Structure of Chiral Au44(2,4-DMBT)26 Nanocluster with an 18-Electron Shell Closure

The 18-electron shell closure structure of Au nanoclusters protected by thiol ligands has not been reported until now. Herein, we synthesize a novel nanocluster bearing the same gold atom number but a different thiolate number as another structurally resolved nanocluster Au-44(TBBT)(28) (TBBTH = 4-tert-butylbenzenelthiol). The new cluster was determined to be Au-44(2,4-DMBT)(26) (2,4-DMBTH = 2,4-dimethylbenzenethiol) using multiple techniques, including mass spectrometry and single crystal X-ray crystallography (SCXC). Au-44(2,4-DMBT)(26) represents the first 18-electron closed-shell gold nanocluster. SCXC reveals that the atomic structure of Au-44(2,4-DMBT)(26) is completely different from that of Au-44(TBBT)(28) but is similar to the structure of AU(38Q). The arrangement of staples (bridging thiolates) and part of the Au-29 kernel atom induces the chirality of Au-44(2,4-DMBT)(26). The finding that a small portion of the gold kernel exhibits chirality is interesting because it has not been previously reported to the best of our knowledge. Although Au-44(2,4-DMBT)(26) bears an 18-electron shell closure structure, it is less thermostable than Au-44(TBBT)(28), indicating that multiple factors contribute to the thermostability of gold nanoclusters. Surprisingly, the small difference in Au/thiolate molar ratio between Au-44(2,4-DMBT)(26) and Au-44(TBBT)(28) leads to a dramatic distinction in Au 4f X-ray photoelectron spectroscopy, where it is found that the charge state of Au in Au-44(2,4-DMBT)(26) is remarkably more positive than that in Au-44(TBBT)(28) and even slightly more positive than the charge states of gold in Au-(2,4-DMBT) or Au-TBBT complexes.