Unveiling Complex Structural Features of Thiolate-Protected Gold Nanoclusters: From Internal Core to External “Staple” Motifs and Overall Charge States

Abstract

The revelation of numerous thiolate-protected gold nanocluster (TP-AuNCs) structures, achieved through a blend of theoretical predictions and experimental detection/validation, presents a vast amount of data for understanding the structural evolution of these nanoclusters. Typically, these clusters featured an internal gold core surrounded by external “staple” motifs SR[Au(SR)]_x (x = 0, 1, 2, 3, ...) at various charge states. In this Account, we outline our Grand Unified Model (GUM) that elucidates the growth mechanism of the internal gold core, the bonding nature of outer “staple” motifs, and a ring model illuminating the intricate interfacial interactions between the motifs and the internal gold core, as well as a simple rule governing charge states.

In GUM, we integrate both the duet and octet rules into TP-AuNCs, and we treat the internal gold core as a combination of triangular Au₃ and tetrahedral Au₄ elementary blocks satisfying the duet rule (Au₃(2e) and Au₄(2e)), along with the icosahedral Au₁₃ secondary blocks following the octet rule (Au₁₃(8e)). By combining different numbers of these blocks, a variety of gold cores of TP-AuNCs can be formed. Consequently, the high stability of many known nanoclusters can be attributed to the inherent stability of each block. By inspecting the “staple” motifs, we can describe their structural characteristics and unique bonding mechanisms with a 3-center 4-electron model, alongside the electron pair repulsion theory and valence bond theory. Our study shows that Au atom within “staple” motif SR[Au(SR)] exhibits hypercoordination, while surrounded by two pairs of bonding electron pairs. The repulsion among bonding electron pairs guarantees the stability of each three-atom center (S–Au–S) at quasi-linear configuration. Additionally, we propose a ring model to understand the intricate interactions between the “staple” motifs and the gold core. Within this framework, “staple” motifs and specific gold core atoms tend to form ring structures, with interactions between the gold core atoms and these rings crucial for the structural stability of TP-AuNCs. Finally, since charge states affect the number of valence electrons and the free electrons are dispersed over the inner-core elementary blocks of Au₃(2e) and Au₄(2e), correlating cluster charge states with their core structure through valence electrons can elucidate the nature of overall charge states in TP-AuNCs. These recently developed models allow us to understand generic structural features of the internal gold core, external “staple” motifs, and overall charge states of TP-AuNCs.

Considering the complex structural features of TP-AuNCs, four key factors contributing to their stability are identified: (1) Inherent stability of elementary blocks crucial for forming the gold cores, (2) aurophilic interactions among these elementary blocks, (3) robust nature of Au–S covalent bonds, and (4) aurophilic interactions among the rings. Furthermore, a theoretical approach for effectively designing TP-AuNCs structures is proposed based on our understanding their stability.