Brønsted Acidic Ionic Liquids and Their Zwitterions: Synthesis, Characterization and pKa Determination

Imidazolium chlorides with one or two carboxylic acid substituent groups, 1‐methyl‐3‐alkylcarboxylic acid imidazolium chloride, [Me{(CH2)nCOOH}im]Cl (n=1, 3), and 1,3‐dialkylcarboxylic acid imidazolium chloride, [{(CH2)nCOOH}2im]Cl (n=1, 3), have been synthesized via their corresponding acid esters. Deprotonation of the carboxylic acid functionalized imidazolium chlorides with triethylamine affords the corresponding zwitterions [Me{(CH2)nCOO}im] (n=1, 3) and [{(CH2)nCOOH}{(CH2)nCOO}im] (n=1, 3). Subsequent reaction of the zwitterions with strong acids gives the new imidazolium salts [Me{(CH2)nCOOH}im]X (n=1, 3; X=BF4, CF3SO3) and [{(CH2)nCOOH}2im]X (n=1, 3; X=BF4, CF3SO3), which exhibit melting points as low as −61 °C. The solid‐state structures of two of the carboxylic acid functionalized imidazolium salts have been determined by single‐crystal X‐ray diffraction analysis. Extensive hydrogen bonding is present between the chloride and the imidazolium, with eight Cl⋅⋅⋅H interactions below 3 Å. The pKa values of all the salts, determined by potentiometric titration, lie between 1.33 and 4.59 at 25 °C. Novel, essentially chloride‐free Brønsted acidic ionic liquids have been synthesized via their corresponding zwitterions. These acidic salts exhibit melting points as low as −61 °C and could thus serve as reaction media. The crystal structures of the precursors indicate extensive hydrogen bonding, as illustrated.