A novel guanidine-based NNN pincer cobalt(II) complex, Co1, was investigated for dioxygen (O2) activation. Upon reaction with O2 at room temperature, a μ-hydroxo-bridged CoII–OH-CoII complex (Co2) was isolated as the final oxygenated product. In-depth spectroscopic investigation revealed that the formation of Co2 occurs via an intermediate superoxide species (Co1–O2•–), by taking advantage of the redox non-innocence behavior of the ligand (3,3′-(pyridine-2,6-diyl)bis(1-methyl-N-phenylimidazolidin-2-imine)), and therefore keeping the cobalt oxidation state unchanged at +2. Interestingly, the zinc analogue, Zn1, was also shown to yield a similar μ-hydroxo-bridged ZnII–OH-ZnII complex (Zn2) as the final product under the same aerobic conditions, providing further confirmation of the ligand influence on metal oxidation and spin states during O2 activation. Further reactivity studies demonstrated that Co1–O2•– reacts with nitric oxide (•NO) to form a CoII-peroxynitrite (Co1–ONO2–) intermediate, which performs an unprecedented intramolecular nitration of the phenyl moieties of the ligand at the para position.
