Regioselective Bond-Forming and Hydrolysis Reactions of Doubly Charged Vanadium Oxide Anions in the Gas Phase

The gas-phase reactivity of vanadium-containing dianions, NaV3O9 and its hydrated form H2NaV3O10, were probed towards sulphur dioxide at room temperature by ion-molecule reaction (IMR) experiments in the collision cell of an ion trap mass spectrometer. The sequential addition of two SO2 molecules to the NaV3O9 dianion leads to the breakage of the stable V3O9 backbone, resulting in a charge separation process with the formation of new V-O and S-O bonds. On the contrary, the H2NaV3O10 hydroxide species reacts with SO2, promoting regioselective hydrolysis and bond-forming processes, the latter similar to that observed for the NaV3O9 reactant anion. Kinetic analysis shows that these reactions are fast and efficient with rate constants of the 10−9 (±30) cm3 s−1 molecule−1 order of magnitude.


Introduction
Vanadium-based compounds find several applications in a wide range of research fields. For example, supported vanadium oxides are largely employed in the metallurgic industry as heterogeneous catalysts for the manufacture of important chemicals [1]. Among these, sulphuric acid is considered an excellent indicator of the "industrial strength" of a nation, being one of the most produced chemicals worldwide [2]. More recently, vanadium hydroxides have also attracted interest as secondary electrode materials in the design of Li-ion and redox flow batteries [3][4][5][6].
Considering the significant potential of vanadium species, the chemical features of these compounds have been extensively investigated both in the solution and in the gasphase environments. The latter represents a designable arena in which a variety of different experiments performed by means of mass spectrometric techniques allow one to elucidate structural and thermochemical features of ionic clusters. The refined control of the cluster size, charge, and stoichiometry in the absence of perturbing effects can indeed contribute to assessing the elementary steps of a complex process at a strictly molecular level, obtaining detailed mechanistic information [7][8][9][10][11][12][13]. Accordingly, the reactivity of vanadium oxide mono-cations and anions was probed towards selected hydrocarbons, water, and SO 2 by ion-molecule reaction (IMR) experiments [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. In particular, it has been reported that the vanadium-oxide kernel of the V 4 O 10 − anion can effectively incorporate SO 2 , leading to a [V 4 O 10 -SO 2 ] − complex with a square pyramidal structure [29], whereas neutral vanadium oxides only give association intermediates or oxidation and reduction products by the reaction with SO 2 [32,33].
Although many studies in the literature describe the gas-phase reactivity of monocharged vanadium oxide ions, the chemistry of the corresponding multi-charged species has been only partially explored. These ions can be generated by electrospray ionization (ESI) processes by the direct transfer of charged species formed in solution under mild Reactions 2022, 3 255 conditions into the gas-phase environment [34]. In particular, we refer both to small doublycharged species (e.g., VO 2+ , VOH 2+ , VOH 2+ ) stabilized as water adducts [35] and large multiply-charged polyoxovanadates in which vanadium can assume different oxidation states and a variety of intriguing geometries [36].
The typical gas-phase reactions of dications are the so-called bond-forming processes where the simultaneous breakage and formation of chemical bonds lead to two singlycharged ions and/or a new doubly-charged species [37][38][39][40][41][42][43][44]. On the contrary, dianions are commonly involved in proton-transfer and substitution/elimination reactions always resulting in the incorporation of the intact dianion in the final product scaffold [45][46][47]. Recently, we reported the unprecedented bond-forming reactivity of doubly-charged vanadium hydroxoanions in the gas phase studied by IMR experiments and theoretical calculations [48,49] formula with SO 2 in favoring a hydrolysis reaction, giving rise to the charge separation of the dianion along with the formation of the HOSO 2 − bisulfite anion [49]. Continuing with our studies focused on the gas-phase activation of SO 2 [50][51][52][53][54][55][56], here we report on the reactivity of two polyoxovanadate dianions, namely NaV 3 O 9 2− and H 2 NaV 3 O 10 2− , towards SO 2 by using IMR mass spectrometry. The reactions are very fast and efficient and lead to singly charged products that do not contain the reactants in the molecular scaffold. Cooperative effects and regioselectivity were demonstrated when addressing different reaction outcomes, providing new interesting examples of bond-forming and hydrolysis reactions in the gas phase.

Materials
The research-grade chemicals and solvents employed in this study were purchased from Sigma-Aldrich and used without further purification. The stated purities of the chemicals, as reported by the vendor, are as follows: NaVO 3 (99.9%), SO 2 (99.9%), whereas H 2 O and CH 3 CN solvents are at the HPLC grade.

Mass Spectrometry
All the experiments were carried out on an LTQ XL linear ion trap mass spectrometer (Thermo Fisher Scientific, Bremen, Germany) that was in-house modified to perform ion-molecule reactions and study the kinetic profiles of gas-phase processes [57]. The instrument was equipped with an electrospray ionization (ESI) source operating in negative ion mode, under the following experimental conditions: source voltage of 2-3 kV and capillary temperature of 275 • C. Nitrogen (N 2 ) was used as sheath and auxiliary gas at a flow rate of 3.7 and 0.74 L min −1 , respectively [58]. Capillary and tube lens voltages were set to low values (in the range of −1 and −10 V) to maximize the formation of multiply-charged ions at the expense of singly-charged species [48,49].
Millimolar solutions of NaVO 3 in H 2 O/CH 3 CN (1/3 v/v) were infused into the ESI source via the on-board syringe pump at a flow rate of 5 µL min −1 , whereas neutral sulphur dioxide (SO 2 ) was separately introduced into the ion trap through a deactivated fused-silica capillary that entered the vacuum chamber from a 6.25 mm hole placed in the backside of the mass spectrometer. The pressure of the neutral gas was kept constant by a metering valve and measured by a Granville−Phillips Series 370 Stabil Ion Vacuum Gauge.
Owing to the position of the Pirani gauge, the actual SO 2 pressure was estimated based on a calibration of the pressure reading [59]. Typical pressures of SO 2 ranged between 1.1 × 10 −7 and 7.6 × 10 −7 Torr.
The NaV 3 O 9 2− and H 2 NaV 3 O 10 2− reactant ions were generated in the ESI source and thermalized by collisions with background helium during the transfer and inside the trap (collision frequency ≈ 10 5 ·s −1 ). After 0.1-0.2 s, they were mass-to-charge isolated and reacted with SO 2 . The signals of the ionic reactant and products were monitored over time as a function of the SO 2 concentration, and for each reaction time an average of 10 scan acquisitions were acquired. The normalized collision energy was set to zero, and the activation Q value was optimized to ensure stable trapping fields for all the ions under investigation.
To measure the rate constants, logarithmic plots of the reactant species concentration vs. time were constructed. Accordingly, all the studied reactions display a pseudo-first-order decay. Experimental data obtained from the kinetic analysis were fitted to a mathematical model built on the basis of the hypothesized reaction mechanism by using Dynafit 4 package [60]. Bimolecular rate constants k (cm 3 molecule −1 s −1 ) were derived as a product of the pseudo-first-order constants (s −1 ) divided by the concentration of neutral reagent gas. To ensure the accuracy of the k values, ca. 10-15 independent measurements for each precursor ion were performed on different days. All rates were measured over a 7-fold SO 2 pressure range, showing a linear correlation with the neutral density and a standard deviation <15%. Nevertheless, a conservative error of 30% was given, due to the uncertainties of the neutral pressure measurements. The reaction efficiencies were expressed as the ratio of the bimolecular rate constants k to the collision rates that were calculated according to the average dipole orientation (ADO) theory [61].
Collision-induced dissociation (CID) experiments were carried out with the MS n function of the LTQ XL mass spectrometer. Helium was used as the collision gas at a pressure of ca. 3 × 10 −3 Torr. Typical normalized collision energies (NCE) were in the range of 10% and 50%, depending on the isolated ion, whereas the activation time and Q value were set to 30 ms and 0.250, respectively.
All spectra were recorded and elaborated using the Xcalibur software (version 2.0.6, Thermo Fisher Scientific, Bremen, Germany) supplied with the instrument.

The NaV 3 O 9 2− and H 2 NaV 3 O 10 2− Reactant Dianions
Trimeric species of NaV 3 O 9 2− and H 2 NaV 3 O 10 2− general formula were generated in the gas phase by electrospraying aqueous solutions of sodium metavanadate. The speciation of this salt was deeply investigated both in solution [62,63] and in the gas phase [64][65][66][67], highlighting the formation of discrete multi-charged anions containing several vanadium atoms organized in higher structures (e.g., dimer, trimer, tetramer).
Before introducing SO 2 into the trap, both NaV 3 O 9 2− and H 2 NaV 3 O 10 2− reactant ions were mass-to-charge selected and exposed to helium buffer gas over long activation times (maximum a. t. = 10 s). Since no remarkable signal loss occurred as a result of the isolation and subsequent accumulation, these doubly-charged species can be considered stable towards spontaneous dissociation in the gas phase. Hence, CID experiments were performed to obtain salient structural information that was interpreted in the light of our previous calculations on similar dianionic species [48,49]. In particular, the NaV 3 O 9 2− dianion can be associated with the analogous HV 3 O 9 2− hydroxovanadate species characterized by a V 2 O 6 four-membered ring connected to a VO 3 moiety [49]. Indeed, pentavalent vanadium derivatives form polyoxovanadates showing repetitive units of VO 3 revealed at m/z 99 by mass spectrometry [67]. Accordingly, the NaV 3 O 9 2− parent ion at m/z 160 dissociates into two singly-charged fragments at m/z 221 and 99 that are respectively consistent with the anions of the general formula: NaV 2 O 6 − and VO 3 − (Figure 1). We can thus suppose that the VO3 moiety is released as a consequence of a V-O cleavage induced by the collision of the parent species with the background helium gas forming a NaV2O6 − counterpart. NaV2O6 − maintains a V2O6-closed structure in agreement with the CID mass spectrum of the similar HV3O9 2− species, which dissociates into VO3 − and HV2O6 − , as theoretically described elsewhere [49].
Passing to the H2NaV3O10 2− ion at m/z 169, it dissociates either by breaking into two singly-charged fragments, respectively corresponding to H2VO4 − (m/z = 117) and NaV2O6 − (m/z = 221) anions, or by losing an H2O molecule with the formation of the doubly-charged daughter species at m/z 160 (Figure 2a).
The latter was, in turn, isolated through the sequence 169 → 160 and submitted to MS 3 dissociation, giving rise to the same fragmentation pattern already observed for the NaV3O9 2− dianion (Figure 2b vs. Figure 1), thus pointing to the same structure. The doubly-charged species at m/z 169 could be thus composed of a mixture of H2NaV3O10 2− and the isomeric hydrated electrostatic complex ion [NaV3O9·H2O] 2− . However, the loss of a water molecule forming a daughter ion with a V2O6 2− four-membered ring scaffold was also observed for the covalent open-chain pirovanadate dianion, H2V2O7 2− . The theoretical structure predicted for this species is characterized by two vicinal OH groups that are involved in the H2O release resulting from a fast proton transfer reaction [48]. In addition, when exposed to H2O in the trap, a naked NaV3O9 2− dianion proved, even for the longest possible time, to be unreactive towards the addition of a water molecule. We can thus suppose that the VO 3 moiety is released as a consequence of a V-O cleavage induced by the collision of the parent species with the background helium gas forming a NaV 2 O 6 − counterpart. NaV 2 O 6 − maintains a V 2 O 6 -closed structure in agreement with the CID mass spectrum of the similar HV 3 O 9 2− species, which dissociates into VO 3 − and HV 2 O 6 − , as theoretically described elsewhere [49]. Passing to the H 2 NaV 3 O 10 2− ion at m/z 169, it dissociates either by breaking into two singly-charged fragments, respectively corresponding to H 2 VO 4 − (m/z = 117) and  We can thus suppose that the VO3 moiety is released as a consequence of a V-O cleavage induced by the collision of the parent species with the background helium gas forming a NaV2O6 − counterpart. NaV2O6 − maintains a V2O6-closed structure in agreement with the CID mass spectrum of the similar HV3O9 2− species, which dissociates into VO3 − and HV2O6 − , as theoretically described elsewhere [49].
Passing to the H2NaV3O10 2− ion at m/z 169, it dissociates either by breaking into two singly-charged fragments, respectively corresponding to H2VO4 − (m/z = 117) and NaV2O6 − (m/z = 221) anions, or by losing an H2O molecule with the formation of the doubly-charged daughter species at m/z 160 (Figure 2a).
The latter was, in turn, isolated through the sequence 169 → 160 and submitted to MS 3 dissociation, giving rise to the same fragmentation pattern already observed for the NaV3O9 2− dianion (Figure 2b vs. Figure 1), thus pointing to the same structure. The doubly-charged species at m/z 169 could be thus composed of a mixture of H2NaV3O10 2− and the isomeric hydrated electrostatic complex ion [NaV3O9·H2O] 2− . However, the loss of a water molecule forming a daughter ion with a V2O6 2− four-membered ring scaffold was also observed for the covalent open-chain pirovanadate dianion, H2V2O7 2− . The theoretical structure predicted for this species is characterized by two vicinal OH groups that are involved in the H2O release resulting from a fast proton transfer reaction [48]. In addition, when exposed to H2O in the trap, a naked NaV3O9 2− dianion proved, even for the longest possible time, to be unreactive towards the addition of a water molecule. The latter was, in turn, isolated through the sequence 169 → 160 and submitted to MS 3 dissociation, giving rise to the same fragmentation pattern already observed for the NaV 3 O 9 2− dianion (Figure 2b vs. Figure 1), thus pointing to the same structure. The doubly-charged species at m/z 169 could be thus composed of a mixture of H 2 NaV 3 O 10 2− and the isomeric hydrated electrostatic complex ion [NaV 3 O 9 ·H 2 O] 2− . However, the loss of a water molecule forming a daughter ion with a V 2 O 6 2− fourmembered ring scaffold was also observed for the covalent open-chain pirovanadate dianion, H 2 V 2 O 7 2− . The theoretical structure predicted for this species is characterized by two vicinal OH groups that are involved in the H 2 O release resulting from a fast proton transfer reaction [48]. In addition, when exposed to H 2 O in the trap, a naked NaV 3 O 9 2− dianion proved, even for the longest possible time, to be unreactive towards the addition of a water molecule.
In light of this experimental evidence and always keeping a +5 oxidation state for the vanadium atoms, we propose that the H2NaV3O10 2− parent ion at m/z 169, presumably formed in solution by a hydrolysis reaction at the expense of the NaV3O92− dianion, may be characterized by an open structure, showing an H2VO4 terminal moiety which is released as the mono-anion of the vanadic acid (H2VO 4− ) through an intracluster rearrangement, according to Scheme 1. The singly charged vanadate, H2VO 4− , is indeed a stable and deeply characterized species, previously obtained as a product of an ion-molecule reaction between the H2V2O7 2− dianions and SO 2 [48].
In light of this experimental evidence and always keeping a +5 oxidation state for the vanadium atoms, we propose that the H2NaV3O10 2− parent ion at m/z 169, presumably formed in solution by a hydrolysis reaction at the expense of the NaV3O9 2− dianion, may be characterized by an open structure, showing an H2VO4 terminal moiety which is released as the mono-anion of the vanadic acid (H2VO4 − ) through an intracluster rearrangement, according to Scheme 1. The singly charged vanadate, H2VO4 − , is indeed a stable and deeply characterized species, previously obtained as a product of an ion-molecule reaction between the H2V2O7 2− dianions and SO2 [48]. Scheme 1. CID mechanism of the H2NaV3O10 2− reactant ion at m/z 169.
The reaction sequence was confirmed by the kinetic analysis reported in Figure 3 showing both the reactivity of the NaV3O9 2− parent ion (Figure 3a) and that of its ligated product, [NaV3O9·SO2] 2− , obtained by the direct addition of the first SO2 molecule ( Figure  3b).

Reactivity of NaV 3 O 9 2− and H 2 NaV 3 O 10 2− Dianions towards SO 2
In the presence of sulphur dioxide in the ion trap, a thermal NaV 3 O 9 2− dianion binds an SO 2 molecule, giving the ligated [NaV 3 O 9 ·SO 2 ] 2− addition product (Equation (1)). It in turn reacts with a second SO 2 , forming two singly-charged product ions according to Equation (2).
The reaction sequence was confirmed by the kinetic analysis reported in Figure 3 showing both the reactivity of the NaV 3 O 9 2− parent ion ( Figure 3a) and that of its ligated product, [NaV 3 O 9 ·SO 2 ] 2− , obtained by the direct addition of the first SO 2 molecule (Figure 3b).
In light of this experimental evidence and always keeping a +5 oxidation state for the vanadium atoms, we propose that the H2NaV3O10 2− parent ion at m/z 169, presumably formed in solution by a hydrolysis reaction at the expense of the NaV3O9 2− dianion, may be characterized by an open structure, showing an H2VO4 terminal moiety which is released as the mono-anion of the vanadic acid (H2VO4 − ) through an intracluster rearrangement, according to Scheme 1. The singly charged vanadate, H2VO4 − , is indeed a stable and deeply characterized species, previously obtained as a product of an ion-molecule reaction between the H2V2O7 2− dianions and SO2 [48]. Scheme 1. CID mechanism of the H2NaV3O10 2− reactant ion at m/z 169.
The reaction sequence was confirmed by the kinetic analysis reported in Figure 3 showing both the reactivity of the NaV3O9 2− parent ion (Figure 3a) and that of its ligated product, [NaV3O9·SO2] 2− , obtained by the direct addition of the first SO2 molecule ( Figure  3b).  The rate constants of the two reactions at 298 K, as measured from the kinetic plots, amount to k 1 = 1.40 × 10 −9 and k 2 = 0.39 × 10 −9 (±30%) cm 3 s −1 molec −1 , whereas the reactions' efficiencies are estimated to be slightly > 100% and 34.5%, respectively.
A possible reaction mechanism can be postulated based on the borderline acid nature of the SO 2 neutral reagent and on the heteronuclear four-membered stable structure pre-dicted for the VO 3 SO 2 − product ion [48]. According to Scheme 2, in fact, after the addition of an SO 2 molecule, the attack of the second SO 2 to the opposite side of the dianionic species triggers the breaking of a V-O bond containing a bridged oxygen atom and the formation of new V-O and S-O bonds, eventually leading to two singly charged products, [NaV 2 O 6 ·SO 2 ] − and VO 3 SO 2 − . Hence, two SO 2 molecules are needed to activate the bond-forming reaction, one of which only plays a spectator role, remaining non-covalently attached to the NaV 2  As shown in the kinetic plot of Figure 4, the whole process is fast and efficient, showing a k 3 = 1.15 × 10 −9 (±30%) cm 3 s −1 molec −1 and an efficiency of 100%. The OH − transfer (Equation (3a)) predominates over the V-O cleavage, as demonstrated by the branching ratio between the two reaction channels (Equation (3a) vs. Equation (3b)), amounting to 3.7. The high rate constant of reaction 3.1, very close to that observed for a similar system [48], seems to confirm our previous experimental evidence of a hydroxide structure for the H 2 NaV 3 O 10 2− dianion, rather than a hydrated one. Hydrolysis reactions, in fact, occur at a much slower rate [49].
Interestingly, reaction 3 is a regioselective process, the outcome of which depends on where the SO 2   Interestingly, reaction 3 is a regioselective process, the outcome of which depends on where the SO2 molecule approaches the H2NaV3O10 2− dianion (Scheme 3). If sulphur dioxide coordinates the O-H bond, then pathway (a) of Scheme 3 takes place (Equation (3a)). Hence, the H2NaV3O10 2− parent ion behaves as an effective HOdonor, forming the bisulfite ionic species HOSO2 -and a second product, HNaV3O9 -, reasonably consisting of a V2O6 four-membered ring connected to a VO3 moiety, as in the case of the similar H2V3O9dianion obtained by the reaction of the HV3O9 2-species with SO2 and H2O [49].
Conversely, when the SO2 molecule attacks the H2NaV3O10 2− dianion on the opposite side, or at least far from the -OH groups, we observe the bond-forming reaction illustrated in Scheme 3b (Equation (3b)). Accordingly, the stable V3O10 kernel is broken with the formation of two singly charged products, the H2VO4 -vanadate and the [NaV2O6·SO2] -anions, the latter being also formed in Equation (2) (Scheme 2).

Scheme 3.
Plausible reaction mechanism of H2NaV3O10 2− ions with SO2 based on the corresponding kinetic plot.
The asymmetric structure of the H2NaV3O10 2− reactant ion may reasonably explain the occurrence of the regioselective process, as the SO2 molecule can approach the dianion from two chemically different extremities. Similar  Conversely, when the SO 2 molecule attacks the H 2 NaV 3 O 10 2− dianion on the opposite side, or at least far from the -OH groups, we observe the bond-forming reaction illustrated in Scheme 3b (Equation (3b)). Accordingly, the stable V 3 O 10 kernel is broken with the formation of two singly charged products, the H 2 VO 4 − vanadate and the [NaV 2 O 6 ·SO 2 ] − anions, the latter being also formed in Equation (2) (Scheme 2).
The asymmetric structure of the H 2 NaV 3 O 10 2− reactant ion may reasonably explain the occurrence of the regioselective process, as the SO 2 molecule can approach the dianion from two chemically different extremities. Similar reactions have already been observed in the gas phase, where a plethora of different factors (e.g., thermodynamic, kinetic, electronic, steric, and orbital) can act by affecting the branching ratios of alternative reaction products [68][69][70]. As to the reactivity of the OH terminal of H 2 NaV 3 O 10 2− , it is worth recalling that hydroxylation of acidic sites of vanadium-oxide-based materials, due to the reaction of the ubiquitous water, is known to change the electronic properties and the structure of the catalytic active sites of such materials [71].
Moreover, the open H 2 NaV 3 O 10 2− dianion is substantially less constrained compared to the other NaV 3 O 9 2− reactant species characterized by a V 2 O 6 four-membered ring. Hence, when SO 2 coordinates the H 2 NaV 3 O 10 2− dianion from the VO 3 side, a free terminal O − can play a nucleophilic attack on the vicinal vanadium atom, driving the formation of the two mono-charged products (Scheme 3, pathway (b)).

Conclusions
Fast and efficient reactions between polyvanadate dianions and sulphur dioxide have been reported in this study. Two SO 2 molecules, once sequentially added to the doubly charged vanadium oxide anion, NaV 3 O 9 2− , lead to the breakage of the stable V 3 O 9 kernel. Otherwise, the hydroxide H 2 NaV 3 O 10 2− dianion reacts with sulphur dioxide in a regioselective process by following two different pathways according to which an SO 2 molecule can promote either the breakage of a terminal V-OH bond or the V 3 O 10 kernel.
All these reactions are associated with charge separation processes that result in two singly charged product ions by the formation of new V-O and S-O bonds. The charge separation occurs exclusively as a result of the chemical reaction with sulfur dioxide, in which the dipole moment plays a crucial role in promoting the ion-neutral bonding.

Data Availability Statement:
The data presented in this study are available on request from the corresponding authors.