Isolating Ligand Steric Effects on Metal Ion Reduction and Gold Nanoparticle Formation Pathways Using Au(I)-Phosphine Complexes.

Steric effects are widely used to control molecular reactivity, yet their role in nanoparticle (NP) synthesis remains difficult to isolate and leverage because changes in ligand structure often alter both precursor electronic structure and resulting reduction thermodynamics. Here, we use three Au(I)-phosphine chloride complexes with systematically increasing ligand cone angles to decouple precursor sterics from electronics and determine how steric bulk influences gold NP formation. Density functional theory and cyclic voltammetry show that the three precursors have comparable frontier orbital energies and standard reduction potentials, while their heterogeneous electron-transfer kinetics differ substantially, with increasing steric bulk progressively slowing reduction. When these precursors are used in otherwise identical NP syntheses, the resulting particle populations differ significantly. The least sterically hindered precursor produces the largest, plasmonically active particles, while the most sterically encumbered precursor yields predominantly non-plasmonic cluster-like products. Transmission electron microscopy, extinction spectra, and a custom-built and time-resolved color-intensity analysis are consistent with this relationship indicating that increasing precursor steric bulk both slows NP formation and also suppresses downstream NP growth into larger particles, likely by hindering post-reduction cluster coalescence as evidenced by the appearance of Au cluster populations detected by matrix-assisted laser desorption ionization mass spectrometry. Taken together, these results indicate that precursor sterics impact gold NP formation through dual, but distinguishable roles in metal ion reduction kinetics and also altering downstream NP growth pathways. More broadly, this work establishes precursor sterics as a distinct molecular design parameter for controlling both the kinetics and outcomes of NP synthesis.