Tuning the locus of oxidation in Cu-diamido-diphenoxo complexes: From Cu(III) to Cu(II)-phenoxyl radical

While N₂O₂ tetraanionic ligands containing a strong N-amidate σ-donor are generally assumed to stabilise metal high valence states, we herein have shown that, in dianionic Cu(II)-diamido-diphenoxo complexes, H-bonding and electronic effects on the phenolate groups may modulate the electronic structure of their oxidised species from Cu(III) to Cu(II)-phenoxyl radical complexes; and so in the negative potential range. We observe that electron-poor phenolate complexes 2²⁻ and 3²⁻ oxidise to Cu(III) species, whereas electron rich phenolate complex 1²⁻ oxidises to a Cu(II)-phenoxyl radical. Our DFT results suggest that π-electron-rich phenolate rings in 1²⁻ are responsible for an increase of the HOMO orbital energy, bringing the HOMO-SOMO gap small enough to favour a ligand-based oxidation process. Further DFT-calculations have also shown that upon changing the o,p-phenol substituent from electron-widthdrawing groups (NO₂) to electron-donating ones (OMe), the favoured oxidised state switches from Cu(III) to Cu(II)-radical. These results emphasize the use of the versatile diamido-diphenoxo backbone as a promising way to novel GO-chemical models, as well as molecular switches.