Extremely Slow Thermally-Induced Spin Crossover in the Two-Dimensional Network [Fe(bbtr)₃ ](BF₄ )₂

Area of Science:

• Materials Science
• Inorganic Chemistry
• Solid-State Physics

Background:

• Spin crossover (SCO) is a phenomenon where iron(II) complexes switch between low-spin and high-spin states.
• The compound [Fe(bbtr)₃ ](BF₄ )₂, a 2D coordination polymer, was previously considered spin crossover silent.
• Understanding SCO behavior is crucial for developing molecular switches and sensors.

Purpose of the Study:

• To investigate the thermally and pressure-induced spin crossover phenomenon in [Fe(bbtr)₃ ](BF₄ )₂.
• To compare the SCO behavior of tetrafluoroborate with its isostructural perchlorate analogue.
• To elucidate the mechanism of SCO under high pressure in mixed crystal systems.

Main Methods:

• Variable temperature magnetic susceptibility measurements.
• High-pressure magnetic studies.
• Variable pressure single crystal X-ray diffraction.

Main Results:

• [Fe(bbtr)₃ ](BF₄ )₂ exhibits slow spin crossover below 80 K with a hysteresis loop (T_1/2^↓ =76 K, T_1/2^↑ =89 K).
• Unlike the perchlorate analogue, tetrafluoroborate does not show a phase transition before SCO.
• Applying 1 GPa pressure shifts SCO to above 270 K, and high-pressure studies reveal a unified SCO mechanism across different compositions.

Conclusions:

• The 2D coordination polymer [Fe(bbtr)₃ ](BF₄ )₂ displays thermally induced spin crossover, contrary to previous assumptions.
• High pressure effectively removes the differentiation in SCO behavior observed between tetrafluoroborate and perchlorate systems.
• The study highlights the slow kinetics of SCO in tetrafluoroborate and the pressure-induced unification of SCO mechanisms.