Magnetic nanoparticles supported bio-stabilised palladium catalyst for the copper-free sonogashira reaction

Abstract

Palladium mediated catalysis has achieved an impressive place in numerous commercial chemical processes. In particular, due to its high surface area to volume ratio, palladium nanoparticles (PdNPs) show high reactivity that makes them a powerful catalyst for many organic transformations. Green synthesis of PdNPs employing plant extract has been suggested as eco-friendly alternatives to chemical and physical methods. In this research, the synthesis of PdNPs using Artocarpus altilis aqueous leaf extract was investigated. The biomolecules present in the Artocarpus altilis leaf extract are believed to act as reducing and as capping agent for the formation of PdNPs. Effect of reaction time, metal ion concentration, volume of leaf extract and pH of the extract on the formation of PdNPs were investigated and monitored using UV-vis spectroscopic analysis. The optimised conditions were used in the synthesis of PdNPs supported on the amine functionalised silica-coated magnetite nanoparticles. The use of magnetite as catalyst support is attractive since magnetic separation has emerged as a robust, highly efficient, and rapid catalyst separation tool. Meanwhile, ligand assisted method employing 3-(2-aminoethylamino)propyl trimethoxysilane (AEAPTS) covalently anchored the PdNPs thus controlled the PdNPs size and prevented agglomeration. The bio-stabilised PdNPs supported on the amine functionalised silica-coated magnetite (Fe3O4-SiO2-AEAPTS-PdNPs) catalyst was characterised using Fourier transform infrared spectroscopy (FTIR), CHN analysis, X-ray diffraction (XRD), flame atomic absorption spectrophotometry (FAAS), high resolution transmission electron microscopy-energy dispersive X-ray spectroscopy (HRTEM-EDX), vibrating-sample magnetometer (VSM), zeta potential and X-ray photoelectron spectroscopy (XPS) analyses. The Fe3O4-SiO2-AEAPTS-PdNPs catalyst was then tested in the copper-free Sonogashira reaction under aerobic condition in water. Effect of base, catalyst amount, and temperature on the reaction conversion was investigated and monitored using gas chromatography-flame ionisation detection (GC-FID). The optimisation reaction between phenylacetylene and iodobenzene to yield diphenylacetylene successfully gave 90% conversion using 0.2 mol% of Fe3O4-SiO2-AEAPTS-PdNPs catalyst with triethylamine as base at 60oC for 24 h. Fe3O4-SiO2-AEAPTS-PdNPs showed an impressive catalytic performance with turnover number of 450 and turnover frequency of 18.8 h-1. In addition, the recycle test result showed that the catalyst can be used up to four cycles without significant loss of catalytic activity. Fe3O4-SiO2-AEAPTS-PdNPs catalyst was further examined in the reaction between phenylacetylene and less reactive aryl halides which reacted well at 80oC and gave desired products with good yields. The coupling of bromobenzene and phenylacetylene gave good conversion of 49% while activated bromobenzene such as 4-bromoacetophenone and 1-bromo-4-nitrobenzene, bearing electron-withdrawing group at their para-positions gave better conversion of 53% and 56%, respectively. All crude products were isolated and purified using column chromatography and were characterised using gas chromatography-mass spectrometry (GC-MS) and FTIR, 1H nuclear magnetic resonance (1H-NMR) and 13C nuclear magnetic resonance (13C-NMR) spectroscopic analyses.