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

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.

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

Structure Transformation among Deca-, Dodeca-and Tridecavanadates
The structure of decavanadates, [HnV10O28] (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 51 V 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].

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 {V 5+ O6} units, {(n-C4H9)4N}4[V10O26] consists eight {V 5+ O4} and two {V 4+ 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].
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 51 V 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).

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 51 V 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 51 V 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 51 V 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 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.

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 51 V NMR.

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].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 51 V NMR spectra, [H3V10O28] 3− and [V12O32] 4− retained their structures and [V13O34] 3− was partly decomposed to decavanadates (Figures A6 and A7).Since decavanadates are less active for the oxidation of thioanisole, [V13O34] 3− is contributed to the high activity.

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

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.

Conclusions
The quantitative conversions among deca-, dodeca-and tridecavanadates were established by monitoring through 51 V 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.

Table 1 .
Oxidation of thioanisole with vanadium species a .