Preparation of Preyssler-type Phosphotungstate with One Central Potassium Cation and Potassium Cation Migration into the Preyssler Molecule to form Di-Potassium-Encapsulated Derivative.

A mono-potassium cation-encapsulated Preyssler-type phosphotungstate, [P5W30O110K]14- (1), was prepared as a potassium salt, K14[P5W30O110K] (1a), by heating mono-bismuth- or mono-calcium-encapsulated Preyssler-type phosphotungstates (K12[P5W30O110Bi(H2O)] or K13[P5W30O110Ca(H2O)]) in acetate buffer. Characterization of the potassium salt 1a by single-crystal X-ray structure analysis, 31P and 183W nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared spectroscopy, high-resolution electrospray ionization mass spectroscopy, and elemental analysis revealed that one potassium cation is encapsulated in the central cavity of the Preyssler-type phosphotungstate molecule with a formal D 5h symmetry. Density functional theory calculations have confirmed that the potassium cation prefers the central position of the cavity over a side position, in which no water molecules are coordinated to the encapsulated potassium cation. 31P NMR and cyclic voltammetry analyses revealed the rapid protonation-deprotonation of the oxygens in the cavity compared to that of other Preyssler-type compounds. Heating of 1a in the solid state afforded a di-K+-encapsulated compound, K13[P5W30O110K2] (2a), indicating that a potassium counter-cation is introduced in one of the side cavities, concomitantly displacing the internal potassium ion from the center to a second side cavity, thus providing a new method to encapsulate an additional cation in Preyssler compounds.

Introduction

Tungsten forms anionic mixed metal oxides with other cationic elements to afford heteropolytungstates. Such compounds display acidic, multielectron redox, and photochemical properties and have thus attracted increasing interest as catalysts and functional materials.[1−3] Preyssler-type phosphotungstates, [P5W30O110M(H2O)](15 (M: encapsulated cation), contain five PO4 tetrahedra surrounded by 30 WO6 octahedra (10 cap tungsten and 20 belt tungsten) forming a doughnut-shaped molecule (Figure ), and have been employed as acid catalysts[4] and functional materials.[5−7] Preyssler compounds can encapsulate different cations (M) such as Na+,[8,9] Ag+,[5,10,11] K+,[12,13] lanthanoid cations,[9,14−18] Ca2+,[9,18,19] Bi3+,[9,18,20] Y3+,[9,18,19] and actinoid metals,[9,14,15,19,21] and their properties such as the redox potentials, magnetic properties, and thermal stability can be tuned by exchanging the encapsulated cations.[9,15,16,18,22−28] Such Preyssler compounds present two types of cavities: (1) one central cavity surrounded by 10 oxygens (Oa) bound to P atoms and (2) two side cavities surrounded by 5 oxygens (Oa) bound to P atoms and 5 bridging oxygens (Ob) between two cap tungsten atoms (Figure ).

Figure 1

(a) Polyhedral representation of a Preyssler-type phosphotungstate molecule with one encapsulated cation. (b) Ball-and-stick representation of one-fifth of the Preyssler-type phosphotungstate [PW6O22] unit. Ball-and-stick representation of a (c) mono-cation-encapsulated Preyssler-type molecule, (d) di-K+-encapsulated Preyssler-type molecule 2, (e) mono-K+-encapsulated Preyssler-type molecule 1, and (f) concentric circles on the oxygen, tungsten, and phosphorus atoms. (g) Polyhedral presentation of the packing of K14[P5W30O110K]·nH2O (1a) in the unit cell. Green, blue, red, orange, gray, black, and violet balls indicate tungsten, phosphorus, oxygen, oxygen surrounding the central cavity, mono-encapsulated cation, water molecule oxygen coordinated to the cation, and potassium atoms, respectively.

In almost all of the Preyssler-type phosphotungstates, the encapsulated cation occupies one of the two side cavities, presenting a coordination with five Oa oxygens, five Ob oxygens, and one coordinating water molecule (Figure c). One exception is the di-K+-encapsulated Preyssler-type phosphotungstate [P5W30O110K2]13– (2), which we reported in a previous paper.[29] In this compound, two potassium cations are located in the side cavities (Figure d). Wang’s group reported a K+ located in the central cavity of a Preyssler-type sulfotungstate, [S5W30O110K]9–, where K+ was coordinated by 10 Oa oxygens (Figure e).[30]

In this paper, we report a new Preyssler-type phosphotungstate with one encapsulated potassium cation in the central cavity, [P5W30O110K]14– (1) (Figure e), and the migration of a potassium counter-cation into the molecule in the solid state to form a di-K+ species (2).

Results and Discussion

Preparation and Isolation of K14[P5W30O110K] (1a)

K13[P5W30O110Ca(H2O)] was dissolved in a potassium acetate buffer (pH 4.7) in the presence of KCl and heated at 170 °C for 24 h (Table , entry 4). Once the reaction mixture had cooled down to room temperature, a colorless solid was obtained. The 31P NMR spectrum of the solid shows three singlets at −8.96, −10.15, and −10.96 ppm (Figure b); the peaks at −8.96 and −10.96 ppm were assigned to the starting material, [P5W30O110Ca(H2O)]13–, and the di-K+-encapsulated derivative, [P5W30O110K2]13– (2), respectively. The peak at −10.15 ppm is not assignable to any known species. The 31P NMR spectrum of the filtrate shows two singlets at 0.75 and −11.47 ppm (Figure c), where the singlet at 0.75 ppm may correspond to phosphate species and the singlet at −11.47 ppm is not assignable. Repeated recrystallization of the solid from hot water afforded colorless crystals showing only the 31P NMR singlet at −10.15 ppm (Figure e,f).

Table 1

Yield of Mono-K- and Di-K-Encapsulated Preyssler-type Phosphotungstates under Different Conditions

	reaction conditions			yielda
entry	solutionb	added KCl	temp. time	[P5W30O110K]14– (1)	[P5W30O110K2]13– (2)	recov.a
Starting Compound: K13[P5W30O110Ca(H2O)] (0.3 mmol)
1	1 M KOAc	0 mmol	170 °C	11	16	52
			24 h
2	1 M KOAc	0 mmol	170 °C	9	44	36
			48 h
3	1 M KOAc	3 mmol	170 °C	14	18	48
			24 h
4	1 M KOAc	6 mmol	170 °C	15	18	35
			24 h
5	1 M KOAc	6 mmol	170 °C	1	61	2
			48 h
6	1 M KOAc	9 mmol	170 °C	13	22	29
			24 h
7	1 M KOAc	18 mmol	170 °C	8	36	6
			24 h
8	1 M LiOAc	0 mmol	170 °C	0	4	33
			48 h
Starting Compound: K12[P5W30O110Bi(H2O)] (0.3 mmol)
9	1 M LiOAc	0 mmol	170 °C	4	28	1
			24 h
10	1 M KOAc	0 mmol	170 °C	0	17	59
			24 h
11	1 M KOAc	5 mmol	170 °C	0	9	60
			24 h
12	1 M KOAc	18 mmol	170 °C	0	49	0
			24 h
13	1 M KOAc	0 mmol	125 °C	0	0	no reactionc
			24 h
14	H2O	5 mmol	170 °C	0	0	no reactionc
			24 h
15	0.1 M HCl	5 mmol	170 °C	0	0	no reactionc
			24 h
Starting Compound: K14[P5W30O110Na(H2O)] (0.3 mmol)
16	1 M KOAc	18 mmol	170 °C	0	0	no reactionc
			24 h

The solid obtained after cooling down the reaction mixture contained only Preyssler species (starting compound, 1a and 2a), and the conversions and yields were estimated based on the 31P NMR integration ratio and the amount of solid isolated. No Preyssler species remained in the solution.

pH = 4.7, 5 mL containing 5 mmol of K+.

Only the starting compound was detected in the solid and the solution obtained after the reaction.

Figure 2

31P NMR spectra of (a) [P5W30O110Ca(H2O)]13–, (b) the solid, and (c) solution obtained after the reaction of K13[P5W30O110Ca(H2O)] in KOAc buffer (pH 4.7) in the presence of KCl (6 mmol, Table , entry 4), and the solid obtained after the (d) first, (e) second, and (f) third recrystallization. The [P5W30O110Ca(H2O)]13– spectra display one singlet or two singlets depending on the pH of the solution. The black, blue, and red arrows indicate the signals for [P5W30O110Ca(H2O)]13–, [P5W30O110K2]13–, and the new species, respectively. The solid (ca. 50 mg) was dissolved in ca. 1.0 mL of D2O.

Single-Crystal Structure Analysis of K14[P5W30O110K]

The single-crystal structure analysis revealed that the isolated colorless crystals belong to a Preyssler-type phosphotungstate with one encapsulated potassium cation (K+) (Figure e). The potassium cation is located in the central cavity on a pseudo-fivefold rotation axis of the molecule coordinated by 10 P–Oa oxygens with bond distances of 2.934–2.962 Å. No water molecules coordinating the central K+ ion were found.

A few reports on mono-K+-encapsulated Preyssler-type compounds have been published. Sun’s group reported Preyssler-type phosphotungstates, where one K+ was placed in one of the side cavities with a coordinated water molecule. On the other hand, Wang’s group reported a Preyssler-type sulfotungstate, where a K+ ion was placed in the central cavity. K14[P5W30O110K] (1a) is the first example in which a K+ ion is placed in the central cavity of a Preyssler-type phosphotungstate. Similar to the potassium salt of the di-K+-encapsulated Preyssler compound, K13[P5W30O110K2] (2a),[29]1a crystallizes in an orthorhombic space group (Figure g), where counter-cations and solvent molecules are located between the molecules.

A summary of the parameters of the [P5W30O110K]14– (1) unit in 1a is presented in Table , together with those of the [P5W30O110K2]13– (2) unit in 2a. The molecular size is quite similar, although the diameter of the circle going through the five Oa atoms in 1 is slightly larger than that in 2.

Table 2

Comparison of the NMR Chemical Shift, Infrared (IR) Wavenumber, and Size of the Two Potassium-Encapsulated Preyssler-type Phosphotungstates

	mono-K, K14[P5W30O110K] (1a)	di-K, K13[P5W30O110K2] (2a)
NMR Chemical Shift (ppm)
31P	–10.15	–10.96
183W	–208.2 (20W), −302.1 (10W)	–205.6 (20W), −265.1 (10W)
IR bands (cm–1)	1173, 1088, 1012, 985, 930, 906, 794	1178, 1088, 1016, 987, 935, 908, 781
Diameter (Å) of the Circle Going through the Given Atomsa
Oa	4.5849	4.4874
Ob	5.3456	5.3800
cap W	6.3485	6.3922
P	7.0447	7.0110
belt W	12.2746	12.2818
Thickness (Å): Distance between Belt or Cap Tungsten Atoms
belt W	3.3233	3.3251
cap W	6.6959	6.7530

Diameter of the concentric circles for the five Oa, Ob, Cap W, or P atoms (Figure f).

IR, Elemental Analysis, High-Resolution Electrospray Ionization Mass Spectroscopy (HR-ESI-MS), and 183W NMR Analysis of K14[P5W30O110K] (1a)

The IR spectrum of 1a shows the characteristic bands of a Preyssler-type phosphotungstate at 1173, 1088, 1012, 985, 930, 906, and 794 cm–1 (Figure b), similar to those of mono-Na+(H2O)-encapsulated Preyssler-type phosphotungstates (Figure a) and 2a (Figure d), but with some bands slightly shifted (Table ). We have previously reported that the IR bands of 2 are sharper than those of mono-Na+(H2O)-encapsulated compounds because the higher symmetry of the di-K+ compound (D5) gives fewer peaks with more degenerate normal modes of vibration than those of the Na+(H2O) compound (C5).[29] The IR bands of 1a are also sharper than those of Na+(H2O) compounds, confirming that 1a bears one central K+ ion with D5 symmetry.

Figure 3

IR spectra of (a) K14[P5W30O110Na], (b) K14[P5W30O110K] (1a), (c) H14[P5W30O110K] (1b), (d) K13[P5W30O110K] (1a) heated at 300 °C, and (e) K13[P5W30O110K2] (2a).

The elemental analysis revealed that the formula of 1a is K14[P5W30O110K]·26H2O. The HR-ESI-MS spectrum of 1a dissolved in a CH3CN/H2O mixture showed the characteristic peaks for H8[P5W30O110K]6–, H9[P5W30O110K]5–, H10[P5W30O110K]6–, and their dehydrated species (Figure S1), indicating also the presence of [P5W30O110K]14– species in the solution.

Figure shows the 183W NMR spectra of 1a, 2a, and K14[P5W30O110Na(H2O)] dissolved in D2O. The spectrum of the mono-Na(H2O)-encapsulated Preyssler-type phosphotungstate shows four singlets with a 2:2:1:1 integration ratio (Figure c) owing to its C5 symmetry, where the belt and cap tungsten atoms close to the encapsulated Na+ are not equivalent to those far from the encapsulated cation (Figure d). The 183W NMR spectrum of [P5W30O110K]14– (1) displays two singlets at −208.2 and −302.1 ppm with a 2:1 integration ratio (Figure a), similar to the 183W NMR spectrum of [P5W30O110K2]13– (2) (Figure b) but with different chemical shifts (Table ). The presence of two singlets in a 2:1 integration ratio indicates that both the cap and belt tungsten atoms are equivalent, suggesting that the isolated Preyssler-type phosphotungstate with one central K+ is stable in the aqueous solution.

Figure 4

183W NMR spectra of (a) K14[P5W30O110K] (1a), (b) K13[P5W30O110K2] (2a), and (c) K14[P5W30O110Na]. Each sample (ca. 0.5 g for 1a and 1.0 g for K13[P5W30O110K2] and K14[P5W30O110Na]) was dissolved in ca. 2.5 mL of D2O using a Li-resin.

Density Functional Theory (DFT) Calculations

The structure of 1a differs from that of other Preyssler-type phosphotungstates, in which one cation is encapsulated in the side cavity coordinated by one water molecule. To confidently assign the obtained structure, we calculated the relative stability of all of the possible structures (Table ). The calculations revealed that the mono-K+-encapsulated Preyssler structure with a K+ ion sitting in the central cavity in the absence of water coordination is 35.74 kcal mol–1 more stable than that with the cation sitting in one of the side cavities with a coordinated water molecule (entry 1). The mono-K+-encapsulated Preyssler structure with the K+ ion in a side cavity without a coordinated water molecule is 20.17 kcal mol–1 more stable than that presenting a solvent coordination (entry 2). These results suggest that the coordination of a water molecule is not the preferred situation when K+ is present inside the Preyssler cluster. Furthermore, the mono-K+-encapsulated Preyssler without coordinated water is 15.6 kcal mol–1 more stable when K+ is in the central cavity than when it is located in a side cavity (entry 3). These results suggest that a structure with one central K+ with no water coordination is theoretically reasonable.

Table 3

DFT Calculation Results

entry	most stable species	least stable species	energy diff. (kcal mol–1)
1	[P5W30O110K]14– (K+ in the central cavity) + H2Oa	[P5W30O110K(H2O)]14– (K+ in the side cavity)b	35.74
2	[P5W30O110K]14– (K+ in the side cavity) + H2Oa	[P5W30O110K(H2O)]14– (K+ in the side cavity)b	20.17
3	[P5W30O110K]14– (K+ in the central cavity)	[P5W30O110K]14– (K in the side cavity)	15.6
4	[P5W30O110K2]13– + water molecules (K+ in the two side cavities)	[P5W30O110K]14– + (K+)aqc (K+ in the central cavity)	66

The H2O molecule is outside the Preyssler molecule.

The H2O molecule is inside the Preyssler molecule and coordinated to the encapsulated K+.

(K+)aq has been computed as a large water cluster surrounding a K+ ion.

In contrast, Sun’s group has reported mono-K+-encapsulated Preyssler-type compounds where one K+ ion is sitting in one of the side cavities and one water molecule is coordinated to the K+ in Preyssler-metal–organic hybrid coordination polymers.[12,13] Thus, the structure with one K+ cation in the side cavity may be stabilized in such a hybrid system.

Preparation and Characterization of the Acid Form H14[P5W30O110K] (1b)

Similar to other Preyssler compounds, the acid form of 1a, H14[P5W30O110K] (1b), can be prepared by the reaction of 1a with an acidic resin (eq ). The elemental analysis confirmed that all of the counter-cations were exchanged with protons except the encapsulated K+, which was not exchanged, affording the formula H14[P5W30O110K]·40H2O. The IR spectrum of 1b is similar to that of 1a in terms of the band position and sharpness (Figure c). These results thus confirmed the successful preparation of the acid form H14[P5W30O110K] (1b) with one central K+.The 31P NMR spectrum of 1b in D2O shows one broad singlet, unusual for Preyssler-type compounds (Figure b), which is converted into the sharp peak of 1a upon neutralization of 1b with 14 equiv of KHCO3 in D2O (eq ). It has been reported that the 31P NMR spectra of Preyssler phosphotungstates, [P5W30O110M(H2O)](15– (M = Ca2+, Bi3+, Eu3+, Y3+), show one sharp singlet or two sharp singlets depending on the pH of the solution. Pope’s group has reported that the protonation of the inner oxygen of mono-cation-encapsulated Preyssler-type phosphotungstates occurs to form mono-cation-and-proton-encapsulated species (eq ),[19] describing the pKa value for a mono-Eu3+(H2O)-encapsulated compound to be below 3. We have reported the pKa value for a mono-Ca2+(H2O)-encapsulated compound to be ca. 6,[18] and López and Poblet’s group has reported pKa values of less than 2 for a mono-Na+(H2O)-encapsulated compound.[23] The inner oxygens (Oa) not coordinating the encapsulated cations have been suggested to be prone to undergo such a protonation (eq ).[19,23] The protonation–deprotonation rate is significantly slower than the NMR timescale, and thus sharp 31P NMR singlets of protonated and deprotonated species are observed. Such a slow proton-exchange rate is explained by the coordination of two oxygens (one Oa not coordinating the encapsulated cation and the O of the water molecule coordinated to the encapsulated cation) to the incorporated proton. In previous paper, we reported that the potassium salt (2a) (Figure h) and acid form of the di-K+ compound show only one sharp singlet at the same chemical shift because no protonation occurs in the cavity.[29]The 31P NMR spectrum of the potassium salt 1a became broader in an acidic solution (Figure c–f), indicating that the changes in the broadness of the signal are reversible. On the other hand, the 31P NMR signal of 2a in the acid solution was sharp (Figure g). We believe that the broad signal of 1b in D2O arises from the protonation–deprotonation processes of the inner oxygen, which are so fast that only one a peak is observed and whose protonation occurs at pH values lower than 2. The absence of a water molecule coordinated to the K+ ion in 1 may be the reason why the protonation–deprotonation processes are faster than in the other Preyssler compounds.

Figure 5

31P NMR spectra of K14[P5W30O110K] (1a) dissolved in (a) D2O, (c) 1 M HCl, (d) 0.1 M HCl, (e) Briton–Robinso buffer (pH 2), and (f) Briton–Robinso buffer (pH 7). 31P NMR spectra of (b) H14[P5W30O110K] (1b) dissolved in D2O and of K13[P5W30O110K2] (2a) in (g) 1 M HCl and (h) D2O. Each sample (ca. 50 mg) was dissolved in ca. 1.0 mL of solution.

Protonation of the inner oxygen has also been demonstrated by cyclic voltammetry (CV). It is known that the redox potentials are shifted by changing the encapsulated cation charge,[9] and the first reduction potential shifts to more positive values with the increasing charge of the incorporated cation.

The Na+(H2O)-encapsulated Preyssler-type phosphotungstate has been reported to exhibit two large redox couples and one small redox couple in 1.0 M HCl, corresponding to four-, four-, and two-electron redox processes of the tungsten atoms, respectively (Figure (pink)).[9,25] The larger two, four-electron redox couples are split into two, two-electron redox couples upon exchanging Na+ with higher-valence cations, such as Ca2+, Y3+, or Th4+, and the first two-electron reduction potential (the most positive redox couple) is shifted to a more positive potential by increasing the cationic valence of the encapsulated cation (the first redox potentials are −0.16, −0.14, and −0.12 V vs Ag/AgCl for the Na+(H2O)-, Ca2+(H2O)-, and Bi3+(H2O)-encapsulated compounds, respectively).

Figure 6

Cyclic voltammograms of K14[P5W30O110K] (1a, black), K13[P5W30O110K2] (2a, red), K14[P5W30O110Na] (pink), K13[P5W30O110Ca] (green), and K12[P5W30O110Bi] (blue). Each sample (ca. 1 mM) was dissolved in 1.0 M HCl. The arrow indicates the direction of the potential scan. (a) Potential window between 0.40 and −0.60 V. (b) An enlarged CV with potential window between 0.00 and −0.25 V.

The inner oxygen of a mono-cation-encapsulated Preyssler compounds is protonated in 1.0 M HCl,[23] and we consider that the observed redox potentials correspond to the protonated species, in which case the first reduction potential of di-K+-encapsulated 2a is similar to that of the Na+(H2O)-H+-encapsulated compound.[29]

The cyclic voltammogram of 1a in 1.0 M HCl shows three large redox couples (Figure a (black)). The first reduction peak current starts to increase at a more positive potential than that of 2a, being similar to the species from the Ca2+(H2O)-H+-encapsulated structure (Figure b), suggesting that two-proton protonation occurs in the 1.0 M HCl solution.

Reaction Conditions

To find optimal synthesis condition for mono-K+-encapsulated species (1) and di-K+-encapsulated species (2) under hydrothermal conditions, we varied the reaction conditions and the obtained species were analyzed by 31P NMR spectroscopy (Table ).

When Ca2+(H2O)-encapsulated compound K13[P5W30O110Ca(H2O)] was heated in KOAc buffer solution at 170 °C for 24 h, 1 and 2 were obtained in 11 and 16% yield, respectively, and the starting Ca(H2O) compound was recovered in 52% yield (entry 1). When KCl was added to the reaction solution, the conversion of the Ca(H2O) compound increased, as did the yield of 2 (entries 3, 4, 6, and 7). The conversion of the Ca(H2O) compound increased with the reaction time, the yield of 1 decreased, and that of 2 increased (entries 2 and 5). When the reaction was performed in LiOAc buffer solution, only 2 was obtained in 4% yield and the starting Ca(H2O) compound was recovered in 33% yield. In the LiOAc buffer, only a small amount of potassium (counter-cation of the starting compound) is present and able to replace the Ca2+ ions. These results indicate that the starting Ca(H2O) compound is consumed in the buffer solution and the yield of both 1 and 2 increases with the increasing amount of potassium cations in the solution.

When a Bi3+(H2O)-encapsulated compound, K12[P5W30O110Bi(H2O)], was heated in the LiOAc buffer solution, 1 and 2 were obtained in 4 and 28% yield, respectively, and almost all of the starting Bi(H2O) compound was consumed (entry 9). The addition of KCl to the reaction mixture increased only the yield of 2, whereas 1 was not detected (entries 10–12). A reaction temperature of 170 °C is required to consume the Bi(H2O) compound (entry 13) and the reaction needs to be performed in the buffer solution (entries 14 and 15).

The same reaction with a Na+(H2O)-encapsulated compound, K14[P5W30O110Na(H2O)], did not produce any new solids, and the Na-encapsulated compound was fully recovered (entry 16).

Thermal Conversion of Mono-K-Encapsulated Compound 1 into Di-K-Encapsulated Compound 2 in Solid State

The thermal stability of the Preyssler compounds is one of the most interesting topics for us. We have reported that the potassium salt of di-K+-encapsulated Preyssler compound 2a is thermally stable up to 450 °C.[29] We observed that heating of 1a produced di-K+-encapsulated species 2a (Figures d and S2), indicating that one potassium counter-cation migrates into one of the side cavities and the K+ in the central cavity moves into the other side cavity (Figure ). Thermogravimetric-differential thermal analysis (TG-DTA) measurements of 1a revealed an exothermal peak at a temperature between 250 and 320 °C (Figure S3), which may be assigned to the conversion of 1a into 2a.

Figure 7

Schematic representation of the conversion of K14[P5W30O110K] (1a) into K13[P5W30O110K2] (2a).

Exchange of the encapsulated cations has thus been achieved under hydrothermal conditions, and this solid-state heating process offers an additional method to introduce and/or exchange the encapsulated cation in the Preyssler compounds.

Taking the mono-K+ compound with K+ in the center of the cavity (1a) as a reference, compound 2a with two K+ ions in the side cavities was found to be much more stable (66 kcal mol–1, Table , entry 4). Although this process is largely exothermic, the conversion of 1a into 2a needs a temperature of ca. 300 °C. Intuitively, a large activation barrier must be overcome to reach the most stable product, [P5W30O110K2]13–. The calculations show that a relatively large (thermal) energy (ca. 36–43 kcal mol–1) is necessary for the cation to pass through the cavity entrance and reach the final position in the di-K structure (Figure S4).

Catalytic Activity as an Acid Catalyst

The acid form 1b exhibits catalytic activity similar to that of other acid forms of mono-cation-encapsulated compounds toward the hydration of ethyl acetate (Table ), and the catalytic activity per weight was found to be similar to that of the mono-Na(H2O)-encapsulated compound[4] and better than that of the well-known Keggin-type phosphotungstic acid H3PW12O40.

Table 4

Catalytic Activity for the Hydrolysis of Ethyl Acetatea

		rate
catalyst	conv. (%)	per weight (mmol g–1 min–1)	per acid amount (mmol acid mol–1 min–1)	ref
H14[P5W30O110Na]	46.2	275.7	164.0	(4)
H13[P5W30O110K2]	55.1	304.8	195.9	(28)
H14[P5W30O110K]	45.6	273.6	162.9	this work
H3PW12O40	49.1	175.2	174.5	(4)
blank	1.4			(4)

Amount of protons: 0.042 mmol, 5 wt % ethyl acetate in D2O (total volume: 3.0 mL, ethyl acetate: 0.15 g), reaction temperature: 80 °C, reaction time: 2 h.

Conclusions

The first Preyssler-type phosphotungstate with one encapsulated potassium cation in the central cavity, [P5W30O110K]14–, was prepared as a potassium salt and characterized by empirical and theoretical methods. Heating of the potassium salt led to the migration of one potassium counter-cation into one of the side cavities, displacing the already encapsulated potassium cation to the other side cavity to form a di-K+-encapsulated derivative. This process represents a new method to introduce cations into the Preyssler compounds.

Experimental Section

Materials

Deionized water (Millipore, Elix) was used in all of the experiments. Compounds K14[P5W30O110Na(H2O)]·23H2O, K13[P5W30O110Ca(H2O)]·25H2O, and K12[P5W30O110Bi(H2O)]·24H2O were prepared according to published procedures[18] and analyzed by 31P NMR and infrared (IR) spectroscopy. All of the other chemicals were reagent grade and used as supplied.

Preparation of K14[P5W30O110K]·17H2O (1a)

K13[P5W30O110Ca(H2O)]·25H2O (2.39 g, W: 9 mmol) and KCl (0.45 g, 6 mmol) were mixed in the potassium acetate buffer (2 M KOAc and 2 M AcOH mixed in a 1:1 ratio, 5 mL, pH 4.7) in a 30 mL Teflon-liner autoclave and the mixture was stirred for 5 min at room temperature. The autoclave was placed in an oven heated at 170 °C for 24 h. Once the reactor had cooled down to room temperature, the resulting colorless crystals were separated from the solution by filtration. The crystals were recrystallized from 25 mL of hot water heated at 90 °C (using a metal bath), and a final third recrystallization afforded a pure product suitable for single-crystal X-ray diffraction (XRD) analysis. The colorless crystals were collected by filtration and dried at 70 °C overnight (0.04 g, 0.005 mmol, yield of 2% based on W). Elemental anal. calcd (found) for K14[P5W30O110K]·17H2O (%): P, 1.86 (1.85); W, 66.3 (66.4); K, 7.05 (7.11); H, 0.48 (0.46).

Preparation of H14[P5W30O110K]·40H2O (1b)

K14[P5W30O110K]·17H2O (0.20 g) was dissolved in H2O (8 mL) and passed through 2.5 g of Dowex 50 WX8 in a protonic form packed in a glass tube (inner diameter: 20 mm) with additional water until the eluent was neutral. The eluent was then evaporated using a rotary evaporator in vacuo at 60 °C. A minimum amount of water was added, and the resulting solution was poured into a glass beaker and dried at 70 °C overnight (0.18 g, 0.13 mmol, yield of 93% based on W). Elemental anal. calcd (found) for H14[P5W30O110K]·40H2O (%): P, 1.89 (1.88); W, 67.2 (67.2); K, 0.485 (0.51); H, 1.06 (1.161).

X-ray Crystallography

Single-crystal X-ray diffraction (XRD) data for the 1a crystals were collected on a Bruker SMART APEX II ULTRA diffractometer at 173 K using a monochromated Mo Kα radiation (λ = 0.71073 Å). The structure was solved via direct methods using a SHELXS-97 and refined via the full-matrix least-squares method on F2 with SHELXL-97.[31] The atoms of the polyoxometalate molecules and counter-cations were refined anisotropically, and the oxygen atoms of crystalline water molecules were located in a difference Fourier map and refined with isotropic thermal parameters. The hydrogen atoms of crystalline water were not located. The number of potassium atoms and water oxygen atoms determined by XRD was lesser than that determined by elemental analysis due to disorder in the structure. The crystallographic data are summarized in Table . Further details of the crystal structure analysis can be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de); http://www.fiz-karlsruhe.de/request_for_deposited_data.html on quoting the deposition number CSD-433840 for K14[P5W30O110K] (1a).

Table 5

Crystal Data of K14[P5W30O110K] (1a)

compound	K14[P5W30O110K]·17H2O
empirical formula	K15P5W30O127H34
molecular weight/g mol–1	7981.28
crystal size/mm	0.10 × 0.10 × 0.08
crystal color and shape	colorless, block
temperature/K	173
crystal system	orthorhombic
space group (no.)	Pnna (52)
a/Å	32.6621(15)
b/Å	21.5252(10)
c/Å	19.0566(9)
volume/Å3	13 397.9(11)
Z	4
data/parameters	15 370/901
R (int)	0.0245
density (calcd)/g cm–1	4.249
abs. coefficient/mm–1	26.220
R1 (I > 2s(I))a	0.0229
wR2 (all data)b	0.0522

R1 = ∑||Fo| – |Fc||/∑|Fo|.

Rw = [∑w(Fo2 – Fc2)2]/∑[w(Fo2)2]1/2.

Other Analytical Techniques

The IR spectra were recorded on a NICOLET 6700 Fourier transform infrared spectrometer (Thermo Fisher Scientific) as KBr pellets. The cyclic voltammetry (CV) was performed on a CHI620D system (BAS Inc.) at ambient temperature. A glassy carbon working electrode (diameter, 3 mm), a platinum wire counter electrode, and an Ag/AgCl reference electrode (203 mV vs the normal hydrogen electrode at 25 °C) (3 M NaCl, BAS Inc.) were used. The approximate formal potential values (E1/2 values) were calculated from the CVs as the average of the cathodic and anodic peak potentials for the corresponding oxidation and reduction waves. The 31P NMR spectra were recorded on a Varian 500 (500 MHz) spectrometer (Agilent, P resonance frequency: 202.333 MHz). The spectra were referenced to external 85% H3PO4 (0 ppm). The 183W NMR spectra were recorded on a Varian 500 (500 MHz) spectrometer (Agilent) (W resonance frequency: 20.825 MHz). The spectra were referenced to an external saturated Na2WO4 (0 ppm). The 1a sample for 183W NMR spectroscopy was treated with lithium resin to increase its solubility in D2O. Elemental analyses were carried out by Mikroanalytisches Labor Pascher (Remagen, Germany). High-resolution ESI-MS spectra were recorded on an LTQ Orbitrap XL instrument (Thermo Fisher Scientific) with an accuracy of 3 ppm. Each sample (5 mg) was dissolved in 5 mL of H2O, and the solutions were diluted with CH3CN (final concentration: ca. 10 μg mL–1).

Hydrolysis of Ethyl Acetate

Hydrolysis of ethyl acetate was carried out at 80 °C with 5 wt % ethyl acetate in D2O (total volume: 3.0 mL, ethyl acetate: 0.15 g) for 2 h.[4] The amount of protons used was maintained at 0.042 mmol.

Electronic Structure Calculations

Density functional theory (DFT) calculations were performed for the [P5W30O110K]14–, [P5W30O110K(H2O)]14–, and [P5W30O110K2]13– structures with the ADF 2016 suite of programs.[32,33] Intermediate geometries were also analyzed to estimate the K+ encapsulation energy profile. Equilibrium geometries were obtained upon full geometry optimization with tight convergence criteria (optimized structures must be very accurate when frequency calculations are to be conducted because molecular vibrations can be highly dependent on the geometrical parameters) and the OPBE functional[34,35] with triple-ζ + double polarization atomic basis sets, using the frozen core approximation for the following shells: 1s–3p for K, 1s–2p for P, 1s–4f for W, and 1s for O. We simulated an aqueous solution (dielectric constant, ε = 78.39) by including the solvent and counterion effects by means of the conductor-like screening model.[36−39]