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Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

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Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
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Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...
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Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
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Coordination compounds and complexes exhibit different colors, geometries, and magnetic behavior, depending on the metal atom/ion and ligands from which they are composed. In an attempt to explain the bonding and structure of coordination complexes, Linus Pauling proposed the valence bond theory, or VBT, using the concepts of hybridization and the overlapping of the atomic orbitals. According to VBT, the central metal atom or ion (Lewis acid) hybridizes to provide empty orbitals of suitable...
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Color in Coordination Complexes
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The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved...
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A large dinuclear Fe(ii) triple helicate demonstrating a two-step spin crossover.

Kyle J Howard-Smith1, Alexander R Craze1, Hikaru Zenno2

  • 1School of Science, Western Sydney University, Locked Bag 1797, Penrith, NSW 2751, Australia. feng.li@westernsydney.edu.au.

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Summary

Researchers synthesized the largest dinuclear iron(II) triple helicate with a 273° helical twist. This system exhibits a rare two-step spin crossover transition, offering new insights into spin crossover materials.

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Area of Science:

  • Coordination Chemistry
  • Materials Science
  • Magnetochemistry

Background:

  • Spin crossover (SCO) in iron(II) complexes is a key phenomenon for molecular switches and sensors.
  • Dinuclear SCO systems offer complex magnetic behaviors and potential for cooperative effects.
  • Helical structures in coordination chemistry can lead to unique packing and electronic properties.

Purpose of the Study:

  • To synthesize and characterize the largest reported dinuclear iron(II) triple helicate.
  • To investigate the spin crossover behavior and helical twist of the synthesized complex.
  • To explore the mechanism of the observed two-step spin transition.

Main Methods:

  • Synthesis of dinuclear iron(II) complex with aromatic spacers.
  • X-ray crystallography for structural determination, including helical twist analysis.
  • Variable-temperature magnetic susceptibility measurements to study spin crossover.
  • Spectroscopic techniques for electronic structure analysis.

Main Results:

  • Successful synthesis of a large dinuclear Fe(II) triple helicate featuring a 273° helical twist.
  • Observation of a distinct two-step spin transition phenomenon.
  • Structural analysis revealed the role of aromatic spacers in achieving the significant helical twist.

Conclusions:

  • The synthesized Fe(II) triple helicate represents a significant advancement in SCO material design.
  • The two-step spin transition highlights the potential for complex magnetic switching in helical systems.
  • This work provides a foundation for designing novel helical SCO materials with tunable properties.