title: Tuning the Reactivity of Terminal Nickel(III)-Oxygen Adducts for C-H Bond Activation.
abstract: Two metastable NiIIIcomplexes, [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 NiIIprecursors ([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-1s-1) compared favorably with the most reactive high-valent metal-oxidants, and showcases the exceptional reactivity of late transition metal-oxygen adducts.
authors.affiliation: School of Chemistry and CRANN/AMBER Nanoscience Institute, Trinity College Dublin, The University of Dublin , College Green, Dublin 2, Ireland.
authors.forename: Paolo
authors.initials: P
authors.lastname: Pirovano
authors.affiliation: Case Western Reserve University Center for Synchrotron Biosciences, National Synchrotron Light Source II, Brookhaven National Laboratory , Upton, New York 11973, United States.
authors.forename: Erik R
authors.initials: ER
authors.lastname: Farquhar
authors.affiliation: ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain.|Institut de Química Computacional i Catàlisi, Facultat de Ciències, Universitat de Girona , Campus Montilivi, 17003 Girona, Spain.
authors.forename: Marcel
authors.initials: M
authors.lastname: Swart
authors.affiliation: School of Chemistry and CRANN/AMBER Nanoscience Institute, Trinity College Dublin, The University of Dublin , College Green, Dublin 2, Ireland.
authors.forename: Aidan R
authors.initials: AR
authors.lastname: McDonald
mesh_terms: D002244:Carbon / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D009532:Nickel / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D010636:Phenols / Q000737:chemistry
chemical_list: D009942:Organometallic Compounds; D010636:Phenols; D002244:Carbon; D009532:Nickel; D006859:Hydrogen; D010100:Oxygen