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Article

Transformation of Tri-Titanium(IV)-Substituted α-Keggin Polyoxometalate (POM) into Tetra-Titanium(IV)-Substituted POMs : Reaction Products of Titanium(IV) Sulfate with the Dimeric Keggin POM Precursor under Acidic Conditions

by
Yuki Mouri
,
Yoshitaka Sakai
,
Yoshitaka Kobayashi
,
Shoko Yoshida
and
Kenji Nomiya
*
Department of Materials Science, Faculty of Science, Kanagawa University, Hiratsuka, Kanagawa 259-1293, Japan
*
Author to whom correspondence should be addressed.
Materials 2010, 3(1), 503-518; https://doi.org/10.3390/ma3010503
Submission received: 27 November 2009 / Revised: 8 January 2010 / Accepted: 13 January 2010 / Published: 15 January 2010
(This article belongs to the Special Issue Polyoxometalate Compounds)
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Abstract

:
Reaction products of titanium(IV) sulfate in HCl-acidic aqueous solution with the dimeric species linked through three intermolecular Ti-O-Ti bonds of the two tri-titanium(IV)-substituted α-Keggin polyoxometalate (POM) subunits are described. Two novel titanium(IV)-containing α-Keggin POMs were obtained under different conditions. One product was a dimeric species through two intermolecular Ti-O-Ti bonds of the two tetra-titanium(IV)-substituted α-Keggin POM subunits, i.e., [[{Ti(H2O)3}2(μ-O)](α-PW9Ti2O38)]26- (1). The other product was a monomeric α-Keggin species containing the tetra-titanium(IV) oxide cluster and two coordinated sulfate ions, i.e., [{Ti4(μ-O)3(SO4)2(H2O)8}(α-PW9O34)]3- (2). Molecular structures of 1 and 2 were also discussed based on host (lacunary site)-guest (titanium atom) chemistry.

Graphical Abstract

1. Introduction

Polyoxometalates (POMs) are molecular metal-oxide clusters, which have attracted considerable attention in the fields of catalysis, medicine, surface science and materials science, since POMs are often considered as molecular analogues of metal oxides in term of structural analogy [1,2,3,4,5,6,7,8]. Site-selective substitution of the WVI atoms in POMs with TiIV atoms is particularly interesting, because of formation of the multicenter active sites with corner- or edge-sharing TiO6 octahedra and, also, generation of oligomeric species through Ti-O-Ti bonds [9,10,11,12,13,14,15,16,17,18]. A number of catalytic reactions of titanium(IV)-containing POMs have also been reported so far [19,20,21].
The ionic radius of TiIV (0.75 Å) is close to that of WVI (0.74 Å), suggesting that TiIV should fit nicely into the POM framework. However, there is a significant consequence in terms of oligomeric Ti-O-Ti anhydride formation resulting from substitution by several TiIV atoms. For instance, tri-TiIV-1,2,3- and di-TiIV-1,2-substituted Keggin POMs heretofore reported have been isolated as dimeric, Ti-O-Ti bridged anhydride forms, e.g., [(β-1,2,3-SiW9Ti3O37)2O3]14- [9], [(α-1,2,3-GeW9Ti3O37)2O3]14- [10], [(β-1,2,3-GeW9Ti3O37)2O3]14- [11], [(α-1,2,3-PW9Ti3O37)2O3]12- [12], and [(α-1,2-PW10Ti2O38)2O2]10- [13]. Di-TiIV-substituted γ-Keggin silicotungstate and germanotungstate, i.e., [{γ-SiTi2W10O36(OH)2}2O2]8- [14] and [{γ-GeTi2W10O36(OH)2}2O2]8- [15], have been also prepared as a dimeric species, while di-TiIV-1,5-substituted β-Keggin POM has been isolated as a tetrameric species, [{β-Ti2SiW10O39}4]24- [16]. In addition, the cyclic tri-TiIV-substituted Keggin trimers such as [(α-Ti3PW9O38)3(PO4)]18- [17], [(α-Ti3SiW9O37OH)3(TiO3(OH2)3)]17- [17], and {K[(Ge(OH)O3) (GeW9Ti3O38H2)3]}14- [18] have been recently reported.
From the viewpoint of host-guest chemistry of Ti-substitution in POM, in contrast to most TiIV-substituted Keggin POMs consisting of a combination of a mono-lacunary site (one host) and an octahedral Ti group (one guest), an unusual host-guest relationship has been found in some recent POMs, [[{Ti(ox)(H2O)}4(μ-O)3](α-PW10O37)]7- (H2ox = oxalic acid) 3 [22] and [{Ti(ox)(H2O)}2(μ-O)](α-PW11O39)]5- 4 [23]. The two host-four guest POM 3 [22] in the solid state is composed of four octahedral Ti groups (four guests), i.e., the two intramolecular Ti-O-Ti bonds linked with the μ-O atom, incorporated to the two adjacent, octahedral vacant sites (two hosts) within the di-lacunary Keggin POM. This POM has been prepared by a 4:1 molar-ratio reaction in HCl-acidic solution (pH 0.08) of the anionic titanium(IV) complex [TiO(ox)2]2- with the dimeric form of the two di-titanium(IV)-substituted, α-Keggin POM units, K10[(α-1,2-PW10Ti2O39)2]·18H2O [13]. The one host-two guest POM 4 [23] has been also obtained by the reactions under strongly acidic conditions of [TiO(ox)2]2- with mono- or tri-lacunary Keggin POMs, [α-PW11O39]7- or [A-PW9O34]9-, and also with the dimeric species of the two mono-titanium(IV)-substituted Keggin POM units, [(PW11TiO39)2OH]7- as precursors. The last reaction was based on the finding of 4 in solution, i.e., its dissociation equilibrium to [(PW11TiO39)2OH]7-, [TiO(ox)2]2- and H+. In preparations of 3 and 4, it should be also noted that the dimeric species of the two mono-and/or di-titanium(IV)-substituted Keggin POM units, but not the lacunary Keggin POMs, can be used as POM precursors.
In this work, to further extend such reactions, we have investigated the reaction products of Ti(SO4)2·4H2O with [(α-1,2,3-PW9Ti3O37)2O3]12- [12] as the dimeric precursor linked through three intermolecular Ti-O-Ti bonds of the two tri-titanium(IV)-substituted α-Keggin POM units under aqueous HCl-acidic conditions. Two novel titanium(IV)-containing α-Keggin POMs were synthesized under different conditions; one was K5H[[{Ti(H2O)3}2(μ-O)](α-PW9Ti2O38)]2·9H2O (K-1) identified as the dimeric species through the two intermolecular Ti-O-Ti bonds of the two tetra-titanium(IV)-substituted Keggin POM units, and the other was K3[{Ti4(μ-O)3(SO4)2(H2O)8}(α-PW9O34)]·6H2O (K-2), characterized as the monomeric species containing tetra-titanium(IV) oxide cluster and two coordinated sulfate ions (note: the corresponding polyoxoanion moieties are abbreviated simply as 1 and 2, respectively). These compounds were unequivocally characterized with complete elemental analysis, TG/DTA, FT-IR, X-ray crystallography, solid-state 31P-CPMAS NMR and solution (31P-, 183W-) NMR spectroscopy. Herein, we report full details of the synthesis, characterization and host-guest chemistry of the novel POMs, K-1 and K-2.

2. Experimental Section

2.1. Materials

The following reactants were used as received: KCl, 1 M aqueous HCl solution (quantitative analysis grade) (all from Wako); Ti(SO4)2·4H2O (Junsei); D2O (Isotec). The POM precursor, K10H2[(α-1,2,3-PW9Ti3O37)2O3]·xH2O (x = 17 [12], 15), was prepared according to the literature and identified by FT-IR, TG/DTA and 31P-NMR.

2.2. Instrumentation/Analytical Procedures

Complete elemental analysis was carried out by Mikroanalytisches Labor Pascher (Remagen, Germany). The sample was dried at room temperature under 10-3–10-4 Torr overnight before analysis. Infrared spectra was recorded on a Jasco 4100 FT-IR spectrometer in KBr disks at room temperature. Thermogravimetric (TG) and differential thermal analyses (DTA) were acquired using a Rigaku Thermo Plus 2 series TG/DTA TG 8120 instrument. TG/DTA measurement was run under air with a temperature ramp of 4 °C/min between 20 and 500 °C.
31P{1H}-NMR (161.70 MHz) spectra in D2O solution were recorded in 5-mm outer diameter tubes on a JEOL JNM-EX 400 FT-NMR spectrometer with a JEOL EX-400 NMR data processing system. 31P-NMR spectra were measured in aqueous solution with reference to an external standard of 25% H3PO4 in H2O in a sealed capillary. The 31P-NMR signals were shifted to +0.544 ppm by using 85% H3PO4 as a reference instead of 25% H3PO4. 183W-NMR (16.50 MHz) spectra were recorded in 10-mm outer diameter tubes on a JEOL JNM-EX 400 FT-NMR spectrometer equipped with a JEOL NM-40T10L low-frequency tunable probe and a JEOL EX-400 NMR data-processing system. 183W-NMR spectra measured in D2O were referenced to an external standard of saturated Na2WO4-D2O solution. The 183W-NMR signals were shifted to –0.787 ppm by using a 2 M Na2WO4 solution as a reference instead of saturated Na2WO4 solution.
Solid-state 31P-CPMAS NMR (121.00 MHz) spectra were recorded in 6-mm outer diameter tubes on a JEOL JNM-ECP 300 FT-NMR spectrometer with a JEOL ECP-300 NMR data-processing system and were referenced to an external standard, (NH4)2HPO4. Chemical shift is reported as negative for resonance upfield of (NH4)2HPO4 (δ 1.60).

2.3. Synthesis

2.3.1. K5H[[{Ti(H2O)3}2(μ-O)](α-PW9Ti2O38)]2·9H2O (K-1)

Ti(SO4)2·4H2O (0.15 g, 0.48 mmol) was dissolved in a vigorously stirred aqueous 1 M HCl solution (10 mL). To the colorless, clear solution was added K10H2[(α-1,2,3-PW9Ti3O37)2O3]·15H2O (0.50 g, 0.09 mmol). This solution was stirred for 10 min in a water bath at 60 °C. To the clear solution solid KCl (0.75 g, 10.0 mmol) was added. After cooling to room temperature, the colorless clear solution was allowed to stand overnight in a refrigerator at 4 °C. The colorless plate crystals formed were collected on a membrane filter (JG 0.2 μm), washed with ice-cooled water (5 mL) and dried in vacuo for 2 h. The white powder obtained in 25.6% yield (0.13 g scale) was soluble in water and insoluble in most organic solvents containing EtOH and Et2O. Anal. {Found: H, 0.47; K, 3.39; O, 26.70; P, 1.12; Ti, 7.07; W, 61.40; total 100.15%. Calc. for H25K5O90P2Ti8W18 or K5H[[{Ti(H2O)3}2(μ-O)](α-PW9Ti2O38)]2 : H, 0.47; K, 3.61; O, 26.59; P, 1.14; Ti, 7.07; W, 61.11%}. A weight loss of 2.40% (weakly solvated or adsorbed water) was observed during the course of drying at room temperature at 10-3–10-4 Torr overnight before analysis, suggesting the presence of 7–8 water molecules. TG/DTA under atmospheric conditions: a weight loss of 6.69% was observed at below 200.0 °C; calc. 6.79% for total of 21 water molecules, i.e., 12 coordinated water molecules plus x = 9 hydrated water molecules in K5H[[{Ti(H2O)3}2(μ-O)](α-PW9Ti2O38)]2·xH2O. IR (KBr) (poly-oxometalate region): 1,097 s, 1,045 s, 960 vs, 895 s, 795 vs, 692 vs, 582 w, 516 w, 497 w cm-1; Solid-state 31P-NMR: δ −9.54; 31P-NMR (22.9 °C, D2O): δ −9.70; 31P-NMR (22.1 °C, 0.1 M HCl aq.): δ −9.61.

2.3.2. K3[{Ti4(μ-O)3(SO4)2(H2O)8}(α-PW9O34)]·6H2O (K-2)

K10H2[(α-1,2,3-PW9Ti3O37)2O3]·15H2O (2.0 g, 0.36 mmol) was added to a clear colorless solution of Ti(SO4)2·4H2O (0.60 g, 1.92 mmol) dissolved in 1 M aqueous HCl solution (40 mL). The colorless clear solution was stirred for 30 min in a water bath at 80 °C. To the solution was added solid KCl (0.20 g, 2.68 mmol). After cooling to room temperature, the clear solution was evaporated at 40 °C to a volume of ca. 5 mL with a rotary evaporator. The resulting white suspension was stored in a refrigerator at 4 °C overnight. The white precipitate formed was filtered off through a membrane filter (JG 0.2 μm). The clear filtrate was slowly evaporated at room temperature. After three days, clear colorless plate crystals formed, which were used for X-ray diffraction measurement. The remaining crystals were collected on a membrane filter (JG 0.2 μm) and dried in vacuo for 2 h. The colorless plate crystals obtained in 40.3% yield (0.88 g scale) were soluble in water and insoluble in most organic solvents. Stability in water is low; it decomposes within a few days at room temperature to produce several unknown materials. Anal. {Found: H, 0.55; K, 4.48; O, 29.5; P, 1.02; S, 2.38; Ti, 6.52; W, 54.80; total 99.25%. Calc. for H16K3O53P1S2Ti4W9 or K3[{Ti4(μ-O)3(SO4)2(H2O)8}(α-PW9O34)]: H,0.55 ; K, 4.01; O, 29.01, P, 1.06; S, 2.19; Ti, 6.55; W, 56.61%}. A weight loss of 3.83% (weakly solvated or adsorbed water) was observed during the course of drying at room temperature at 10-3–10-4 Torr overnight before analysis, suggesting the presence of 6 water molecules. TG/DTA under atmospheric conditions: a weight loss of 7.92% was observed at below 201.0 °C; calc. 7.78% for total of 14 water molecules, i.e., 8 coordinated water molecules plus x = 6 hydrated water molecules in K3[{Ti4(μ-O)3(SO4)2(H2O)8}(α-PW9O34)]·xH2O. IR (KBr) (polyoxometalate region) 1,232 m 1,200 w 1,128 m 1,092 s, 1,028 m, 962 s, 926 s, 793 vs, 607 m, 519 m, 476 m cm-1; Solid-state 31P-NMR: δ −14.43; 31P-NMR (22.4 °C, D2O): δ −14.96; 31P-NMR (23.4°C, 0.5 M H2SO4 aq.): δ −15.07. 31P-NMR (22.5 °C, 1 M HNO3 aq.): δ −14.86; 183W-NMR (22.0 °C, 0.5 M H2SO4 aq.): δ −120.8 (3W × 1), −179.1 (6W × 1).

2.4. X-Ray Crystallography

A colorless plate crystal of K-1 (0.29 × 0.08 × 0.03 mm) and a colorless plate crystal of K-2 (0.23 × 0.18 × 0.14 mm) were surrounded by liquid paraffin (Paratone-N) to prevent their degradation. Data collection was done by Bruker SMART APEX CCD diffractometer at 90 K in the range of 1.15 ° < θ < 28.30 ° (K-1) and 1.15 ° < θ < 28.34 ° (K-2). The intensity data were automatically collected for Lorentz and polarization effects during integration. The structure was solved by direct methods (program SHELXS-97) [24] followed by subsequent difference Fourier calculation and refined by full-matrix, least-square procedure on F2 (program SHELXL-97) [25]. Absorption correction was performed with SADABS (empirical absorption correction) [26]. The composition and formula of the POM containing many counterions and many hydrated water molecules have been determined with complete elemental analysis and TG/DTA analysis. Refinements of the positions and temperature factors of many solvent molecules and countercations in the POM are limited because of their disorder. Consequently, the residual electron densities in the final difference maps for 1 and 2 were rather large. We can reveal only the molecular structure of the POM, but not the crystal structure. These features are too common in the POM crystallography [9,10,11,12,13,14,15,16,17,18].

2.4.1. Crystal data for K-1

H43K5O99P2Ti8W18, M = 5,577.28, triclinic, space group P-1, a = 11.8044 (10), b = 13.7073 (12), c = 18.5727 (16) Å, α = 72.0090 (10), β = 84.4570 (10), γ = 65.245 (2) °, V = 2,593.7 (4) Å3, Z = 1, Dc = 3.571 Mg m-3, μ(Mo-Kα) = 20.781 mm-1. R1 = 0.0606, wR2 = 0.1465 (for all data). Rint = 0.0333, R1 = 0.0553, wR2 = 0.1429, GOF = 1.110 (25,624 total reflections, 12,729 unique reflections where I > 2σ(I)). The maximum and minimum residual density (9.764 and −5.468 eÅ-3) holes were located at 2.79 Å from O(4W) and 0.69 Å from W(3), respectively. The polyoxoanion 1 consisting of 18 tungsten atoms, eight titanium atoms, two phosphorus atoms, and 90 oxygen atoms, including the oxygen atoms due to 12 coordinated water molecules and two μ-O atoms, and five potassium cations treated with a disorder model, per formula unit, were identified, but the location of nine hydrated water molecules per formula unit were not determined as a result of disorder. CSD number 421237.

2.4.2. Crystal data for K-2

H28K3O59PS2Ti4W9, M = 3,030.86, monoclinic, space group P21/c, a = 17.8107 (11), b = 13.4314 (9), c = 23.1442(15) Å, β = 93.5690(10) °, V = 5525.9(6) Å3, Z = 4, Dc = 3.643 Mg m-3, μ(Mo-Kα) = 19.646 mm-1. R1 = 0.0404, wR2 = 0.1012 (for all data). Rint = 0.0348, R1 = 0.0354, wR2 = 0.0981, GOF = 1.043 (51,341 total reflections, 13,749 unique reflections where I > 2σ(I)). The maximum and minimum residual density (6.443 and −2.312 eÅ-3) holes were located at 2.74 Å from O(8W) and 0.78 Å from W(3), respectively. The polyoxoanion 2 consisting of nine tungsten atoms, four titanium atoms, one phosphorus atom, two sulfur atoms, and 53 oxygen atoms, including the oxygen atoms due to the eight coordinated water molecules, two coordinated sulfate ions, per formula unit, were identified. The location of three potassium cations treated with a disorder model and six hydrated water molecules, per formula unit, were also identified. CSD number 421238.

3. Results and Discussion

3.1. Synthesis and Compositional Characterization

The water-soluble potassium salts K-1 and K-2 were obtained in 25.6% (0.13 g scale) and 40.3% (0.88 g scale) yields, respectively, under the different conditions using approximately 1:5-molar ratio solution of K10H2[(α-1,2,3-PW9Ti3O37)2O3]·15H2O and Ti(SO4)2·4H2O. Compound K-1 was derived by a 10-min reaction at 60 °C, while K-2 was obtained by a 30-min reaction at 80 °C. The crystalline samples were unequivocally characterized by complete elemental analysis including O analysis, TG/DTA, FT-IR, solid-state and solution 31P-NMR spectroscopy and X-ray crystallography. The formation of polyoxoanions 1 and 2 can be represented in Equations (1) and (2):
[(α-1,2,3-PW9Ti3O37)2O3]12- + 2Ti(SO4)2 + 13H2O
→ [[{Ti(H2O)3}2(μ-O)](α-PW9Ti2O38)]26- (1) + 4SO42- + 2H+
[(α-1,2,3-PW9Ti3O37)2O3]12- + 2Ti(SO4)2 + 13H2O + 6H+
→ 2[{Ti4(μ-O)3(SO4)2(H2O)8}(α-PW9O34)]3- (2)
For complete elemental analysis the two samples, K-1 and K-2, were dried at room temperature under a vacuum of 10-3–10-4 Torr overnight. All elements (H, K, O, P, Ti and W for K-1 and H, K, O, P, S, Ti and W for K-2) were observed for a total analysis of 100.15% for K-1 and 99.25% for K-2. The data we found were in good accord with the calculated values for the formula with five potassium cations, 12 coordinated water molecules, two μ-O groups and without any hydrated water molecules for K-1, and for the formula with three potassium cations, eight coordinated water molecules, three μ-O groups, two coordinated sulfate ions and without any hydrated water molecules for K-2 (see Experimental section). The weight losses observed during drying before analysis were 2.40% for K-1 and 3.83% for K-2, which corresponded to ca. 7–8 and six hydrated water molecules, respectively. Thus, the elemental analyses indicated a presence of a total of 19–20 water molecules for K-1 and of a total of 14 water molecules for K-2 under atmospheric conditions.
On the other hand, in the TG/DTA measurements carried out under atmospheric conditions, the weight loss of 6.69% was observed at below 200.0 °C for K-1 and that of 7.92% was observed at below 201.0 °C for K-2. The former value corresponded to a total of ca. 21 water molecules (calc. 6.79%) for K-1, which were assigned to 12 coordinated water molecules plus nine hydrated water molecules, whereas the latter corresponded to a total of ca. 14 water molecules (calc. 7.78%) for K-2, which were assigned to eight coordinated water molecules plus six hydrated water molecules. Thus, the total water molecules observed by TG/DTA measurements under atmospheric conditions are approximately agreed with the total water molecules found by elemental analysis. The formulas presented herein are based on the results of TG/DTA measurements.
The solid FT-IR spectra, measured as KBr disks, of K-1 (Figure 1a), K-2 (Figure 1b) and the precursor K10H2[(α-1,2,3-PW9Ti3O37)2O3]·15H2O (Figure 1c), showed the characteristic vibrational bands of Keggin-type “PW12O40n-” polyoxotungstate framework (Figure 1) [27]. In the FT-IR spectra of K-1 and the precursor, the bands based on the Ti-O-Ti vibration between the two Keggin units are observed at 692 and 718 cm-1, respectively, suggesting that they are dimeric species, while in the spectrum of K-2 no Ti-O-Ti vibrational band was observed, suggesting that it is monomeric species. In the spectrum of K-2, bands due to the coordinated sulfate ions are observed at 1,232, 1,200 and 1,128 cm-1.
Materials 03 00503 g001 1024
Figure 1. The FT-IR spectra in the region (1,300–400 cm-1), measured as KBr disks, of (a) K5H[[{Ti(H2O)3}2(μ-O)](α-PW9Ti2O38)]2·9H2O (K-1), (b) K3[{Ti4(μ-O)3(SO4)2(H2O)8}(α-PW9O34)]·6H2O (K-2), in which the coordinated sulfates are seen as bands at 1,232, 1,200 and 1,128 cm-1, and (c) K10H2[(α-1,2,3-PW9Ti3O37)2O3]·15H2O as a precursor.
Figure 1. The FT-IR spectra in the region (1,300–400 cm-1), measured as KBr disks, of (a) K5H[[{Ti(H2O)3}2(μ-O)](α-PW9Ti2O38)]2·9H2O (K-1), (b) K3[{Ti4(μ-O)3(SO4)2(H2O)8}(α-PW9O34)]·6H2O (K-2), in which the coordinated sulfates are seen as bands at 1,232, 1,200 and 1,128 cm-1, and (c) K10H2[(α-1,2,3-PW9Ti3O37)2O3]·15H2O as a precursor.
Materials 03 00503 g001

3.2. Molecular Structures of 1 and 2

The molecular structure of polyoxoanion 1 in K-1, its polyhedral representation, and the partial structure around the Ti8 center are shown in Figure 2a, Figure 2b and Figure 2c, respectively. Selected bond lengths (Å) and angles (°) around the Ti8 center in 1 are given in Table 1, while other bond lengths (Å) and angles (°) in 1 (Table S1) and the bond valence sum (BVS) calculations of the W, Ti, O and P atoms (Table S3) are deposited in the Supplementary material.
Materials 03 00503 g002 1024
Figure 2. (a) Molecular structure of the dimeric polyoxoanion [[{Ti(H2O)3}2(μ-O)](α-PW9Ti2O38)]26- (1) in K-1, (b) its polyhedral representation, and (c) the partial structure around the Ti8 center. In (b), the four octahedral groups (green) of the eight octahedral TiO6 units are in the class of one host-two guest type coordination and the remaining four octahedral groups (blue) are in the class of one host-one guest type coordination. The two intermolecular Ti-O-Ti bonds are formed between the octahedra (blue octahedra) of the one host-one guest coordination. All 18 WO6 octahedra are shown in gray and the two central PO4 groups are shown in yellow. Total 12 coordinated water molecules are exhibited as open circles in the terminal positions in the four TiO6 (green) octahedra.
Figure 2. (a) Molecular structure of the dimeric polyoxoanion [[{Ti(H2O)3}2(μ-O)](α-PW9Ti2O38)]26- (1) in K-1, (b) its polyhedral representation, and (c) the partial structure around the Ti8 center. In (b), the four octahedral groups (green) of the eight octahedral TiO6 units are in the class of one host-two guest type coordination and the remaining four octahedral groups (blue) are in the class of one host-one guest type coordination. The two intermolecular Ti-O-Ti bonds are formed between the octahedra (blue octahedra) of the one host-one guest coordination. All 18 WO6 octahedra are shown in gray and the two central PO4 groups are shown in yellow. Total 12 coordinated water molecules are exhibited as open circles in the terminal positions in the four TiO6 (green) octahedra.
Materials 03 00503 g002
The composition and formula of K-1 containing five potassium counterions, 12 coordinated water molecules and nine hydrated water molecules were determined by complete elemental analysis and TG/DTA analysis. In X-ray crystallography, polyoxoanion 1, five potassium cations treated with a disorder model and 12 hydrated water molecules based on BVS calculations, per formula unit, were identified in the crystal structure (see Experimental section), but the location of nine hydrated water molecules per formula unit were not determined as a result of disorder.
The molecular structure of 1 is composed of two “PW9Ti4O45” Keggin POM halves linked via two Ti-O-Ti bonds between them. Each half contains the Ti4 center consisting of two Ti atoms (blue octahedra) of one host (mono-lacunary site)-one guest (one Ti atom) coordination and two Ti atoms (green octahedra) of one host-two guest coordination. The Ti4 center is composed of four corner-sharing Ti octahedra. The two Ti4 centers are linked through the two Ti-O-Ti bonds, each bond being formed between the two Ti atoms (blue octahedra) of one host-one guest coordination. In each Keggin POM half unit, six coordinated water molecules shown by BVS calculations occupy the six terminal positions of the two Ti atoms (green octahedra) of one host-two guest coordination, which are shown as open circles in Figure 2b. The whole symmetry of the molecule is represented by point group C2h.
Bond angles around the Ti8 center of 1 can be compared with those around the Ti6 center of the precursor POM [(α-1,2,3-PW9Ti3O37)2O3]12- [12]; (1) the two Ti-O-Ti bonds between the two Keggin units of 1 [136.0(5)°] vs the three Ti-O-Ti bonds between the two Keggin units of the precursor [131.0(7)–131.3(7)°] and (2) the four Ti-O-Ti bonds within the Keggin unit of 1 (between two blue octahedra [155.7(5)°], between two green octahedra [136.9(5)°], and between one blue and one green octahedra [165.9(6)–169.8(6)°]) vs the three Ti-O-Ti bonds within the Keggin unit of the precursor [141.7(7) –147.4(8)°].
Bond lengths around the Ti8 center of 1 can be also compared with those around the Ti6 center of the precursor. The Ti-O lengths in the two Ti-O-Ti bonds between the two Keggin units [1.806(9)−1.832(9), average 1.819 Å], and the Ti-O lengths in the four Ti-O-Ti bonds within the Keggin units [1.747(9)−1.915(9), average 1.839 Å] of 1, which have been obtained at the present refinement, can be compared with those of the precursor, i.e., the Ti-O lengths in the three Ti-O-Ti bonds (between the two Keggin units) [1.80(1)−1.86(1), average 1.83 Å] and the Ti-O lengths in the three Ti-O-Ti bond lengths (within the Keggin units) [1.79(1)−1.92(1), average 1.87 Å].
In both 1 and the precursor, the W-Ot (Ot: terminal oxygen), W-Oc (Oc: corner sharing oxygen), W-Oe (Oe: edge-sharing oxygen), and W-Oa (Oa: oxygen coordinated to P atom) lengths were almost the same and in the normal range [2].
The molecular structure of polyoxoanion 2 in K-2, its polyhedral representation (side view and top view), and the partial structure around the Ti4 center are shown in Figure 3a, (Figure 3b and Figure 3c) and Figure 3d, respectively. Selected bond lengths (Å) and angles (°) around the Ti4 centers in 2 are also given in Table 1, while other bond lengths (Å) and angles (°) in 2 (Table S2) and the bond valence sum (BVS) calculations of the W, Ti, S, O and P atoms (Table S4) are deposited in the Supplementary material.
The composition and formula of K-2 containing three potassium counterions, eight coordinated water molecules and six hydrated water molecules were determined by complete elemental analysis and TG/DTA analysis. In X-ray crystallography, polyoxoanion 2, three potassium cations treated with a disorder model, eight coordinated water molecules based on BVS calculations and six hydrated water molecules, per formula unit, were identified in the crystal structure (see Experimental section).
Table
Table 1. Selected bond lengths (Å) and angles (°) around the titanium(IV) centers in K-1 and K-2.
Table 1. Selected bond lengths (Å) and angles (°) around the titanium(IV) centers in K-1 and K-2.
lengthsK-1 K-2
Ti(1)-O(25)2.014(9)Ti(1)-O(25)1.871(6)
Ti(1)-O(26)2.001(9)Ti(1)-O(26)1.813(6)
Ti(1)-O(31)1.823(9)Ti(1)-O(1M)1.823(6)
Ti(1)-O(32)1.912(9)Ti(1)-O(1W)2.154(6)
Ti(1)-O(34)2.382(9)Ti(1)-O(2W)2.100(6)
Ti(1)-O(38)1.806(9)Ti(1)-O(1X)2.069(6)
average1.990average1.972
Ti(2)-O(29)2.053(9)Ti(2)-O(27)1.898(5)
Ti(2)-O(30)1.974(9)Ti(2)-O(28)1.888(6)
Ti(2)-O(31)1.803(9)Ti(2)-O(2M)1.745(5)
Ti(2)-O(33)1.915(9)Ti(2)-O(3W)2.097(6)
Ti(2)-O(36)2.294(9)Ti(2)-O(4W)2.139(5)
Ti(2)-O(38)i1.832(9)Ti(2)-O(4X)2.053(5)
average1.9785average1.970
Ti(3)-O(27)1.907(10)Ti(3)-O(29)1.873(6)
Ti(3)-O(32)1.753(9)Ti(3)-O(30)1.860(6)
Ti(3)-O(1X)1.897(10)Ti(3)-O(3M)1.749(6)
Ti(3)-O(1W)2.080(11)Ti(3)-O(5W)2.135(6)
Ti(3)-O(2W)2.179(10)Ti(3)-O(6W)2.124(6)
Ti(3)-O(3W)2.101(11)Ti(3)-O(7W)2.166(7)
average1.9862average1.9845
Ti(4)-O(28)1.871(10)Ti(4)-O(1M)1.791(6)
Ti(4)-O(33)1.747(9)Ti(4)-O(2M)1.862(6)
Ti(4)-O(1X)1.864(10)Ti(4)-O(3M)1.860(6)
Ti(4)-O(4W)2.119(11)Ti(4)-O(8W)2.088(6)
Ti(4)-O(5W)2.162(9)Ti(4)-O(3X)2.082(6)
Ti(4)-O(6W)2.097(9)Ti(4)-O(5X)2.102(5)
average1.9767average1.9642
anglesK-1 K-2
Ti(1)-O(31)-Ti(2)155.7(5)Ti(1)-O(1M)-Ti(4)153.0(4)
Ti(1)-O(32)-Ti(3)169.8(6)Ti(2)-O(2M)-Ti(4)153.0(3)
Ti(2)-O(33)-Ti(4)165.9(6)Ti(3)-O(3M)-Ti(4)164.8(4)
Ti(3)-O(1X)-Ti(4)136.9(5)
Ti(1)-O(38)-Ti(2)i136.0(5)
Symmetry operations: i = -x+1,-y+1,-z+1.
Materials 03 00503 g003 1024
Figure 3. (a) Molecular structure of the monomeric polyoxoanion [{Ti4(μ-O)3(SO4)2(H2O)8}(α-PW9O34)]3- (2) in K-2, polyhedral representations ((b) side view and (c) top view), and (d) the partial structure around the Ti4 center. In (b) and (c), the four octahedral TiO6 units are shown in blue, all nine WO6 octahedra are shown in gray, and the one central PO4 group is shown in yellow. The two sulfate ions coordinated to titanium(IV) octahedra are shown as yellow circles (S) and red circles (O), and the coordinated water molecules to titanium(IV) octahedra are exhibited by open circles.
Figure 3. (a) Molecular structure of the monomeric polyoxoanion [{Ti4(μ-O)3(SO4)2(H2O)8}(α-PW9O34)]3- (2) in K-2, polyhedral representations ((b) side view and (c) top view), and (d) the partial structure around the Ti4 center. In (b) and (c), the four octahedral TiO6 units are shown in blue, all nine WO6 octahedra are shown in gray, and the one central PO4 group is shown in yellow. The two sulfate ions coordinated to titanium(IV) octahedra are shown as yellow circles (S) and red circles (O), and the coordinated water molecules to titanium(IV) octahedra are exhibited by open circles.
Materials 03 00503 g003
The molecular structure of 2 is the monomeric POM composed of the Ti4 cluster accompanied with the two coordinated sulfate ions constructed on the tri-lacunary Keggin unit. Regardless of the coordination of the sulfate ions, an arrangement of the Ti4 cluster can be considered as a new type of host-guest relation, i.e., three host (tri-lacunary site)-four guest (four Ti octahedra) coordination. Although, in 1, the Ti4 center contained in the Keggin POM half unit may be also considered as a kind of three host-four guest coordination, it is only a combination of two TiIV atoms based on one host-one guest coordination and two TiIV atoms based on one host-two guest coordination. The previously reported, two host-four guest coordination can be also reconsidered as a combination of one host-two guest coordination [22]. The two sulfate ions bridged between the terminal positions of two Ti octahedra (central Ti4 octahedron and peripheral Ti1 or Ti2 octahedron). In total, eight coordinating water molecules in 2 are assigned to two each on the two peripheral Ti1 and Ti2 octahedra, one on the central Ti4 octahedron and three on the peripheral Ti3 octahedron without the coordination of sulfate ion, which are shown as open circles in Figure 3b and Figure 3c. Thus, whole symmetry of this molecule is exhibited by point group C1, i.e., POM 2 takes chiral configuration. In fact, an enantiomeric pair was found in the unit cell.
Two bonds (Ti2-O2M-Ti4 and Ti1-O1M-Ti4) of three Ti-O-Ti bonds containing the central Ti4 octahedron are almost equivalent and distinguished from the remaining one (Ti3-O3M-Ti4 bond), which contains a Ti3 octahedron without coordination to the sulfate ion. This fact is reflected on the angles [153.0(4), 153.0(3) and 164.8(4)°]. However, the Ti-O lengths in all Ti-O-Ti bonds were almost the same. As a matter of fact, the Ti-O lengths in the two almost equivalent Ti-O-Ti bonds (Ti2-O2M-Ti4 and Ti1-O1M-Ti4) were in the range of 1.745(5)−1.862(6) Å, while those of the remaining Ti3-O3M-Ti4 bond were in the range of 1.749(6)−1.860(6) Å. Thus, coordination of sulfate ion affects the bond angles of Ti-O-Ti, rather than the bond lengths.
The bond valence sum (BVS) calculations [28,29] (Table S3 and Table S4), based on the observed bond lengths, suggest that all atoms, except for the six doubly protonated oxygen atoms (O1W-O6W: 0.374–0.489) in 1 and the eight doubly protonated oxygen atoms (O1W-O8W: 0.387–0.478) in 2, i.e., both due to water molecules, maintain formal valences (W6+, Ti4+, P5+, S6+ and O2-). In 1 and 2, no protonation was confirmed in any Ti-O-Ti bonds between the Keggin units and, also, within the Keggin units. In 2, BVS values of the oxygen atoms bonded to sulfur atoms (O1X-O8X: 1.500–1.943) suggest formal valence O2-, i.e., no protonation to sulfate ions.

3.3. Solid-State and Solution NMR

Solid-state 31P-CPMAS NMR of K-1, K-2 and the precursor (Figure 4) showed a single resonance at −9.54, −14.43 and −10.64 ppm, respectively; K-1 and the precursor should correspond to the dimeric structures determined by X-ray analysis, and K-2 to the monomeric structure. On the other hand, solution 31P-NMR spectra of K-1, K-2 and the precursor in D2O (Figure 5) showed only one resonance at −9.70, −14.96 and −10.52 ppm, respectively, confirming the formation of a single phosphorus-containing compound. The dimeric nature in aqueous solution of the precursor has been confirmed with the molecular weight measurements based on ultracentrifugation sedimentation equilibrium [12]. 31P-NMR of K-1 in 0.1 M HCl aqueous solution observed at −9.61 ppm, and those of K-2 in 0.5 M H2SO4 and 1 M HNO3 aqueous solutions observed at −15.07 and −14.86 ppm, respectively, were almost the same as the 31P-NMR spectra observed in D2O.
Solid-state and solution 31P-NMR suggest that the dimeric structure of 1 and the monomeric structure of 2 will be maintained in aqueous solution. However, it is not clear whether the coordinated sulfate ions in the solid state are kept in aqueous solution.
Materials 03 00503 g004 1024
Figure 4. Solid-state 31P-CPMAS NMR spectra of (a) K5H[[{Ti(H2O)3}2(μ-O)](α-PW9Ti2O38)]2·9H2O (K-1), (b) K3[{Ti4(μ-O)3(SO4)2(H2O)8}(α-PW9O34)]·6H2O (K-2) and (c) K10H2[(α-1,2,3-PW9Ti3O37)2O3]·15H2O as a precursor.
Figure 4. Solid-state 31P-CPMAS NMR spectra of (a) K5H[[{Ti(H2O)3}2(μ-O)](α-PW9Ti2O38)]2·9H2O (K-1), (b) K3[{Ti4(μ-O)3(SO4)2(H2O)8}(α-PW9O34)]·6H2O (K-2) and (c) K10H2[(α-1,2,3-PW9Ti3O37)2O3]·15H2O as a precursor.
Materials 03 00503 g004
Materials 03 00503 g005 1024
Figure 5. 31P-NMR spectra of (a) K5H[[{Ti(H2O)3}2(μ-O)](α-PW9Ti2O38)]2·9H2O (K-1) in 0.1 M aqueous HCl, (b) K3[{Ti4(μ-O)3(SO4)2(H2O)8}(α-PW9O34)]·6H2O (K-2) in aqueous 0.5 M aqueous H2SO4, and (c) K10H2[(α-1,2,3-PW9Ti3O37)2O3]·15H2O as a precursor in D2O.
Figure 5. 31P-NMR spectra of (a) K5H[[{Ti(H2O)3}2(μ-O)](α-PW9Ti2O38)]2·9H2O (K-1) in 0.1 M aqueous HCl, (b) K3[{Ti4(μ-O)3(SO4)2(H2O)8}(α-PW9O34)]·6H2O (K-2) in aqueous 0.5 M aqueous H2SO4, and (c) K10H2[(α-1,2,3-PW9Ti3O37)2O3]·15H2O as a precursor in D2O.
Materials 03 00503 g005
183W-NMR measurement in HCl-acidic aqueous solution of K-1 was unsuccessful, because not enough concentration was obtained for the measurement. In order to increase the solubility of 1, the cation exchange was attempted with a batch method using the cation-exchange resin column in Na+ form and, also, with direct cation exchange using excess amount of NaClO4. However, compound K-1 seriously decomposed. On the other hand, 183W-NMR of K-2 in 0.5 M aqueous H2SO4 solution was successfully measured (Figure S1). In comparison with the 183W-NMR of the precursor, it is clear that the monomeric species is present in aqueous solution, also as suggested by solution 31P-NMR, although it is unclear if the solid-state structure containing two coordinated sulfate ions is kept.

4. Conclusions

The dimeric species linked through three intermolecular Ti-O-Ti bonds of the two tri-titanium(IV)-substituted α-Keggin polyoxometalate (POM) subunits, K10H2[(α-1,2,3-PW9Ti3O37)2O3]·15H2O, has been considered as a very stable form of the Ti-substituted Keggin POMs, because the monomeric form has been difficult to derive [12]. Nevertheless, this POM has shown effective epoxidation catalysis of olefins with aqueous hydrogen peroxide [19]. In this work, a new reaction of this POM with Ti(SO4)2·4H2O was found. Depending upon reaction conditions, it gave the two new Ti-containing products; one was obtained as K5H[[{Ti(H2O)3}2(μ-O)](α-PW9Ti2O38)]2·9H2O (K-1) by the 10 min-reaction at 60 °C, while the other was obtained as K3[{Ti4(μ-O)3(SO4)2(H2O)8}(α-PW9O34)]·6H2O (K-2) by the 30 min-reaction at 80°C. Interestingly, the former product contained a dimeric species of the tetra-titanium(IV) substituted Keggin units via two intermolecular Ti-O-Ti bonds, while the latter contained a monomeric tetra-titanium(IV) oxide cluster constructed on the tri-lacunary Keggin unit. In the solid state, the latter contained two coordinated sulfate ions. These compounds have been unequivocally characterized in the solid state and in solution. In the viewpoint of host-guest chemistry of Ti-substitution in POM, the Ti4 center in 1 was composed of a combination of one host-one guest and one host-two guest coordination, whereas the Ti4 center in 2 comprised a new type of host-guest relation, i.e., three host-four guest coordination. The polyoxoanions 1 and 2 are also of interest as possible solid oxidation catalysts. Studies in this direction are in progress.

Acknowledgements

This work was supported by a Grant-in-Aid for Scientific Research (C) and also by a High-tech Research Center Project, both from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

Supplementary Materials

Supplementary File 1Supplementary File 2
Supplementary material (solution 183W-NMR of 2 and the precursor (Figure S1); bond lengths (Å) and angles (°) for 1 and 2 (Tables S1 and S2); bond valence sum (BVS) calculations for 1 and 2 (Table S3 and S4)) can be downloaded online at https://www.mdpi.com/1996-1944/3/1/503/s1. CSD reference numbers 421237 for K-1 and 421238 for K-2. For crystallographic data in CIF or other electronic format, see https://www.mdpi.com/1996-1944/3/1/503/s2.

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MDPI and ACS Style

Mouri, Y.; Sakai, Y.; Kobayashi, Y.; Yoshida, S.; Nomiya, K. Transformation of Tri-Titanium(IV)-Substituted α-Keggin Polyoxometalate (POM) into Tetra-Titanium(IV)-Substituted POMs : Reaction Products of Titanium(IV) Sulfate with the Dimeric Keggin POM Precursor under Acidic Conditions. Materials 2010, 3, 503-518. https://doi.org/10.3390/ma3010503

AMA Style

Mouri Y, Sakai Y, Kobayashi Y, Yoshida S, Nomiya K. Transformation of Tri-Titanium(IV)-Substituted α-Keggin Polyoxometalate (POM) into Tetra-Titanium(IV)-Substituted POMs : Reaction Products of Titanium(IV) Sulfate with the Dimeric Keggin POM Precursor under Acidic Conditions. Materials. 2010; 3(1):503-518. https://doi.org/10.3390/ma3010503

Chicago/Turabian Style

Mouri, Yuki, Yoshitaka Sakai, Yoshitaka Kobayashi, Shoko Yoshida, and Kenji Nomiya. 2010. "Transformation of Tri-Titanium(IV)-Substituted α-Keggin Polyoxometalate (POM) into Tetra-Titanium(IV)-Substituted POMs : Reaction Products of Titanium(IV) Sulfate with the Dimeric Keggin POM Precursor under Acidic Conditions" Materials 3, no. 1: 503-518. https://doi.org/10.3390/ma3010503

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