Trifluoromethylation of a Well‐Defined Square‐Planar Aryl‐NiII Complex involving NiIII/CF3. and NiIV−CF3 Intermediate Species

Abstract

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 Ni 0 to obtain well-defined aryl-Ni II -Br complexes ([L n −Ni II ]Br). Abstraction of the halide with AgX (X=OTf − or ClO 4 − ) 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 CF 3 + sources afforded L n −CF 3 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 L 1 −Ni III /CF 3 . adduct, which rapidly recombines to form a [L 1 -Ni IV -CF 3 ](X) 2 intermediate species. A final facile reductive elimination affords L 1 −CF 3 . 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