Trifluoromethylation of a Well-Defined Square-Planar Aryl-Ni^II Complex involving Ni^III/CF₃ ^. and Ni^IV−CF₃ Intermediate Species

Ni-mediated trifluoromethylation of an aryl−Br bond in model macrocyclic ligands (L_n−Br) has been thoroughly studied, starting with an oxidative addition at Ni⁰ to obtain well-defined aryl-Ni^II-Br complexes ([L_n−Ni^II]Br). Abstraction of the halide with AgX (X=OTf⁻ or ClO₄ ⁻) thereafter provides [L_n−Ni^II](OTf). The nitrate analogue has been obtained through a direct C−H activation of an aryl−H bond using Ni^II salts, and this route has been studied by X-ray absorption spectroscopy (XAS). Crystallographic XRD and XAS characterization has shown a tight macrocyclic coordination in the aryl−Ni^II complex, which may hamper direct reaction with nucleophiles. On the contrary, enhanced reactivity is observed with oxidants, and the reaction of [L_n−Ni^II](OTf) with CF₃ ⁺ sources afforded L_n−CF₃ products in quantitative yield. A combined experimental and theoretical mechanistic study provides new insights into the operative mechanism for this transformation. Computational analysis indicates the occurrence of an initial single electron transfer (SET) to 5-(trifluoromethyl)dibenzothiophenium triflate (TDTT), producing a transient L₁−Ni^III/CF₃ ^. adduct, which rapidly recombines to form a [L₁-Ni^IV-CF₃](X)₂ intermediate species. A final facile reductive elimination affords L₁−CF₃. The well-defined square-planar model system studied here permits to gain fundamental knowledge on the rich redox chemistry of nickel, which is sought to facilitate the development of new Ni-based trifluoromethylation methodologies.