Tuning the Reactivity of Terminal Nickel(III)-Oxygen Adducts for C-H Bond Activation

Two metastable Ni^III complexes, [Ni^III(OAc)(L)] and [Ni^III(ONO₂)(L)] (L = N,N'-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate, OAc = acetate), were prepared, adding to the previously prepared [Ni^III(OCO₂H)(L)], with the purpose of probing the properties of terminal late-transition metal oxidants. These high-valent oxidants were prepared by the one-electron oxidation of their Ni^II precursors ([Ni^II(OAc)(L)]^- and [Ni^II(ONO₂)(L)]^-) with tris(4-bromophenyl)ammoniumyl hexachloroantimonate. Fascinatingly, the reaction between any [Ni^II(X)(L)]^- and NaOCl/acetic acid (AcOH) or cerium ammonium nitrate ((NH₄)₂[Ce^IV(NO₃)₆], CAN), yielded [Ni^III(OAc)(L)] and [Ni^III(ONO₂)(L)], respectively. An array of spectroscopic characterizations (electronic absorption, electron paramagnetic resonance, X-ray absorption spectroscopies), electrochemical methods, and computational predictions (density functional theory) have been used to determine the structural, electronic, and magnetic properties of these highly reactive metastable oxidants. The Ni^III-oxidants proved competent in the oxidation of phenols (weak O-H bonds) and a series of hydrocarbon substrates (some with strong C-H bonds). Kinetic investigation of the reactions with di-tert-butylphenols showed a 15-fold enhanced reaction rate for [Ni^III(ONO₂)(L)] compared to [Ni^III(OCO₂H)(L)] and [Ni^III(OAc)(L)], demonstrating the effect of electron-deficiency of the O-ligand on oxidizing power. The oxidation of a series of hydrocarbons by [Ni^III(OAc)(L)] was further examined. A linear correlation between the rate constant and the bond dissociation energy of the C-H bonds in the substrates was indicative of a hydrogen atom transfer mechanism. The reaction rate with dihydroanthracene (k₂ = 8.1 M^-1 s^-1) compared favorably with the most reactive high-valent metal-oxidants, and showcases the exceptional reactivity of late transition metal-oxygen adducts.