Environmentally benign catalyst: Synthesis, characterization, and properties of pyridinium dicationic molten salts (ionic liquids) and use of application in esterification

A series of pyridinium based dicationic [C-6(Mpy)(2)], [C-10(Epy)(2)], [C-10(Bpy)(2)] with traditional counter anions such as Br-, BF4-, PF6- and Tf2N- molten salts (ionic liquids (ILs)) were synthesized. Most of them can be classified as ionic liquids (mp < 100 degrees C). Key physicochemical properties, such as melting point, thermal stability and solubility in common solvents were determined. The synthesized dicationic salt 1,1'-hexane-1,6-diylhis (3-methylpyridinium) dibromide tested for the esterification of carboxylic acid derivatives with a variety of alkyl halides in the presence of triethylamine. The esterification was carried out under mild reaction conditions and without any additional organic solvent. The results showed that carboxylic acids were successfully converted into esters with good yields and high selectivity. The produced esters were easily recovered due to immiscibility with the ionic liquid as green reaction medium. The ionic liquid was recycled and reused. Thus, reported dicationic ionic liquid was a promising catalyst for esterification reaction. (C) 2012 Elsevier B.V. All rights reserved.