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 Au314 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 1H 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.

With the objective of better understanding the Brust synthesis reaction, this paper examines the evolution of the core sizes of hexanethiolate monolayer-protected Au clusters (MPCs) in a typical synthesis reaction mixture, at time intervals over the course of 125 h. Transmission electron microscopy shows that the average MPC core diameter gradually increases over the first 60 h of reaction and then remains largely unchanged afterward at ∼3.0 nm. Differential pulse voltammetry of purified MPC aliquots removed from the synthesis reaction exhibit quantized double-layer (QDL) charging peaks. QDL charging peaks have been previously shown to be a strong function of MPC core size and dispersity and reveal (i) the presence of several discernible core sizes in each sample and (ii) an increase in cluster capacitance (CCLU) with longer reaction times, consistent with the electron microscopy results.