The effect of counteranions on the molecular structures of phosphanegold(i) cluster cations formed by polyoxometalate (POM)-mediated clusterization

Abstract

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, [{(AuL_Cl)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)(L_Cl)] (RS-Hpyrrld = RS-2-pyrrolidone-5-carboxylic acid; L_Cl = 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 [{(AuL_Cl)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 [{(AuL_Cl)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. [{(AuL_Cl)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, ³¹P{¹H} NMR, ¹H 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 [{(AuL_F)₂(μ-OH)}₂]₃[PMo₁₂O₄₀]₂·3EtOH (4-PMo₁₂: L_F = tris(4-fluoro phenyl)phosphane) were successfully converted to the POM-free crystalline OTf⁻ salt [{(AuL_F)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 Au⋯O_POM and hydrogen-bonding (C–H⋯O_POM and O–H⋯O_POM) 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.