*2.1. Synthesis of Compounds* **1** *and* **2**

In the presence of the triol ligand, V2O5 and Na2MoO4·2H2O were added to water, which resulted in the formation of a light brown solution under heating conditions. Crystals of compounds **1** and **2** were obtained by standing the solutions for a week. Several factors are supposed to play important roles in the successful synthesis of compounds. Firstly, the molar ratio of V/Mo in the reactants is one such factor. Only when the molar ratio of V/Mo is in the range of 5:2 and 5:3 can the targeted products be obtained. If a higher percentage of V exists in the reactants, a triol-ligand-modified hexavanadate cluster is obtained, which has the same architecture as that reported in the literature [25]. If reactants with lower percentages of V are used, some VMo clusters are obtained and most of them cannot provide high-quality crystals for further accurate determination of their structures. Secondly, the control of pH is another essential factor. When the pH of the reaction solution is controlled between 4–5, the quality of the crystals is good for giving high-quality X-ray diffraction data for analysis. If the pH decreases to 3–4, the quality of the crystals becomes poor and is not very stable in air, which quickly degenerates into powder after the crystals leave the mother liquid due to weathering. In a more acidic environment with a smaller pH value, crystals cannot be obtained. On the other hand, when pH is higher than 5, the solubility of the reactants in the water becomes poor, and nearly no reaction occurs. In addition, the amount of triol ligand seems to have so little influence on the product that excessive organic species cannot change its number on the cluster, and only one triol ligand remains attached to the polyanion. On the other hand, the presence of the triol ligand is the essential factor for the formation of the present architecture. We have also conducted experiments under similar conditions without a triol ligand, and as a result, only some VMo species with usual structures were obtained. All these factors indicate that the compounds presented here are only successfully synthesized under relatively harsh conditions, which need good modulation to the reactions.
