Reduction of α,ω-Dihalohexanes with Electrogenerated Nickel(I) Salen

Abstract

A recent study has described the electrocatalytic reduction of 1,6-dibromohexane at a silver electrode.1 To mitigate the costs of silver electrodes, glassy carbon electrodes can be used in the presence of a solution-phase catalyst. [[2,2′-[1,2-Ethanediylbis-(nitrilomethylidyne)]bis[phenolato]-N,N′,O,O′]nickel(I), further referred to as nickel(I) salen, has been used as an electrogenerated catalyst in tandem with glassy carbon electrodes due to its ability to reductively cleave carbon—halogen bonds in organic compounds. Previous studies at glassy carbon electrodes with nickel(I) salen or at silver electrodes have primarily focused on reduction of monohalogenated compounds or linear hydrocarbons substituted at both primary positions with mixed halogens. Catalytic reduction of such straight-chain alkyl halides with electrogenerated nickel(I) salen yielded both straight-chain monomeric and dimeric products in high yields.2 In this study, electrogenerated nickel(I) salen was used to reduce 1,6-dibromohexane, 1,6-diiodohexane, and 1-bromo-6-iodohexane catalytically. These substrates were electrolyzed in the presenece of 0.10 M tetramethylammonium tetrafluoroborate (TMABF4) in N,N-dimethylformamide at a glassy carbon electrode. Cyclic voltammetry indicated that direct reduction of 1,6-dibromohexane and 1,6-diiodohexane occurs respectively at –1.64 and –1.56 V vs. a cadmium mercury amalgam reference electrode (Cd(Hg)) with a potential of –0.76 V vs. an aqueous saturated calomel electrode. In contrast, catalytic cathodic peaks are at –0.92 and –0.97 V vs. Cd(Hg) for 1,6-dibromohexane and 1,6-diiodohexane, respectively, indicating that nickel(I) salen does indeed reduce these substrates electrocatalytically. Controlled-potential (bulk) electrolyses of the halogenated substrates led to minor conversion to cyclohexane and halogenated dimers with significant amounts of starting material remaining in solution. This drastic decrease in yield compared to previous studies is hypothesized to be the result of alkylation of the imino bond on the salen ligand.2,3 Though nickel(II) salen can undergo a reversible, one-electron reduction to form nickel(I) salen, reduction of nickel(II) salen can also create a ligand-reduced radical at the imino bond, thus resulting in alkylation of both imino bonds on the salen ligand during electrolysis.3 This dialkylation is responsible for the decreased yield of reaction products and was confirmed with the aid of liquid chromatography—mass spectrometry. A mechanism is proposed with radical intermediates which result in dialkylated nickel(II) salen. Martin, E. T.; Strawsine, L. M.; Mubarak, M. S.; Peters, D. G., Direct Reduction of 1,2- and 1,6-Dibromohexane at Silver Cathodes in Dimethylformamide. Electrochim. Acta 2015, 186, 369–376.Mubarak, M. S.; Peters, D. G., Homogeneous Catalytic Reduction of α,ω-Dihaloalkanes with Electrogenerated Nickel(I) Salen. J. Electroanal. Chem. 1995, 388, 195–198.Martin, E. T.; McGuire, C. M.; Peters, D. G., Catalytic Reduction of Organic Halides by Electrogenerated Nickel(I) Salen. Electrochem. Soc. Interface 2016, 25, 41–45. Figure 1