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

Abstract

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 13 C NMR spectroscopic signature that clearly distinguishes them from the mono-NHC Au(I) surface sites or supported imidazoliums. We find that bisligated 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.

Mechanism, Chemoselectivity, and Stereoselectivity of an NHC-Catalyzed Reaction of Aldehydes and Hydrazones: A DFT Study

“…Heterocyclic carbenes (NHCs), first isolated and characterized by Arduengo and coworkers in 1991, have gathered considerable interest because of their tremendous potential for synthetic chemistry. During the past few decades, NHCs have been used as potent ligands for transition metals, − nanoparticles, and surfaces. − Such species can also serve as organocatalysts. As powerful organocatalysts, NHCs can catalyze a wide range of reactions, including annulation, − benzoin, − C–C − /C–heteroatom − bond formation, C–H and C–N activation, − stetter, − and radical reactions. , As a result, a broad range of functionalized carbocycles and heterocycles could be constructed by utilizing NHC-catalyzed transformations.…”