**4. Conclusions**

The synthesis of Ge-V-O clusters, especially secondary metal substituted Ge-V-O clusters is still a great challenge for chemists. In this manuscript, we synthesized compounds **1** and **2**, which are the first examples formed by Ge-V-O clusters and transition metal complexes of aromatic organic ligands. Compounds **1** and **2** are also the first secondary metal substituted Ge-V-O clusters of aromatic organic ligands. Compound **3** is a novel 3-D framework with interesting channel structure. The catalytic properties of these compounds and two previously reported compounds have been investigated. We plan to apply these compounds in other oxidation catalytic reactions and hope to find applications of them in electrochemistry as well.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules27144424/s1; Figure S1: IR spectra of compounds **1**–**4**; Figure S2: Simulated and experimental XRD patterns of compounds **1**–**4**; Figure S3: UV-Vis spectra of compounds **1**–**4**; Figure S4: EPR spectra of compounds **1**–**4**; Figure S5: Simulated, experimental XRD patterns and XRD patterns after three cycles of compounds **3**; Figure S6: FT-IR spectrum of compound **3** and FT-IR spectra of compound **3** after three cycles; Table S1: Bond valence sum calculations for Ge, V and O in compounds **1**–**3**; Table S2: Comparison of the catalytic performances of our compounds and other reported POMs [83,84]. References [85,86] are mentioned in Supplementary Materials.

**Author Contributions:** X.-B.C. and H.-Y.G. conceived the research and designed the experiments. H.-Y.G. performed the experiments. X.-B.C. and X.Z. analyzed and interpreted the data. X.-B.C. and H.Q. wrote and revised the article. All authors contributed to the final approval of the article. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Jilin Provincial Department of Science and Technology (Grant 20190802027ZG) and A Project Supported by the Scientific Research Fund of Zhejiang Provincial Education Department, grant number 00321JYT01AL.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Date is available form Corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are not available from the authors.

#### **References**

