Catalytic reduction of 1-iodooctane by nickel(I) salen electrogenerated at carbon cathodes in dimethylformamide: Effects of added proton donors and a mechanism involving both metal- and ligand-centered one-electron reduction of nickel(II) salen

In dimethylformamide containing tetramethylammonium tetrafluoroborate, 1-iodooctane is reduced catalytically by nickel(I) salen electrogenerated at a glassy carbon cathode. Cyclic voltammograms for the nickel(II) salen-1-iodooctane system recorded in the absence as well as in the presence of a proton or deuteron donor (1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), phenol, or D₂O) exhibit enhanced cathodic peaks and diminished anodic peaks for the nickel(II) salen-nickel(I) salen couple, along with a new cathodic peak attributed to reduction of a nickel(II) salen species for which an imino bond of the ligand is octylated. Without a proton donor, controlled-potential catalytic reduction of 1-iodooctane by nickel(I) salen affords hexadecane, octane, and 1-octene. For electrolyses performed in the presence of either HFIP or phenol, the yields of hexadecane and octane are decreased and increased, respectively, whereas that of 1-octene remains unchanged. Bulk electrolyses done in the presence of D₂O give a product distribution similar to that obtained when no proton donor is added; none of the octane is deuterated, indicating that octyl radicals (not octyl carbanions) are precursors for the formation of octane. Theoretical calculations involving density functional theory have been employed to establish that nickel(II) salen can undergo either a metal- or ligand-centered one-electron reduction. A mechanistic scheme is proposed that invokes both metal- and ligand-centered reduction of nickel(II) salen to explain the effects of proton donors on the catalytic reduction of alkyl halides as well as the pathway for alkylation of the imino bonds of the ligand.