Tetranuclear Palladacycles of 3-Acetyl-7-methoxy-2 H-chromen-2-one Derived Schiff Bases: Efficient Catalysts for Suzuki-Miyaura Coupling in an Aqueous Medium.

ABSTRACT

Tetranuclear organopalladium(II) complexes 1-3 and mononuclear complex 4 have been synthesized by the complexation of 3-acetyl-7-methoxy-2 H-chromen-2-one derived Schiff bases with potassium tetrachloropalladate K2[PdCl4]. Structural confirmation for the complexes (1-3) has been achieved by single-crystal X-ray diffraction analysis. The ligands are found to bind with the palladium ion through its azomethine nitrogen, thiolate sulfur, and C4 carbon atom of the coumarin moiety subsequent to C-H activation. The monomeric nature of complex 4 was confirmed from its mass spectroscopic data. In complex 4, coordination occurred via the lactone oxygen, azomethine nitrogen, and thiolate sulfur atoms. Computational study has been used to determine the optimized molecular structures of the complexes. An explanation on the energies of their highest occupied and lowest unoccupied molecular orbital levels and their electronic spectra has also been provided on the basis of the theoretical calculations. A systematic study of the application of these complexes as catalysts in Suzuki-Miyaura coupling (SMC) has been done with different aryl halides and phenyl boronic acid in an aqueous medium. Optimization of the reaction indicated that complex 2 exhibits greater efficiency than other complexes. An appreciable yield of the coupled products was observed with the minimum use of catalyst (µmol), and the C-C coupling has been confirmed by GC/GC-MS. An interesting result of our catalyst is the coupling of four different chloroquinolines with phenyl boronic acid to afford the coupled products in good yields.