Reaction Mechanism Governing the Formation of 1,3-bis(diphenylphosphino)propane-protected Gold Nanoclusters
Jeffrey W. Hudgens, John M. Pettibone, Thomas P. Senftle, Ryan N. Bratton
This report outlines the determination of a reaction mechanism that can be manipulated to develop directed syntheses of gold monolayer protected clusters (MPCs) prepared by the reduction of solutions containing 1,3-bis(diphenylphosphino)propane (L3) ligand and Au(PPh3)Cl. Nanocluster synthesis was initiated by the reduction of two-coordinate phosphine-ligated [AuILL']+ complexes (L, L' = PPh3, L3), resulting in free radical complexes. The [Au0LL']. free radicals nucleated, forming a broad size distribution of ligated clusters. Timed ultraviolet-visible spectroscopy and electrospray ionization mass spectrometry monitored the ligated Aux, 6 ≤ x ≤ 13 clusters, which comprise reaction intermediates and final products. By employing different solvents and reducing agents, reaction conditions were varied to highlight the largest portion of the reaction mechanism. We have identified several solution-phase reaction classes, including dissolution of the gold precursor, reduction, continuous nucleation/core growth, ligand exchange, ion-molecule reactions, and etching of colloids and larger clusters. Simple theories can account for the reaction intermediates and final products. The initial distribution of the nucleation products contains mainly neutral clusters. However, the rate of reduction controls the amount of reaction overlap occurring in the system, allowing a clear distinction between reduction/nucleation and subsequent solution phase processing. During solution phase processing, the complexes undergo core etching and core growth reactions, including reactions that convert neutral clusters to cations, in a cyclic process that promotes the formation of stable clusters of specific metal nuclearity. These processes comprise "size-selective" processing that can narrow a broad distribution into specific nuclearities, enabling the development of tunable syntheses.