Size evolution and ligand effects on the structures and stability of (AuL)n (L = Cl, SH, SCH3, PH2, P(CH3)2, n = 1–13) clusters

Abstract

The synthesis and characterization of ligand protected gold nanoclusters (Au_mL_n) have attracted great interest. After the crystallization of Au₁₀₂(SR)₄₄ and Au₂₅(SR)₁₈⁻ clusters, the syntheses and theoretical predictions of Au_mL_n clusters have been greatly accelerated. To date, there are few systematic studies on the size evolution and ligand effects of Au–L binary systems. Herein, using a stoichiometric (AuL)_n (n = 1–13) system as a test case, we theoretically investigate the ligand effects (L = Cl, SH, SCH₃, PH₂, and P(CH₃)₂) on these structures and size evolution. The method of genetic algorithm combined with density functional theory is used to perform an extensive global search of the potential energy surface to locate the global minima (GM) and low-lying isomers. For each ligand, the structural features are roughly similar to (AuSR)_n, that is, the GMs change from single ring to catenane structures. Besides, a new folding mode (ring-at-ring) is revealed in the GMs at n = 12–13. The GM structures are very similar for L = SH and SCH₃ and for L = PH₂ and P(CH₃)₂, which indicate that the R groups can be directly replaced by H in the calculations. However, there are obvious differences in the GM structures for L = Cl, SH and PH₂. It is found that the origin of the ligand effects is the polarity of the Au–L bond. The Au–Cl bond is of the highest plorarity, and the noncovalent interaction index approach reveals that the Au⋯Au aurophilic interaction is the strongest for L = Cl, followed by L = SH and L = PH₂. Moreover, the polarity of the Au–L bond may affect the preferred Au–L–Au bond angle, which is an important geometric parameter. The linearity of Cl–Au–Cl is the easiest to be broken for more Au⋯Au contacts, which is viewed in the GMs of (AuCl)_n at n = 7, 8 and 12.