Monomeric and Polymeric Mesoionic N-Heterocyclic Carbene-Tethered Silver Nanoparticles: Synthesis, Stability, and Catalytic Activity

Abstract

In recent years, N-heterocyclic carbenes (NHCs) have garnered significant attention as promising alternatives to thiols to stabilize metallic nanoparticles and planar surfaces. While most studies thus far have focused on NHC-functionalized gold nanoparticles (AuNPs), as an ideal platform to investigate the role of NHCs in stabilizing such nanoparticles, their ability to protect more unstable coinage metal nanoparticles, such as silver nanoparticles (AgNPs), has been largely overlooked. This is despite the fact that AgNPs possess a much more sensitive optical response that, upon their enhanced stability, can broaden their scope of application in various fields, including nanomedicine and catalysis. In this study, the synthesis and use of monomeric and polymeric mesoionic NHC–Ag(I) complexes as precursors to mono- and multidentate NHC-tethered AgNPs are reported. The polymeric analog was obtained by first synthesizing a polymer, containing 1,2,3-triazole repeat units, employing the copper-catalyzed alkyne–azide cycloaddition click polymerization of monomers containing diazide- and dialkyne functional groups. Subsequent quaternization of the triazole moieties and Ag insertion yielded the target NHC–Ag-containing polymer. Using this polymer as well as its monomeric analog as substrates, AgNPs with either catenated networks of NHCs or monomeric NHCs were fabricated by their reduction using borane–tert-butylamine complex. Our stability studies demonstrate that while monomeric NHCs impart some degree of stability to AgNPs, particularly at elevated temperatures in aqueous as well as organic medium, their polymeric analogs further enhance their stability in acidic environment (pH = 2) and against glutathione (3 mM), as an example of a biologically relevant thiol, in aqueous media. To highlight the application of these NHC-functionalized AgNPs in catalysis, we explore the aqueous phase reduction of methyl orange and 4-nitrophenol.

Insight into the nature of carbon–metal bonding for N-heterocyclic carbenes in gold/silver complexes and nanoparticles using DFT-correlated Raman spectroscopy: strong evidence for π-backbonding

“…This type of carbene-sandwich structure, contrary to the mono-carbene gold complex C 12 NHC-AuBr , has been noted previously in the literature. 31 The mono-carbene bound silver complex, C 12 NHC-AgBr , was not observed. According to DFT calculations, the formation of a dimer instead of a monomer for the studied complex appears more energetically favorable in accordance with the experimental observation (Fig.…”

Mesoionic carbene-based self-assembled monolayers on gold

“…36 Mesoionic carbenes (MICs) 37,38 are superior s-donors compared to NHCs and are powerful ligands in organometallic chemistry. However, apart from the work of the Nazemi group highlighting the use of MICs on nanoparticles, 39,40 MICs are absent in the eld of surface chemistry.…”

“…36 Mesoionic carbenes (MICs) 37,38 are superior σ-donors compared to NHCs and are powerful ligands in organometallic chemistry. However, apart from the work of the Nazemi group highlighting the use of MICs on nanoparticles, 39,40 MICs are absent in the field of surface chemistry. Given the lack of experimental studies with this important class of carbene, and the possibility for enhanced properties of the resulting SAMs, we set out to explore these structural variants as ligands for self-assembled monolayers on gold.…”

Cyclic(alkyl)(amino)carbene-Stabilized Gold Nanoparticles for Selective CO2 Reduction

“…The NHC-functionalized AuNP is a milestone discovery from the main group chemistry perspective . Apart from imidazolium- and benzimidazolium-based NHCs, acyclic and mesoionic − NHCs have also found their use as surface capping ligands very recently. The stabilization of AuNPs by NHCs can be best rationalized using the hard–soft Lewis acid–base concept .…”