Structure of Silver–N-Heterocyclic Carbenes in Solution: Evidence of Equilibration in DMSO at Very Different Time Scales by ¹H NMR Experiments

Ag–N-heterocyclic carbene (NHC) complexes can occur as neutral [(NHC)­AgX] and/or ionic [(NHC)₂Ag]­[AgX₂] species in solution. Herein, the behavior of four complexes bearing common bulky <i>N</i>,<i>N</i>′-diaryl NHCs was examined in DMSO. ¹H NMR experiments performed systematically in CDCl₃ and in DMSO-<i>d</i>₆ were used to detect DMSO or chloride ion induced equilibration between the neutral and ionic forms at very different time scales. We observed that solubilization of IMesAgCl and SIMesAgCl in DMSO at 300 K led in less than a few minutes to a complete equilibration. Here, magnetization exchange rate constants were determined from EXSY experiments. In contrast, IPrAgCl and SIPrAgCl, which possess more bulky NHCs, were found to exhibit much lower exchange rates (or no exchange) at 300 K. IPrAgCl was shown to slowly generate an increasing amount of homoleptic cationic complex in DMSO for several hours before reaching equilibrium. In this case, the initial rate of formation of the cationic complex was determined

Supracrystals of <i>N</i>‑Heterocyclic Carbene-Coated Au Nanocrystals

Controlling the generation of organized 3D assemblies of individual nanocrystals, called supracrystals, as well as their properties, is an important challenge for the design of new materials in which the coating agent plays a major role. We present herein a new generation of structured fcc Au supracrystals made of <i>N</i>-heterocyclic carbene (NHC)-coated Au nanocrystals. The 3D assemblies were achieved by using benzimidazole-derived NHCs tailored with long alkyl chains at different positions. The average size of the nanocrystal precursors (4, 5, or 6 nm) and their ability to self-assemble were found to be dependent on the length, orientation, and number of alkyl chains on the NHC. Thick and large supracrystal domains were obtained from 5 nm Au nanocrystals coated with NHCs substituted by C14 alkyl chains on the nitrogen atoms. Here, the geometry of both the C_carbene–Au and N–C_alkyl bonds induces a specific orientation of the alkyl chains, different from that of alkylthiols, resulting in Au surface covering by the chains. However, the edge-to-edge distances in the supracrystals suggest that the supracrystals are stabilized by interdigitation of neighboring nanocrystals alkyl chains, whose terminal part must point outward with the appropriate geometry

Nanocrystals: Why Do Silver and Gold N‑Heterocyclic Carbene Precursors Behave Differently?

Synthesizing stable Au and Ag nanocrystals of narrow size distribution from metal–N-heterocyclic carbene (NHC) complexes remains a challenge, particularly in the case of Ag and when NHC ligands with no surfactant-like properties are used. The formation of nanocrystals by one-phase reduction of metal–NHCs (metal = Au, Ag) bearing common NHC ligands, namely 1,3-diethylbenzimidazol-2-ylidene (<b>L</b>^<b>1</b>), 1,3-bis­(mesityl)­imidazol-2-ylidene (<b>L</b>^<b>2</b>), and 1,3-bis­(2,6-^<i>i</i>Pr₂C₆H₃)­imidazol-2-ylidene (<b>L</b>^<b>3</b>), is presented herein. We show that both Au and Ag nanocrystals displaying narrow size distribution can be formed by reduction with amine–boranes. The efficiency of the process and the average size and size distribution of the nanocrystals markedly depend on the nature of the metal and NHC ligand, on the sequence in the reactant addition (i.e., presence or absence of thiol during the reduction step), and on the presence or absence of oxygen. Dodecanethiol was introduced to produce stable nanocrystals associated with narrow size distributions. A specific reaction is observed with Ag–NHCs in the presence of thiols whereas Au–NHCs remain unchanged. Therefore, different organometallic species are involved in the reduction step to produce the seeds. This can be correlated to the lack of effect of NHCs on Ag nanocrystal size. In contrast, alteration of Au nanocrystal average size can be achieved with a NHC ligand of great steric bulk (<b>L</b>^<b>3</b>). This demonstrates that a well-defined route for a given metal cannot be extended to another metal