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

The combination of NiIIX2 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 Ni0 is proposed as the catalytic species. Nonetheless, in none of these studies has a NiII to Ni0 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 [NiII 2(μ-OH2)­(dtbbpy)2(BPCO2)4] complex 1 integrating a benzophenone chromophore is reduced into a diamagnetic NiI dimer of the general formula [NiI 2(dtbbpy)2(BPCO2)2]. In marked contrast, in THF, photolysis led to the fast formation of Ni0, 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 1 combined with UVA irradiation catalyzes cross-couplings, that is, C­(sp3)–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.