RESUMEN
The bottom-up synthesis of plasmonic NHC@CuNPs from common starting reagents, via the formation of the synthetically accessible NHC-Cu(I)-Br complex and its reduction by NH3·BH3 is reported. The resulting NHC@CuNPs have been characterized in detail by XPS, TEM and NMR spectroscopy. The stability of NHC@CuNPs was investigated under both inert and ambient conditions using UV-Vis analysis. While the NHC@CuNPs are stable under inert conditions for an extended period of time, the NPs oxidize under air to form CuxO with concomitant release of the stabilizing NHC ligand.
RESUMEN
Highly active and selective heterogeneous catalysis driven by metallic nanoparticles relies on a high degree of stabilization of such nanomaterials facilitated by strong surface ligands or deposition on solid supports. In order to tackle these challenges, N-heterocyclic carbene stabilized gold nanoparticles (NHC@AuNPs) emerged as promising heterogeneous catalysts. Despite the high degree of stabilization obtained by NHCs as surface ligands, NHC@AuNPs still need to be loaded on support structures to obtain easily recyclable and reliable heterogeneous catalysts. Therefore, the combination of properties obtained by NHCs and support structures as NHC bearing "functional supports" for the stabilization of AuNPs is desirable. Here, we report the synthesis of hyper-crosslinked polymers containing benzimidazolium as NHC precursors to stabilize AuNPs. Following the successful synthesis of hyper-crosslinked polymers (HCP), a two-step procedure was developed to obtain HCP·NHC@AuNPs. Detailed characterization not only revealed the successful NHC formation but also proved that the NHC functions as a stabilizer to the AuNPs in the porous polymer network. Finally, HCP·NHC@AuNPs were evaluated in the catalytic decomposition of 4-nitrophenol. In batch reactions, a conversion of greater than 99% could be achieved in as little as 90 s. To further evaluate the catalytic capability of HCP·NHC@AuNP, the catalytic decomposition of 4-nitrophenol was also performed in a flow setup. Here the catalyst not only showed excellent catalytic conversion but also exceptional recyclability while maintaining the catalytic performance.
RESUMEN
Solid supported or ligand capped gold nanomaterials (AuNMs) emerged as versatile and recyclable heterogeneous catalysts for a broad variety of conversions in the ongoing catalytic 'gold rush'. Existing at the border of homogeneous and heterogeneous catalysis, AuNMs offer the potential to merge high catalytic activity with significant substrate selectivity. Owing to their strong binding towards the surface atoms of AuMNs, NHCs offer tunable activation of surface atoms while maintaining selectivity and stability of the NM even under challenging conditions. This work summarizes well-defined catalytically active NHC capped AuNMs including spherical nanoparticles and atom-precise nanoclusters as well as the important NHC design choices towards activity and (stereo-)selectivity enhancements.
RESUMEN
N-Heterocyclic carbene-stabilized metal nanoparticles have drawn much attention over the last decade due their strong carbon metal bond. Although several reports show increased stability of such N-heterocyclic carbene-stabilized metal nanoparticles, only limited examples of water-soluble N-heterocyclic carbene stabilized metal nanoparticles are known to date. However, water dispersibility and stability in biologically relevant solvents would be a prerequisite for any biological applications. Drawing from the natural amino acid chiral pool, L-histidine was utilized for preparing chiral NHC ligands in the synthesis of water soluble NHC-stabilized gold nanoparticles. For this purpose, N-acetyl-L-histidine ethyl ester was converted into its imidazolium salt either using methyl iodide or 2-iodopropane as alkylation agent. Subsequent reaction of the imidazolium salt with [Au(SMe2)Cl] yielded the corresponding organometallic gold chloride complex. Histidine-2-ylidene stabilized gold nanoparticles were first generated in organic solvents; the histidine derived capping ligand bore ethyl ester moieties which were saponified, affording water soluble pH-responsive NHC-stabilized gold nanoparticles. These gold nanoparticles show remarkable stability in aqueous solutions, with gold nanoparticle solutions remaining stable after months of storage.
