Reaction of Au55(PPh3)12Cl6 with thiols yields thiolate monolayer protected Au75 clusters

This paper describes the reaction of the phosphine-protected Au nanoparticle Au-55(PPh3)(12)Cl-6 (1, "Au55") with hexanethiol (2) and other thiols. The voltammetry of the reaction product 2 displays a well-defined pattern of peaks qualitatively reminiscent of AU(38) nanoparticles, but with quite different spacing (0.74 +/- 0.01 V) between the potentials of initial oxidation and reduction steps (electrochemical gap). Correction of this "molecule-like" gap for charging energy indicates a HOMO-LUMO gap energy of about 0.47 V. Voltammetry of the products (3 and 4) of reaction of 1 with C3H7SH and PhC2H4SH, respectively, is similar. Laser desorption/ionization mass spectrometry (LDI-MS) shows that 2 contains a high proportion of a core mass in the 14-15 kDa range, which is proposed to be AU(75). UV-vis spectra of 2-4 are relatively featureless, similar to previous reports of thiolate-protected AU(75) nanoparticles. HPLC analysis of 2 shows a AU75 content of ca. 73%; the electrochemical purity estimate is also high, about 55%. Combining the mass spectrometric result with thermogravimetric analysis of 2 leads to a preliminary formulation AU75(SC6H13)(40). This AU(75) synthesis complements a previous Brust-type synthesis and is unusual in the apparent provocation in the reaction of an increase in core size.