Catalytic C–H Trifluoromethylation of Arenes and Heteroarenes via Visible Light Photoexcitation of a Co(III)–CF₃ Complex

Abstract

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^•)Co^III(CF₃)(THF)(OTf)] (<b>II</b>) containing an open-shell [OCO^•]^1- radical ligand bound to a <i>S</i> = 0 Co(III) center. <b>II</b> 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 <b>II</b>. The latter comprises a competitive degradation pathway, which is overcome under catalytic conditions by using excess substrate. Accordingly, generation of <b>II</b> from the reaction of [(OCO)Co^IIL] (<b>III</b>) (L = THF, MeCN) with Umemoto's dibenzothiophenium trifluoromethylating reagent (<b>1</b>) 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 <b>II</b> occurs via an ligand-to-metal charge-transfer (LMCT) (OCO⁰)Co^II(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.