**1. Introduction**

Polyoxometalates (POMs), as anionic metal-oxide clusters with diverse nuclearities, elemental compositions and physicochemical properties, have usually been constructed through the self-assembly of reactive oxometallate precursors in aqueous or organic reaction systems. [1–4] POMs can serve as crucial intermediates in the reaction pathway from water-soluble metal ions to insoluble metal oxides, and isolation of these molecular intermediates enable insightful elucidation on the formation mechanism and control over reaction pathways. POMs exhibit special characteristics of high negative charges, rich redox properties, good thermal stability, and readily available organic grafting [5,6], leading to wide applications in catalysis [7], magnetism [8], material science [9], electrochemistry [10], luminescence [11], etc.

As an important derivative of plenary POMs, lacunary POMs can be easily formed by removing one to several [MO6] (M = Mo, W) building blocks from prototypal architectures such as the Keggin or Wells–Dawson type POMs [12]. These lacunary POMs usually show high coordination reactivity and oxidative and thermal stability. Their high negative charge and nucleophilic oxygen-enriched surfaces render them suitable inorganic, diamagnetic, multidentate nucleophilic ligands toward the electrophilic center. Transition metals (TM) or lanthanide (Ln) cations can be easily incorporated into the defect sites of lacunary POM ligands to construct metal-substituted POMs, which can exhibit unique physicochemical properties depending on the types of incorporated metal ions [13–20]. Metal-substituted POMs (MSPs) typically possess a higher negative charge density than that of the plenary POMs due to the substitution of a high oxidation state M6+ ion (e.g., W6+, Mo6+) with a low oxidation state Mn+ ion (usually *n* = 1–3) [21]. To date, a wide variety of MSPs have been prepared, especially by the transition metals like manganese, iron, cobalt, nickel, copper and zinc in the fourth period and the lanthanides in the sixth period of the periodic table [22,23]. In contrast, the research on the syntheses of titanium- and zirconium-substituted POMs is still in a very early stage, which could be mainly attributed to the following two reasons: (a) the easy hydrolysis of Ti4+/Zr4+ salts in

**Citation:** Ni, Z.; Lv, H.; Yang, G. Recent Advances of Ti/Zr-Substituted Polyoxometalates: From Structural Diversity to Functional Applications. *Molecules* **2022**, *27*, 8799. https://doi.org/ 10.3390/molecules27248799

Academic Editor: Xiaobing Cui

Received: 20 November 2022 Accepted: 9 December 2022 Published: 12 December 2022

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aqueous synthesis, and (b) the high tendency to formation of isolate oligomeric structures through intermolecular dehydration of terminal hydroxyls.

In this review, we have mainly focused on the structural diversities of Ti/Zr-substituted POMs according to the polymerization number of POM building blocks and the number of titanium or zirconium atoms. The representative catalytic application of Ti/Zr-substituted POMs has also been discussed. Finally, a perspective of this research area is also proposed. It is expected that this review could provide research insights into the controllable design and syntheses of Ti/Zr-substituted POMs derivatives with interesting catalytic properties.
