[(MeCN)Ni(CF3)3]1– and [Ni(CF3)4]2– : Foundations Towards the Development of Trifluoromethylations at Unsupported Nickel
The nickel anions [(MeCN)Ni(CF<sub>3</sub>)<sub>3</sub>]<sup>1</sup><sup>–</sup> and [Ni(CF<sub>3</sub>)<sub>4</sub>]<sup>2</sup><sup>–</sup> were prepared by formal addition of three and four equivalents of [AgCF<sub>3</sub>] to [(dme)NiBr<sub>2</sub>] in the presence of supporting [PPh<sub>4</sub>] 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(CF<sub>3</sub>)<sub>3</sub>]<sup>1–</sup> and [Ni(CF<sub>3</sub>)<sub>4</sub>]<sup>2–</sup>, these complexes are better described as physically d<sup>9</sup> metal complexes. [(MeCN)Ni(CF<sub>3</sub>)<sub>3</sub>]<sup>1</sup><sup>–</sup> is electrophilic and reacts with other nucleophiles like phenoxide to yield the unsupported [(PhO)Ni(CF<sub>3</sub>)<sub>3</sub>]<sup>2–</sup> salt, revealing the broader potential of [(MeCN)Ni(CF<sub>3</sub>)<sub>3</sub>]<sup>1–</sup> in the development of ligandless trifluoromethylations at nickel. Proof-in-principle experiments show that reaction of [(MeCN)Ni(CF<sub>3</sub>)<sub>3</sub>]<sup>1– </sup>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(CF<sub>3</sub>)<sub>4</sub>(SO<sub>4</sub>)][PPh<sub>4</sub>]<sub>2</sub>, which was derived through the two electron oxidation of [Ni(CF<sub>3</sub>)<sub>4</sub>]<sup>2–</sup>.