A Brønsted-Ligand-Based Iron Complex as a Molecular Switch with Five Accessible States

Area of Science:

• Coordination Chemistry
• Materials Science
• Molecular Electronics

Background:

• Spin crossover (SCO) complexes are crucial for molecular switches.
• Tuning SCO properties requires adaptable ligand systems.
• Mononuclear iron complexes offer a platform for exploring spin states.

Purpose of the Study:

• To synthesize and characterize a mononuclear iron complex with a Brønsted diacid ligand.
• To investigate the spin crossover behavior and electronic states of the complex and its analogues.
• To demonstrate the tunability of electronic states in molecular systems using proton donor/acceptor ligands.

Main Methods:

• Synthesis of mononuclear iron(II) and iron(III) complexes.
• Characterization of electronic properties and spin states.
• In situ switching experiments in solution and solid-state.

Main Results:

• A mononuclear Fe(II) complex, [Fe(H2L)2](BF4)2 (1A), exhibits abrupt SCO at 258 K.
• Deprotonation yields [Fe(HL)2] (1B) with gradual SCO above 350 K.
• Fe(III) analogues show different spin states (S=5/2 and S=1/2) depending on deprotonation.
• In situ switching between multiple states was demonstrated in solution and solid-state.

Conclusions:

• Proton donor/acceptor ligands significantly expand the accessible states in switchable molecular systems.
• This mononuclear system demonstrates versatility for developing advanced molecular devices.
• The ability to tune electronic states via ligand modification is key for molecular electronics applications.