**2. The Application of Anderson-Type POMs in Catalytic Synthesis**

*2.1. Catalyzation of Anderson-Type POMs (P)*

[I]+P

In 2016, Kadam and Gokavi investigated the dynamics and catalytic mechanism of the Anderson-type POMs, chromium (III) molybdate catalyst, which originally formed via the reaction between [O=CrV(OH)6Mo6O18] <sup>3</sup><sup>−</sup> from the oxidation of H[CrIII(OH)6Mo6O18] 2− and hydrazide without free radicals in the course of the reaction process. This work provides good ideas and inspiration for oxidation hydrazide by Anderson-type POMs [35].

In 2017, the Wei group reported a simple, mild and efficient method of aerobic oxidation amine by inorganic ligand-loaded non-precious metal catalyst (NH)4)n[MMo6O18(OH)6] (M=Cu2+; Fe3+; Co3+; Ni2+; Zn2+, n = 3 or 4) in water at 100 ◦C via one step, demonstrating that the catalytic activity and selectivity can be significantly improved by transforming the central metal atoms. In the presence of Anderson-type POMs, with O2 as the unique oxidant, the catalytic oxidation of fundamental amine and secondary amines, and the coupling reaction of alcohol and amines can be achieved to generate the relevant imines. This new catalytic system provides a new method for catalytic oxidation reaction through inorganic ligand-supported metal catalysts [36].

Soon after, Wei and collaborators proposed a Cu-based Anderson-type POM, (NH4)4 [Cu(OH)6Mo6O18] used for oxidation of carboxylic acid to aldehyde in water (Figure 4). This system used oxygen as the sole oxidant in the mild condition and was also suitable for various aldehyde derivatives with different functional groups [37]. In this catalytic process, the catalyst (NH4)4[Cu(OH)6Mo6O18] can be recycled at least six times and maintain high catalytic activity. This method of preparing for carboxylic acid is not only simple to operate, but also avoids the use of expensive and toxic raw materials. The versatility of this catalytic route makes it possible for industrial applications.

In 2018, the Wei Group used inorganic ligand-supported Fe-based Anderson-type POMs (NH)4)3[Fe(OH)6Mo6O18] to prepare imines by aerobic oxidation of aldehyde/ketone and amine oxidation coupling with oxygen as the sole oxidation [38]. The catalyst is comprised of an inexpensive system (NH4)6Mo7O24·4H2O and Fe2(SO4)3. This system can be applied in broad substrates with low loss of catalytic activity, proving that this catalytic system has great potential in catalytic chemical transformation. Additionally, the stably inorganic skeleton for the catalyst provides good stability and is reusable in the course of the reaction process; for this reason, the catalytic oxidation of halide and amine can be easily improved to the gram level and has the potential to apply in industry.

**Figure 4.** A method for preparing carboxylic acids by oxidizing aldehydes in water using (NH4)4 [Cu(OH)6Mo6O18].

After that, the Wei group published an inorganic ligand-supported iodine catalyst (NH4)5[IMo6O24] using an efficient method of aerobic alcohol oxidation. This system is compatible with multiple functional groups with high selectivity and good stability [39]. Based on experimental results, they proposed a preliminary mechanism of iodine catalyst (NH4)5[IMo6O24] for alcohol oxidation (Figure 5). This process is similar to the enzymatic oxidation reaction, which can be divided into two independent semi-reactions: (NH4)5[IVIIMo6 VIO24] mediated alcohol oxidation reactions and dioxygen-coupled oxidation reaction with [IVMo6 VIO24]. For the iodo-molybdic acid catalyst by the inorganic ligands supported, two oxidizing equivalents required for oxidation are stored at the iodine center. Meanwhile, the addition of additives exerts a significant influence on the progress of the reaction, because the presence of Cl<sup>−</sup> and Ac<sup>−</sup> as an electron transfer medium promotes the electron transfer efficiency of (NH4)5[IVIIMo6 VIO24]. This proposed mechanism is meaningful for the application of new Anderson-type POMs in organic reactions. This catalyst is a kind of efficient and mild inorganic ligand coordination catalytic system with high valence of iodine. The system has a wide range of substrate tolerance, good selectivity and recyclability, which avoids using the toxic organic ligands and toxic oxidant. Compared with the expensive organic iodine reagent, this system is convenient to apply in medicine, spices and food additives. Besides, the structure of (NH4)5[IMo6O24] also provides more insight on designing the new Anderson-type POMs by replacing the metal ion in the backbone.

**Figure 5.** Proposed mechanism for the I-POM-catalyzed oxidation of alcohols.

Subsequently, Wei and co-worker prepared a Zn-based Anderson-type POM, (NH4)4 [Zn(OH)6Mo6O18], which was used as a catalyst for oxide cross-coupling reaction of halide and amine, oxide self-coupling reaction of amine, and the halide oxide reaction [40]. In this catalytic system (Figure 6), they chose benzyl amine and benzyl chloride as model substrates for the oxidative cross-coupling reaction with generating N-benzylidenebenzylamine in 32% yield, 47% selectivity. By screening the solvent, acetonitrile showed the best yield and selectivity with 89% and 90%, respectively. Additionally, shortening and extending the reaction time also affected the yield. In addition, the optimal reaction temperature at 60 °C and 1.0 mol% was the optimal amount of catalyst. Although the reaction uses O2 as an oxygen source, relevant imines, aldehyde and ketone can still be efficiently prepared. The inorganic ligand-supported zinc-based Anderson-type POMs are not only easy to prepare and synthesize by the hydrothermal method, but also easy to recover due to the heterogeneity of the reaction. Compared with precious noble metal catalysts such as rhodium, ruthenium, and palladium, this catalyst avoids the use of toxic, air and water-sensitive organic ligands. So, Anderson-type POMs still have great potential as a heterogeneous catalyst in organic reactions.

**Figure 6.** Zn-based Anderson-type POMs catalyze the oxidation of halides to aldehydes or ketones, and amines to sub-units.

In the same year, Sawant and co-worker investigated the application of Co-based Anderson-type POMs, CoIII(OH)6Mo6O18] <sup>3</sup>−, in catalyzed oxidation of acetaminophen in the aqueous media when the pH values are 1 or 2 [41]. In this reaction, the electron transfer from neutral acetaminophen to anion, and then in the step of rate determination, the free radicals are further oxidized to N-acetylquinone imide as an intermediate, and what is more, hydrolyze to obtain benzoquinone and acetic acid. The experimental results reveal that the formation of a weak complex among the reactants promotes the reaction.

In 2019, the Wei group reported Cu-based Anderson-type POMs, (NH4)4[Cu(OH)6 Mo6O18], which were used to study the aerobic oxidation of alcohol [42]. The classical transition metal complex catalytic system required complex organic ligands or nitroso radicals as auxiliary catalysts, and even strong oxidants. Nevertheless, when many organic compounds are in contact with these strong oxidants, it can easily result in exploding. Therefore, the traditional oxidation reaction of alcohol possesses complex process, highprice, severe and dangerous reaction conditions. Although, this catalyst is synthesized in water from cheap ammonium heptamolybdate and copper sulfate. The results showed that this catalytic system was suitable for various substrates in the oxidation of alcohol with excellent selectivity and activity. This method is not only safe and efficient, but also environmental. It has the potential to achieve industrial application. The experimental results show that the catalytic mechanism of the catalytic system is as follows (Figure 7) [42]: The two oxidation equivalents required for oxidation are not only stored in the copper center, but

can be transferred to six marginalized MoO6 on the unit for (NH4)4[Cu(OH)6Mo6O18]. The Cu-based Anderson-type POMs react with alcohol to generate active substance A, while A and E, as a pair of isoforms, can interconvert, possibly due to the transfer of electrons to the inorganic ligands MoO6 via the intramolecular oxygen bridge Cu-O-Mo. Thus, one of the Mo cells goes from positive hexavalent to positive pentavalent. Anderson-type POMs A firstly activate molecular oxygen generation activity species B. Subsequently, species B hybridizes by the O=O bond to obtain the highly reactive metal oxygen species C as an active oxidant. During this catalytic reaction, the detection by MS indicates that water is involved in the reaction, producing hydrogen peroxide, so that the catalytic system is closer to the galactose oxidase reaction. CuII and the hydroxyl radical act together as a single-electron oxidant to be a two-electron alcohol oxidation reaction. In the reaction of galactose oxidase, a single center which is copper reacts with oxygen, producing hydrogen peroxide as a by-product. High active species C reacts with ethanol to generate intermediate D, and then H atoms are extracting to generate aldehyde and regenerate E. Finally, the intermediate E (CuII) was re-oxidized by hydrogen peroxide to substance A (CuI ).

**Figure 7.** The catalytic mechanism of alcohol oxidation by using (NH4)4[Cu(OH)6Mo6O18].

At the same time, the Wei group also prepared Fe-based Anderson-type POMs, (NH4)3[Fe(OH)6Mo6O18] and applied them in olefination epoxidation reaction with 30% H2O2 as oxidant. This catalytic system does not require additional reductant or free radicals as initiators, and the experimental operation is simple, completely avoiding the use of expensive, toxic precious metal catalysts. Moreover, it does not need air/water sensitive and commercially unavailable organic ligands or tungsten POMs. (NH4)3[Fe(OH)6Mo6O18] can be obtained by one-step reaction in 100 ◦C water. This system has successfully converted the various aromatic and aliphatic alkenes into the corresponding epoxy compounds, with a good yield and selectivity [43]. Subsequently, Wei and collaborators further used this system in oxidative esterification reaction of various aldehydes with alcohol, and the corresponding esters were achieved under mild conditions with high yields, including several drug molecules and natural products [44].

After that, the Wei group and co-worker developed Cr-based Anderson-type POMs, (NH)4)3[CrMo6O18(OH)6]. The various primary and secondary amines, and even dual primary amine, were successfully converted into the corresponding formamides and methylated using this catalyst with methanol as a potential formylation reagent. Compared with the high-valence Cr-based catalyst including CrO3 and K2Cr2O7, etc. This one with low-valent chromium catalyst is more effective, safe, and green environmental [45]. Wei then reported another work using (NH4)3[Fe(OH)6Mo6O18] catalyzed formic acid and amine coupling to generate formamide in mild conditions [46]. In the same year, the

Xu group also developed Fe-based Anderson-type POMs, Na3Fe(OH)6Mo6O18·5H2O to prepare for 5-formyl-2-furanformic acid from 5-hydroxyl methylfuranic acid. The related results also further proved that the catalyst has high catalytic activity under an aerobic environment [47].

In 2020, Wei and co-worker proposed Co-based Anderson-type POMs (NH4)3[Co(OH)6 Mo6O18]. In the presence of this catalyst, it is effective in achieving from alcohol to esters when KCl is used as an additive and 30% H2O2 as oxidant (Figure 8). As the important complexes in the fields of biology, medicine, and fine chemicals, esters are usually prepared by carboxylic acids or active derivatives (acyl chlorine and anhydride) reacted with alcohol. The Co-based Anderson-type POMs catalytic system is cheap, stable, safe and efficient and can successfully make alcohol to esters under mild conditions. They first explored the substrate range of benzyl alcohol and fatty alcohol oxidation coupled with methanol to generate methyl esters. The results demonstrate that various substituted benzyl alcohols as well as fatty alcohols were performed efficiently and highly selectively; the highest yield can reach 99%. Subsequently, on the basis of the above work, they further extended the catalytic system to the esterification reaction of benzyl alcohol and fatty alcohol with other alcohols, the corresponding ester products were also obtained. More importantly, the natural product octranactone can also be prepared by this method with 66% yield. In this catalytic system, the experiment shows that the additive can significantly enhance the selectivity and activity of the catalytic reaction system. The single-crystal X-ray diffraction test showed that the chloride ion in the additive linked with CoMo6 forming a supramolecular dimer 2 (CoMo6Cl) served as the key catalytic intermediate, which then achieves alcohol-to-ester conversion through nucleophilic addition and β-H elimination. The catalyst enables to achieve an oxidation coupling reaction of various alcohols (aromatic and aliphatic) under mild conditions and produce the corresponding esters with high yield [48]. This work shows that Anderson-type POMs, as an important class of single-metal ion inorganic molecule carriers, are an excellent and environmentally friendly catalyst with great potential applications in organic reactions. As a multifunctional catalyst, it changes the structure and properties of the central metal, so that Anderson-type POMs show excellent catalytic activity and extremely high catalytic efficiency in the green catalytic oxidation of alcohol.

**Figure 8.** The supramolecular structure of dimer 2(CoMo6Cl) and its catalytic route.
