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^•-] (<b>1a</b>) and [L₂Ni^II-O₂^•-]^- (<b>2a</b>) [where L₁ = CH{(CMe)(2,6-iPr₂C₆H₃N)}₂ and L₂ = MeN{C(═O)(2,6-iPr₂C₆H₃N)}₂]. In <b>1a</b>, the β-diketiminato ligand stabilizes the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>π</mml:mi></mml:mrow><mml:mrow><mml:mi>xz</mml:mi></mml:mrow><mml:mrow><mml:mo>*</mml:mo></mml:mrow></mml:msubsup></mml:math> (O₂^•-) (HOMO) and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>d</mml:mi><mml:mrow><mml:msup><mml:mi>x</mml:mi><mml:mn>2</mml:mn></mml:msup><mml:mo>-</mml:mo><mml:msup><mml:mi>y</mml:mi><mml:mn>2</mml:mn></mml:msup></mml:mrow></mml:msub></mml:math> (LUMO), resulting in higher electrophilicity (ω = 1.69 eV) and lower nucleophilicity (<i>N</i> = 4.05 eV), which enhance <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>σ</mml:mi></mml:math>_C-H (nucleophile) → <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>π</mml:mi></mml:mrow><mml:mrow><mml:mi>xz</mml:mi></mml:mrow><mml:mrow><mml:mo>*</mml:mo></mml:mrow></mml:msubsup></mml:math> (catalyst) donation in the transition state, promoting greater reactivity toward nucleophiles. Conversely, the more electron-rich biuret ligand in <b>2a</b> destabilizes <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>π</mml:mi></mml:mrow><mml:mrow><mml:mi>xz</mml:mi></mml:mrow><mml:mrow><mml:mo>*</mml:mo></mml:mrow></mml:msubsup></mml:math>(O₂^•-) (HOMO), leading to high nucleophilicity (<i>N</i> = 6.77 eV) and low electrophilicity (ω = 0.04 eV), thereby strengthening <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow><mml:mi>π</mml:mi></mml:mrow><mml:mrow><mml:mi>xz</mml:mi></mml:mrow><mml:mrow><mml:mo>*</mml:mo></mml:mrow></mml:msubsup></mml:math>(O₂^•-) → π* (electrophile) interactions and enhancing the greater reactivity toward electrophiles. Electronic structure calculations on the ethylene-forming enzyme (EFE) model give <i>N</i> = 4.00 eV, similar to <b>1a</b>, where moderate nucleophilicity still enables α-keto carbon attack coupled with concerted C-C bond cleavage that lowers the barrier.