Structure, dynamics and catalytic activity of palladium(II) complexes with imidazole ligands

Several palladium complexes of the type [Pd(im)2Cl2], [Pd(im)3Cl]Cl, and [Pd(im)4]Cl2 (im = imidazole 1, 1-methylimidazole 2, 1,2-dimethylimidazole 3, 1-butylimidazole 4, 4a, 1-phenylimidazole 6, 1-phenylimidazoline 7, and 1-methylimidazoline 8) were prepared and structurally characterized. The square planar structure of two new complexes with the composition [Pd(im)4]Cl2 (2b, 4b) was confirmed by X-ray analysis. In solution, exchange of imidazole ligands leading to heteroleptic products was evidenced by ESI-MS studies. Two bis-ligated complexes, bearing 1-methylimidazole (2a) and 1-propoxymethylimidazole (5) ligands, were obtained in the reaction of palladium with imidazoles formed by deprotection of one nitrogen atom in the respective imidazolium halides. Catalytic Suzuki–Miyaura reactions were carried out using the obtained palladium complexes in isopropanol–water solution. High yields of the cross-coupling products were obtained at 40 and 60 °C when 2-bromotoluene, 4-bromotoluene, and 4-bromoanizole were used as substrates.

Graphical abstractNew palladium–imidazole complexes have been obtained and characterized in solid and in a solution. ESI-MS measurements performed in a solution demonstrated a presence of two different species, namely Pd(im)2Cl2 and [Pd(im)3Cl]Cl. Imidazole ligands undergo substitution by free imidazoles added to the solution and exchange of imidazole ligands between two palladium complexes was also observed. The square planar structure of two complexes of [Pd(im)4]Cl2 composition have been confirmed by X-ray. The obtained compounds used in Suzuki–Miyaura reaction of bromoaryls with phenylboronic acid in isopropanol–water mixture produced ca. 100% of cross-coupling products at 40–60 °C.Figure optionsDownload full-size imageDownload as PowerPoint slide