Dynamic Stabilization of the Ligand–Metal Interface in Atomically Precise Gold Nanoclusters Au₆₈ and Au₁₄₄ Protected by meta-Mercaptobenzoic Acid

Ligand-stabilized, atomically precise gold nanoclusters with a metal core of a uniform size of just 1–3 nm constitute an interesting class of nanomaterials with versatile possibilities for applications due to their size-dependent properties and modifiable ligand layers. The key to extending the usability of the clusters in applications is to understand the chemical bonding in the ligand layer as a function of cluster size and ligand structure. Previously, it has been shown that monodispersed gold nanoclusters, stabilized by meta-mercaptobenzoic acid (m-MBA or 3-MBA) ligands and with sizes of 68–144 gold atoms, show ambient stability. Here we show that a combination of nuclear magnetic resonance spectroscopy, UV–vis absorption, infrared spectroscopy, molecular dynamics simulations, and density functional theory calculations reveals a distinct chemistry in the ligand layer, absent in other known thiol-stabilized gold nanoclusters. Our results imply a low-symmetry C1 ligand layer of 3-MBA around the gold core of Au68 and Au144 and suggest that 3-MBA protects the metal core not only by the covalent S–Au bond formation but also via weak π–Au and O═C–OH···Au interactions. The π–Au and −OH···Au interactions have a strength of the order of a hydrogen bond and thus are dynamic in water at ambient temperature. The −OH···Au interaction was identified by a distinct carbonyl stretch frequency that is distinct for 3-MBA-protected gold clusters, but is missing in the previously studied Au102(p-MBA)44 cluster. These thiol–gold interactions can be used to explain a remarkably low ligand density on the surface of the metal core of these clusters. Our results lay a foundation to understand functionalization of atomically precise ligand-stabilized gold nanoclusters via a route where weak ligand–metal interfacial interactions are sacrificed for covalent bonding.