Hydrogen-bonded assemblies of iron(II) spin crossover complexes

Area of Science:

• Coordination Chemistry
• Materials Science
• Magnetochemistry

Background:

• Spin crossover (SCO) materials containing iron(II) complexes exhibit distinct magnetic properties.
• The [Fe(bpp)₂]²⁺ cation, where bpp is 2,6-bis(pyrazol-3-yl)pyridine, is a key component in SCO systems.
• Rigid polycarboxylate anions influence the structural and SCO behavior of metal complexes.

Purpose of the Study:

• To synthesize and characterize novel spin crossover (SCO) salts with [Fe(bpp)₂]²⁺ cations and various rigid polycarboxylate anions.
• To investigate the impact of dehydration and temperature on the crystal structure and magnetic properties of these SCO salts.
• To understand the relationship between molecular structure, guest molecules (water), and spin transition phenomena.

Main Methods:

• Synthesis of novel iron(II) coordination compounds.
• Single-crystal X-ray diffraction for structural analysis.
• Thermogravimetric analysis (TGA) to study dehydration and thermal properties.
• Variable-temperature magnetic susceptibility measurements to probe spin crossover behavior.

Main Results:

• The anthracene-9,10-dicarboxylate (ADC) salt, [Fe(bpp)₂](ADC)·9H₂O, exhibits a porous structure and undergoes SCO upon dehydration, remaining in the high-spin state upon cooling.
• The partially protonated benzene-1,3,5-tricarboxylate (BTC) salt, [Fe(bpp)₂](HBTC)·5H₂O, shows SCO concurrent with dehydration, with partial spin transition occurring on cooling.
• The biphenyl-4,4'-dicarboxylate (BPDC) salt, [Fe(bpp)₂](BPDC)·5H₂O, maintains a high-spin ground state in its hydrated form.

Conclusions:

• The choice of rigid polycarboxylate anion significantly influences the SCO properties of [Fe(bpp)₂]²⁺ salts.
• Dehydration is a critical trigger for spin crossover in these porous SCO materials.
• Structural features, including hydrogen bonding and water content, play a crucial role in modulating SCO behavior.