New redox-active ligands on iron and cobalt for C-C bond forming reactions
Bayless, Michael Bruce
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Redox-active ligands deliver redox equivalents to impart multi-electron functionality at 3d metals that typically undergo to one electron redox events. It was proposed that 3d metals with redox-active ligands could form unusually well-defined catalysts for C-C bond forming reactions to mimic palladium-type reactivity. Therefore, several new complexes containing an iron or cobalt with redox-active ligands were synthesized and tested for their ability to form new C-C bonds. A bis(iminosemiquinone) iron (III) complex was able to homocouple aryl Grignards using dioxygen as the terminal oxidant. However, ligand redistribution prevented detailed mechanistic study of the C-C bond forming reaction and led to catalyst degradation. To address the challenges seen in the iron catalyst a new cobalt electron transfer (ET) series containing a pincer-type bis(phenolate) N-heterocyclic carbene ligand (CoNHC) was synthesized. Studies indicate the CoNHC ET series spans multiple-electrons by corporative metal and ligand redox. These complexes were evaluated for cross-coupling of alkyl halides and aryl Grignards. Mechanistic studies imply that the low cross-coupling yields were due to ligand degradation. However, CoNHC catalytically activate cross-couples ether nitriles and aryl Grignards via a novel C-O bond activation leading to a new C-C bond. Findings concerning redox-active ligands on iron and cobalt for C-C bond forming reactions and implications for future research are discussed.