**1. Introduction**

Protons play a key role in a wide range of water-associated processes, from geochemistry to biology [1]. The appearance of the Theory of Coupled Electron and Proton Transfer Reactions [2–4] opened rich prospects for chemical reactions design [5–8]. Currently, proton coupled electron transfer (PCET) processes play crucial roles in synthesis and catalysis [9,10], e.g., artificial photosynthesis systems [11–14] and PCET at interfaces [15–18].

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 [19–27]. 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 [28–30].

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]. Such POM catalysts as [{RuIV 4(OH)2(H2O)4}(γ-SiW10O34)2] <sup>10</sup><sup>−</sup> [33–35] and [CoII 4(H2O)2(B-α-PW9O34)2] <sup>10</sup><sup>−</sup> have become classics [36,37]. Recently, [V6O13(TRIOLNO 2)2] <sup>2</sup><sup>−</sup> was applied to achieve concerted transfer of protons and electrons. Fully reduced clusters can induce 2e<sup>−</sup>/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 [40–45]. One of the central complexes in this chemistry is (Bu4N)4[β-Mo8O26] (Scheme 1), which

**Citation:** Volchek, V.V.; Kompankov, N.B.; Sokolov, M.N.; Abramov, P.A. Proton Affinity in the Chemistry of Beta-Octamolybdate: HPLC-ICP-AES, NMR and Structural Studies. *Molecules* **2022**, *27*, 8368. https:// doi.org/10.3390/molecules27238368

Academic Editor: Xiaobing Cui

Received: 29 October 2022 Accepted: 25 November 2022 Published: 30 November 2022

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is a standard precursor of all reactions in organic media, leading to a huge number of materials with different properties [46–49]. Our ongoing research focuses on the use of the coordination chemistry of the [β-Mo8O26] <sup>4</sup><sup>−</sup> anion in the study of silver chemistry in non-aqueous solutions [50–52]. Karoui and Ritchie used (Bu4N)4[β-Mo8O26] in the microwave-assisted synthesis of tris(alkoxo)molybdovanadates [V3Mo3O16(O3-R)]2<sup>−</sup> (R = C5H8OH or C4H6NH2) by the reaction between [β-Mo8O24] <sup>4</sup>−, [H3V10O28] <sup>3</sup><sup>−</sup> 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.

**Scheme 1.** The structure of the [β-Mo8O26] <sup>4</sup><sup>−</sup> anion.

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 [55–57]. In the present research, this technique helps us to have control over products' formation in different conditions.
