How Does Iridium(III) Photocatalyst Regulate Nickel(II) Catalyst in Metallaphotoredox-Catalyzed C–S Cross-Coupling? Theoretical and Experimental Insights

Photoredox-mediated iridium/nickel dual catalysis has successfully triggered a series of traditionally challenging carbon–heteroatom cross-coupling reactions. However, detailed mechanisms, such as the catalytic cycles for dual catalysts and the role of base additive, remain controversy in these reactions. In this study, a highly chemoselective C–S cross-coupling of thiols with heteroaryl iodides has been investigated by density functional theory (DFT) calculations and emission quenching experiments. Interestingly, the oxidation state modulation mechanism merging oxidative quenching (IrIII–*IrIII–IrIV–IrIII) and nickel catalytic cycles (NiII–NiI–NiIII–NiI–NiII) is favorable. It is consisted of four major steps: pyridine mediated proton-coupled electron transfer, oxidative addition of heteroaryl iodides with Ni(I)–halide complex, reductive elimination, and single-electron transfer. In contrast, the radical mechanism initiated by reductive quenching of *IrIII with thiols is impractical, because oxidative addition or σ-bond metathesis from Ni(II)–thiolate intermediate is highly energy-demanding. This study will hopefully benefit the future understanding of such photoredox-mediated dual catalytic systems.