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(<i>o</i>-(NCH₂P^<i>i</i>Pr₂)C₆H₄)₃)]^- (<b>[2-H</b>_<b>2</b><b>]</b>^<b>-</b>), 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 <b>[2-H</b>_<b>2</b><b>]</b>^<b>-</b> to form the 'naked' Co(-I) species <b>[2]</b>^<b>-</b> was observed by ultrafast transient absorption spectroscopy. In combination with its reactivity profile, <b>[2]</b>^<b>-</b> 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 (<b>2</b>^<b>ox</b><b>-H</b>_<b>2</b>), which is characterized by EPR spectroscopy, re-enters the catalytic cycle via a putative anionic Co(0) hydride intermediate <b>[2</b>^<b>ox</b><b>-H]</b>^<b>-</b>. To the best of our knowledge, this is the first example of a 3<i>d</i> metal photoredox catalyst for C-F bond activation.