Probing the Ligand Exchange of N-Heterocyclic Carbene-Capped Ag2S Nanocrystals with Amines and Carboxylic Acids.

Abstract

N-Heterocyclic carbenes (NHCs) are versatile L-type ligands that have been shown to stabilize coinage metal chalcogenide nanocrystals, such as Ag2S, remarkably well. However, very little research has been done on the interaction between NHC ligands and coinage metal chalcogenide nanocrystal surfaces and subsequent ligand exchange reactions. Herein, solution 1H nuclear magnetic resonance methods were used to monitor ligand exchange reactions on stoichiometric Ag2S nanocrystal platforms with various primary amine and carboxylic acid ligands. Despite the introduction of new ligands, the native NHC ligands remain tightly bound to the Ag2S nanocrystal surface and are not displaced at room temperature. Primary amine and carboxylic acid ligands demonstrated quantitative ligand exchange only after the samples had been heated with an excess incoming ligand, which implies a strong NHC-Ag binding energy. Density functional theory affirms that a model NHC ligand binds the strongest to a Ag12S6 cluster surface, followed by amine and carboxylic acid binding; computational analysis is therefore in line with the absence of NHC displacement observed in experiments. Both the bulky sterics of the C14-alkyl chains on the NHC and the high energies for the binding of NHC to the Ag2S surface contribute to the superior colloidal stability over conventional long-chain amine or carboxylic acid ligands (many months vs hours to days).