Bronsted acidic ionic liquids and their zwitterions: Synthesis, characterization and pK(a) determination

Imidazolium chlorides with one or two carboxylic acid substituent groups, 1-methyl-3-alkylcarboxylic acid imidazolium chloride, [Me{(CH2)(n)COOH}im]Cl (n=l, 3), and 1,3-dialkylcarboxylic acid imidazolium chloride, [{(CH2)(n)COOH}(2)im]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)(n)COO}im] (n=1, 3) and [{(CH2),COOH}{(CH2)(n)COO}im] (n= 1, 3). Subsequent reaction of the zwitterions with strong acids gives the new imidazolium salts [Me{(CH2)(n)COOH }im]X (n=1, 3; X=BF4, CF3SO3) and [{(CH2)(n)COOH)(2)im]X (n=1, 3; X=BF4, CF3SO3), which exhibit melting points as low as -61 degreesC. The solidstate 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 (ClH)-H-... interactions below 3Angstrom. The pK(a) values of all the salts, determined by potentiometric titration, lie between 1.33 and 4.59 at 25 degreesC.