Gold Cluster Formation with Phosphine Ligands: Etching as a Size-Selective Synthetic Pathway for Small Clusters?

Triphenylphosphine (Pph(3)) is commonly used during syntheses of stable, dosed-shell monolayer protected dusters (MPCs). Models of transition metal (TM) cluster and nanoparticle syntheses commonly assign PPh3 a passive role as a chemical placeholder, electron balancing species, or surfactant. This study provides the first direct evidence that PPh3 is a proactive etching agent that promotes the formation of specific dosed-shell cluster sizes. To observe this effect, we developed a colorimetric tool that simultaneously monitors size distribution and population of PPh3-protected clusters as a function of time. The distribution of the clusters is assigned to different bin sizes by chemical conversion with L-3 (L-3 = 1,3-bis(diphenylphosphino)propane): (i) total conversion of PPh3-protected Au-8 and Au-9 dusters into [Au6L43)(2+) and (ii) ligand exchange with (Au-x(PPh3)(y)](z+) (10 <= x <= 13) clusters to form L-3-protected Au-10 and Au-11 clusters. Evolution of the nascent cluster distribution in ethanol and methanol solvent systems was monitored by the colorimetric assay, which revealed a cyclic process of growth and etching reactions around the most stable duster species to form nearly monodisperse product distributions. We formally define the population growth of specific dusters through cyclic processing of the Au MPCs as size selective processing. The current study highlights the need for incorporating bidirectional processing, including relative rate information, into TM kinetic models for ligands with growth and etching efficacy.