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

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Abstract
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.