Crystal Structures of Au2 Complex and Au25 Nanocluster and Mechanistic Insight into the Conversion of Polydisperse Nanoparticles into Monodisperse Au25 Nanoclusters

We previously reported a size-focusing conversion of polydisperse gold nanoparticles capped by phosphine into monodisperse [Au25(PPh3)10(SC2H4Ph)5Cl2]2+ nanoclusters in the presence of phenylethylthiol. Herein, we have determined the crystal structure of [Au25(PPh3)10(SC2H4Ph)5Cl2]2+ nanoclusters and also identified an important side-producta Au(I) complex formed in the size focusing process. The [Au25(PPh3)10(SC2H4Ph)5Cl2]2+ cluster features a vertex-sharing bi-icosahedral core, resembling a rod. The formula of the Au(I) complex is determined to be [Au2(PPh3)2(SC2H4Ph)]+ by electrospray ionization (ESI) mass spectrometry, and its crystal structure (with SbF6– counterion) reveals Au–Au bridged by −SC2H4Ph and with terminal bonds to two PPh3 ligands. Unlike previously reported [Au2(PR3)2(SC2H4Ph)]+ complexes in the solid state, which exist as tetranuclear complexes (i.e., dimers of [Au2(PR3)2(SC2H4Ph)]+ units) through a Au···Au aurophilic interaction, in our case we found that the [Au2(PPh3)2(SC2H4Ph)]+ complex exists as a single entity, rather than being dimerized to form a tetranuclear complex. The observation of this Au(I) complex allows us to gain insight into the intriguing conversion process from polydisperse Au nanoparticles to monodisperse Au25 nanoclusters.