Synthesis Of Disazo-palladium(Ⅱ) Pincers And Theirs Catalytic Activity In Sonogashira And Suzuki Coupling

Pincer complexes are widely used in many fields. Recently, pincer complexeshave attracted much attention because of their widely utility as catalysts in organicsynthesis for their unique structures. Four N^C^N disazo ligands3a-3d and amonoazo ligand8were synthesized after coupling reactions of m-bromophenol withsubstituted diazonium salt. Treatment of these ligands with Pd2(dba)3in toluene byoxidative addition of C－Br bond resulted in the corresponding pincer complexes4a-4d and cyclopalladated azo compound9, respectively. Then pincer complexes5a-5d and5a’-5d’ were obtained from the reactions4a-4d with AgNO3and AgClO4,respectively. And complexe6was obtained from the reaction4c with PPh3. ThePdII-N^C^N pincers and their precursors as mentioned above were fully characterizedby elemental analysis, IR,1H NMR,13C NMR spectroscopy. The structures of5c,6,9were further confirmed by X-ray single crystal diffraction.The application of PdII－N^C^N pincers in Sonogashira and Suzuki couplingreactions were also studied. The impact of R substituents as well as type of pincers onthe catalytic activities in Sonogashira and Suzuki coupling was investigated. Inaddition, the different halo atoms, the substituent categories on the aryl halidescoupling reagent were assessed. The comparison of pincers (4a,5a,5a’) led to thebest catalyst4a. But complex6was the best catalyst among all of palladiumcompounds in the carbon-carbon cross coupling reaction. No trend was obviouslyobserved regards to the substituents effect in4a-4d on the Sonogashira and Suzukicoupling reactions. In the coupling reactions, the iodobenzene normally gave thehighest coupling yield, while the chlorobenzene was the lowest, which was inconsistence with the C－X bond energy. A phenomenon was also observed thathalobenznes containing the electron-withdrawing groups improved the catalytic yieldsthan those bearing electron-donating groups.