Visible-light-induced C–N bond formation by Ce(III)-containing antimonotungstate with Ce-to-polyanion charge transfer

Yufeng Liu† ^a, Hanjin Shi†^a, Miao Zhang†^c, Haoqi Liu^a, Pengtao Ma*^c and Guoping Yang*^ab
aJiangxi Province Key Laboratory of Functional Organic Polymers, Jiangxi Key Laboratory for Mass Spectrometry and Instrumentation, East China University of Technology, Nanchang 330013, China. E-mail: erick@ecut.edu.cn
^bHubei Key Laboratory of Processing and Application of Catalytic Materials, Huanggang Normal University, Huanggang, Hubei 438000, China
^cHenan Key Laboratory of Polyoxometalate Chemistry, College of Chemistry and Molecular Sciences, Henan University, Kaifeng, Henan 475004, China. E-mail: mpt@henu.edu.cn

First published on 12th August 2025

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Introduction

The construction of C–N bonds represents one of the most pivotal reactions in organic synthesis, extensively utilized in the synthesis of natural products, pharmaceuticals, functional materials, and so on.^1–6 A wide variety of methodologies have been developed to construct C–N bonds; however, one reaction that has garnered renewed attention after being overlooked is the formation of acyl hydrazides from azodicarboxylates and aldehydes.⁷ The reaction employs readily available aldehydes as starting materials, and the resulting hydrazides are widely applied in the synthesis of high-value-added chemicals, such as the drugs vorinostat and moclobemide.^8,9 Therefore, significant efforts have been dedicated to advancing the hydroacylation of azodicarboxylates with aldehydes as an efficient route for C–N bond formation. Traditional strategies for synthesizing acyl hydrazides typically involve transition metal catalysis, alongside the use of strong acids or bases and high temperatures.^10–13 Achieving hydroacylation of azodicarboxylates under green and mild conditions remains both interesting and challenging. Photocatalysis, owing to its potential to utilize sunlight as a clean and renewable light source, has emerged as an environmentally benign and sustainable approach for organic transformations.^14–17 The photocatalyzed hydroacylation of azodicarboxylates through C–H bond activation of aldehydes is recognized as a mild and efficient method for C–N bond formation. A series of photocatalysts, including tetrabutylammonium decatungstate, graphite flakes, and benzoylformic acid, have been utilized to realize this transformation.^18–23 Despite the notable advantages of these catalytic systems, challenges such as the poor durability of photocatalysts and the use of toxic solvents continue to drive the development of new photocatalytic systems that better align with the principles of green and sustainable chemistry.

Polyoxometalates (POMs) are a class of structurally well-defined metal oxide clusters that exhibit superior thermal and oxidative stability compared to organic or organometallic complexes.^24–28 Moreover, POMs possess unique photoinduced charge-transfer and redox properties due to intramolecular ligand-to-metal charge transfer (LMCT).^29,30 However, the current LMCT process requires ultraviolet light irradiation because of the substantial energy gaps involved, thereby limiting the widespread application of POMs in photocatalysis.^31–33 Visible-light-responsive POMs can be achieved by regulating the HOMO and/or LUMO of POMs through the introduction of metal cations into the vacant sites.^34–36 Trivalent lanthanide (Ln) cations, owing to their high coordination number and flexible coordination environment, can flexibly connect lacunary POMs to construct novel Ln-containing POMs (Ln-POMs).^37–41 Some Ln-POMs have demonstrated prominent metal-to-POM charge transfer, enabling visible-light-induced chemical transformations.^42,43 For example, Streb et al. utilized visible-light-responsive [(Ce(DMSO)₄)₂V₁₁O₃₀Cl] as a catalyst for the photocatalytic oxidative degradation of organic dye molecules.⁴⁴ The Mizuno group synthesized a visible-light-responsive Ce-POM: TBA₆[{Ce(H₂O)}₂{Ce(CH₃CN)}₂(μ₄-O)(γ-SiW₁₀O₃₆)₂], which exhibits good photocatalytic activity for the visible-light-promoted oxidative coupling of benzylamine, selective oxidation of thioethers, and α-cyanation of tertiary amines.⁴⁵ Given the pivotal role of C–N bond formation in organic synthesis, there is an urgent need to develop visible-light-responsive POM catalysts for efficient C–N bond construction.

Herein, we successfully synthesized a Ce-containing antimonotungstate, [(CH₃)₂NH₂]₄Na₂₁{[Ce₄(H₂O)₈W₆O₁₂(tar)₃(OAc)][B-α-SbW₉O₃₃]₄}·51H₂O (CeSbW), via a one-step method from an acetate buffer solution at room temperature. CeSbW consists of four [B-α-SbW₉O₃₃]⁹⁻ units linked through a unique central [Ce₄(H₂O)₈W₆O₁₂(tar)₃(OAc)]¹¹⁺ cluster core modified by two carboxylate groups. This structure represents a rare POM integrated with mixed organic carboxylate ligands. CeSbW exhibits a significant response to visible light due to the presence of prominent intramolecular Ce^III-to-polyanion (W^VI) charge transfer. Upon irradiation with visible light, CeSbW exhibits excellent catalytic performance for the conversion of C–H to C–N bonds by hydroacylation of dialkyl azodicarboxylates. The hydroacylation product could be obtained in up to 99% yield with broad substrate scope in an environmentally friendly manner.

Results and discussion

Crystal structure description of CeSbW

CeSbW crystallized in the triclinic space group P (Table S1). CeSbW consists of the organic–inorganic hybrid tetrameric anion {[Ce₄(H₂O)₈W₆O₁₂(tar)₃(OAc)][B-α-SbW₉O₃₃]₄}²⁵⁻ (Fig. 1a and b), twenty-one Na⁺ cations, four [(CH₃)₂NH₂]⁺ cations, and about fifty-one lattice water molecules. The bond-valence sum (BVS) calculations for CeSbW indicate that the oxidation states of the Sb, W, and Ce centers are +3, +6, and +3, respectively (Table S2). In CeSbW, two {SbW₉} units are linked together by one asymmetric [Ce₂(H₂O)₄W₃O₆]¹²⁺ {Ce₂W₃} segment to construct the half-polyanion {[Ce₂(H₂O)₄W₃O₆][SbW₉O₃₃]₂}⁶⁻ (Fig. 1c). The dimeric half-polyanion presents an open C-shaped sandwich structure, providing an ideal cavity for the pentacore {Ce₂W₃} fragment. Vacancies in each three-vacancy {SbW₉} unit are occupied by one W atom and one Ce atom via two W–O–W bonds and two Ce–O–W bonds. Two such {CeW(SbW₉)} units are connected by a third W (W11 or W32) atom, which occupies the remaining vacant oxygen position, thereby forming a dimer (Fig. 1d and e). There are two types of crystallographically independent Ce³⁺ ions in 1a; one type (Ce1 and Ce3) is coordinated by three μ₂-O atoms from three centrally bridged {WO₆} octahedra, in addition to two terminal O atoms from {SbW₉}. The other type (Ce2 and Ce4) is similarly coordinated to the two terminal oxygen atoms from {SbW₉}, but in addition, each ion is simultaneously bound to the three centrally bridged {WO₆} octahedra by three μ₂-O atoms and three coordinated water ligands. In the {Ce₂W₃} cluster, the three {WO₆} octahedra are isolated from each other and do not interact, while the two Ce centers are positioned on opposite sides of a triangle formed by three W atoms. This configuration results in the formation of a triangular bipyramidal structure composed of these five atoms (Fig. 1f).

In the structural unit of the polyanion CeSbW, there are four crystallographically unique Ce³⁺ ions. The coordination numbers and geometries of Ce1/Ce3 and Ce2/Ce4 are nearly identical. The Ce1 and Ce3 ions are nine-coordination monocapped tetragonal antiprisms (Fig. S1), coordinated by three carboxyl oxygen atoms from the tartrate and acetate ligands, and two μ₂-O atoms from the three {SbW₉} units, two μ₂-O atoms from three {WO₂} groups, and one water molecule. The Ce2 and Ce4 ions exhibit an eight-coordinated dodecahedral configuration (Fig. S1), coordinated by two μ₂-O atoms from the three {SbW₉} units, two μ₂-O atoms from three {WO₂} groups, and three water molecules.

Alternatively, CeSbW can be described as a rectangular tetramer, consisting of the decanuclear Ce–W bimetallic fragment [Ce₄(H₂O)₈W₆O₁₂(tar)₃(OAc)]¹¹⁺ {Ce₄W₆(tar)₃(OAc)} (Fig. 1g) and four {SbW₉} fragments. The two pentanuclear {Ce₂W₃} fragments are connected by tartrate and acetate ligands to form the {Ce₄W₆(tar)₃(OAc)} fragment. Specifically, the {Ce₄W₆(tar)₃(OAc)} fragment consists of two subunits: a {Ce₂(H₂O)₂W₂O₂(tar)₂(OAc)}⁵⁺ {Ce₂W₂(tar)₂(OAc)} fragment (Fig. 1h) and a {Ce₂(H₂O)₆W₄O₁₀(tar)}⁶⁺ {Ce₂W₄(tar)} fragment (Fig. 1i). In {Ce₂W₂(tar)₂(OAc)}, the W and Ce atoms (W22 and Ce1 or W1 and Ce3) are bridged by the tartrate ligand. Two oxygen atoms from the carboxyl and hydroxyl groups on the tartrate ligand coordinate with the W22 or W1 atoms. The Ce1 and Ce3 atoms are linked together by two oxygen atoms on the acetate ligand. In {Ce₂W₄(tar)}, two {Ce(H₂O)₃W₂O₅} fragments are connected by a tartrate ligand, with four oxygen atoms from the tartrate ligand coordinated to the W21 and W42 atoms, respectively. Additionally, the free CeSbW units are arranged regularly along the a, b, and c axes in the –AAA– motif (Fig. S2). Notably, in the bc plane, CeSbW units in the same column are parallel, while units in adjacent columns are arranged in an interleaving fashion.

Photophysical properties

First, we tested the photoelectric properties of CeSbW to evaluate its feasibility in photocatalysis. To prepare samples for optical property testing, the Cs salt of CeSbW with poor water solubility (recorded as CeSbW-Cs) was synthesized by reacting CeSbW with CsCl in water. FT-IR spectra indicated that the polyanion structure remained unchanged (Fig. S4). Solid-state UV-vis testing showed that CeSbW-Cs had a wide light absorption range between 200 and 500 nm, indicating its effective utilization of visible light during photocatalysis (Fig. 2a). The optical band gap (E_g) of CeSbW-Cs was determined to be about 2.70 eV using the Kubelka–Munk function and Tauc plot (Fig. 2a inset). Mott–Schottky experiments at 1500, 2000, and 2500 Hz were conducted to calculate the lowest unoccupied molecular orbital (LUMO) of CeSbW-Cs. As shown in Fig. 2b, the LUMO position of CeSbW-Cs is estimated to be −1.35 V vs. Ag/AgCl (−1.15 V vs. NHE). The highest occupied molecular orbital (HOMO) position is 1.44 V vs. NHE according to E_HOMO = E_g + E_LUMO (Fig. 2b inset). As shown in Fig. 2c, the transient photocurrent measurements of CeSbW-Cs exhibited a fast and periodic response during five consecutive rounds of intermittent irradiation. In addition, the electrochemical impedance spectrum (EIS) of CeSbW-Cs was recorded, and a semicircular plot was obtained (Fig. S7). These results indicate efficient spatial separation and migration of photogenerated charge carriers in CeSbW-Cs. In conclusion, the photoelectric performance tests suggested that CeSbW-Cs has great potential to be an excellent photocatalyst.

Photocatalyzed C–N bond formation

The observed excellent optical and electrochemical properties of CeSbW encouraged us to investigate its potential as a photocatalyst for the conversion of C–H to C–N bonds via hydroacylation of dialkyl azodicarboxylates. Initially, a series of solvents, including EtOH, dimethyl carbonate (DMC), ethyl acetate (EA), propylene carbonate (PC), H₂O, and MeCN, were screened using benzaldehyde (1a) and di-tert-butyl azodicarboxylate (2a) as substrates (Table 1, entries 1–6). The results revealed that H₂O was the optimized solvent, affording the product 3a with 75% yield. To further enhance this hydroacylation reaction, various light sources were examined with H₂O as the solvent (Table 1, entries 7–10). The results suggested that a 420 nm LED was the optimal light source and achieved a yield of 99% (turnover number [TON] = 247.5, turnover frequency [TOF] = 82.5 h⁻¹) (Table 1, entry 7). This TOF value was the highest among all reported photocatalysts (Table S3). Additionally, lower yields were observed when decreasing the amount of 1a or shortening the reaction time, respectively (Table 1, entries 11 and 12). The optimized reaction conditions were established as follows: 1a (0.6 mmol), 2a (0.2 mmol), and CeSbW (0.4 mol%) were stirred at room temperature for 3 h under the irradiation of a 420 nm LED (10 W) in H₂O (2 mL). Moreover, the CeSbW-Cs prepared from CeSbW also exhibited good catalytic performance under identical conditions, indicating that the counter-cations had a negligible effect on catalytic activity (Table 1, entry 13).

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The substrate scope of various aldehydes and azodicarboxylates was examined under optimal reaction conditions to demonstrate the broad applicability of the CeSbW-based photocatalytic system (Table 2). First, benzaldehydes bearing electron-donating groups (–Me, –ⁱPr, and –OMe) reacted smoothly with 2a to generate the corresponding products 3a–3e in good yields (80–99% yields). The reaction also proceeded efficiently with a series of halo-substituted benzaldehydes (–F, –Cl, –Br), affording the desired products 3f–3i in high yields (84–93%). Notably, even strongly electron-withdrawing groups such as –CN, –CF₃, and –COOMe were also compatible with this photocatalytic system, producing the hydroacylation products 3j–3l in good yields (80–88%). In addition, naphthaldehydes and N-heterocycle aldehydes served as effective substrates, resulting in the formation of the desired products 3m–3o in excellent yields. Furthermore, diethyl azodicarboxylate and diisopropyl azodicarboxylate exhibited similar reactivity to 2a, achieving yields of 96% and 98%, respectively (3p and 3q).

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H₂O, as a reaction solvent, is not only environmentally benign but also facilitates the separation of products from the reaction mixture, thereby enhancing the feasibility of recycling the CeSbW catalyst. Following the reaction, organic compounds can be readily separated from the aqueous phase by simple extraction with organic solvents, while the aqueous phase containing dissolved CeSbW can be reused for subsequent runs. As shown in Fig. 3a, CeSbW in the aqueous phase was successfully recycled five times without significant deactivation using 1a and 2a as model substrates. The UV-vis absorption spectra before and after catalysis show no obvious changes (Fig. 3b), indicating the excellent reusability and photostability of CeSbW.

A series of control experiments were conducted to investigate the reaction mechanism. As shown in Fig. 3c, CeCl₃·6H₂O exhibited some catalytic activity, albeit lower than that observed for a mixture of Na₉[SbW₉O₃₃]·19.5H₂O ({SbW₉}) and CeCl₃·6H₂O. This indicates a possible charge transfer between the Ce^III centers and the lacunary {SbW₉} ions in solution. The high catalytic efficiency of CeSbW indicates that the crystal framework facilitates the charge transfer from Ce to POMs, enhancing its catalytic activity. When radical inhibitors 2,2,6,6-tetramethylpiperidin-1-yl-oxidanyl (TEMPO) and 2,6-di-tert-butyl-4-methylphenol (BHT) were introduced into the reaction system, the reaction was significantly inhibited, suggesting that this catalytic process may involve radical intermediates. Moreover, the corresponding adduct 4 was detected by high-resolution mass spectrometry, which further indicated that the benzoyl radical was generated in this transformation (Fig. S8). To further investigate the charge transfer from Ce to the polyanion, UV-vis spectroscopy experiments were performed. Upon visible light irradiation (420 nm) of an aqueous solution containing benzaldehyde and CeSbW under a nitrogen atmosphere, the solution color changed from pale yellow to purple, suggesting the possible formation of W^V species (Fig. 3d inset). The intensity of the band around 820 nm, attributed to the intervalence charge transfer from W^V to W^VI, gradually increased under visible light irradiation (Fig. 3d), providing additional evidence for the formation of W^V species. Moreover, when 2a was added to the photoreduced system, the W^V species was rapidly reoxidized within 2 min in the absence of light, with the UV-vis spectrum closely resembling that of the original CeSbW (Fig. 3e). These results indicate that visible-light-induced charge transfer occurs from Ce^III to the polyanion (W^VI) in CeSbW, with 1a participating in the formation of W^V species and 2a facilitating the reoxidation process.

Based on the experimental results and relevant literature,^20,22,45 a plausible mechanism for the CeSbW-catalyzed hydroacylation reaction was proposed (Fig. 3f). Under light irradiation, Ce^IIISbW^VI undergoes intramolecular charge transfer to form an excited state [Ce^IVSbW^V], which subsequently reacts with benzaldehyde 1a via hydrogen atom transfer (HAT) to generate benzoyl radicals A and H⁺[Ce^IIISbW^V]. The radical addition of A to the NN bond of 2a generates the nitrogen radical B. B abstracts the hydrogen proton from H⁺[Ce^IIISbW^V] and is subsequently reduced by [Ce^IIISbW^V], leading to the formation of the desired product along with the regeneration of the photocatalyst.

Conclusions

In conclusion, we successfully synthesized a visible-light-responsive Ce(III)-modified antimonotungstate through a one-pot assembly reaction. The compound exhibits unique intramolecular Ce^III-to-polyanion (W^VI) charge transfer under visible light irradiation, which enables CeSbW to show good visible light absorption and good photoelectric properties. These features endow CeSbW with excellent photocatalytic activity for the visible-light-induced conversion of C–H to C–N bonds. This catalytic reaction proceeds under mild conditions in water with broad substrate scope of aldehydes. The photocatalyst shows good stability and reusability. This work provides guidance for the development of new visible-light-responsive POM catalysts for valuable chemical transformations.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI. SI including materials and instrumentation, synthetic and experimental procedures, characterisation data, details of X-ray crystallographic analysis, and NMR spectra. See DOI: https://doi.org/10.1039/d5qi01526j.

CCDC 2428624 contains the supplementary crystallographic data for this paper.⁴⁶

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22371040 and 22301033), the Jiangxi Provincial Natural Science Foundation (20232ACB213005 and 20242BAB25125), and the Open Fund of Hubei Key Laboratory of Processing and Application of Catalytic Materials (202441804).

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Footnote

† These authors contributed equally to this work.

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