Palladium(II)@Zirconium-Based Mixed-Linker Metal-Organic Frameworks as Highly Efficient and Recyclable Catalysts for Suzuki and Heck Cross-Coupling Reactions

Two series of amine-functionalized Zr-based mixed-linker metal-organic frameworks (MOFs; UiO-66-Mix and UiO-67-Mix) have been synthesized with different ratios of 1,4-benzenedicarboxylate/2-amino-1,4-benzenedicarboxylate and 4,4-biphenyldicarboxylic acid/2-amino-biphenyl-4,4-dicarboxylic acid incorporated into their structures. The pendant amino groups were postmodified by the condensation reaction with pyridine-2-carboxaldehyde, and Pd metal centers were anchored onto the MOFs by coordination to the pyridylimine moieties. The two series of heterogeneous Pd catalysts (UiO-66-Mix-PI-Pd and UiO-67-Mix-PI-Pd) were utilized for the Suzuki and Heck cross-coupling reactions in ethanol/water under mild reaction conditions and they showed a remarkably high catalytic activity. The porosity of the MOFs and the density of the metal binding sites of the Pd catalysts had great influence on the Suzuki and Heck cross-coupling reactions. The best results were achieved with a very low Pd loading (0.054mol%), and the use of UiO-67-3-PI-Pd catalyst led to the complete conversion of reaction substrates. Moreover, the catalysts were readily recovered and recycled in at least 10cycles without significant leaching or loss of catalytic activity. The catalysts were generally applicable for different reaction substrates with various substituents in both Suzuki and Heck reactions.