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

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.