Self-assembly of a giant molybdenum titanium-oxo cluster [Mo₄₂Ti₁₂(O₂)₂₄] for bifunctional oxidation catalysis

Abstract

The exploration of high-nuclearity molecular molybdenum titanium-oxo clusters (MoTOCs) and their reactivity is a great challenge for polyoxometalate chemistry and materials science. Herein, we report a giant MoTOC [K₈(H₂O)₈][Ti₁₂(O₂)₆(OH)₁₂Mo₄₂O₁₂₄(O₂)₁₈(H₂O)₁₇]·31H₂O (1) by self-assembly of degraded lacunary isopolymolybdate fragments and peroxide-stabilized titanium ions in aqueous solution. Compound 1 features 54 metal centers and a rare pure inorganic triangular prism structure with a size of 1.8 × 1.8 × 1.5 nm³, which is the first and largest water-soluble MoTOC found to date. More importantly, it contains multiple peroxo groups on the surface, which makes it exhibit superior benzyl alcohol/benzaldehyde (photo)catalytic oxidation performance. This work opens an unusual avenue for the synthesis of giant MoTOCs.

---

Introduction

Polyoxometalates (POMs) are well-established metal-oxo cluster anions composed of early transition metals (W^VI, Mo^VI, V^V, Nb^V, Ta^V), which have unique physical–chemical properties and far-ranging applications in catalysis, magnetism, medicine, electro-optics, nanotechnology, and materials science.^1–7 They can serve as excellent inorganic multi-dentate ligands to incorporate appropriate organic molecules or various metal ions to construct high-nuclear POM-based functional materials due to the well-defined structures, sizes, compositions, nucleophilic O-enriched surface, and rich electronic properties.^8–10 Currently, the high-nuclear polyoxomolybdates chemistry is particularly impressive, which is heavily dominated by organic-ligand functionalization and giant molybdenum blues or browns.^11–14 Although the successful development of high-nuclear polyoxomolybdates has been achieved, metal-substituted high-nuclear polyoxomolybdates are much less developed because of the poor stability in aqueous solution and difficulty in isolating lacunary polyoxomolybdate species.^15–17 The discovery of new metal-substituted high-nuclear polyoxomolybdate architectures and understanding the assembly mechanism in an attempt to achieve controllable preparation of targeted products with expected structures or certain properties have long been an attractive but challenging research field.

The introduction of metals into polyoxomolybdates can construct abundant heterometallic high-nuclear POMs with various compositions and structures, which can result in diverse functionalities depending on the structure and the type of introduced metal cation.^18–20 Titanium-containing polyoxomolybdates are extremely important due to their good application prospect in catalysis.^21–23 To date, some intriguing molybdenum titanium-oxo clusters (MoTOCs) have been reported, but the known examples are limited to small nuclearities, including {Mo₂Ti₂}, {Mo₅Ti}, {Mo₄Ti₄}, {Mo₄Ti₆}, {Mo₆Ti₄}, and {Mo₄Ti₈} (Fig. 1 and Table S1†),^24–29 and all of them are constructed from titanium ions bridged by oxo bridges from alkoxides or carboxylates in organic solvents because titanium ions are prone to fast and spontaneous hydrolysis in water and generate an uncontrolled amorphous TiO₂ precipitate. Actually, the lacunary POM building blocks (BBs) worked well in the construction of novel POM structures (Table S2†).^30–33 However, due to the lack of stable lacunary polyoxomolybdate fragments, the synthesis of new-type MoTOCs, especially giant high-nuclear MoTOCs via a green aqueous route, is a great challenge for POM chemistry. Fortunately, we recently found that lacunary polyoxomolybdate intermediate BBs could be effectively obtained by degrading saturated polyoxomolybdates under the specific pH conditions and reported the first water-soluble titanium-containing heteromolybdate anion [{PMo₉O₃₄TiO}₂]¹⁴⁻.³⁴ Meanwhile, we considered that titanium ions can be effectively protected through peroxide ions since the introduction of peroxy groups will increase the number of surface oxygen atoms and lower the surface charge density, but this concept has not been used in MoTOC architecture design.^35–37

Fig. 1 Polyhedral structures of the reported representative molybdenum titanium-oxo clusters.

Herein, we report a rare and giant high-nuclear triple-prismatic shaped MoTOC, [K₈(H₂O)₈][Ti₁₂(O₂)₆(OH)₁₂Mo₄₂O₁₂₄(O₂)₁₈(H₂O)₁₇]·31H₂O ({Mo₄₂Ti₁₂}) (1), by the reaction of (NH₄)₆Mo₇O₂₄ with TiCl₄ under the assistance of hydrogen peroxide (H₂O₂) under green and mild conditions (Fig. S1†). As expected, (NH₄)₆Mo₇O₂₄ degraded into lacunary polyoxomolybdate fragments under the specific pH conditions and coordinated with peroxide-stabilized Ti⁴⁺ to form the high-nuclear MoTOC 1. With 54 metal centers, 1 is by far the first and largest water-soluble MoTOC. A fascinating feature of 1 is that 24 peroxo groups are grafted on 18 Mo and 12 Ti atoms simultaneously, which enables 1 to serve as an excellent catalyst for the effective conversion from benzyl alcohol to benzaldehyde to benzoic acid. Moreover, the assembly mechanism of 1 has also been described, which can guide the synthesis of novel giant MoTOC structures to achieve function-oriented structural assembly.

Results and discussion

Single-crystal X-ray diffraction (SCXRD) analysis revealed that compound 1 crystallized in the monoclinic P2₁/n space group and presented an unprecedented triangular prism-shaped pure inorganic polyperoxo-titanmolybdate giant cluster with nanoscale sizes of ca. 1.8 × 1.8 × 1.5 nm³ (Fig. S2, S3 and Table S3†). The structure of 1 is composed of three octagonal [Mo₁₂Ti₄(O₂)₈O₄₀]ⁿ⁻ (denoted as {Mo₁₂Ti₄}) BBs and three binuclear BBs [Mo₂O₁₁]ⁿ⁻ (denoted as {Mo₂}) (Fig. 2a–c). The {Mo₁₂Ti₄} BB is built up by two [Mo₅Ti₂O₂₆]ⁿ⁻ (denoted as {Mo₅Ti₂}) BBs and two [Mo(O₂)O₅]ⁿ⁻ (denoted as {Mo₁}) BBs sharing four edges and six μ₃-O vertices, showing C_2V symmetry. Different from the most reported MoTOCs, such as {Mo₄Ti₆} and {Mo₄Ti₈}, the structure of 1 does not require extra organic ligands to maintain structural integrity and stability. The K⁺ ions connect with the terminal and bridging oxygen atoms of 1 and the K–O bonds act as linkers to form a 3D conformation (Fig. S4–S6†). Bond valence sum (BVS) calculations (Tables S4 and S5†) indicate that the oxidation states of all Mo and Ti centers are +6 and +4, respectively.

Fig. 2 Crystal structure of Mo₄₂Ti₁₂. Ball-and-stick and polyhedron views of the (a) {Mo₄₂Ti₁₂}, (b) {Mo₁₂Ti₄}, (c) {Mo₂}, {Mo₁}, {Mo₅Ti₂} and (d) {Mo₇}, {Mo₅}, {Mo₅Ti₂} building blocks. Color code: blue ball, Mo; sky blue ball, Mo; green ball, Ti; red ball, O.

{Mo₁₂Ti₄} as the core structure includes four different Mo⁶⁺ ions’ coordination environments apart from the symmetrical unit, such as six-coordinated Mo-1/Mo-3 and seven-coordinated Mo-2/Mo-4 (Fig. S7 and S8†). In addition, eight Mo atoms are connected to Ti atoms by μ₃-O (the bond angles of Mo–O–Ti are in the range of 97.2(5)°–110.5(5)°) and four Mo atoms are connected to Ti atoms by μ₄-O (the bond angles of Mo–O–Ti are in the range of 148.8(7)°–163.0(7)°). The Mo⋯O and O_peroxo⋯O_peroxo bond lengths are in the range of 1.591(15)–2.515(11) Å and 1.162(8)–1.216(8) Å, respectively. All Ti ions displayed a seven-coordinate mode due to the presence of the μ₂-peroxo bond between Ti ions. It is worth noting that the known examples of Ti-containing molecules/clusters with μ₂-peroxo bonds are still quite a few, with representative examples such as the recently reported [Ti₂₀(μ-O)₈(HO₂)₈(O₂)₁₂(R,R-tart)₁₂]¹⁶⁻ complex.³⁸ An interesting feature is that the four neighboring Ti atoms are linked by four μ₃-O and two μ₂-peroxo groups to form a {Ti₄(O₂)₂O₄} (denoted as {Ti₄}) core (Fig. S9†). The core structure of such {Ti₄} BB is similar to that of [Ti₄(C₂H₂O₃)₄(C₂H₃O₃)₂(O₂)₄O₂]⁶⁻ (denoted as {Ti₄-1}), which was reported by Kakihana.³⁹ Compared to {Ti₄-1}, the Ti1 and Ti3 atoms of {Ti₄} are connected with the Mo4 atom by the O1 atom (μ₃-O), which further shortens the bond angle Ti1–O1–Ti3 of the {Ti₄} core from 155 to 147°. The biggest difference between the two structures is that the O2′⋯O3′ bond length of {Ti₄-1} is 2.38 Å, but the O2⋯O3 bond length of the {Ti₄} core is 1.46 Å, which confirmed that the μ₂-peroxo bond existed in the Ti1 and Ti2 atoms.

Combining detailed crystal structure analysis and specific experimental steps (see the ESI†), the assembly mechanism of 1 is proposed. Firstly, the saturated (NH₄)₆Mo₇O₂₄ lost two Mo ions and degraded into [Mo₅O₂₁]ⁿ⁻ (denoted as {Mo₅}) BBs in the alkaline aqueous solution containing Ti ions and H₂O₂ (Fig. 2d). The formed lacunary sites were automatically replenished by two Ti⁴⁺ in aqueous solution to obtain {Mo₅Ti₂} BBs. Meanwhile, the lost Mo ions formed two types of BBs, {Mo₁} and {Mo₂} (Fig. S10†). Two {Mo₅Ti₂} BBs are further combined with two {Mo₁} BBs to dimerize into {Mo₁₂Ti₄} BBs (Fig. S11†). In the end, every two {Mo₁₂Ti₄} BBs were connected by one {Mo₂}, ultimately forming the {Mo₄₂Ti₁₂} structure (Fig. S12†). Such assembly mechanisms may provide opportunities and guidance for the synthesis of more new giant MoTOCs with special functions.

The Fourier-transform infrared (FT-IR) spectrum was first used to identify the multiple chemical bonds from different BBs. It is noted that the strong band at 880 cm⁻¹ is attributed to the stretching vibrations of peroxo groups (Fig. S13†).⁴⁰ The phase purity of 1 was further confirmed by powder XRD analysis (Fig. S14†), which proved that the giant high-nuclearity 1 is stable under ambient conditions. Moreover, 1 is water-stable, which can be proved by UV–vis spectra (Fig. S15†). The energy dispersive X-ray (EDX) spectra and elemental mappings of 1 confirmed the existence and uniform distribution of Ti, Mo, O and K (Fig. S16 and 17†). The full-scan X-ray photoelectron spectrum (XPS) further revealed the existence of these elements (Fig. S18†). The high-resolution Mo 3d XPS spectrum of 1 can be deconvoluted into two peaks corresponding to Mo⁶⁺-3d_3/2 (236.0 eV) and Mo⁶⁺-3d_5/2 (232.8 eV), demonstrating the Mo⁶⁺ in 1 (Fig. 3a). The high-resolution Ti 2p XPS spectra exhibit two peaks at 464.4 eV (Ti 2p_1/2) and 458.6 eV (Ti 2p_3/2) with about 5.8 eV binding energy difference, which are assigned to typical Ti⁴⁺ (Fig. 3b). The O 1s XPS spectra (Fig. 3c) display two peaks at 531.8 and 530.4 eV, corresponding to the binding energies of O–O and Mo–O/Ti–O, respectively.^41–44 The dark-field STEM image shows uniform bright dots with a size of 1.8 ± 0.2 nm, which matches well with the core size of 1 (Fig. S19†). TGA was further performed under a N₂ atmosphere to investigate the thermal stability of 1 (Fig. S20†).

Fig. 3 (a–c) The XPS high resolution scans of Mo 3d, Ti 2p and O 1s for Mo₄₂Ti₁₂. (d) The UV/Vis diffuse reflectance. (e) SPV and (f) I–t spectrum of Mo₄₂Ti₁₂.

It is well-recognized that titanium-containing solids and semiconductors can show efficient (photo)catalytic oxidation of benzyl alcohol (BA).^45,46 However, the reports of MoTOCs as catalysts for the selective oxidation of BA are scarce. The rich peroxo groups in 1 may enable it to exhibit strong oxidizing properties. Moreover, 1 displays strong absorption in the ultraviolet and visible region, from which the band gap (E_g) can be determined to be 2.45 eV, demonstrating semiconductor-like properties (Fig. 3d and S21†).⁴⁷ The surface photovoltage (SPV) and Mott–Schottky spectrum further confirmed its n-type semiconductor feature (Fig. 3e and S22†).^48,49 The high photocurrent obtained from the time-resolved photocurrent response curves indicates the good separation efficiency of photo-generated electron–hole pairs in 1 (Fig. 3f). Thus, we used 1 as a catalyst for the selective oxidation of BA to benzaldehyde (BAD) and further used it as a photocatalyst for the oxidation of BAD to benzoic acid (BZA) under visible light.

In the selective oxidation of BA systems, 3 mol% of catalyst 1 was added to 3 mL of deionized water in the presence of 1.63 mmol of 30% H₂O₂ and 0.21 mmol of BA at 65 °C for 12 h without any other additives; we were delighted to observe that the conversion rate of BA is up to 99%, and the selectivity of BAD as the major product is 91.5%. A longer reaction time showed that a higher selectivity of 96.4% of BAD was obtained when the reaction time was extended to 24 h (Table S6†). Therefore, we selected 24 h as the optimal reaction time in the following experiments. In contrast, only 3.8% conversion rate of BA and 1.1% selectivity of BAD were obtained in the absence of catalyst 1 under the same conditions, which reveals that catalyst 1 can significantly improve the conversion of BA (Table 1). Except for the excellent catalytic oxidation capacity, catalyst 1 also showed good stability and reusability for catalyzed oxidation of BA, which can be proved by the minor variations in the yields of BAD after five cycles (Fig. 4a and Table S7†). For better comparison, we have thoroughly summarized the related literature on catalytic oxidation BA systems. Our present work shows highly competitive selectivity and conversion rate among the reported crystalline POM-based catalysts (Table S8†).

Fig. 4 (a) Catalytic BA oxidation: BA conversion and BAD selectivity (b) Photocatalytic BAD oxidation: BAD conversion and BZA selectivity.

Table 1 The reaction parameters for catalytic oxidation of BA and photocatalytic oxidation of BAD^a

Entry	Substrate	Cat.	Light	Con.^c/%	Sel.^c/%
a Reaction conditions: catalytic oxidation of BA: 1 (3.0 mol%), BA (0.21 mmol), H2O2 (1.63 mmol), H2O (3 mL), T (65 °C), t (24 h); photocatalytic oxidation of BAD: 1 (0.3 mol%), BAD (1 mmol), H2O2 (16.3 mmol), H2O (3 mL), hv (AM 1.5G), T (65 °C), t (12 h). b TiO2 (0.3 mol%). c Conversion and selectivity were calculated by gas chromatography.
1	Benzyl alcohol	Mo42Ti12	None	99	96.4
2	Benzyl alcohol	None	None	3.8	1.1
3	Benzaldehyde	Mo42Ti12	AM 1.5	92.6	99
4	Benzaldehyde	None	AM 1.5	10.3	99
5^b	Benzaldehyde	TiO2	AM 1.5	78.6	99

---

Subsequently, compound 1 was further used as a photocatalyst for the photocatalytic oxidation of BAD under simulated sunlight irradiation owing to the suitable bandgap as an n-type semiconductor. By adding 0.3 mol% of 1, 3 mL of deionized water in the presence of 16.3 mmol of 30% H₂O₂ and 1 mmol of BAD at 55 °C for 12 h, 92.6% conversion rate of BAD and 99% selectivity of BZA were detected. When photocatalyst 1 was removed from the catalytic system, only 10.3% conversion of BAD was obtained under the same conditions, indicating the excellent photocatalytic performance of 1. As a comparison, when traditional titanium dioxide is used as a photocatalyst, the conversion rate of BAD is only 78.6% (Table 1), suggesting that the reported MoTOC with a unique and well-defined structure has greater potential for application in photocatalytic oxidation of BAD.

To explore the photocatalytic reaction mechanism, a series of control experiments were performed to investigate the dominant reactive species in the photocatalytic oxidation reaction. Specifically, isopropanol, ammonium oxalate, and 1,4-benzoquinone were introduced into the reaction system to quench ˙OH, h⁺, and ˙O₂⁻, respectively.^50,51 It should be noted that the conversion rate of BAD was significantly decreased after adding 1,4-benzoquinone scavengers, indicating that ˙O₂⁻ plays a key role in the photocatalytic oxidation process. The slight decline in the conversion rates was also observed with the addition of isopropanol and ammonium oxalate, which demonstrates that ˙OH and h⁺ are also involved in the photocatalytic oxidation process (Table S9†). Based on the above results, a possible mechanism for the photocatalytic oxidation of BAD by 1 is proposed and depicted in Fig. S23 and 24.† Additionally, five cycle experiments of photocatalyst 1 showed that the negligible change of BAD conversion rate was noted (Fig. 4b and Table S10†). Meanwhile, the FT-IR and XRD patterns of 1 before and after the catalytic reaction are nearly coincident except for a slight decrease in crystallinity (Fig. S25 and 26†), which means that photocatalyst 1 has good stability and recyclability in the photocatalytic oxidation reaction.

Experimental

Synthesis of Mo₄₂Ti₁₂ (1)

Solution A: 1 M TiCl₄ (5 mL) was dissolved in H₂O (25 mL); after stirring for 10 min, H₂O₂ (10 mL) was added to this solution at 0 °C with stirring, and then the pH of the solution was adjusted to 6.3 with 5 M KOH. Solution B: (NH₄)₆Mo₇O₂₄·4H₂O (1.0600 g, 0.85 mmol) dissolved in H₂O (15 mL). 5 mL of solution A was added to solution B and the final pH was maintained at 4.66 with 3 M HCl. This solution was stirred for another 60 min at 80 °C, then filtered and left to evaporate slowly. One week later, light yellow block-shaped crystals were obtained, collected by filtration and dried in air. Yield: (0.72 g, yield: 68%). Elemental analysis (%): calculated for Mo₄₂K₈O₂₀₉Ti₁₂H₆₂ K 3.75, Ti 6.90, Mo 48.41; found, K 3.51, Ti 6.72, Mo 48.23. FT-IR (KBr, 4000–400 cm⁻¹): 3166 (m), 1617 (s), 1412 (s), 938 (w), 880 (s), 843 (m), 708 (s), 649 (w), 587 (w).

Catalytic experiments

The oxidation of benzyl alcohol: 1.63 mmol 30% H₂O₂, 0.21 mmol benzyl alcohol, and 3.0 mol% 1 were added to 3 mL of deionized water and stirred at 65 °C for 12, 24, 36, 48, and 60 hours, respectively, to optimize the reaction time. After the oxidation reaction is completed, 1 was recovered by extraction experiment and reused for an additional cycle. For comparison, the control experiment was also conducted by the same reaction route without the addition of 1.

The photocatalytic oxidation of benzaldehyde: 16.3 mmol 30% H₂O₂, 1 mmol benzaldehyde, and 0.3 mol% 1 were added to a quartz bottle containing 3 mL of deionized water and then the reaction system was sealed. Then the sealed quartz bottle was stirred at 55 °C for 12 hours under AM 1.5 G simulated sunlight to make them fully react. 1 was recovered by extraction experiment and reused for an additional cycle. For comparison, the control experiment was also conducted by the same reaction without the addition of 1. Besides, the photocatalytic oxidation activity of TiO₂ was also investigated for comparison by using the same number of moles of TiO₂ instead of 1.

Conclusions

In summary, a new giant molybdenum titanium-oxo cluster 1 with the largest nuclear number and a unique pure inorganic triangular prism structure was synthesized by a simple self-assembly procedure in aqueous solution for the first time. 1 was fully structurally characterized and its assembly mechanism was analyzed in detail. For 1, 42 molybdenum and 12 titanium atoms are connected by bridging oxo and peroxo groups, showing controllable and selective catalytic oxidation of BA to BAD and photocatalytic oxidation of BAD to BZA with high selectivity and conversion rate. This work broadens the MoTOC chemistry and opens new perspectives for the design of multifunctional catalytic materials.

Author contributions

M. Xu, W.-J. Zhang and P. Wang performed the synthesis and characterization. M. Xu and K.-K. Guo performed the crystallographic analysis. T. Wang and X.-L. Wang designed the study and supervised the project. All the authors discussed the results and co-wrote the manuscript.

Data availability

Some or all data, models, or code that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by the Program for the Development of Science and Technology of Jilin Province (Item No. YDZJ202201ZYTS395 and YDZJ202201ZYTS313) and the National Natural Science Foundation of China (Grant No. 22201097 and 21771035).

References

1. H. L. Zhang, A. Li, K. Li, Z. P. Wang, X. C. Xu, Y. X. Wang, M. V. Sheridan, H. S. Hu, C. Xu, E. V. Alekseev, Z. Y. Zhang, P. Yan, K. C. Cao, Z. F. Chai, T. E. Albrecht-Schönzart and S. A. Wang, Ultrafiltration separation of Am(VI)-polyoxometalate from lanthanides, Nature, 2023, 616, 482–487.
2. D. H. Guan, X. X. Wang, L. N. Song, C. L. Miao, J. Y. Li, X. Y. Yuan, X. Y. Ma and J. J. Xu, Polyoxometalate Li₃PW₁₂O₄₀ and Li₃PMo₁₂O₄₀ Electrolytes for High-energy All-solid-state Lithium Batteries, Angew. Chem., Int. Ed., 2024, 63, e202317949.
3. M. A. AlDamen, J. M. Clemente-Juan, E. Coronado, C. Martí-Gastaldo and A. Gaita-Ariño, Mononuclear Lanthanide Single-Molecule Magnets Based on Polyoxometalates, J. Am. Chem. Soc., 2008, 130(28), 8874–8875.
4. Y. Q. Feng, F. Y. Fu, L. L. Zeng, M. Y. Zhao, X. Xin, J. K. Liang, M. Zhou, X. K. Fang, H. J. Lv and G. Y. Yang, Atomically Precise Silver Clusters Stabilized by Lacunary Polyoxometalates with Photocatalytic CO₂ Reduction Activity, Angew. Chem., Int. Ed., 2024, 63, e202317341.
5. J. Liu, N. Jiang, J. M. Lin, Z. B. Mei, L. Z. Dong, Y. Kuang, J. J. Liu, S. J. Yao, S. L. Li and Y.-Q. Lan, Structural Evolution of Giant Polyoxometalate: From “Keplerate” to “Lantern” Type Mo₁₃₂ for Improved Oxidation Catalysis, Angew. Chem., Int. Ed., 2023, 62, e202304728.
6. H. P. Xiao, Y. S. Hao, X. X. Li, P. Xu, M. D. Huang and S. T. Zheng, A Water-Soluble Antimony-Rich Polyoxometalate with Broad-Spectrum Antitumor Activities, Angew. Chem., Int. Ed., 2022, 61, e202210019.
7. F. Y. Zhao, T. Cheng, X. L. Lu, N. Ghorai, Y. W. Yang, Y. V. Geletii, D. G. Musaev, C. L. Hill and T. Q. Lian, Charge Transfer Mechanism on a Cobalt-Polyoxometalate-TiO₂ Photoanode for Water Oxidation in Acid, J. Am. Chem. Soc., 2024, 146, 14600–14609.
8. T. Minato, K. Suzuki, K. Yamaguchi and N. Mizuno, Alkoxides of Trivacant Lacunary Polyoxometalates, Chem. – Eur. J., 2017, 23, 14213–14220.
9. M. Dufaye, S. Duval, G. Stoclet, X. Trivelli, M. Huvé, A. Moissette and T. Loiseau, Uranyl Cation Incorporation in the [P₈W₄₈O₁₈₄]⁴⁰⁻ Macrocycle Phosphopolytungstate, Inorg. Chem., 2019, 58, 1091–1099.
10. P. Y. Zhang, Y. Wang, L. Y. Yao and G. Y. Yang, Hepta-Zr-Incorporated Polyoxometalate Assembly, Inorg. Chem., 2022, 61, 10410–10416.
11. D. H. Yang, Y. F. Liang, P. T. Ma, S. Z. Li, J. P. Wang and J. Y. Niu, Ligand-Directed Conformation of Inorganic-Organic Molecular Capsule and Cage, Inorg. Chem., 2014, 53, 3048–3053.
12. Y. F. Liang, S. Z. Li, D. H. Yang, P. T. Ma, J. Y. Niu and J. P. Wang, Controllable assembly of multicarboxylic acids functionalized heteropolyoxomolybdates and allochroic properties, J. Mater. Chem. C, 2015, 3, 4632–4639.
13. W. M. Xuan, R. Pow, D. L. Long and L. Cronin, Exploring the Molecular Growth of Two Gigantic Half-Closed Polyoxometalate Clusters {Mo₁₈₀} and {Mo₁₃₀Ce₆}, Angew. Chem., Int. Ed., 2017, 56, 9727–9731.
14. X. X. Li, C. H. Li, M. J. Hou, B. Zhu, W. C. Chen, C. Y. Sun, Y. Yuan, W. Guan, C. Qin, K. Z. Shao, X. L. Wang and Z. M. Su, Ce-mediated molecular tailoring on gigantic polyoxometalate {Mo₁₃₂} into half-closed {Ce₁₁Mo₉₆} for high proton conduction, Nat. Commun., 2023, 14, 5025.
15. J. C. Liu, J. W. Zhao, C. Streb and Y. F. Song, Recent advances on high-nuclear polyoxometalate clusters, Coord. Chem. Rev., 2022, 471, 214734.
16. S. T. Zheng and G. Y. Yang, Recent advances in paramagnetic-TM-substituted polyoxometalates (TM = Mn, Fe, Co, Ni, Cu), Chem. Soc. Rev., 2012, 41, 7623–7646.
17. D. L. Long, R. Tsunashima and L. Cronin, Polyoxometalates: Building Blocks for Functional Nanoscale Systems, Angew. Chem., Int. Ed., 2010, 49, 1736–1758.
18. Z. W. Guo, L. H. Lin, J. P. Ye, Y. Chen, X. X. Li, S. Lin, J. D. Huang and S. T. Zheng, Core-Shell-Type All-Inorganic Heterometallic Nanoclusters: Record High-Nuclearity Cobalt Polyoxoniobates for Visible-Light-Driven Photocatalytic CO₂ Reduction, Angew. Chem., Int. Ed., 2023, 62, e202305260.
19. P. Yang, M. Alsufyani, A. H. Emwas, C. Q. Chen and N. M. Khashab, Lewis Acid Guests in a {P₈W₄₈} Archetypal Polyoxotungstate Host: Enhanced Proton Conductivity via Metal-Oxo Cluster within Cluster Assemblies, Angew. Chem., Int. Ed., 2018, 57, 13046–13051.
20. M. J. Turo, L. F. Chen, C. E. Moore and A. M. Schimpf, Co²⁺-Linked [NaP₅W₃₀O₁₁₀]¹⁴⁻: A Redox-Active Metal Oxide Framework with High Electron Density, J. Am. Chem. Soc., 2019, 141, 4553–4557.
21. W. J. Xu, M. Xu, Y. Zheng, X. X. Wang, F. Y. Li and L. Xu, Hydrogen bonding assisted formation of sandwich-type titanium-containing heteropolymolybdates: water-soluble and photoelectroactive, Inorg. Chem. Front., 2020, 7, 3667–3673.
22. M. Paesa, F. Almazán, C. Yus, V. Sebastián, M. Arruebo, L. M. Gandía, S. Reinoso, I. Pellejero and G. Mendoza, Gold Nanoparticles Capped with a Novel Titanium(IV)-Containing Polyoxomolybdate Cluster: Selective and Enhanced Bactericidal Effect Against Escherichia coli, Small, 2024, 20, 2305169.
23. W. Y. Wang, Z. Jing, Y. M. Hong, X. Y. Ma, K. L. Li, P. T. Ma, J. Y. Niu and J. P. Wang, Synthesis and Characterization of 6-Ti-Substituted Polyoxomolybdate with High Catalytic Activity for Sulffde Oxidation, Inorg. Chem., 2024, 63, 6268–6275.
24. H. Akashi, J. Chen, H. Hasegawa, M. Hashimoto, T. Hashimoto, T. Sakuraba and A. Yagasaki, Synthesis and structural characterization of [H_xCp^*TiMo₅O₁₈]^(3−x)- (x=0, 1, 2); new insights into protonation patterns in polyoxometalates, Polyhedron, 2003, 22, 2847–2854.
25. S. Eslava, B. P. R. Goodwill, M. McPartlin and D. S. Wright, Extending the Family of Titanium Heterometallicoxoalkoxy Cages, Inorg. Chem., 2011, 50, 5655–5662.
26. L. Yang, X. P. Shu, M. Y. Fu, H. Y. Wang, Q. Y. Zhu and J. Dai, Molybdenum-titanium oxo-cluster, an efficient electrochemical catalyst for the facile preparation of black titanium dioxide film, Dalton Trans., 2020, 49, 10516–10522.
27. H. Uchiyama, D. Puthusseri, J. Grins, D. Gribble, G. A. Seisenbaeva, V. G. Pol and V. G. Kessler, Single-Source Alkoxide Precursor Approach to Titanium Molybdate, TiMoO₅, and Its Structure, Electrochemical Properties, and Potential as an Anode Material for Alkali Metal Ion Batteries, Inorg. Chem., 2021, 60, 3593–3603.
28. W. Z. Chen, X. F. Yi, J. Zhang and L. Zhang, Heterometallic Mo-Ti oxo clusters with metal–metal bonds: Preparation and visible-light absorption behaviors, Polyoxometalates, 2023, 2, 9140013.
29. D. X. Wang, Y. S. Liu, G. J. Chen, F. F. Gao, G. Y. Zhang, G. Wang, C. H. Tung and Y. F. Wang, Ligation of Titanium-oxide and {Mo₂} Units for Solar CO₂ Storage, Inorg. Chem., 2023, 62, 21074–21082.
30. L. Huang, S. S. Wang, J. W. Zhao, L. Cheng and G. Y. Yang, Synergistic Combination of Multi-ZrIV Cations and Lacunary Keggin Germanotungstates Leading to a Gigantic Zr₂₄-Cluster-Substituted Polyoxometalate, J. Am. Chem. Soc., 2014, 136, 7637–7642.
31. Y. J. Kikukawa, K. Yamaguchi and N. Mizuno, Zinc(II) Containing γ-Keggin Sandwich-Type Silicotungstate: Synthesis in Organic Media and Oxidation Catalysis, Angew. Chem., Int. Ed., 2010, 49, 6096–6100.
32. X. F. Yi, N. V. Izarova, M. Stuckart, D. Guérin, L. Thomas, S. Lenfant, D. Vuillaume, J. V. Leusen, T. Duchoň, S. Nemšák, S. D. M. Bourone, S. Schmitz and P. Kögerler, Probing Frontier Orbital Energies of {Co₉(P₂W₁₅)₃} Polyoxometalate Clusters at Molecule-Metal and Molecule-Water Interfaces, J. Am. Chem. Soc., 2017, 139, 14501–14510.
33. T. Minato, K. Suzuki, K. Yamaguchi and N. Mizuno, Synthesis and Disassembly/Reassembly of Giant Ring-Shaped Polyoxotungstate Oligomers, Angew. Chem., Int. Ed., 2016, 55, 9630–9633.
34. M. Xu, T. Wang, F. Y. Li, W. J. Xu, Y. Zheng and L. Xu, Water-soluble titanium-polyoxomolybdate with external μ₃ bridging oxygen coordination on a lacunary Keggin structure, Chem. Commun., 2020, 56, 1097–1100.
35. M. Kakihana, M. Tada, M. Shiro, V. Petrykin, M. Osada and Y. Nakamura, Structure and Stability of Water Soluble (NH₄)₈[Ti₄(C₆H₄O₇)₄(O₂)₄]·8H₂O, Inorg. Chem., 2001, 40, 891–894.
36. Z. H. Zhou, Y. F. Deng, Q. X. Liu, H. L. Zhang, T. C. W. Mak and Y. L. Chow, Selective Ligand Conversion of Ethylenediamine Tetraacetate to Its Triacetate on Peroxotitanate(IV), Inorg. Chem., 2007, 46, 6846–6848.
37. Y. C. Yang, Q. X. Liu, Z. H. Zhou and H. L. Wan, Regioselective conversions of H₄pdta (1,2-propanediaminetetraacetic acid) and H₄eed3a to their triacetates on peroxotitanates, Dalton Trans., 2019, 48, 16943–16951.
38. W. T. Jin, F. Yang, L. Deng, M. L. Chen, J. F. Chen, H. B. Chen and Z. H. Zhou, Wheel-Like Icosanuclear Peroxotitanate A Stable Water-Soluble Catalyst for Oxygen Transfer Reactions, Inorg. Chem., 2018, 57, 14116–14122.
39. K. Tomita, V. Petrykin, M. Kobayashi, M. Shiro, M. Yoshimura and M. Kakihana, A Water-Soluble Titanium Complex for the Selective Synthesis of Nanocrystalline Brookite, Rutile, and Anatase by a Hydrothermal Method, Angew. Chem., Int. Ed., 2006, 45, 2378–2381.
40. J. M. Stauber and C. C. Cummins, Terminal Titanyl Complexes of Tri- and Tetrametaphosphate: Synthesis, Structures, and Reactivity with Hydrogen Peroxide, Inorg. Chem., 2017, 56, 3022–3029.
41. N. Li, J. Liu, J. J. Liu, L. Z. Dong, S. L. Li, B. X. Dong, Y. H. Kan and Y. Q. Lan, Self-Assembly of a Phosphate-Centered Polyoxo-Titanium Cluster: Discovery of the Heteroatom Keggin Family, Angew. Chem., Int. Ed., 2019, 58, 17260–17264.
42. J. M. Lin, N. Li, S. P. Yang, M. J. Jia, J. Liu, X. M. Li, L. An, Q. W. Tian, L. Z. Dong and Y. Q. Lan, Self-Assembly of Giant Mo₂₄₀ Hollow Opening Dodecahedra, J. Am. Chem. Soc., 2020, 142, 13982–13988.
43. C. B. Ma, Y. P. Xu, L. X. Wu, Q. Wang, J. J. Zheng, G. X. Ren, X. Y. Wang, X. F. Gao, M. Zhou, M. Wang and H. Wei, Guided Synthesis of a Mo/Zn Dual Single-Atom Nanozyme with Synergistic Effect and Peroxidase-like Activity, Angew. Chem., Int. Ed., 2022, 61, e202116170.
44. S. Y. Xu, W. X. Shi, J. R. Huang, S. Yao, C. Wang, T. B. Lu and Z. M. Zhang, Single-cluster Functionalized TiO₂ Nanotube Array for Boosting Water Oxidation and CO₂ Photoreduction to CH₃OH, Angew. Chem., Int. Ed., 2024, 63, e202406223.
45. C. M. Crombie, R. J. Lewis, R. L. Taylor, D. J. Morgan, T. E. Davies, A. Folli, D. M. Murphy, J. K. Edwards, J. Z. Qi, H. Y. Jiang, C. J. Kiely, X. Liu, M. S. Skjøth-Rasmussen and G. J. Hutchings, Enhanced Selective Oxidation of Benzyl Alcohol via In Situ H₂O₂ Production over Supported Pd-Based Catalysts, ACS Catal., 2021, 11, 2701–2714.
46. C. J. Li, G. R. Xua, B. H. Zhang and J. R. Gong, High selectivity in visible-light-driven partial photocatalytic oxidation of benzyl alcohol into benzaldehyde over single-crystalline rutile TiO₂ nanorods, Appl. Catal., B, 2012, 115–116, 201–208.
47. G. Y. Zhang, C. Y. Liu, D. L. Long, L. Cronin, C.-H. Tung and Y. F. Wang, Water-Soluble Pentagonal-Prismatic Titanium-Oxo Clusters, J. Am. Chem. Soc., 2016, 138, 11097–11100.
48. G. F. Hou, L. H. Bi, B. Li and L. X. Wu, Reaction Controlled Assemblies of Polyoxotungstates (-molybdates) and Coordination Polymers, Inorg. Chem., 2010, 49, 6474–6483.
49. Q. Y. Hu, S. S. Chen, T. Wågberg, H. S. Zhou, S. J. Li, Y. D. Li, Y. L. Tan, W. Q. Hu, Y. Ding and X. B. Han, Developing Insoluble Polyoxometalate Clusters to Bridge Homogeneous and Heterogeneous Water Oxidation Photocatalysis, Angew. Chem., Int. Ed., 2023, 62, e202303290.
50. Y. Liu, P. Zhang, B. Z. Tian and J. L. Zhang, Core-Shell Structural CdS@SnO₂ Nanorods with Excellent Visible-Light Photocatalytic Activity for the Selective Oxidation of Benzyl Alcohol to Benzaldehyde, ACS Appl. Mater. Interfaces, 2015, 7, 13849–13858.
51. Y. H. Zhang, N. Zhang, Z. R. Tang and Y. J. Xu, Identification of Bi₂WO₆ as a highly selective visible light photocatalyst toward oxidation of glycerol to dihydroxyacetone in water, Chem. Sci., 2013, 4, 1820–1824.

---

Footnote

† Electronic supplementary information (ESI) available: Synthesis, characterization, crystal structure, catalytic oxidation of BA and photocatalytic oxidation BAD properties of 1; IR, EDX, elemental mapping, XPS, dark-field STEM images, Mott–Schottky and TGA analysis. CCDC 2068105 for 1. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi01795a

---