Palladium Nanoparticles within Core-Cross-Linked Polymer Gels for Suzuki Coupling Reactions: from Monomers to Ready-to-Use Catalysts in Two-Steps

Abstract

This study shows that polymer nanogels (well-defined cross-linked nanoparticles behaving as highly hydrated unimolecular objects) are suitable capping agents for palladium nanoparticle catalyst systems. In spite of the structurally complex nature of the catalysts, it was possible to go from acrylate-based monomers to ready-to-use and recyclable catalysts in only two-steps. First, nanogels were synthesized by activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) method with sequential addition of monomers. Then, in the second step, palladium acetate was reduced to obtain ready-to-use catalysts (ca. 25 nm nanogel structures containing ca. 7 nm palladium nanoparticles). The catalytic activity was confirmed in p-nitrophenol (Nip) reduction and Suzuki cross-coupling reactions. The observed rate constants (k obs ) for Nip reduction were in the range of 0.8-10.0 × 10 -2 s -1 depending on the polymeric capping agent structure, with the highest value being found for nanogel-based systems. These composite catalysts also mediated the cross-coupling Suzuki reaction providing the expected products in quantitative yields under relatively mild conditions after 4 h at 50 °C. The simplicity of catalyst preparation protocol that ensures excellent activity and the low concentration of polymer applied during the synthesis are a step forward in terms of environmental and economic prospects.

Role of Polymer Structures in Catalysis by Transition Metal and Metal Oxide Nanoparticle Composites

“…In the other work, several pHresponsive hydrogels have been used for syntheses of Pd NPs for testing in the 4-NP reduction and Suzuki coupling, but the catalytic data were unremarkable. 189 On the other hand, PNIPAM-based core−shell microgel particles with a cross-linked core and a linear phase transition region and containing Ag NPs showed excellent properties in the 4-NP reduction. 190 The rate constants were strongly dependent on temperature which is a powerful stimulus for PNIPAM and on the microgel particle architecture.…”

“…However, the authors did not explore different temperatures which would be reasonable for temperature sensitive microgels. In the other work, several pH-responsive hydrogels have been used for syntheses of Pd NPs for testing in the 4-NP reduction and Suzuki coupling, but the catalytic data were unremarkable …”