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(SCH2CH2Ph)(18) with a cationic thiol HS(CH2)(11)N(CH3)(3)(+), which resulted in different populations of the two thiolate ligands (SR0 and SR+), that is, Au-25(SR0)(18-x)(SR+)(x). The ligand-exchange process was monitored by electrospray ionization mass spectrometry, and the reaction kinetics is discussed in combination with 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.