Unexpected reactivity of Au25(SCH2CH2Ph)18 nanoclusters with salts

We report some interesting results of the chemical reactivity of thiolate-protected [Au-25(SCH2CH2Ph)(18)](0) nanoclusters with two types of salts, including tetraoctylammonium halide (TOAX) and NaX. At the early stage of the reaction, [Au-25(SCH2CH2Ph)(18)](0) was found to spontaneously convert to its anionic form ([Au-25(SCH2CH2Ph)(18)](-)) in the presence of either type of salt. However, a large difference was observed in the second stage of the reaction. With NaX, we observed decomposition of anionic clusters, while with TOAX, the clusters show excellent stability. We have gained some insight into the reaction mechanism. The X- ions seem to attack [Au-25(SCH2CH2Ph)(18)](q) surface and displace some thiolates, evidenced by the observation of halide-bound clusters such as Au-25(SCH2CH2Ph)(18-x)Br-x in mass spectrometry analysis. These halide-bound clusters show a reduced stability, and their decomposition into Au(I) complexes leads to the release of gold valence electrons of the clusters; concurrently, the non-halide-bound [Au-25(SCH2CH2Ph)(18)](0) clusters are reduced into [Au-25(SCH2CH2Ph)(18)](-). For the second stage of reaction with organic salts such as TOA-Br, after [Au-25(SCH2CH2Ph)(18)](0) clusters are converted to [Au-25(SCH2CH2Ph)(18)](-)) the TOA(+) counterions surprisingly protect the anionic clusters from further attack by halide ions, hence, TOA(+) cations can stabilize [Au-25(SCH2CH2Ph)(18)](-) clusters. In contrast, with NaX salts the Na+ ions do not provide any steric stabilization of the [Au-25(SCH2CH2Ph)(18)](-) clusters, hence, degradation occurs when being further attacked by halide ions, especially Br- and I-.