Efficient catalytic activation of Suzuki-Miyaura C-C coupling reactions with recyclable palladium nanoparticles tailored with sterically demanding di-n-alkyl sulfides

摘要

n-Bromodocosane reacts with Na2S, generated in situ by the reduction of elemental sulfur with NaBH4, to give n-didocosyl sulfide (L1), which acts as a protector for palladium nanoparticles (2-7) that are prepared using different palladium precursors in the presence of L1 (Pd : L1 ratio 1 : 2 and 4 : 1). The NPs have been characterized with powder X-ray diffraction, SEM, EDX, UV-vis spectroscopy and HRTEM. The size (nm) ranges for the majority of spherical NPs 2-7 are similar to 18-19, 4-5, 5-7, 4-6, 7-9 and 4-6 respectively. The precursor of palladium affects the size, shape and dispersion of the NPs. When [Pd(CH3CN)(2)Cl-2]/Na2PdCl4 was used as a precursor, uniformly dispersed NPs of narrow size range were obtained. L1 and its complex [Pd(L1)(2)Cl-2] (1) have also been synthesized by the reaction of Na2PdCl4 with L1 and characterized with H-1 and C-13{H-1} NMR spectroscopy. The NPs show good catalytic activity for the Suzuki-Miyaura coupling (SMC) of various aryl chlorides/bromides with phenylboronic acid at low catalyst loading (0.1-0.5 mol% of Pd). The conversion is good for some aryl halides in a short reaction time of the order 1-2 h. Among 2-7, the highest activity is observed for Pd NPs obtained from Na2PdCl4, which is probably due to uniformity in their size and dispersion. The distinct advantage of NPs 2-7 is that they can be separated and reused at least up to five times. The complex 1, equivalent to 0.001 mol% Pd, is efficient for the SMC of some aryl halides, as good conversion into coupled products has been observed. Two phase tests, conducted for 1 and 3, suggest the contribution of both homogeneous and heterogeneous catalytic pathways in overall catalysis.