Coordination Chemistry ofo‐Semiquinones

Abstract

Abstracto‐Semiquinone (SQ) radicals are ideally suited to chelate with transition metal ions. Ligand paramagnetism results in spin‐coupling interactions with paramagnetic metal ions, and complexes containing multiple SQ ligands have magnetic properties resulting from inter‐ligand coupling. The unpaired SQ electron resides in a ligand‐localized π orbital close in energy with the valence d orbitals of many transition metal ions. Electron transfer between ligand and metal may be observed under equilibrium conditions by monitoring the temperature‐dependence of spectral and magnetic properties in a process known as valence tautomerism. The redox potentials of common SQ ligands overlap with the redox potentials of metal ions contributing to redox series that may involve reversible electron transfer steps at both the metal and SQ ligands. Multielectron redox reactions involving SQ ligands are useful in small molecule transformations that involve substrate oxidation or reduction over a narrow range in electrochemical potential. These properties are unique to complexes containing ligands capable of changing their charged state by either intramolecular electron transfer within the complex molecule or by an exchange of charge with an external redox component. Reactions and processes of this type will be the focus of this review on the coordination chemistry ofo‐semiquinonate ligands.