Interplay between spin crossover and proton migration along short strong hydrogen bonds

Area of Science:

• Inorganic Chemistry
• Materials Science
• Solid-State Physics

Background:

• Spin crossover (SCO) complexes exhibit bistability between high-spin (HS) and low-spin (LS) states.
• SCO phenomena are influenced by crystal packing, hydrogen bonding, and external stimuli like light.
• Understanding SCO mechanisms is crucial for developing molecular switches and memory devices.

Purpose of the Study:

• To investigate the spin crossover behavior of the iron(II) salt [Fe(bpp)2](isonicNO)2·HisonicNO·5H2O.
• To explore the role of hydrogen bonding and proton dynamics in SCO.
• To study the light-induced excited-state spin trapping (LIESST) and its effect on crystal structure and proton motion.

Main Methods:

• Single-crystal X-ray diffraction at variable temperatures.
• Neutron crystallography.
• Photocrystallography.
• Variable-temperature magnetic susceptibility measurements.

Main Results:

• The compound exhibits a partial spin crossover at 167 K to a metastable mixed-spin phase (50% HS, 50% LS).
• Annealing leads to a 100% LS phase with altered hydrogen bonding (HB) and proton displacement in short strong hydrogen bonds (SSHB).
• LIESST induces a 100% HS phase, breaking the O4W⋯O5W HB and causing proton disorder in the SSHB, with stepped relaxation at 68 K and 76 K.

Conclusions:

• The iron(II) SCO complex displays complex spin transitions influenced by hydrogen bonding and proton dynamics.
• LIESST demonstrates light-induced activation barriers for proton motion, suggesting potential for light-controlled molecular devices.
• The interplay between spin states, hydrogen bonding, and proton disorder is key to the material's functionality.