Electrochemistry of substituted salen complexes of nickel(II): Nickel(I)-catalyzed reduction of alkyl and acetylenic halides

Dimethyl-, diethyl-, and diphenyl-substituted analogues of nickel(II) salen have been synthesized, and the cyclic voltammetric behavior of each compound at a glassy carbon electrode in dimethylformamide (DMF) containing tetra-n-butylammonium tetrafluoroborate (TBABF4) has been compared with that of nickel(II) salen itself. Differences in the cathodic peak potentials for these species have been rationalized with the aid of theoretical computations based on density functional theory. Cyclic voltammograms for reduction of dimethylated nickel(II) salen reveal that placing a methyl group on the carbon atom of each imino (CN) bond of the ligand improves the performance of electrogenerated dimethylated nickel(I) salen as a catalyst for reduction of 1-iodobutane. This conclusion is supported by experiments that combine controlled-potential electrolysis with high-performance liquid chromatography–electrospray ionization–mass spectrometry (HPLC–ESI–MS) to detect and analyze the post-electrolysis nickel-containing species arising from the bulk catalytic reduction of 1-iodooctane. Additional studies have been made of the use of electrogenerated dimethylated nickel(I) salen for the catalytic reduction of alkyl halides and for the reductive intramolecular cyclization of acetylenic halides.