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Distinct metal-exchange pathways of doped Ag25 nanoclusters.

Atomically precise metal nanoclusters (NCs) containing more than one type of metal atom (i.e., doped or alloyed), due to synergistic effects, open new avenues for engineering the catalytic and optical properties of NCs in a manner that homometal NCs cannot. Unfortunately, it is still a major challenge to controllably introduce multimetallic dopants in NCs, understanding the dopants' positions, mechanism, and synergistic effects. To overcome these challenges, we designed a metal-exchange approach involving NCs as molecular templates and metal ions as the source of the incoming dopant. In particular, two structurally similar monodoped silver-rich NCs, [MAg24(SR)18]2- (M = Pd/Pt and SR: thiolate), were synthesized as templates to study their mechanistic transformation in response to the introduction of gold atoms. The controllable incorporation of Au atoms into the MAg24 framework facilitated the elucidation of distinct doping pathways through high-resolution mass spectrometry, optical spectroscopy and elemental analysis. Interestingly, gold replaced the central Pd atom of [PdAg24(SR)18]2- clusters to produce predominantly bimetallic [AuAg24(SR)18]- clusters along with a minor product of an [Au2Ag23(SR)18]- cluster. In contrast, the central Pt atom remained intact in [PtAg24(SR)18]2- clusters, and gold replaced the non-central Ag atoms to form trimetallic [AuxPtAg24-x(SR)18]2- NCs, where x = 1-2, with a portion of the starting [PtAg24(SR)18]2- NCs remaining. This study reveals some of the unusual metal-exchange pathways of doped NCs and the important role played by the initial metal dopant in directing the position of a second dopant in the final product.