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.

Water-soluble titanium-polyoxomolybdate with external μ3 bridging oxygen coordination on a lacunary Keggin structure

The first water-soluble titanium-containing polyoxomolybdate was synthesized and structurally characterized, showing photocatalytic activity in benzaldehyde oxidation.

Synthesis, X-ray Crystal Structure, and Photochromism of a Sandwich-Type Mono-Aluminum Complex Composed of Two Tri-Lacunary α-Dawson-Type Polyoxotungstates

The synthesis and molecular structure of a dimeric, mono-aluminum complex composed of two tri-lacunary α-Dawson polyoxometalates, [H₁₄Al(B-α-P₂W₁₅O₅₆)₂]^7- (1), is described herein. The tetra-n-butylammonium salt of 1, [(n-C₄H₉)₄N]₇[H₁₄Al(B-α-P₂W₁₅O₅₆)₂] (TBA-1) was prepared by passing an aqueous solution of K₆[B-α-H₃P₂W₁₅O₅₉{Al(OH₂)}₃]⋅14H₂O through an ion-exchange resin column (H⁺-form), followed by addition of tetra-n-butylammonium bromide. Analytically pure and colorless crystals of TBA-1 were obtained via vapor diffusion from acetonitrile/methanol at ~25 °C. Single-crystal X-ray structure analysis revealed that a six-coordinate aluminum ion was sandwiched between two tri-lacunary α-Dawson-type units, resulting in an overall C_2h symmetry. The characterization of TBA-1 was accomplished by elemental analyses, thermogravimetric/differential thermal analyses, Fourier-transform infrared spectroscopy, and solution ³¹P nuclear magnetic resonance spectroscopy. The photochromic properties of TBA-1 were also characterized in methanol under light irradiation (λ = 365 nm and ≥400 nm).

Hybrid dimers based on metal-substituted Keggin polyoxometalates (metal = Ti, Ln) for cyanosilylation catalysis

Six hybrid dimers 1–6 constructed from [PW₁₀Ti₂O₄₀]⁷⁻/[PW₁₁LnO₃₉]⁴⁻ and lanthanide–organic units are reported, which as heterogeneous catalysts show high activity for cyanosilylation under solvent-free conditions.

Organic–inorganic hybrid assemblies based on Ti-substituted polyoxometalates for photocatalytic dye degradation

New organic–inorganic hybrid compounds based on Ti-substituted polyoxometalates exhibit high photocatalytic dye (MB) degradation activities.

Cyclic oligomer of oxide clusters through a siloxane bond. Synthesis and structure of reaction products of alpha2-mono-lacunary Dawson polyoxometalate with tetrachlorosilane and tetraethoxysilane

The title compound with the formula (Me2NH2)13H5[{alpha2-P2W17O61(Si2O)}3(micro-O)3].39H2O Me2NH(2)-1 was obtained in 12.8% (0.258 g scale) yield by a 1:2 molar ratio reaction of alpha2-mono-lacunary Dawson polyoxometalate (POM) [alpha2-P2W17O61]10- with SiCl4 in water and unequivocally characterized by complete elemental analysis, TG/DTA, FTIR, (solution and solid-state) 31P NMR, solid-state 29Si NMR and X-ray crystallography. [Note: the moieties of the polyoxoanions in Me2NH(2)-1 and Me2NH(2)-2 are abbreviated simply as 1 and 2, respectively.] X-Ray crystallography revealed that the molecular structure of polyoxoanion 1 was a cyclic trimer consisting of three alpha2-mono-lacunary Dawson POM subunits constituted through intra- and intermolecular siloxane bonds. The compound resulted in an approximate Cs symmetry, but not the idealized C3v symmetry, because one Dawson unit was unequivalently incorporated into the cyclic framework. This compound is the first example of a siloxane-bonding cyclic trimer of POMs and its formation suggests that the oxygen atoms in the lacunary site of POM are reactive just like the surface oxygen atoms of silica. The formation of the cyclic trimer was also clarified by two control experiments: (1) the reaction of [alpha2-P2W17O61]10- with Si(OEt)4 under acidic conditions (pH 1), and (2) oligomerization under 6 M aqueous HCl acidic conditions (pH approximately 0) of the reactive monomer units (RMUs: tentatively assigned as "[alpha2-P2W17O61{O(SiOH)2}]6-"), derived from the cleavage of Ph-Si bonds by the thermal degradation of [alpha2-P2W17O61{O(SiPh)2}]6-. On the other hand, the reactions of alpha2-mono-lacunary Dawson POMs with GeX4 (X=OEt, Cl) gave the monomeric species of mono-germanium-substituted Dawson POM (Me2NH2)7[alpha2-P2W17O61(GeOH)].5H2O Me2NH(2)-2, which was characterized by CHN analysis, TG/DTA, FTIR, X-ray crystallography, and (solid-state and solution) 31P NMR spectroscopy. X-ray crystallography revealed that compound Me2NH(2)-2 was a monomeric species, but the Ge site per formula unit was not determined due to disorder of alpha-Dawson structure.