*3.4. 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.

#### *3.5. 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 (ProntoSIL 120-5-C18AQ, 2 × 75 mm), eluents: A—0.06% tetrabutylammonium hydroxide (for (Bu4N)4[β-Mo8O26] and (Bu4N)4H2[V10O28]), 0.02% tetrabutylammonium hydroxide (for (Bu4N)4[β-Mo8O26] and [H4SiW12O40]·14H2O); B—acetonitrile. Gradient elution with a gradual increase in acetonitrile concentration was employed to resolve the species. ICP-AES spectrometer iCap 6500 Duo (Thermo Scientific, Waltham, MA, USA) with concentric nebulizer was applied as detector in hyphenated HPLC-ICP-AES. For the element detection Mo 281.6 nm, V 292.4 nm and W 239.7 nm, spectral lines were selected. All measurements were performed in three replicates.

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−<sup>1</sup> and the eluent velocity of 3 mL min−<sup>1</sup> (peristaltic pump speed—75 rpm).

## **4. Conclusions**

This manuscript describes an affinity of [β-Mo8O26] <sup>4</sup><sup>−</sup> lacunes for interaction with H-atoms possessing some N–H or even C–H (in Me4N+) acidity. We demonstrated this in the case of 1D polymeric chains formation via py-NH2 and [β-Mo8O26] <sup>4</sup><sup>−</sup> interaction. This example illustrates a general approach to the formation of soft matters based on such types of interactions. The reaction of [β-Mo8O26] <sup>4</sup><sup>−</sup> with diluted acids generates a set of unknown complexes, according to the HPLC-ICP-AES data. Moreover, there was a new type of reactivity of [β-Mo8O26] <sup>4</sup><sup>−</sup> combining: (i) proton transfer from another type of polyoxometalates in solution, (ii) backbone breaking and (iii) transformation into mixed Lindqvist type complexes has been demonstrated. In the case of (Bu4N)4H2[V10O28], this reaction gives [V2Mo4O19] <sup>4</sup>−. [H4SiW12O40] plays a role as a proton and W source, producing [W2Mo4O19] <sup>2</sup>−. The key study here is proton transfer into the lacune of [β-Mo8O26] <sup>4</sup>−, generating the reactive transition state. At the current stage, it is impossible to deduce the mechanism, which is not as simple as [Mo2O7] <sup>2</sup>−-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.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules27238368/s1, Table S1: SCXRD Experimental details; Figure S1: Comparison of experimental and calculated powder diffraction patterns for **1**; Figure S2: 51V NMR spectrum of (Bu4N)3Na[V2Mo4O19] (DMSO-*d*6, r.t.); Figure S3: 51V NMR spectrum of the reaction mixture containing 70 mg **Mo8** (300 μL DMSO-*d*6) + 55 mg **V10** (300 μL DMSO-*d*6); Figure S4: 51V NMR spectrum of the reaction mixture containing 70 mg **Mo8** (300 μL DMSO-*d*6) + 22 mg **V10** (300 μL DMSO-*d*6), after 10 min at 60 ◦C.

**Author Contributions:** Conceptualization, P.A.A.; methodology, P.A.A.; validation, P.A.A., V.V.V. and N.B.K.; formal analysis, V.V.V. and N.B.K.; data curation, V.V.V. and N.B.K.; writing—original draft preparation, P.A.A.; writing—review and editing, P.A.A. and M.N.S.; visualization, V.V.V. and N.B.K.; supervision, P.A.A.; project administration, P.A.A.; funding acquisition, P.A.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the grant of the President of the Russian Federation for young scientists—Doctors of Sciences MD-396.2021.1.3.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The crystallographic data have been deposited in the Cambridge Crystallographic Data Centre under the deposition codes CCDC 2215910.

**Acknowledgments:** The authors thank the Ministry of Science and Higher Education of the Russian Federation for access to the XRD facilities of the Nikolaev Institute of Inorganic Chemistry. The technical staff of the Institute is also thanked for their assistance.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
