Hydrogen Atom Transfer by a High-Valent Nickel-Chloride Complex

Abstract

Oxo-metal-halide moieties have often been implicated as C–H bond activating oxidants with the terminal oxo-metal entity identified as the electrophilic oxidant. The electrophilic reactivity of metal-halide species has not been investigated. We have prepared a high-valent nickel-halide complex [Ni III (Cl)(L)] ( 2, L = N, N ′-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamide) by one-electron oxidation of a [Ni II (Cl)(L)] − precursor. 2 was characterized using electronic absorption, electron paramagnetic resonance, and X-ray absorption spectroscopies and mass spectrometry. 2 reacted readily with substrates containing either phenolic O–H or hydrocarbon C–H bonds. Analysis of the Hammett, Evans–Polanyi, and Marcus relationships between the determined rate constants and substrate p K a, X–H bond dissociation energy, and oxidation potential, respectively, was performed. Through this analysis, we found that 2 reacted by a hydrogen atom transfer (HAT) mechanism. Our findings shine light on enzymatic high-valent oxo-metal-halide oxidants and open new avenues for oxidative halogenation catalyst design.