title: Brønsted acidic ionic liquids and their zwitterions: synthesis, characterization and pKa determination.
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 degrees 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 A. The pK(a) values of all the salts, determined by potentiometric titration, lie between 1.33 and 4.59 at 25 degrees C.
authors.affiliation: Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne, EPFL-BCH, 1015 Lausanne, Switzerland.
authors.forename: Zhaofu
authors.initials: Z
authors.lastname: Fei
authors.forename: Dongbin
authors.initials: D
authors.lastname: Zhao
authors.forename: Tilmann J
authors.initials: TJ
authors.lastname: Geldbach
authors.forename: Rosario
authors.initials: R
authors.lastname: Scopelliti
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Dyson