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 Au 25 (SCH 2 CH 2 Ph) 18 with a cationic thiol HS(CH 2 ) 11 N(CH 3 ) 3 +, which resulted in different populations of the two thiolate ligands (SR 0 and SR + ), that is, Au 25 (SR 0 ) 18– x (SR + ) x . The ligand-exchange process was monitored by electrospray ionization mass spectrometry, and the reaction kinetics is discussed in combination with 1 H 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.