Structural Changes in Five‐Coordinate Bromido‐bis(o‐iminobenzo‐semiquinonato)iron(III) Complex: Spin‐Crossover or Ligand‐Metal Antiferromagnetic Interactions?

Spin‐crossover metal complexes represent important building blocks for a future generation of electronic and optical devices. Pentacoordinated o‐iminosemiquinonate iron(III) complexes are able to demonstrate spin transitions between high spin (HS) and intermediate spin (IS) states under the influence of temperature or irradiation. Studied (MeimSQ)2FeBr sample showed a broad magnetic transition in the temperature region from 30 K to 300 K. Remarkably that observed behavior of magnetization can be interpreted with two controversial models. In the first model, the values of the effective magnetic moment at low temperature and high temperature can be assigned to the IS and HS magnetic moment of ferric ion coupled antiferromagnetically to radical anion ligands. In the second model, the metal spin on metal center remains IS in the whole temperature interval, while the mutual orientation of three magnetic moments in the molecule undergo changes due to exchange interactions. In this work we apply density functional theory and X‐ray absorption spectroscopy to unravel the origin of magnetic properties of the complex. Temperature‐induced changes of interatomic distances in the first coordination sphere support the second model. Such comprehensive analysis of the magnetic properties makes it possible to shed light on the nature of spin transitions in complexes of transition metals with open‐shell ligands, which are often complicated by strong exchange interactions. Unusual temperature‐dependent magnetization in the pentacoordinate ferric bis‐o‐iminosemiquinonate complex is not related to the spin crossover. X‐ray absorption spectroscopy quantified structural deformations originating from spin transition upon cooling. DFT simulations in broken symmetry approach explain these geometry changes by strong exchange interactions between spins on Fe and ligands.