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 (2ox-H2), which is characterized by EPR spectroscopy, re-enters the catalytic cycle via a putative anionic Co(0) hydride intermediate 2ox-H-. To the best of our knowledge, this is the first example of a 3d metal photoredox catalyst for C-F bond activation.