Magnetically recoverable nickel-palladium alloy nanocatalysts for direct C–H arylation reactions

Abstract

Novel magnetically recoverable nanocatalyst comprising nickel-palladium (NiPd) alloy nanoparticles (NPs) supported on reduced graphene oxide (rGO) modified with cobalt ferrite (CoFe₂O₄) NPs was fabricated for the direct C–H arylation of imidazopyridine, imidazole, indolizine and furan with aryl halides. To prepare the presented catalyst, rGO nanosheets were first modified with as-synthesized CoFe₂O₄ NPs and then the obtained CoFe₂O₄-rGO nanocomposites served as a support material for the synthesis of bimetallic NiPd alloy NPs at various compositions. The obtained CoFe₂O₄-rGO/NiPd nanocatalysts were characterized by many advanced analytical techniques including TEM, STEM-EDS, XRD, XPS, and ICP-MS. Next, to optimize the reaction conditions, CoFe₂O₄-rGO/NiPd nanocatalysts with different alloy compositions and their monometallic counterparts (CoFe₂O₄-rGO/Ni and CoFe₂O₄-rGO/Pd) were initially tested in the direct C–H arylation of imidazopyridine with bromobenzene. Among all tested nanocatalysts under the optimum reaction conditions, CoFe₂O₄-rGO/Ni₂₀Pd₈₀ showed the best catalytic activity in terms of the isolated product yields. The C–H arylation reactions were studied over a broad substrate scope (35 examples from 36 substrates) and gave the related biaryl products in good to excellent yields. Besides a broad substrate scope, the late-stage C–H arylation of zolimidine, a gastroprotective drug, was realized under the optimized reaction conditions. Moreover, the CoFe₂O₄-rGO/Ni₂₀Pd₈₀ nanocatalysts were recovered from the reaction medium using a simple magnet and reused in the C–H arylation reactions up to five consecutive runs without a significant drop in the product yield. This study shows that magnetically recoverable Pd nanoalloys are promising heterogeneous catalysts to be used in sustainable C–H functionalization reactions.