“Naked Gold Core Atoms” in Thiolate-Protected Gold Nanoclusters: The Exploration of Structural Mechanisms and Introduction of Phosphine Ligands

We present novel insights into the structural mechanisms of four crystallized thiolate-protected gold nanoclusters, namely, Au₁₆(SR)₁₂, Au₂₁(SR)₁₅, Au₂₉(SR)₁₉, and Au₃₀(SR)₁₈. These nanoclusters exhibit one or two surface gold core atoms that lack thiolate group protection, termed "naked gold core atoms". The existence of "naked gold core atoms" can be explained by the presence of an individual surface gold core atom that cannot establish a bond with the staple motif (-[SR(AuSR)<i>_n</i>]-, where <i>n</i> = 0, 1, 2, ...) since each staple motif requires two surface gold core atoms for bonding. Alternatively, it can be attributed to the situation in which the two surface gold atoms are too widely separated to be linked by a staple motif. Further structural analyses of the coordination environment surrounding the "naked gold core atoms" reveal the presence of a nearby Au₅ ring. The average bond length between the "naked gold core atom" and the gold atoms in the Au₅ ring is determined to be 3.0 Å, indicating robust aurophilic interactions among them. Hence, the Au₅ ring can be regarded as a protective motif that shields the "naked gold core atom" from requiring an additional SR motif for binding. This is also the rationale behind the use of quotation marks for the term "naked gold core atoms". After comprehending the structural mechanism of these "naked gold core atoms", we can design the structures of highly stable thiolate- and phosphine-protected gold nanoclusters by introducing phosphine ligands onto the "naked gold core atoms". Our work not only contributes to the understanding of the intricate interfacial interactions in thiolate-protected gold nanoclusters but also provides valuable insights for synthesizing a greater variety of hybrid ligand-protected gold nanoclusters experimentally.