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 (L1), 1,3-bis(mesityl)imidazol-2-ylidene (L2), and 1,3- bis(2,6-i Pr2 C6H3) imidazol-2-ylidene (L3), is presented herein. We show that both Au and Ag nanocrystals displaying narrow size distribution can be formed by reduction with amine− boranes. The effi ciency 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, diff erent organometallic species are involved in the reduction step to produce the seeds. This can be correlated to the lack of eff ect 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 (L3). This demonstrates that a well-defi ned route for a given metal cannot be extended to another metal.
Source : Why Do Silver and Gold N-Heterocyclic Carbene Precursors Behave Differently ? » X.Ling, N. Schaeffer, S. Roland and M.P. Pileni Langmuir,2013, 29, 12647−12656.