Redox non-innocent bis(phenoxide) pincer ligand cobalt complexes for selective radical C–H (trifluoro)alkylation through photoinduced cobalt–R(f) bond weakening
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Kuehner, Chris S.
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Abstract
Photoredox catalysis has become an important tool for bond-breaking and -making methods via efficient conversion of light into chemical energy. However, many methods utilize later row transition metals which have adverse economic, biologic, and environmental impacts, thus motivating efforts to explore cheaper more sustainable catalysts such as earth abundant metals. The use of 3d metals for bimolecular single electron transfer has been challenged by their ultra-short-lived excited states. My PhD thesis research harnesses LMCT to promote ligand lability in a Co chromophore for bond-making processes as an alternative strategy to utilize first row metals for photoredox catalysis.
Chemical oxidation of a previously reported (OCO)Co complex that contains a redox-active [OCO] pincer ligand affords a Co–CF3 complex two oxidation states above Co(II), computational and structural data is consistent with formulation as [(OCO•–)CoIII(CF3)(THF)OTf]. This complex is thermodynamically stable but upon exposure to blue (440 nm) light induces Co–CF3 bond homolysis and release of •CF3 which is trapped by radical acceptors such as TEMPO•, (hetero)arenes, or the [OCO•] ligand. The radical trapping by the ligand backbone is a competitive pathway which is overcame by utilizing catalytic conditions. Alternatively, II can be synthesized by treating (OCO)CoII(THF) with Umemoto’s dibenzothiophenium trifluoromethylating reagent completes a photoredox catalytic cycle for C–H (hetero)arene trifluoromethylation utilizing visible light. The rearomatization of the cyclohexadienyl radical by the Co containing byproduct negates the need or a sacrificial or substrate derived oxidant, thus increasing the overall atom-economy of the catalytic trifluoromethylation and the (OCO)Co core can act as both the chromophore and the redox-center.
Efforts to expand this observed VLIH reactivity of the Co–CF3 core to alkylation focused on radical decarboxylation of carboxylates via Co–O VLIH. A class of four new Co(III)–carboxylate complexes supported by the redox active [OCO] ligand were synthesized. Computational data suggests that these (OCO)CoIIIO2CR (1) complexes retain the photophysical properties for low-energy Co–O bond homolysis. Exposure of 1 to red (660 nm) light results in alkylation of radical acceptors such as TEMPO• or (hetero)arenes. Co–O bond homolysis occurs in either coordinating or non-coordinating solvents, but the use of coordinating solvents suppresses formation of the photoinert dimer [(OCO)Co]2O2CR. However, the monomer dimer equilibrium is highly sensitive to the presence of coordinating solvents such that full conversion of the dimer back to the monomeric species is observed using as little as 1.75 equivalents of a coordinating solvent.
The sum of this thesis demonstrates the utility of the (OCO)Co core as a chromophore for stoichiometric and catalytic trifluoro(alkylation) of (hetero)arenes. In terms of organic product yields and distributions, these reactions are not advantageous to the current state-of-the art methods, but our catalytic approach is a distinct strategy to activate inherently strong M–R(f) bonds for applications in photocatalysis.
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2023-11-10
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Dissertation