Reversible Carbon-Carbon Bond Formation Induced by Oxidation and Reduction at a Redox-Active Cobalt Complex

The electronic structure of the diamagnetic pyridine imine enamide cobalt dinitrogen complex, ((iPr)PIEA)-CoN2 ((iPr)PIEA = 2-(2,6-Pr-i(2)-C6H3N=CMe)-6-(2,6-Pr-i(2)-C6H3NC=CH2)C5H3N), was determined and is best described as a low-spin cobalt(II) complex antiferromagnetically coupled to an imine radical anion. Addition of potential radical sources such as NO, PhSSPh, or Ph3Cl resulted in C-C coupling at the enamide positions to form bimetallic cobalt compounds. Treatment with the smaller halocarbon, PhCH2Cl, again induced C-C coupling to form a bimetallic bis(imino)pyridine cobalt chloride product but also yielded a monomeric cobalt chloride product where the benzyl group added to the enamide carbon. Similar cooperative metal ligand addition was observed upon treatment of ((iPr)PIEA)CoN2 with CH2=CHCH2Br, which resulted in allylation of the enamide carbon. Reduction of Coupled-((PDI)-P-iPr)CoCl (Coupled-((PDI)-P-iPr)CoCl = [2-(2,6-Pr-i(2)-C6H3N=CMe)-C5H3N-6-(2,6-Pr-i(2)-C6H3N=CCH2-)CoCl](2)) with NaBEt3H led to quantitative formation of ((iPr)PIEA)CoN2, demonstrating the reversibility of the C-C bond forming reactions. The electronic structures of each of the bimetallic cobalt products were also elucidated by a combination of experimental and computational methods.