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Solid Versus Solution Spin Crossover and the Importance of the        Fe-N≡C(X) Angle.

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This study introduces new iron(II) complexes that exhibit spin crossover (SCO) behavior. Interestingly, these complexes show SCO in solution, contrasting with their solid-state properties, highlighting solvent effects on SCO.

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Area of Science:

  • Coordination Chemistry
  • Materials Science
  • Physical Chemistry

Background:

  • Iron(II) complexes are known to exhibit spin crossover (SCO), a phenomenon where the spin state of the iron ion changes in response to external stimuli.
  • The ligand field strength and crystal packing significantly influence SCO behavior in the solid state.
  • Understanding the interplay between solid-state and solution properties is crucial for designing SCO materials.

Purpose of the Study:

  • To synthesize and characterize a new family of mononuclear [FeII(Rdpt)2(NCE)2] complexes.
  • To investigate the spin crossover properties of these complexes in both solid state and solution.
  • To elucidate the influence of solvent and crystal packing on SCO behavior.

Main Methods:

  • Synthesis of mononuclear iron(II) complexes with a pyridyl triazole ligand (tolpyph).
  • Single-crystal X-ray diffraction for structural characterization.
  • Solid-state and solution magnetic measurements (UV-vis, magnetic susceptibility) to determine spin crossover properties.

Main Results:

  • Three new iron(II) complexes, [FeII(tolpyph)2(NCS)2] (1), [FeII(tolpyph)2(NCSe)2] (2), and [FeII(tolpyph)2(NCBH3)2] (3), were synthesized and characterized.
  • Complex 3 is the first structurally characterized [FeII(Rdpt)2(NCE)2]-type complex with an NCBH3 co-ligand.
  • While complexes 1 and 2 remain high-spin in the solid state, all three complexes exhibit spin crossover in solution, with transition temperatures (T1/2) near room temperature.

Conclusions:

  • The synthesized iron(II) complexes display distinct spin crossover behaviors in the solid state versus solution.
  • Solvation effects play a significant role in promoting spin crossover in solution, even for complexes that do not exhibit SCO in the solid state.
  • These findings emphasize the importance of considering both solid-state and solution environments when studying and designing spin crossover materials.