Direct self-focusing synthesis of monodisperse [Au₈(PPh₃)₇]²⁺ nanoclusters

Direct and scalable synthesis of monodisperse gold nanoclusters is highly desired but remains a great challenge due to the complexity of chemical reactions. In this regard, a suitable precursor is important as it can simplify the synthesis processes and offer controllability in tuning the product. In this study, we found that Au(PPh₃)₂Cl could be used as an effective precursor for the direct synthesis of atomically monodisperse [Au₈(PPh₃)₇]²⁺ nanoclusters without the need of tedious post-synthetic purification steps. The Au(PPh₃)₂Cl precursor could be directly reduced by NaBH₄ (0.25 molar equivalent) in a dichloromethane solution; this produced Au₈ clusters with a 35% reaction yield. Time-dependent mass spectrometry and in situ UV-vis absorption spectroscopy reveal that the synthesis process is initiated by the rapid formation of the Au₆-Au₈ mixture, followed by a slow spontaneous self-focusing process that converges the mixture into atomically monodisperse Au₈. The success of this direct synthesis has been hypothesized to arise from the relatively stronger Au(i)Au(i) aurophilic attraction between Au(i)-PPh₃ complexes that facilitates the aggregation of Au(i)-PPh₃ on Au(0) cores.