DUGi

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

http://dugi.udg.edu/item/http:@@@@hdl.handle.net@@2072@@299131

Two 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

This publication has emanated from research supported by the European Union (FP7-333948, ERC-2015-STG-678202). Research in the McDonald lab is supported in part by a research grant from Science Foundation Ireland (SFI/12/RC/2278), and in the Swart lab by the Ministerio de Economia y Competitividad (MINECO, Projects CTQ2014-59212-P and CTQ2015-70851-ERC), the DIUE of the Generalitat de Catalunya (Project 2014SGR1202), and the European Fund for Regional Development (FEDER, UNGI10-4E-801). XAS experiments were conducted at SSRL beamline 2-2 (SLAC National Accelerator Laboratory), with support from the DOE Office of Science (DE-AC02-76SF00515 and DE-SC0012704) and NIH (P30-EB-009998)

American Chemical Society