Palladium(II) complexes bearing 2-(1H-imidazol/oxazol-2-yl)-pyridines: Synthesis, structures and ligand effects in Suzuki–Miyaura cross-coupling

A series of dichloropalladium(II) complexes (Pd1–Pd4ox) based on derivatives of the hemilabile 2-(1H-imidazol-2-yl)pyridine and 2-(oxazol-2-yl)pyridine ligands were synthesized and characterized. The ligands were designed to introduce a variety of electronic and structural features and crystal structures of Pd1, Pd1Me, Pd3, and Pd3Me were determined. Pd1 and Pd1Me both crystallize in the monoclinic space group C2/c. Pd3 crystallizes in the monoclinic space group P21/c while Pd3Me crystallizes in the orthorhombic space group P212121. All complexes were utilized as pre-catalysts in a comparative study to probe relevant features of the palladium active species during high temperature Suzuki–Miyaura cross-coupling catalysis. Relatively higher catalytic activities were obtained for complexes bearing the bidentate ligands that possess weaker azole donor arms. Results obtained suggest that ligand dissociation steps may be important for generating active species with high efficiency. In particular, the observed ligand effects in the investigated series indicate that higher catalyst efficiencies were observed with complexes for which the coordination of the ligand at the palladium center could readily be converted into a monodentate mode.