Reversible Guest Binding in a Non-Porous Fe(II) Coordination Polymer Host Toggles Spin Crossover

Area of Science:

• Coordination Chemistry
• Materials Science
• Spin Crossover Phenomena

Background:

• Iron(II) complexes with [1,2-bis(pyridin-2-ylmethyl)thio]ethane (bpte) ligand can form dimeric compounds or 1D coordination polymers.
• The formation pathway and properties of these complexes are influenced by the choice of counteranion and guest molecules.
• Spin crossover (SCO) is a phenomenon where iron(II) complexes switch between low-spin and high-spin states, often triggered by external stimuli.

Purpose of the Study:

• To investigate the role of counteranions in controlling the formation of adiponitrile adducts of [Fe(bpte)](2+).
• To characterize the spin crossover behavior of the resulting coordination polymers.
• To explore the influence of guest molecules, such as acetone, on the spin state and structural integrity of the complexes.

Main Methods:

• Synthesis of iron(II) complexes with varying counteranions (SbF6 and BPh4).
• Structural characterization of dimeric and polymeric complexes.
• Variable temperature studies to monitor spin state transitions and guest molecule interactions (e.g., acetone absorption/desorption).

Main Results:

• The choice of counteranion dictates the formation of either a dimeric complex or a 1D coordination polymer.
• The polymeric complex {[Fe(bpte)(μ2 -NC(CH2 )4 CN)](BPh4 )2 ⋅Me2 CO} exhibits abrupt and hysteretic spin crossover (SCO) S=0↔S=2 behavior upon desolvation/resolvation of acetone.
• Desolvation at 80°C leads to a high-spin state, while cooling the desolvate to 205 K induces SCO; reversible acetone absorption regenerates the low-spin solvated phase.

Conclusions:

• Counteranion engineering provides a route to control the dimensionality and spin crossover properties of iron(II) coordination polymers.
• The non-porous polymer demonstrates reversible guest-responsive spin crossover, highlighting potential applications in molecular switches and sensors.
• The observed hysteresis in spin crossover behavior suggests potential for memory effects in these materials.