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 nano clusters 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 C-1 ligand layer of 3-MBA around the gold core of Au-68 and Au-144 and suggest that 3-MBA protects the metal core not only by the covalent S-Au bond formation but also via weak pi-Au and O=C-OH center dot center dot center dot Au interactions. The pi-Au and -OH center dot center dot center dot Au interactions have a strength of the order of a hydrogen bond and thus are dynamic in water at ambient temperature. The -OH center dot center dot center dot 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 Au-102(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.