Catalytic C-H Trifluoromethylation of Arenes and Heteroarenes via Visible Light Photoexcitation of a Co(III)-CF 3 Complex.

A cobalt photocatalyst for direct trifluoromethylation of (hetero)arene C(sp2 )-H bonds is described and shown to operate via visible light activation of a Co-CF3 intermediate, which functions as a combined chromophore and organometallic reaction center. Chemical oxidations of previously reported (OCO)Co complexes containing a redox-active [OCO] pincer ligand afford a Co-CF3 complex two oxidation states above Co(II). Computational and spectroscopic studies are consistent with formulation of the product as [(OCO• )CoIII (CF3 )(THF)(OTf)] ( II ) containing an open-shell [OCO• ]1- radical ligand bound to a S = 0 Co(III) center. II is thermodynamically stable, but exposure to blue (440 nm) light induces Co-CF3 bond homolysis and release of • CF3 , which is trapped by radical acceptors including TEMPO• , (hetero)arenes, or the radical [OCO• ] ligand in II . The latter comprises a competitive degradation pathway, which is overcome under catalytic conditions by using excess substrate. Accordingly, generation of II from the reaction of [(OCO)CoII L] ( III ) (L = THF, MeCN) with Umemoto's dibenzothiophenium trifluoromethylating reagent ( 1 ) followed by photolytic Co-CF3 bond activation completes a photoredox catalytic cycle for C-H (hetero)arene trifluoromethylation utilizing visible light. Electronic structure and photophysical studies, including time-dependent density functional theory (TDDFT) calculations, suggest that Co-CF3 bond homolysis at II occurs via an ligand-to-metal charge-transfer (LMCT) (OCO0 )CoII (CF3 ) state, revealing ligand redox activity as a critical design feature and establishing design principles for the use of base metal chromophores for selectivity in photoredox bond activations occurring via free radical intermediates.