Dynamics of place-exchange reactions on monolayer-protected gold cluster molecules

Monolayer-protected gold clusters (Au MPCs) are stable, easily synthesized, organic solvent-soluble, nanoscale materials. MPCs with protecting monolayers composed of alkanethiolate ligands (RS) can be functionalized (R'S) by ligand place-exchange reactions, i.e., x(R'SH) + (RS)(m)MPC --> x(RSH) + (R'S)(m)(RS)(m-x)MPC, where x is the number of ligands place-exchanged (1 to 108) and m is the original number (ca. 108) of alkanethiolate ligands per Au-314 cluster. The dynamics and mechanism of this reaction were probed by determining its kinetic order and final equilibrium position relative to incoming (R'S) and initial (RS) protecting thiolate ligands. The reactions were characterized by H-1 NMR and IR spectroscopy, and the dispersity of place-exchange reaction products was preliminarily inspected by chromatography. The results of these experiments show that ligand exchange is an associative reaction and that the displaced thiolate becomes a thiol solution product. Disulfides and oxidized sulfur species are not involved in the reaction. Cluster-bound thiolate ligands differ widely in susceptibility to place-exchange, presumably owing to differences in binding sites (Au core edge and vertice sites are presumably more reactive than terrace sites). The rate of place-exchange decreases as the chain length and/or steric bulk of the initial protecting ligand shell is increased. The exchange results and proposed mechanism are compared to those for place-exchange reactions on self-assembled monolayers confined to flat gold surfaces.