A DMSO-soluble intercluster compound consisting of a tetra{phosphanegold(I)}oxonium cation and an α-Keggin polyoxometalate (POM) anion, [{Au(PPh 3 )} 4 (μ 4 -O)] 3 [α-PW 12 O 40 ] 2 ( 1), was found to be an effective precatalyst for the silver-and acid-free catalysis of diphenylacetylene hydration (0.67 mol % catalyst; conversions 36.1%, 55.2%, and 93.7% after 4, 6, and 24 h reactions, respectively). The reaction proceeded in the suspended system in 6 mL of 1,4-dioxane/water (4:1) at 80°C because of the low solubility of 1. Similar POM-based phosphanegold(I) compounds (5), which is composed of a heptakis{triphenylphosphanegold(I)}-dioxonium cation and an α-Keggin POM anion, and [Au(CH 3 CN)-(PPh 3 )] 3 [α-PMo 12 O 40 ] (6), which consists of an acid-free monomeric phosphanegold(I) acetonitrile cation and an α-Keggin molybdo-POM anion, also exhibited acid-free catalysis for the hydration of diphenylacetylene. An induction period was observed in the catalysis by 5. On the other hand, their component species, or phosphanegold(I) species without the POM anion, such as [{Au(PPh 3 )} 4 (μ 4 -O)](BF 4 ) 2 (2) and [{Au(PPh 3 )} 3 (μ 3 -O)]BF 4 (3), and the monomeric phosphanegold(I) complex [Au(RS-pyrrld)(PPh 3 )](4) (RS-Hpyrrld = RS-2-pyrrolidone-5-carboxylic acid), the last of which has been used as a precursor for the preparation of 1, 5, and 6, showed poor activities in this reaction (0.67 mol % catalysts; conversions 1.8%, 1.7%, and 0.5% after 24 h reactions, respectively). However, upon adding the free-acid form of Keggin POM, i.e., H 3 [α-PW 12 O 40 ]·7H 2 O (H-POM: 0.67 mol %), 2−4 exhibited remarkably enhanced activities (conversion 97.6% each after 24 h reactions). In contrast, the activities were not enhanced after adding either the sodium salt of the Keggin POM, Na 3 [α-PW 12 O 40 ]·8H 2 O (Na-POM; 0.67 mol %), or a Brønsted acid 10% HBF 4 aqueous solution (0.67 mol %). Both H-POM and Na-POM themselves exhibited no activity. Catalysis by the phosphanegold(I) species for diphenylacetylene hydration was influenced significantly under the free-acid form or sodium salt of the Keggin POM. Acid-free catalytic hydration by 1 of other alkynes, such as phenylacetylene and 1-phenyl-1-butyne, was also examined. ■ INTRODUCTIONPolyoxometalates (POMs) are anionic, molecular metal− oxygen bonding clusters that resemble the discrete fragments of solid metal oxides and mimic soluble metal oxides. These properties have led to a range of applications in different fields, especially in catalysis, medicine, biology, electrochromism, magnetism, and material science. 1 A combination of POMs with cluster cations or macrocations has resulted in the formation of various interesting intercluster compounds from the viewpoints of ionic crystals, crystal growth, crystal engineering, structure, sorption properties, etc. 2−4 Recently, we reported the formation of a novel intercluster compound consisting of a tetrakis{triphenylphosphanegold-(I)}oxonium cluster cation and the Keggin POM anion, i.e., [{Au(PPh 3 )} 4 (μ 4 -O)] 3 [α-PW 12 O ...

Novel phosphanegold(I) cluster cations combined with polyoxometalate (POM) anions, i.e., intercluster compounds, [(Au{P(m-FPh)3})4(μ4-O)]2[{(Au{P(m-FPh)3})2 (μ-OH)}2][α-PMo12O40]2•EtOH (1), [(Au{P(m-FPh)3})4(μ4-O)]2[α-SiMo12O40]•4H2O (2), [(Au{P(m-MePh)3})4(μ4-O)]2[α-SiM12O40] (M = W (3), Mo (4)) and [{(Au {P(p-MePh)3})4(μ4-O)}{(Au{P(p-MePh)3})3(μ3-O)}][α-PW12O40] (5) were synthesized by POM-mediated clusterization, and unequivocally characterized by elemental analysis, TG/DTA, FT-IR, X-ray crystallography, solid-state CPMAS 31 P NMR and solution (1 H, 31 P{ 1 H}) NMR. Formation of the these gold(I) cluster cations was strongly dependent upon the charge density and acidity of the POMs, and the substituents and substituted positions on the aryl group of triarylphosphane ligands. These gold(I) cluster cations contained various bridged-oxygen atoms such as μ4-O, μ3-O and μ-OH groups.

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The effect of counteranions on the molecular structures of phosphanegold(i) cluster cations formed by polyoxometalate (POM)-mediated clusterization was investigated. A novel intercluster compound, [{(AuLCl)2(μ-OH)}2]3[α-PMo12O40]2·3EtOH (1-PMo12), was obtained as orange-yellow plate crystals in 12.0% yield from a 6 : 1 molar ratio reaction of the monomeric phosphanegold(i) carboxylato complex [Au(RS-pyrrld)(LCl)] (RS-Hpyrrld = RS-2-pyrrolidone-5-carboxylic acid; LCl = tris(4-chlorophenyl)phosphane) in CH2Cl2 with the free acid-form of Keggin polyoxometalate (POM), H3[α-PMo12O40]·14H2O. An EtOH/H2O (5 : 1, v/v) solvent mixture was used. The dimeric cation [{(AuLCl)2(μ-OH)}2](2+) in 1-PMo12 was in a parallel-edge arrangement that was formed by self-assembly through the inter-cationic aurophilic interactions of the μ-OH-bridged dinuclear phosphanegold(i) cation. The POM anion in 1-PMo12 was successfully exchanged with a smaller PF6(-) anion by the use of an anion-exchange resin. POM-free, colorless block crystals of [{(AuLCl)3(μ3-O)}2](PF6)2·4CH2Cl2 (2-PF6) were obtained by vapor diffusion in 14.1% yield. During the synthesis of 2-PF6, a compound with mixed counteranions (one POM and one PF6(-) anion), i.e. [{(AuLCl)4(μ4-O)}]2[α-PMo12O40]PF6 (3-PMo12PF6), was obtained in 66.4% yield. Both products were characterized by elemental analysis, TG/DTA, FT-IR, (31)P{(1)H} NMR, (1)H NMR, and X-ray crystallography. X-ray crystallography revealed that the countercation in 2-PF6 was the dimeric cation of the μ3-O-bridged tris{phosphanegold(i)} species, whereas that in 3-PMo12PF6 consisted of an unusual μ4-O-bridged tetragonal-pyramidal tetrakis{phosphanegold(i)} cation. Therefore, we concluded that the POM anion significantly contributed to the stabilization of these countercations (parallel-edged arrangement in 1-PMo12 and μ4-O-bridged tetragonal-pyramid in 3-PMo12PF6). Moreover, the previously reported yellow crystals of [{(AuLF)2(μ-OH)}2]3[PMo12O40]2·3EtOH (4-PMo12: LF = tris(4-fluoro phenyl)phosphane) were successfully converted to the POM-free crystalline OTf(-) salt [{(AuLF)2(μ-OH)}2](OTf)2·0.5Et2O (4-OTf) by the use of an anion-exchange resin. X-ray crystallography also revealed that the parallel-edge arrangement of the dimeric cation in 4-PMo12 was converted to the crossed-edge arrangement of that in 4-OTf. These results illustrate that the AuOPOM and hydrogen-bonding (C-HOPOM and O-HOPOM) interactions between the phosphanegold(i) cluster cation and the Keggin POM anion in the solid state significantly contribute to the structure, composition, and stability of the phosphane gold(i) cluster cations in 4-PMo12.