Nucleophilic reactivity and electrocatalytic reduction of halogenated organic compounds by nickel o-phenylenedioxamidate complexes

Abstract

A growing number of halogenated organic compounds have been identified as hazardous pollutants. Although numerous advanced oxidative processes have been developed to degrade organohalide compounds, reductive and nucleophilic molecular approaches to dehalogenate organic compounds have rarely been reported. In this manuscript, we employ nickel(II)-ate complexes bearing the o-phenylenebis(N-methyloxamide) (Me₂opba) tetraanionic ligand as nucleophilic reagents that can react with alkyl halides (methyl up to the bulky isobutyl) by O-alkylation to give their respective imidate products. Four new nickel(II) complexes have been characterized by X-ray crystallography, and the salient structural parameters and FT-IR vibrational bands (∼1655 cm⁻¹) concur with their assignment as the imidate tautomeric form. To the best of our knowledge, this is the first report on the nucleophilic reactivity of Ni^II(Me₂opba) with halogenated organic compounds. The parent nickel(II) Me₂opba complex exhibits reversible electrochemical oxidation and reduction behavior. As a proof of concept, Ni^II(Me₂opba) and its alkylated congeners were utilized for the electrocatalytic reduction of chloroform, as a representative, simple polyhalogenated organic molecule that could arise from the oxidative treatment of organic compounds by chlorination. Modest turnover numbers of up to 6 were recorded, with dichloromethane identified as one of the possible products. Future efforts are directed towards bulkier -ate complexes that possess metal-centered instead of ligand-centered nucleophilic activity to create more effective electrocatalysts for the reduction of halogenated organic compounds.