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, (iPrPIEA)­CoN2 (iPrPIEA = 2-(2,6-iPr2–C6H3NCMe)-6-(2,6-iPr2–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 (iPrPIEA)­CoN2 with CH2CHCH2Br, which resulted in allylation of
the enamide carbon. Reduction of Coupled-(iPrPDI)­CoCl (Coupled-(iPrPDI)­CoCl = [2-(2,6-iPr2–C6H3NCMe)-C5H3N-6-(2,6-iPr2–C6H3NCCH2−)­CoCl]2) with NaBEt3H led to
quantitative formation of (iPrPIEA)­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.