Département de chimie moléculaire

A review. Salen ligands are ubiquitous ligands because they can be readily prepd. by condensation of a diamine with two equiv. of salicylaldehyde. They form stable complexes with a great variety of metal ions and find applications in various fields, esp. catalysis. The introduction of chirality at the bridge and the adjunction of sterically demanding tert-Bu groups in ortho and para positions of the phenols allow for efficient enantioselective catalysis. However, early investigations on the oxidn. chem. of phenols highlighted that the incorporation of tert-Bu 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 are discussed in this perspective article. The authors will focus the authors' 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. Addnl., 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 oxidn. The substituents, metal coordination sphere, solvent and temp. are crucial factors that dictate the electronic structure of these one-electron oxidized salen complexes. [on SciFinder(R)]