Disparate reactivity of a chiral iron(II) tetracarbene complex with organic azides.

A chiral tetra-NHC iron(II) complex and its disparate reactivity with multiple organic azides is reported. Both aryl and alkyl azides react with the iron(II) complex yielding three distinct products: an iron(IV) imide, an iron(IV) tetrazene, and a surprising and unprecedented double imide insertion complex.

Solvent-Assisted Control of Metal-Organic Nanotube Size and Morphology.

While intensive studies have focused on the synthesis and characterization of new metal-organic nanotube (MONT) structures, the lack of size and morphology control remains an obstacle in broadening applications for this class of materials. Herein, we demonstrate control of MONT crystallite size and morphology by tuning polarity and the protic/aprotic nature of solvents, including dimethylformamide, N-methyl-2-pyrrolidone, ethanol, and 2-methyltetrahydrofuran, for the isostructural syntheses of two MONTs. Through a combination of transmission electron microscopy, powder X-ray diffraction, and selected area electron diffraction, we find that MONT crystallite sizes can be tuned while maintaining control over the relative dispersity without significantly altering the underlying crystal structure.

Unmasking the Electrochemical Stability of N-Heterocyclic Carbene Monolayers on Gold.

N-heterocyclic carbene (NHC) monolayers are transforming electrocatalysis and biosensor design via their increased performance and stability. Despite their increasing use in electrochemical systems, the integrity of the NHC monolayer during voltage perturbations remains largely unknown. Herein, we deploy surface-enhanced Raman spectroscopy (SERS) to measure the stability of two model NHCs on gold in ambient conditions as a function of applied potential and under continuous voltammetric interrogation. Our results illustrate that NHC monolayers exhibit electrochemical stability over a wide voltage window (-1 V to 0.5 V vs Ag|AgCl), but they are found to degrade at strongly reducing (< -1 V) or oxidizing (>0.5 V) potentials. We also address NHC monolayer stability under continuous voltammetric interrogation between 0.2 V and -0.5 V, a commonly used voltage window for sensing, showing they are stable for up to 43 hours. However, we additionally find that modifications of the backbone NHC structure can lead to significantly shorter operational lifetimes. While these results highlight the potential of NHC architectures for electrode functionalization, they also reveal potential pitfalls that have not been fully appreciated in electrochemical applications of NHCs.

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

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

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.

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.

Statistical copolymer metal organic nanotubes.

Metal-organic nanotubes (MONTs) are 1-dimensional crystalline porous materials that are formed from ligands and metals in a manner identical to more typical 3-dimensional metal-organic frameworks (MOFs). MONTs form anisotropically in one dimension making them excellent candidates for linker engineering for control of chemical composition and spacing. A novel series of MONTs was synthesized utilizing a mixture of 1,2,4-ditriazole ligands containing both a fully protonated aryl moiety and its tetrafluorinated analog in ratios of, 0 : 1, 1 : 4, 1 : 1, 4 : 1, and 1 : 0, respectively. All MONTs were characterized by both bulk and nanoscale measurements, including SCXRD, PXRD, ssNMR and TEM, to determine the resulting co-polymer architecture (alternating, block, or statistical) and the ligand ratios in the solid materials. All characterization methods point towards statistical copolymerization of the materials in a manner analogous to 3D MOFs, all of which notably could be achieved without destructive analytical methods.

Giving Gold Wings: Ultrabright and Fragmentation Free Mass Spectrometry Reporters for Barcoding, Bioconjugation Monitoring, and Data Storage.

The widespread application of laser desorption/ionization mass spectrometry (LDI-MS) highlights the need for a bright and multiplexable labeling platform. While ligand-capped Au nanoparticles (AuNPs) have emerged as a promising LDI-MS contrast agent, the predominant thiol ligands suffer from low ion yields and extensive fragmentation. In this work, we develop a N-heterocyclic carbene (NHC) ligand platform that enhances AuNP LDI-MS performance. NHC scaffolds are tuned to generate barcoded AuNPs which, when benchmarked against thiol-AuNPs, are bright mass tags and form unfragmented ions in high yield. To illustrate the transformative potential of NHC ligands, the mass tags were employed in three orthogonal applications: monitoring a bioconjugation reaction, performing multiplexed imaging, and storing and reading encoded information. These results demonstrate that NHC-nanoparticle systems are an ideal platform for LDI-MS and greatly broaden the scope of nanoparticle contrast agents.

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

The discovery of N-heterocyclic carbenes (NHCs) revolutionized organometallic chemistry due to their strong metal-ligand bonds. These strong bonds also lend enhanced stability to gold surfaces and nanoparticles. This stability and high degree of synthetic tunability has allowed NHCs to supplant thiols as the ligand of choice when functionalizing gold surfaces. This review article summarizes the basic science and applications of NHCs on gold surfaces and gold nanoparticles. Additionally, scientific questions that are unique to gold-NHC systems are discussed, such as the NHC adatom binding motif and the NHC surface mobility. Finally, new applications for NHCs on gold are covered with particular attention to biomedicine, catalysis, and microelectronics.

Toward asymmetric aziridination with an iron complex supported by a D2-symmetric tetra-NHC.

A neutral D2-symmetric macrocyclic tetra-N-heterocyclic carbene ligand was synthesized. The macrocycle was ligated to iron(II) via transmetalation from an isolated silver complex that has two conformers. The iron complex catalyzed the first stereospecific aziridination between aryl azides and aliphatic alkenes, albeit with low ee's.

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.

Late Lanthanide Macrocyclic Tetra-NHC Complexes.

An isostructural set of macrocyclic tetra-N-heterocyclic carbene (NHC) complexes were synthesized on late lanthanides including Lu, Yb, Ho, Dy, and Gd. They were characterized by single-crystal X-ray diffraction, multinuclear NMR, electrochemistry, and SQUID magnetometry. Solid-state structures show that all complexes are in a highly distorted square-pyramidal geometry with an axial HMDS ligand. 1H NMR for Lu, Yb, and Dy demonstrates that these geometries are maintained in solution. Electrochemical measurements on the Yb complex show that the NHCs are very strong σ-donors compared to other organometallic Yb complexes. Magnetic measurements of the Yb and Dy complexes reveal slow relaxation of the magnetization in both complexes. The highly anisotropic Dy complex possesses an energy barrier to spin reversal of 52.42 K/36.43 cm-1 and waist-restricted hysteresis up to 2.8 K. Finally, an 18-atom macrocycle variant of the Lu complex was synthesized for comparison in reactivity and stability. These complexes are the first lanthanides prepared with macrocyclic NHCs and suggest that NHCs may be a promising ligand for developing single-molecule magnets.

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.

Actinide tetra-N-heterocyclic carbene 'sandwiches'.

Highly-symmetrical, thorium and uranium octakis-carbene 'sandwich' complexes have been prepared by 'sandwiching' the An(iv) cations between two anionic macrocyclic tetra-NHC ligands, one with sixteen atoms and the other with eighteen atoms. The complexes were characterized by a range of experimental methods and DFT calculations. X-ray crystallography confirms the geometry at the metal centre can be set by the size of the macrocyclic ring, leading to either square prismatic or square anti-prismatic shapes; the geometry of the latter is retained in solution, which also undergoes reversible, electrochemical one-electron oxidation or reduction for the uranium variant. DFT calculations reveal a frontier orbital picture that is similar to thorocene and uranocene, in which the NHC ligands show almost exclusively σ-donation to the metal without π-backbonding.

Imidazolinium N-Heterocyclic Carbene Ligands for Enhanced Stability on Gold Surfaces.

N-heterocyclic carbenes (NHCs) have emerged as versatile and robust ligands for noble metal surface modifications due to their ability to form compact, self-assembled monolayers. Despite a growing body of research, previous NHC surface modification schemes have employed just two structural motifs: the benzimidazolium NHC and the imidazolium NHC. However, different NHC moieties, including saturated NHCs, are often more effective in homogenous catalysis chemistry than these aforementioned motifs and may impart numerous advantages to NHC surfaces, such as increased stability and access to chiral groups. This work explores the preparation and stability of NHC-coated gold surfaces using imidazolium and imidazolinium NHC ligands. X-ray photoelectron spectroscopy and surface-enhanced Raman spectroscopy demonstrate the attachment of NHC ligands to the gold surface and show enhanced stability of imidazolinium compared to the traditional imidazolium under harsh acidic conditions.

In Situ Monitoring of the Seeding and Growth of Silver Metal-Organic Nanotubes by Liquid-Cell Transmission Electron Microscopy.

Metal-organic nanotubes (MONTs) are highly ordered one-dimensional crystalline porous frameworks. Despite being nanomaterials, virtually all studies of MONTs rely on characterization of the bulk crystalline material (micron-sized) by single-crystal X-ray diffraction. For MONTs to achieve their raison d'être as tunable one-dimensional nanomaterials, individual tubes or small finite bundles of tubes must be synthesized and characterized. Therefore, to directly observe their formation under a variety of reaction conditions in solution, we employ liquid-cell transmission electron microscopy (LCTEM), which allows the early stages of MONT assembly to be monitored in real time. Notably, changing the metal-to-ligand ratio alters the local concentrations of reactant monomers, resulting in multiple nucleation and growth pathways and diverse morphologies at the nanoscale. These various initial seeds grow to form the same nanocrystalline needle phase. This approach of employing LCTEM to study these nanomaterials is analogous to monitoring typical homogeneous solution phase reactions by NMR for controlled nanomaterial formation.

A Benchtop Method for Appending Protic Functional Groups to N-Heterocyclic Carbene Protected Gold Nanoparticles.

The remarkable resilience of N-heterocyclic carbene (NHC) gold bonds has quickly made NHCs the ligand of choice when functionalizing gold surfaces. Despite rapid progress using deposition from free or CO2 -protected NHCs, synthetic challenges hinder the functionalization of NHC surfaces with protic functional groups, such as alcohols and amines, particularly on larger nanoparticles. Here, we synthesize NHC-functionalized gold surfaces from gold(I) NHC complexes and aqueous nanoparticles without the need for additional reagents, enabling otherwise difficult functional groups to be appended to the carbene. The resilience of the NHC-Au bond allows for multi-step post-synthetic modification. Beginning with the nitro-NHC, we form an amine-NHC terminated surface, which further undergoes amide coupling with carboxylic acids. The simplicity of this approach, its compatibility with aqueous nanoparticle solutions, and its ability to yield protic functionality, greatly expands the potential of NHC-functionalized noble metal surfaces.

A Chiral Macrocyclic Tetra-N-Heterocyclic Carbene Yields an "All Carbene" Iron Alkylidene Complex.

The first chiral macrocyclic tetra-N-heterocyclic carbene (NHC) ligand has been synthesized. The macrocycle, prepared in high yield and large scale, was ligated onto palladium and iron to give divalent C2 -symmetric square planar complexes. Multinuclear NMR and single crystal X-ray diffraction demonstrated that there are two distinct NHCs on each ligand, due to the bridging chiral cyclohexane. Oxidation of the iron(II) complex with trimethylamine N-oxide yielded a bridging oxo complex. Diazodiphenylmethane reacted with the iron(II) complex at room temperature to give a paramagnetic diazoalkane complex; the same reaction yielded the "all carbene" complex at elevated temperature. Electrochemical measurements support the assignment of the "all carbene" complex being an alkylidene. Notably, the diazoalkane complex can be directly transformed into the alkylidene complex, which had not been previously demonstrated on iron. Finally, a test catalytic reaction with a diazoalkane on the iron(II) complex does not yield the expected cyclopropane, but actually the azine compound.

One macrocyclic ligand, four oxidation states: A 16-atom ringed dianionic tetra-NHC macrocycle and its Cr(II) through Cr(V) complexes.

Despite chromium being among the first transition metals ever reported to bind to an NHC, chromium NHC complexes, especially in mid and high oxidation states, have received scant attention. Herein, the synthesis, characterization, and reactivity of a series of Cr(II) to Cr(V) complexes bearing a 16-atom ringed dianionic tetra-NHC macrocycle are reported. The Cr(II) dimer is diamagnetic and displays a very short Cr-Cr quadruple bond, unprecedented for Cr-NHC complexes to date. Oxidative cleavage of the Cr-Cr bond leads to the formation of a highly stable diamagnetic Cr(IV) oxo complex. Similar reactions with organic azides lead to paramagnetic Cr(IV) imide complexes. Notably, the Cr(IV) oxo can be oxidized in a reversible reaction to yield a Cr(V) cationic oxo complex, which is a very rare high oxidation state Cr-NHC-compound. This Cr(V) oxo undergoes stoichiometric oxygen atom transfer. Similar reactions were attempted with molybdenum and tungsten to form macrocyclic NHC complexes, but only a molybdenum dimer could be isolated.

Elucidation of the Reaction Mechanism of C2 + N1 Aziridination from Tetracarbene Iron Catalysts.

A combined computational and experimental study was undertaken to elucidate the mechanism of catalytic C2 + N1 aziridination supported by tetracarbene iron complexes. Three specific aspects of the catalytic cycle were addressed. First, how do organic azides react with different iron catalysts and why are alkyl azides ineffective for some catalysts? Computation of the catalytic pathway using density functional theory (DFT) revealed that an alkyl azide needs to overcome a higher activation barrier than an aryl azide to form an iron imide, and the activation barrier with the first-generation catalyst is higher than the activation barrier with the second-generation variant. Second, does the aziridination from the imide complex proceed through an open-chain radical intermediate that can change stereochemistry or, instead, via an azametallacyclobutane intermediate that retains stereochemistry? DFT calculations show that the formation of aziridine proceeds via the open-chain radical intermediate, which qualitatively explains the formation of both aziridine diastereomers as seen in experiments. Third, how can the formation of the side product, a metallotetrazene, be prevented, which would improve the yield of aziridine at lower alkene loading? DFT and experimental results demonstrate that sterically bulky organic azides prohibit formation of the metallotetrazene and, thus, allow lower alkene loading for effective catalysis. These multiple insights of different aspects of the catalytic cycle are critical for developing improved catalysts for C2 + N1 aziridination.