Bidentate organochalcogen ligands (N, E; E = S/Se) as stabilizers for recyclable palladium nanoparticles and their application in Suzuki–Miyaura coupling reactions

Abstract

Chalcogeno-substituted secondary amines L1 and L2 have been synthesized by the reduction of corresponding imines. Ligands L1 and L2 have been characterized using 1H and 13C{1H} NMR spectroscopy. The Schiff base precursors used for the synthesis of L1 has also been characterized using single crystal x-ray diffraction technique. Both the ligands (L1 and L2) have been used as stabilizers for palladium nanoparticles (PdNPs) 1–4 in two different ratios of Pd:L (1:1 and 1:4). The NPs have been characterized by UV–Vis spectroscopy, powder X-ray diffraction, scanning electron microscopy (SEM), SEM-EDX, and transmission electron microscopy (TEM). The chalcogen donor site showed remarkable role in the stabilization of these nanoparticles. The Pd:L ratio in NPs has been noticed to affect the size and shape of particles and also their catalytic behavior. Size of the NPs has been found to be in a range of 2–3 nm (NPs 1); 4–5 nm (NPs 2); 3–4 nm (NPs 3); and 4–6 nm (NPs 4). When explored as catalyst, the low loading of these NPs gave significant conversions for the coupling of various aryl halides with phenylboronic acid (Suzuki–Miyaura coupling) in a short reaction time of 3 h. The highest catalytic activity has been observed for Pd NPs 1 (Pd:L ratio 1:1) due to the uniformity in the dispersion of particles. The distinct advantage associated with these NPs (1–4) is that they retain catalytic activity after the reaction and are recyclable up to three times. Attempts were made to gain mechanistic insights of catalysis and it was found that both homogeneous and heterogeneous catalytic processes contribute to the catalysis.