Coupled crystallographic order-disorder and spin state in a bistable molecule: multiple transition dynamics

Area of Science:

• Coordination Chemistry
• Materials Science
• Solid-State Chemistry

Background:

• Spin-crossover (SCO) complexes are known for their potential in molecular switches and memory devices.
• Understanding the interplay between SCO phenomena and structural changes is crucial for designing functional materials.
• Previous studies have explored SCO behavior, but the direct experimental demonstration of coupled crystallographic order-disorder and SCO bistability remained elusive.

Purpose of the Study:

• To synthesize and characterize a novel Fe(II) SCO complex with a bispyrazolylpyridine ligand.
• To investigate the relationship between spin transition, crystallographic transformations, and molecular dynamics.
• To experimentally demonstrate the bistability arising from the coupling of SCO and crystallographic order-disorder.

Main Methods:

• Synthesis of a novel Fe(II) SCO complex, [Fe(H(4)L)(2)][ClO(4)](2)⋅H(2)O⋅2(CH(3))(2)CO (1), featuring a bispyrazolylpyridine ligand with phenol groups.
• Variable-temperature structural analysis (X-ray diffraction) to probe crystal structure changes during SCO.
• Magnetic susceptibility measurements to determine SCO transition temperatures and hysteresis.
• Differential scanning calorimetry (DSC) to study thermal variations and heat capacity changes.

Main Results:

• The complex exhibits a wide hysteresis (40 K) in magnetization due to the spin transition, with transition temperatures (T(0.5)) at 133 K and 173 K.
• Structural analysis reveals an unsymmetrical and rich structure, with disorder in the high-spin (HS) state and order in the low-spin (LS) state.
• Experimental evidence confirms the bistability of crystallographic order-disorder coupled to SCO for the first time, with hysteresis observed in cell parameters.

Conclusions:

• The study successfully demonstrates the direct experimental coupling between spin crossover and crystallographic order-disorder phenomena in an Fe(II) complex.
• This coupling leads to a bistable system where the crystal structure order is intrinsically linked to the spin state.
• The findings highlight the critical role of molecular dynamics in governing the asymmetric SCO behavior and coupled transitions.