Doping Silver Increases the Au₃₈(SR)₂₄ Cluster Surface Flexibility

Multiple Ag atoms were doped inside Au₃₈­(SCH₂­CH₂­Ph)₂₄ nanoclusters using the metal exchange method for the first time for the synthesis of Ag_<i>x</i>­Au_{38–<i>x</i>}­(SCH₂­CH₂­Ph)₂₄. MALDI-TOF mass spectrometry revealed the time dependence of the synthesis. Cluster species with different numbers of Ag atoms (different <i>x</i> values) migrate differently on a chromatography (HPLC) column, which allows one to isolate cluster samples with a narrowed distribution of exchanged metal atoms. The enantiomers of selected Ag_<i>x</i>­Au_{38–<i>x</i>}­(SCH₂­CH₂­Ph)₂₄ samples (average <i>x</i> = 6.5 and 7.9) have been separated by HPLC. Doping changes the electronic structure, as is evidenced by the significantly different CD spectra. UV–vis spectra of the doped sample also show diminished features. The temperature required for complete racemization follows Au₃₈ > Ag_<i>x</i>Au_{38–<i>x</i>} (<i>x</i> = 6.5) > Ag_<i>x</i>Au_{38–<i>x</i>} (<i>x</i> = 7.9). To our surprise, the racemization of Ag_<i>x</i>­Au_{38–<i>x</i>}­(SCH₂­CH₂­Ph)₂₄ (<i>x</i> = 7.9) occurred even at 20 °C. Racemization involves a rearrangement of the staple motifs at the cluster surface. The results therefore show an increased flexibility of the cluster with increasing silver content. The weaker Ag–S bonds compared to Au–S are proposed to be at the origin of this observation. The experimentally determined activation energy for the racemization is ca. 21.5 kcal/mol (<i>x</i> = 6.5) and 19.5 kcal/mol (<i>x</i> = 7.9), compared to 29.5 kcal/mol for Au₃₈­(SCH₂­CH₂­Ph)₂₄, suggesting no complete metal–S bond breaking in the process