Quantifying the Lewis Acidity of Mono-, Di-, and Trivalent Cations in Anhydrous Bis(trifluoromethylsulfonyl)imide Salts

While the bis(trifluoromethylsulfonyl)imide anion (TFSI⁻; formula [N(SO₂CF₃)₂]⁻) has been noted for its practical utility, the use of TFSI⁻ salts as sources of Lewis acidic metal cations for studies of cation-driven tuning effects has not been reported. Here, the effective Lewis acidity of mono-, di-, and trivalent cations (namely, K⁺, Na⁺, Li⁺, Ba²⁺, Ca²⁺, Mg²⁺, Zn²⁺, La³⁺, Y³⁺, Lu³⁺, and Sc³⁺) in the form of their TFSI⁻ salts is described, along with quantitative comparisons to salts of several other weakly coordinating anions (namely, SO₃CF₃⁻, PF₆⁻, and BArF₂₄⁻). Triphenylphosphine oxide (TPPO) was used as a ³¹P NMR probe in titration experiments for quantification of key parameters describing the effective Lewis acidities of the salts in acetonitrile (CH₃CN) solutions. Notably, the TFSI⁻ salts of di- and trivalent cations were found to display strong binding to TPPO with significant speciation and were found to be quite hygroscopic. Taken together, the measurements demonstrate that TFSI⁻ salts are systematically better/stronger effective Lewis acids than their triflate analogues. And, considering the excellent solubility of TFSI⁻ salts, these materials appear attractive for further use and development in Lewis-acidity-dependent applications, including catalysis and tuning of multimetallic materials.