Unexpected reactivity of Au25(SCH2CH2Ph)18 nanoclusters with salts

We report some interesting results of the chemical reactivity of thiolate-protected Au(25)(SCH(2)CH(2)Ph)(18)](0) nanoclusters with two types of salts, including tetraoctylammonium halide (TOAX) and NaX. At the early stage of the reaction, Au(25)(SCH(2)CH(2)Ph)(18)](0) was found to spontaneously convert to its anionic form (Au(25)(SCH(2)CH(2)Ph)(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)(SCH(2)CH(2)Ph)(18)](q) surface and displace some thiolates, evidenced by the observation of halide-bound clusters such as Au(25)(SCH(2)CH(2)Ph)(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)(SCH(2)CH(2)Ph)(18)](0) clusters are reduced into Au(25)(SCH(2)CH(2)Ph)(18)](-). For the second stage of reaction with organic salts such as TOA-Br, after Au(25)(SCH(2)CH(2)Ph)(18)](0) clusters are converted to Au(25)(SCH(2)CH(2)Ph)(18)](-)) the TOA(+) counterions surprisingly protect the anionic clusters from further attack by halide ions, hence, TOA(+) cations can stabilize Au(25)(SCH(2)CH(2)Ph)(18)](-) clusters. In contrast, with NaX salts the Na(+) ions do not provide any steric stabilization of the Au(25)(SCH(2)CH(2)Ph)(18)](-) clusters, hence, degradation occurs when being further attacked by halide ions, especially Br(-) and I(-).