Ammonia-Induced Size Convergence of Atomically Monodisperse Au₆ Nanoclusters

Developing effective synthetic protocols for atomically monodisperse Au nanoclusters is pivotal to their fundamental science and applications. Here, we present a novel synthetic protocol toward atomically monodisperse [Au₆(PPh₃)₆]²⁺ nanoclusters (abbreviated as Au₆) via ammonia-induced size convergence from polydisperse Au_<i>x</i> (<i>x</i> = 6–11) nanocluster mixture. The analogous ammonia-induced size conversion reactions starting from individually prepared Au₇ and Au₉ nanoclusters to Au₆ were traced by time-dependent ultraviolet–visible absorption and electrospray ionization mass spectra. It is observed that in both cases the size conversion is achieved through gradual release of the ion–molecule complex [NH₄AuPPh₃Cl]⁺ from the larger Au nanoclusters until the formation of thermodynamically stable Au₆ nanoclusters with the stability against the etching reaction. The role of ammonia ions in this size convergence synthesis is to accelerate the depletion of [Au­(PPh₃)]⁺ fragments from the PPh₃-protected Au nanoclusters, by the formation of the stable complex [NH₄AuPPh₃Cl]⁺