Kinetics of Cationic-Ligand-Exchange Reactions in Au₂₅ Nanoclusters

Herein, we report the first investigation into the kinetics of cationic-ligand-exchange processes through the reaction of Au₂₅(SCH₂CH₂Ph)₁₈ with a cationic thiol HS­(CH₂)₁₁N­(CH₃)₃⁺, which resulted in different populations of the two thiolate ligands (SR⁰ and SR⁺), that is, Au₂₅(SR⁰)_{18–<i>x</i>}(SR⁺)_<i>x</i>. The ligand-exchange process was monitored by electrospray ionization mass spectrometry, and the reaction kinetics is discussed in combination with ¹H NMR results; these techniques reveal that the kinetics of the cationic-ligand-exchange process, which is different from a typical neutral-thiol-to-neutral-thiol ligand exchange, is strongly dependent on how the SR⁺ ligands interact with each other during the ligand-exchange process. There are two main factors that determine the unique reaction kinetics, namely, Coulombic repulsions (i) between the attached SR⁺ ligands and free ligands in solution, which hinder further ligand exchange, and (ii) among the surface SR⁺ ligands on the thiolate monolayer, which promote the thermal decomposition of the nanocluster