“Sergeants-and-soldiers” principle in the synthesis of intrinsically chiral Au ₁₃ clusters

Surface ligand modification on atomically precise metal clusters is one of the important strategies not only to stabilize monodisperse clusters, but also to derive unique optical and electronic properties. The chiral coordination of surface ligands on clusters controls the global chirality in clusters giving rise to unambiguous optical activity including circular dichroism (CD) and circularly polarized luminescence (CPL). Stereoselective synthesis of intrinsically chiral clusters has been achieved by the use of chiral ligands stabilizing one-handed clusters selectively. Herein, we investigate the effect of chiral ligand coordination on the deracemization of intrinsically chiral Au₁₃ superatom clusters. The mixture of achiral bis(diphenylphosphino)ethane (DPPE) and chiral (R,R)-1,2-bis[(2-methoxyphenyl)phenylphosphino]ethane (R-DIPAMP) led to the formation of a mixture of [Au₁₃(R-DIPAMP)_x(DPPE)_5-xCl₂]³⁺ clusters with varied x-values (x = 0-5). Reverse phase HPLC successfully fractionalized the mixture into solutions composed of single component Au₁₃ clusters depending on the x-values. The separated Au₁₃ clusters afforded similar optical activity to that of [Au₁₃(R-DIPAMP)₅Cl₂]³⁺ in the CD study regardless of x-values (x = 1-4). The DFT calculation supports that the coordination of a single DIPAM ligand is enough to bias the twisting direction in the Au₁₃ superatom with dictating the tortional orientation of other four DPPE ligands. The emergence of the "sergeants-and-soldiers principle", wherein a small number of chiral ligands determine the chiral orientation of other achiral ligands, is thus demonstrated in the synthesis of intrinsically chiral Au₁₃ clusters.