Philicity-Guided Reactivity in Metal-Superoxo Species: Bridging Enzymatic Function to Biomimetic Design

Understanding metal-superoxide formation and reactivity is vital, as enzymes like 1-aminocyclopropane-1-carboxylic acid oxidase (ACCO) and α-ketoglutarate (α-KG)-dependent oxygenases exploit Fe^III-superoxide species for electrophilic and nucleophilic reactions, inspiring biomimetic models whose development remains limited by the complex factors governing their reactivity. To elucidate these factors, we conducted a comprehensive investigation into the reactivity of two Ni^II-superoxide complexes, [[L₁Ni^II-O₂^•-] (1a) and [L₂Ni^II-O₂^•-]^- (2a) [where L₁ = CH{(CMe)(2,6-iPr₂C₆H₃N)}₂ and L₂ = MeN{C(═O)(2,6-iPr₂C₆H₃N)}₂]. In 1a, the β-diketiminato ligand stabilizes the ${\pi }_{\mathrm{xz}}^{*}$ (O₂^•-) (HOMO) and $d_{x^2-y^2}$ (LUMO), resulting in higher electrophilicity (ω = 1.69 eV) and lower nucleophilicity (N = 4.05 eV), which enhance $\sigma$_C-H (nucleophile) → ${\pi }_{\mathrm{xz}}^{*}$ (catalyst) donation in the transition state, promoting greater reactivity toward nucleophiles. Conversely, the more electron-rich biuret ligand in 2a destabilizes ${\pi }_{\mathrm{xz}}^{*}$(O₂^•-) (HOMO), leading to high nucleophilicity (N = 6.77 eV) and low electrophilicity (ω = 0.04 eV), thereby strengthening ${\pi }_{\mathrm{xz}}^{*}$(O₂^•-) → π* (electrophile) interactions and enhancing the greater reactivity toward electrophiles. Electronic structure calculations on the ethylene-forming enzyme (EFE) model give N = 4.00 eV, similar to 1a, where moderate nucleophilicity still enables α-keto carbon attack coupled with concerted C-C bond cleavage that lowers the barrier.