Addressable Carbene Anchors for Gold Surfaces

Organometallic Chemistry Tools for Building Biologically Relevant Nanoscale Systems

The recent emergence of organometallic chemistry for modification of biomolecular nanostructures has begun to rewrite the long-standing assumption among practitioners that small-molecule organometallics are fundamentally incompatible with biological systems. This Perspective sets out to clarify some of the existing misconceptions by focusing on the growing organometallic toolbox for biomolecular modification. Specifically, we highlight key organometallic transformations in constructing complex biologically relevant systems on the nanomolecular scale, and the organometallic synthesis of hybrid nanomaterials composed of classical nanomaterial components combined with biologically relevant species. As research progresses, many of the challenges associated with applying organometallic chemistry in this context are rapidly being reassessed. Looking to the future, the growing utility of organometallic transformations will likely make them more ubiquitous in the construction and modification of biomolecular nanostructures.

IPr# Complexes-Highly-Hindered, Sterically-Bulky Cu(I) and Ag(I) N-Heterocyclic Carbenes: Synthesis, Characterization, and Reactivity

Metal-N-heterocyclic carbene (M-NHC) complexes are well-known as an important class of organometallic compounds widely used in transition-metal catalysis. Taking into account that the steric hindrance around the metal center is one of the major effects in M-NHC catalysis, the development of new, sterically hindered M-NHC complexes is an ongoing interest in this field of research. Herein, we report the synthesis and characterization of exceedingly sterically hindered, well-defined, air- and moisture-stable Cu(I) and Ag(I) complexes, [Cu(NHC)Cl] and [Ag(NHC)Cl], in the recently discovered IPr# family of ligands that hinge upon modular peralkylation of anilines. The complexes in both the BIAN and IPr families of ligands are reported. X-ray crystallographic analyses and computational studies were conducted to determine steric effects, Frontier molecular orbitals, and bond orders. The complexes were evaluated in the model hydroboration of the alkynes. We identified [Cu(BIAN-IPr#)Cl] and [Ag(BIAN-IPr#)Cl] as highly reactive catalysts with the reactivity outperforming the classical IPr and IPr*. Considering the attractive features of well-defined Cu(I)-NHC and Ag(I)-NHC complexes, this class of sterically bulky yet wingtip-flexible complexes will be of interest for catalytic processes in various areas of organic synthesis and catalysis.

Fabrication of azido-PEG-NHC stabilized gold nanoparticles as a functionalizable platform

Rapid and precise detection of biochemical markers is vital for accurate medical diagnosis. Gold nanoparticles (AuNPs) have emerged as promising candidates for diagnostic sensing due to their biocompatibility and distinctive physical properties. However, AuNPs functionalized with selective targeting vectors often suffer from reduced stability in complex biological environments. To address this, (N)-heterocyclic carbene (NHC) ligands have been investigated for their robust binding affinity to AuNP surfaces, enhancing stability. This study outlines an optimized top-down synthesis route for highly stable, azide-terminal PEGylated NHC (PEG-NHC) functionalized AuNPs. This process employs well-defined oleylamine-protected AuNPs and masked PEGylated NHC precursors. The activation and attachment mechanisms of the masked NHCs were elucidated through the identification of intermediate AuNPs formed during incomplete ligand exchange. The resulting PEG-NHC@AuNPs exhibit exceptional colloidal stability across various biologically relevant media, showing no significant aggregation or ripening over extended periods. These particles demonstrate superior stability compared to those synthesized via a bottom-up approach. Further functionalization of azide-terminal PEG-NHC@AuNPs was achieved through copper-catalyzed click- and bioorthogonal strain-promoted azide-alkyne cycloaddition reactions. The maintained colloidal stability and successful conjugation highlight the potential of azide-functionalized PEG-NHC@AuNPs as a versatile platform for a wide range of biomedical applications.

Highly Stable Carboranyl Ligated Gold Nano-Catalysts for Regioselective Aromatic Bromination

Achieving electronic/steric control and realizing selectivity regulation in nanocatalysis remains a formidable challenge, as the dynamic nature of metal-ligand interfaces, including dissolution (metal leaching) and structural reconstruction, poses significant obstacles. Herein, we disclose carboranyls (CBs) as unprecedented carbon-bonded functional ligands (Eads.CB-Au(111)=-2.90 eV) for gold nanoparticles (AuNPs), showcasing their exceptional stabilization capability that is attributed by strong Au-C bonds combined with B-H⋅⋅⋅Au interactions. The synthesized CB@AuNPs exhibit core(Aun)-satellite(CB2Au-) structure, showing high stability towards multiple stimuli (110 °C, pH=1-12, thiol etchants). In addition, different from conventional AuNP catalysts such as triphenylphosphine (PPh3) stabilized AuNPs, dissolution of catalytically active gold species was suppressed in CB@AuNPs under the reaction conditions. Leveraging these distinct features, CB@AuNPs realized outstanding p : o selectivities in aromatic bromination. Unbiased arenes including chlorobenzene (up to >30 : 1), bromobenzene (15 : 1) and phenyl acrylate were examined using CB@AuNPs as catalysts to afford highly-selective p-products. Both carboranyl ligands and carboranyl derived counterions are crucial for such regioselective transformation. This work has provided valuable insights for AuNPs in realizing diverse regioselective transformations.

Metal-ligand bond in group-11 complexes and nanoclusters

Density functional theory is used to study geometric, energetic, and electronic properties of metal-ligand bonds in a series of group-11 metal complexes and ligand-protected metal clusters. We study complexes as the forms of M-L (L = SCH3, SC8H9, PPh3, NHCMe, NHCEt, NHCiPr, NHCBn, CCMe, CCPh) and L1-M-L2 (L1 = NHCBn, PPh3, and L2 = CCPh). Furthermore, we study clusters denoted as [M13L6Br6]- (L = PPh3, NHCMe, NHCEt, NHCiPr, NHCBn). The systems were studied at the standard GGA level using the PBE functional and including vdW corrections via BEEF-vdW. Generally, Au has the highest binding energies, followed by Cu and Ag. PBE and BEEF-vdW functionals show the order Ag-L > Au-L > Cu-L for bond lengths in both M-L complexes and metal clusters. In clusters, the smallest side group (CH3) in NHCs leads to the largest binding energy whereas no significant variations are seen concerning different side groups of NHC in M-L complexes. By analyzing the projected density of states and molecular orbitals in complexes and clusters, the M-thiolate bonds were shown to have σ and π bond characteristics whereas phosphines and carbenes were creating σ bonds to the transition metals. Interestingly, this analysis revealed divergent behavior for M-alkynyl complexes: while the CCMe group displayed both σ and π bonding features, the CCPh ligand was found to possess only σ bond properties in direct head-to-head binding configuration. Moreover, synergetic effects increase the average binding strength to the metal atom significantly in complexes of two different ligands and underline the potential of adding Cu to synthesize structurally richer cluster systems. This study helps in understanding the effects of different ligands on the stability of M-L complexes and clusters and suggests that PPh3 and NHCs-protected Cu clusters are most stable after Au clusters.

Energetics and Redox Kinetics of Pure Ferrocene-Terminated N-Heterocyclic Carbene Self-Assembled Monolayers on Gold

N-heterocyclic carbene (NHC) self-assembled monolayers (SAMs) on gold have received considerable attention, but little is known about the lateral interactions between neighboring NHC molecules, their stability when subjected to aggressive oxidizing/reducing conditions, and their interactions with solution ions, all of which are essential for their use in a wide range of applications. To address these deficiencies, we present a comprehensive investigation of two different ferrocene (Fc)-terminated NHC SAMs with different chain lengths and linking groups. Pure monolayers of Fc-terminated NHCs display only a single, symmetrical pair of redox peaks, implying the formation of a homogeneous SAM structure with uniformly distributed Fc/Fc+ redox centers. By comparison, pure Fc-alkylthiol SAMs exhibit complex and impractical redox chemistry and require surface dilution in order to achieve reproducible properties. The NHC SAMs examined in this study exhibit very fast Fc redox kinetics and comparable or even superior stability against the application of multiple potential cycles or long-time holding at constant potential compared to alkylthiol SAMs. Furthermore, ion pairing of Fc+ and hydrophobic perchlorate and other hydrophilic anions is observed with Fc-NHC SAMs, highlighting conditions favorable for future applications of these monolayers. This study should therefore shed light on the very promising characteristics of redox-active NHC SAMs as an alternative to traditional Fc-alkylthiol SAMs for multiple practical applications, including in sensors and electrocatalysis.

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.

Minimizing Contact Resistance and Flicker Noise in Micro Graphene Hall Sensors Using Persistent Carbene Modified Gold Electrodes

Scalable micro graphene Hall sensors (μGHSs) hold tremendous potential for highly sensitive and label-free biomagnetic sensing in physiological solutions. To enhance the performance of these devices, it is crucial to optimize frequency-dependent flicker noise to reduce the limit of detection (LOD), but it remains a great challenge due to the large contact resistance at the graphene-metal contact. Here we present a surface modification strategy employing persistent carbene on gold electrodes to reduce the contact resistivity by a factor of 25, greatly diminishing μGHS flicker noise by a factor of 1000 to 3.13 × 10-14 V2/Hz while simultaneously lowering the magnetic LOD SB1/2 to 1440 nT/Hz1/2 at 1 kHz under a 100 μA bias current. To the best of our knowledge, this represents the lowest SB1/2 reported for scalable μGHSs fabricated through wafer-scale photolithography. The reduction in contact noise is attributed to the π-π stacking interaction between the graphene and the benzene rings of persistent carbene, as well as the decrease in the work function of gold as confirmed by Kelvin Probe Force Microscopy. By incorporating a microcoil into the μGHS, we have demonstrated the real-time detection of superparamagnetic nanoparticles (SNPs), achieving a remarkable LOD of ∼528 μg/L. This advancement holds great potential for the label-free detection of magnetic biomarkers, e.g., ferritin, for the early diagnosis of diseases associated with iron overload, such as hereditary hemochromatosis (HHC).

Beyond the Gold-Thiol Paradigm: Exploring Alternative Interfaces for Electrochemical Nucleic Acid-Based Sensing

Nucleic acid-based electrochemical sensors (NBEs) use oligonucleotides as affinity reagents for the detection of a variety of targets, ranging from small-molecule therapeutics to whole viruses. Because of their versatility in molecular sensing, NBEs are being developed broadly for diagnostic and biomedical research applications. Benchmark NBEs are fabricated via self-assembly of thiol-based monolayers on gold. Although robust for rapid prototyping, thiol monolayers suffer from limitations in terms of stability under voltage modulation and in the face of competitive ligands such as thiolated molecules naturally occurring in biofluids. Additionally, gold cannot be deployed as an NBE substrate for all biomedical applications, such as in cases where molecular measurements coupled to real-time, under-the-sensor tissue imaging is needed. Seeking to overcome these limitations, the field of NBEs is pursuing alternative ligands and electrode surfaces. In this perspective, I discuss new interface fabrication strategies that have successfully achieved NBE sensing, or that have the potential to allow NBE sensing on conductive surfaces other than gold. I hope this perspective will provide the reader with a fresh view of how future NBE interfaces could be constructed and will serve as inspiration for the pursuit of collaborative developments in the field of NBEs.

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.

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.

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.

Chiral NHC Ligands for Enantioselective Gold(I) Catalysis Under Aerobic Conditions: the Importance of Conformational Flexibility and Steric Hindrance of NHC Ligand on Reactivity

Gold(I) catalysis has been recognized as a valuable tool for the unique transformation of multiple carbon-carbon bonds. Enantioselective π-catalysis based on gold(I) complexes is, however, still underdeveloped due to lack of privileged ligands. Herein, we present an accessible method to a new family of stable yet catalytically active chiral NHC-Au(I)-Cl complexes. The key to preserving a simultaneous fine balance between reactivity and stability in this newly developed family appears to be sterically hindered, but conformationally flexible NHC ligands. These could be easily accessed on a multigram scale by merging sterically hindered anilines with commercially available amino alcohols and amines via a four-steps synthetic sequence without the need for chromatographic purification. Further investigations of the catalytic activity of NHC-Au-Cl complexes identified the OH functionality incorporated into the NHC core as crucial for the level of enantioselectivity as well as the TsO- anion responsible for the activation of NHC-Au(I)-Cl. Finally, NMR studies and X-ray investigations revealed for the first time that the widely accepted ion metathesis (NHC-Au-Cl to NHC-Au-OSO2 R) responsible for the activation of NHC-Au-Cl complexes does not take place (or it is very slow) in commonly used MeNO2 in contrast to DCM.

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.

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.

IPr*Oxa - a new class of sterically-hindered, wingtip-flexible N,C-chelating oxazole-donor N-heterocyclic carbene ligands

N-heterocyclic carbenes (NHCs) have emerged as a major direction in ancillary ligand development for stabilization of reactive metal centers in inorganic and organometallic chemistry. In particular, wingtip-flexible NHCs have attracted significant attention due to their unique ability to provide a sterically-demanding environment for transition metals in various oxidation states. Herein, we report a new class of sterically-hindered, wingtip-flexible NHC ligands that feature N,C-chelating oxazole donors. These ligands are readily accessible through a modular arylation of oxazole derivatives. We report their synthesis and complete structural and electronic characterization. The evaluation of steric, electron-donating and π-accepting properties and coordination chemistry to Ag(I), Pd(II) and Rh(I) is described. Preliminary studies of catalytic activity in Ag, Pd and Rh-catalyzed coupling and hydrosilylation reactions are presented. This study establishes the fluxional behavior of a freely-rotatable oxazole unit, wherein the oxazolyl ring adjusts to the steric and electronic environment of the metal center. Considering the tremendous impact of sterically-hindered NHCs and their potential to stabilize reactive metals by N-chelation, we expect that this class of NHC ligands will be of broad interest in inorganic and organometallic chemistry.

[Au(Np#)Cl]: Highly Reactive and Broadly Applicable Au(I)─NHC Catalysts for Alkyne π-Activation Reactions

Cationic Au(I)─NHC (NHC = N-heterocyclic carbene) complexes have become an important class of catalysts for alkyne π-activation reactions in organic synthesis. In particular, these complexes are characterized by high stability of catalytic species engendered by strong σ-donation and metal backbonding. Herein, we report the synthesis and characterization of well-defined [Au(NHC)Cl] complexes featuring recently discovered IPr# family of ligands that hinge upon modular peralkylation of aniline. These ligands have been commercialized in collaboration with MilliporeSigma (IPr#: 915653; Np#: 915912; BIAN-IPr#: 916420). Evaluation of the [Au(NHC)Cl] complexes in a series of Au(I)─NHC-catalyzed π-functionalizations of alkynes, such as hydrocarboxylation, hydroamination and hydration, resulted in the identification of wingtip-flexible [Au(Np#)Cl] as a highly reactive and broadly applicable catalyst with the re-activity outperforming the classical [Au(IPr)Cl] and [Au(IPr*)Cl] complexes. The utility of this catalyst has been demonstrated in the direct late-stage derivatization of complex pharmaceuticals. Structural and computational studies were conducted to determine steric effects, frontier molecular orbitals and bond orders of this class of catalysts. Considering the attractive features of well-defined Au(I)─NHC complexes, we anticipate that this class of bulky and wingtip-flexible Au(I)─NHCs based on the modular peralkylated naphthylamine scaffold will find broad application in π-functionalization of alkynes in various areas of organic synthesis and catalysis.

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.

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.

Stability of N-Heterocyclic Carbene Monolayers under Continuous Voltammetric Interrogation

N-Heterocyclic carbenes (NHCs) are promising monolayer-forming ligands that can overcome limitations of thiol-based monolayers in terms of stability, surface functionality, and reactivity across a variety of transition-metal surfaces. Recent publications have reported the ability of NHCs to support biomolecular receptors on gold substrates for sensing applications and improved tolerance to prolonged biofluid exposure relative to thiols. However, important questions remain regarding the stability of these monolayers when subjected to voltage perturbations, which is needed for applications with electrochemical platforms. Here, we investigate the ability of two NHCs, 1,3-diisopropylbenzimidazole and 5-(ethoxycarbonyl)-1,3-diisopropylbenzimidazole, to form monolayers via self-assembly from methanolic solutions of their trifluoromethanesulfonate salts. We compare the electrochemical behavior of the resulting monolayers relative to that of benchmark mercaptohexanol monolayers in phosphate-buffered saline. Within the -0.15 to 0.25 V vs Ag|AgCl voltage window, NHC monolayers are stable on gold surfaces, wherein they electrochemically perform like thiol-based monolayers and undergo similar reorganization kinetics, displaying long-term stability under incubation in buffered media and under continuous voltammetric interrogation. At negative voltages, NHC monolayers cathodically desorb from the electrode surface at lower bias (-0.1 V) than thiol-based monolayers (-0.5 V). At voltages more positive than 0.25 V, NHC monolayers anodically desorb from electrode surfaces at similar voltages to thiol-based monolayers. These results highlight new limitations to NHC monolayer stability imposed by electrochemical interrogation of the underlying gold electrodes. Our results serve as a framework for future optimization of NHC monolayers on gold for electrochemical applications, as well as structure-functionality studies of NHCs on gold.

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

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

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.

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.

[Ni(Np#)(η5-Cp)Cl]: Flexible, Sterically Bulky, Well-Defined, Highly Reactive Complex for Nickel-Catalyzed Cross-Coupling

Ni-NHCs (NHC = N-heterocyclic carbene) have become an increasingly important class of complexes in catalysis and organometallic chemistry owing to the beneficial features of nickel as an abundant 3d metal. However, the development of well-defined and air-stable Ni-NHC complexes for cross-coupling has been more challenging than with Pd-NHC catalysis because of less defined reactivity trends of NHC ancillary ligands coordinated to Ni. Herein, we report the synthesis and catalytic activity of well-defined [Ni(NHC)(η5-Cp)Cl] complexes bearing recently commercialized IPr# family of ligands (Sigma Aldrich) and versatile cyclopentadienyl throw-away ligand. The NHC ligands, IPr#, Np# and BIAN-IPr#, are prepared by robust and modular peralkylation of anilines. Most crucially, we identified [Ni(Np#)(η5-Cp)Cl] as a highly reactive [Ni(NHC)(η5-Cp)Cl] complex, with the reactivity outperforming the classical [Ni(IPr)(η5-Cp)Cl] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). These [Ni(NHC)(η5-Cp)Cl] precatalysts were employed in the Suzuki and Kumada cross-coupling of aryl chlorides and aryl bromides. Computational studies were conducted to determine steric effect and bond order analysis. Considering the attractive features of well-defined Ni-NHCs, we anticipate that this class of bulky and flexible Ni-NHC catalysts will find broad application in organic synthesis and catalysis.

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.

N-Heterocyclic Carbene Complexes of Nickel(II) from Caffeine and Theophylline: Sustainable Alternative to Imidazol-2-ylidenes

Xanthines, such as caffeine and theophylline, are abundant natural products that are often present in foods. Leveraging renewable and benign resources for ligand design in organometallic chemistry and catalysis is one of the major missions of green and sustainable chemistry. In this Special Issue on Sustainable Organometallic Chemistry, we report the first nickel-N-heterocyclic carbene complexes derived from Xanthines. Well-defined, air- and moisture-stable, half-sandwich, cyclopentadienyl [CpNi(NHC)I] nickel-NHC complexes are prepared from the natural products caffeine and theophylline. The model complex has been characterized by x-ray crystallography. The evaluation of steric, electron-donating and π-accepting properties is presented. High activity in the model Suzuki-Miyaura cross-coupling is demonstrated. The data show that nickel-N-heterocyclic carbenes derived from both Earth abundant 3d transition metal and renewable natural products represent a sustainable alternative to the classical imidazol-2-ylidenes.

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.

N-Heterocyclic Carbene Based Nanolayer for Copper Film Oxidation Mitigation

The wide use of copper is limited by its rapid oxidation. Main oxidation mitigation approaches involve alloying or surface passivation technologies. However, surface alloying often modifies the physical properties of copper, while surface passivation is characterized by limited thermal and chemical stability. Herein, we demonstrate an electrochemical approach for surface-anchoring of an N-heterocyclic carbene (NHC) nanolayer on a copper electrode by electro-deposition of alkyne-functionalized imidazolium cations. Water reduction reaction generated a high concentration of hydroxide ions that induced deprotonation of imidazolium cations and self-assembly of NHCs on the copper electrode. In addition, alkyne group deprotonation enabled on-surface polymerization by coupling surface-anchored and solvated NHCs, which resulted in a 2 nm thick NHC-nanolayer. Copper film coated with a NHC-nanolayer demonstrated high oxidation resistance at elevated temperatures and under alkaline conditions.

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.

Plasmon-Driven Oxidative Coupling of Aniline-Derivative Adsorbates: A Comparative Study of para-Ethynylaniline and para-Mercaptoaniline

Plasmon-driven photocatalysis has emerged as a paradigm-shifting approach, based upon which the energy of photons can be judiciously harnessed to trigger interfacial molecular transformations on metallic nanostructure surfaces in a regioselective manner with nanoscale precision. Over the past decade, the formation of aromatic azo compounds through plasmon-driven oxidative coupling of thiolated aniline-derivative adsorbates has become a testbed for developing detailed mechanistic understanding of plasmon-mediated photochemistry. Such photocatalytic bimolecular coupling reactions may occur not only between thiolated aniline-derivative adsorbates but between their nonthiolated analogues as well. How the nonthiolated adsorbates behave differently from their thiolated counterparts during the plasmon-driven coupling reactions, however, remains largely unexplored. Here, we systematically compare an alkynylated aniline-derivative, para-ethynylaniline, to its thiolated counterpart, para-mercaptoaniline, in terms of their adsorption conformations, structural flexibility, photochemical reactivity, and transforming kinetics on Ag nanophotocatalyst surfaces. We employ surface-enhanced Raman scattering as an in situ spectroscopic tool to track the detailed structural evolution of the transforming molecular adsorbates in real time during the plasmon-driven coupling reactions. Rigorous analysis of the spectroscopic results, further aided by density functional theory calculations, lays an insightful knowledge foundation that enables us to elucidate how the alteration of the chemical nature of metal-adsorbate interactions profoundly influences the transforming behaviors of the molecular adsorbates during plasmon-driven photocatalytic reactions.

NHC-Stabilized Au10 Nanoclusters and Their Conversion to Au25 Nanoclusters

Herein, we describe the synthesis of a toroidal Au10 cluster stabilized by N-heterocyclic carbene and halide ligands via reduction of the corresponding NHC-Au-X complexes (X = Cl, Br, I). The significant effect of the halide ligands on the formation, stability, and further conversions of these clusters is presented. While solutions of the chloride derivatives of Au10 show no change even upon heating, the bromide derivative readily undergoes conversion to form a biicosahedral Au25 cluster at room temperature. For the iodide derivative, the formation of a significant amount of Au25 was observed even upon the reduction of NHC-Au-I. The isolated bromide derivative of the Au25 cluster displays a relatively high (ca. 15%) photoluminescence quantum yield, attributed to the high rigidity of the cluster, which is enforced by multiple CH-π interactions within the molecular structure. Density functional theory computations are used to characterize the electronic structure and optical absorption of the Au10 cluster. 13C-Labeling is employed to assist with characterization of the products and to observe their conversions by NMR spectroscopy.

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.

Systematic investigation of the influence of electronic substituents on dinuclear gold(I) amidinates: synthesis, characterisation and photoluminescence studies

Dinuclear gold(I) compounds are of great interest due to their aurophilic interactions that influence their photophysical properties. Herein, we showcase that gold-gold interactions can be influenced by tuning the electronic properties of the ligands. Therefore, various para substituted (R) N,N'-bis(2,6-dimethylphenyl)formamidinate ligands (pRXylForm; Xyl = 2,6-dimethylphenyl and Form = formamidinate) were treated with Au(tht)Cl (tht = tetrahydrothiophene) to give via salt metathesis the corresponding gold(I) compounds [pRXylForm₂Au₂] (R = -OMe, -Me, -Ph, -H, -SMe, and -CO₂Me). All complexes showed intense luminescence properties at low temperatures. Alignment with the Hammett parameter σ_p revealed the trends in the ¹H and ¹³C NMR spectra. These results showed the influence of the donor-acceptor abilities of different substituents on the ligand system which were confirmed with calculated orbital energies. Photophysical investigations showed their lifetimes in the millisecond range indicating phosphorescence processes and revealed a redshift with the decreasing donor ability of the substituents in the solid state.

N-Heterocyclic Carbene Ligand Stability on Gold Nanoparticles in Biological Media

The ability to functionalize gold nanoparticle surfaces with target ligands is integral to developing effective nanosystems for biomedical applications, ranging from point-of-care diagnostic devices to site-specific cancer therapies. By forming strong covalent bonds with gold, thiol functionalities can easily link molecules of interest to nanoparticle surfaces. Unfortunately, thiols are inherently prone to oxidative degradation in many biologically relevant conditions, which limits their broader use as surface ligands in commercial assays. Recently, N-heterocyclic carbene (NHC) ligands emerged as a promising alternative to thiols since initial reports demonstrated their remarkable stability against ligand displacement and stronger metal-ligand bonds. This work explores the long-term stability of NHC-functionalized gold nanoparticles suspended in five common biological media: phosphate-buffered saline, tris-glycine potassium buffer, tris-glycine potassium magnesium buffer, cell culture media, and human serum. The NHCs on gold nanoparticles were probed with surface-enhanced Raman spectroscopy (SERS) and X-ray photoelectron spectroscopy (XPS). SERS is useful for monitoring the degradation of surface-bound species because the resulting vibrational modes are highly sensitive to changes in ligand adsorption. Our measurements indicate that imidazole-based NHCs remain stable on gold nanoparticles over the 21 days of examination in all tested environments, with no observed change in the molecule's SERS signature, XPS response, or UV-vis plasmon band.

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.

N-Heterocyclic carbenes and their precursors in functionalised porous materials

Though N-heterocyclic carbenes (NHCs) have emerged as diverse and powerful discrete functional molecules in pharmaceutics, nanotechnology, and catalysis over decades, the heterogenization of NHCs and their precursors for broader applications in porous materials, like metal-organic frameworks (MOFs), porous coordination polymers (PCPs), covalent-organic frameworks (COFs), porous organic polymers (POPs), and porous organometallic cages (POMCs) was not extensively studied until the last ten years. By de novo or post-synthetic modification (PSM) methods, myriads of NHCs and their precursors containing building blocks were designed and integrated into MOFs, PCPs, COFs, POPs and POMCs to form various structures and porosities. Functionalisation with NHCs and their precursors significantly expands the scope of the potential applications of porous materials by tuning the pore surface chemical/physical properties, providing active sites for binding guest molecules and substrates and realizing recyclability. In this review, we summarise and discuss the recent progress on the synthetic methods, structural features, and promising applications of NHCs and their precursors in functionalised porous materials. At the end, a brief perspective on the encouraging future prospects and challenges in this contemporary field is presented. This review will serve as a guide for researchers to design and synthesize more novel porous materials functionalised with NHCs and their precursors.

A single-molecule blueprint for synthesis

Chemical reactions that occur at nanostructured electrodes have garnered widespread interest because of their potential applications in fields including nanotechnology, green chemistry and fundamental physical organic chemistry. Much of our present understanding of these reactions comes from probes that interrogate ensembles of molecules undergoing various stages of the transformation concurrently. Exquisite control over single-molecule reactivity lets us construct new molecules and further our understanding of nanoscale chemical phenomena. We can study single molecules using instruments such as the scanning tunnelling microscope, which can additionally be part of a mechanically controlled break junction. These are unique tools that can offer a high level of detail. They probe the electronic conductance of individual molecules and catalyse chemical reactions by establishing environments with reactive metal sites on nanoscale electrodes. This Review describes how chemical reactions involving bond cleavage and formation can be triggered at nanoscale electrodes and studied one molecule at a time.

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.

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.

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.