Palladium-scavenging self-assembled hybrid hydrogels - reusable highly-active green catalysts for Suzuki-Miyaura cross-coupling reactions.

Petr Slavík,David K Smith,Dustin W Kurka

doi:10.1039/c8sc04561e

Abstract

A hybrid hydrogel based on 1,3:2,4-dibenzylidene sorbitol (DBS) modified with acyl hydrazides combined with agarose was used for in situ reduction and binding of palladium from aqueous mixtures without the need for an external reducing agent. Palladium uptake was monitored and the formation of Pd nanoparticles (PdNPs) trapped within the gel and located close to the nanofibres was confirmed. This gel effectively scavenges palladium from solution to concentrations < 0.04 ppm - well below the recommended limits for pharmaceutical products. The resulting hybrid hydrogel with embedded PdNPs was used as a catalyst for Suzuki-Miyaura cross-coupling reactions. The gel network stabilises PdNPs, preventing aggregation/leaching and giving excellent catalytic lifetimes. The gel acts as a simple reaction dosing form, being simply added to reactions performed in green solvents in air. Once reactions are complete, the gel can be simply removed, recycled and reused (>10 times). Reactions were purified by simple washing protocols, and leaching of Pd from the gels is limited (<1 ppm). The gels were also used in flow-through mode, giving efficient, rapid reactions, with easy work-up. These catalytic gels combine advantages of homogeneous and heterogeneous catalysts - they are solvent compatible with the reaction taking place in a solution-like environment, while the solid-like gel network enables catalyst recycling. In summary, these hydrogels scavenge 'waste' palladium and convert it into gel 'wealth' capable of efficient, environmentally-friendly Suzuki-Miyaura catalysis.

Highlights

Supramolecular gels are colloidal so materials based on the self-assembly of carefully-designed low-molecular-weight gelators (LMWGs) into a nanoscale solid-like network that spans a liquid-like phase
From the resulting concentration of Pd2+ in solution we could determine the amount of Pd loaded into the gel, which was typically 7–8 mmol. This implies that one equivalent of dibenzylidene sorbitol (DBS)-CONHNH2 is capable of reducing two equivalents of palladium, suggesting each acylhydrazide is responsible for the reduction of one equivalent of palladium
We have shown that DBS-CONHNH2 is capable of converting waste-to-wealth by scavenging palladium from solution, converting it in situ and without external reducing agent into nanoparticle form, and that the resulting gels have the capacity to catalyse Suzuki–Miyaura cross-coupling reactions

Summary

Introduction

Supramolecular gels are colloidal so materials based on the self-assembly of carefully-designed low-molecular-weight gelators (LMWGs) into a nanoscale solid-like network that spans a liquid-like phase.1 They have applications based on their rheological properties, and have transformative potential in high-tech applications.2 Given that less than 1% of additive is typically able to immobilise bulk solvent, supramolecular gels are highly solvent compatible, porous materials, and small molecules can rapidly diffuse within them. To incorporate PdNPs inside the gel, blocks of the hybrid hydrogel (formed from 2.00 mg of DBS-CONHNH2, 2.50 mg of agarose and 0.55 mL of deionised water) were immersed in an aqueous solution of PdCl2 (5 mM) and allowed to stand for 48 hours at room temperature (Fig. 2c).

Results

Conclusion