Synthesis of tetranuclear complex of Pd(II) with thiosemicarbazone ligands derived from 2‐quinolone and its catalytic evaluation in Suzuki–Miyaura‐type coupling reactions and alkoxylation of chloroquinolines

A tetranuclear palladium(II) complex [(Pd(H‐6MOQtsc‐Ph))4] was obtained from the reaction between 6‐methyl‐2‐oxo‐1,2‐dihydroquinoline‐3‐carboxaldehyde‐4(N)‐phenylthiosemicarbazone [H2‐6MOQtsc‐Ph] and K2[PdCl4]. The ligand and the Pd(II) complex were characterized by Fourier transform infrared spectroscopy (FT‐IR), UV–visible and 1H NMR spectroscopy. X‐ray diffraction studies confirmed the tetrameric nature of the complex with the coordination of ligand through quinolone carbonyl, azomethine nitrogen and thiolate sulfur atoms, and the fourth site is occupied by 2‐quinolone nitrogen atom of the adjacent ligand. The synthesized complex was tested as catalyst in Suzuki–Miyaura coupling reaction between various chloroquinoline derivatives with phenylboronic acid. The reactions afforded unexpected C‐alkoxylated (CO coupling) products instead of more expected C‐arylated (CC coupling) products in the respective alcoholic mediums. However, the reactions with traditional aryl halides probed with very good yield of the corresponding CC coupling products.