**1. Introduction**

As an important and basic type of polyoxometalates (POMs), Anderson-Evans clusters have been synthesized and characterized for nearly one hundred years, with a general formula of [Xn+M6O24Hm] (12−n−m)−, in which X expresses the heteroatom and M the addenda atoms (Mo or W in most cases) [1–3]. Compared with the terminal O atoms in Anderson-Evans polyanion, the bi- or tri-bridging O atoms (μ2- or μ3-O) have a higher reactive activity and can be replaced by some organic species with hydroxyl groups under specific circumstances to form various decoration types (Figure S1, Supplementary Materials) [4–6]. Through this method, different organic functional groups can be introduced into the inorganic skeleton, which not only enriches the structural figures of the final adducts but also integrates the properties of both parts, resulting in novel functionalities [7–11]. Because the organic functional groups have different hydrophilic and hydrophobic properties, they need to be introduced into polyanions under different solvent environments, such as water or organic solvents [12,13], while due to the use of TBA4Mo8O26 (TBA = tetrabutylammonium cation) as the Mo source in most cases, triol-ligand modified Anderson-Evans polyanions are mostly prepared in organic solvents such as CH3CN, CH3OH and *N*,*N*dimethylformamide, which limits their applications [14,15]. So, it is necessary to find new Mo sources, for example Na2MoO4, (NH4)6Mo7O24, and Na4Mo8O26, suitable for reactions in aqueous solution, which may also bring new architectures. On the other hand, compared with the wide range of investigations on organic components covalently

**Citation:** Wang, Y.; Duan, F.; Liu, X.; Li, B. Cations Modulated Assembly of Triol-Ligand Modified Cu-Centered Anderson-Evans Polyanions. *Molecules* **2022**, *27*, 2933. https://doi.org/ 10.3390/molecules27092933

Academic Editor: Santiago Reinoso

Received: 16 April 2022 Accepted: 3 May 2022 Published: 4 May 2022

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modified Anderson-Evans polyanions with trivalent heteroatoms, examples with divalent center atoms are less focused on and reported [16–18]. Aside from the initial instances with Zn2+ and Ni2+ used as heteroatoms [19], investigations on the triol-ligand covalently modified CuII-, CoII- and Ni II-centered Anderson-Evans polyanions are conducted, including finding new modification types, their transformation between different modification architectures and the co-modification of triol ligand and methanol or acetic acid [20–24].

Except developing new modification manners for discrete polyanions, it is also important to construct extended structures by employing the triol-ligand modified Anderson-Evans polyanions as building blocks in appropriate ways. As a comparison, one dimensional (1D) to three dimensional (3D) structures based on undecorated Anderson-Evans polyanions have been reported, in which terminal O atoms of the clusters are used to coordinate with transition metal ions for the formation of the extended organic-inorganic hybrids [25–28]. For the triol-ligand decorated Anderson-Evans polyanions, only a few cases have been reported. For example, through the introduction of pyridine groups, the discrete polyanions can be assembled into an extended structure through M–N bonds, where N atoms are sourced from the pyridine groups [29–32]. Another important strategy is linking amino functionalized Anderson-Evans clusters with 4-connected building units though imine condensation to form metal–organic frameworks [33,34]. As a more common method, rare earth ions (Ln3+) can also be used as nodes to link this type of cluster for the formation of coordination polymers through Ln–O bonds [35]. However, there are still no systematical investigations available on the triol-ligand modified XII-centered Anderson-Evans polyanions in aqueous solution, especially with regard to the influence of cations on the assembled structures of polyanions [36].

Considering the investigations in this field, in the present contribution, we investigate the synthesis of triol-ligand modified CuII-centered Anderson-Evans polyanions in detail in an aqueous solution. By selecting (NH4)6Mo7O24, MoO3 and Na4Mo8O26 as our Mo sources, which have not been widely applied before, we synthesized a series of divalent metal ion-centered clusters, and also realized the controlled modulation of an assembly of polyanions with NH4 +, Cu2+ and Na<sup>+</sup> as cations (Scheme 1). In addition, the influences of the size and connecting mode of linkers was discussed, as well as a Hirshfeld analysis of the building blocks with various decoration types. The presented results not only provide an efficient synthetic route for triol-ligand modified Anderson-Evans polyanions with divalent heteroatoms, but also express the important role of cations in the assembly of polyanions.

**Scheme 1.** Schematic illustration for the synthetic parameters in different decoration fashions and extended structures in the preparation of Cu-centered Anderson–Evans POMs **1**–**9**. All H atoms except those attaching to N and μ3-O atoms are omitted for clarity. The bold numbers are added to represent the corresponding compounds.
