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, <b>Co1</b>, was investigated for dioxygen (O₂) activation. Upon reaction with O₂ at room temperature, a μ-hydroxo-bridged Co^II-OH-Co^II complex (<b>Co2</b>) was isolated as the final oxygenated product. In-depth spectroscopic investigation revealed that the formation of <b>Co2</b> occurs via an intermediate superoxide species (<b>Co1-O</b> _<b>2</b> ^<b>•-</b> ), by taking advantage of the redox non-innocence behavior of the ligand (3,3'-(pyridine-2,6-diyl)-bis-(1-methyl-<i>N</i>-phenylimidazolidin-2-imine)), and therefore keeping the cobalt oxidation state unchanged at +2. Interestingly, the zinc analogue, <b>Zn1</b>, was also shown to yield a similar μ-hydroxo-bridged Zn^II-OH-Zn^II complex (<b>Zn2</b>) 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 <b>Co1-O</b> _<b>2</b> ^<b>•-</b> reacts with nitric oxide (^•NO) to form a Co^II-peroxynitrite (<b>Co1-O</b> <b>NO₂</b> ^<b>-</b> <b>)</b> intermediate, which performs an unprecedented intramolecular nitration of the phenyl moieties of the ligand at the para position.