Electrophilic Substitution\nas a Mechanism for Ligand\nExchange Reactions on Silver Monolayer-Protected Clusters

Chemical research on noble metal monolayer-protected\nclusters (MPCs)\nhas typically focused on discovering new sizes and compositions. However,\nthe discovery of the fundamental chemical principles that govern MPC\nreactions, which are required for the rational chemical transformation\nof MPCs, has received significantly less attention. Here, we study\nin detail the electronic nature of the mechanism of postsynthetic\nligand exchange reactions using M<sub>4</sub>Ag<sub>44</sub>(<i>p</i>-MBA)<sub>30</sub> as a model system, where M is a monocationic\ncounterion and <i>p</i>-MBA is <i>para</i>-mercaptobenzoic\nacid. The systematic exchange of aryl thiol ligands, with different\nelectron-withdrawing and -donating substituents in different ring\npositions, was studied in detail by electrospray-ionization mass spectrometry\nmeasurements of equilibrium product distributions and density functional\ntheory (DFT) calculations of fully ligand-exchanged clusters. We found\nthat (i) these ligand exchange reactions are driven by the relative\nelectrophilicity of the incoming ligands, (ii) the electrophilic driving\nforce was stronger than modest steric effects, and (iii) site-specific\nsubstitution due to ligand shell bonding geometries was not observed,\nrevealing certain limitations of rational synthesis. DFT calculations\nuncovered the complexity of the charge distributions on the clusters,\ncontrasting with currently used simple heuristic models of charge\nwithdrawal and calling attention to the subtlety and intricacy of\nthe effects of charge redistribution upon substitution. By carefully\nand systematically establishing such basic chemical principles to\nbuild a catalog of MPC reactions, in analogy to organic chemistry,\nthe rational synthesis of new MPC materials using MPCs as reagents\nin transformation reactions could become possible.