Ligand-centred oxidative chemistry in sterically hindered salen complexes: an interesting case with nickel

Salen ligands are ubiquitous ligands because they can be readily prepared by condensation of a diamine with two equivalents of salicylaldehyde. They form stable complexes with a great variety of metal ions and find applications in various fields, especially catalysis. The introduction of chirality at the bridge and the adjunction of sterically demanding tert-butyl groups in ortho and para positions of the phenols allow for efficient enantioselective catalysis. On the other hand, early investigations on the oxidation chemistry of phenols highlighted that the incorporation of tert-butyl groups in ortho and para positions can stabilize enormously the one-electron oxidized product e.g. the phenoxyl radical. The redox-activity of sterically hindered salen ligands will be discussed in this perspective article. We will focus our attention on nickel salen complexes since both the metal and the ligand are potentially redox-active, while the oxidized products are stable enough to be characterized by EPR and NIR spectroscopies. Additionally, the one-electron oxidized species could be isolated as single crystals in some instances, giving detailed pictures of their electronic structure. Both the Ni(II)-radical and Ni(III) bis(phenolate) valence tautomers are accessible upon one-electron oxidation. The substituents, metal coordination sphere, solvent and temperature are crucial factors that dictate the electronic structure of these one-electron oxidized salen complexes.