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

The spectroscopic, electrochemical, and crystallographic characterization of [(^Me,HPyTACN)Ni^II(CH₃CN)₂](OTf)₂ (1) (^Me,HPyTACN = 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,HPyTACN, 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.