The fate of Au25(SR)18 clusters upon ligand exchange with binaphthyl-dithiol: interstaple binding vs. decomposition.

The ligand exchange reaction between monodisperse Au25(2-PET)18 (2-PET: 2-phenylethylthiolate) clusters and 1,1'-binaphthyl-2,2'-dithiol (BINAS) was long thought to induce decomposition of the cluster (Si et al., J. Phys. Chem. C, 2009). We repeated the experiment and analyzed the reaction products using MALDI-TOF mass spectrometry. The spectra clearly indicate successful ligand exchange, bidentate binding of the BINAS ligand and intact Au25 clusters. The reaction products are identified as Au25(2-PET)18-2x(BINAS)x (x = 1-4) for a 24 h reaction with a 50-fold molar excess of BINAS. Two likely binding motifs are discussed. Analysis of atomic distances in both the cluster and the free ligand indicates interstaple binding connecting the central sulfur atom of the protecting (SRAu)2SR with the outer sulfur atom of a second unit. The results presented have implications on the binding position of BINAS in Au38(SR)24-2x(BINAS)x clusters.