Ionic Liquid Mediated Graphene-based Pd Nanocomposites for Coupling Reactions

Abstract

Aims: In search of a ligand-free, recyclable, selective, and stable catalytic system, we engineered both Pd/GO and Pd/rGO composites and tested them as catalysts for Heck and Suzuki reactions in [bmim] NTf2 ionic liquid medium. Background: Various reports and reviews have been published on exploring the application of ionic liquids as a reaction medium for different organic transformations. Recently, graphene-supported Pt nanoparticles have immobilized with the 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene bis(trifluoromethylsulfonyl) imide ionic liquid [MTBD][bmsi] and further tested to study the oxygen reduction reaction. Surprisingly, [MTBD][bmsi] immobilized system was found highly active towards electrocatalytic reaction. Objective: In various reports, palladium nanoparticles were immobilized with graphene oxide (GO) or with reduced graphene oxide (rGO), and these two types of graphene were further tested as a catalyst for different coupling reactions such as Suzuki-Miyaura, Heck, and Suzuki reaction. Both Pd/- GO and Pd/rGO were found attractive concerning catalyst specific property, i.e., high surface area, and because of that, graphene immobilized palladium was found to be similar to other commercially available palladium catalysts (e.g., Pd on charcoal), but collectively, both hybrid materials (Pd/- GO and Pd/rGO) possess various drawbacks, like high catalyst loading, catalyst leaching (via agglomeration of Pd metals into the clusters) during the recycling test (especially in case of Pd/GO), limited substrate scope, and the requirement of polar solvents, etc. Methods: All the chemicals were purchased from Sigma Aldrich, Acros, or Fluka. NMR spectra were recorded on a standard Bruker 300WB spectrometer with an Avance console at 300 and 75 MHz for 1H and 13C NMR, respectively. Pd/O and Pd/rGO were synthesized as per the reported procedure. The residue was purified by flash chromatography (FC) with hexane/ethyl acetate. The detailed 1H and 13C NMR of each Heck and Suzuki reaction product were found similar to the reported analytical data. 1-butyl-3-methylimidazolium bis (trifluoromethyl sulfonyl) imide ([bmim]NTf2) was synthesized as per the reported procedure. Results and Conclusion: We have successfully developed a highly efficient ligand-free method for Heck and Suzuki reaction, using Pd/rGO catalysts in an ionic liquid medium which afforded the coupling products with excellent yield. One of the major advantages of the proposed protocol is that the catalyst system can be easily re-usable without the loss of catalytic activity, thereby multiplying catalyst turnover. Another advantage is that the reaction proceeds without phosphine ligands, which are expensive, toxic, and contaminants of the product. The green nature of ionic liquid and the simplicity of its operation make the present Heck and Suzuki reactions more attractive.