Synthesis and structural studies of Pd(II) complexes of bidentate Schiff bases and their catalytic activities as pre-catalysts in the Mizoroki-Heck reaction

Abstract

Pd(II) complexes 5a-u were synthesized from the bidentate Schiff base ligands 4a-u and Pd(OAc)2 in acetonitrile under an argon atmosphere and their catalytic activities were evaluated in the Mizoroki-Heck reaction. The bidentate Schiff base ligands 4a-u were synthesized by the condensation of 3,5-di-tert-butylsalicylaldehyde, 3-tert-butylsalicylaldehyde, 5-tert-butylsalicylaldehyde or salicylaldehyde with aniline or its derivatives. The Pd(II) Schiff base complexes were characterized by 1H NMR, 19F NMR, 13C NMR and FT-IR spectroscopy, HRMS and CHNS analysis. The molecular structures of complexes 5d and 5u were confirmed by the single crystal XRD method. The trans-[Pd(II)(N,O)2] complexes were obtained after the complexation of the ligands derived from condensation of 3,5-di-tert-butylsalicylaldehyde and aniline, 2-methylaniline, 2-fluoroaniline, 2-bromoaniline, 2-hydroxyaniline, 4-tert-butylaniline or 2,3,4,5,6-pentafluoroaniline, 5-tert-butylsalicylaldehyde and aniline or 4-methoxyaniline, and salicylaldehyde and aniline, 2-methoxyaniline or 4-methoxyaniline. A mixture of cis- and trans-(N,O)2Pd(II) complexes were obtained with the ligands derived from 3,5-di-tert-butylsalicylaldehyde and 4-methylaniline, 4-fluoroaniline, 4-bromoaniline, 4-chloroaniline, 2-methoxyaniline, 4-methoxyaniline or 4-hydroxyaniline, and 3-tert-butylsalicylaldehyde and aniline or 4-methoxyaniline. The interconversions of the cis–trans isomers of the Pd(II) complexes show that the thermal stability of the cis-Pd(II) and trans-Pd(II) complexes is dependent on the structure of the ligands in acetonitrile. The complex 5a (5 mol%) acts as an efficient pre-catalyst for the Mizoroki-Heck reaction of aryl halide (iodide and bromide) with alkenes (styrene and acrylate) using NaHCO3 as a base in DMF at 140 °C, where the corresponding trans-alkenes were afforded in 21–99% yields.