Scalable synthesis of size- and composition-selected clusters with predetermined properties for applications in catalysis, energy conversion and storage, and chemical separations will be facilitated by a mechanistic understanding of cluster nucleation, growth, and etching in solution. We have characterized the formation of triphenylphosphine (PPh₃) ligated gold clusters in the Au_1-22 size range using online reaction monitoring combined with high-mass-resolution electrospray ionization mass spectrometry. To improve the sensitivity and mass resolution of larger clusters for unambiguous identification, we increased the number of scan averages and reduced the width of mass collection windows, thereby mitigating mass and abundance bias resulting from small gold-ligand species present in high abundance. We have identified new clusters including Au₅(PPh₃)₅⁺, Au₁₂(PPh₃)₉HCl²⁺, Au₁₅(PPh₃)₉Cl²⁺, Au₁₆(PPh₃)₁₀Cl₂²⁺, Au₁₇(PPh₃)₁₁³⁺, Au₁₈(PPh₃)₁₀²⁺, Au₁₉(PPh₃)₁₀Cl²⁺, Au₂₀(PPh₃)₁₂H₃³⁺, Au₂₁(PPh₃)₁₀Cl²⁺, and Au₂₂(PPh₃)₁₀Cl₂²⁺, indicating that a full range of clusters between Au_1–22 is present in solution. We present evidence that the Au_12–14 size range is a transition point in cluster nucleation where smaller clusters exhibit a one-to-one gold-to-ligand ratio while larger clusters feature additional gold atoms without an equal number of accompanying ligands. Our results show that differently functionalized phosphines may be used to direct the size and abundance of gold clusters in solution. While for PPh₃-ligated clusters the most abundant species are Au_6–9, for methyldiphenylphosphine (PPh₂Me) and dimethylphenylphosphine (PPhMe₂)-ligated clusters the most abundant species are Au_10–11 and Au_12–14, respectively. We also present evidence that small hydrogen-containing intermediates (Au₂(PPh₃)₂H⁺ and Au₄(PPh₃)₄H⁺) are responsible for the growth of larger clusters. Degradation of these intermediates and growth of larger clusters are shown to be dependent on irradiation of the solution with light. Together, our findings advance the mechanistic understanding of the size-selective synthesis of clusters in the subnanometer size regime where “each atom counts” toward tuning the properties of clusters for eventual applications.

Michael Hewitt

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