109Ag NMR chemical shift as a descriptor for Brønsted acidity from molecules to materials

13C Chemical Shift of N-Heterocyclic Carbenes in Coinage Metal(I) Complexes and Clusters: trans-Influence and Spin-Orbit Coupling

N-Heterocyclic carbenes (NHCs) are versatile ligands in organometallic chemistry, prized for their strong σ-donating and tunable electronic properties. They are used to stabilize a wide range of motifs, including clusters and nanoparticles, based in particular on coinage metals─Cu, Ag, and Au. Notably, the carbene 13C NMR isotropic chemical shift (δiso) of NHC-coinage metal complexes varies significantly across these elements, reflecting the nuanced interplay of electronic and structural factors. Here, we study the carbene carbon chemical shift in NHC-Au(I)-X complexes (X = H, OH, halides, CN, N3, and neutral ligands such as pyridine and NHC) compared to the Cu and Ag congeners. Density functional theory calculations are used to analyze the chemical shielding tensor components, revealing that stronger σ-donor X-ligands lead to greater deshielding of δiso through enhanced paramagnetic contributions and, for Au, spin-orbit contributions of comparable magnitude. Moreover, a correlation between the spin-orbit contribution to the chemical shift (σso) and the Au-carbene bond distance highlights the critical role of trans-influence in modulating spin-orbit coupling and the overall chemical shift. Analysis of σso shows that stronger σ-donor ligands, associated with a greater trans-influence and elongated Au-carbene bond, lead to a higher-lying NHC-Au σ-bond and lower-lying π*-orbital, ultimately yielding greater deshielding and higher 13C chemical shift. This work provides insight into how structural and electronic factors govern carbene chemical shifts in NHC-based Au complexes and clusters, establishing a direct link between NMR spectroscopic descriptors and electronic structure, thus opening avenues for developing structure-activity relationships in catalysis and materials science.

Phosphine/sulfoxide-carbone, a ligand with a flexible bonding mode for early to late transition metals

In recent years, carbones have emerged as a new exciting class of carbon-based ligands. We report here a series of organometallic complexes demonstrating the versatility in coordination mode of phosphine/sulfoxide carbone 1. Indeed, 1 is able to chelate both early and late transition metals, in a mono- or bidentate fashion depending on the oxyphilic character of metal center thanks to the presence of the sulfoxide moiety. All complexes have been fully characterized by X-ray diffraction analysis and by NMR spectroscopy (except hafnium complex because of its extreme insolubility). It is noteworthy that silver(I) and zirconium(IV) complexes are efficient transmetalating reagents toward copper(I) complexes.

Surface Coordination Chemistry of Graphitic Carbon Nitride from Ag Molecular Probes

Graphitic carbon nitride (g-C3N4) has gained significant attention for its catalytic properties, especially in the development of Single Atom Catalysts (SACs). However, the surface chemistry underlying the formation of these isolated metal sites remains poorly understood. In this study we employ Surface OrganoMetallic Chemistry (SOMC) together with advanced microscopic and spectroscopic techniques for an in-depth analysis of functionalized g-C3N4 materials, where tailored organosilver probe molecules are used to monitor surface processes and characterize resulting surface species. A multi-technique approach - including high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), X-ray absorption spectroscopy (XAS), and multinuclear solid-state Nuclear Magnetic Resonance spectroscopy (ssNMR), coupled with density functional theory (DFT) calculations - identifies three primary surface species in Ag-functionalized g-C3N4: bis-NHC-Ag+, dispersed Ag+ sites, and physisorbed molecular precursor. These findings highlight a dynamic grafting process and provide insights into the surface coordination chemistry of functionalized g-C3N4 materials.

Antimicrobial Activity of Anionic Bis(N-Heterocyclic Carbene) Silver Complexes

The antimicrobial properties of a series of anionic bis(carbene) silver complexes Na3[Ag(NHCR)2] were investigated (2a-2g and 2c', where NHCR is a 2,2'-(imidazol-2-ylidene)dicarboxylate-type N-heterocyclic carbene). The complexes were synthesized by the interaction of imidazolium dicarboxylate compounds with silver oxide in the presence of aqueous sodium hydroxide. Complexes 2f,g were characterized analytically and spectroscopically, and the ligand precursor 1f and complexes 2c and 2g were structurally identified by X-ray diffraction methods. The anions of 2c and 2g, [Ag(NHCR)2]3-, showed a typical linear disposition of Ccarbene-Ag-Ccarbene atoms and an uncommonly eclipsed conformation of carbene ligands. The antimicrobial properties of complexes 2a-g, which contains chiral (2b-2e and 2c') and non-chiral derivatives (2a,f,g), were evaluated against Gram-negative bacteria, Escherichia coli and Pseudomonas aeruginosa, and a Gram-positive bacterium, Staphylococcus aureus. From the observed values of the minimal inhibitory concentration and minimal bactericidal concentration, complexes 2a and 2b showed the best antimicrobial activity against all strains. An interesting chirality-antimicrobial relationship was found, and eutomer 2c' showed better activity than its enantiomer 2c against the three bacteria. Furthermore, these complexes were investigated experimentally and theoretically by 109Ag nuclear magnetic resonance, and the electronic and steric characteristics of the dianionic carbene ligands were also examined.

Synthesis and Luminescence Studies of a Tethered, Trigonal, Silver(I) Tris(alkyne) Complex

The synthesis and characterization of a tris(alkyne) ligand, tris[2-(trimethylsilyl)ethynyl-4-tert-butylbenzyl]amine (1), and its silver(I) hexafluorophosphate complex, 1-Ag, are reported. The solid-state structure and luminescence properties of 1-Ag indicate relatively strong silver(I)-alkyne interactions between the metal cation and 1. No significant changes in the bond angles or lengths were observed upon metalation of 1 with Ag+, indicating a relatively unstrained ligand-metal motif. The luminescence properties of 1 and 1-Ag are also disclosed, showing attenuation in the luminescence intensity upon Ag+ metalation, with Stokes shifts of ∼3700 and ∼3200 cm-1 for 1 and 1-Ag, respectively. The lifetimes of 1-Ag (τ1 = 8.383 ± 0.053 ns and τ2 = 4.665 ± 0.061 ns) were longer than those of 1 (τ1 = 6.708 ± 0.085 ns and τ2 = 3.689 ± 0.025 ns), possibly indicating multiple conformers of 1-Ag in solution. This new silver alkyne platform has potential applications in studies of catalysis, luminescent compounds, and sensing.

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.