Generation and Nitric Oxide Reactivity of a Cobalt(II) Superoxide Complex via Guanidine-Based Ligand Non-Innocence

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 Co^II-OH-Co^II 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-O ₂ ^•- ), 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 Zn^II-OH-Zn^II 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-O ₂ ^•- reacts with nitric oxide (^•NO) to form a Co^II-peroxynitrite (Co1-O NO₂ ^- ) intermediate, which performs an unprecedented intramolecular nitration of the phenyl moieties of the ligand at the para position.