Effect of subtle changes of isomeric ligands on the synthesis of atomically precise water-soluble gold nanoclusters

The subtle structural change of hydrophilic ligands on the size control of metal nanoclusters (NCs) is unclear but critically important for fundamental understanding. Herein, we report our findings that subtle changes of isomeric ligands lead to a dramatic difference in the size of water-soluble Au NCs. By using isomeric para-mercaptobenzoic acid (p-MBA), m-MBA, and o-MBA as model ligands, it was found that both the steric hindrance and the electronic effect of isomeric ligands significantly influences the size of Au NCs, resulting in the formation of different sized Au44(p-MBA)26 NCs, Au25(m-MBA)18 NCs, and Au37/43(o-MBA)22/26 NCs. Besides this, by collocating any two of the isomeric MBAs as ligand pairs to compare their protecting capability for Au NCs, the protecting abilities of such ligands were found to follow the trend: m-MBA > o-MBA > p-MBA. In addition, the growth process of Au44(p-/o-MBA)26 NCs from Au(i)-MBA complexes in the NaBH4 reduction system was also monitored by real-time UV-vis absorption spectroscopy and ESI mass spectrometry, which complies with the 2e- hopping growth principle, indicating the universal applicability of this principle in the synthesis of thiolated metal NCs. This study provides a fundamental understanding of the effect of ligands' steric hindrance and electronic factors on the size control of water-soluble metal NCs and sheds light on the formation of metal NCs in the NaBH4 reduction system.