Photoinduced Cobalt(III)−Trifluoromethyl Bond Activation Enables Arene C−H Trifluoromethylation

Abstract

Visible-light capture activates a thermodynamically inert Co^III -CF₃ bond for direct C-H trifluoromethylation of arenes and heteroarenes. New trifluoromethylcobalt(III) complexes supported by a redox-active [OCO] pincer ligand were prepared. Coordinating solvents, such as MeCN, afford green, quasi-octahedral [(^S OCO)Co^III (CF₃ )(MeCN)₂ ] (2), but in non-coordinating solvents the complex is red, square pyramidal [(^S OCO)Co^III (CF₃ )(MeCN)] (3). Both are thermally stable, and 2 is stable in light. But exposure of 3 to low-energy light results in facile homolysis of the Co^III -CF₃ bond, releasing ^. CF₃ radical, which is efficiently trapped by TEMPO^. or (hetero)arenes. The homolytic aromatic substitution reactions do not require a sacrificial or substrate-derived oxidant because the Co^II by-product of Co^III -CF₃ homolysis produces H₂ . The photophysical properties of 2 and 3 provide a rationale for the disparate light stability.