Supramolecular Enhancement of Electrochemical Nitrate Reduction Catalyzed by Cobalt Porphyrin Organic Cages for Ammonia Electrosynthesis in Water.

The electrochemical nitrate (NO3 - ) reduction reaction (NO3 RR) to ammonia (NH3 ) represents a sustainable approach for denitrification to balance global nitrogen cycles and an alternative to traditional thermal Haber-Bosch processes. Here, we present a supramolecular strategy for promoting NH3 production in water from NO3 RR by integrating two-dimensional (2D) molecular cobalt porphyrin (CoTPP) units into a three-dimensional (3D) porous organic cage architecture. The porphyrin box CoPB-C8 enhances electrochemical active site exposure, facilitates substrate-catalyst interactions, and improves catalyst stability, leading to turnover numbers and frequencies for NH3 production exceeding 200,000 and 56 s-1 , respectively. These values represent a 15-fold increase in NO3 RR activity and 200-mV improvement in overpotential for the 3D CoPB-C8 box structure compared to its 2D CoTPP counterpart. Synthetic tuning of peripheral alkyl substituents highlights the importance of supramolecular porosity and cavity size on electrochemical NO3 RR activity. These findings establish the incorporation of 2D molecular units into 3D confined space microenvironments as an effective supramolecular design strategy for enhancing electrocatalysis.

Synergistic Porosity and Charge Effects in a Supramolecular Porphyrin Cage Promote Efficient Photocatalytic CO2 Reduction.

We present a supramolecular approach to catalyzing photochemical CO2 reduction through second-sphere porosity and charge effects. An iron porphyrin box (PB) bearing 24 cationic groups, FePB-2(P), was made via post-synthetic modification of an alkyne-functionalized supramolecular synthon. FePB-2(P) promotes the photochemical CO2 reduction reaction (CO2 RR) with 97 % selectivity for CO product, achieving turnover numbers (TON) exceeding 7000 and initial turnover frequencies (TOFmax ) reaching 1400 min-1 . The cooperativity between porosity and charge results in a 41-fold increase in activity relative to the parent Fe tetraphenylporphyrin (FeTPP) catalyst, which is far greater than analogs that augment catalysis through porosity (FePB-3(N), 4-fold increase) or charge (Fe p-tetramethylanilinium porphyrin (Fe-p-TMA), 6-fold increase) alone. This work establishes that synergistic pendants in the secondary coordination sphere can be leveraged as a design element to augment catalysis at primary active sites within confined spaces.

Multifunctional Charge and Hydrogen-Bond Effects of Second-Sphere Imidazolium Pendants Promote Capture and Electrochemical Reduction of CO2 in Water Catalyzed by Iron Porphyrins.

Microenvironments tailored by multifunctional secondary coordination sphere groups can enhance catalytic performance at primary metal active sites in natural systems. Here, we capture this biological concept in synthetic systems by developing a family of iron porphyrins decorated with imidazolium (im) pendants for the electrochemical CO2 reduction reaction (CO2 RR), which promotes multiple synergistic effects to enhance CO2 RR and enables the disentangling of second-sphere contributions that stem from each type of interaction. Fe-ortho-im(H), which poises imidazolium units featuring both positive charge and hydrogen-bond capabilities proximal to the active iron center, increases CO2 binding affinity by 25-fold and CO2 RR activity by 2000-fold relative to the parent Fe tetraphenylporphyrin (Fe-TPP). Comparison with monofunctional analogs reveals that through-space charge effects have a greater impact on catalytic CO2 RR performance compared to hydrogen bonding in this context.

Identification of stomatal-regulating molecules from de novo arylamine collection through aromatic C-H amination.

Stomata-small pores generally found on the leaves of plants-control gas exchange between plant and the atmosphere. Elucidating the mechanism that underlies such control through the regulation of stomatal opening/closing is important to understand how plants regulate photosynthesis and tolerate against drought. However, up-to-date, molecular components and their function involved in stomatal regulation are not fully understood. We challenged such problem through a chemical genetic approach by isolating and characterizing synthetic molecules that influence stomatal movement. Here, we describe that a small chemical collection, prepared during the development of C-H amination reactions, lead to the discovery of a Stomata Influencing Molecule (SIM); namely, a sulfonimidated oxazole that inhibits stomatal opening. The starting molecule SIM1 was initially isolated from screening of compounds that inhibit light induced opening of dayflower stomata. A range of SIM molecules were rapidly accessed using our state-of-the-art C-H amination technologies. This enabled an efficient structure-activity relationship (SAR) study, culminating in the discovery of a sulfonamidated oxazole derivative (SIM*) having higher activity and enhanced specificity against stomatal regulation. Biological assay results have shed some light on the mode of action of SIM molecules within the cell, which may ultimately lead to drought tolerance-conferring agrochemicals through the control of stomatal movement.

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.

N-Heterocyclic carbene and thiol micropatterns enable the selective deposition and transfer of copper films.

Microcontact printed patterns of N-heterocyclic carbenes (NHCs) and thiols were prepared on gold substrates and utilized as templates for the creation of metallic Cu structures using electroplating. The presence of the NHC in the pattern is essential to enable the transfer of the resulting copper microstructures to a second substrate.

Robust, Highly Luminescent Au13 Superatoms Protected by N-Heterocyclic Carbenes.

Gold superatom nanoclusters stabilized entirely by N-heterocyclic carbenes (NHCs) and halides are reported. The reduction of well-defined NHC-Au-Cl complexes produces clusters comprised of an icosahedral Au13 core surrounded by a symmetrical arrangement of nine NHCs and three chlorides. X-ray crystallography shows that the clusters are characterized by multiple CH-π and π-π interactions, which rigidify the ligand and likely contribute to the exceptionally high photoluminescent quantum yields observed, up to 16.0%, which is significantly greater than that of the most luminescent ligand-protected Au13 superatom cluster. Density functional theory analysis suggests that clusters are 8-electron superatoms with a wide HOMO-LUMO energy gap of 2 eV. Consistent with this, the clusters have high stability relative to phosphine stabilized clusters.

N-heterocyclic carbene-functionalized magic-number gold nanoclusters.

Magic-number gold nanoclusters are atomically precise nanomaterials that have enabled unprecedented insight into structure-property relationships in nanoscience. Thiolates are the most common ligand, binding to the cluster via a staple motif in which only central gold atoms are in the metallic state. The lack of other strongly bound ligands for nanoclusters with different bonding modes has been a significant limitation in the field. Here, we report a previously unknown ligand for gold(0) nanoclusters-N-heterocyclic carbenes (NHCs)-which feature a robust metal-carbon single bond and impart high stability to the corresponding gold cluster. The addition of a single NHC to gold nanoclusters results in significantly improved stability and catalytic properties in the electrocatalytic reduction of CO2. By varying the conditions, nature and number of equivalents of the NHC, predominantly or exclusively monosubstituted NHC-functionalized clusters result. Clusters can also be obtained with up to five NHCs, as a mixture of species.

N-Heterocyclic Carbenes in Materials Chemistry.

N-Heterocyclic carbenes (NHCs) have become one of the most widely studied class of ligands in molecular chemistry and have found applications in fields as varied as catalysis, the stabilization of reactive molecular fragments, and biochemistry. More recently, NHCs have found applications in materials chemistry and have allowed for the functionalization of surfaces, polymers, nanoparticles, and discrete, well-defined clusters. In this review, we provide an in-depth look at recent advances in the use of NHCs for the development of functional materials.

Generation and conversion of an N-heterocyclic carbene on Pt(111).

The formation of N-heterocyclic carbenes on a platinum surface is demonstrated. Surface vibrational spectroscopy measurements isolate steps in the transformation from 1,3-diisopropylbenzimidazolium hydrogen carbonate to surface carbene. As the temperature is further increased, the reactive metal transforms the isopropyl groups leading to the effective formation of a new surface carbene.

Self-Assembled N-Heterocyclic Carbene-Based Carboxymethylated Dextran Monolayers on Gold as a Tunable Platform for Designing Affinity-Capture Biosensor Surfaces.

Sensor surfaces play a predominant role in the development of optical biosensor technologies for the analysis of biomolecular interactions. Thiol-based self-assembled monolayers (SAMs) on gold have been widely used as linker layers for sensor surfaces. However, the degradation of the thiol-gold bond can limit the performance and durability of such surfaces, directly impacting their performance and cost-effectiveness. To this end, a new family of materials based on N-heterocyclic carbenes (NHCs) has emerged as an alternative for surface modification, capable of self-assembling onto a gold surface with higher affinity and superior stability as compared to the thiol-based systems. Here we demonstrate three applications of NHC SAMs supporting a dextran layer as a tunable platform for developing various affinity-capture biosensor surfaces. We describe the development and testing of NHC-based dextran biosensor surfaces modified with each of streptavidin, nitrilotriacetic acid, and recombinant Protein A. These affinity-capture sensor surfaces enable oriented binding of ligands for optimal performance in biomolecular assays. Together, the intrinsic high stability and flexible design of the NHC biosensing platforms show great promise and open up exciting possibilities for future biosensing applications.

N-Heterocyclic Carbene Self-Assembled Monolayers on Gold as Surface Plasmon Resonance Biosensors.

Surface plasmon resonance (SPR)-based biosensing is a powerful tool to study the recognition processes between biomolecules in real-time without need for labels. The use of thiol chemistry is a critical component in surface functionalization of various SPR biosensor surfaces on gold. However, its use is hampered by the high propensity for oxidation of the gold-thiol linkage even in ambient atmosphere, resulting in a short lifetime of SPR sensor chips unless strict precautions are taken. Herein, we describe an approach to overcome this limitation by employing highly robust self-assembled monolayers (SAMs) of alkylated N-heterocyclic carbenes (NHCs) on gold. An alkylated NHC sensor surface was developed and its biosensing capabilities were compared to a commercial thiol-based analogue-a hydrophobic association (HPA) chip-in terms of its ability to act as a reliable platform for biospecific interaction analysis under a wide range of conditions. The NHC-based SPR sensor outperforms related thiol-based sesnsors in several aspects, including lower nonspecific binding capacity, better chemical stability, higher reproducibility, shorter equilibration time, and longer life span. We also demonstrate that the NHC-based sensor can be used for rapid and efficient formation of a hybrid lipid bilayer for use in membrane interaction studies. Overall, this work identifies the great promise in designing NHC-based surfaces as a new technology platform for SPR-based biosensing.

Amphiphilic N-Heterocyclic Carbene-Stabilized Gold Nanoparticles and Their Self-Assembly in Polar Solvents.

Soft matter-directed self-assembly of amphiphilic inorganic nanoparticles (NPs) has recently emerged as a promising approach to access NP ensembles with superior collective properties. While thiol-terminated molecules are primarily employed to tether the amphiphilic ligand to the metal, concerns remain regarding the stabilities of the resulting NPs and their corresponding aggregates. As an alternative, we report amphiphilic N-heterocyclic carbene (NHC)-functionalized gold nanoparticles (AuNPs). To accomplish this, an amphiphilic NHC-AuI complex based on an asymmetric triethylene glycol-/dodecyl-functionalized benzimidazole was first synthesized and used to prepare the corresponding stable amphiphilic NHC-decorated AuNPs. The resulting NPs were comprehensively characterized using both solution- and solid-state-based techniques such as proton nuclear magnetic resonance spectroscopy, dynamic light scattering, transmission electron microscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy. By optimizing the self-assembly behavior of these amphiphilic AuNPs in deionized water, ethanol, and their mixtures, we were able to fine-tune the plasmonic properties of the AuNPs in the wide range of 525-640 nm. Furthermore, when treated with thiols, the ensembles showed greater stability compared to their parent discrete AuNP counterparts at room temperature.

Carboxymethylated Dextran-Modified N-Heterocyclic Carbene Self-Assembled Monolayers on Gold for Use in Surface Plasmon Resonance Biosensing.

Surface chemistry is a key enabler for various biosensing applications. Biosensors based on surface plasmon resonance routinely employ thiol-based chemistry for the linker layer between gold-coated support surfaces and functional biosensor surfaces. However, there is a growing awareness that such sensor surfaces are prone to oxidation/degradation problems in the presence of oxygen, and previous efforts to improve the stability have shown limited advancements. As an alternative, recent studies employing N-heterocyclic carbene (NHC) self-assembled monolayers (SAMs) deposited on gold have shown significant promise in this area. Here, we describe a sensor surface employing an NHC SAM to couple a modified carboxymethylated dextran onto a gold surface. Such a dextran matrix is also used for affinity chromatography, and it is the most commonly employed matrix for commercial biosensor surfaces today. The performance reliability of the dextran-modified NHC chip to act as an alternative biosensing platform is compared with that of a thiol-based commercial chip in the proof-of-concept tests. The resultant NHC sensor surface shows a higher thermal stability compared to thiol analogues. Moreover, the plasma protein/drug and antibody/antigen interactions were validated on the NHC-based dextran chip and showed similar performance as compared to the thiol-based commercial chip. Ultimately, this study shows the strong potential applicability of chemical modifications to gold surfaces using NHC ligands for biosensing applications.

N-Heterocyclic Carbene Self-assembled Monolayers on Copper and Gold: Dramatic Effect of Wingtip Groups on Binding, Orientation and Assembly.

Self-assembled monolayers of N-heterocyclic carbenes (NHCs) on copper are reported. The monolayer structure is highly dependent on the N,N-substituents on the NHC. On both Cu(111) and Au(111), bulky isopropyl substituents force the NHC to bind perpendicular to the metal surface while methyl- or ethyl-substituted NHCs lie flat. Temperature-programmed desorption studies show that the NHC binds to Cu(111) with a desorption energy of Edes =152±10 kJ mol-1 . NHCs that bind upright desorb cleanly, while flat-lying NHCs decompose leaving adsorbed organic residues. Scanning tunneling microscopy of methylated NHCs reveals arrays of covalently linked dimers which transform into adsorbed (NHC)2 Cu species by extraction of a copper atom from the surface after annealing.

Simple direct formation of self-assembled N-heterocyclic carbene monolayers on gold and their application in biosensing.

The formation of organic films on gold employing N-heterocyclic carbenes (NHCs) has been previously shown to be a useful strategy for generating stable organic films. However, NHCs or NHC precursors typically require inert atmosphere and harsh conditions for their generation and use. Herein we describe the use of benzimidazolium hydrogen carbonates as bench stable solid precursors for the preparation of NHC films in solution or by vapour-phase deposition from the solid state. The ability to prepare these films by vapour-phase deposition permitted the analysis of the films by a variety of surface science techniques, resulting in the first measurement of NHC desorption energy (158±10 kJ mol(-1)) and confirmation that the NHC sits upright on the surface. The use of these films in surface plasmon resonance-type biosensing is described, where they provide specific advantages versus traditional thiol-based films.

High-energy collision-induced dissociation tandem mass spectrometry of regioisomeric lactose palmitic acid monoesters using matrix-assisted laser desorption/ionization.

RATIONALE: Structural characterization and differentiation of three newly synthesized lactose monopalmitate regioisomers at positions O-3, O-3' and O-6' were realized by single-stage matrix-assisted laser desorption/ionization time-of-flight/time-of-flight mass spectrometry (MALDI-TOF/TOF-MS) in the positive ion mode and by high-energy collision-induced dissociation tandem mass spectrometry (CID-MS/MS). METHODS: A MALDI-TOF/TOF analyzer was utilized for the analysis of these isobaric lactose monopalmitate regioisomers. The CID-MS/MS spectra were acquired using high-energy cid with a 2 kV potential difference between the source acceleration voltage and the collision cell. RESULTS: High-energy (CID) tandem mass spectrometry (MS/MS) analyses of the sodiated molecules, [M + Na](+), showed distinguishing cross-ring product ions and characteristic fingerprint product ions, which allowed the straight-forward mass spectrometric characterization of these different regiosiomers. CONCLUSIONS: This investigation allowed us to unravel the novel fragmentation behavior of the sodiated regioisoimer molecules obtained from the mono-substituted D-lactose fatty acid esters using high-energy CID-TOF/TOF-MS/MS analyses. The high-energy CID of the [M + Na](+) ions from the isobaric lactose monopalmitate regioiosmers promoted the formation of characteristic (0,2) A2 cross-ring cleavage product ions.