Structure Transformation among Deca-, Dodeca- and Tridecavanadates and Their Properties for Thioanisole Oxidation

Abstract

The transformation of three types of polyoxovanadates, {(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈], {(n-C₄H₉)₄N}₄[V₁₂O₃₂] and {(n-C₄H₉)₄N}₃[V₁₃O₃₄] have been investigated according to the rational chemical equations, and the best transformation conditions were reported. By the reaction of [H₃V₁₀O₂₈]³⁻ with 0.33 equivalents of {(n-C₄H₉)₄N}OH in acetonitrile at 80 °C, [V₁₂O₃₂]⁴⁻ was formed with 92% yield. The reaction in nitroethane with 0.69 equivalents of p-toluenesulfonic acid gave [V₁₃O₃₄]³⁻ with 91% yield. The ⁵¹V NMR observation of each reaction suggests the complete transformations of [H₃V₁₀O₂₈]³⁻ to [V₁₂O₃₂]⁴⁻ and to [V₁₃O₃₄]³⁻ proceeded without the formation of any byproducts and it provides the reliable synthetic route. Decavanadates were produced by the hydrolysis of [V₁₂O₃₂]⁴⁻ or [V₁₃O₃₄]³⁻. While the direct transformation of [V₁₃O₃₄]³⁻ to [V₁₂O₃₂]⁴⁻ partly proceeded, the reverse one could not be observed. For the thioanisole oxidation, [V₁₃O₃₄]³⁻ showed the highest activity of the three.

1. Introduction

Polyoxometalates have been attracting much attention in broad fields such as catalyst chemistry, magnetochemistry, and pharmaceutical chemistry [1,2,3]. Polyoxovanadates have a unique structural chemistry due to the availability of various coordination environments of tetrahedral {VO₄}, square-pyramidal {VO₅} and octahedral {VO₆} units [4]. Due to the high reactivity, the chemistry of polyoxovanadates is limited in decavanadate species. The only stable example of isopolyoxovanadates in aqueous solution is decavanadates, which consist of {VO₆} units, in addition to the metavanadate species, [VO₃]_nⁿ⁻ with {VO₄} units [5]. The employment of non-aqueous solvent increases the diversity of the polyoxovanadates. Up to now, various isopolyoxovanadates with only fully oxidized V⁵⁺, such as [V₃O₉]³⁻, [V₄O₁₂]⁴⁻ and [V₅O₁₄]³⁻ with {VO₄} units; [V₁₁O₂₉F₂]⁴⁻, [HV₁₂O₃₂(Cl)]⁴⁻, [V₁₂O₃₂]⁴⁻, [V₁₂O₃₂(Cl)]⁵⁻ and [H₂V₁₄O₃₈(Cl)]⁵⁻ with {VO₅} units; [V₇O₁₉F]⁴⁻ with both {VO₄} and {VO₅} units; and [V₆O₁₉]⁸⁻ derivatives, [H_nV₁₀O₂₈]⁽⁶⁻ⁿ⁾⁻ and [V₁₃O₃₄]³⁻ with {VO₆} units have been synthesized [6,7,8,9,10,11,12,13,14,15,16,17,18,19]. Since the properties such as redox and optics of polyoxovanadates depend on the anion structures, it is crucially important to control those cluster frameworks [14,19].

The half spherical dodecavanadates [V₁₂O₃₂]⁴⁻ consist of twelve {VO₅} units and have an open cavity into which several molecules and anions, such as nitriles, dichloromethane, NO⁻ and Cl⁻, are incorporated and the host-guest interactions have been theoretically investigated (Figure 1) [9,10,20,21,22,23]. To clarify the role of an electron-rich guest and the anion, further investigations including systematic synthesis are required.

Tridecavanadate [V₁₃O₃₄]³⁻ was reported as an oxidation catalyst for a number of organic substrates (Figure 1) [19]. After the pioneering work on the chemistry of [V₁₃O₃₄]³⁻, few further investigations were reported. Our group successfully synthesized the four electron reduced tridecavanadate [H₄V₁₃O₃₄]³⁻ [24]. During our investigation, the electrochemical oxidation of [H₄V₁₃O₃₄]³⁻ gave [V₁₂O₃₂]⁴⁻ instead of [V₁₃O₃₄]³⁻. The protonation may prevent the oxidation of [H₄V₁₃O₃₄]³⁻ to form [V₁₃O₃₄]³⁻. Cronin’s group synthesized the manganese-substituted polyoxovanadate with the same anion structure of [V₁₃O₃₄]³⁻ [25]. The hetero-metal containing polyoxometalates have the potential to show unique catalytic and magnetic properties [26,27,28,29]. As like synthesis of Cronin’s cluster from the related manganese-containing polyoxovanadate, the control of the structure transformation is important.

In this work, we report on the structure transformation among [H₃V₁₀O₂₈]³⁻, [V₁₂O₃₂]⁴⁻ and [V₁₃O₃₄]³⁻, including the improved synthesis of {(n-C₄H₉)₄N}₃[V₁₃O₃₄]. In addition, oxidation of thioanisole using these polyoxovanadates is discussed.

Figure 1. Anion structures of (a) [H₃V₁₀O₂₈]³⁻, (b) [V₁₂O₃₂]⁴⁻ and (c) [V₁₃O₃₄]³⁻. Yellow octahedra and orange square-pyramids represent {VO₆} and {VO₅} units, respectively.

Figure 1. Anion structures of (a) [H₃V₁₀O₂₈]³⁻, (b) [V₁₂O₃₂]⁴⁻ and (c) [V₁₃O₃₄]³⁻. Yellow octahedra and orange square-pyramids represent {VO₆} and {VO₅} units, respectively.

2. Results and Discussion

2.1. Structure Transformation among Deca-, Dodeca- and Tridecavanadates

The structure of decavanadates, [H_nV₁₀O₂₈]⁽⁶⁻ⁿ⁾⁻, consists of ten octahedral {VO₆} units stacked together in one molecule (Figure 1). While tridecavanadate [V₁₃O₃₄]³⁻ also has a similar structure with three additional {VO₆} octahedra in a triangular arrangement as add-on units on one of the surfaces of the decavanadate. Dodecavanadate [V₁₂O₃₂]⁴⁻ has a totally different structure which is based on twelve square-pyramidal {VO₅} units in a cage arrangement that has a cavity to incorporate a small molecule like acetonitrile at the center of the anion (Figure 1). These three complexes have distinct structures, yet the size of the anions are similar, and we tried to find the reaction conditions which can transform one molecule into another by adjusting the stoichiometry according to the molecular formula. The transformation process is monitored through ⁵¹V NMR since all chemical shifts for each of those complexes were already reported [17,19,30]. In the transformation process, the employment of decavanadates as a starting material is especially beneficial because they are one of the most widely available species and thoroughly investigated in water as well as acetonitrile [8,31].

2.1.1. Transformation between Deca- and Dodecavanadates

The synthesis of the dodecavanadates was achieved in two different routes, even if the precursors are different in oxidation states, formulas and total charges on the cluster. In the first report by Klemperer’s group, [V₁₂O₃₂]⁴⁻ was prepared by refluxing an acetonitrile solution of the decavanadate with only two protonation sites, {(n-C₄H₉)₄N}₄[H₂V₁₀O₂₈], for 1–2 min with >80% yield [10]. Later, by our group, [V₁₂O₃₂]⁴⁻ was prepared by the oxidation of another type of decavanadate {(n-C₄H₉)₄N}₄[V₁₀O₂₆] for 1 h at room temperature with 90% yield [9]. While {(n-C₄H₉)₄N}₄[H₂V₁₀O₂₈] consists ten {V⁵⁺O₆} units, {(n-C₄H₉)₄N}₄[V₁₀O₂₆] consists eight {V⁵⁺O₄} and two {V⁴⁺O₅} units. These differences show that the precursors are not important and provide us an opportunity to investigate the transformation of polyoxovanadates in accordance with the reaction equations. The triprotonated decavanadate [H₃V₁₀O₂₈]³⁻ was selected as a precursor for investigation since the solution state of [H₃V₁₀O₂₈]³⁻ in acetonitrile and water is well discussed before [17]. The conventional equation for the synthesis of {(n-C₄H₉)₄N}₄[V₁₂O₃₂] from {(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈] can be written by adjusting the stoichiometry as expressed in Equation (1).

6{(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈] + 2{(n-C₄H₉)₄N}OH ⇄ 5{(n-C₄H₉)₄N}₄[V₁₂O₃₂] + 10H₂O

(1)

By the reaction of {(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈] with 0.33 equivalents of {(n-C₄H₉)₄N}OH in acetonitrile at room temperature for 24 h, ⁵¹V NMR spectra showed signals due to [H₃V₁₀O₂₈]³⁻ and [H₂V₁₀O₂₈]⁴⁻, and [V₁₂O₃₂]⁴⁻ as a minor product (Figure A1). The transfer reaction was slow at room temperature, and to accelerate the reaction, heating the solution was necessary. After refluxing for 1 h, only signals due to [V₁₂O₃₂]⁴⁻ were observed with >99% chemoselectivity, and the isolated yield was 92% (Figure 2). IR spectrum of the isolated product show a perfect match with that of {(n-C₄H₉)₄N}₄[V₁₂O₃₂] (Figure 3).

In the presence of a stoichiometric amount of water, [V₁₂O₃₂]⁴⁻ was stable with retention of the structure. An excess amount of water leads to the transformation of [V₁₂O₃₂]⁴⁻ to decavanadates. This is consistent with the established distribution study in aqueous media that dodecavanadates are not involved in the complicated equilibrium of polyoxovanadates [5]. In a mixed solvent of acetonitrile and water (3:1, v/v), the mixture of [H₃V₁₀O₂₈]³⁻ and [H₂V₁₀O₂₈]⁴⁻ was obtained (Figure A2). By addition of 0.4 equivalents of p-toluenesulfonic acid (TsOH) with respect to [V₁₂O₃₂]⁴⁻, only [H₃V₁₀O₂₈]³⁻ was detected by ⁵¹V NMR and the isolated yield was 83% (Figure 2). IR spectrum of the isolated product shows convincing agreement with that of {(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈] (Figure 3).

Figure 2. ⁵¹V NMR spectra of (a) [H₃V₁₀O₂₈]³⁻, (d) [V₁₂O₃₂]⁴⁻ and (g) [V₁₃O₃₄]³⁻ crystals dissolved in acetonitrile and of the reaction solution after the transformations of (b) [V₁₂O₃₂]⁴⁻ to [H₃V₁₀O₂₈]³⁻, (c) [V₁₃O₃₄]³⁻ to [H₃V₁₀O₂₈]³⁻, (e) [H₃V₁₀O₂₈]³⁻ to [V₁₂O₃₂]⁴⁻, (f) [V₁₃O₃₄]³⁻ to [V₁₂O₃₂]⁴⁻ and (h) [H₃V₁₀O₂₈]³⁻ to [V₁₃O₃₄]³⁻.

Figure 2. ⁵¹V NMR spectra of (a) [H₃V₁₀O₂₈]³⁻, (d) [V₁₂O₃₂]⁴⁻ and (g) [V₁₃O₃₄]³⁻ crystals dissolved in acetonitrile and of the reaction solution after the transformations of (b) [V₁₂O₃₂]⁴⁻ to [H₃V₁₀O₂₈]³⁻, (c) [V₁₃O₃₄]³⁻ to [H₃V₁₀O₂₈]³⁻, (e) [H₃V₁₀O₂₈]³⁻ to [V₁₂O₃₂]⁴⁻, (f) [V₁₃O₃₄]³⁻ to [V₁₂O₃₂]⁴⁻ and (h) [H₃V₁₀O₂₈]³⁻ to [V₁₃O₃₄]³⁻.

Figure 3. IR spectra of (a) [H₃V₁₀O₂₈]³⁻, (d) [V₁₂O₃₂]⁴⁻ and (g) [V₁₃O₃₄]³⁻ crystals and of the powder obtained after the transformations of (b) [V₁₂O₃₂]⁴⁻ to [H₃V₁₀O₂₈]³⁻, (c) [V₁₃O₃₄]³⁻ to [H₃V₁₀O₂₈]³⁻, (e) [H₃V₁₀O₂₈]³⁻ to [V₁₂O₃₂]⁴⁻, (f) [V₁₃O₃₄]³⁻ to [V₁₂O₃₂]⁴⁻ and (h) [H₃V₁₀O₂₈]³⁻ to [V₁₃O₃₄]³⁻.

Figure 3. IR spectra of (a) [H₃V₁₀O₂₈]³⁻, (d) [V₁₂O₃₂]⁴⁻ and (g) [V₁₃O₃₄]³⁻ crystals and of the powder obtained after the transformations of (b) [V₁₂O₃₂]⁴⁻ to [H₃V₁₀O₂₈]³⁻, (c) [V₁₃O₃₄]³⁻ to [H₃V₁₀O₂₈]³⁻, (e) [H₃V₁₀O₂₈]³⁻ to [V₁₂O₃₂]⁴⁻, (f) [V₁₃O₃₄]³⁻ to [V₁₂O₃₂]⁴⁻ and (h) [H₃V₁₀O₂₈]³⁻ to [V₁₃O₃₄]³⁻.

2.1.2. Transformation between Deca- and Tridecavanadates

In the reported procedure, {(n-C₄H₉)₄N}₃[V₁₃O₃₄] was obtained by refluxing 2.6 M {(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈] acetonitrile solution for 7 h under dry nitrogen [19]. In our investigation under the same condition, [V₁₃O₃₄]³⁻ was formed as a minor product. The time dependent observation of ⁵¹V NMR spectra at 80 °C reveals that the formation of [V₁₂O₃₂]⁴⁻ is faster than that of [V₁₃O₃₄]³⁻ (Figure A3). Once [V₁₂O₃₂]⁴⁻ is formed, [V₁₃O₃₄]³⁻ is no longer obtained in acetonitrile (see below). Although the solubility of [V₁₃O₃₄]³⁻ is lower than [V₁₂O₃₂]⁴⁻ and [V₁₃O₃₄]³⁻ is selectively obtained by crystallization, the synthesis of [V₁₃O₃₄]³⁻ is difficult in our hands. Therefore, the improved synthesis of [V₁₃O₃₄]³⁻ is suggested in this paper. Since acetonitrile acts as a template to stabilize the host anion [V₁₂O₃₂]⁴⁻, the choice of the appropriate solvent is important for the rational synthesis of [V₁₃O₃₄]³⁻. {(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈] was dissolved in nitroethane and stirred for 3 h at 80 °C. Despite the indication that [V₁₃O₃₄]³⁻ is selectively formed by ⁵¹V NMR, the solution color turned to green during the reaction, suggesting that some of the vanadium atoms are reduced by hot nitroethane. The reduced byproducts are no longer detectable by ⁵¹V NMR. After the removal of [V₁₃O₃₄]³⁻ as crystals, addition of diethyl ether gave green precipitates. IR spectrum of this precipitate was in agreement with that of reported [V₁₈O₄₆(NO₃)]⁵⁻ (Figure A4) [32]. The transformation of {(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈] into {(n-C₄H₉)₄N}₃[V₁₃O₃₄] may be accomplished according to Equation (2).

13{(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈] ⇄ 10{(n-C₄H₉)₄N}₃[V₁₃O₃₄] + 9{(n-C₄H₉)₄N}OH + 15H₂O

(2)

The formation of a base according to Equation (2) prompted the reductive condition for those polyoxovanadates. In the presence of 0.69 equivalents of TsOH with respect to [H₃V₁₀O₂₈]³⁻ to neutralize {(n-C₄H₉)₄N}OH, the solution color remained brown. ⁵¹V NMR spectrum of the reaction solution showed pure signals due to [V₁₃O₃₄]³⁻ (Figure 2). IR spectrum of the brown precipitates formed by addition of an excess amount of diethyl ether into the reaction solution, was the same as that of [V₁₃O₃₄]³⁻ crystals, suggesting that the complete transformation of [H₃V₁₀O₂₈]³⁻ to [V₁₃O₃₄]³⁻ is achieved with 91% isolated yield (Figure 3). We find this reaction useful for further investigation of [V₁₃O₃₄]³⁻ chemistry.

By the reaction of [V₁₃O₃₄]³⁻ with 0.9 equivalents of {(n-C₄H₉)₄N}OH in a mixed solvent of acetonitrile and water (2:1, v/v), quantitative formation of [H₃V₁₀O₂₈]³⁻ was detected by ⁵¹V NMR and IR spectra (Figure 2 and Figure 3). The transformation of [V₁₃O₃₄]³⁻ to [H₃V₁₀O₂₈]³⁻ was ca. 10 times faster than that of [V₁₂O₃₂]⁴⁻ partly due to the structural resemblance between [H₃V₁₀O₂₈]³⁻ and [V₁₃O₃₄]³⁻. Both of the structures of [H₃V₁₀O₂₈]³⁻ and [V₁₃O₃₄]³⁻ consist of {VO₆} units sharing the edge of the octahedra. Only difference in the [V₁₃O₃₄]³⁻ structure from [H₃V₁₀O₂₈]³⁻ is the successive capping of the additional three {VO₆} octahedra in a triangular arrangement as add-on units on top of the same decavanadate framework.

2.1.3. Transformation between Dodeca- and Tridecavanadates

In addition to the transformation between {(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈] and {(n-C₄H₉)₄N}₄[V₁₂O₃₂] or {(n-C₄H₉)₄N}₃[V₁₃O₃₄], transformation between {(n-C₄H₉)₄N}₄[V₁₂O₃₂] and {(n-C₄H₉)₄N}₃[V₁₃O₃₄] was investigated based on the Equation (3).

13{(n-C₄H₉)₄N}₄[V₁₂O₃₂] + 8H₂O ⇄ 12{(n-C₄H₉)₄N}₃[V₁₃O₃₄] + 16{(n-C₄H₉)₄N}OH

(3)

In acetonitrile, [V₁₂O₃₂]⁴⁻ was not converted to [V₁₃O₃₄]³⁻ even by addition of water and TsOH with heating. The resemblance of stoichiometry between Equations (1) and (3) also make it difficult to control the selective conversion of [V₁₂O₃₂]⁴⁻ to [V₁₃O₃₄]³⁻. Decavanadates are thermodynamically stable in the presence of water and [V₁₂O₃₂]⁴⁻ is hardly transformed to other species in acetonitrile without water even when a stoichiometric amount of acid was added. To stimulate the formation of [V₁₃O₃₄]³⁻, we tried to use a different solvent from acetonitrile and nitroethane was successful in converting [V₁₂O₃₂]⁴⁻ into other forms. However, only the decomposition reaction was observed under the reaction condition with 1.2 equivalents of TsOH in nitroethane at 80 °C, and the formation of [V₁₃O₃₄]³⁻ could not be detected by ⁵¹V NMR.

Without any additives, [V₁₃O₃₄]³⁻ was stable even in the refluxing condition in acetonitrile. According to Equation (3), the addition of 1.3 equivalents of {(n-C₄H₉)₄N}OH to the solution of {(n-C₄H₉)₄N}₃[V₁₃O₃₄] was investigated. From ⁵¹V NMR spectrum, decavanadates and [V₁₂O₃₂]⁴⁻ were formed in 2 h at room temperature (Figure 4). The formation of decavanadates could be proceeded via Equation (2). Without heating, [V₁₂O₃₂]⁴⁻ was hardly obtained via Equation (1) for 2 h (Figure A1). Therefore, [V₁₃O₃₄]³⁻ was directly converted to [V₁₂O₃₂]⁴⁻ via Equation (3). To achieve high yield of [V₁₂O₃₂]⁴⁻, the reaction solution was refluxed and the formation of [V₁₂O₃₂]⁴⁻ was detected by ⁵¹V NMR with >95% chemoselectivity (Figure 2). The isolated yield was 86%. IR spectrum of the isolated product was identical to that of {(n-C₄H₉)₄N}₄[V₁₂O₃₂] (Figure 3).

Figure 4. ⁵¹V NMR spectra of the reaction solution of {(n-C₄H₉)₄N}₃[V₁₃O₃₄] with 1.3 equivalents of {(n-C₄H₉)₄N}OH after 2 h (a) at room temperature and (b) in reflux condition.

Figure 4. ⁵¹V NMR spectra of the reaction solution of {(n-C₄H₉)₄N}₃[V₁₃O₃₄] with 1.3 equivalents of {(n-C₄H₉)₄N}OH after 2 h (a) at room temperature and (b) in reflux condition.

2.2. Oxidation of Thioanisole

Next, to clarify the differences among [H₃V₁₀O₂₈]³⁻, [V₁₂O₃₂]⁴⁻ and [V₁₃O₃₄]³⁻ in the activity for oxidation, the thioanisole oxidation with t-butyl hydroperoxide (TBHP) was carried out (Table 1). The oxidation of sulfides to sulfoxides and sulfones has been a widely researched subject due to the importance of products as intermediates in organic synthesis [33]. This method is also useful for oxidative desulfurization of oil [34]. TBHP and H₂O₂ are usual oxidants for the oxidation of sulfides. It is reported that the redox properties of [H₃V₁₀O₂₈]³⁻, [V₁₂O₃₂]⁴⁻ and [V₁₃O₃₄]³⁻ are different from one another [19]. Although there are some reports on oxidation catalysis for several organic substrates with [H₃V₁₀O₂₈]³⁻ and [V₁₃O₃₄]³⁻, the comparison of the activity in these compounds is not investigated [19,35,36].

Table 1. Oxidation of thioanisole with vanadium species ^a.

Entry	Vanadium Species (μmol)	Total Yield (%)	Ratio of Sulfoxide:Sulfone
1	{(n-C4H9)4N}3[V13O34] (7.7)	91	93:7
2	{(n-C4H9)4N}4[V12O32] (8.4)	79	79:21
3	{(n-C4H9)4N}3[H3V10O28] (10)	33	94:6
4 b	V2O5 (20)	67	97:3
5	-	3	-

^a Reaction condition: thioanisole (1 mmol), TBHP (1 mmol), acetonitrile (2 mL), 25 °C, 2.5 h. Yields were determined by GC with an internal standard. Total yield (%) = {sulfoxide (mol) + sulfone (mol) × 2}/initial TBHP (mol) × 100; ^b V₂O₅ was suspended.

Table 1. Oxidation of thioanisole with vanadium species ^a.

Entry	Vanadium Species (μmol)	Total Yield (%)	Ratio of Sulfoxide:Sulfone
1	{(n-C4H9)4N}3[V13O34] (7.7)	91	93:7
2	{(n-C4H9)4N}4[V12O32] (8.4)	79	79:21
3	{(n-C4H9)4N}3[H3V10O28] (10)	33	94:6
4 b	V2O5 (20)	67	97:3
5	-	3	-

^a Reaction condition: thioanisole (1 mmol), TBHP (1 mmol), acetonitrile (2 mL), 25 °C, 2.5 h. Yields were determined by GC with an internal standard. Total yield (%) = {sulfoxide (mol) + sulfone (mol) × 2}/initial TBHP (mol) × 100; ^b V₂O₅ was suspended.

Each polyoxovanadate retains their anion structures in acetonitrile at 25 °C. In the presence of 20 equivalents of TBHP with respect to polyoxovanadates, their anion structures were maintained (Figure A5). In the presence of [V₁₃O₃₄]³⁻, 91% total yield of methyl phenyl sulfoxide and methyl phenyl sulfone was achieved in 2.5 h (Table 1). The ratio of sulfoxide and sulfone was 93:7. Under the same reaction conditions, oxidation reaction hardly proceeded in the absence of vanadium species. [H₃V₁₀O₂₈]³⁻ showed a lower activity for this reaction (33% yield in 2.5 h). The reactivity of [V₁₂O₃₂]⁴⁻ was different from that of [V₁₃O₃₄]³⁻. The yield with [V₁₂O₃₂]⁴⁻ in 0.5 h (59%) was higher than that with [V₁₃O₃₄]³⁻ (25%), and the reaction ended in 90 min with ca. 80% yield. Successive oxidation of sulfoxide to sulfone proceeded from the initial stage of the reaction in the presence of [V₁₂O₃₂]⁴⁻. While the [V₁₂O₃₂]⁴⁻ framework is closely related to layered V₂O₅, in which vanadium atoms adopt the square-pyramidal coordination geometry, [V₁₂O₃₂]⁴⁻ showed the higher activity for the formation of sulfone than V₂O₅. The reactivity depends on the electrophilicity of the active oxidant [37].

After the reaction, the precipitates of polyoxovanadates were formed by addition of a large amount of ethyl acetate and the powder was collected by filtration. From IR and ⁵¹V NMR spectra, [H₃V₁₀O₂₈]³⁻ and [V₁₂O₃₂]⁴⁻ retained their structures and [V₁₃O₃₄]³⁻ was partly decomposed to decavanadates (Figure A6 and Figure A7). Since decavanadates are less active for the oxidation of thioanisole, [V₁₃O₃₄]³⁻ is contributed to the high activity.

3. Experimental Section

3.1. Chemicals and Instruments

All solvents were purchased from Wako Pure Chemical Industries (Osaka, Japan) and used as received. p-Toluenesulfonic acid monohydrate (TsOH) and 40% tetra-n-butylammonium hydroxide aqueous solution were purchased from Wako Pure Chemical Industries and Sigma-Aldrich (Tokyo, Japan) and used after the dilution to 0.1 M solution with acetone (for the transformations to [V₁₃O₃₄]³⁻) or acetonitrile (for the other transformations). Thioanisole, naphthalene, methyl phenyl sulfoxide, methyl phenyl sulfone and 5.5 M t-butyl hydroxide in decane were obtained from Wako Pure Chemical Industries, Tokyo Chemical Industry (Tokyo, Japan) and Sigma-Aldrich and used as received. {(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈] was synthesized according to the reported procedure [17].

IR spectra were measured on Jasco FT/IR-4200 (Hachioji, Japan) using KBr disks. NMR spectra were recorded with JEOL JNM-LA400 (Akishima, Japan). ⁵¹V NMR spectra were measured at 105.15 MHz at 25 °C unless otherwise noted. The chemical shift reference standard for ⁵¹V NMR spectroscopy is VOCl₃. Elemental analyses of C, H and N were performed by the Research Institute for Instrumental Analysis at Kanazawa University (Kanazawa, Japan). GC analyses were performed on Shimadzu GC-2014 (Kyoto, Japan) with a flame ionization detector (FID) equipped with a NEUTRABOND-1 capillary column (internal diameter = 0.25 mm, length = 30 m).

3.2. Transformation of {(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈] to {(n-C₄H₉)₄N}₄[V₁₂O₃₂]

{(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈] (100 mg, 0.06 mmol) was dissolved in acetonitrile (3 mL), followed by addition of 0.1 M {(n-C₄H₉)₄N}OH (0.2 mL, 0.02 mmol) and refluxed for 1 h. After cooling, the solution was added to diethyl ether (40 mL) and stirred for 10 min. Then, the precipitates formed were collected and dried to afford 98 mg of {(n-C₄H₉)₄N}₄[V₁₂O₃₂] (92% yield based on vanadium atoms). ⁵¹V NMR of the reaction solution: δ −590, −597, −605 ppm. Anal. Calcd. for {(n-C₄H₉)₄N}₄[V₁₂O₃₂]: C,36.72; H, 6.93; N, 2.68; found: C, 35.33; H, 6.62; N, 2.67. IR (KBr pellet; 1500–400 cm⁻¹): 1483, 1379, 994, 860, 792, 761, 711, 649, 609, 549, 519 cm⁻¹.

3.3. Transformation of {(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈] to {(n-C₄H₉)₄N}₃[V₁₃O₃₄]

{(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈] (100 mg, 0.06 mmol) was dissolved in nitroethane (3 mL), followed by addition of 0.1 M TsOH (0.4 mL, 0.04 mmol) and stirred for 3 h at 80 °C. After cooling, the solution was added to diethyl ether (40 mL) and stirred for 10 min. Then, the precipitates formed were collected and dried to afford 81 mg of {(n-C₄H₉)₄N}₃[V₁₃O₃₄] (91% yield based on vanadium atom). ⁵¹V NMR of the reaction solution: δ −331, −451, −496, −500 ppm. Anal. Calcd. for {(n-C₄H₉)₄N}₃[V₁₃O₃₄]: C, 29.82; H, 5.63; N, 2.17; found: C, 29.64; H, 5.59; N, 2.34. IR (KBr pellet; 1500–400 cm⁻¹): 1482, 1379, 1001, 991, 859, 816, 786, 605, 523, 457 cm⁻¹.

3.4. Transformation of {(n-C₄H₉)₄N}₄[V₁₂O₃₂] to {(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈]

{(n-C₄H₉)₄N}₄[V₁₂O₃₂] (106 mg, 0.05 mmol) was dissolved in a mixed solvent of acetonitrile and water (4 mL, 3:1, v/v), followed by addition of 0.1 M TsOH (0.2 mL, 0.02 mmol) and stirred for 10 h at 60 °C. After cooling, the solution was added to the mixed solution of diethyl ether and tetrahydrofuran (45 mL, 8:1, v/v) and stirred for 10 min. Then, the precipitates formed were collected and dried to afford 84 mg of {(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈] (83% yield based on vanadium atom). ⁵¹V NMR of the reaction solution: δ −397, −431, −506, −524 ppm. IR (KBr pellet; 1500–400 cm⁻¹): 1482, 1379, 987, 972, 876, 843, 808, 771, 721, 609, 550, 506, 461 cm⁻¹.

3.5. Transformation of {(n-C₄H₉)₄N}₃[V₁₃O₃₄] to {(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈]

{(n-C₄H₉)₄N}₃[V₁₃O₃₄] (19 mg, 0.01 mmol) was dissolved in a mixed solvent of acetonitrile and water (3 mL, 2:1, v/v), followed by addition of 0.1 M {(n-C₄H₉)₄N}OH (0.09 mL, 0.009 mmol) and stirred for 1 h at 60 °C. After cooling, the solution was added to the mixed solution of diethyl ether and tetrahydrofuran (45 mL, 8:1, v/v) and stirred for 10 min. Then, the precipitates formed were collected and dried to afford 19 mg of {(n-C₄H₉)₄N}₃[H₃V₁₀O₂₈] (86% yield based on vanadium atom). ⁵¹V NMR of the reaction solution: δ −397, −430, −506, −525 ppm. IR (KBr pellet; 1500–400 cm⁻¹): 1482, 1379, 987, 973, 875, 843, 809, 770, 719, 609, 550, 507, 461 cm⁻¹.

3.6. Transformation of {(n-C₄H₉)₄N}₃[V₁₃O₃₄] to {(n-C₄H₉)₄N}₄[V₁₂O₃₂]

{(n-C₄H₉)₄N}₃[V₁₃O₃₄] (39 mg, 0.02 mmol) was dissolved in acetonitrile (3 mL), followed by addition of 0.1 M {(n-C₄H₉)₄N}OH (0.26 mL, 0.026 mmol) and refluxed for 1 h. After cooling, the solution was added to diethyl ether (40 mL) and stirred for 10 min. Then, the precipitates formed were collected and dried to afford 40 mg of {(n-C₄H₉)₄N}₄[V₁₂O₃₂] (86% yield based on vanadium atom). ⁵¹V NMR of the reaction solution: δ − 590, −597, −605 ppm. IR (KBr pellet; 1500–400 cm⁻¹): 1483, 1380, 993, 860, 789, 762, 711, 649, 605, 553, 519 cm⁻¹.

3.7. Oxidation of Thioanisole

The oxidations of thioanisole were carried out in an 18 mL glass tube reactor containing a magnetic stir bar. A procedure was as follows: Vanadium species, thioanisole, naphthalene as internal standard, acetonitrile and TBHP were successively placed into the glass tube reactor. The reaction mixture was stirred at 25 °C. The yields of the products were periodically determined by GC analysis using an internal standard technique. All products are known and confirmed by comparison of their GC retention times with the authentic samples.

4. Conclusions

The quantitative conversions among deca-, dodeca- and tridecavanadates were established by monitoring through ⁵¹V NMR to determine the optimized transformation conditions. The inorganic host cage complex [V₁₂O₃₂]⁴⁻ is converted from [H₃V₁₀O₂₈]³⁻ by adjusting the amount of base. Decavanadate is self-condensed into [V₁₃O₃₄]³⁻ in nitroethane by utilizing proton on the decavanadate for the dehydration condensation and the further addition of the controlled amount of acid helps to prevent the byproduct formation. The quantitative formation of [V₁₃O₃₄]³⁻ was achieved with one of the most available [H₃V₁₀O₂₈]³⁻ as a starting material. Transformation of [V₁₂O₃₂]⁴⁻ or [V₁₃O₃₄]³⁻ to decavanadates proceeded by the hydrolysis reaction. While the direct transformation of [V₁₃O₃₄]³⁻ to [V₁₂O₃₂]⁴⁻ proceeded by addition of base, conversion of [V₁₂O₃₂]⁴⁻ to [V₁₃O₃₄]³⁻ could not be observed during our investigation. These transformation reactions proceeded according to the equations and provide the reliable synthetic pathway for the synthesis of [V₁₃O₃₄]³⁻. The studies shows that a careful control of the amount of acid or base in appropriate solvents is able to achieve the transformation among those isopolyoxovanadates, which is fundamentally important species in polyoxovanadate chemistry.

We also demonstrated that [V₁₃O₃₄]³⁻ shows the highest activity for the oxidation of thioanisole among these polyoxovanadates.