[(MeCN)Ni(CF3)3]1– and [Ni(CF3)4]2– : Foundations Towards the Development of Trifluoromethylations at Unsupported Nickel

The nickel anions [(MeCN)Ni(CF3)3]1 and [Ni(CF3)4]2 were prepared by formal addition of three and four equivalents of [AgCF3] to [(dme)NiBr2] in the presence of supporting [PPh4] counter-ion. Detailed insights into the electronic properties of these new compounds were obtained through the use of density functional theory (DFT) calculations, spectroscopy-oriented configuration interaction (SORCI) calculations, X-ray absorption spectroscopy, and cyclic voltammetry. The data collectively show that trifluoromethyl complexes of nickel, even in the most common oxidation state of nickel(II), are highly covalent systems whereby a hole is distributed on the trifluoromethyl ligands and surprisingly rendering the metal to a physically more reduced state. In the cases of [(MeCN)Ni(CF3)3]1– and [Ni(CF3)4]2–, these complexes are better described as physically d9 metal complexes. [(MeCN)Ni(CF3)3]1 is electrophilic and reacts with other nucleophiles like phenoxide to yield the unsupported [(PhO)Ni(CF3)3]2– salt, revealing the broader potential of [(MeCN)Ni(CF3)3]1– in the development of ligandless trifluoromethylations at nickel. Proof-in-principle experiments show that reaction of [(MeCN)Ni(CF3)3]1– with an aryl iodonium salt yields trifluoromethylated arene, presumably via a high valent, unsupported, and formally organonickel(IV) intermediate. Evidence for the feasibility of such intermediates is provided with the structurally characterized [Ni(CF3)4(SO4)][PPh4]2, which was derived through the two electron oxidation of [Ni(CF3)4]2–.