Binding vs Etching: N-Heterocyclic Carbene Ligands on Gold Nanoparticles

Abstract

Thiols on gold are the quintessential self-assembled monolayer, providing molecule-level interfacial control to solid surfaces. The Au–S bond is reasonably strong but labile. The Au–C bonds from N-heterocyclic carbene (NHC) ligands on gold surfaces are stronger and generally more inert than Au–S bonds, and it is known that NHCs can displace thiol ligands on gold, suggesting that NHCs might provide more chemically stable self-assembled monolayers than thiols on gold. Herein, we describe the substitution kinetics of two sizes of gold nanoparticles, originally coated with thiol ligands, with a family of NHC ligands that feature a series of electron-donating and -withdrawing substituents. It is found that the most electron-withdrawing NHC substituents led to etching of the gold nanoparticles to yield bis-NHC-Au(I) products, while the most electron-donating substituents led to stable self-assembled monolayers, as judged by experiments over a 7-day period. Computational modeling suggests that the pKa of the NHC is a good predictor of the Au–C bond strength and hence the outcome of binding versus etching. Thus, the electronic nature of the NHC ligand must be taken into account for the creation of stable self-assembled monolayers on gold in solution.