**4. Conclusions**

In summary, two novel vanadium-containing Keggin-type PONbs modified by a Ni-complex have been successfully synthesized by controlling temperature and the vanadium source. In **1**, the five-capped {PNb12O40(VO)5} were connected by [Ni(en)2] 2+ into a 3D framework; in **2**, the discrete bicapped {PNb12O40(VO)2} was isolated with [Ni(en)3] 2+ as counter cations. Compounds **1** and **2** as organic-inorganic hybrid materials exhibit good pH stability in aqueous solution and thermal stability in organic solvent. Importantly, under alkaline additive free conditions, compounds **1** and **2** are highly active for the selective oxidation of BA. The control experiments show that both the Brønsted basicity of PONb and the redox activity of the V caps play an important role in the electrocatalytic process. This study not only enriches the structure data base of vanadium-containing PONbs but also extends their catalytic application.

**Supplementary Materials:** CCDC 2164660 (**1**), 2164659 (**2**) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac. uk/conts/retrieving.html, accessed on 13 April 2022. The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules27092862/s1, Table S1: Crystal data and structure refinement for **1** and **2**; Tables S2–S5: Selected bond lengths and bond angles for **1** and **2**; Tables S6 and S7: BVS results of Ni and V atoms for **1** and **2**; Table S8: Electrocatalytic oxidation of benzyl alcohol (BA) catalyzed by **1** in the presence of radical scavengers; Figure S1: Digital photographs of **1** and **2**; Figure S2: the IR spectra of **1** and **2**; Figure S3: the PXRD patterns of **1** and **2**; Figure S4: The TG curves of **1** and **2**; Figure S5: The XPS spectra for Ni(2p) and V(2p) in **1**; Figure S6: The XPS spectra for Ni(2p) and V(2p) in **2**; Figure S7: The IR spectra of **1** and **2** after being soaked in aqueous solutions with different pH values for 24 h; Figures S8 and S9: The PXRD patterns and IR spectra of **1** and **2** after being soaked in different solvents; Figures S10 and S11: The PXRD patterns and IR spectra of **1** and **2** after heating in acetonitrile for 2 h; Figure S12: Gas chromatogram of the benzyl alcohol oxidation by **1**; Figure S13: The gas chromatograph of benzyl alcohol oxidation before and after reaction; Figure S14: GC-MS spectrum of the by-product; Figure S15: CO2-TPD for **1** and **2**; Figure S16: CV curves of **1** and **2** at different scan rates; Figure S17: The IR spectra of **1** after the recycle test; Figure S18: UV-vis spectra of postreaction solution, BA, benzyl aldehyde, and PNb12V2; Figure S19: Proposed mechanism for the electrocatalytic selective oxidation of BA to benzaldehyde by **1**.

**Author Contributions:** Conceptualization, Y.C. and C.H.; methodology, Y.C. and C.H.; software, N.Z., C.L. and D.Z.; validation, X.L.; formal analysis, X.L., N.Z. and C.L.; investigation, X.L. and N.Z.; resources, Y.C., C.H. and J.D.; data curation, X.L., N.Z. and J.D.; writing—original draft preparation, X.L.; writing—review and editing, N.Z., C.L. and Y.C.; funding acquisition, Y.C. and C.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was financially supported by the National Natural Science Foundation of China (Nos. 21971010, 21871026, and 22101012).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are contained within the article and supplementary materials.

**Acknowledgments:** The authors thank the Analysis and Testing Center, Beijing Institute of Technology, for device preparation and characterization.

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

**Sample Availability:** Samples of compounds **1** and **2** are available from the authors.

#### **References**

