Unraveling the Mechanism of a Co(−I)–Ga Photoreductant in the Catalytic Hydrodefluorination of Electron-Rich Fluoroarenes

Strong photoreductants based on first-row transition metals are rare. We report an anionic heterobimetallic Co(−I)–Ga complex, [(H₂)CoGa(N(o-(NCH₂PⁱPr₂)C₆H₄)₃)]⁻ ([2-H₂]^–), that catalyzes the hydrodefluorination of electron-rich fluoroarenes under violet light irradiation (390 nm), in the presence of stoichiometric alkoxide base, substoichiometric crown ether, and 1 atm H₂. The photodissociation of H₂ from [2-H₂]^– to form the ‘naked’ Co(−I) species [2]^– was observed by ultrafast transient absorption spectroscopy. In combination with its reactivity profile, [2]^– is proposed to be photoexcited to form an active photoreductant that reduces fluoroarenes via outer-sphere single-electron transfer. Detailed mechanistic studies suggest that the resultant Co(0) complex (2^ox-H₂), which is characterized by EPR spectroscopy, re-enters the catalytic cycle via a putative anionic Co(0) hydride intermediate [2^ox-H]^–. To the best of our knowledge, this is the first example of a 3d metal photoredox catalyst for C–F bond activation.