Dual Photoredox Ni/Benzophenone Catalysis: A Study of the Ni Precatalyst Photoreduction Step.

The combination of NiX salts with a bipyridine-type ligand and aromatic carbonyl-based chromophores has emerged as a benchmark precatalytic system to efficiently conduct cross-couplings mediated by light. Mechanistic studies have led to two scenarios in which Ni is proposed as the catalytic species. Nonetheless, in none of these studies has a Ni to Ni photoreduction been evidenced. By exploiting UV-visible, nuclear magnetic resonance, resonance Raman, electron paramagnetic resonance, and dynamic light scattering spectroscopies and also transmission electron microscopy, we report that, when photolyzed by UVA in alcohols, the structurally defined [Ni(μ-OH)(dtbbpy)(BPCO)] complex integrating a benzophenone chromophore is reduced into a diamagnetic Ni dimer of the general formula [Ni(dtbbpy)(BPCO)]. In marked contrast, in THF, photolysis led to the fast formation of Ni, which accumulates in the form of metallic ultrathin Ni nanosheets characterized by a mean size of ∼100 nm and a surface plasmon resonance at 505 nm. Finally, it is shown that combined with UVA irradiation catalyzes cross-couplings, that is, C(sp)-H arylation of THF and O-arylation of methanol. These results are discussed in light of the mechanisms proposed for these cross-couplings with a focus on the oxidation state of the catalytic species.