Intercluster Compound between a Tetrakis{triphenylphosphinegold(I)}oxonium Cation and a Keggin Polyoxometalate (POM): Formation during the Course of Carboxylate Elimination of a Monomeric Triphenylphosphinegold(I) Carboxylate in the Presence of POMs

The preparation and structural characterization of a novel intercluster compound, [{Au(PPh3)}4(μ4-O)]3[α-PW12O40]2·4EtOH (1), constructed between a tetrakis{triphenylphosphinegold(I)}oxonium cation and a saturated α-Keggin polyoxometalate (POM) are described. The tetragold(I) cluster oxonium cation was formed during the course of carboxylate elimination of a monomeric phosphinegold(I) carboxylate complex, i.e., [Au((R,S)-pyrrld)(PPh3)] [(R,S)-Hpyrrld = (R,S)-2-pyrrolidone-5-carboxylic acid], in the presence of the free acid form of a Keggin POM, H3[α-PW12O40]·7H2O. The liquid−liquid diffusion between the upper water/EtOH phase containing the Keggin POM and the lower CH2Cl2 phase containing the monomeric gold(I) complex gave a pure crystalline sample of 1 in good yield (42.1%, 0.242 g scale). Complex 1 was formed by ionic interaction between the tetragold(I) cluster cation and the Keggin POM anion. As a matter of fact, the POM anion in 1 can be exchanged with the BF4− anion using an anion-exchange resin (Amberlyst A-27) in BF4− form. By using other Keggin POMs, such as H4[α-SiW12O40]·10H2O and H3[α-PMo12O40]·14H2O, the same tetragold(I) cluster cation was also formed, i.e., in the forms of [{Au(PPh3)}4(μ4-O)]2[α-SiW12O40]·2H2O (2) and [{Au(PPh3)}4(μ4-O)]3[α-PMo12O40]2·3EtOH (3). Compounds 1−3, as dimethyl sulfoxide-soluble, EtOH- and Et2O-insoluble dark-yellowish white solids, were characterized by complete elemental analysis, thermogravimetric and differential thermal analyses, Fourier transform IR, X-ray crystallography, and solid-state (CPMAS 31P and 29Si) and solution (31P{1H} and 1H) NMR spectroscopy. The molecular structures of 1 and 2 were successfully determined. The tetragold(I) cluster cation was composed of four PPh3AuI units bridged by a central μ4-oxygen atom in the geometry of a trigonal pyramid or distorted tetrahedron.