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Download fileTuning the Reactivity of Terminal Nickel(III)–Oxygen Adducts for C–H Bond Activation
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posted on 2016-10-14, 00:00 authored by Paolo Pirovano, Erik R. Farquhar, Marcel Swart, Aidan R. McDonaldTwo
metastable NiIII complexes, [NiIII(OAc)(L)]
and [NiIII(ONO2)(L)] (L = N,N′-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate,
OAc = acetate), were prepared, adding to the previously prepared [NiIII(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 NiII precursors ([NiII(OAc)(L)]− and [NiII(ONO2)(L)]−) with
tris(4-bromophenyl)ammoniumyl hexachloroantimonate. Fascinatingly,
the reaction between any [NiII(X)(L)]− and NaOCl/acetic acid (AcOH) or cerium ammonium
nitrate ((NH4)2[CeIV(NO3)6], CAN), yielded [NiIII(OAc)(L)] and [NiIII(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 NiIII-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 [NiIII(ONO2)(L)] compared to [NiIII(OCO2H)(L)] and [NiIII(OAc)(L)], demonstrating the effect
of electron-deficiency of the O-ligand on oxidizing power. The oxidation
of a series of hydrocarbons by [NiIII(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.