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 acidity of the salts in acetonitrile (CH3CN) solutions. Notably, the TFSI– salts of di- and triva-lent 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 ana-logues. 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.