Synthesis Of NCN Diimine-palladium (â…¡) Pincers And Theirs Catalytic Activity In Sonogashira Coupling

Pincer complexes are widely used in many fields. Recently, pincer complexeshave attracted much attention because of their widely used as catalysts in organicsynthesis for their unique structures. Three series NCN diimine ligands were obtainedafter condensation of5But2bromoisophthalaldehyde,2bromoisophthalaldehydeand5nitro2bromoisophthalaldehyde with substituted anilines. Treatment of theseligands with Pd2(dba)3in anhydrous toluene by oxidative addition of C Br bondresulted in the corresponding PdII-NCN-But, PdII-NCN-H and PdII-NCN-NO2pincercomplexes. The PdII-NCN pincers and their precursors as mentioned above were fullycharacterized by elemental analysis, IR,1H NMR,13C NMR spectroscopy. Thestructures of4tert butyl2,6bis(imino)isophthalaldimin2yl palladium(II) bromide(9c) and4nitro2,6bis(imino)isophthalaldimin2yl palladium(II) bromide (13c)were further confirmed by X-ray single crystal diffraction. The influences of theelectronic effect and spatial effect of substituent on the formation of these PdII-NCNpincers were also discussed.The application of PdII-NCN pincers in Sonogashira coupling reactions was alsostudied. The impact of substituents Z=But, H, NO2as well as R on the catalyticactivities in Sonogashira coupling were investigated. In addition, the different haloatoms, the nature of substituent and co-catalyst (PPh3) were assessed. Finally, thereaction conditions were optimized. The comparison of these three series pincers ledto the best catalyst system. All these results demonstrated that the PdII-NCN-NO2pincers exhibited the highest coupling yields. No trend was obviously observedregards to the substituents effect in aniline ring on the Sonogashira coupling. In thecoupling, the iodobenzene normally gave the highest coupling yield, while the thechlorobenzene was the lowest, which was in consistence with the C X bond energy.With the help of co-catalyst (PPh3), the Sonogashira coupling always gave the highercoupling yields. A phenomenon that halobenznes containing the electron-withdrawinggroups improved the catalytic yields than those bearing electron-donating groups wasalso observed.