Unveiling the Mechanistic Dichotomy in Nickel–Carboxamidate-Mediated Hydrogen Atom Abstraction: Coupled PCET and ET/PT Pathways

Selective oxidative C–H functionalization is central to upgrading simple hydrocarbons into value-added products, but controlling hydrogen atom abstraction with cost-effective first-row transition-metal catalysts remains challenging. Here, we examine four complexes, [LNi(X)] (X = –OCO2H (1a), –OC(O)CH3 (2a), –ONO2 (3b), and tBu-terpy (4b); L = N,N′-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate), to elucidate how electronic and steric properties of ancillary ligands modulate HAA mechanisms. DFT analysis reveals a mechanistic bifurcation: complexes with anionic ancillary ligands (1b–3b) activate DTBP via PCET, where proton transfer to the ligand is coupled with β-electron transfer to the metal, with the strongly electron-withdrawing –ONO2 group in 3b enhancing PCET asynchronicity and lowering the barrier. In contrast, the neutral, bulky tBu-terpy ligand in 4b weakens the Ni···Ncarb bond, favoring an oxidative asynchronous HAT pathway in which both protons and electrons transfer to the primary carboxamide nitrogen. This assignment is supported by IBO analysis, projected dipole-moment evolution, Hirshfeld spin populations, asynchronicity parameters, and condensed Fukui functions, which distinguish PCET from HAT and quantify oxidative asynchronicity. Overall, this work establishes design principles for tuning PCET and HAT reactivity through a ligand-controlled electronic structure in first-row transition-metal catalysts.