Tuning C-N Ullmann coupling with anion-exchanged ionic liquid-functionalized nanoparticle catalysts

Abstract

Modern organic chemistry demands efficient and sustainable synthetic methods. Copper-catalyzed Ullmann coupling reactions, adept at constructing biaryl and aryl-heteroaryl motifs, have gained significant traction. However, traditional methods suffer from limitations. Nanoparticle catalysts, particularly copper nanoparticles (CuNPs), offer improved efficiency, but their performance is hampered by agglomeration and leaching. Hybrid supports incorporating ionic liquids (ILs) address this issue by providing a polar environment stabilizing CuNPs and enhancing catalytic activity and selectivity. This study investigates the impact of anion exchange in ionic liquids on the catalytic performance of C-N Ullmann couplings. Anion exchange modifies interionic interactions, hydrogen bonding, and dipolar properties, ultimately influencing the reaction's outcome. We synthesized CuNPs impregnated in silica supports functionalized with a triazolium-based ionic liquid possessing different anions (BF4−, PF6−, NTf2−) to elucidate the effect of anion exchange.Characterization techniques like FT-IR, XPS, TGA, and XPS confirmed successfully incorporating the triazole group and varying anions onto the material. TEM analysis showed the presence of CuNPs (4.5–5.7 nm) impregnated on the triazole functionalized silica supports.Catalytic studies yielded fascinating insights into the impact of different anions on conversion and selectivity. Iodide (I−) promoted higher conversion (100%), while tetrafluoroborate (BF4−) offered superior selectivity (57%). Computational studies provided valuable explanations, shedding light on the reactivity of the nanostructures and the interaction mechanism between substrates and the nanocatalyst. The theoretical results suggest that coordinative covalent bonds and highly polarized interactions govern the substrate interaction, with the aniline path achieving higher stabilization.Furthermore, the choice of anion influences the reactivity, substrate stabilization, and spin density distribution. These properties directly correlate with the conversion and selectivity observed in the catalytic studies.