Gold-thiolate ring as a protecting motif in the Au20(SR)16 nanocluster and implications.

Understanding how gold nanoclusters nucleate from Au(I)SR 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 Au20(TBBT)16 (TBBT = SPh-t-Bu). The structure features a vertex-sharing bitetrahedral Au7 kernel and an unprecedented "ring" motif-Au8(SR)8. This large ring protects the Au7 kernel through strong Auring-Aukernel bonding but does not involve S-Aukernel bonding, in contrast to the common "staple" motifs in which the S-Aukernel bonding is dominant but the Austaple-Aukernel interaction is weak (i.e., aurophilic). As the smallest member in the TBBT "magic series", Au20(TBBT)16, together with Au28(TBBT)20, Au36(TBBT)24, and Au44(TBBT)28, reveals remarkable size-growth patterns in both geometric structure and electronic nature. Moreover, Au20(TBBT)16, together with the Au24(SR)20 and Au18(SR)14 nanoclusters, forms a "4e" nanocluster family, which illustrates a trend of shrinkage of bitetrahedral kernels from Au8(4+) to Au7(3+) and possibly to Au6(2+) with decreasing size.