent.
• DFT calculations revealed that methoxy substituents decrease the t2g-eg orbital energy gap, while nitro substituents increase it.

Conclusions:

• Substituent effects on SalEen ligands significantly modulate the spin crossover behavior of Fe(III) complexes.
• The observed changes in T1/2 are directly correlated with alterations in the electronic structure, specifically the t2g-eg orbital energy gap.
• This study provides insights into rational design strategies for SCO materials.