Various Oxygen-Centered Phosphanegold ( I ) Cluster Cations Formed by Polyoxometalate ( POM )-Mediated Clusterization : Effects of POMs and Phosphanes

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 31P NMR and solution (1H, 31P{1H}) 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. OPEN ACCESS Inorganics 2014, 2 661


Introduction
Polyoxometalates (POMs) are discrete metal oxide clusters that are of current interest as soluble metal oxides and for their applications in catalysis, medicine and materials science [1][2][3][4][5][6].The preparation of POM-based materials is therefore an active field of research.One of the intriguing aspects of POMs is that their combination with cluster cations or macrocations has resulted in the formation of various intercluster compounds that are interesting from the viewpoints of conducting research on ionic crystals, crystal engineering, structure, sorption properties and so on.In many compounds, POMs have been combined with the independently prepared metal cluster cations [7,8].
As continued work, we examined the POM-mediated clusterization of monomeric phosphanegold(I) units using the Keggin tungsto-and molybdo-POMs with heteroatoms P and Si, and the gold(I) carboxylate precursors with the X-substituted triarylphosphane ligands (X = m-F, m-Me and p-Me), [Au(RS-pyrrld){P(XPh)3}].
In this paper, we report the syntheses and characterization of several novel intercluster compounds, . These compounds were formed by reactions of the Keggin POMs having varied charge densities and different acidities with the phosphanegold(I) complexes containing varied substituents on the aryl group.
All gold(I) cluster cations in these intercluster compounds 1-5 contained bridged-oxygen atoms such as μ4-O, μ3-O and μ-OH groups, which were originated from water contained in the reaction system and/or the hydrated water molecules of the POMs.
The solid-state FT-IR spectra of 1-5 showed the characteristic vibrational bands on the basis of coordinating PR3 ligands (Figures S6-S10).The FT-IR spectra also showed prominent vibrational bands owing to the α-Keggin molybdo-and tungsto-POMs [22].In these spectra, the carbonyl vibrational bands of the anionic RS-pyrrld ligand in the [Au(RS-pyrrld)(PR3)] precursors disappeared, showing that the carboxylate ligand was eliminated.Elimination of the carboxylate ligand was also confirmed by 1 H NMR in DMSO-d6.The carboxylate plays a role of only the leaving group.In fact, not only pyrrolidone carboxylate, but also other carboxylates such as 5-oxotetrahydrofran-2-carboxylate and acetylglycinate can serve as the leaving groups in the formation of the tetragold(I) clusters in the presence of POMs [10].

H} NMR
The solid-state CPMAS 31 P and solution 31 P{ 1 H} NMR in DMSO-d6 signals of 1-5 are listed in Table 2.The solid-state CPMAS 31  The signal at −3.4 ppm is assignable to the heteroatom phosphorus in the Keggin molybdo-POM anion, and the signal at 24.4 ppm is assignable to the overlapped signals of the tetragold(I) cluster cations and the dimer-of-dinuclear gold(I) cluster cation.The solid-state CPMAS 31 P NMR of 2, 3 and 4 showed two broad signals at 19.6 and 24.4 ppm for 2, 18.3 and 28.5 ppm for 3, 17.4 and 27.7 ppm for 4 originating from the inequivalent phosphane groups.The three signals at 19.6, 18.3 and 17.4 ppm are assignable to one apical phosphane group, respectively, and the other three signals at 24.4, 28.5 and 27.7 ppm are assignable to the three basal phosphane groups in the trigonal-pyramidal structure, respectively [10,13].The solid-state CPMAS 31 P NMR of 5 showed two broad signals at −14.6 and 23.1 ppm due to the Keggin tungsto-POM anion and phosphane groups of the heptagold(I) cluster cation, respectively.Although all phosphane groups of the heptagold(I) cluster cation are inequivalent as shown in X-ray analysis, their signals were observed as one broad peak at 23.1 ppm.The solution 31 P{ 1 H} NMR signals of 1, 2 and 5 in DMSO-d6 were observed as major sharp signals at 26.31, 25.67 and 22.39 ppm, respectively.These signals were shifted to a higher field from the monomeric phosphanegold(I) precursors (29.20 and 25.35 ppm), respectively.In general, the 31 P{ 1 H} NMR signals of oxygen-centered phosphanegold(I) clusters are observed in the higher field in comparison with those of the monomeric phosphanegold(I) precursors [10][11][12].The peak at −3.23 ppm for 1 and −14.88 ppm for 5 are assignable to the heteroatom phosphorus in the Keggin POMs (M = Mo, W).Because Keggin molybdo-POMs are unstable in DMSO, the minor signals at −0.40, 43.57ppm of 1 and 43.38 ppm of 2 are assignable to the decomposition species.Because 3 and 4 are insoluble in any solvents, the solution 31 P{ 1 H} NMR data of 3 and 4 were not obtained.

Instrumentation and Analytical Procedures
CHN elemental analyses were carried out with a Perkin-Elmer (Waltham, MA, USA) 2400 CHNS Elemental Analyzer II (Kanagawa University, Kanagawa, Japan).IR spectra were recorded on a Jasco (Tokyo, Japan) 4100 FT-IR spectrometer in KBr disks at room temperature.TG/DTA were acquired using a Rigaku (Tokyo, Japan) Thermo Plus 2 series TG 8120 instrument. 1 H NMR (500.00MHz) and 31 P{ 1 H} NMR (202.00MHz) spectra in a DMSO-d6 solution were recorded in 5-mm-outer-diameter tubes on a JEOL (Tokyo, Japan) JNM-ECP 500 FT-NMR spectrometer with a JEOL (Tokyo, Japan) ECP-500 NMR data processing system.The 1 H NMR spectra were referenced to an internal standard of tetramethylsilane (SiMe4).The 31 P{ 1 H} NMR spectra were referenced to an external standard of 25% H3PO4 in H2O in a sealed capillary.The 31 P{ 1 H} NMR data with the usual 85% H3PO4 reference are shifted to +0.544 ppm from our data.Solid-state CPMAS 31 P NMR (121.00MHz) spectra were recorded in 6-mm-outer-diameter rotors on a JEOL JNM-ECP 300 FT-NMR spectrometer with a JEOL ECP-300 NMR data processing system.The spectra were referenced to an external standard of (NH4)2HPO4 (δ 1.60).
Crystallization.The pale-yellow white powder (0.100 g) was dissolved in 20 mL of a CH2Cl2-EtOH (3:1, v/v) mixed solvent and was filtered through a folded filter paper (Whatman No. 5).The pale-yellow clear filtrate was slowly evaporated at room temperature in the dark.After 3 days, colorless plate crystals were formed and collected on a membrane filter (JV 0.1 μm), washed with EtOH (10 mL × 2) and Et2O (10 mL × 2), and dried in vacuo for 2 h.Yield: 0.026 g (26.0%).The crystalline samples were soluble in DMSO and sparingly soluble in CH2Cl2, but insoluble in H2O

X-ray Crystallography
Single crystals with dimensions of 0.30 × 0.08 × 0.07 mm 3 for 1, 0.06 × 0.06 × 0.04 mm 3 for 2, 0.23 × 0.13 × 0.07 mm 3 for 3, 0.31 × 0.26 × 0.15 mm 3 for 4 and 0.08 × 0.07 × 0.02 mm 3 for 5 were mounted on cryoloops using liquid paraffin and cooled by a stream of cooled N2 gas.Data collection was performed on a Bruker (Madison, WI, USA) SMART APEX CCD diffractometer at 100 K for 1, 4 and 5, and Rigaku (Tokyo, Japan) VariMax with Saturn CCD diffractometer at 120 K for 2 and 3.The intensity data were automatically collected for Lorentz and polarization effects during integration.The structure was solved by direct methods (program SHELXS-97) followed by subsequent difference Fourier calculation and refined by a full-matrix, least-squares procedure on F 2 (program SHELXL-97) [28].Absorption correction was performed with SADABS (empirical absorption correction) [29].The compositions and formulae of the POMs containing many solvated molecules were determined by CHN elemental analysis, TG/DTA and 1 H NMR. Any solvent molecules in the structure were highly disordered and impossible to refine by using conventional discrete-atom models.To resolve these issues, the contribution of the solvent electron density was removed by using the SQUEEZE routine in PLATON for 1 [30].The details of the crystallographic data for 1-5 are listed in Table 1, and bond lengths (Å) and angles (°) for 1-5 are shown in Tables S1-S5.CCDC 1028278 (1), 1028279 (2), 1028280 (3), 1028281 (4) and 1028282 (5), respectively.Polyhedral representation in Figure 1 was drawn by using the VESTA 3 series [31].

Conclusions
In this paper, we prepared and characterized various novel intercluster compounds 1-5 by POM-mediated clusterization.Formation of these gold(I) cluster cations in 1-5 was strongly dependent upon the charge density and acidity of the POMs, and substituted position on the aryl group of triarylphosphane ligands as well.The structures of phosphanegold(I) cluster cations were stabilized by the intra-cluster and inter-cluster aurophilic interactions, and also interactions between the gold(I) cluster cations and POM anions.The two free-acid forms of Keggin POMs with heteroatom Si provided the trigonal-pyramidal structures of the tetraphosphanegold(I) cluster cations in the intercluster compounds, i.

Figure 1 .
Figure 1.(a) Molecular structure of 1; (b) The partial structure of the tetragold(I) cluster cation moiety; (c) The partial structure of the dimer-of-dinuclear gold(I) cluster cation moiety; (d) The interactions among the tetragold(I) cluster cations, dimer-of-dinuclear gold(I) cluster cation and Keggin polyoxometalate (POM) anions.

Figure 2 .
Figure 2. (a) Molecular structure of 5; (b) The partial structure of the heptagold(I) cluster cation.
P NMR of 1 observed two broad signals at −3.4 and 24.4 ppm.