Novel redox non-innocent bis(phenoxide) pincer complexes of cobalt: Connecting electronic structures and reactivity
Harris, Caleb Frank
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In this thesis, I describe the synthesis, characterization, and reactivity of a variety of new cobalt complexes containing redox-active, pincer-type ligands based on an N-heterocyclic carbene (NHC) centered, bis(phenoxide) scaffold (OCO). The complexes span formal oxidation states from CoII to CoV within a 1.5 V applied potential range. Experimental and computational data combine to describe the physical oxidation states of the non-innocent ligand/metal combinations as well as the effects of altering the degree of saturation in the backbone of the NHC moiety. Preliminary efforts towards utilization of these complexes for C–C and C–N coupling reactions are described. It was determined that treating the complexes with one and two electron oxidants generally leads to the decomposition of the ligand, suggesting alternative strategies are needed to protect the OCO ligand backbone from deleterious attack by reactive small molecule fragments. (OCO)Co complexes were then treated with a radical CF3 source and their products were characterized. In one case, the ligand forms an unexpected C-CF3 bond, but in another, a new complex containing a Co-CF3 bond is formed. The isolated Co-CF3 complex is either 5- or 6-coordinate depending on the solvent and/or temperature. The 5-coordinate complex was found to be photosensitive and capable of Co-CF3 bond homolysis upon exposure to visible light. This allows for direct trifluoromethylation of unactivated arene and heteroarene C-H bonds. Interestingly, the 6-coordinate species is unaffected by visible light. The differences in reactivity can be explained by the observed variations in geometry and electronic structure. A κ2-trifluoroacetate complex was also synthesized and observed to undergo Co-O bond homolysis under visible light. This complex is also competent to trifluoromethylate aryl C-H bonds, presumably through a radical decarboxylation mechanism. Catalytic trifluoromethylation of silyl enol ethers, using commercially available electrophilic CF3 reagents, was also achieved using of one of the (OCO)Co complexes. A single electron transfer mechanism is proposed to impart radical character on the CF3 reagents, thus destabilizing the compounds and causing them to deliver a CF3 radical. The radical attacks the unsaturated bond of the enol ether, producing a tertiary organic radical which reduces the oxidized metal back to its original oxidation state. The organic compound then forms an α-trifluoromethyl ketone and an ionic salt byproduct. The cobalt complex is capable of facilitating this SET mechanism in multiple oxidation states, demonstrating the unique versatility of the ligand/metal platform for catalytic C–C bond formation. Both of these cases represent the first examples of cobalt involved C-H bond trifluoromethylation reported in literature and are both resultant of two individual properties associated with this of this distinctive ligand scaffold.