Proton Affinity in the Chemistry of Beta-Octamolybdate: HPLC-ICP-AES, NMR and Structural Studies

The affinity of [β-Mo8O26]4− toward different proton sources has been studied in various conditions. The proposed sites for proton coordination were highlighted with single crystal X-ray diffraction (SCXRD) analysis of (Bu4N)3[β-{Ag(py-NH2)Mo8O26]}] (1) and from analysis of reported structures. Structural rearrangement of [β-Mo8O26]4− as a direct response to protonation was studied in solution with 95Mo NMR and HPLC-ICP-AES techniques. A new type of proton transfer reaction between (Bu4N)4[β-Mo8O26] and (Bu4N)4H2[V10O28] in DMSO results in both polyoxometalates transformation into [V2Mo4O19]4−, which was confirmed by the 95Mo, 51V NMR and HPLC-ICP-AES techniques. The same type of reaction with [H4SiW12O40] in DMSO leads to metal redistribution with formation of [W2Mo4O19]2−.

In polyoxometalate (POM) chemistry, protonation affects the formation, stability and reactivity of polyoxoanions. Most self-assembly cascade reactions are pH driven when fast protonation-deprotonation processes provoke rapid species transformation/organization into various associates up to nanoscopic size. The study of self-assembly processes is one of the top subjects in modern chemical science [19][20][21][22][23][24][25][26][27]. Such a specific organization of the matter in different solutions is a research focus for a large number of research groups. For example, research groups led by T. Mak and Di Sun successfully merged polyoxometalate chemistry with that of coinage metal clusters using the self-assembly approach [28][29][30].
The electronic structure of polyoxoanions together with low-energy protonation makes such objects very attractive for PCET reactions. The most important catalytic process in this field is water oxidation [31,32] [36,37]. Recently, [V 6 O 13 (TRIOL NO 2 ) 2 ] 2− was applied to achieve concerted transfer of protons and electrons. Fully reduced clusters can induce 2e − /2H + transfer reactions from surface hydroxide ligands [38].
In the chemistry of group 6 polyoxometalates, the polyoxomolybdates are significantly more labile than the polyoxotungstates, thus making researchers favor the latter in their studies of POM chemistry. However, several studies of polyoxomolybdates' reactivity [39] and catalytic performance (electron transfer reactions) appeared [40][41][42][43][44][45]. One of the central complexes in this chemistry is (Bu 4 N) 4 [β-Mo 8 O 26 ] (Scheme 1), which reactivity [39] and catalytic performance (electron transfer reactions) appeared [40][41][42][43][44][45]. One of the central complexes in this chemistry is (Bu4N)4[β-Mo8O26] (Scheme 1), which is a standard precursor of all reactions in organic media, leading to a huge number of materials with different properties [46][47][48][49]. Our ongoing research focuses on the use of the coordination chemistry of the [β-Mo8O26] 4− anion in the study of silver chemistry in nonaqueous solutions [50][51][52]. Karoui and Ritchie used (Bu4N)4[β-Mo8O26] in the microwaveassisted synthesis of tris(alkoxo)molybdovanadates [V3Mo3O16(O3-R)] 2− (R = C5H8OH or C4H6NH2) by the reaction between [β-Mo8O24] 4− , [H3V10O28] 3− and pentaerythritol or tris(hydroxymethyl)aminomethane [53]. These results show the possibility of the reaction between two different types of polyoxometalates producing mixed-metal compounds based on a different structural type. Such reactions are practically unknown and can generate interesting mixed metal complexes. This is very important and can be used for various materials preparation applied in catalysis (different Mo/V oxides), photochemistry, solid-state devices (capacitors), biochemistry and biomedicine. An important question is what is the trigger and the driving force of such metal redistribution reactions? In this research, we focused on the behavior of the [β-Mo8O26] 4− anion toward protonation to answer this question. Some years ago, we suggested a straightforward hyphenated HPCL-ICP-AES technique [54] as an efficient tool to study the reaction products in different polyoxometalate systems [55][56][57]. In the present research, this technique helps us to have control over products' formation in different conditions.

Structural Analysis
The structure of [β-Mo8O26] 4− is preorganized for the coordination of different metal cations due to the presence of two trans-located lacunes (Scheme 1). During the study of complexation in the Ag + /[β-Mo8O26] 4− /L (L = auxiliary ligand) systems [50,51,58], we found a large number of equilibria that can be easily shifted by the addition of different ligands. In the present case, we tested 4-aminopyridine (py-NH2) as an auxiliary ligand in order to produce a 1D {-Mo8-Ag-py-NH2-Ag-Mo8-} coordination polymer. Instead of this, the reaction gives (Bu4N)3[Ag(py-NH2)Mo8O26] as the main product (phase purity was confirmed by XRPD, see Supplementary Materials, Figure S1). In the crystal structure (SCXRD details are collected in Supplementary Materials Table S1) Ag + , [β-Mo8O26] 4− and py-NH2 combine into another type of 1D coordination polymer when [Ag(py-NH2)Mo8O26] 3− anions stack together via py-NH2…O=Mo interactions ( Figure 1). An important question is what is the trigger and the driving force of such metal redistribution reactions? In this research, we focused on the behavior of the [β-Mo 8 O 26 ] 4− anion toward protonation to answer this question. Some years ago, we suggested a straightforward hyphenated HPCL-ICP-AES technique [54] as an efficient tool to study the reaction products in different polyoxometalate systems [55][56][57]. In the present research, this technique helps us to have control over products' formation in different conditions.

Structural Analysis
The structure of [β-Mo 8 O 26 ] 4− is preorganized for the coordination of different metal cations due to the presence of two trans-located lacunes (Scheme 1). During the study of complexation in the Ag + /[β-Mo 8 O 26 ] 4− /L (L = auxiliary ligand) systems [50,51,58], we found a large number of equilibria that can be easily shifted by the addition of different ligands. In the present case, we tested 4-aminopyridine (py-NH 2 ) as an auxiliary ligand in order to produce a 1D {-Mo 8 -Ag-py-NH 2 -Ag-Mo 8 -} coordination polymer. Instead of this, the reaction gives (Bu 4 N) 3 [Ag(py-NH 2 )Mo 8 O 26 ] as the main product (phase purity was confirmed by XRPD, see Supplementary Materials, Figure S1). In the crystal structure (SCXRD details are collected in Supplementary Materials Table S1 (7) and d(Ag1-O13) = 2.689(7) Å indicate CN = 3+2 for Ag + . These distances are in agreement with the previously published pyridinium complexes of this type [50]. The distances for py-NH2…O=Mo interactions fill the interval between 2.974 and 3.285 Å. The shortest N…O contacts 2.974 and 3.021 Å are depicted in blue in Figure 1.
The formation of this coordination polymer via NH2…POM interactions is very interesting. We did the structural search for bonding between the oxoligands of the  According to the structural analysis, R3NH + , R2NH2 + and NH4 + interact with terminal O=Mo groups of [β-Mo8O26] 4− lacunes. Moreover, even Me4N + can interact with the lacune (VEHTAF). In the crystal structure of GEBYER, the [β-Mo8O26] 4− lacunes interact with two H2O molecules. In the case of 1, we detected interaction between the neutral NH2-group protons with the O=Mo groups of polyoxomolybdate. This illustrates strong attraction between the lacune terminal oxoligands and H-atoms possessing some acidity (chiefly N-H, but also C-H in Me4N + ). Considering this, we can suggest direct proton transfer exactly to these oxoligands-producing terminal Mo-OH group, which is highly reactive (M-O πbonding breaking) and initiates further rearrangement of octamolybdate into hexamolybdate. The detailed mechanistic studies of this transformation are still absent. In this research, we used this channel to initiate the reaction between [β-Mo8O26] 4− and different protonated polyoxometalates serving as proton source. Such direct reactions between two different polyoxometalates are poorly studied. The HPLC-ACP-AES technique was used to control the products.

Reactivity of [β-Mo8O26] 4−
The first candidate for this type of reaction was easily prepared (Bu4N)4H2[V10O28]. The HPLC-ICP-AES chromatogram of pure (Bu4N)4[β-Mo8O26] in acetonitrile shows a major molybdenum peak (tR = 3.6 min), corresponding to the octamolybdate anion [β-Mo8O26] 4− , and a minor peak (tR = 4.8 min), which can be assigned as a hexamolybdate anion [Mo6O19] 2− (Figure 3a) [59]. The profile of the major peak is asymmetric due to the presence of [α-Mo8O26] 4− , according to the previous ESI-MS data, demonstrating the absence of any other molybdates in the solution [50]. The HPLC-ICP-AES chromatogram of a freshly prepared solution of (Bu4N)4H2[V10O28] shows a single peak containing vanadium (tR = 3.0 min), which confirms the presence of individual vanadate anion [V10O28] 6− in the solution (Figure 3b). Moreover, the addition of 2 eq of Bu4NOH to the solution of (Bu4N)4H2[V10O28] does not reflect any POM transformation. According to the structural analysis, R 3 NH + , R 2 NH 2 + and NH 4 + interact with terminal O=Mo groups of [β-Mo 8 O 26 ] 4− lacunes. Moreover, even Me 4 N + can interact with the lacune (VEHTAF). In the crystal structure of GEBYER, the [β-Mo 8 O 26 ] 4− lacunes interact with two H 2 O molecules. In the case of 1, we detected interaction between the neutral NH 2 -group protons with the O=Mo groups of polyoxomolybdate. This illustrates strong attraction between the lacune terminal oxoligands and H-atoms possessing some acidity (chiefly N-H, but also C-H in Me 4 N + ). Considering this, we can suggest direct proton transfer exactly to these oxoligands-producing terminal Mo-OH group, which is highly reactive (M-O π-bonding breaking) and initiates further rearrangement of octamolybdate into hexamolybdate. The detailed mechanistic studies of this transformation are still absent. In this research, we used this channel to initiate the reaction between [β-Mo 8 O 26 ] 4− and different protonated polyoxometalates serving as proton source. Such direct reactions between two different polyoxometalates are poorly studied. The HPLC-ACP-AES technique was used to control the products.

Reactivity of [β-Mo 8 O 26 ] 4−
The first candidate for this type of reaction was easily prepared (Bu 4 N) 4 (Figure 3a) [59]. The profile of the major peak is asymmetric due to the presence of       [59]. Since the viscosity of DMSO is 5 times that of acetonitrile, we were forced to reduce the concentration of the ion-pair reagent in the HPLC eluent to prevent column overpressure. Therefore, the peak retention times in DMSO increased. The HPLC-ICP-AES technique was used to investigate the reaction products between (Bu4N)4[β-Mo8O26] and [H4SiW12O40]•14H2O at different molar ratios. For the Mo/W = 10/1 molar ratio at Co of (Bu4N)4[β-Mo8O26] = 3 mM, we observed four peaks (Figure 6a): (i) unreacted octamolybdate (tR = 4.5 min), (ii) poorly separated peak with atomic ratio Mo:W = 2.3 (tR = 4.7 min), (iii) hexamolybdate (tR = 5.7 min) and (iv) Mo-free peak (tR = 6.2 min) from unreacted silicotungstic acid. With an increase in the tungstate concentration (Mo/W = 10/2 molar ratio), the same major peak with atomic ratio Mo:W = 2.3 was observed (Figure 6b).
In addition, the chromatogram shows minor W-free peaks (tR = 4.5 min, tR = 5.6 min) and a single peak containing tungsten (tR = 6.2 min), which may indicate an excess of the tungstate anion. Further increase in the concentration of tungstate (Mo/W = 10/4 molar ratio) leads to the disappearance of the first molybdenum peak ([β-Mo8O26] 4− , tR = 4.5 min) and an increase in the intensity of the peak of unreacted tungstate.  [59]. Since the viscosity of DMSO is 5 times that of acetonitrile, we were forced to reduce the concentration of the ion-pair reagent in the HPLC eluent to prevent column overpressure. Therefore, the peak retention times in DMSO increased. The HPLC-ICP-AES technique was used to investigate the reaction products between (Bu 4 N) 4 (Figure 6a): (i) unreacted octamolybdate (t R = 4.5 min), (ii) poorly separated peak with atomic ratio Mo:W = 2.3 (t R = 4.7 min), (iii) hexamolybdate (t R = 5.7 min) and (iv) Mo-free peak (t R = 6.2 min) from unreacted silicotungstic acid. With an increase in the tungstate concentration (Mo/W = 10/2 molar ratio), the same major peak with atomic ratio Mo:W = 2.3 was observed (Figure 6b).
In addition, the chromatogram shows minor W-free peaks (t R = 4.5 min, t R = 5.6 min) and a single peak containing tungsten (t R = 6.   (Figure 7, peak no. 5, t R = 5.6 min) in the ratio of 95:5. Addition of 0.001 M acetic acid decreases the octamolybdate peak intensity, while causing an increase in the hexamolybdate peak intensity and the appearance of a new peak (peak no. 4, t R = 5.1 min). Further increase in the acetic acid concentration continues to reduce the intensity of the octamolybdate peak and leads to an increase in the intensity of peak no. 4, as well as the appearance of two minor peaks (peak no. 1,2) of smaller molybdates. At an acetic acid concentration of 0.008 M, the intensity ratio of the peaks corresponding to octamolybdate (peak no. 3), the new product (peak no. 4), and hexamolydate (peak no. 5), is 1.8:3.4:1, respectively. No further changes in the ratio of species in solution was observed with an increase in the concentration of acetic acid from 0.008 M to 0.01 M; however, the intensity of all peaks decreases by 1.5, and further acidification leads to the formation of a white precipitate, which makes the HPLC analysis unapplicable.  (Figure 7, peak no. 5, tR = 5.6 min) in the ratio of 95:5. Addition 0.001 M acetic acid decreases the octamolybdate peak intensity, while causing an incre in the hexamolybdate peak intensity and the appearance of a new peak (peak no. 4, t 5.1 min). Further increase in the acetic acid concentration continues to reduce the intens of the octamolybdate peak and leads to an increase in the intensity of peak no. 4, as w as the appearance of two minor peaks (peak no. 1,2) of smaller molybdates. At an ace acid concentration of 0.008 M, the intensity ratio of the peaks corresponding to octam lybdate (peak no. 3), the new product (peak no. 4), and hexamolydate (peak no. 5) 1.8:3.4:1, respectively. No further changes in the ratio of species in solution was observ with an increase in the concentration of acetic acid from 0.008 M to 0.01 M; however, intensity of all peaks decreases by 1.5, and further acidification leads to the formation a white precipitate, which makes the HPLC analysis unapplicable.

NMR
NMR spectroscopy was anticipated to be an informative tool to study the reaction between (Bu 4 N) 4 51 V and 95 Mo NMR active isotopes. We measured 95 Mo NMR spectra for the following solutions to study the effects of acidification of Mo8 by Hpts (Hpts = p-toluenesulfonic acid) (Figure 8).
solutions to study the effects of acidification of Mo8 by Hpts (Hpts = p-toluenesulfonic acid) (Figure 8). The signals from such species should appear in a negative region, in comparison with the literature [63].
The reaction between Mo8 and V10 was studied using both 95 Mo and 51 V NMR ( Figure  9). The signals from such species should appear in a negative region, in comparison with the literature [63].
The reaction between Mo8 and V10 was studied using both 95 Mo and 51 V NMR ( Figure 9). The 51 V NMR spectra ( Figures S2-S4) show exclusive formation of [V2Mo4O19] 4− , to the detriment of other mixed metal Lindqvist molybdovanadates, meaning that such reactions can offer a straightforward way to this anion. Two signals in the 95 Mo NMR spectrum of (Bu4N)4[β-Mo8O26] indicate an equilibrium between α and β isomers, as described   O 26 ] indicate an equilibrium between α and β isomers, as described in the literature [64]. In the case of spectrum b (Figure 9), the baseline correction was not as accurate, and the peaks from Mo8 have slightly negative intensities. Moreover, due to this problem, the profile of the main signal is also not as correct. Nevertheless

XRPD
X-ray powder diffraction patterns were measured on a Bruker D8 Advance diffractometer using LynxEye XE T-discriminated CuKα radiation. Samples were layered on a flat plastic specimen holder.

HPLC-ICP-AES and HPLC
Separation was performed with the HPLC system Milichrom A-02 (EcoNova, Novosibirsk, Russia), equipped with a two-beam spectrophotometric detector at the wavelength range of 190−360 nm in the ion-pair mode of reversed phase chromatography (Pron-toSIL 120-5-C18AQ, 2 × 75 mm), eluents: A-0.06% tetrabutylammonium hydroxide (for (Bu 4 N) 4  The data acquisition and processing were carried out with iTEVA (Thermo Scientific, Waltham, MA, USA) software. In order to eliminate plasma quenching, we diluted the liquid coming out of the column into the spray chamber with deionized water. The steady state of the plasma and the optimal values of the analytical signals were finally achieved at the eluent flow rate of 0.25 mL min −1 and the eluent velocity of 3 mL min −1 (peristaltic pump speed-75 rpm). At the current stage, it is impossible to deduce the mechanism, which is not as simple as [Mo 2 O 7 ] 2− -elimination. In comparison with the microwave synthesis reported by Karoui and Ritchie, simple thermal activation does not need any special equipment. The addition of any triol type organic ligands into the reaction mixture will be the next step in such reactivity studies. Such an approach opens a way to new mixed functionalized complexes.