**4. Conclusions and Perspectives**

In summary, this review has mainly addressed the development of Ti/Zr-substituted POMs with an emphasis on structural diversity, synthetic approaches, and potential catalytic applications. According to the overview of these reported Ti/Zr-substituted POMs, we can conclude that (1) the solution-based synthetic approach is an effective method for the syntheses of Ti/Zr-substituted POMs; (2) interesting and attracting POM structures could be often obtained via the hydrothermal synthetic strategy. These successful synthetic strategies and the persistent and dedicated efforts of chemists over the past decades have greatly contributed to the vast and beautiful array of Ti/Zr-substituted POMs. However, there are still bottlenecks in Ti/Zr-substituted POM chemistry with respect to the controllable assembly of target POM structures, the exploration of novel synthetic approaches, as well as the insightful understanding of catalytic mechanisms using Ti/Zr-substituted POM catalysts. Therefore, we believe that the development of other synthetic methods, for instance, the mixed solvent diffusion method, ionothermal approach, templated modular assembly method or the combination with existing solution/hydrothermal approaches, would provide new blood to the synthetic chemistry of Ti/Zr-substituted POMs. Moreover, the ground-breaking exploration of new catalytic functionalities of these Ti/Zr-substituted POMs should also be strengthened in the future. Finally, we hope this critical review could provide research insights into the controllable design and syntheses of Ti/Zr-substituted POMs derivatives and, in the meantime, attract more researchers to join the research community of POM Chemistry or the related interdisciplinary research areas.

**Author Contributions:** Literature research, Z.N.; writing—original draft preparation, Z.N.; writing review and editing, H.L. and G.Y.; funding acquisition, Z.N., H.L., and G.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** Project supported by Key Projects of Science and Technology of Henan Province (No. 212102210442), the National Natural Science Foundation of China (Nos. 21871025 and 21831001).

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

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** The authors would like to express their gratitude to Cambridge Crystallographic Data Centre (CCDC) for providing access to the CSD Enterprise.

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

#### **References**

