2.2.2. [O]+PO

In 2017, the Yu group developed the first organic modified hybrid Fe-based Andersontype POMs [N (C4H9)4]3[FeMo6O18(OH)3{(OCH2)3CNH2}] for the aerobic oxidation of aldehydes to acids in water (Figure 9) [50] with O2 (1 atm). They found that the alkalinity of additives played a great impact on yield. Benzaldehyde, which was used as substrate, reacted with different additives at 50 ◦C. When Na2CO3 (pKb = 3.67) was used as additive, the yield of benzoic acid was 95%. When NaHCO3 (pKb = 7.95) was added in the system, the yield of benzoic acid decreased to 63%. However, the additive replaced by Na2SO3 (pKb = 6.8) the yield reduced to 26%. While Na2SO4(pKb = 12.0) was chosen as additive, the lowest yield of benzoic acid was 5%. For organic alkaline additives, such as CH3COONa and Et3N, the yield of benzoic acid was 83% and 89%, respectively. Regarding additives containing neutral salts, such as NaBr, KCl, and NaCl, the desired product was obtained in moderate yield. The acid additive, NH4Cl, avoided the process reaction. These results indicate that the additive to Fe-based Anderson-type POMs, FeIIIMo6, has a great effect on the activity of the catalyst, perhaps because of the high tunability of the acid-based properties of POMs. Subsequently, they also studied the effect of the catalyst dosage and the reaction temperature on catalytic activity. Changing the amount of the catalyst has little effect on the yield of the product. The yield of benzoic acid reached the maximum at 50 ◦C, but increasing or decreasing the temperature showed the different result. When the reaction temperature increased to 70 ◦C, the yield decreases to 96%, possibly due to reduced contact surface between the oxygen (gas phase) and the catalyst. When the reaction was performed in a nitrogen sphere, the yield of the obtained products was 5% less. As a control experiment, the experiments conducted without FeIIIMo6, only a small amount of product generation can be detected. This Fe-based Anderson-type POMs system was carried out with oxygen as the sole oxidant under extremely mild aqueous solution conditions, and suitable to a variety of functional group aldehydes. This method is environmentally friendly, has a low cost and high catalytic efficiency, as well as the potential application in industry.

The Wei group and co-worker also found that halogen ions (X−) can be incorporated with sub-nanoscale organoalkoxyl ligands-modified Al-based Anderson-type POMs([(n-C)4H9)4N]3{AlMo6O18(OH)3[(OCH2)3CCH3]}) in solution, forming the stable supramolecular complex with the binding constant K = 1.53 × 103. This system was used in oxidation to aldehydes from alcohol. Crystal structure analysis demonstrated this behavior that binding occurs between the halogen ion X<sup>−</sup> and the unmodified three hydroxyl groups on the surface of {AlMo6O18(OH)3[(OCH2)3CCH3]}<sup>3</sup>−, forming multiple X···H-O [51]. The interaction in this supramolecular sub-nano-cluster system means that its catalytic activity for oxidation of aldehydes can be adjusted by introduction of halogen–halogen ions and water. Chlorine ions inhibit by blocking the active center of the cluster, and the catalytic activity of the cluster can be reactivated by replacing the chloride with water supra-molecules. The result shows that the presence of multiple synergistic hydrogen bonds is key to overcome the electrostatic repulsion between halogen ions and Al-based Anderson-type POMs. This work not only enriches the supramolecular chemistry of Anderson-type POMs, but also provides a new way of selective catalysts for oxidation reactions.

**Figure 9.** Catalyst anion structure and the catalytic reaction route.
