Uranyl-silicotungstate-containing hybrid building units {α-SiW₉} and {γ-SiW₁₀} with excellent catalytic activities in the three-component synthesis of dihydropyrimidin-2(1H)-ones

Abstract

A novel uranyl-containing polytungstate, {K_4.2Na_18.9H_0.9(H₂O)₂₀[K₄(UO₂)₃(H₂O)₆(α-SiW₉O₃₄)(γ-SiW₁₀O₃₆)₃]}·ca.16H₂O (U₃), was synthesized using the Keggin-type precursor [γ-SiW₁₀O₃₆]⁸⁻ ({γ-SiW₁₀}) and UO₂(OAc)₂ and was further characterized by single-crystal X-ray diffraction and various techniques. Single crystal X-ray diffraction analysis showed a quadrahedron-like structure of U₃ with interesting tarot-like (top view) and crown-like (side view) views, among which one central {K₄(UO₂)₃(H₂O)₃} cluster was surrounded by one {α-SiW₉} and three {γ-SiW₁₀} building units. In addition, a more significant aspect is that U₃ shows good catalytic activity in the dehydration condensation of aldehydes, acetoacetates, and urea to green synthesize dihydropyrimidin-2(1H)-ones. The advantages of this transformation include solvent-free conditions, water as the sole by-product, available starting materials, good compatibility, and operational simplicity.

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Introduction

Self-assembly of metal–oxygen clusters with definite structures makes it possible to design and synthesize metal oxide materials from micro- to macro-scale. Polyoxometalates (POMs) are a kind of molecular metal–oxygen-cluster with various structures and important properties involving applications in catalysis, photoelectric materials, medicine, etc.^1–9 In general, in situ and precursor strategies are the mainstream methods used in the synthesis of 3d/4f-metal-containing POMs. Thousands of 3d/4f-metal-containing POMs have been reported and show fascinating properties. The in situ method offers more driving powers to intrinsically build new POMs by adjusting the reaction steps and conditions, while the usage of lacunary POMs precursors makes it easier to predict the structures of the obtained POMs. However, the development of actinide-containing POMs may be limited by synthetic methods, and the study still lags far behind 3d/4f-metal-containing POMs comparing the research contents and depth, which includes structure-based research and applications.¹⁰ Actinides that have unique 5f electrons and bigger atomic radii may show more abundant oxidation states and are good members to synthesize POMs.^11–16 For these stable actinides, thorium and uranium are most accessible to study, and U-containing heteropolytungstates (U-POWs) are the most explored. But it is disappointing to note that less than ten cases were in situ synthesized and dozens of U-POWs were all synthesized using lacunary POM precursors among these U-POWs.^17–22 Thus, the knowledge of the structural diversities of actinide-containing POMs still needs exploitation.

It should be noted that the synthesis of POMs that contain more than one lacunary POM building unit is quite challenging due to the complexity of the self-assembly. Most of the U-POWs are based on one type of building unit and there are only four cases that contain more than one type of lacunary POM building unit, named hybrid structures. The first structure reported by Pope in 2001 was [(UO₂)₃(H₂O)₅As₃W₂₉O₁₀₄]¹⁹⁻, which consisted of a trivacant Keggin-{AsW₉} and a Dawson-like {(AsW₉)₂(W₂)} building units.²³ The second one, [(UO₂)₃(H₂O)₄As₃W₂₆O₉₄]¹⁷⁻, was based on Keggin-{AsW₉} and a rare Dawson-like {(AsW₈)₂W} building units.²⁴ The third showed an iso-heteropolytungstate hybrid structure, [(H₃Sb^IIIW₁₇O₅₉)U^IV(HW₅O₁₈)]¹¹⁻, which was in situ constructed from simple materials with monovacant Lindqvist-{W₅} and monovacant Dawson-{SbW₁₇} building units.¹⁸ Recently, Zhang's and our group have reported the fourth hybrid U-POW, K_1.5Na_13.25[(UO₂)₃(SeO₃)₃Na₅(H₂O)₆(SeW₆O₂₁)(SeW₉O₃₃)₃]·26H₂O, which was in situ synthesized with one Anderson-{SeW₆} and three Keggin-{SeW₉} building units.²²

The usage of lacunary POM precursors indeed makes it efficient to synthesize new POMs, but the relatively strong stabilities of lacunary POM precursors also reduce the structural uncertainties of the obtained new POMs.^25–27 Thus, these changeful and relatively unstable lacunary POM precursors with low stabilities are preferred to build new U-POWs.^28–33 For example, Kortz reported a horseshoe-like U-POW, e.g., K₆Li₁₉[Li(H₂O)K₄(H₂O)₃{(UO₂)₄(O₂)₄(H₂O)₂}₂(PO₃OH)₂P₆W₃₆O₁₃₆]·74H₂O, which was synthesized using the precursor [H₆P₄W₂₄O₉₄]¹⁸⁻ ({P₄W₂₄}).³⁴ {P₄W₂₄} transformed into a rare {P₂W₁₂}-trimeric {P₆W₃₆} building unit in an LiAc buffer in the synthetic procedure. Another representative case is the third hybrid U-POW, (NH₄)₁₇[(UO₂)₃(H₂O)₄As₃W₂₆O₉₄]·16H₂O, as mentioned earlier, which was synthesized from {AsW₉} (Na₉[AsW₉O₃₃]·27H₂O).²⁴ A part of {AsW₉} lost another {WO₆} and assembled into a rare Dawson-like {(AsW₈)₂W} building unit. {γ-SiW₁₀} is one of the most changeful lacunary POM precursors and could undergo complicated transformations into monovacant {SiW₁₁}, trivacant {SiW₉}, and other species in solution.³⁵ Thus, new U-POWs may be obtained using {γ-SiW₁₀} under specific conditions.

Regarding the aforementioned cases, we report a new U-POW with hybrid polyanions: {K_4.2Na_18.9H_0.9(H₂O)₂₀[K₄(UO₂)₃(H₂O)₆(α-SiW₉O₃₄)(γ-SiW₁₀O₃₆)₃]}·ca.16H₂O (U₃), which is synthesized by a one-pot method using K₈[γ-SiW₁₀O₃₆]·12H₂O and UO₂(OAc)₂. A part of {γ-SiW₁₀} transformed into trivacant {α-SiW₉} in the aqueous solution and joined in the assembly of the polyanion. The polyanion of U₃ shows a quadrahedron-like configuration based on a {K₄(UO₂)₃(H₂O)₆} cluster chromophore and two kinds of lacunary silicotungstate building units, e.g., trivacant {α-SiW₉} and bivacant {γ-SiW₁₀}. This work provides the first case of U-POW that is constructed from two kinds of Keggin-type lacunary building units. Besides, U₃ shows good catalytic activity in the synthesis of dihydropyrimidin-2(1H)-ones (DHPMs) via the dehydration condensation of aldehydes, acetoacetates, and urea. The solvent-free conditions, water as the sole by-product, and simple starting materials make this method suitable for the green synthesis of DHPMs.

Experimental

Materials and methods

The precursor K₈[γ-SiW₁₀O₃₆]·12H₂O was synthesized according to the method reported by Hervéa.³⁶ Other reagents were purchased from the reagent manufacturers and used without further purification. CAUTION! Although the uranyl salt used is depleted, it is still radioactive and toxic. Appropriate protective measures are still necessary for handling all radioactive materials.

Synthesis of U₃

In a NaCl aqueous solution (0.25 M, 20 mL), UO₂(OAc)₂·2H₂O (0.1 mmol, 0.0424 g) and K₈[γ-SiW₁₀O₃₆]·12H₂O (0.4 mmol, 1.1884 g) were dissolved successively. The pH value of the mixture was then adjusted to 5.9 using 1 M HCl. The cloudy solution was heated and stirred at 85 °C for 30 min. During the heating procedure, this cloudy solution would transform from cloudy to clear and then cloudy again. After cooling down to room temperature, NaCl (4 g) was added to the solution and stirred for 5 min. The resulting solution was filtered and the filtrate was left to evaporate at room temperature. A cold room temperature (below 10 °C) is necessary for the formation of U₃. Unknown and cubic crystals will appear if the room temperature is higher.³⁷ Yellow-green crystals of U₃ with rod-like shapes were collected after two weeks (yield: 11% based on UO₂(OAc)₂·2H₂O). Elemental analysis (ICP-OES, %) for H_84.9K_8.2Na_18.9O₁₉₀Si₄U₃W₃₉, found: Na, 3.37; K, 2.49; W, 55.68. IR (cm⁻¹): 3388 (s), 1624 (m), 1005 (w), 945 (m), 847 (vs), 787 (s), 722 (vs), 689 (vs), 558 (m).

Typical procedure of dehydration condensation reaction catalyzed by U₃

Benzaldehyde (1 mmol), ethyl acetoacetate (1.5 mmol), urea (1 mmol), and U₃ (0.2 mol%) were added to a 4 mL reaction vial. Then the reaction was carried out at 100 °C for 3 h. After cooling down to room temperature, the crude product was dissolved in hot EtOH (10 mL). After simple filtration, the filtrate was cooled in an ice bath to promote the recrystallization of the desired product. The purified products were all characterized by NMR and the corresponding data are provided in the ESI.†

Results and discussion

Analysis of U₃

The single-crystal analysis demonstrates that U₃ crystallizes in the P1̄ space group. The polyanion in U₃ consists of three parts, including one {K₄(UO₂)₃(H₂O)₆} chromophore at the centre of the polyanion, one {α-SiW₉} and three {γ-SiW₁₀} moieties (Fig. 1A). In the triangular {K₄(UO₂)₃(H₂O)₆} chromophore, all the three U(VI) atoms (U1/U2/U3) show a similar 7-coordinated distorted pentagonal bipyramidal geometry at the peak of the triangle. The polar oxygen atoms from the U(VI) pentagonal bipyramid could be divided into two groups based on the positions relative to U(VI) atoms (Fig. S1†). The bond lengths of three inner polar oxygen atoms and the corresponding U(VI) atoms (O1 for U1, O3 for U2, and O5 for U3) are in the range of 1.795(18) to 1.811(17) Å, which are slightly longer than the typical O=U=O bonds (1.75–1.79 Å), while the outside polar oxygen atoms (O2 for U1, O4 for U2 and O6 for U3) show normal U=O bond lengths (1.736(16) Å for O2–U1, 1.794(18) Å for O4–U2 and 1.781(17) for O6–U3). The bond angles for O=U=O bonds are also distorted compared with a standard uranyl ion (178.048(13)° for U1, 179.418(10)° for U2 and 178.997(11)° for U3). For the five equatorial oxygen atoms, four oxygen atoms coordinated with K(I) ions are offered by {α-SiW₉} and {γ-SiW₁₀}, while the other oxygen atoms (O7/O8/O9) are coordinated with water molecules. The five equatorial oxygen atoms (O7/O10/O11/O16/O17) for U1 are almost coplanar and the deviation is less than 0.05 Å. But O8 is 0.0848 Å away from the plane constructed by O12/O13/O18 for U2. For U3, the four equatorial oxygen atoms (O14/O15/O20/O21) offered by polyanions are almost coplanar and O9 is 0.1433 Å away from the plane of O14/O15/O20.

Fig. 1 (A) Schematic depiction of the units in the polyanion of U₃; (B) top view and (C) side view of the polyanion in U₃; and (D) and (E) the simplified schematic depiction of the polyanion in U₃.

In the typical coordination bond lengths of K–O bonds (2.9 Å), K1 is coordinated by four equatorial oxygen atoms (O10/O12/O17/O18), one coordinated water molecule (O151), and two other oxygen atoms (bridging oxygen atoms offered by {γ-SiW₁₀}) (Fig. S1†). K2 is coordinated by three equatorial oxygen atoms (O14/O19/O20), one coordinated water molecule (O152), and one oxygen atom offered by {γ-SiW₁₀}. Similarly, K3 is also coordinated by three equatorial oxygen atoms (O11/O16/O21), one coordinated water molecule (O153) and the other two oxygen atoms offered by {γ-SiW₁₀}. Thus, a triangular arrangement is constructed by the connection of three U(VI) pentagonal bipyramids and three K(I) ions. K1 to K3 have weak coordination interactions with adjacent oxygen with distances from 2.9161(3) to 3.2179(3) Å. K4 is located at the position above the centre of this triangle and is coordinated by two inner polar oxygen atoms (O1/O5) from U1 and U3 with bond lengths of 2.8974(2) and 2.8993(2) Å, respectively. The distance from K4 to the plane of U1/U2/U3 is 2.3507 Å. Besides, K4 also has weak coordination interactions with adjacent oxygen atoms and two halves of water molecules (O154/O155) in the range of 2.9188(3)–3.2768(3) Å. In short, a triangular {K₄(UO₂)₃(H₂O)₆} chromophore is constructed dexterously by the connection of U(VI) and K(I) ions and oxygen atoms offered by {α-SiW₉} and {γ-SiW₁₀}. Three U(VI) ions are at the peaks of the triangle and K1 to K3 are located at the centre of the edges.

In addition, one {α-SiW₉} offers all the six peak oxygen atoms (O16–O21) to coordinate with three U(VI) and K(I) ions from the direction perpendicular to the plane of the triangle at the opposite side from K4. All the three {γ-SiW₁₀} building units that act as inorganic multidentate ligands only offer two terminal oxygen atoms and one or two bridging oxygen atoms to coordinate with U(VI) and K(I) ions from directions perpendicular to the edges of the triangle, respectively. Thus, the three {γ-SiW₁₀} units are all unsaturated. The plane constructed by Si2 to Si4 from three {γ-SiW₁₀} units is not parallel to the plane formed by U1 to U3; the dihedral angle is 2.768°, while the distance between K4 and plane Si2 to Si4 is only 0.0930 Å. As a result, a hybrid quadrahedron-like polyanion in U₃ is constructed dexterously from a one {α-SiW₉}, three {γ-SiW₁₀} and one {K₄(UO₂)₃(H₂O)₆} cluster with a formula of [K₄(UO₂)₃(H₂O)₆(α-SiW₉O₃₄)(γ-SiW₁₀O₃₆)₃]²⁴⁻ (Fig. 1B–E and 2A). Besides, adjacent [K₄(UO₂)₃(H₂O)₆(α-SiW₉O₃₄)(γ-SiW₁₀O₃₆)₃]²⁴⁻ ions are alternately connected by K(I) and Na(I) ions and coordinating water molecules to form a one-dimensional chain in the typical coordination bond lengths of K–O bonds (2.9 Å) and Na–O bonds (2.6 Å) (Fig. S2†).

Fig. 2 (A) Ball-and-stick view of the polyanion in U₃ and (B) schematic depiction of the hydrogen bonds in U₃.

It is worth noting that there are strong hydrogen bonds between the six coordinated water molecules (O7/O8/O9/O151/O152/O153) and six uncoordinated peak oxygen atoms (O145–O150) from three {γ-SiW₁₀} (Fig. 2B). The distances between the donors and the acceptors are in the range of 2.67(3)–3.17(3) Å (Table S4†). Twelve oxygen atoms are connected in a zigzag manner by hydrogen bonds to form a heart-like arrangement. These hydrogen bonds help stabilize the unsaturated coordination structure.

The formula of U₃ was determined by various characterization methods. The ICP-OES results reveal that the mass fractions of K, Na, and W in U₃ were 2.49%, 3.37%, and 55.68%, respectively. Thus, the number of K : Na : W should be 8.2 : 18.9 : 39 and there was 0.9 hydrogen ion for charge balance. The TGA curve of U₃ gives a 5.86% weight loss from room temperature to 300 °C, which corresponds to about 42 water molecules (Fig. S3†). The number of lattice water molecules removed by the SQUEEZE procedure is 16. Thus, the formula of U₃ should be {K_4.2Na_18.9H_0.9(H₂O)₂₀[K₄(UO₂)₃(H₂O)₆(α-SiW₉O₃₄)(γ-SiW₁₀O₃₆)₃]}·ca.16H₂O. The elementary composition of U₃ was also confirmed by energy dispersive spectroscopy (EDS) mapping results (Fig. S4†). Electrospray ionization mass spectrometry (ESI-MS) results demonstrated the chemical behaviours of U₃ in water (Fig. S5 and Table S6†). The unsaturated coordination structure of U₃ partly decomposed in electrospray ionization, but the polyanion of U₃ could also maintain a degree of stability. U₃ could be used as a catalyst under mild conditions. Besides, Fourier transform infrared, Raman, and solid-state luminescence spectra of U₃ were also recorded to assess the properties of U₃ (Fig. S7–S9†).

Evaluation of the catalytic activity of U₃

The evaluation of the catalytic activity of U₃ was performed in the synthesis of DHPMs through the dehydration condensation of aldehydes, acetoacetates, and urea.^37–40 DHPMs play an extremely important role in organic synthesis and pharmaceutical chemistry, and have received great attention from organic chemists.^41–45 Certain derivatives have good effects in anti-tumour, anti-inflammatory, and anti-bacterial activities, etc., and especially, some functional DHPMs are considered to have great development potential in the clinical treatment of calcium channel block, adrenergic antagonism, neuropeptide Y antagonism, and so on.^46–49 This research would provide a promising catalytic system for the synthesis of DHPMs in good yields through a green method.

The reaction was carried out with benzaldehyde (1a), ethyl acetoacetate (2a), and urea (3a) as the model substrates, U₃ as the catalyst, and various reaction conditions were optimized including the solvent, reaction temperature and time, the dosage of the catalyst, and the proportion of substrates (Fig. 3A). From Fig. 3B, there is no doubt that the catalytic activity of U₃ is better under solvent-free conditions than in other solvents. Besides, from Fig. 3C–F, it is obvious that the product 4a yield increases with an increase of temperature, time, the dosage of the catalyst, and the proportion of the substrates. By screening, the optimum reaction conditions of the catalytic system were obtained as follows: 0.2 mol% U₃, solvent-free, 100 °C, 3 h, and 1a : 2a : 3a = 1 : 1.5 : 1.

Fig. 3 (A) Model reaction: benzaldehyde (1a, 1 mmol), ethyl acetoacetate (2a, 1 mmol), urea (3a, 1 mmol), U₃ (0.1 mol%), solvent (1 mL), 100 °C, and 2 h; (B) screening of the reaction solvent (EG: ethylene glycol and DMF: N, N-dimethylformamide); (C and D) optimizing reaction temperature and time; (E) exploring the dosage of the catalyst; and (F) investigating the proportion of substrates (1a : 2a : 3a).

Having established the optimal reaction conditions, the scope of this transformation was further investigated (Table 1). A range of benzaldehyde derivatives (1) having both electron-donating groups (p-Me, m-Me, o-Me, p-ⁱPr, p-OMe, and p-OEt) and electron-withdrawing groups (p-F, p-Cl, p-Br, and p-N(Me)₂) were tested with ethyl acetoacetate (2a) and urea (3a). Generally, all the reactions proceeded smoothly with good yields of the corresponding products (4b–4j, and 4m). In addition, it was found that 2-naphthaldehyde and 5-methyl furfural can also react with ethyl acetoacetate (2a) and urea (3a) to generate the desired products 4k and 4l in moderate yields. Subsequently, we studied the reaction of benzaldehyde (1a), methyl acetoacetate (2b), and urea (3a), giving the product 4n in a yield of 86%. More interestingly, our protocol is applicable to the reaction of benzaldehyde (1a), ethyl acetoacetate (2a), and thiourea (3b), delivering the corresponding product 4o in a yield of 87%.

Table 1 Synthesis of DHPMs catalyzed by U₃ ^a

a Reaction conditions: benzaldehydes (1, 1.0 mmol), ethyl acetoacetates (2, 1.5 mmol), urea (3, 1.0 mmol), U₃ (0.2 mol%), solvent-free conditions, 100 °C, and 3 h.

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Conclusions

In summary, we have synthesized and fully characterized a new hybrid U-POW based on two kinds of Keggin-type building units. A part of {γ-SiW₁₀} transformed into trivacant {α-SiW₉} in the aqueous solution and joined in the assembly to access the polyanion. U₃ represents a rare hybrid U-POW that is built based on one {α-SiW₉} and three {γ-SiW₁₀} building units. Besides, U₃ was demonstrated to be a good catalyst for the synthesis of dihydropyrimidin-2(1H)-ones via the condensation of aldehydes, acetoacetates, and urea. Importantly, the solvent-free conditions, water as the sole by-product, and available starting materials mean that this protocol provides a green and efficient multicomponent reaction for the synthesis of DHPMs. This work provides another case of rare hybrid U-POWs and the application of U-POWs in catalytic synthesis chemistry and may benefit the development of related research.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22001034), the Jiangxi Provincial Natural Science Foundation (20212BAB213001), and the Natural Science Foundation of Guangxi (2022GXNSFGA035003).

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Footnotes

† Electronic supplementary information (ESI) available: Bond lengths/angles, tables, and additional figures. CCDC 2132862. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi02653h

‡ These authors contributed equally to this work.

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