A Bowl-Type Dodecavanadate as a Halide Receptor.

The dodecavanadate framework, [V12O32]4-, exhibits a unique bowl-type structure with an open molecular oxide cage having a cavity diameter of 4.4 Å, and different synthetic paths were required to construct the bowl-type structure with a different guest. A new dodecavanadate, {(n-C4H9)4N}4[V12O32(CH3NO2)] (1), is synthesized with a nitromethane guest, which is stacked above the entrance of the hemisphere rather than fully occupying the cavity, and it enables a guest-capturing reaction, while retaining the anionic cage structure. Compound 1 is a good precursor for halide-centered dodecavanadates, {(C2H5)4N}5[V12O32(X)] (X = Cl- (2), Br- (3), and I- (4)). The position of the halide inside the cavity correlates with the ionic radius of the guest; the small chloride ion sat at the far bottom, and the large iodide floated at the entrance. The inclusion reaction rates were estimated through 51V NMR time-course measurements in nitromethane. The reaction rates increase in the order I- < Br- < Cl-.

Introduction

Anion receptor chemistry is applicable in a large variety of fields, such as biological, environmental, medicinal, and materials chemistry.[1] Until now, a number of artificial anion receptors with high affinity and/or selectivity have been reported. Anions can be recognized by several binding interactions, such as electrostatic interactions, hydrogen-bonding interactions between anions and NH or CH groups, anion−π interactions, and Lewis acid–anion interactions. Among anion receptors, a halide receptor is one of the important research topics because it can act as sensitive on/off switches for functional materials as well as biological functioning.[2] The design of receptors for the simplest anion is difficult because of its spherical symmetry. For the development of a halide receptor, a clever design of the binding site is necessary to accommodate a halide guest, and the host has to provide a highly symmetric spherical cavity with the right size for the halide ion.

In general, anion receptors have positive or neutral charges on their host frameworks.[2] Although vanadium–oxygen clusters possess a high negative charge, they act as hosts that can stabilize an anion guest through electrostatic interaction with positively charged vanadium ions.[4] Anionic metal–oxygen clusters of highly oxidized early transition metals, such as vanadium, molybdenum, and tungsten, are the so-called polyoxometalates.[3] They are employed in a wide range of fields of science, such as analytical chemistry, medicinal chemistry, electrochemistry, photochemistry, and catalyst chemistry.[3] Although there are many structures of polyoxometalates, typical anions such as F–, Cl–, Br–, I–, CN–, CO32–, NO–, NO2–, NO3–, N3–, HS–, SCN–, HCO2–, and CH3CO2– are only stabilized in vanadium-based polyoxometalates (polyoxovanadates), showing structural versatility.[4] The geometrical shape of the anions governs the resulting cluster structures through the template reaction. Without anion templates, the unique frameworks of polyoxovanadates are not formed.

From these spherical structures, the encapsulated anions are unable to dissociate freely due to the closed-cage structures. The guest anions should be able to dissociate from or associate with the cavity for an application through an open site in the spheres. Such polyoxovanadates have the potential to act as an inorganic host molecule that may exhibit properties required for applications such as molecular recognition, ion separation, or molecular switching.[1,5] A bowl-type dodecavanadate, [V12O32]4–, was first reported by Yaghi et al.[6] This framework possesses a 4.4 Å cavity entrance surrounded by eight oxygen atoms in the same plane in the optimized structure. The bowl structure is constructed from two parts, which are the bottom of four corner-shared VO5 units and the rim of eight edge-shared VO5 units. The line connecting the centroids of eight vanadium atoms at the rim and of four vanadium atoms at the bottom is a quasi-four-fold axis. After the first report, two types of dodecavanadates with Cl– or NO– guests were synthesized by quite different procedures.[7,8] The nitric oxide anion-centered dodecavanadate was synthesized from [VO3]− and NO gas.[8] The chloride-centered dodecavanadate, [V12O32(Cl)]5– (2), was synthesized from a cyclic octavanadate, [Cu2V8O24]4–.[7] The removal of Cu2+ ions from the cluster liberates the V8 cyclic ring that is in close proximity to the rim part of the dodecavanadate cage, and spontaneous condensation leads to the formation of 2.[7] We also developed the structure transformation reaction of a chloride-centered dodecavanadate, which changes from a bowl shape to a ball shape by the addition of acid or base.[9]

The computational approach by Bénard et al. revealed the presence of a unique attractive electrostatic interaction between the inside of the dodecavanadate bowl and the nitrile group.[10] This interaction is strong and the previous guest-exchange reaction was limited among nitrile derivatives.[11] An acetonitrile-containing complex, [V12O32(CH3CN)]4–, is the dominant species formed in acetonitrile. The presence of excess acetonitrile as a solvent prevented the exchange of bounded acetonitrile for halides. Our strategy for the systematic halide incorporation reaction into the anionic bowl-type dodecavanadate and for the elucidation of the host property consists in avoiding the use of a nitrile group during the reaction and finding a good precursor that can be easily exchanged for a guest. We seek a guest that forms a weak interaction with the cavity because of steric bulkiness, and nitromethane is suited for this purpose due to its triangular shape that is difficult to fit in the circular cavity and the fact that it is a good solvent for dodecavanadate. In this article, the unique host properties of the dodecavanadate framework as a halide receptor were investigated. Each guest-containing dodecavanadate was isolated and fully characterized. The halide incorporation reaction rates are also discussed.

Results and Discussion

Synthesis and Structures of Guest-Exchangeable Bowl-Type Dodecavanadate

The guest-exchangeable dodecavanadate, {(n-C4H9)4N}4[V12O32(CH3NO2)] (1), was synthesized by an oxidation reaction of a reduced polyoxovanadate, [V10O26]4–, in nitromethane. Addition of t-BuOOH as an oxidant to a [V10O26]4– solution gave a reddish brown solution.[12] The UV–vis spectrum of the isolated product showed no absorption bands above 600 nm, suggesting that all of the vanadium species are oxidized to V5+. The IR spectrum showed the same pattern as that of the cage-type dodecavanadate framework with additional peaks from nitromethane. Single crystals suitable for the X-ray crystallographic analysis were obtained from a mixed solvent of nitromethane and diethyl ether (1:2, v/v, Figure ). One nitromethane exists on top of the dodecavanadate cavity, and only one oxygen atom from nitromethane is barely incorporated at the entrance of the cavity. The deviation of the principal axis of nitromethane from the equatorial plane defined by the eight entrance oxygen atoms of dodecavanadate is ca. 60°. The size of the nitrogroup prohibits complete insertion of the nitromethane molecule into the cavity because the size with respect to the interatomic distance of two oxygen atoms (4.9 Å) is larger than the entrance of the dodecavanadate framework (4.4 Å). The electrostatic balance between attractive and repulsive forces affects the tilting angles of nitromethane on the cavity. Whereas one oxygen atom of the nitrogroup sits at the same level as the vanadium plane at the rim (attractive interaction with positively charged vanadium ions), the other sits at the same level as the entrance of the oxygen plane (repulsive interaction with negative oxygens). The acetonitrile guest can be easily inserted into the cavity due to its linear structure with no steric bulkiness, and in fact, when compound 1 is dissolved in acetonitrile solvent, {(n-C4H9)4N}4[V12O32(CH3CN)] is obtained in 92% yield based on vanadium.[6] In addition, dissolution of 1 in 1,2-dichloroethane gives {(n-C4H9)4N}4[V12O32(C2H4Cl2)] (5) in 85% yield (Figure ), and this demonstrates the guest-capturing capability of 1. Acetonitrile and 1,2-dichloroethane guests are vertically positioned so as to orient a polar functional group to the cavity. Whereas a nitrogen atom of acetonitrile locates at a depth of 1.58 Å from the entrance oxygen atom plane, a chlorine atom in 5 sits at a depth of 0.85 Å.[6] The weak polarity and steric hindrance of the alkyl group prevented the insertion of 1,2-dichloroethane into the cavity.

Figure 1

Ball-and-stick representation of (a) 1, (b) [V12O32(CH3CN)]4–, and (c) 5. The guest, nitromethane, was incorporated with a deviation of ca. 60° from the equatorial plane. Orange, red, blue, black, and green spheres represent the vanadium, oxygen, nitrogen, carbon, and chlorine atoms, respectively.

Halide Incorporation Reaction into the Dodecavanadate Framework

Compound 1 can effectively behave as a guest-exchangeable bowl molecule, and it is proved to be a good starting material for the synthesis of different halide-centered dodecavanadate compounds. The halide-centered compounds, {(C2H5)4N}5[V12O32(X)] (X = Cl– (2), Br– (3), and I– (4)), were obtained simply by addition of tetraethlyammonium halide in nitromethane. Under the same reaction conditions, a fluoride-centered bowl molecule was not isolated. The yields of 2–4 were over 90% based on vanadium. Anion-containing polyoxovanadates are usually formed by the self-condensation of vanadium–oxygen species around the template anions.[4] This is the first report on the post-incorporation of anion guests into preassembled polyoxovanadate frameworks.

An inorganic dodecavanadate host shows unique size selectivity. The sizes of chloride (3.3 Å), bromide (3.6 Å), and iodide (4.1 Å) are smaller than the entrance hole of the dodeavanadate framework (4.4 Å). The larger ion, BF4– (4.6 Å), is unable to penetrate into the framework even with the addition of 30 equiv of {(n-C4H9)4N}BF4 in nitromethane, showing only signals due to 1 in 51V NMR.[13] In addition, we tried to utilize the open-cage structure by testing whether outside reagents can interact with the guest at the center. The reaction with silver cations allows the interaction with halides at the center through the open cavity, and the guest is successfully removed as silver salts in nitromethane solution. The three characteristic 51V NMR peaks from the bowl-type framework are observed in all complexes, showing the retention of the cage structure after the guest removal (Figure S1).

The crystallographic data for 2–4 are collectively shown in Table S1. Compound 2 has already been reported using a different synthetic path, but the improved synthetic method is described in this article.[7] The structures of 2–4 are isomorphous to each other and consist of a dodecavanadate bowl with a halide guest at the center of the cavity (Table ). The overall structures of the dodecavanadate frameworks are similar to each other. No direct bonds between the guest and the cluster cage are observed. The shortest V···X distances are 3.10, 3.32, and 3.47 Å for chloride, bromine, and iodide, respectively, and these values are larger than the sums of the ionic radii of the halide and vanadium ions, suggesting that these anions are stabilized by the electrostatic interaction without direct bonds. In spite of different halide radii, the average distances between the halide ion and eight rim vanadium atoms are in a similar range (3.75 Å for 2, 3.73 Å for 3, and 3.72 Å for 4).

Table 1

Structural Comparison of 2–4

compound	guest	a (Å)a	b (Å)b	c (Å)c
[V12O32(CH3CN)]4–	CH3CN	1.109	1.577d	3.794
1	CH3NO2	1.107	1.054e	3.792
2	Cl–	1.122	1.727	3.785
3	Br–	1.121	1.477	3.784
4	I–	1.112	1.249	3.771
5	(CH2)2Cl2	1.114	0.851	3.806

The distance from the plane defined by the entrance oxygen atoms to the plane defined by the rim vanadium atoms.

The distance from the plane defined by the entrance oxygen atoms to the central halide.

The distance from the plane defined by the entrance oxygen atoms to the plane defined by the bottom vanadium atoms.

The average distances between the nitrogen atom of acetonitrile and the vanadium atoms.

The average distances between the incorporated oxygen atom of nitromethane and vanadium atoms.

The smaller guest sits deeper in the cavity, whereas the larger guest locates shallower in the cavity. The average distances between the halide ion and the four vanadium atoms at the bottom are shorter than those between the halide ion and the eight rim vanadium atoms, and the incorporation depths are different according to the halide radii (Table ). The average distances between the halide ion and four vanadium atoms at the bottom are 3.16, 3.34, and 3.49 Å, respectively. This value for chloride is larger than those in the previously reported polyoxovanadates with the similar opened [V4O8] units to that of 2 (3.00 Å in [V12P20(H2O)12(Ph2CHPO3)8(Cl)2]2– and 2.85 Å in [V4O8(CH3CO2)4(Cl)]–).[14]

Table 2

Average Distances between the Incorporated Guests and Either the Rim Vanadium Atoms or the Vanadium Atoms at the Bottom

compound	guest	Vrim···X (Å)	Vbottom···X (Å)
[V12O32(CH3CN)]4–a	CH3CN	3.747	3.283
1b	CH3NO2	3.756	3.708
2	Cl–	3.754	3.156
3	Br–	3.728	3.337
4	I–	3.721	3.491
5c	(CH2)2Cl2	3.729	3.834

The average distances between the nitrogen atom of acetonitrile and the vanadium atoms.

The average distances between the incorporated oxygen atom of nitromethane and vanadium atoms.

The average distances between the inserted chlorine atom of 1,2-dichloroethane and the vanadium atoms.

51V NMR Spectroscopic Studies

To examine the host–guest interaction, 51V NMR spectra of 1–5 were recorded at 25 °C, and they are shown in Figure . 51V NMR spectrum of 1 shows three peaks at −591, −596, and −605 ppm with an integration ratio of 1:1:1. The peak at −591 ppm in 1 is assigned to vanadium atoms at the bottom.[15] The peaks at −596 and −605 ppm are assigned to vanadium atoms at the rim. The eight rim vanadium atoms can be divided into two groups according to their environments; one is connected to two bottom vanadium atoms through the corner-shared oxygen atom, and the other is connected to one bottom vanadium atom through edge-shared oxygen atoms. Inclusion of the halide guests led to chemical shifts to lower fields. The lowest field peak of 4 has a shoulder, and this indicates that the two lower field peaks overlap with each other. Although the size of the halide ions varies from 3.3 Å for chloride to 4.1 Å for iodide, the diameter of the rim ring and the average distances between the halide ion and vanadium atoms are comparable, and the signals due to the vanadium atoms of the rim are shifted downfield with an increase in the anion radius of the halide ion. It is known that the differences in NMR chemical shifts of endohedral rigid cage frameworks such as fullerene reflect the radii of the encapsulated species.[16] On the other hand, at the bottom, increasing the radius increases the V···X distances, and the signal due to the vanadium atoms of the bottom is shifted upfield with the increase in the anion radius of the halide ion. Thus, 51V NMR chemical shifts essentially depend on the charge density of the halide ions, rather than on the vanadium–halide distance or on the radii of the halide ions.

Figure 2

51V NMR spectra of 1–4measured in nitromethane and of 5 measured in 1,2-dichloroethane.

The halide guests remain trapped even at 70 °C in nitromethane solution. By increasing the temperature, the peaks of 2–4 shifted to lower fields, but otherwise, the spectra remained the same (Figure S2). The spectra simply retrieve the original lines on cooling to room temperature, without any sign of decomposition. Importantly, the dissociation reaction of halide was not observed during our investigation, suggesting that the halide incorporation is a one-way reaction.

Kinetic Study by 51V NMR Spectroscopy

To examine the guest incorporation property, a time course of 51V NMR spectra was collected after the addition of a halide ion to a nitromethane solution of 1. To compare the simple incorporation reaction rates of each halide ion, the reaction was carried out in the presence of 30 equiv of halide. After the addition of halide, the peak intensity due to 1 decreases and that due to the corresponding halide-centered dodecavanadate 2–4 increases with time (Figure ). No other peaks were detected during the reaction. On the basis of the observation, a simple reaction path for the guest incorporation reaction of 1 was proposed as follows.The halide incorporation reactions to form 2–4 completed in 120, 2100, and 6300 s, respectively (Figure S3). From the kinetic plots, the rates for the chloride, bromide, and iodide inclusion reactions based on 51V NMR measurements were 2.7 × 10–2, 2.0 × 10–3, and 8.8 × 10–4 s–1, respectively (Figure ). The rates decreased with increasing anion radius due to its increasing steric hindrance. The incorporation reactions of chloride, bromide, and iodide were carried out at different temperatures. The reaction rates at −10, 0, and 10 °C were 1.8 × 10–2, 2.7 × 10–2, and 4.9 × 10–2 for chloride, 1.1 × 10–3, 2.0 × 10–3, and 3.4 × 10–3 for bromide, and 3.0 × 10–4, 8.8 × 10–4, and 2.8 × 10–3 for iodide, respectively. From the Arrhenius equation, the activation energies for the respective incorporation reaction of chloride, bromide, and iodide were estimated to be 30, 35, and 69 kJ/mol, respectively (Figure S4).

Figure 3

Evolution of 51V NMR spectra in nitromethane at 0 °C, showing the time-dependent spectral changes after the addition of 30 equiv of (a) chloride, (b) bromide, and (c) iodide ions to the solution of 1 to form 2–4, respectively. (d) Kinetic plots for the reaction of 1 with X (X = Cl–, Br–, and I–) to form 2 (green), 3 (blue), and 4 (orange). [1]0 represents the initial concentration of 1.

Density Functional Theory (DFT) Calculation

To compare the affinity of the central halide to the dodecavanadate framework, DFT calculation was performed. The geometries of the vacant dodecavanadate host were optimized using the anion structure of 5 without 1,2-dichloroethane as the starting model. The energies of the host–guest systems were calculated by placing the guest anion at several different positions on the line connecting the centroids of the eight vanadium atoms at the rim and four vanadium atoms at the bottom. The results are plotted as a function of the distance of the guest anion from the plane of the entrance oxygen atoms in Figure S5. The figure clearly shows the energy minima for all chloride, bromide, and iodide ions at 1.61, 1.40, and 1.08 Å, respectively. These values are comparable to those observed in the crystal structures. Figure S5 also shows the energy maxima. The energy gap value between the minima and the maxima is the largest for chloride (103 kJ mol–1) and smallest for iodide (42 kJ mol–1). The gap for bromide is 78 kJ mol–1. These values are large enough to trap the halide guests inside once it enters the cavity of the dodecavanadate bowl even at 70 °C in nitromethane solution.

Conclusions

A bowl-type dodecavanadate with nitromethane as a packing molecule was synthesized and characterized. By dissolution of the nitromethane-containing dodecavanadate into a polar solvent, the guest-exchange reaction proceeds to form a solvent-containing dodecavanadate, retaining the bowl structure. The bowl-type dodecavanadate framework shows a unique halide receptor property. Three kinds of halides were stabilized in the cavity of the bowl by addition of halide salts to the dodecavanadate solution. The guest halide positions in the cavity are related to the halide sizes, and the chloride ion was the most deeply incorporated one inside the cage. The incorporation reaction rate for each halide ion was estimated from 51V NMR measurements in nitromethane. The rates decrease with increasing anion radius. The Arrhenius analysis and DFT calculations confirm that the small chloride ion is incorporated more easily into the cavity by overcoming the negative–negative repulsion between X···O at the entrance of the cavity, and once it is incorporated, it is more difficult to dissociate it due to the electrostatic interaction between X···V from the unique polyoxovanadate framework. The bowl-type polyoxovanadate host and the halide guest possess anionic charge. Nevertheless, the bowl acts as an effective halide receptor, and it captures a halide without dissociation, even if it is an open cavity.

Experimental Section

Materials

All reagents were obtained from commercial suppliers and were used without further purification unless otherwise noted. {(n-C4H9)4N}4[V10O26] was synthesized following the literature method.[12]

Measurements

IR spectra were recorded in KBr pellets on a Horiba FT-720 IR spectrometer or a JASCO FT/IR 4200 spectrometer. 51V NMR spectra were recorded on a JEOL JNM-LA400 at 105.04 MHz. Chemical shifts were externally referenced to pure VOCl3 (δ = 0 ppm). Elemental analyses of C, H, and N were performed by the Research Institute for Instrumental Analysis at Kanazawa University. Elemental analyses of Cl, Br, and I were performed by the Center for Organic Elemental Microanalysis Laboratory at Kyoto University.

Synthesis of {(n-C4H9)4N}4[V12O32(CH3NO2)] (1)

t-BuOOH (80%, 90 μL, 0.72 mmol) was added to a purple solution of {(n-C4H9)4N}4[V10O26] (1.00 g, 0.528 mmol) in nitromethane (4 mL). The resulting red–brown solution was stirred for 1 h at room temperature, and diethyl ether (20 mL) was added to yield a brown powder. The powder was collected and suspended in 2 mL of dichloromethane to dissolve any impurity. (Note that the exposure to humid air in this step leads to the formation of a yellow decavanadate species [HV10O28](6–.) The suspended product was collected by filtration and immediately dissolved in 2 mL of nitromethane. Then, 4 mL of diethyl ether was added to it, and the solution was allowed to stand for 1 h at room temperature to give block-shaped brown crystals of 1. Yield: 355 mg (37.5% based on V); IR (KBr, 400–1000 cm–1): 520, 548, 647, 707, 756, 794, 861, 952, 994 cm–1 (Figure S6); 51V NMR (CH3NO2): δ = −591 (4V), −596 (4V), −605 (4V) ppm; Anal. Calcd for C65H147N5O34V12: C, 36.32%, H, 6.82%, N, 3.24%; found: C, 36.24%, H, 6.88%, N, 3.25%.

Synthesis of {(C2H5)4N}5[V12O32(X)]·CH3NO2 (X = Cl– (2), X = Br– (3), and X = I– (4))

Ten equivalents of dried {(C2H5)4N}X (X = Cl– (for 2), Br– (for 3), I– (for 4)) with respect to 1 was added to a brown solution of 1 (30 mg, 0.014 mmol) in nitromethane (0.4 mL). The solution was allowed to stand for 24 h at room temperature to give block-shaped brown crystals.

{(C2H5)4N}5[V12O32(Cl)]·CH3NO2 (2)

Yield: 22.2 mg (85.2% based on V); IR (KBr, 400–1000 cm–1): 514, 543, 606, 646, 710, 757, 784, 851, 981 cm–1 (Figure S6); 51V NMR (CH3NO2): δ = −580 (4V), −591 (4V), −601 (4V) ppm; Anal. Calcd for C41H103ClN5O34V12: C, 26.32%, H, 5.55%, N, 4.49%, Cl, 1.89%; found: C, 26.10%, H, 5.55%, N, 4.51%, Cl, 1.92%.

{(C2H5)4N}5[V12O32(Br)]·CH3NO2 (3)

Yield: 24.0 mg (90.0% based on V); IR (KBr, 400–1000 cm–1): 512, 542, 602, 645, 709, 757, 784, 853, 982 cm–1 (Figure S6); 51V NMR (CH3NO2): δ = −583 (4V), −589 (4V), −601 (4V) ppm; Anal. Calcd for C41H103BrN5O34V12: C, 25.71%, H, 5.42%, N, 4.39%, Br, 4.17%; found: C, 25.49%, H, 5.53%, N, 4.54%, Br, 4.37%.

{(C2H5)4N}5[V12O32(I)]·CH3NO2 (4)

Yield: 24.0 mg (87.8% based on V); IR (KBr, 400–1000 cm–1): 510, 539, 596, 642, 704, 753, 780, 851, 980 cm–1 (Figure S6); 51V NMR (CH3NO2): δ = −584 (4V), −586 (4V), −600 (4V) ppm; Anal. Calcd for C41H103IN5O34V12: C, 25.09%, H, 5.29%, N, 4.28%, I, 6.47%; found: C, 24.93%, H, 5.30%, N, 4.36%, I, 6.50%.

Synthesis of {(n-C4H9)4N}4[V12O32(C2H4Cl2)] (5)

Compound 1 (108 mg, 0.05 mmol) was dissolved in a mixed solvent of nitromethane (0.2 mL) and 1,2-dichloroethane (4.5 mL). The insoluble material was filtered off. Ethyl acetate was added to the filtrate, and it was allowed to stand for 12 h at 5 °C to give block-shaped brown crystals of 5. Yield: 101 mg (85% based on V); IR (KBr, 400–1000 cm–1): 521, 550, 649, 708, 759, 794, 862, 953, 995 cm–1 (Figure S6); 51V NMR (1,2-C2H4Cl2): δ = −591 (4V), −597 (4V), −607 (4V) ppm; Anal. Calcd for C66H148Cl2N4O32V12: C, 35.39%, H, 6.57%, N, 2.36%; found: C, 35.54%, H, 6.59%, N, 2.45%.

X-ray Crystallography

Single-crystal structure analyses of 2–5 were performed using a Rigku/MSC Mercury diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71070 Å) and with 0.5° ω-scans at 0 and 90° in ϕ. Details of data collection and structure refinement, including the final cell constants, are listed in Table S1. Data were collected and processed using the CrystalClear program (Rigaku) (Supporting Information). Absorption corrections were applied on the basis of face indexing. The structures were solved by direct methods and refined by full-matrix least squares using the SHELX-97 and SHELX2013 program suite.[17] The program PLATON was used to evaluate the accuracy of the refinements, space group choice, and the lack of higher symmetry.[18]

Kinetic Studies

A nitromethane solution of {(n-C4H9)4N}X (X = Cl–, Br–, I–; 1.2 M, 0.25 mL) was mixed with a nitromethane solution of 1 (0.04 M, 0.25 mL) in an NMR tube at −20 °C. 51V NMR spectral change was measured at −10, 0, and 10 °C.

Calculation Methods

All calculations were performed using the ADF program package.[19] The basis functions consisted of a triple-ζ basis set with polarization functions from the ADF basis-set library (TZP). The core–shells up to 1s for O, 2p for Cl, 3p for V, 4p for Br, and 4d for I were kept frozen. The geometries of the dodecavanadate host were fixed to those of the optimized host during the calculations.