Palladium(II)-Schiff base complex supported on multi-walled carbon nanotubes: A heterogeneous and reusable catalyst in the Suzuki- Miyaura and copper-free Sonogashira-Hagihara reactions

The palladium-catalyzed coupling of aryl halides with aryl boronic acids (Suzuki-Miyaura coupling) or terminal alkynes (Sonogashira-Hagihara coupling) reactions have shown to have widespread applications in the synthesis of natural products, biologically active molecules, and materials science. Many catalytic systems have been developed for the Suzuki-Miyaura and Sonogashira-Hagihara cross-coupling reactions using different palladium catalysts such as Pd(PPh₃)₄, PdCl₂(PPh₃)₂, and PdCl₂(CH₃CN)₂. However, phosphine ligands used in these reactions are sensitive to air oxidation. Moreover, many palladium catalysts used in the Suzuki-Miyaura and Sonogashira-Hagihara reactions are homogeneous; these catalysts are impossible to be recovered. In recent years, great efforts have been devoted to the introduction and application of effective heterogeneous catalysts. Studies on the isolation, characterization and catalytic activities of functionalized carbon nanotubes (CNTs) have received particular attention during the last decade owing to their specific catalytic applications compared to homogeneous complexes. Schiff bases, which are an important class of ligands with extensive applications in different fields, also showed excellent catalytic activity when grafted on CNTs. In this work, we have presented a multi-walled carbon nanotubes anchored Pd(II)-Schiff base complex as a heterogeneous catalyst for Suzuki-Miyaura cross-coupling reactions and copper- and phosphorous-free Sonogashira-Hagihara cross-coupling reactions in aqueous media under aerobic condition. The catalyst was air-stable and could be recovered and reused for four consecutive runs in Suzuki-Miyaura reaction and three successive runs in Sonogashira-Hagihara reaction.