PdII square planar complexes of the type [IL]2[PdX4] as catalyst precursors for the Suzuki–Miyaura cross-coupling reaction. The first in situ ESI-MS evidence of [(IL)xPd3] clusters formation

PdII anionic, square planar complexes of the type [IL]2[PdX4], where IL = imidazolium cations, X = Cl, Br, have been applied for the first time as catalyst precursors for the Suzuki–Miyaura (S–M) reaction carried out in alcohols at 40 °C. Depending on the structure of the catalyst precursor used, different yields of 2-methylbiphenyl (2-MePh–Ph) were obtained in a test reaction of 2-bromotoluene (2-MePhBr) with phenyl boronic acid (PhB(OH)2). The highest yield, ca. 95%, was obtained for [dmiop]2[PdCl4] (dmiop-1,2-dimethyl-3-propoxymethylene imidazolium cation) in methanol in the presence of KOH as a base. It was found that during the reaction Pd0 nanoparticles of 4–7 nm have been formed in situ from the palladium(II) precursor. In the first stage of this process, [(IL)3Pd(OH)2]+-type species have been formed from [IL]2[PdX4] precursors. Small Pd0 clusters, [(IL)5Pd3(H2O)]+ and [(IL)3Pd3(H2O)7]+, were also identified by ESI-MS. These species reacted with ArB(OH)2 forming palladium aryl complexes of the type [(IL)2PdAr]+, key intermediates in the catalytic process. The first experimental evidence of the IL stabilized Pd0 species in S–M reaction is presented.

In situ ESI-MS evidence of [Pd0]x nanoparticles formation and stabilization with ionic liquids during Suzuki–Miyaura reaction catalyzed with [IL]2[PdX4] precursors.Figure optionsDownload as PowerPoint slide