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trans-Configured Ligands Boost Spin Crossover to Room Temperature in Mononuclear Fe(II) Complexes.

Guang-Wen Fu1, Yan Kong1, Xin Chen1

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Chemistry, an Asian Journal
|July 21, 2025
PubMed
Summary

Seven new iron complexes were synthesized and studied for spin-crossover (SCO) properties. Complex 3, featuring a trans-configuration, showed enhanced SCO characteristics, highlighting ligand influence on spin transitions.

Keywords:
DFT calculationsMononuclear Fe(II) complexesSpin transition temperaturesSpin‐crossovertrans‐Configured ligands

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Area of Science:

  • Coordination Chemistry
  • Materials Science
  • Solid-State Physics

Background:

  • Spin-crossover (SCO) complexes are materials that can switch between low-spin and high-spin states, offering potential for advanced electronic devices.
  • The ligand environment significantly influences the SCO properties of iron(II) complexes, including transition temperatures and cooperativity.
  • Understanding the relationship between molecular structure and SCO behavior is crucial for designing novel functional materials.

Purpose of the Study:

  • To synthesize and characterize seven new mononuclear iron(II) complexes with varying ligands: 8-aminoquinoline (aqin), 2-picolylamine (2-pic), and tris(2-pyridylmethyl)amine (tpa).
  • To investigate the structural and magnetic properties, particularly the spin-crossover (SCO) behavior, of these complexes.
  • To elucidate the influence of ligand field strength and molecular configuration (cis vs. trans) on SCO transition temperatures (T1/2).

Main Methods:

  • Synthesis and single-crystal X-ray diffraction to determine the precise molecular structures of the iron(II) complexes.
  • Magnetic susceptibility measurements to identify SCO behavior and determine transition temperatures (T1/2).
  • Theoretical calculations, including Gaussian and DFT+U+D3 methods, to model the electronic structure and rationalize the observed SCO phenomena.

Main Results:

  • Seven new mononuclear iron(II) complexes were successfully synthesized: [Fe(aqin)2(NCS)2] (1), [Fe(aqin)2(NCSe)2] (2), [Fe(aqin)2(NCBH3)2] (3), [Fe(2-pic)2(NCS)2] (4), [Fe(2-pic)2(NCSe)2] (5), [Fe(2-pic)(NCBH3)2] (6), and [Fe(tpa)(NCBH3)2] (7).
  • Complexes 1, 2, and 4 remained in the high-spin state, while complexes 3, 5, 6, and 7 exhibited SCO with T1/2 values of 310 K, 162 K, 262 K, and 400 K, respectively.
  • Complex 3, possessing a trans-configuration, displayed an unusually high T1/2 (310 K) compared to related cis-configured complexes, attributed to reduced structural distortion and LS stabilization.

Conclusions:

  • The crystallographic configuration (cis vs. trans) significantly impacts the spin-crossover properties of iron(II) complexes.
  • The trans-configuration in complex 3 leads to enhanced SCO behavior, with a transition temperature near room temperature.
  • Theoretical modeling supports the experimental findings, indicating that reduced distortion in trans-complexes stabilizes the low-spin state, increasing T1/2.