Toward Regiochemical Control of Ligands on Thiol-Protected Gold Clusters

Thiolate-protected gold clusters attract interest due to their straightforward synthesis, facile handling, and ability to be easily functionalized via ligand exchange mechanisms. Because of their well-defined structure and low symmetry, a “grand challenge” is controlling the regioselectivity of ligand exchange. Such control enables placement of specific ligands at specific symmetry-unique sites on gold clusters. Early reports suggested that solvent exposure of the gold atoms could predict their reactivity toward ligand exchange; however, subsequent observations show that such ligands do not remain in place and are of short lifetimes. In this perspective, we discuss the complex nature of ligand exchange, which proceeds with multiple kinetic environments on gold clusters with many ligand symmetry environments. We highlight recent observations of inter- and intracluster ligand exchange that appear responsible for the short lifetimes of initial ligand exchange products. We discuss possible avenues for limiting inter- and intracluster ligand exchange, allowing regiochemical specificity to be retained. We postulate a set of 6 rules that appear to govern regiochemical control of ligand locations on gold clusters.