CTAB-assisted synthesis of reduced graphene oxide supported Pd nanoparticles (Pd@rGO) as a sustainable heterogeneous catalyst for C-2 arylation of indoles with arylboronic acids

Abstract

A series of Pd nanoparticles (NPs) incorporated in reduced graphene oxide (rGO), viz. Pd@rGO0.16, Pd@rGO0.32, Pd@rGO0.48, and Pd@rGO1 were synthesized employing water as a solvent. The subscript values correspond to the millimoles of cetyltrimethylammonium bromide (CTAB) utilized in the synthesis process. The quantity of CTAB was varied to finely tune both the morphology and size of the resulting nanoclusters. Transmission Electron Microscopy (TEM) analysis indicated that the average particle sizes of Pd@rGO0.16 and Pd@rGO0.32 fall in the range of 4.5–5.0 nm and 20–25 nm, respectively. On the other hand, particles were found to be agglomerated in Pd@rGO0.48 and Pd@rGO1. The Pd@rGO0.16 composite was exhaustively characterized by TEM, Scanning Electron Microcopy-Energy Dispersive X-ray analysis (SEM-EDX), powder X-ray diffraction(XRD), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements. ICP-AES analysis of Pd@rGO0.16 indicated that 0.01 g of Pd@rGO0.16 contains 0.09 mol% Pd. The catalytic potentiality of these NPs was investigated for direct C(sp[1])-H bond activation of various indoles with aryl boronic acids. Among the four composites, Pd@rGO0.16 exhibited the best activity for the abovementioned organic transformation. Different indoles with diverse electronic groups underwent coupling with aryl boronic acids giving up to 86 % product yield. It was retrievable for up to five consecutive catalytic cycles without compromising its catalytic activity.