[(MeCN)Ni(CF <sub>3</sub> ) <sub>3</sub> ] <sup>−</sup> and [Ni(CF <sub>3</sub> ) <sub>4</sub> ] <sup>2–</sup> : Foundations toward the Development of Trifluoromethylations at Unsupported Nickel
Scott T. Shreiber, Ida M. DiMucci, Mikhail Khrizanforov, Charles J. Titus, Dennis Nordlund, Yulia B. Dudkina, Roger E. Cramer, Yulia H. Budnikova, Kyle M. Lancaster, David A. Vicic
Abstract
Nickel anions [(MeCN)Ni(CF3)3]− and [Ni(CF3)4]2– were prepared by the formal addition of 3 and 4 equiv, respectively, of AgCF3 to [(dme)NiBr2] in the presence of the [PPh4]+ counterion. 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, surprisingly rendering the metal to a physically more reduced state. In the cases of [(MeCN)Ni(CF3)3]− and [Ni(CF3)4]2–, these complexes are better physically described as d9 metal complexes. [(MeCN)Ni(CF3)3]− is electrophilic and reacts with other nucleophiles such as phenoxide to yield the unsupported [(PhO)Ni(CF3)3]2– salt, revealing the broader potential of [(MeCN)Ni(CF3)3]− in the development of “ligandless” trifluoromethylations at nickel. Proof-in-principle experiments show that the reaction of [(MeCN)Ni(CF3)3]− with an aryl iodonium salt yields trifluoromethylated arene, presumably via a high-valent, unsupported, and formal organonickel(IV) intermediate. Evidence of the feasibility of such intermediates is provided with the structurally characterized [PPh4]2[Ni(CF3)4(SO4)], which was derived through the two-electron oxidation of [Ni(CF3)4]2–.