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

Advancements in silver-based nanocatalysts for organic transformations and other applications: a comprehensive review (2019-2024)

Over time, nanocomposites have revolutionized materials science, offering numerous applications in fields such as catalysis, environmental purification and treatment, biomedicine and various industries. Among these, silver-based nanocomposites are particularly notable for their remarkable stability, reusability, biocompatibility, and multifunctional medicinal properties. Hence, we present a comprehensive summary of recent developments (2019-2024) in silver-based nanomaterials, focusing on their applications across multiple domains, including catalytic organic transformations, biomedical uses, environmental remediation, and industrial sectors such as food packaging, agriculture and textiles. By highlighting recent advancements and emerging trends, we aim to provide a thorough understanding of the role of silver-based nanocomposites in contemporary science and technology, emphasizing their potential to drive innovation across diverse disciplines.

Self-Assembled Monolayer of N-Heterocyclic Carbene as a Primer in a Dual-Layer Coating for Corrosion Protection on Iron

The development of highly stable coatings on iron is essential for mitigating corrosion formation. Herein, it is demonstrated that a self-assembled monolayer of N-Heterocyclic Carbene (NHC) can be electrodeposited on iron foil and function as a binder for a secondary, crosslinked polymer network coating. The dual layer coating, constructed of a monolayer of NHCs and a polymer film, as a primary and a secondary coating, respectively, effectively preventes corrosion formation with a protective efficiency of 99.6 ± 0.2 %, as determined by polarization measurements in 3.5 wt.% NaCl solution. Spectroscopic analysis identified the formation of a chemical interaction between the NHC monolayer and the polymer film. The strong anchoring of NHC to iron along with its chemical interaction with the polymer film induced high stability and durability of the dual-layer coating to effectively protect the coated iron from corrosion formation.

Self-Assembled Monolayers of Triazolylidenes on Gold and Mixed Gold/Dielectric Substrates

N-Heterocyclic carbenes (NHCs) have emerged as valuable ligands for surface chemistry. They can be used to prepare robust self-assembled monolayers (SAMs) for a variety of applications, including as small-molecule inhibitors (SMIs) for metal surfaces in the fabrication of next-generation integrated circuits with angstrom precision. However, little work has been performed to assess the effect of structural and electronic modifications to the basic NHC structure. Herein, we report the design and deposition of a series of 1,2,3-triazolylidene (Tz)-type carbenes on gold (Au) and Au/SiO2 patterned substrates. Triazolylidenes are an important class of stable carbenes that can be prepared with ease by using click chemistry. In this work, we studied the selective deposition of 1,2,3-triazolium hydrogen carbonate salts. The thermal properties of these precursors were measured and shown to be appropriate for either solution or vapor phase deposition. Tz-SAM stability was studied by time-of-flight secondary-ion mass spectrometry (ToF-SIMS) of Tz SAMs before and after exposure to various conditions, leading to the conclusion that Tz SAMs have thermal stabilities greater than that of NHC SAMs reported to date. Tz SAMs were analyzed by using X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS) and contact angle measurements. High selectivity for deposition on metal regions over dielectric regions on patterned Au/SiO2 substrates enabled the use of Tzs as an entirely new class of SMIs on preventing ZnO deposition, providing considerable potential utility in microelectronics fabrication methods. Structure-property relationships were studied and provided key insight into the effectiveness of the SAM as a blocking agent.

Poly(N-Heterocyclic Carbene)-Capped Alloy and Core-Shell AuAg Bimetallic Nanoparticles

N-Heterocyclic carbene (NHC)-stabilized metal nanoparticles (NPs) have recently attracted considerable attention. While most efforts in the field have been devoted to the development of NHC-tethered monometallic NPs and enhancing their stabilities under various conditions, their bimetallic counterparts are rare in the literature. Herein, we demonstrate that the covalent immobilization of Au and Ag atoms on polymerized NHCs is a powerful method to access bimetallic AuAg NPs. In addition, we show that while AuAg alloy NPs are often obtained via this method, the use of bimetallic polymeric substrates with lower Ag content, relative to Au, results in the formation of core-shell NPs with Au core and Ag shell. Application of these nanomaterials for oxygen reduction reaction is demonstrated with all materials exhibiting electrocatalytic activity. This work demonstrates for the first time that while bimetallic poly(NHC-metal)s are viable substrates to access NHC-stabilized bimetallic NPs, careful adjustment of metal content in the polymeric substrates can finetune the microstructure of the resulting NPs, i.e. alloy vs. core-shell.

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

N-Heterocyclic carbenes (NHCs) have emerged as promising ligands for stabilizing metallic complexes, nanoclusters, nanoparticles (NPs) and surfaces. The carbon-metal bond between NHCs and metal atoms plays a crucial role in determining the resulting material's stability, reactivity, function, and electronic properties. Using Raman spectroscopy coupled with density functional theory calculations, we investigate the nature of carbon-metal bonding in NHC-silver and NHC-gold complexes as well as their corresponding NPs. While low wavenumbers are inaccessible to standard infrared spectroscopy, Raman detection reveals previously unreported NHC-Au/Ag bond-stretching vibrations between 154-196 cm-1. The computationally efficient r2SCAN-3c method allows an excellent correlation between experimental and predicted Raman spectra which helps calibrate an accurate description of NHC-metal bonding. While π-backbonding should stabilize the NHC-metal bond, conflicting reports for the presence and absence of π-backbonding are seen in the literature. This debate led us to further investigate experimental and theoretical results to ultimately confirm and quantify the presence of π-backbonding in these systems. Experimentally, an observed decrease in the NHC's CN stretching due to the population of the π* orbital is a good indication for the presence of π-backbonding. Using energy decomposition analysis - natural orbitals for chemical valence (EDA-NOCV), our calculations concur and quantify π-backbonding in these NHC-bound complexes and NPs. Surprisingly, we observe that NPs are less stabilized by π-backbonding compared to their respective complexes-a result that partially explains the weaker NHC-NP bond. The protocol described herein will help optimize metal-carbon bonding in NHC-stabilized metal complexes, nanoparticles and surfaces.

Mesoionic carbene-based self-assembled monolayers on gold

N-Heterocyclic carbenes (NHC) have been widely studied as ligands for surface chemistry, and have shown advantages compared to existing ligands (e.g. thiols). Herein, we introduce mesoionic carbenes (MICs) as a new type of surface ligand. MICs exhibit higher σ-donor ability compared to typical NHCs, yet they have received little attention in the area of surface chemistry. The synthesis of MICs derived from imidazo[1,2-a]pyridine was established and fully characterized by spectroscopic methods. The self-assembly of these MICs on gold was analyzed by X-ray photoelectron spectroscopy (XPS). Additionally, XPS was used to compare bonding ability in MICs compared to the typical NHCs. These results show that MIC overlayers on gold are robust, resistant to replacement by NHCs, and may be superior to NHCs for applications that require even greater levels of robustness.

Self-Assembled Monolayers of N-Heterocyclic Olefins on Au(111)

Self-assembled monolayers (SAMs) of N-heterocyclic olefins (NHOs) have been prepared on Au(111) and their thermal stability, adsorption geometry, and molecular order were characterized by X-ray photoelectron spectroscopy, polarized X-ray absorption spectroscopy, scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. The strong σ-bond character of NHO anchoring to Au induced high geometrical flexibility that enabled a flat-lying adsorption geometry via coordination to a gold adatom. The flat-lying adsorption geometry was utilized to further increase the surface interaction of the NHO monolayer by backbone functionalization with methyl groups that induced high thermal stability and a large impact on work-function values, which outperformed that of N-heterocyclic carbenes. STM measurements, supported by DFT modeling, identified that the NHOs were self-assembled in dimers, trimers, and tetramers constructed of two, three, and four complexes of NHO-Au-adatom. This self-assembly pattern was correlated to strong NHO-Au interactions and steric hindrance between adsorbates, demonstrating the crucial influence of the carbon-metal σ-bond on monolayer properties.