Efficient Heck reactions catalyzed by a highly recyclable palladium(II) complex of a pyridyl-functionalized imidazolium-based ionic liquid

The reactions of 2-(2-pyridyl)imidazole with alkyl iodides at 25 degrees C in the presence of base gave rise to 1-alkyl-2-(2-pyridyl)imidazole. Subsequent neat reactions with alkyl or polyfluoroalkyl halides at 100 degrees C, followed by anion exchange with LiN(SO2CF3)(2), generated the mono-quaternary ionic liquids 3a-31. All of them have excellent thermal stability and wide liquid range. Most of the salts with asymmetric N-substituents are liquid at room temperature. The effect of N-substituent variation and symmetry on NMR, TGA and DSC is discussed. Reaction of 3f with palladium(II) chloride produced a mononuclear palladium ionic liquid complex 4, the structure of which was confirmed by single-crystal X-ray diffraction analysis. The Heck cross-coupling reactions using 4 in ionic liquid 3f demonstrated excellent stability and recyclability.