Palladium complexes with 3-phenylpropylamine ligands: synthesis, structures, theoretical studies and application in the aerobic oxidation of alcohols as heterogeneous catalysts

Abstract

The reaction of 3-phenylpropylamine with Pd(OAc)₂ by heating in toluene resulted in the nearly square-planar complex trans-[Pd(C₆H₅(CH₂)₃NH₂)₂(OAc)₂] (1). Complex 1 reacted with NaCl in methanol to obtain the corresponding product trans-[Pd(C₆H₅(CH₂)₃NH₂)₂Cl₂] (2). Treatment of 2 with triphenylphosphine in dichloromethane afforded trans-[Pd(C₆H₅(CH₂)₃NH₂)₂(PPh₃)₂]2Cl⁻ (3). All the palladium(II) complexes (1–3) were fully characterized by IR and NMR spectroscopy. In addition, the crystal structures of 1 and 2 were determined by single-crystal X-ray diffraction analysis. In these structures, the acetate and chloride ligands are in trans geometry. Density functional theory (DFT) calculations gave bond lengths and angles that were noted as experimental values. Palladium nanoparticles that were derived from complexes (1–3) were supported on cucurbit[6]uril (CB[6]) and identified by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), inductively coupled plasma analysis (ICP) and high-resolution X-ray powder spectroscopy (HR-XPS). CB[6]-supported palladium nanoparticles (NPs) were used as heterogeneous catalysts for the aerobic oxidation of alcohols to the corresponding aldehydes or ketones without over-oxidation. CB[6]-Pd NPs (3) (prepared from complex 3) show better catalytic activity than CB[6]-Pd NPs (1), (2), as a higher yield was observed with them in a relatively short time. Factors such as the amount of catalyst, solvent, temperature and reaction time were all systematically investigated to determine their effects on the yield of catalytic alcohol oxidation reactions. This catalytic system displayed high activity and selectivity toward alcohols in mild conditions. The catalyst was reused five times without any significant loss of catalytic activity.