Determination of the structure and geometry of N-heterocyclic carbenes on Au(111) using high-resolution spectroscopy

Synthesis and characterization of low-dimensional N-heterocyclic carbene lattices

The covalent interaction of N-heterocyclic carbenes (NHCs) with transition metal atoms gives rise to distinctive frontier molecular orbitals (FMOs). These emergent electronic states have spurred the widespread adoption of NHC ligands in chemical catalysis and functional materials. Although formation of carbene-metal complexes in self-assembled monolayers on surfaces has been explored, design and electronic structure characterization of extended low-dimensional NHC-metal lattices remains elusive. Here we demonstrate a modular approach to engineering one-dimensional (1D) metal-organic chains and two-dimensional (2D) Kagome lattices using the FMOs of NHC-Au-NHC junctions to create low-dimensional molecular networks exhibiting intrinsic metallicity. Scanning tunneling spectroscopy and first-principles density functional theory reveal the contribution of C-Au-C π-bonding states to dispersive bands that imbue 1D- and 2D-NHC lattices with exceptionally small work functions.

N-Heterocyclic Carbene Monolayers on Metal-Oxide Films: Correlations between Adsorption Mode and Surface Functionality

N-Heterocyclic carbene (NHC) ligands have been self-assembled on various metal and semimetal surfaces, creating a covalent bond with surface metal atoms that led to high thermal and chemical stability of the self-assembled monolayer. This study explores the self-assembly of NHCs on metal-oxide films (CuOx, FeOx, and TiOx) and reveals that the properties of these metal-oxide substrates play a pivotal role in dictating the adsorption behavior of NHCs, influencing the decomposition route of the monolayer and its impact on work function values. While the attachment of NHCs onto CuOx is via coordination with surface oxygen atoms, NHCs interact with TiOx through coordination with surface metal atoms and with FeOx via coordination with both metal and oxygen surface atoms. These distinct binding modes arise due to variances in the electronic properties of the metal atoms within the investigated metal-oxide films. Contact angle and ultraviolet photoelectron spectroscopy measurements have shown a significantly higher impact of F-NHC adsorption on CuOx than on TiOx and FeOx , correlated to a preferred, averaged upright orientation of F-NHC on CuOx.

The Electron-Rich and Nucleophilic N-Heterocyclic Imines on Metal Surfaces: Binding Modes and Interfacial Charge Transfer

The strongly electron-donating N-heterocyclic imines (NHIs) have been employed as excellent surface anchors for the thermodynamic stabilization of electron-deficient species due to their enhanced nucleophilicity. However, the binding mode and interfacial property of these new ligands are still unclear, representing a bottleneck for advanced applications in surface functionalization and catalysis. Here, NHIs with different side groups have been rationally designed, synthesized, and analyzed on various metal surfaces (Cu, Ag). Our results reveal different binding modes depending on the molecular structure and metal surface. The molecular design enables us to achieve a flat-lying or upright configuration and even a transition between these two binding modes depending on the coverage and time. Importantly, the two binding modes exhibit different degrees of interfacial charge transfer between the molecule and the surface. This study provides essential microscopic insight into the NHI adsorption geometry and interfacial charge transfer for the optimization of heterogeneous catalysts in coordination chemistry.

NMR Spectroscopic Signatures of Cationic Surface Sites from Supported Coinage Metals Interacting with N-Heterocyclic Carbenes

N-heterocyclic carbenes (NHCs) have been extensively studied to modulate the reactivity of molecular catalysts, colloids, and their supported analogues, being isolated sites, clusters, or nanoparticles. While the interaction of NHCs on metal surfaces has been discussed in great detail, showing specific coordination chemistry depending on the type of NHC ligands, much less is known when the metal is dispersed on oxide supports, as in heterogeneous catalysts. Herein, we study the interaction of NHC ligands with Au surface sites dispersed on silica, a nonreducible oxide support. We identify the easy formation of bis-NHC ligated Au(I) surface sites parallel to what is found on metallic Au surfaces. These species display a specific 13C NMR spectroscopic signature that clearly distinguishes them from the mono-NHC Au(I) surface sites or supported imidazoliums. We find that bis-ligated surface species are not unique to supported Au(I) species and are found for the corresponding Ag(I) and Cu(I) species, as well as for the isolobal surface silanols. Furthermore, the interaction of NHC ligand with silica-supported Au nanoparticles also yields bis-NHC ligated Au(I) surface sites, indicating that metal atoms can also be easily extracted from nanoparticles, further illustrating the dynamics of these systems and the overall favorable formation of such bis-ligated species across a range of systems, besides what has been found on crystalline metal facets.

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.

N-heterocyclic carbene adsorption states on Pt(111) and Ru(0001)

N-heterocyclic carbene ligands (NHCs) are increasingly used to tune the properties of metal surfaces. The generally greater chemical and thermal robustness of NHCs on gold, as compared to thiolate surface ligands, underscores their potential for a range of applications. While much is now known about the adsorption geometry, overlayer structure, dynamics, and stability of NHCs on coinage elements, especially gold and copper, much less is known about their interaction with the surfaces of Pt-group metals, despite the importance of such metals in catalysis and electrochemistry. In this study, reflection absorption infrared spectroscopy (RAIRS) is used to probe the structure of benzimidazolylidene NHC ligands on Pt(111) and Ru(0001). The experiments exploit the intense absorption peaks of a CF3 substituent on the phenyl ring of the NHC backbone to provide unprecedented insight into adsorption geometry and chemical stability. The results also permit comparison with literature data for NHC ligands on Au(111) and to DFT predictions for NHCs on Pt(111) and Ru(0001), thereby greatly extending the known surface chemistry of NHCs and providing much needed molecular information for the design of metal-organic hybrid materials involving strongly reactive metals.

Tailored Monolayers of N-Heterocyclic Carbenes by Kinetic Control

Despite the substantial success of N-heterocyclic carbenes (NHCs) as stable and versatile surface modification ligands, their use in nanoscale applications beyond chemistry is still hampered by the failure to control the carbene binding mode, which complicates the fabrication of monolayers with the desired physicochemical properties. Here, we applied vibrational sum-frequency generation spectroscopy to conduct a pseudokinetic surface analysis of NHC monolayers on Au thin films under ambient conditions. We observe for two frequently used carbene structures that their binding mode is highly dynamic and changes with the adsorption time. In addition, we demonstrate that this transition can be accelerated or decelerated to adjust the binding mode of NHCs, which allows fabrication of tailored monolayers of NHCs simply by kinetic control.

Selective Deposition of N-Heterocyclic Carbene Monolayers on Designated Au Microelectrodes within an Electrode Array

The incorporation of organic self-assembled monolayers (SAMs) in microelectronic devices requires precise spatial control over the self-assembly process. In this work, selective deposition of N-heterocyclic carbenes (NHCs) on specific electrodes within a two-microelectrode array is achieved by using pulsed electrodeposition. Spectroscopic analysis of the NHC-coated electrode arrays reveals that each electrode is selectively coated with a designated NHC. The impact of NHC monolayers on the electrodes' work function is quantified using Kelvin probe force microscopy. These measurements demonstrate that the work function values of each electrode can be independently tuned by the adsorption of a specific NHC. The presented deposition method enables to selectively coat designated microelectrodes in an electrode array with chosen NHC monolayers for tuning their chemical and electronic functionality.

N-Heterocyclic Olefins on a Silicon Surface

The adsorption of N-heterocyclic olefins (NHOs) on silicon is investigated in a combined scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory study. We find that both of the studied NHOs bind covalently, with ylidic character, to the silicon adatoms of the substrate and exhibit good thermal stability. The adsorption geometry strongly depends on the N-substituents: for large N-substituents, an upright adsorption geometry is favored, while a flat-lying geometry is found for the NHO with smaller wingtips. These different geometries strongly influence the quality and properties of the obtained monolayers. The upright geometry leads to the formation of ordered monolayers, whereas the flat-lying NHOs yield a mostly disordered, but denser, monolayer. The obtained monolayers both show large work function reductions, as the higher density of the flat-lying monolayer is found to compensate for the smaller vertical dipole moments. Our findings offer new prospects in the design of tailor-made ligand structures in organic electronics and optoelectronics, catalysis, and material science.

Thermal Stability and Electronic Properties of N-Heterocyclic Carbene-Protected Au13 Nanocluster and Phosphine-Protected Analogues

Despite significant advances in manufacturing atomically precise gold nanoclusters protected by various ligands, there is a limited understanding of the thermal stability dynamics and electronic properties of ligand effects. We conducted ab initio molecular dynamics (AIMD) simulations on the well-characterized [Au13(NHCMe)9Cl3]2+ nanocluster and its counterpart [Au13(PMe3)9Cl3]2+ cluster to evaluate the thermal stability induced by N-heterocyclic carbene (NHC) and phosphine ligands. The result shows that under vacuum conditions, [Au13(PMe3)9Cl3]2+ is more stable than [Au13(NHCMe)9Cl3]2+, and both lead to metal nucleation decomposition, breaking into the Au12 fragment and L-Au-Cl (L = NHCMe or PMe3) complexes eventually. The optical and electronic properties of these two clusters change significantly due to ligand alteration. Furthermore, we have designed a novel [Au13(NHCMe)(PMe3)8Cl3]2+ cluster coprotected by NHC and phosphine ligands, displaying higher thermal stability than the homoligand protected [Au13(NHCMe)9Cl3]2+ and [Au13(PMe3)9Cl3]2+. Our hypothetical species are an interesting model for nanostructured materials, facilitating the experimental exploration of cluster synthesis and catalytic applications.

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.

Robust Sub-5 Nanometer bis(Diarylcarbene)-Based Thin Film for Molecular Electronics and Plasmonics

In miniaturized electronic and optoelectronic circuits, molecular tunnel junctions have attracted enormous research interest due to their small footprint, low power consumption, and rich molecular functions. However, the most popular building blocks used in contemporary molecular tunnel junctions are thiol molecules, which attach to electrode surfaces via a metal-thiolate (MS) bond, showing low stability and usually quick degradation within several days. To pave the way for more widely applicable and stable molecular tunnel junctions, there is a need to develop new molecular anchoring groups. Here, this work demonstrates robust and air-stable molecular tunnel junctions with a sub-5 nanometer bis(diarylcarbene)-based thin film as the tunneling barrier, which anchors to the electrode surface via a AuC bond. The bis(diarylcarbene)-based molecular tunnel junctions exhibit high thermal stability against heating up to 200 °C and long storage lifetime over 5 months in an ambient environment. Both electrical and optical performance of these bis(diarylcarbene)-based molecular junctions are characterized systematically, showing similar behaviors to thiol-based junctions as well as largely improved emission stability. This research highlights the excellent performance of bis(diarylcarbene)-based molecular tunnel junctions, which could be useful for applications in molecular electronics and plasmonics.

Using Surface-Enhanced Raman Spectroscopy to Unravel the Wingtip-Dependent Orientation of N-Heterocyclic Carbenes on Gold Nanoparticles

N-Heterocyclic carbenes (NHCs) are an attractive alternative to thiol ligands when forming self-assembled monolayers on noble-metal surfaces; however, relative to the well-studied thiol monolayers, comparatively little is known about the binding, orientation, and packing of NHC monolayers. Herein, we combine surface-enhanced Raman spectroscopy (SERS) and first-principles theory to investigate how the alkyl "wingtip" groups, i.e., those attached to the nitrogens of N-heterocyclic carbenes, affect the NHC orientation on gold nanoparticles. Consistent with previous literature, smaller wingtip groups lead to stable flat configurations; surprisingly, bulkier wingtips also have stable flat configurations likely due to the presence of an adatom. Comparison of experimental SERS results with the theoretically calculated spectra for flat and vertical configurations shows that we are simultaneously detecting both NHC configurations. In addition to providing information on the adsorbate geometry, this study highlights the extreme SERS enhancement of vibrational modes perpendicular to the surface.

Surface Ligands Dictate the Mechanical Properties of Inorganic Nanomaterials

The ability for organic surface chemistry to influence the properties of inorganic nanomaterials is appreciated in some instances but is poorly understood in terms of mechanical behavior. Here we demonstrate that the global mechanical strength of a silver nanoplate can be modulated according to the local binding enthalpy of its surface ligands. A continuum-based core-shell model for nanoplate deformation shows that the interior of a particle retains bulk-like properties while the surface shell has yield strength values that depend on surface chemistry. Electron diffraction experiments reveal that, relative to the core, atoms at the nanoplate surface undergo lattice expansion and disordering directly related to the coordinating strength of the surface ligand. As a result, plastic deformation of the shell is more difficult, leading to an enhancement of the global mechanical strength of the plate. These results demonstrate a size-dependent coupling between chemistry and mechanics at the nanoscale.

N‐Heterocyclic Carbenes: Molecular Porters of Surface Mounted Ru‐Porphyrins

Ru-porphyrins act as convenient pedestals for the assembly of N-heterocyclic carbenes (NHCs) on solid surfaces. Upon deposition of a simple NHC ligand on a close packed Ru-porphyrin monolayer, an extraordinary phenomenon can be observed: Ru-porphyrin molecules are transferred from the silver surface to the next molecular layer. We have investigated the structural features and dynamics of this portering process and analysed the associated binding strengths and work function changes. A rearrangement of the molecular layer is induced by the NHC uptake: the NHC selective binding to the Ru causes the ejection of whole porphyrin molecules from the molecular layer on silver to the layer on top. This reorganisation can be reversed by thermally induced desorption of the NHC ligand. We anticipate that the understanding of such mass transport processes will have crucial implications for the functionalisation of surfaces with carbenes.

Functionalized N-Heterocyclic Carbene Monolayers on Gold for Surface-Initiated Polymerizations

Although N-heterocyclic carbenes (NHCs) are superior to thiol adsorbates in that they form remarkably stable bonds with gold, the generation of NHC-based self-assembled monolayers (SAMs) typically requires a strong base and an inert atmosphere, which limits the utility of such films in many applications. Herein, we report the development and use of bench-stable NHC adsorbates, benzimidazolium methanesulfonates, for the direct formation of NHC films on gold surfaces under an ambient atmosphere at room temperature without the need for extraordinary precautions. The generated NHC SAMs were fully characterized using ellipsometry, X-ray photoelectron spectroscopy (XPS), polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and contact angle measurements, and they were compared to analogous SAMs generated from an NHC bicarbonate adsorbate. Based on these findings, a unique radical initiator α,ω-bidentate azo-terminated NHC adsorbate, NHC15AZO[OMs], was designed and synthesized for the preparation of SAMs on gold surfaces with both NHC headgroups bound to the surface. The adsorbate molecules in NHC15AZO SAMs can exist in a hairpin or a linear conformation depending on the concentration of the adsorbate solution used to prepare the SAM. These conformations were studied by a combination of ellipsometry, XPS, PM-IRRAS, and scanning electron microscopy using gold nanoparticles (AuNPs) as a tag material. Moreover, the potential utility of these unique radical-initiating NHC films as surface-initiated polymerization platforms was demonstrated by controlling the thickness of polystyrene brush films grown from azo-terminated NHC monolayer surfaces simply by adjusting the reaction time of the photoinitiated radical polymer growth process.

Covalent Adsorption of N-Heterocyclic Carbenes on a Copper Oxide Surface

Tuning the properties of oxide surfaces through the adsorption of designed ligands is highly desirable for several applications, such as catalysis. N-Heterocyclic carbenes (NHCs) have been successfully employed as ligands for the modification of metallic surfaces. On the other hand, their potential as modifiers of ubiquitous oxide surfaces still needs to be developed. Here we show that a model NHC binds covalently to a copper oxide surface under UHV conditions. In particular, we report the first example of a covalent bond between NHCs and oxygen atoms from the oxide layer. This study demonstrates that NHC can also act as a strong anchor on oxide surfaces.

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.

Thermopower of Molecular Junction in Harsh Thermal Environments

Molecular junctions can be miniaturized devices for heat-to-electricity conversion application, yet these operate only in mild thermal environments (less than 323 K) because thiol, the most widely used anchor moiety for chemisorption of active molecules onto surface of electrode, easily undergoes thermal degradation. N-Heterocyclic carbene (NHC) can be an alternative to traditional thiol anchor for producing ultrastable thermoelectric molecular junctions. Our experiments showed that the NHC-based molecular junctions withstood remarkably high temperatures up to 573 K, exhibiting consistent Seebeck effect and thermovoltage up to approximately |1900 μV|. Our work advances our understanding of molecule-electrode contact in the Seebeck effect, providing a roadmap for constructing robust and efficient organic thermoelectric devices.

Highly Ordered N-Heterocyclic Carbene Monolayers on Cu(111)

The benzannulated N-heterocyclic carbene, 1,3-dibenzylbenzimidazolylidene (NHC^DBZ) forms large, highly ordered domains when adsorbed on Cu(111) under ultrahigh vacuum conditions. A combination of scanning tunnelling microscopy (STM), high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) calculations reveals that the overlayer consists of vertical benzannulated NHC moieties coordinating to Cu adatoms. Long-range order results from the placement of the two benzyl substituents on opposite sides of the benzimidazole moiety, with their aromatic rings approximately parallel to the surface. The organization of three surface-bound benzyl substituents from three different NHCs into a triangular array controls the formation of a highly ordered Kagome-like surface lattice. By comparison with earlier studies of NHCs on Cu(111), we show that the binding geometry and self-assembly of NHC^DBZ are influenced by intermolecular and adsorbate-substrate interactions and facilitated by the flexibility of the methylene linkage between the N-heterocycle and the aromatic wingtip substituents.

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.

Adatom mediated adsorption of N-heterocyclic carbenes on Cu(111) and Au(111)

The adsorption of N-heterocyclic carbenes (NHCs) on Cu(111) and Au(111) surfaces is studied with density-functional theory. The role of the molecular side groups as well as the surface morphology in determining the adsorption geometry are explored in detail. Flat-laying NHCs, as observed experimentally for NHC with relatively small side groups, result from the adsorption at adatoms and give rise to the so-called ballbot configurations, which are more stable than adsorption on flat surfaces and provide an efficient precursor for the formation of bis(NHC) dimers. On Au(111), the resulting (NHC)₂ Au complexes are purely physisorbed and thus mobile. On the more reactive Cu(111), in contrast, the central Cu atom in the (NHC)₂ Cu dimer is still covalently bound to the surface, resulting in a mobility, which has to be thermally activated.

Probing N-Heterocyclic Carbene Surfaces with Laser Desorption Ionization Mass Spectrometry

The proliferation of N-heterocyclic carbene (NHC) self-assembled monolayers (SAMs) on gold surfaces stems from their exceptional stability compared to conventional thiol-SAMs. The prospect of biological applications for NHC-SAMs on gold shows the need for biocompatible techniques (e.g., large biomolecule detection and high throughput) that assesses SAM molecular composition. Herein, we demonstrate that laser desorption ionization mass spectrometry (LDI-MS) is a powerful and facile probe of NHC surface chemistry. LDI-MS of prototypical imidazole-NHC- and benzimidazole-NHC-functionalized AuNPs yields exclusively [NHC₂Au]⁺ ions and not larger gold clusters. Employing benzimidazole-NHC isotopologues, we explore how monolayers pack on a single AuNP and the lability of the NHCs once ligated. Quantitative analysis of the homoleptic and heteroleptic [NHC₂Au]⁺ ions is performed by comparing to a binomial model representative of a randomized monolayer. Lastly, the reduction of nitro-NHC-AuNPs to amine-NHC-AuNPs is tracked via LDI-MS signals, illustrating the ability of LDI-MS to probe postsynthetic modifications of the anchored NHCs, which is critical for current and future applications of NHC surfaces.

Recent advances in the chemistry and applications of N-heterocyclic carbenes

N-Heterocyclic carbenes, despite being isolated and characterized three decades ago, still capture scientists' interest as versatile, modular and strongly coordinating moieties. In the last decade, driven by the increasingly refined fundamental understanding of their behaviour, the emergence of new carbene frameworks and cogent sustainability issues, N-heterocyclic carbenes have experienced a tremendous increase in utilization across several disparate fields. In this Review, a concise overview of N-heterocyclic carbenes encompassing their history, properties and applications in transition metal catalysis, on-surface chemistry, main group chemistry and organocatalysis is provided. Emphasis is placed on developments emerging in the last seven years and on envisaging future directions.

Self-assembly of N-heterocyclic carbenes on Au(111)

Although the self-assembly of organic ligands on gold has been dominated by sulfur-based ligands for decades, a new ligand class, N-heterocyclic carbenes (NHCs), has appeared as an interesting alternative. However, fundamental questions surrounding self-assembly of this new ligand remain unanswered. Herein, we describe the effect of NHC structure, surface coverage, and substrate temperature on mobility, thermal stability, NHC surface geometry, and self-assembly. Analysis of NHC adsorption and self-assembly by scanning tunneling microscopy and density functional theory have revealed the importance of NHC-surface interactions and attractive NHC-NHC interactions on NHC monolayer structures. A remarkable way these interactions manifest is the need for a threshold NHC surface coverage to produce upright, adatom-mediated adsorption motifs with low surface diffusion. NHC wingtip structure is also critical, with primary substituents leading to the formation of flat-lying NHC2Au complexes, which have high mobility when isolated, but self-assemble into stable ordered lattices at higher surface concentrations. These and other studies of NHC surface chemistry will be crucial for the success of these next-generation monolayers.

Controlled growth of ordered monolayers of N-heterocyclic carbenes on silicon

N-Heterocyclic carbenes (NHCs) are promising modifiers and anchors for surface functionalization and offer some advantages over thiol-based systems. Because of their strong binding affinity and high electron donation, NHCs can dramatically change the properties of the surfaces to which they are bonded. Highly ordered NHC monolayers have so far been limited to metal surfaces. Silicon, however, remains the element of choice in semiconductor devices and its modification is therefore of utmost importance for electronic industries. Here, a comprehensive study on the adsorption of NHCs on silicon is presented. We find covalently bound NHC molecules in an upright adsorption geometry and demonstrate the formation of highly ordered monolayers exhibiting good thermal stability and strong work function reductions. The structure and ordering of the monolayers is controlled by the substrate geometry and reactivity and in particular by the NHC side groups. These findings pave the way towards a tailor-made organic functionalization of silicon surfaces and, thanks to the high modularity of NHCs, new electronic and optoelectronic applications.

The ligand effect on the interface structures and electrocatalytic applications of atomically precise metal nanoclusters

Metal nanoclusters, also known as ultra-small metal nanoparticles, occupy the gap between discrete atoms and plasmonic nanomaterials, and are an emerging class of atomically precise nanomaterials. Metal nanoclusters protected by different types of ligands, such as thiolates, alkynyls, hydrides, and N-heterocyclic carbenes, have been synthesized in recent years. Moreover, recent experiment and theoretical studies also indicated that the metal nanoclusters show great promise in many electrocatalytic reactions, such as hydrogen evolution, oxygen reduction, and CO₂reduction. The atomically precise nature of their structures enables the elucidation of structure-property relationships and the reaction mechanisms, which is essential if nanoclusters with enhanced performances are to be rationally designed. Particularly, the ligands play an important role in affecting the interface bonding, stability and electrocatalytic activity/selectivity. In this review, we mainly focus on the ligand effect on the interface structure of metal nanoclusters and then discuss the recent advances in electrocatalytic applications. Furthermore, we point out our perspectives on future efforts in this field.

Using silyl protecting group to enable post-deposition C–C coupling reactions of alkyne-functionalized N-heterocyclic carbene monolayers on Au surfaces

Alkyne-functionalized NHC ligands were protected by TIPS group that enabled surface-anchoring of NHCs with chemically-sensitive functionality and providing access, following TIPS removal, to on-surface Sonogashira cross–coupling reactions.

Cooperation of N-Heterocyclic Carbenes on a Gold Surface

Atomically precise tailoring of interface structures is crucial for developing functional materials. We demonstrate an N-heterocyclic carbene (NHC) based molecular tool, which modifies the structure of a gold surface with atomic accuracy by the formation of gold nanorods. After adsorption on the gold surface, individual surface atoms are pulled out by the NHCs, generating single-atom surface defects and mobile NHC-Au species. Atomistic calculations reveal that these molecular "ballbots" can act as assembling tools to dislocate individual surface atoms. The predicted functionality of these carbene-based complexes is confirmed by scanning tunneling microscopy measurements. Cooperative operation of these NHC-Au species induces a step-wise formation of gold nanorods. Consequently, the surface is re-structured by a zipper-type mechanism. Our work presents a foundation to utilize molecular-based nanotools to design surface structures.

Electrochemical deposition of N-heterocyclic carbene monolayers on metal surfaces

N-heterocyclic carbenes (NHCs) have been widely utilized for the formation of self-assembled monolayers (SAMs) on various surfaces. The main methodologies for preparation of NHCs-based SAMs either requires inert atmosphere and strong base for deprotonation of imidazolium precursors or the use of specifically-synthesized precursors such as NHC(H)[HCO₃] salts or NHC–CO₂ adducts. Herein, we demonstrate an electrochemical approach for surface-anchoring of NHCs which overcomes the need for dry environment, addition of exogenous strong base or restricting synthetic steps. In the electrochemical deposition, water reduction reaction is used to generate high concentration of hydroxide ions in proximity to a metal electrode. Imidazolium cations were deprotonated by hydroxide ions, leading to carbenes formation that self-assembled on the electrode’s surface. SAMs of NO₂-functionalized NHCs and dimethyl-benzimidazole were electrochemically deposited on Au films. SAMs of NHCs were also electrochemically deposited on Pt, Pd and Ag films, demonstrating the wide metal scope of this deposition technique.

N-heterocyclic carbenes (NHCs) have been widely used for the formation of monolayers but self-assembly methods come with drawbacks such as need for dry environment or using specifically-synthesized precursors. Here, the authors demonstrate an approach for surface-anchoring of NHCs which overcomes these limitations by using electrochemically-assisted deprotonation.

Self-Assembled Monolayers of Nitron: Self-Activated and Chemically Addressable N-Heterocyclic Carbene Monolayers with Triazolone Structural Motif

N-heterocyclic carbenes (NHCs) have emerged as a unique molecular platform for the formation of self-assembled monolayers (SAMs) on various surfaces. However, active carbene formation requires deprotonation of imidazolium salt precursors, which is mostly facilitated by exposure of the salt to exogenous base. Base residues were found to be adsorbed on the metal surface and hindered the formation of well-ordered carbene-based monolayers. Herein, we show that nitron, a triazolone-based compound that freely tautomerizes to a carbene, can spontaneously self-assemble into monolayers on Pt and Au surfaces, which obviates the necessity for base-induced deprotonation for active carbene formation. SAMs of nitron were found to be thermally stable and could not be displaced by thiols, and thus their high chemical stability was demonstrated. The amino group in surface-anchored nitron was shown to be chemically available for S_N 2 reactions, and makes surface-anchored nitron a chemically addressable cross-linking reagent for surface modifications.

Simulations of X-ray absorption spectroscopy and energetic conformation of N-heterocyclic carbenes on Au(111)

It has recently been demonstrated that N-heterocyclic carbenes (NHCs) form self-assembled monolayers (SAMs) on metal surfaces. Consequently, it is important to both characterize and understand their binding modes to fully exploit NHCs in functional surface systems. To assist with this effort, we have performed first-principles total energy calculations for NHCs on Au(111) and simulations of X-ray absorption near edge structure (XANES). The NHCs we have considered are N,N-dimethyl-, N,N-diethyl-, N,N-diisopropylbenzimidazolylidene (BNHCX, with X = Me, Et, and iPr, respectively) and the bis-BNHCX-Au complexes derived from these molecules. We present a comprehensive analysis of the energetic stability of both the BNHCX and the complexes on Au(111) and, for the former, examine the role of the wing group in determining the attachment geometry. Further structural characterization is performed by calculating the nitrogen K-edge X-ray absorption spectra. Our simulated XANES results give insight into (i) the relationship between the BNHCX/Au geometry and the N(1s) → π*/σ*, pre-edge/near-edge, absorption intensities, and (ii) the contributions of the molecular deformation and molecule-surface electronic interaction to the XANES spectrum. These simulated spectra work not only as a map to the BNHCX conformation, but also, combined with electronic structure calculations, provide a clear understanding of recent experimental XANES findings on BNHCX/Au.

N-Heterocyclic Carbenes Reduce and Functionalize Copper Oxide Surfaces in One Pot

Benzimidazolium hydrogen carbonate salts have been shown to act as N-heterocyclic carbene precursors, which can remove oxide from copper oxide surfaces and functionalize the resulting metallic surfaces in a single pot. Both the surfaces and the etching products were fully characterized by spectroscopic methods. Analysis of surfaces before and after NHC treatment by X-ray photoelectron spectroscopy demonstrates the complete removal of copper(II) oxide. By using ¹³ C-labelling, we determined that the products of this transformation include a cyclic urea, a ring-opened formamide and a bis-carbene copper(I) complex. These results illustrate the potential of NHCs to functionalize a much broader class of metals, including those prone to oxidation, greatly facilitating the preparation of NHC-based films on metals other than gold.

An Electron-Rich Cyclic (Alkyl)(Amino)Carbene on Au(111), Ag(111), and Cu(111) Surfaces

The structural properties and binding motif of a strongly σ-electron-donating N-heterocyclic carbene have been investigated on different transition-metal surfaces. The examined cyclic (alkyl)(amino)carbene (CAAC) was found to be mobile on surfaces, and molecular islands with short-range order could be found at high coverage. A combination of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations highlights how CAACs bind to the surface, which is of tremendous importance to gain an understanding of heterogeneous catalysts bearing CAACs as ligands.

Strong Metal-Adsorbate Interactions Increase the Reactivity and Decrease the Orientational Order of OH-Functionalized N-Heterocyclic Carbene Monolayers

Fundamental understanding of the correlation between the structure and reactivity of chemically addressable N-heterocyclic carbene (NHC) molecules on various surfaces is essential for the design of functional NHC-based self-assembled monolayers. In this work, we identified the ways by which the deposition of chemically addressable OH-NHCs on Au(111) or Pt(111) surfaces modified the anchoring geometry and chemical reactivity of surface-anchored NHCs. The properties of surface-anchored NHCs were probed by conducting X-ray photoelectron spectroscopy and polarized near-edge X-ray absorption fine structure measurements. While no preferred orientation was identified for OH-NHCs on Pt(111), the anchored molecules adopted a preferred flat-lying position on Au(111). Dehydrogenation and aromatization of the imidazoline ring along with partial hydroxyl oxidation were detected in OH-NHCs that were anchored on Au(111). The dehydrogenation and aromatization reactions were facilitated, along with partial decomposition, for OH-NHCs that were anchored on Pt(111). The spectroscopic results reveal that stronger metal-adsorbate interactions increase the reactivity of surface-anchored OH-NHCs while decreasing their molecular orientational order.

Transition metal-based catalysts for the electrochemical CO2 reduction: from atoms and molecules to nanostructured materials

An overview of the main strategies for the rational design of transition metal-based catalysts for the electrochemical conversion of CO₂, ranging from molecular systems to single-atom and nanostructured catalysts.

Synergistic adsorptions of Na2CO3 and Na2SiO3 on calcium minerals revealed by spectroscopic and ab initio molecular dynamics studies

FTIR, XPS, and ab initio molecular dynamics studies demonstrated that sodium silicate (Na₂SiO₃) adsorbs on fluorite with a higher affinity when they are treated beforehand by sodium carbonate (Na₂CO₃) due to proton exchange(s).

The synergistic effects between sodium silicate (Na₂SiO₃) and sodium carbonate (Na₂CO₃) adsorbed on mineral surfaces are not yet understood, making it impossible to finely tune their respective amounts in various industrial processes. In order to unravel this phenomenon, diffuse reflectance infrared Fourier transform and X-ray photoelectron spectroscopies were combined with ab initio molecular dynamics to investigate the adsorption of Na₂SiO₃ onto bare and carbonated fluorite (CaF₂), an archetypal calcium mineral. Both experimental and theoretical results proved that Na₂CO₃ adsorbs onto CaF₂ with a high affinity and forms a layer of Na₂CO₃ on the surface. Besides, at low Na₂SiO₃ concentration, silica mainly physisorbs in a monomeric protonated form, Si(OH)₄, while at larger concentration, significant amounts of polymerised and deprotonated forms are identified. Prior surface carbonation induces an acid–base reaction on the surface, which results in the formation of the basic forms of the monomers and the dimers, i.e. SiO(OH)₃^– and Si₂O₃(OH)₄^2–, even at low coverage. Their adsorption is highly favoured compared to the acid forms, which explains the synergistic effects observed when Na₂SiO₃ is used after Na₂CO₃. The formation of the basic form on the bare surface is observed only by increasing the surface coverage to 100%. Hence, when Na₂CO₃ is used during a separation process, lower Na₂SiO₃ concentrations are needed to obtain the same effect as with lone Na₂SiO₃ in the separation process.

Flexible NO2 -Functionalized N-Heterocyclic Carbene Monolayers on Au (111) Surface

The formation of flexible self-assembled monolayers (SAMs) in which an external trigger modifies the geometry of surface-anchored molecules is essential for the development of functional materials with tunable properties. In this work, it is demonstrated that NO₂ -functionalized N-heterocyclic carbene molecules (NHCs), which were anchored on Au (111) surface, change their orientation from tilted into flat-lying position following trigger-induced reduction of their nitro groups. DFT calculations identified that the energetic driving force for reorientation was the lower steric hindrance and stronger interactions between the chemically reduced NHCs and the Au surface. The trigger-induced changes in the NHCs' anchoring geometry and chemical functionality modified the work function and the hydrophobicity of the NHC-decorated Au surface, demonstrating the impact of a chemically tunable NHC-based SAM on the properties of the metal surface.