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

Visible-light capture activates a thermodynamically inert CoIII -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)CoIII (CF3 )(MeCN)2 ] (2), but in non-coordinating solvents the complex is red, square pyramidal [(S OCO)CoIII (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 CoIII -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 CoII by-product of CoIII -CF3 homolysis produces H2 . The photophysical properties of 2 and 3 provide a rationale for the disparate light stability.