title: Spectroscopic Analyses on Reaction Intermediates Formed during Chlorination of Alkanes with NaOCl Catalyzed by a Nickel Complex.
abstract: The spectroscopic, electrochemical, and crystallographic characterization of [((Me,H)PyTACN)Ni(II)(CH3CN)2](OTf)2 (1) ((Me,H)PyTACN = 1-(2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane, OTf = CF3SO3) is described together with its reactivity with NaOCl. 1 catalyzes the chlorination of alkanes with NaOCl, producing only a trace amount of oxygenated byproducts. The reaction was monitored spectroscopically and by high resolution electrospray-mass spectrometry (ESI-MS) with the aim to elucidate mechanistic aspects. NaOCl reacts with 1 in acetonitrile to form the transient species [(L)Ni(II)-OCl(S)](+) (A) (L = (Me,H)PyTACN, S = solvent), which was identified by ESI-MS. UV/vis absorption, electron paramagnetic resonance, and resonance Raman spectroscopy indicate that intermediate A decays to the complex [(L)Ni(III)-OH(S)](2+) (B) presumably through homolytic cleavage of the O-Cl bond, which liberates a Cl(•) atom. Hydrolysis of acetonitrile to acetic acid under the applied conditions results in the formation of [(L)Ni(III)-OOCCH3(S)](2+) (C), which undergoes subsequent reduction to [(L)Ni(II)-OOCCH3(S)](2+) (D), presumably via reaction with OCl(-) or ClO2(-). Subsequent addition of NaOCl to [(L)Ni(II)-OOCCH3(S)](+) (D) regenerates [(L)Ni(III)-OH(S)](2+) (B) to a much greater extent and at a faster rate. Addition of acids such as acetic and triflic acid enhances the rate and extent of formation of [(L)Ni(III)-OH(S)](2+) (B) from 1, suggesting that O-Cl homolytic cleavage is accelerated by protonation. Overall, these reactions generate Cl(•) atoms and ClO2 in a catalytic cycle where the nickel center alternates between Ni(II) and Ni(III). Chlorine atoms in turn react with the C-H bonds of alkanes, forming alkyl radicals that are trapped by Cl(•) to form alkyl chlorides.
authors.affiliation: Stratingh Institute for Chemistry, Faculty of Mathematics and Natural Sciences, University of Groningen , Nijenborgh 4, 9747AG Groningen, The Netherlands.
authors.forename: Apparao
authors.initials: A
authors.lastname: Draksharapu
authors.affiliation: Institut de Química Computacional i Catalisi (IQCC) and Departament de Química, University of Girona , Campus de Montilivi, Girona 17071, Spain.
authors.forename: Zoel
authors.initials: Z
authors.lastname: Codolà
authors.affiliation: Institut de Química Computacional i Catalisi (IQCC) and Departament de Química, University of Girona , Campus de Montilivi, Girona 17071, Spain.|Serveis Tècnics de Recerca (STR), Universitat de Girona, Parc Cientı́fic i Tecnològic de la UdG , Pic de Peguera 15, E17003 Girona, Catalonia, Spain.
authors.forename: Laura
authors.initials: L
authors.lastname: Gómez
authors.affiliation: Institut de Química Computacional i Catalisi (IQCC) and Departament de Química, University of Girona , Campus de Montilivi, Girona 17071, Spain.
authors.forename: Julio
authors.initials: J
authors.lastname: Lloret-Fillol
authors.affiliation: Stratingh Institute for Chemistry, Faculty of Mathematics and Natural Sciences, University of Groningen , Nijenborgh 4, 9747AG Groningen, The Netherlands.
authors.forename: Wesley R
authors.initials: WR
authors.lastname: Browne
authors.affiliation: Institut de Química Computacional i Catalisi (IQCC) and Departament de Química, University of Girona , Campus de Montilivi, Girona 17071, Spain.
authors.forename: Miquel
authors.initials: M
authors.lastname: Costas