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.
When irradiated by UVA, a Ni(II) precatalyst integrating a benzophenone chromophore in its structure catalyzes the regioselective cycloaddition of alkynes and azides to produce 1,5‐disubsituted 1,2,3‐triazoles with 1,5:1,4 regioisomer ratios ranging from 1.7:1 to >20:1. Speciation and reactivity studies revealed that the reaction most likely proceeds through a Ni(I)/Ni(III) catalytic cycle.
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