ESI-MS Identification of the Cationic Phosphine-Ligated Gold Clusters Au1-22: Insight into the Gold-Ligand Ratio and Abundance of Larger Clusters.

Triphenylphosphine (PPh3)-ligated gold nanoclusters are valuable for a number of potential applications due to their relative ease of synthesis and usefulness in forming advanced cluster architectures. While previous studies have reported cationic PPh3-ligated gold clusters with core sizes of Au1-4, Au6-11, and Au13-14, there has not been definitive identification by mass spectrometry of many larger clusters in the Au12-25 range. Herein, we survey a polydisperse solution of cationic PPh3-ligated gold clusters using high-mass-resolution (M/ΔM = 60,000) electrospray ionization mass spectrometry (ESI-MS). To improve the sensitivity and mass resolution of larger clusters for unambiguous identification, we increased the number of scan averages and reduced the range of mass collection windows to 200 m/z, thereby mitigating potential mass and ion abundance bias resulting from smaller "building block" gold clusters that are present in much higher abundance in solution. In addition to the previously reported clusters, we identify several new species 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 Au1-22 may be observed in a single polydisperse solution. Considering all of the clusters observed, our findings provide evidence that the Au12-14 size range is a critical transition point in cluster nucleation. While smaller clusters exhibit a 1:1 gold-to-ligand ratio, larger clusters (beginning Au12-14) feature additional gold atoms without an equal number of accompanying ligands. Our results support previous evidence in the literature indicating that the "magic number" icosahedral Au13 geometry is the smallest cluster size where a ligand-less central gold atom is coordinated by a complete shell of 12 surrounding ligated gold atoms, thereby creating a stable "one-shell" cluster. Furthermore, our findings reinforce growing evidence that ligands may be used to actively direct gold cluster size and abundance during synthesis. While for PPh3-ligated systems the most abundant species are Au6-9 clusters, we find that for related methyldiphenylphosphine (PPh2Me) and dimethylphenylphosphine (PPhMe2)-ligated systems the most abundant cluster sizes are Au10-11 and Au12-14, respectively. Together, we demonstrate that reducing the range of m/z collection windows and increasing the number of scan averages dramatically improves instrument sensitivity for cationic gold clusters, enabling thorough characterization of polydisperse solutions that is not possible using conventional techniques.