Structural and Theoretical Basis for Ligand Exchange on Thiolate Monolayer Protected Gold Nanoclusters

Ligand exchange reactions are widely used for imparting
new functionality
on or integrating nanoparticles into devices. Thiolate-for-thiolate
ligand exchange in monolayer protected gold nanoclusters has been
used for over a decade; however, a firm structural basis of this reaction
has been lacking. Herein, we present the first single-crystal X-ray
structure of a partially exchanged Au102(p-MBA)40(p-BBT)4 (p-MBA = para-mercaptobenzoic acid, p-BBT = para-bromobenzene thiol) with p-BBT as the incoming ligand. The crystal structure shows that 2 of
the 22 symmetry-unique p-MBA ligand sites are partially
exchanged to p-BBT under the initial fast kinetics
in a 5 min timescale exchange reaction. Each of these ligand-binding
sites is bonded to a different solvent-exposed Au atom, suggesting
an associative mechanism for the initial ligand exchange. Density
functional theory calculations modeling both thiol and thiolate incoming
ligands postulate a mechanistic pathway for thiol-based ligand exchange.
The discrete modification of a small set of ligand binding sites suggests
Au102(p-MBA)44 as a powerful
platform for surface chemical engineering.