Supported Au Nanoparticles with N-Heterocyclic Carbene Ligands as Active and Stable Heterogeneous Catalysts for Lactonization

Revealing the Mechanism of Combining Best Properties of Homogeneous and Heterogeneous Catalysis in Hybrid Pd/NHC Systems

The formation of transient hybrid nanoscale metal species from homogeneous molecular precatalysts has been demonstrated by in situ NMR studies of catalytic reactions involving transition metals with N-heterocyclic carbene ligands (M/NHC). These hybrid structures provide benefits of both molecular complexes and nanoparticles, enhancing the activity, selectivity, flexibility, and regulation of active species. However, they are challenging to identify experimentally due to the unsuitability of standard methods used for homogeneous or heterogeneous catalysis. Utilizing a sophisticated solid-state NMR technique, we provide evidence for the formation of NHC-ligated catalytically active Pd nanoparticles (PdNPs) from Pd/NHC complexes during catalysis. The coordination of NHCs via C(NHC)-Pd bonding to the metal surface was first confirmed by observing the Knight shift in the 13C NMR spectrum of the frozen reaction mixture. Computational modeling revealed that as little as few NHC ligands are sufficient for complete ligation of the surface of the formed PdNPs. Catalytic experiments combined with in situ NMR studies confirmed the significant effect of surface covalently bound NHC ligands on the catalytic properties of the PdNPs formed by decomposition of the Pd/NHC complexes. This observation shows the crucial influence of NHC ligands on the activity and stability of nanoparticulate catalytic systems.

Single-gold etching at the hypercarbon atom of C-centred hexagold(I) clusters protected by chiral N-heterocyclic carbenes

Chemical etching of nano-sized metal clusters at the atomic level has a high potential for creating metal number-specific structures and functions that are difficult to achieve with bottom-up synthesis methods. In particular, precisely etching metal atoms one by one from nonmetallic element-centred metal clusters and elucidating the relationship between their well-defined structures, and chemical and physical properties will facilitate future materials design for metal clusters. Here we report the single-gold etching at a hypercarbon centre in gold(I) clusters. Specifically, C-centred hexagold(I) clusters protected by chiral N-heterocyclic carbenes are etched with bisphosphine to yield C-centred pentagold(I) (CAuI5) clusters. The CAuI5 clusters exhibit an unusually large bathochromic shift in luminescence, which is reproduced theoretically. The etching mechanism is experimentally and theoretically suggested to be a tandem dissociation-association-elimination pathway. Furthermore, the vacant site of the central carbon of the CAuI5 cluster can accommodate AuCl, allowing for post-functionalisation of the C-centred gold(I) clusters.

Heterogeneous Rhodium Single-Atom-Site Catalyst Enables Chemoselective Carbene N-H Bond Insertion

Transition-metal-catalyzed carbene insertion reactions of a nitrogen-hydrogen bond have emerged as robust and versatile methods for the construction of C-N bonds. While significant progress of homogeneous catalytic metal carbene N-H insertions has been achieved, the control of chemoselectivity in the field remains challenging due to the high electrophilicity of the metal carbene intermediates. Herein, we present an efficient strategy for the synthesis of a rhodium single-atom-site catalyst (Rh-SA) that incorporates a Rh atom surrounded by three nitrogen atoms and one phosphorus atom doped in a carbon support. This Rh-SA catalyst, with a catalyst loading of only 0.15 mol %, exhibited exceptional catalytic performance for heterogeneous carbene insertion with various anilines and heteroaryl amines in combination with diazo esters. Importantly, the heterogeneous catalyst selectively transformed aniline derivatives bearing multiple nucleophilic moieties into single N-H insertion isomers, while the popular homogeneous Rh2(OAc)4 catalyst produced a mixture of overfunctionalized side products. Additionally, similar selectivities for N-H bond insertion with a set of stereoelectronically diverse diazo esters were obtained, highlighting the general applicability of this heterogeneous catalysis approach. On the basis of density functional theory calculations, the observed selectivity of the Rh-SA catalyst was attributed to the insertion barriers and the accelerated proton transfer assisted by the phosphorus atom in the support. Overall, this investigation of heterogeneous metal-catalyzed carbene insertion underscores the potential of single-atom-site catalysis as a powerful and complementary tool in organic synthesis.

Molar excess of coordinating N-heterocyclic carbene ligands triggers kinetic digestion of gold nanocrystals

There has been much interest in evaluating the strength of the coordination interactions between N-heterocyclic carbene (NHC) molecules and transition metal ions, nanocolloids and surfaces. We implement a top-down core digestion test of Au nanoparticles (AuNPs) triggered by incubation with a large molar excess of poly(ethylene glycol)-appended NHC molecules, where kinetic dislodging of surface atoms and formation of NHC-Au complexes progressively take place. We characterize the structure and chemical nature of the generated PEG-NHC-Au complexes using 1D and 2D 1H-13C NMR spectroscopy, supplemented with matrix assisted laser desorption/ionization mass spectrometry, and transmission electron microscopy. We further apply the same test using thiol-modified molecules and find that though etching can be measured the kinetics are substantially slower. We discuss our findings within the classic digestion of transition metal ores and colloids induced by interactions with sodium cyanide, which provides an insight into the strength of coordination between the strong σ-donating (soft Lewis base) NHC and Au surfaces (having a soft Lewis acid character), as compared to gold-to-gold covalent binding.

Seeking a Au-C stretch on gold nanoparticles with 13C-labeled N-heterocyclic carbenes

Gold nanoparticles were functionalized with natural abundance and 13C-labeled N-heterocyclic carbenes (NHCs) to investigate the Au-C stretch. A combinatorial approach of surface enhanced Raman spectroscopy (SERS) and density-functional theory (DFT) calculations highlighted vibrational modes significantly impacted by isotopic labeling at the carbene carbon. Critically, no isotopically-impacted stretching mode showed majority Au-C character.

Immobilizing glucose oxidase on AuCu hydrogels for enhanced electrochromic biosensing

Development of highly sensitive and accurate biosensors still faces a great challenge. Herein, glucose oxidase (GOx) is efficiently immobilized on the AuCu hydrogels owing to their porous structure and interfacial interaction, demonstrating enhanced catalytic activity, satisfactory stability and recyclability. Besides, by integration of AuCu@GOx and electrochromic material of Prussian blue, a sensitive and stable biosensing platform based on the excellent electrochromic property of Prussian blue and the enhanced enzyme activity of AuCu@GOx is developed, which enables the electrochemical and visual dual-mode detection of glucose. The as-constructed biosensing platform possesses a wide linear range, and good selectivity for glucose detection with a limit of detection of 0.82 μM in visual mode and 0.84 μM in electrochemical mode. This easy-to-operate biosensing platform opens a door for the practical application of the multi-mode strategy for glucose detection.

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.

Surface Modification of ITO with N-Heterocyclic Carbene Precursors Results in Electron Selective Contacts in Organic Photovoltaic Devices

Surface modification of indium tin oxide (ITO) electrodes with organic molecules is known to tune their work function which results in higher charge carrier selectivity in corresponding organic electronic devices and hence influences the performance of organic solar cells. In recent years, N-heterocyclic carbenes (NHCs) have also been proven to be capable to modify the work function of metals and semimetals compared to the unfunctionalized surface via the formation of strong covalent bonds. In this report, we have designed and performed the modification of the ITO surface with NHC by using the zwitterionic bench stable IPr-CO2 as the NHC precursor, applied via spin coating. Upon modification, the work function of ITO electrodes was reduced significantly which resulted in electron selective contacts in corresponding organic photovoltaic devices. In addition, various characterization techniques and analytical methods are used to elucidate the nature of the bound species and the corresponding binding mechanism of the material to the ITO surface.

Carbonylative Self-Coupling of Aryl Boronic Acids Using a Confined Pd Catalyst within Melamine Dendron and Fibrous Nano-Silica: A CO Surrogate Approach

Development of heterogeneous catalysts with tunable activity and selectivity has posed a persistent challenge. This study addresses this challenge by fabricating a hybrid environment through the combination of mesoporous silica and N-rich melamine dendron via covalent grafting, allowing for controllable growth and encapsulation of Pd NPs. This catalyst presented an excellent catalytic activity for the oxidative carbonylative self-coupling of aryl boronic acids to afford symmetric biaryl ketones using N-formyl saccharin as a sustainable solid CO source and Cu as a co-catalyst.

N-Heterocyclic Carbene Boranes: Dual Reagents for the Synthesis of Gold Nanoparticles

N-Heterocyclic carbenes (NHCs) have drawn considerable interest in the field of nanomaterials chemistry as highly stabilizing ligands enabling the formation of strong and covalent carbon-metal bonds. Applied to gold nanoparticles synthesis, the most common strategy consists of the reduction of a preformed NHC-AuI complex with a large excess of a reducing agent that makes the particle size difficult to control. In this paper, we report the straightforward synthesis of NHC-coated gold nanoparticles (NHC-AuNPs) by treating a commercially available gold(I) precursor with an easy-to-synthesize NHC-BH3 reagent. The latter acts as both the reducing agent and the source of surface ligands operating under mild conditions. Mechanistic studies including NMR spectroscopy and mass spectrometry demonstrate that the reduction of gold(I) generates NHC-BH2 Cl as a by-product. This strategy gives efficient control over the nucleation and growth of gold particles by varying the NHC-borane/gold(I) ratio, allowing unparalleled particle size variation over the range of 4.9±0.9 to 10.0±2.7 nm. Our strategy also allows an unprecedented precise and controlled seeded growth of gold nanoparticles. In addition, the as-prepared NHC-AuNPs exhibit narrow size distributions without the need for extensive purification or size-selectivity techniques, and are stable over months.

NHC stabilized copper nanoparticles via reduction of a copper NHC complex

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.

Hyper crosslinked polymer supported NHC stabilized gold nanoparticles with excellent catalytic performance in flow processes

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.

Gold Nanoparticles Immobilized in Porous Aromatic Frameworks with Abundant Metal Anchoring Sites as Heterogeneous Nanocatalysts

Porous aromatic frameworks (PAFs) with rich metal coordination sites are highly effective support materials for gold nanoparticles (AuNPs), which would not only prevent AuNPs agglomeration but also facilitate mass transfer during the catalytic process. In this work, PAF-160, -161, and -162 bearing diphosphine units are synthesized via the Friedel-Crafts alkylation reaction to act as efficient platforms for AuNPs immobilization. These PAFs possess high surface areas (up to 655 m2 g-1) together with excellent stabilities, and the different linkage lengths between P centers allow more scattered and accessible sites for gold coordination. In the resultant Au-PAFs, AuNPs with uniform sizes are stabilized dispersedly. The catalytic performances of these Au-PAFs are monitored by the reduction of 4-nitrophenol (4-NP), and all materials exhibit excellent catalytic activities on the reduction of 4-NP, especially Au-PAF-162 with the apparent rate constant (kapp) up to 0.019 s-1. Additionally, the reductions of various nitroarenes with different functional groups are explored and all Au-PAFs can convert most nitroaromatic derivatives to the corresponding arylamines with high conversions of 99%, in which the reaction mechanism is also proposed. Furthermore, a continuous catalytic device with Au-PAF-160 catalyst is explored, and Au-PAF-160 can convert 1-chloro-4-nitrobenzene, 2,6-dichoronitrobenzene and 1-chloro-2,4-dinitrobenzene into the corresponding amines in sequence in the continuous flow catalytic experiments. This work has enriched the variety of porous materials for noble metal immobilization and promotes their applications in heterogeneous catalysis.

Coordination-Induced Trigger for Activity: N-Heterocyclic Carbene-Decorated Ceria Catalysts Incorporating Cr and Rh with Activity Induction by Surface Adsorption Site Control

A coordination-induced trigger for catalytic activity is proposed on an N-heterocyclic carbene (NHC)-decorated ceria catalyst incorporating Cr and Rh (ICy-r-Cr_0.19Rh_0.06CeO_z). ICy-r-Cr_0.19Rh_0.06CeO_z was prepared by grafting 1,3-dicyclohexylimidazol-2-ylidene (ICy) onto H₂-reduced Cr_0.19Rh_0.06CeO_z (r-Cr_0.19Rh_0.06CeO_z) surfaces, which went on to exhibit substantial catalytic activity for the 1,4-arylation of cyclohexenone with phenylboronic acid, whereas r-Cr_0.19Rh_0.06CeO_z without ICy was inactive. FT-IR, Rh K-edge XAFS, XPS, and photoluminescence spectroscopy showed that the ICy carbene-coordinated Rh nanoclusters were the key active species. The coordination-induced trigger for catalytic activity on the ICy-bearing Rh nanoclusters could not be attributed to electronic donation from ICy to the Rh nanoclusters. DFT calculations suggested that ICy controlled the adsorption sites of the phenyl group on the Rh nanocluster to promote the C-C bond formation of the phenyl group and cyclohexenone.

Polymer N-Heterocyclic Carbene (NHC) Ligands for Silver Nanoparticles

Polymer N-heterocyclic carbenes (NHCs) are a class of robust surface ligands to provide superior colloidal stability for metal nanoparticles (NPs) under various harsh conditions. We report a general method to prepare polymeric NHCs and demonstrate that these polymer NHC-AgNPs are stable against oxidative etching and show high peroxidase activity. We prepared three imidazolium-terminated poly(methyl methacrylate) (PMMA), polystyrene (PS), and poly(2-(2-methoxyethoxy)ethyl methacrylate) (PMEO₂MA) through atom-transfer radical polymerization with an imidazole-containing initiator. The imidazolium end group was further converted to NHC-Ag(I) in the presence of Ag₂O at room temperature. Polymer NHC-Ag(I) can transmetalate to AgNPs through ligand exchange at the interface of oil/water within 2 min. All the three polymers can modify metal NPs, such as AgNPs, Ag nanowires, and AuNPs, providing excellent thermal, oxidative, and chemical stabilities for AgNPs. As an example, in the presence of hydrogen peroxide, AgNPs modified by polymer NHCs were resistant against oxidative etching with a rate of ∼700 times slower than those grafted with thiolates. AgNPs modified by polymer NHCs also showed higher peroxidase activity, 4 times more active than those capped by citrate and polyvinylpyrrolidone (PVP) and 2 times more active than those with polymer thiolate. Our studies demonstrate a great potential of using polymer NHCs to stabilize metallic NPs for various applications.

Bottom-up Synthesis of Water-Soluble Gold Nanoparticles Stabilized by N-Heterocyclic Carbenes: From Structural Characterization to Applications

N-heterocyclic carbenes (NHCs) have become attractive ligands for functionalizing gold nanoparticle surfaces with applications ranging from catalysis to biomedicine. Despite their great potential, NHC stabilized gold colloids (NHC@AuNPs) are still scarcely explored and further efforts should be conducted to improve their design and functionalization. Here, the 'bottom-up' synthesis of two water-soluble gold nanoparticles (AuNP-1 and AuNP-2) stabilized by hydrophilic mono- and bidentate NHC ligands is reported together with their characterization by various spectroscopic and analytical methods. The NPs showed key differences likely to be due to the selected NHC ligand systems. Transmission electron microscopy (TEM) images showed small quasi-spherical and faceted NHC@AuNPs of similar particle size (ca. 2.3-2.6 nm) and narrow particle size distribution, but the colloids featured different ratios of Au(I)/Au(0) by X-ray photoelectron spectroscopy (XPS). Furthermore, the NHC@AuNPs were supported on titania and fully characterized. The new NPs were studied for their catalytic activity towards the reduction of nitrophenol substrates, the reduction of resazurin and for their photothermal efficiency. Initial results on their application in photothermal therapy (PTT) were obtained in human cancer cells in vitro. The aforementioned reactions represent important model reactions towards wastewater remediation, bioorthogonal transformations and cancer treatment.

N-Heterocyclic Carbene Modified Palladium Catalysts for the Direct Synthesis of Hydrogen Peroxide

Heterogeneous palladium catalysts modified by N-heterocyclic carbenes (NHCs) are shown to be highly effective toward the direct synthesis of hydrogen peroxide (H₂O₂), in the absence of the promoters which are typically required to enhance both activity and selectivity. Catalytic evaluation in a batch regime demonstrated that through careful selection of the N-substituent of the NHC it is possible to greatly enhance catalytic performance when compared to the unmodified analogue and reach concentrations of H₂O₂ rivaling that obtained by state-of-the-art catalysts. The enhanced performance of the modified catalyst, which is retained upon reuse, is attributed to the ability of the NHC to electronically modify Pd speciation.

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

Zeolite chemistry and catalysis are expected to play a decisive role in the next decade(s) to build a more decentralized renewable feedstock-dependent sustainable society owing to the increased scrutiny over carbon emissions. Therefore, the lack of fundamental and mechanistic understanding of these processes is a critical "technical bottleneck" that must be eliminated to maximize economic value and minimize waste. We have identified, considering this objective, that the chemistry related to the first-generation reaction intermediates (i.e., carbocations, radicals, carbenes, ketenes, and carbanions) in zeolite chemistry and catalysis is highly underdeveloped or undervalued compared to other catalysis streams (e.g., homogeneous catalysis). This limitation can often be attributed to the technological restrictions to detect such "short-lived and highly reactive" intermediates at the interface (gas-solid/solid-liquid); however, the recent rise of sophisticated spectroscopic/analytical techniques (including under in situ/operando conditions) and modern data analysis methods collectively compete to unravel the impact of these organic intermediates. This comprehensive review summarizes the state-of-the-art first-generation organic reaction intermediates in zeolite chemistry and catalysis and evaluates their existing challenges and future prospects, to contribute significantly to the "circular carbon economy" initiatives.

Growth of N-Heterocyclic Carbene Assemblies on Cu(100) and Cu(111): From Single Molecules to Magic-Number Islands

N‐Heterocyclic carbenes (NHCs) have superior properties as building blocks of self‐assembled monolayers (SAMs). Understanding the influence of the substrate in the molecular arrangement is a fundamental step before employing these ligands in technological applications. Herein, we study the molecular arrangement of a model NHC on Cu(100) and Cu(111). While mostly disordered phases appear on Cu(100), on Cu(111) well‐defined structures are formed, evolving from magic‐number islands to molecular ribbons with coverage. This work presents the first example of magic‐number islands formed by NHC assemblies on flat surfaces. Diffusion and commensurability are key factors explaining the observed arrangements. These results shed light on the molecule‐substrate interaction and open the possibility of tuning nanopatterned structures based on NHC assemblies.

Single atoms and small clusters of atoms may accompany Au and Pd dendrimer-encapsulated nanoparticles

We report the presence of small clusters of atoms (<1 nm) (SCs) and single atoms (SAs) in solutions containing 1-2 nm dendrimer-encapsulated nanoparticles (DENs). Au and Pd DENs were imaged using aberration-corrected scanning transmission electron microscopy (ac-STEM), and energy dispersive spectroscopy (EDS) was used to identify and quantify the SAs/SCs. Two main findings have emerged from this work. First, the presence or absence of SAs/SCs depends on both the terminal functional group of the dendrimer (-NH₂ or -OH) and the elemental composition of the DENs (Au or Pd). Second, dialysis can be used to remove the majority of SAs/SCs in cases where a high density of SAs/SCs are present. The foregoing conclusions provide insights into the mechanisms for Au and Pd DEN synthesis and stability. Ultimately, these results demonstrate the need for careful characterization of systems containing nanoparticles to ensure that SAs/SCs, which may be below the detection limit of most analytical methods, are taken into consideration (especially for catalysis experiments).

Reversible Self-Assembly of an N-Heterocyclic Carbene on Metal Surfaces

Self-assembly of cyclohexyl cyclic (alkyl)(amino)carbenes (cyCAAC) can be realized and reversibly switched from a close-packed trimer phase to a chainlike dimer phase, enabled by the ring-flip of the cyclohexyl wingtip. Multiple methods including scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations identified a distinct isomer (axial or equatorial chair conformer) in each phase, and consequently support the conclusion regarding the determination of molecular surface geometry on the self-assembly of cyCAAC. Moreover, various substrates such as Ag (111) and Cu (111) are tested to elucidate the importance of cyCAAC-surface interactions on cyCAAC based nanopatterns. These investigations of patterned surfaces prompted a deep understanding of cyCAAC binding mode, surface geometry and reversible self-assembly, which are of paramount significance in the areas of catalysis, biosensor design and surface functionalization.

Asymmetric Twisting of C-Centered Octahedral Gold(I) Clusters by Chiral N-Heterocyclic Carbene Ligation

Asymmetric induction of metal clusters by ligation of chiral ligands is intriguing in terms of the mechanism of chirality transfer and the stability of the resulting chiral structure. Here we report the asymmetric induction of C-centered hexagold(I) CAu^I₆ clusters into an asymmetrically twisted structure through monodentate, chiral benzimidazolylidene-based N-heterocyclic carbene (NHC) ligands. X-ray diffraction analysis revealed that the NHC-ligated CAu^I₆ cluster was diastereoselectively twisted with directionally selective, bond length expansion, and contraction of the Au···Au contacts and that the original cluster with high symmetry was transformed into an optically pure, asymmetric CAu^I₆ cluster with C₁ symmetry. Moreover, the circular dichroism spectroscopy and the time-dependent density functional theory calculation confirmed that the asymmetrically twisted CAu^I₆ structure was maintained even in solution. Such asymmetric induction of configurationally stable metal clusters would greatly expand the molecular design possibilities of asymmetric catalysts and chiroptical materials by utilizing library chiral NHC ligands.

Fundamentals and applications of N-heterocyclic carbene functionalized gold surfaces and nanoparticles

Improved stability and higher degree of synthetic tunability has allowed N-heterocyclic carbenes to supplant thiols as ligands for gold surface functionalization.  This review article summarizes the basic science and applications of NHCs on gold.

Tailoring graphene-supported Ru nanoparticles by functionalization with pyrene-tagged N-heterocyclic carbenes

Controlling the reactivity and stability of graphene-supported Ru NPs by modifying their surface with pyrene-tagged N-heterocyclic carbene ligands.

Convenient synthesis of tricyclic N(1)-C(2)-fused oxazino-indolones via [Au(I)] catalyzed hydrocarboxylation of allenes

A new [Au(I)] catalyzed intramolecular hydrocarboxylation of allenes is presented as a valuable synthetic route to oxazino-indolones. The employment of 3,5-(CF3)2-C6H3-ImPyAuSbF6 as the optimal catalyst (5 mol%) was necessary to...

Catalytic Asymmetric Hydroalkoxylation of C-C Multiple Bonds

Asymmetric hydroalkoxylation of alkenes constitutes a redox-neutral and 100% atom-economical strategy toward enantioenriched oxygenated building blocks from readily available starting materials. Despite their great potential, catalytic enantioselective additions of alcohols across a C-C multiple bond are particularly underdeveloped, especially compared to other hydrofunctionalization methods such as hydroamination. However, driven by some recent innovations, e.g., asymmetric MHAT methods, asymmetric photocatalytic methods, and the development of extremely strong chiral Brønsted acids, there has been a gratifying surge of reports in this burgeoning field. The goal of this review is to survey the growing landscape of asymmetric hydroalkoxylation by highlighting exciting new advances, deconstructing mechanistic underpinnings, and drawing insight from related asymmetric hydroacyloxylation and hydration. A deep appreciation of the underlying principles informs an understanding of the various selectivity parameters and activation modes in the realm of asymmetric alkene hydrofunctionalization while simultaneously evoking the outstanding challenges to the field moving forward. Overall, we aim to lay a foundation for cross-fertilization among various catalytic fields and spur further innovation in asymmetric hydroalkoxylations of C-C multiple bonds.

Catalytically Active Gold Nanomaterials Stabilized by N-heterocyclic Carbenes

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.

N-Heterocyclic Carbene-Stabilized Ultrasmall Gold Nanoclusters in a Metal-Organic Framework for Photocatalytic CO2 Reduction

Ultrafine gold nanoclusters (Au-NCs) are susceptible to migrate and aggregate, even in the porosity of many crystalline solids. N-heterocyclic carbenes (NHCs) are a class of structurally diverse ligands for the stabilization of Au-NCs in homogeneous chemistry, showing catalytic reactivity in CO₂ activation. Herein, for the first time, we demonstrate a heterogeneous nucleation approach to stabilize ultrasmall and highly dispersed gold nanoclusters in an NHC-functionalized porous matrix. The sizes of gold nanoclusters are tunable from 1.3 nm to 1.8 nm based on the interpenetration of the metal-organic framework (MOF) topology. Control experiments using amine or imidazolium-functionalized MOFs afforded the aggregation of Au species. The resultant Au-NC@MOF composite exhibits a steady and excellent activity in photocatalytic CO₂ reduction, superior to control mixtures without NHC-ligand stabilization. Mechanistic studies reveal the synergistic catalytic effect of MOFs and Au-NCs through the MOF-NHC-Au covalent-bonding bridges.

The influence of surface proximity on photoswitching activity of stilbene-functionalized N-heterocyclic carbene monolayers

Self-assembly of photo-responsive molecules is a robust technology for reversibly tuning the properties of functional materials. Herein, we probed the crucial role of surface-adsorbate interactions on the adsorption geometry of stilbene-functionalized N-heterocyclic carbenes (stilbene-NHCs) monolayers and its impact on surface potential. Stilbene-NHCs on Au film accumulated in a vertical orientation that enabled high photoisomerization efficiency and reversible changes in surface potential. Strong metal-adsorbate interactions led to flat-lying adsorption geometry of stilbene-NHCs on Pt film, which quenched the photo-isomerization influence on surface potential. It is identified that photo-induced response can be optimized by positioning the photo-active group in proximity to weakly-interacting surfaces.

Nitrogen-Doped Carbon Enables Heterogeneous Asymmetric Insertion of Carbenoids into Amines Catalyzed by Rhodium Nanoparticles

Development of stable heterogeneous catalyst systems is a crucial subject to achieve sustainable society. Though metal nanoparticles are robust species, the study of asymmetric catalysis by them has been restricted because methods to activate metal nanoparticles without causing metal leaching were limited. We developed Rh nanoparticle catalysts (NCI-Rh) supported on nitrogen-doped carbon as a solid ligand to interact with metals for asymmetric insertion of carbenoids into N-H bonds cocatalyzed by chiral phosphoric acid. Nitrogen dopants played a crucial role in both catalytic activity and enantioselectivity while almost no catalysis was observed with Rh nanoparticles immobilized on supports without nitrogen dopants. Various types of chiral α-amino acid derivatives were synthesized in high yields with high enantioselectivities and NCI-Rh could be reused in seven runs. Furthermore, we demonstrated the corresponding continuous-flow reaction using a column packed with NCI-Rh. The desired product was obtained efficiently for over 90 h through the reactivation of NCI-Rh and the chiral source could be recovered.

Multicomponent Synthesis of Unsymmetrical 4,5-Disubstituted Imidazolium Salts as N-Heterocyclic Carbene Precursors: Applications in Palladium-Catalyzed Cross-Coupling Reactions

Various novel (a)chiral 4,5-disubstituted 1-aryl-3-alkyl-imidazolium salts were synthesized via the multicomponent reaction of diketone derivatives, sterically congested arylamines, and alkylamines. Moreover, two novel unsymmetrical bulky cycloalkyl-based NHC-Pd complexes proved highly active as catalysts for Suzuki-Miyaura and Negishi cross-coupling reactions.

Nanocatalysts for High Selectivity Enyne Cyclization: Oxidative Surface Reorganization of Gold Sub-2-nm Nanoparticle Networks

Ultrasmall gold nanoparticles (NPs) stabilized in networks by polymantane ligands (diamondoids) were successfully used as precatalysts for highly selective heterogeneous gold-catalyzed dimethyl allyl(propargyl)malonate cyclization to 5-membered conjugated diene. Such reaction usually suffers from selectivity issues with homogeneous catalysts. This control over selectivity further opened the way to one-pot cascade reaction, as illustrated by the 1,6-enyne cycloisomerization-Diels-Alder reaction of dimethyl allyl propargyl malonate with maleic anhydride. The ability to assemble nanoparticles with controllable sizes and shapes within networks concerns research in sensors, medical diagnostics, information storage, and catalysis applications. Herein, the control of the synthesis of sub-2-nm gold NPs is achieved by the formation of dense networks, which are assembled in a single step reaction by employing ditopic polymantanethiols. By using 1,1'-bisadamantane-3,3'-dithiol (BAd-SH) and diamantane-4,9-dithiol (DAd-SH), serving both as bulky surface stabilizers and short-sized linkers, we provide a simple method to form uniformly small gold NPs (1.3 ± 0.2 nm to 1.6 ± 0.3 nm) embedded in rigid frameworks. These NP arrays are organized alongside short interparticular distances ranging from 1.9 to 2.7 nm. The analysis of gold NP surfaces and their modification were achieved in joint experimental and theoretical studies, using notably XPS, NMR, and DFT modeling. Our experimental studies and DFT analyses highlighted the necessary oxidative surface reorganization of individual nanoparticles for an effective enyne cycloisomerization. The modifications at bulky stabilizing ligands allow surface steric decongestion for the alkyne moiety activation but also result in network alteration by overoxidation of sulfurs. Thus, sub-2-nm nanoparticles originating from networks building create convenient conditions for generating reactive Au(I) surface single-sites-in the absence of silver additives-useful for heterogeneous gold-catalyzed enyne cyclization. These nanocatalysts, which as such ease organic products separation, also provide a convenient access for building further polycyclic complexity, owing to their high reactivity and selectivity.

A simple synthesis of [RuCl2(NHC)(p-cymene)] complexes and their use in olefin oxidation catalysis

An operationally simple synthetic route is designed to access the [RuCl₂(NHC)(p-cymene)] family of complexes.

Perylene diimide-tagged N-heterocyclic carbene-stabilized gold nanoparticles: How much ligand desorbs from surface in presence of thiols?

N-Heterocyclic carbenes (NHCs) have recently emerged as viable alternatives to commonly used thiols to stabilize a variety of metal surfaces and nanoparticles. In this context, thanks to their biocompatibility and novel optical properties, NHC-stabilized gold nanoparticles (AuNPs) have been extensively studied. It has been shown that such materials exhibit improved stabilities in acidic and basic solutions, high temperatures, electrolyte solutions, cell culture media, and to some extent to nucleophilic thiols. Despite intense efforts, instability of NHC-functionalized AuNPs to thiols has been an ongoing challenge. In order to circumvent this problem, quantification of NHC desorption from nanoparticle surface by the invading thiols would constitute a necessary first step. To do this, we have first developed water-soluble azide decorated NHC-stabilized "clickable" AuNPs. Optically active perylene diimide (PDI)-tagged AuNP hybrids are then obtained by means of Cu-catalyzed alkyne-azide cycloaddition between these AuNPs and an alkyne-decorated PDI derivative. Investigation of photophysical properties of these AuNP/PDI hybrids revealed that the fluorescence of PDI molecules is effectively quenched by AuNPs in aqueous solution. The extent of NHC desorption from AuNP surface in presence of glutathione (4 mM), as a biologically relevant thiol, is then quantified by means of fluorescence emission restoration of PDI molecules upon detachment from AuNP surfaces. Our results demonstrate that while ∼20% of surface NHCs are displaced by glutathione within the first 24 h of their exposure to the thiol, ligand desorption reaches ∼45% after one week. We believe that these findings will provide more insight on true stability of NHC-stabilized materials.

Miniaturized solid-phase extraction using a mesoporous molecular sieve SBA-15 as sorbent for the determination of triterpenoid saponins from Pulsatilla chinensis by ultrahigh-performance liquid chromatography-charged aerosol detection

A simple, sensitive and efficient solid-phase extraction method, combined with ultrahigh-performance liquid chromatography-charged aerosol detection, was applied to the pre-concentration and determination of four triterpenoid saponins from Pulsatilla chinensis (P. chinensis) ultrasound extract samples. Mesoporous molecular sieve Santa Barbara Amorphous 15 (SBA-15) displayed higher selectivity compared to Mobil Composition of Matter 41 (MCM-41), NH₂-SBA-15 and mesoporous carbon when it comes to being used in pretreatment methods. It was applied as an effective sorbent in the SPE for the enrichment of the target analytes. Additionally, several key experimental parameters including the kinds of sorbents, the amount of SBA-15, the elute pH and types of elution solvent were investigated in detail. Under the optimized conditions, the satisfactory linearity (r² ≥ 0.9940) was acquired and the limits of detection reached 0.461-0.976 μg/mL for the target analytes. The recoveries ranged from 95.1%-103.2%. The experimental results showed that SBA-15 was a candidate material for the purification and concentration of target triterpenoid saponins from complex P. chinensis samples. The study provided theoretical support for the application of mesoporous materials in the field of drug separation and provided references for the extraction and determination of trace compounds in the complex systems of traditional Chinese medicine.