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

Visible-light capture activates a thermodynamically inert Co -CF 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 [( OCO)Co (CF )(MeCN) ] (2), but in non-coordinating solvents the complex is red, square pyramidal [( OCO)Co (CF )(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 -CF bond, releasing CF 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 by-product of Co -CF homolysis produces H . The photophysical properties of 2 and 3 provide a rationale for the disparate light stability.