Synthesis of gold organometallics at the nanoscale

We report the synthesis of aryldiazonium tetrachloroaurate(III) salts [X-4-C₆H₄N≡N]AuCl₄ (X = F, Cl, Br, I, CN, NO₂) and their mild reduction to covalently functionalized nanoparticles. The synthesis of the salts was carried out by the oxidation of anilines, dissolved in HCl, using sodium nitrite followed by the exchange of the counter anion with [AuCl₄]^- in water. In another procedure, the anilines were protonated with H[AuCl₄] in acetonitrile followed by one-electron oxidation using nitrosonium salt [NO]X (X = BF₄⁻, PF₆⁻). Raman and ATR-FTIR spectroscopy displayed the diazonium ν_N≡N and Raman showed the tetrachloroaurate ν_Au-Cl stretching frequencies. X-ray crystal structure of [NO₂-4-C₆H₄N≡N]AuCl₄ salt showed N≡N bond distance typical of a triple bond. Gold-aryl nanoparticles were constructed by the mild chemical reduction of the diazonium gold(III) salts using 9-borabicyclo[3.3.1]nonane (9-BBN). Nanoparticles hydrodynamic diameter, elemental composition, dynamics information, and stability data were determined. Transmission electron microscopy (TEM) measurements showed limited aggregation due to the small suppressing organic shell. However, dynamic light scattering (DLS) measurements showed considerable aggregation in acetonitrile. X-ray photoelectron spectrometry (XPS) showed the gold core in zero oxidation state and manifested the gold-organic shell connectivity. The –N=N-aryl interfacial formation on the gold surface can be ruled out since the N1s peak assigned to the diazonium moiety showed no sign in the XPS nitrogen area in addition to its absence in the ATR-FTIR spectra. Raman spectroscopy measurements showed a peak assigned to gold-carbon bonding supported by density functional calculations (DFT) on Au₂₀-C₆H₄-CN model.