Fluoro‐ and Perfluoralkylsulfonylpentafluoroanilides: Synthesis and Characterization of NH Acids for Weakly Coordinating Anions and Their Gas‐Phase and Solution Acidities

Fluoro‐ and perfluoralkylsulfonyl pentafluoroanilides [HN(C₆F₅)(SO₂X); X=F, CF₃, C₄F₉, C₈F₁₇] are a class of imides with two different strongly electron‐withdrawing substituents attached to a nitrogen atom. They are NH acids, the unsymmetrical hybrids of the well‐known symmetrical bissulfonylimides and bispentafluorophenylamine. The syntheses, the structures of these perfluoroanilides, their solvates, and some selected lithium salts give rise to a structural variety beyond the symmetrical parent compounds. The acidities of representative subsets of these novel NH acids have been investigated experimentally and quantum‐chemically and their gas‐phase acidities (GAs) are reported, as well as the pK_a values of these compounds in acetonitrile (MeCN) and DMSO solution. In quantum chemical investigations with the vertical and relaxed COSMO cluster‐continuum models (vCCC/rCCC), the unusual situation is encountered that the DMSO‐solvated acid Me₂SO–H‐N(SO₂CF₃)₂, optimized in the gas phase (vCCC model), dissociates to Me₂SO‐H⁺–N(SO₂CF₃)₂^? during structural relaxation and full optimization with the solvation model turned on (rCCC model). This proton transfer underlines the extremely high acidity of HN(SO₂CF₃)₂. The importance of this effect is studied computationally in DMSO and MeCN solution. Usually this effect is less pronounced in MeCN and is of higher importance in the more basic solvent DMSO. Nevertheless, the neglect of the structural relaxation upon solvation causes typical changes in the computational pK_a values of 1 to 4 orders of magnitude (4–20 kJ mol^?1). The results provide evidence that the published experimental DMSO pK_a value of HN(SO₂CF₃)₂ should rather be interpreted as the pK_a of a Me₂SO‐H⁺–N(SO₂CF₃)₂^? contact ion pair.