Dynamics and Extent of Ligand Exchange Depend on Electronic Charge of Metal Nanoparticles

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Abstract
Both the rate and extent of ligand place exchange reactions between the hexanethiolate monolayer of Au140 monolayer protected clusters (C6 MPCs) and dissolved 6-mercapto-1-hexanol thiol (HOC6SH) increase with increasing positive electronic charge on the Au cluster core. The rate constant of the ligand place exchange, taken at the early stage of the exchange, is increased by ca. 2-fold for reaction of +3 charged Au140 cores as compared to neutral ones. The initially exchanged ligands are thought to reside mainly on edge and vertex sites of the Au140 core, where the lability of the slightly more ionic Au−S bonds there becomes further enhanced by removing electrons from the core. The reactions slow markedly after 35−50% of the original ligands have been replaced, continuing at a much slower pace for some time to reach an apparent reaction equilibrium. On +2 charged Au140 cores, 85% of the C6 ligands have been exchanged with HOC6H12SH after 20 h. The slower phase of the reaction includes exchange of thiolate ligands on terrace lattice sites most of whichowing to the small sizes of the nanoparticle's Au(111) facesare no more than one Au atom row removed from the nanoparticle edge sites. This slower exchange, the extent of which is also enhanced by positively charging the core, occurs either by intramolecular place exchange with edge sites that subsequently place-exchange with solution thiol or by direct place-exchange with solution thiol. Acid−base studies show that thiolate is more reactive in place exchange reactions than the corresponding thiol.