Ligand Exchange Reactions on Au38 and Au40 Clusters: A Combined Circular Dichroism and Mass Spectrometry Study

The thiolate-for-thiolate ligand exchange reaction between the stable Au-38(2-PET)(24) and Au-40(2-PET)(24) (2-PET: 2-phenylethanethiol) clusters and enantiopure BINAS (BINAS: 1,1'-binaphthyl-2,2'-dithiol) was investigated by circular dichroism (CD) spectroscopy in the UV/vis and MALDI mass spectrometry (MS). The ligand exchange reaction is incomplete, although a strong optical activity is induced to the resulting clusters. The clusters are found to be relatively stable, in contrast to similar reactions on [Au-25(2-PET)(18)](-) clusters. Maximum anisotropy factors of 6.6 x 10(-4) are found after 150 h of reaction time. During the reaction, a varying ratio between Au-38 and Au-40 clusters is found, which significantly differs from the starting material. As compared to Au-38, Au-40 is more favorable to incorporate BINAS into its ligand shell. After 150 h of reaction time, an average of 1.5 and 4.5 BINAS ligands is found for Au-38 and Au-40 clusters, respectively. This corresponds to exchange of 3 and 9 monodentate 2-PET ligands. To show that the limited exchange with BINAS is due to the bidentate nature of the ligand, exchange with thiophenol was performed. The monodentate thiophenol exchange was found to be faster, and more ligands were exchanged when compared to BINAS.