Electrophilic Substitution as a Mechanism for Ligand Exchange Reactions on Silver Monolayer-Protected Clusters

Abstract

Chemical research on noble metal monolayer-protected clusters (MPCs) has typically focused on discovering new sizes and compositions. However, the discovery of the fundamental chemical principles that govern MPC reactions, which are required for the rational chemical transformation of MPCs, has received significantly less attention. Here, we study in detail the electronic nature of the mechanism of postsynthetic ligand exchange reactions using M₄Ag₄₄(p-MBA)₃₀ as a model system, where M is a monocationic counterion and p-MBA is para-mercaptobenzoic acid. The systematic exchange of aryl thiol ligands, with different electron-withdrawing and -donating substituents in different ring positions, was studied in detail by electrospray-ionization mass spectrometry measurements of equilibrium product distributions and density functional theory (DFT) calculations of fully ligand-exchanged clusters. We found that (i) these ligand exchange reactions are driven by the relative electrophilicity of the incoming ligands, (ii) the electrophilic driving force was stronger than modest steric effects, and (iii) site-specific substitution due to ligand shell bonding geometries was not observed, revealing certain limitations of rational synthesis. DFT calculations uncovered the complexity of the charge distributions on the clusters, contrasting with currently used simple heuristic models of charge withdrawal and calling attention to the subtlety and intricacy of the effects of charge redistribution upon substitution. By carefully and systematically establishing such basic chemical principles to build a catalog of MPC reactions, in analogy to organic chemistry, the rational synthesis of new MPC materials using MPCs as reagents in transformation reactions could become possible.