Spectroscopic Analyses on Reaction Intermediates Formed during Chlorination of Alkanes with NaOCI Catalyzed by a Nickel Complex

• Published in: Inorganic chemistry Vol. 54; no. 22; pp. 10656 - 10666
• Main Authors: Draksharapu, Apparao, Codola, Zoel, Gomez, Laura, Lloret-Fillol, Julio, Browne, Wesley R, Costas, Miquel
• Format: Journal Article
• Language: English
• Published: 16.11.2015
• Subjects: Acetonitrile; Alkanes; Chemical bonds; Chlorination; Cleavage; Formations; Nickel; Spectroscopy
• ISSN: 0020-1669
• DOI: 10.1021/acsJnorgchem.5b01463

The spectroscopic, electrochemical, and crystallographic characterization of [( super(Me,H)PyTACN)Ni super(II)(CH sub(3)CN) sub(2)](OTf) sub(2) (1) ( super(Me, H)PyTACN = 1-(2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane, OTf = CF sub(3)SO sub(3)) 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 super(II)-OCl(S)] super(+) (A) (L = super(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 super(III)-OH(S)] super(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 super(III)-OOCCH sub(3)( S)] super(2+) (C), which undergoes subsequent reduction to [(L)Ni super(II)-OOCCH sub(3)(S )] super(2+) (D), presumably via reaction with OCl super(-) or ClO sub(2) super(-). Subsequent addition of NaOCl to [(L)Ni super(II)-OOCCH sub(3)(S )] super(+) (D) regenerates [(L)Ni super(III)-OH(S)] super(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 super(III)-OH(S)] super(2+) (B) from 1, suggesting that O-Cl homolytic cleavage is accelerated by protonation. Overall, these reactions generate Cl* atoms and ClO sub(2) 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.