*Article* **Organic/Inorganic Species Synergistically Supported Unprecedented Vanadomolybdates**

**Tian Chang, Di Qu, Bao Li \* and Lixin Wu**

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, China

**\*** Correspondence: libao@jlu.edu.cn

**Abstract:** Vanadomolybdates (VMos), comprised of Mo and V in high valences with O bridges, are one of the most important types of polyoxometalates (POMs), which have high activity due to their strong capabilities of gaining/losing electrons. Compared with other POMs, the preparation of VMos is difficult due to their relatively low structural stability, especially those with unclassical architectures. To overcome this shortcoming, in this study, triol ligands were applied to synthesize VMos through a beaker reaction in the presence of V2O5, Na2MoO4, and organic species in the aqueous solution. The single-crystal X-ray diffraction results indicate that two VMo clusters, Na4{V5Mo2O19[CH3C(CH2O)3]}·13H2O and Na4{V5Mo2O19[CH3CH2C(CH2O)3]}·13H2O, with a similar architecture, were synthesized, which were both stabilized by triol ligand and {MoO6} polyhedron. Both clusters are composed of five V ions and one Mo ion in a classical Lindqvist arrangement with an additional Mo ion, showing an unprecedented hepta-nuclear VMo structure. The counter Na+ cations assemble into one-dimensional channels, which facilitates the transport of protons and was further confirmed by proton conductivity experiments. The present results provide a new strategy to prepare and stabilize VMos, which is applicable for developing other compounds, especially those with untraditional architectures.

**Keywords:** polyoxometalate; vanadomolybdate; triol ligand; covalent modification

**1. Introduction**

Organic species covalent functionalization on polyoxometalates (POMs) is an efficient method of constructing POMs that possess novel architectures and properties, and results in abundant achievements in the fields of organic and inorganic hybrids as well as materials science [1,2]. To date, several strategies have been developed in accordance with the structural features of POMs, and the classical structures are shown in Figure 1. By utilizing the strong Mo≡N bond, some N-containing organic molecules are capable of being anchored on a Lindqvist-type cluster [Mo6O19] <sup>2</sup><sup>−</sup> (Figure 1a) [3,4]. In some lacunary POMs, P, Si, Sn, or Ge occupy the vacant position and introduce organically functional groups through P–C, Si–C, Sn–C, or Ge–C bonds (Figure 1b) [5]. As a comparison, modification through M–O–C (M = Mo, W, V) is a more convenient and more common method which provides volatile connection modes between organic and inorganic parts and is applicable to different types of POMs. The simplest modification through an M–O–C bond is that methanol or ethanol molecules replace bi- or tri-bridging O atoms of clusters, resulting in partially substituted POMs, which is applicable to nearly all types of POMs (Figure 1c) [6]. Beyond a single M–O–C bond, carboxyl is also a good candidate to covalently bond on the cluster through two M–O–C bonds sourced from one molecule, which provides a stronger combination (Figure 1d) [7,8]. In addition, triol ligands have the ability to supply three M–O–C bonds linking to the same C center through three methene units, which largely improves the stability of the formed complexes, and therefore has obtained rapid progress in the last two decades [9–11]. To date, Lindqvist-(Figure 1e), Anderson-(Figure 1f), Keggin-(Figure 1g),

**Citation:** Chang, T.; Qu, D.; Li, B.; Wu, L. Organic/Inorganic Species Synergistically Supported Unprecedented Vanadomolybdates. *Molecules* **2022**, *27*, 7447. https:// doi.org/10.3390/molecules27217447

Academic Editor: Santiago Reinoso

Received: 13 October 2022 Accepted: 30 October 2022 Published: 2 November 2022

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and Dawson-type clusters (Figure 1h) have all been involved in the successful modification by triol ligands with various architectures, which express excellent properties and various applications [12–14]. The unique coordination behavior of the triol ligand endows its ability to stabilize structures and makes it a good candidate for building POMs with novel or metastable clusters.

**Figure 1.** Ball-and-stick representation of several classical organo-functionalized clusters. (**a**) Imidofunctionalized Lindqvist-type polyanion, (**b**) organophosphorus-functionalized lacunary Keggin-type polyanion, (**c**) methanol-functionalized lacunary Keggin-type polyanion, (**d**) carboxyl-functionalized Anderson-like-type polyanion; triol-ligand-functionalized (**e**) Lindqvist-type, (**f**) Anderson-type, (**g**) lacunary Keggin-type, and (**h**) Dawson-type polyanions. All H atoms are omitted for clarity. R represents the functional group. Dark cyan ball: V, olive ball: Mo, dark green ball: W, purple ball: P, light grey ball: C, red ball: O, blue ball: N, light blue ball: heteroatom.

Vanadomolybdates (VMos), as one of the important branches of POM chemistry, have attracted extensive attention due to their diverse structures and multi-functions in the fields of electrochemistry, magnetism, and catalysis [15–17]. Compared with other POMs, VMos have a stronger ability to gain or lose electrons, and therefore possess a higher activity, resulting in their excellent properties [18,19]. On the other hand, the structures of VMos have relatively low stability, especially those of lacunary species, which need additional components to support their architectures. For instance, with the aid of a transition metal ion dimer {Mn2}, two unstable trivacant Keggin-type VMo clusters were linked to form a sandwich structure, which shows an unusual ferromagnetic coupling [20]. Another strategy for preparing VMos is utilizing the organic species, which are covalently anchored on the clusters and stabilize the obtained hybrids. A classical case was reported by Wei et al., where an unprecedented {V4Mo3} was synthesized in the presence of anilines as stabilizers, which replaced six terminal or bi-bridging O atoms of the {Mo3} cluster [21]. With a similar strategy, an Anderson-type VMo cluster was also isolated, in which two triol ligands replaced all hydroxyls surrounding the heteroatom V, resulting in doubly decorated hybrids [22]. In another example, triethanolamine was applied to stabilize a {VMo6O25} cluster, forming an organic component single-functionalized product [23]. We have also made a contribution in this field showing that a triol ligand is used to anchor on the top position of [VMo9O34] <sup>9</sup><sup>−</sup> cluster with the maintaining of the vacant sites, which are considered to be more active in the reaction [24]. These results indicate that organically covalent modification is an efficient method to prepare VMos, especially those without stable structures in the isolated state. More importantly, with the help of the coordination ability of organic species, VMos with novel structures are expectable.

During our investigation on triol-ligand-modified VMos, we discovered one type of novel Lindqvist-type derivative with V2O5, Na2MoO4, and triol ligand as reactants. In this work, the synthesis and structural characterization of Na4{V5Mo2O19[CH3C(CH2O)3]}·13H2O (**1**) and Na4{V5Mo2O19[CH3CH2C(CH2O)3]}·13H2O (**2**) are presented. The polyanions of compounds **1** and **2** are isostructural except the terminal group of the triol ligand, which can both be seen as a {MoO6} polyhedron attached to a Lindqvist cluster by sharing three O atoms. As far as we know, this type of hepta-nuclear VMo has never been reported in the POM family. The results presented here would provide a new route for preparing triol-ligand-modified VMos with unusual architectures.
