Decoupled spin crossover and structural phase transition in a molecular iron(II) complex.

Area of Science:

• Coordination Chemistry
• Materials Science
• Solid-State Physics

Background:

• Spin-crossover (SCO) in iron(II) complexes is a phenomenon sensitive to external stimuli.
• Crystallographic phase transitions can influence SCO behavior and cooperativity.
• Light-induced excited-spin-state trapping (LIESST) is a key characteristic of SCO materials.

Purpose of the Study:

• To investigate the spin-crossover behavior and crystallographic phase transitions of novel iron(II) complexes.
• To elucidate the role of specific substituents in modulating SCO cooperativity and phase transitions.
• To study the LIESST effect and relaxation kinetics in these SCO materials.

Main Methods:

• Synthesis and single-crystal X-ray diffraction of iron(II) complexes with 4-(methylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine (bppSMe), 4-bromo-2,6-di(pyrazol-1-yl)pyridine (bppBr), and 4-iodo-2,6-di(pyrazol-1-yl)pyridine (bppI).
• Variable-temperature magnetic susceptibility measurements to probe spin-crossover events.
• UV-Vis spectroscopy and variable-temperature powder X-ray diffraction to study phase transitions and LIESST effect.

Main Results:

• Crystalline [Fe(bppSMe)2][BF4]2 exhibits an abrupt SCO at 265 K and a separate phase transition near 205 K, involving a unit-cell contraction.
• Powdered [Fe(bppSMe)2][BF4]2 retains the high-temperature phase over a broad temperature range with SCO behavior similar to single crystals.
• The bromo and iodo analogues ([Fe(bppBr)2][BF4]2 and [Fe(bppI)2][BF4]2) show gradual SCO and adopt the low-temperature crystallographic phase in both spin states.
• Intermolecular steric interactions involving the methylsulfanyl group are identified as the cause for the enhanced cooperativity and phase transition in the bppSMe complex.
• All three compounds display the LIESST effect but exhibit complex relaxation kinetics with both exponential and sigmoidal components.

Conclusions:

• The methylsulfanyl substituent significantly influences SCO cooperativity and induces a distinct crystallographic phase transition in iron(II) complexes.
• The interplay between SCO and crystallographic phase transitions is crucial for understanding material properties.
• Complex LIESST relaxation kinetics highlight the sensitivity of these SCO materials to light stimuli.