Metal Doping of Au25(SR)18- Clusters : Insights and Hindsights

Abstract

The structure, properties, and applications of atomically precise gold nanoclusters are the object of active research worldwide. Over the last few years, research has been also focusing on selective doping of metal nanoclusters through introduction of foreign-metal atoms. Doping has been studied for several clusters, especially the atomically precise Au25(SR)18. Doping has been carried out with noble metals, such as platinum, and less noble metals, such as cadmium and mercury, also because of the ease by which monodoping can be achieved with these metals. Previous studies, which relied extensively on the use of mass spectrometry and single crystal X-ray crystallography, led to assign the specific locations where these single foreign-metal atoms go. Our study provides new insights into this topic and, particularly, compelling evidence about the actual position of the selected metal atoms M = Pt, Pd, Hg, and Cd in the structure of Au24M(SR)18. To make sure that the results were not dependent on the thiolate, for SR we used both butanethiolate (SC4) and the usual "reference" ligand phenylethanethiolate (SC2Ph). The clusters were prepared according to the different literature procedures that led to identify the apparent position of the doping metal, which previously was thought to be dependent on the specific synthetic protocol. Our extensive use of NMR spectroscopy and isotope effects, with the support of mass spectrometry, electrochemistry, and single crystal X-ray crystallography, led us to confirm that noble metals indeed dope the cluster at its central position, whereas no matter how the doping reaction is conducted and the nature of the ligand, the position of both Cd and Hg is always on the icosahedron shell, rather than at the central or staple position, as very often reported. Our results not only provide a reassessment of previous conclusions, but also highlight the importance of NMR spectroscopy studies and cast doubts on drawing conclusions mostly based on single crystal X-ray crystallography.