Electrospray ionization mass spectra of the reactions of NaAuBr 4 and related aurates with nucleophiles

Electrospray ionization mass spectra in the negative ion mode, ESI( )MS, of aqueous solutions of AuBr3 at different pH values have been reported by us, confirming the rapid equilibria (speciation) that were observed by spectrophotometry: AuBr4 and [AuBr3 (OH)] predominate at pH4 and [AuBr2(OH)2] in neutral solutions. Independently, other authors have reported similar ESI mass spectra of [AuCln(OH)4 n] . Thus, gold seems to be unique among all the elements of the periodic table, not only because of the properties of themetal, known from the beginning of the mankind, and of the gold nanoparticles, but also because gold(III) salts do not precipitate easily Au(OH)3 under neutralization, particularly in the presence of anions such as bromide or cyanide. With the aim of uncovering further features of aurate(III) ions, we carried out a series of MS experiments that are reported for the first time: (1) the characterization of commercially available samples of NaAuBr4 and NaAuCl4 at different pH values, (2) the detection by ESI( )MS of the exchanges of ligands (Cl/Br/CN) in aurates and (c) a study of the reactivity (or lack of reactivity) of NaAuBr4 with alcohols, phenols, carbonyl compounds, 1,3-dicarbonyl or 1,3dicarboxyl compounds, and oximes, also followed by ESI( )MS. We first analyzed by ESI( )MS aqueous solutions of NaAuBr4 · H2O and NaAuCl4 · 2H2O at different pH values. The spectra were recorded with an Agilent LC/MSD TOF instrument set under the following conditions: 20-V fragmentation voltage, 3500-V capillary voltage, 3ml/min flow and capillary temperature of 350 °C, with N2 as the nebulization gas at 20 psi. Several spectra were also registered at lower temperatures (150 °C) with the same results (although the sensitivity decreased). The solutions were prepared in milli-Q water, at 0.10M concentrations, and were allowed to equilibrate until the pH value (Crison pH-meter) was stable. They were then diluted (1:100) with ultrapure water and injected immediately. At the beginning of this work, we examined the effect of this dilution (0.001M); in spite of the expected fact that the pH values were closer to 7, the spectra were identical to those at 0.10M. The halido-to-hydroxido exchanges must be slowed down by dilution (with initial reaction rates 10 4 times lower), which explains that the relative intensities of the peaks were practically the same as those in the equilibria at 0.10M. In ultrapure water, the commercially available samples of NaAuBr4 · H2O (from Alfa Aesar and from Strem) and NaAuCl4 · 2 H2O (from Aldrich) have pH values below 3 (1.81, 1.70 and 2.49, respectively, at 0.1M), and around one equiv of NaOH had to be added to neutralize the solutions to pH 7. By ESI( ), both samples of ‘NaAuBr4 hydrate’ in pure water showed only an