Recent Advances of Ti/Zr-Substituted Polyoxometalates: From Structural Diversity to Functional Applications

Polyoxometalates (POMs), a large family of anionic polynuclear metal-oxo clusters, have received considerable research attention due to their structural versatility and diverse physicochemical properties. Lacunary POMs are key building blocks for the syntheses of functional POMs due to their highly active multidentate O-donor sites. In this review, we have addressed the structural diversities of Ti/Zr-substituted POMs based on the polymerization number of POM building blocks and the number of Ti and Zr centers. The synthetic strategies and relevant catalytic applications of some representative Ti/Zr-substituted POMs have been discussed in detail. Finally, the outlook on the future development of this area is also prospected.

Roles of Organic Fragments in Redirecting Crystal/Molecular Structures of Inorganic-Organic Hybrids Based on Lacunary Keggin-Type Polyoxometalates

Lacunary polyoxometalates (LPOMs) are key precursors for the synthesis of functional POMs. To date, reviews dedicated to behavioral studies of LPOMs often comprise the role of metal ions, including transition metal (TM) and rare earth (RE) ions, in extending and stability of high-nuclearity clusters. In contrast, the role of organic ligands in the structures and properties of lacunary-based hybrids has remained less explored. In this review, we focus on the role of organic fragments in the self-assembling process of POM-based architectures and discuss relationships between the nature and structure of organic ligand and properties such as the topology of hybrid inorganic–organic material in RE and TM-RE heterometallic derivatives of lacunary Keggin-type POMs. The effects of organic fragment in mixed ligand hybrids are also briefly reviewed.

Three ring-shaped Zr(iv)-substituted silicotungstates: syntheses, structures and their properties

Three Zr(iv)-substituted polyoxometalate nanoclusters, H₁₆(H₂dap)₄[{Zr₂(OH)₂(α-SiW₁₀O₃₈)}₂{Zr₂(OH)₂(β-SiW₁₀O₃₈)}₂]·8H₂O (1, dap = 1,3-diaminopropane), H₁₇Na₉[(Zr₂(OH)₂)₂(Zr₂BO(OH)₄)₂(β-SiW₁₀O₃₈)₄]·50H₂O (2) and H₂₀Na₈[(Zr₂BO(OH)₄)₂(Zr₂B₂O₂(OH)₅)₂(β-SiW₁₀O₃₈)₄]·40H₂O (3), were made by a hydrothermal reaction of trivacant [A-α-SiW₉O₃₄]^10-, Zr⁴⁺ ions, and H₃BO₃ under different alkaline conditions. Single-crystal X-ray structure analysis revealed that they are ring-shaped polyoxotungstate aggregates built by Zr₂(OH)₂ ({Zr₂}) clusters and two types of dilacunary α-/β-SiW₁₀O₃₈ ({α-/β-SiW₁₀}) fragments for 1, {Zr₂}, {β-SiW₁₀}, and Zr₂BO(OH)₄ ({Zr₂B}) clusters for 2, and {β-SiW₁₀}, {Zr₂B}, and Zr₂B₂O₂(OH)₅ ({Zr₂B₂}) clusters for 3, respectively, showing that the Zr-B-O clusters, {Zr₂B} and {Zr₂B₂}, have been introduced to the lacunary sites of polyoxometalates for the first time. The electrochemistry and electrocatalytic properties of 2 were studied, and it exhibited effective catalytic activities for the reduction of H₂O₂, NO₂^-, and BrO₃^-. Moreover, the catalysis of 2 for the oxidation reactions of various thioethers was investigated, and it was found that 2 possessed high conversion and remarkable selectivity.

Syntheses, Structures, and Electrochemical Properties of Three New Acetate-Functionalized Zirconium-Substituted Germanotungstates: From Dimer to Tetramer

Under hydrothermal conditions, three new acetate-functionalized zirconium-substituted polyoxometalates, H₄Na₂[Na₆(H₂O)₂₂][Zr₄(μ₃-O)₂(OH)₂(OAc)₂(α-GeW₁₀O₃₇)₂]·32H₂O (1), H₈K₃Na₅[Zr₆(μ₃-O)₃(OH)₃(OAc)(H₂O)(β-GeW₁₀O₃₇)₃]·20H₂O (2), H₆K₄Na₁₂[{Zr₅(μ₃-OH)₄(OH)₂}@{Zr₂(OAc)₂(α-GeW₁₀O₃₈)₂}₂]·22H₂O (3), were synthesized and characterized. In 1, the sandwiched dimer [Zr₄(μ₃-O)₂(OH)₂(OAc)₂(α-GeW₁₀O₃₇)₂]^12- was linked to a hexameric [Na₆(H₂O)₂₂]⁶⁺ cluster to form a 1D chain. In 2, the trimer was constructed from three [β₂-GeW₁₀O₃₇]^10- units and a new [Zr₆O₃(OH)₃(OAc)(H₂O)]¹⁴⁺ cluster. In 3, the tetramer was built by two novel sandwich-type dimers [Zr₂(OAc)₂(α₂-GeW₁₀O₃₈)₂]^18- and one unique [Zr₅(μ₃-OH)₄(OH)₂]¹⁴⁺ core in an approximately orthogonal fashion, showing a staggered tetrahedral polyanion. Also, the electrochemistry and electrocatalytic properties of 3 were studied, exhibiting good catalytic activities toward the reduction of H₂O₂, BrO₃^-, and NO₂^-.

Zr4-Substituted polyoxometalate dimers decorated by d-tartaric acid/glycolic acid: syntheses, structures and optical/electrochemical properties

Two Zr₄-sandwiched Keggin polyoxometalates functionalized by chiral d-tartaric acid and glycolic acid were hydrothermally synthesized and structurally characterized. Their optical and electrochemical properties have been investigated in detail.