Gold-Thiolate Ring as a Protecting Motif in the Au20(SR)16 Nanocluster and Implications

Understanding how gold nanoclusters nucleate from (AuSR)-S-1 complexes necessitates the structural elucidation of nanoclusters with decreasing size. Toward this effort, we herein report the crystal structure of an ultrasmall nanocluster formulated as Au-20(TBBT)(16) (TBBT = SPh-t-Bu). The structure features a vertex-sharing bitetrahedral Au-7 kernel and an unprecedented ring motif-Au-8(SR)(8). This large ring protects the Au7 kernel through strong Au-ring-Au-kernel bonding but does not involve S-Au-kernel bonding, in contrast to the common staple motifs in which the S-Au-kernel bonding is dominant but the Au-staple-Au-kernel interaction is weak (i.e., aurophilic). As the smallest member in the TBBT magic series, Au-20(TBBT)(16), together with Au-28(TBBT)(20), Au-36(TBBT)(24), and Au-44(TBBT)(28), reveals remarkable size-growth patterns in both geometric structure and electronic nature. Moreover, Au-20(TBBT)(16), together with the Au-24(SR)(20) and Au-18(SR)(14) nanoclusters, forms a 4e nanocluster family, which illustrates a trend of shrinkage of bitetrahedral kernels from Au-8(4+) to Au-7(3+) and possibly to Au-6(2+) with decreasing size.