**1. Introduction**

The Anderson type of polyoxometalates (abbreviated as POMs) are an important class of structures in oxygen-bridged polymetallic cluster compounds, which can be expressed as [XM6O24] <sup>n</sup><sup>−</sup> or [Hx(XO6)M6O18] <sup>n</sup><sup>−</sup> (M=Mo or W); its central heteroatom X can be replaced with the majority of transition metal atoms like Fe, Co, Cu, Ni and Mn, etc. The stable configuration of Anderson-type POMs is a flat and circular structure (Figure 1a,b). There are three different types of oxygen atoms on the surface of Anderson-type POMs, with different coordination modes, so the reactivity of different sites on the surface is fairly different. Additionally, the class of protonation of the surface oxygen atoms of the POMs are also significantly varied, which can be divided into the following two types (Figure 1c): one non-protonation μ3-O on the surface of Anderson-type POMs called type A; the other possesses protonated μ3-O called type B. Provided that the valence state of the central atom is high (oxidation state > 4), Anderson-type POMs are also found to exist in a fold isomer configuration. This is similar to ammonium heptamolybdate (Figure 1d,e). However, for the central metal atoms in the low valence states, this structure requires organic ligand protection to keep steady (Figure 1f) [1–5].

The oxygen atoms of Anderson-type POMs are surrounded by six octahedrons around the central hetero-atoms by sharing an edge to form a common planar ring structure, otherwise a folded-twisted structure. Moreover, the central hetero-atoms are varied and modified easily, so the structure has a high coordination activity. Their structure and properties can be more multifarious by further modification.

Classical structures of POMs are mostly uniform particles with nano, otherwise sub-nano, size. Due to the octahedral connection and easily modified characteristics

**Citation:** Wei, Z.; Wang, J.; Yu, H.; Han, S.; Wei, Y. Recent Advances of Anderson-Type Polyoxometalates as Catalysts Largely for Oxidative Transformations of Organic Molecules. *Molecules* **2022**, *27*, 5212. https://doi.org/10.3390/ molecules27165212

Academic Editor: Xiaobing Cui

Received: 13 July 2022 Accepted: 11 August 2022 Published: 16 August 2022

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of Anderson-type POMs, this leads them to become a good sub-nano building unit. They can be used to design and synthesize a variety of special sizes and properties of organic– inorganic POMs compounds which show exceedingly important application value in materials, medicine, catalysis and other fields.

**Figure 1.** (**a**) Structure diagram of α-Anderson-type POMs; (**b**,**c**) Ball and stick model of α-Andersontype POMs; (**d**) Structure diagram of β-Anderson-type POMs; (**e**) Ball and stick model of β-Andersontype POMs; (**f**) Organic modified β-Anderson-type POMs.

The previous research about Anderson-type POMs mainly attach great importance to the synthesis and preparation of new compounds and characterization, etc.; the Wang group, Wei group, Zhou group, Niu group and Kreb group have made an army of contributions in this field.

In 2014, Wei groups developed a liquid-phase diffusion single-crystal growth sample addition method and a special and sample addition tube [6]. The invention can significantly improve the working efficiency of the single-crystal cultivation, and the quality of the obtained single crystal by this method is also significantly improved. The Wei group designed a large number of different Anderson-type POMs and crystal of their organic derivatives via this approach (Figure 2). They afterward defined a series of structure of new Anderson-type POMs and the relevant derivatives [7–13] through single-crystal X-ray diffraction, infrared spectrum, NMR spectrum and liquid chromatography, which greatly promoted the development of synthesis Anderson-type POMs and structural chemistry.

With the structural development of Anderson-type POMs, a mounting number of researchers began to study the modification of POMs which controlled the application in the catalytic field [14–19]. These studies illustrate that Anderson-type POMs not only possess acid-based properties, but also excellent redox performance for the contributor that is present in central atoms. Owing to their structural stability, they can also be used as catalysts and applied in homogeneous and heterogeneous reactions, and even through phase-transfer catalyst or ionic liquid catalyst after appropriate chemical modification. Therefore, Anderson-type POMs are regarded as new sub-nano molecules with high catalytic activity. Nowadays, the research about Anderson-type POMs materials and their functions has gradually become a hot field in polyacid chemistry [20–25]. Nevertheless, the early work in the catalytic application of Anderson-type POMs mainly focused on desulfurization of oxidative and the treatment of industrial wastewater [26–30]. The applications in organic reaction are still the confronting great challenge.

**Figure 2.** Several Anderson-type POMs crystals prepared by the special sample tube for liquid-phase diffusion technique [6–13].

It is universally acknowledged that progress of society depends on the development of organic synthetic chemistry. Reducing or avoiding producing harmful products is the question of organic synthesis reaction and the focus of "green" chemical research [31–34]. With the development of Anderson-type POMs in the catalytic reaction, the researchers found that the replacement of a traditional metal or non-metal catalyst is feasible with Anderson-type POMs or their derivatives [35–51]. This system not only can improve the catalytic efficiency, but also generate less by-product in the process. This is exactly an environmentally friendly, green and efficient catalytic system.

The variety of Anderson-type POMs catalysts have been reported. This review summarizes previous research studies based on recent literature. These works are roughly divided into two major categories according to the anion structure (Figure 3). For convenience of description, we define the relevant letter for the following categories: (1) The catalytic application of simple Anderson-type POMs (abbreviated as P); (2) Catalytic application of organic modified Anderson-type POMs derivatives (abbreviated as PO). In these two categories, the catalysts are further divided into two minor categories according to the cationic form of Anderson-type POMs: (1) Simple inorganic cations (such as NH4 +.Na+.H+.K+, the following is abbreviated as [I]+); (2) Organic cations ([O]+).

**Figure 3.** Tree-structure classification of Anderson-type POMs in this paper.
