English

• Esterification represents a very common and one of the most essential reaction in the synthesis of pharmaceuticals, polymers, solvents and fragrances [1–3]. In comparison, conventional esterification reactions involve the use of protonic acid (H₂SO₄, HCl, H₃PO₄, etc.) as catalyst to catalyze the coupling of alcohols and carboxylic acids or activated derivatives [1], which is very corrosive to equipment and troublesome to post-treat accompanied by high energy consumption, many by-products and environmental problems. Amberlyst-15, as a solid acid catalyst in place of corrosive strong acids exhibits excellent performance in oxidative esterification of aldehydes with accompanied by the use of large amount of additives [4]. With the advancement of science and technology, selective esterification systems can be achieved efficiently in the presence of a series of transition metal-based catalysts under mild conditions. Esterification catalyzed by rare noble metal (Ru [5–8], Rh [9], Ir [10], Os [11], Pd [12–16]) catalysts ligated with organic ligands generally associated with the shortcomings of being expensive, complicated/commercially unavailable organic ligands, and difficult to recycle (Scheme 1a). In addition, coin metal (Au [17–22]) nanomaterial-catalyzed esterification also suffers from plenty of drawbacks such as being expensive and large deletion of active sites due to uneven loading while copper-based nanomaterials often suffer from low universality of substrates, difficulties in the study of catalytic sites and catalytic mechanisms [23,24]. Meanwhile, organo-catalyst such as L-proline was developed for the esterification of alcohols due to the inherent non-metal feature, mild reaction conditions and broad functional group tolerance. However, a large number of unsalvageable expensive halogenated reagents (NBS) and catalysts were used, thus generating in copious waste and resulting in poor atom-economy [25]. Therefore, it is still promising to develop new environmentally friendly and atomically economical green catalysts for the synthesis of esters.

  Scheme 1

  

  Scheme 1.  Catalytic systems for esterification reaction. (a) Previous catalytic systems. (b) Previous catalytic systems of POM. (c) This work: esterification reaction catalyzed by POV-based molecular catalysts.

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  Polyoxometalates [26–33], as a class of multinuclear pre-transition metal-oxygen clusters with unparalleled structural diversity and outstanding physicochemical properties have been widely used in the field of catalytic oxidation [34–47], especially for the catalyzed esterification due to their metal ions in the highest valence states being able to act as electron storage containers. Yet, Keggin-type and transition metal-substituted single-lacunary polyoxometalate derivatives are by far the predominant polyoxometalate-based esterification catalysts accompanied by disadvantages such as high temperature, poor substrate applicability and atomic diseconomy (Scheme 1b) [48–56]. The combination of vanadium catalysts and H₂O₂ for the oxidative esterification of aldehydes has been developed for many years. However, the unrecyclability of catalysts and the use of large quantities of strong acid reagents [57], complex catalyst synthesis steps and poor substrate universality [58] limited its application. Meanwhile, imidazole ionic liquids are rapidly gaining in popularity in the field of organic catalysis with the advantages of high boiling point, non-volatile and excellent stability [59] in recent years. Forming N-heterocyclic carbenes (NHCs) under basic conditions is a crucial factor to induce aldehydes to undergo polarity reversal and thus the intermediates were capable of nucleophilic substitution reactions [60–68]. However, difficulty to recover and reuse in catalytic systems restrict their practical applications. Thus, a novel catalytic system combines polyoxometalates with imidazole ionic liquids which is expected to improve their respective catalytic performance and enhance the recyclability of the catalyst was developed. Herein, six POV-based molecular crystals {(C₈H₁₅N₂)₂[V₆O₁₃{(OCH₂)₃CCH₂OH}₂] (1, CCDC: 2131494), (C₁₀H₁₉N₂)₂[V₆O₁₃{(OCH₂)₃CCH₂OH}₂] (2, CCDC: 2118473), (C₁₁H₁₃N₂)₂[V₆O₁₃{(OCH₂)₃CCH₂OH}₂] (3, CCDC: 2,118,470), (C₁₁H₂₁N₂)₂[V₆O₁₃{(OCH₂)₃CCH₂OH}₂]^.2DMF (4, CCDC: 2130958), (C₁₇H₁₇N₂)₂[V₆O₁₃{(OCH₂)₃CCH₂OH}₂]^.2ether^.4/3DMF (5, CCDC: 2118471), (C₇H₉O₄N₂)₂[V₆O₁₃{(OCH₂)₃CCH₂OH}₂]^.4DMF (6, CCDC: 2118474)} have been successfully synthesized in this work, which can efficiently catalyze the esterification of aldehydes and alcohols using 30% H₂O₂ as the sole oxidant with advantages of mild condition, retrievability, rich substrate applicability, environmental friendly, synthetically cheap, noble-metal free and prospects for industrial applications (Scheme 1c). These six catalysts were characterized by single crystal X-ray diffraction, ¹H NMR and FT-IR, respectively. Interactions between imidazole ionic and POVs were also investigated by single crystal X-ray diffraction while proposed mechanism for the POV-Based Ionic Liquid-catalyzed esterification of aldehydes with alcohols was investigated by single crystal X-ray diffraction, control experiments, EPR and cyclic voltammetry studies.

  Synthesis of POV-based molecular catalysts: In order to investigate the effect of different structure cations on the reaction, C₈N₂H₁₅Br (abbreviated as (MeBuIm)Br), C₁₀N₂H₁₉Br (abbreviated as (MeHeIm)Br), C₁₁N₂H₁₃Br (abbreviated as (MeBnIm)Br), C₁₁N₂H₂₁Br (abbreviated as (Bu₂Im)Br), C₁₇N₂H₁₇Br (abbreviated as (Bn₂Im)Br) and C₇N₂O₄H₉Br (abbreviated as (Act₂Im)Br) were synthesised by mixing imidazole/1-methylimidazole with excess haloalkane at 60 ℃ for 24 h (Figs. S1-S12 in Supporting information), we subsequently conducted the synthesis of compounds 1–6 by electrostatic assembly between imidazolium-based ILs and (C₁₆H₃₆N)₂[V₆O₁₃{(OCH₂)₃CCH₂OH}₂]{abbreviated as (TBA)₂V₆} in excellent yields and characterized by single-crystal X-ray diffraction (Tables S1-S3 in Supporting information and Fig. 1), NMR (Figs. S13-S18 in Supporting information), FT-IR (Figs. S19-S24 in Supporting information) and ESI-MS (Fig. S25 in Supporting information).

  Figure 1

  

  Figure 1.  Crystal structure of compounds 1–6.

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  The selective aerobic oxidation of benzaldehyde and methanol to form methyl benzoate was chosen as a template reaction. Initially, compounds 1–6 were conducted to catalyze the template reaction (Table 1). The yields of methyl benzoate are 57%/54%/60% with catalysts 1, 2 and 3, respectively, which might be ascribed to the same substituent at position 1 and different substituents at position 3 on the imidazole ring have little effect on the reaction (Table 1, entries 8–10). Methyl benzoate can be generated in higher yield when compounds 4–6 were employed as catalysts due to the better electronic and potential for asymmetric induction of this structure in carrying out the induction of aldehydes (Table 1, entries 11–13). Benzaldehyde and methanol produced methyl benzoate in a high yield of 82% catalysed by cat. 5 due to the kinetic stabilization effect from steric bulk (Table 1, entry 12) [69–71]. Notably, the highest products yield of up to 99% was achieved with cat.6 while (TBA)₂V₆ and (Act₂Im)Br exhibits much lower catalytic activity (Table 1, entries 1 and 7), respectively, that indicates synergistically catalytic effect from both POV and Act₂lm. Moreover, compared to catalysts 1–5, the catalytic activity is also much lower when (MeBuIM)Br, (MeHeIM)Br, (MeBnIM)Br. (Bu₂IM)Br and (Bn₂IM)Br are used as catalysts, respectively (Table 1, entries 2–6). It might be ascribed to supramolecular interactions caused by hydrogen bond (O13…H16) between POV and Act₂Im (Fig. 2) which facilitate the catalysis process. Also, the change in absorbance can be attributed to supramolecular interactions between the POV and Act₂Im in solution (Fig. S26 in Supporting information). Methyl benzoate can also be generated but in much lower yield when the amount of hydrogen peroxide is reduced indicating that the optimal amount of hydrogen peroxide is 4.0 equiv. (Table 1, entries 14–16).

  Table 1

  

  Table 1.  Optimization of reaction conditions.^a

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  Figure 2

  

  Figure 2.  Different hydrogen bonds of cat. 6.

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  With the above optimized conditions in hand, we examined the selective oxidation of various aldehydes and methanol/ethanol required to determine the generalizability of the current conditions (Fig. 3). Some readily available benzaldehyde derivatives bearing electron-rich groups gave lower yields (compounds 8–13) and electron-deficient groups gave excellent yields (compounds 14–16) of corresponding product. The large dipole moment, high polarity and high intermolecular attraction of the nitro compound resulted in an extremely low yield of 37% (compounds 17). It is very pleasing that both 2-naphthaldehyde and 1,4-phthalaldehyde were able to produce the corresponding esters/diesters in yields of 85% and 84%, respectively (compounds 18 and 19). We further investigated the aerobic oxidative esterification of the less reactive ethanol with aldehyde derivatives to produce the corresponding esters (compounds 21–30), ethanol can also undergo facile oxidative esterification to produce the corresponding products smoothly.

  Figure 3

  

  Figure 3.  Esterification of aldehydes and alcohols catalyzed by cat. 6. Reaction conditions: Cat. 6 (0.7 mol%), benzaldehyde (1 mmol), CH₃OH (2.0 mL), 30% H₂O₂ (4.0 equiv.) at 50 ℃. The yield of isolated products.

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  Surprisingly, conversion of benzaldehyde, 4-fluorobenzaldehyde, 4-chlorobenzaldehyde and 2-naphthaldehyde to the corresponding esters in yields of up to 86%/92%/79%/80%, respectively. Linear aliphatic aldehyde and cyclohexanecarboxaldehyde can also undergo mild oxidation to give the corresponding products in good yields (compounds 31–34). To further demonstrate the industrial practicality of this catalytic system, a 10 mmol reaction of benzaldehyde was conducted to give 1.348 g of methyl benzoate in 99% isolated yield, all these results indicate that the catalytic system is unparalleled in the selective oxidation of aldehydes to afford the corresponding esters even has the prospect of industrial application.

  After the reaction, the reaction solution is directly spun off, filtered, and the filtrate is rinsed with a large amount of ether and deionised water and then dried in a vacuum drying oven. In order to verify whether the catalyst can be recycled and reused after the reaction, we first characterized the anionic skeleton of the catalyst by FT-IR (Fig. S29 in Supporting information), the strong and narrow IR absorption band with a maximum at 941 cm⁻¹ is undoubtedly belonging to the v(V=O) vibrations, and the absorption band at 813.08, 790.96, 709.34 cm⁻¹ belonging to the v(V-O-V) vibrations, a narrow and weak absorption peak at 1024 cm⁻¹ comes from the C—O vibration on the anion backbone implies that Tris-OH was successfully modified to the POV, the result shows the IR spectra of the catalyst before and after the reaction were basically identical. The cations of recovered catalyst were characterized by ¹H NMR (Fig. S30 in Supporting information) and ⁵¹V NMR (Fig. 4), proving that the cations also did not change before and after reaction. Furthermore, the single-crystal X-ray diffraction experiments also indicates that the structure of the catalyst remains totally unchanged from its original state. In addition, catalytic performance of recycled catalyst was also conducted for aerobic oxidation of benzaldehyde and methanol, methyl benzoate was obtained in 98% yield.

  Figure 4

  

  Figure 4.  The ⁵¹V NMR spectra of cat. 6 before and after reaction.

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  In order to understand the mutual influence among the studied substrates, the successive CV investigation are carried out with the groups of cat. 6, cat. 6 + H₂O₂ and V₆ + H₂O₂ + benzaldehyde, respectively. As shown in Fig. 5, the cyclic voltammetry of cat. 6 exhibits one well-defined oxidation wave at −84 mV which is attributed to the V^V/IV couple. The oxidation wave of V₆ shift to negative direction from −84 mV to −205 mV with H₂O₂ addition indicating an easier oxidation process due to confirmed interaction between cat. 6 and H₂O₂. Furthermore, more negatively shift happened to the oxidation wave of cat. 6 from −205 mV to −275 mV when reaction substrate benzaldehyde added into cat. 6 + H₂O₂ solution suggesting strong activation upon benzaldehyde with V₆ catalytic system. Otherwise, the necessity of cat. 6 to the studied reaction activity is verified through the CVs of H₂O₂ + benzaldehyde on reverse cyclovoltammograms that no detectable redox waves are observed (Fig. S28 in Supporting information). Furthermore, the oxidative esterification of aldehydes can proceed via simple radical-based oxidation of hemiacetal. In order to confirm whether it generates free radical active species during the catalytic process, EPR spin trapping experiments investigation was used to explore the mechanism of catalytic reaction (Fig. 6), the results show an EPR signal with a g-value of 2.0023 which is caused by a free electron in the anion.

  Figure 5

  

  Figure 5.  CVs comparison of the cat. 6 (black curve), cat. 6 + H₂O₂ (green curve), cat. 6 + H₂O₂ + PhCHO (green curve). Cyclic voltammetry was carried out under CH₃OH solution with 0.1 mol/L TBAPF₆, 1 × 10⁻⁴ mol/L related analyst, respectively. Scan rate: 100 mV/s.

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  Figure 6

  

  Figure 6.  Normalized EPR spectra of DMPO spin adducts.

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  Based on previous reports in the literature [45,72] and studies by CV, EPR, single crystal X-ray diffraction and control experiments (Fig. S31 in Supporting information), a tentative mechanism for the selective oxidation of benzaldehyde and methanol is proposed (Fig. 7). Initially, N-heterocyclic carbene species (I) was generated in the presence of POV which were used as soft base to absorbed proton by pre-forming hydrogen bond (O13…H16). NHeterocyclic carbene attacks benzaldehyde to form a typical Breslow intermediate (II) which will further generate complex (III) containing a α-substituted hydroxyl group. Subsequently, complex (III) transforms to a ketonic intermediate (IV), the desired esters and 6 are released via a nucleophilic substitution reactions between Ⅳ and methanol. In addition, POV are capable of forming HOO-POV (V) intermediates from hydrogen peroxide with strong oxidizing properties and were able to oxidize benzaldehyde and methanol to the corresponding esters.

  Figure 7

  

  Figure 7.  Proposed mechanism for the 6-catalyzed esterification of aldehydes with alcohols.

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  In conclusion, we have demonstrated highly efficient selective oxidative esterification of aldehydes with alcohols catalyst by POV-based molecular catalysts which is nontoxic, green and recyclable. Supramolecular interactions between POV and Act₂Im facilitate catalysis and enabling the conversion of a wide range of aldehydes to the corresponding esters. Furthermore, such POV-based ionic liquid catalysts can be used for large scale preparation of methyl benzoate and recycled with minimal loss of the activity. The generality of this methodology gives it the potential to be used on an industrial scale

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  The work was financially supported by the National Natural Science Foundation of China (NSFC, Nos. 22371158, 21801153 and 21225103), 2021–2023 Intergovernmental S & T Cooperation Project No. 1 between China and Serbia, Tsinghua University Initiative Foundation Research Program (No. 20131089204), and the State Key Laboratory of Natural and Biomimetic Drugs (No. K202008).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.109061.

  
  
• 1. [1]

     J. Otera, Esterification Methods, Reactions and Applications, Wiley-VCH, 2003, pp. 3–144.

  2. [2]

     J. Otera, J. Nishikido, Esterification: Methods, Reactions, and Applications, John Wiley & Sons, 2009.

  3. [3]

     A. Das, P. Theato, Chem. Rev. 116 (2016) 1434–1495. doi: 10.1021/acs.chemrev.5b00291

  4. [4]

     E.M. Gayakwad, V.V. Patil, G.S. Shankarling, New J. Chem. 41 (2017) 2695–2701. doi: 10.1039/C6NJ03831J

  5. [5]

     M. Nielsen, H. Junge, A. Kammer, M. Beller, Angew. Chem. Int. Ed. 124 (2012) 5809–5811. doi: 10.1002/ange.201200625

  6. [6]

     J. Zhang, G. Leitus, Y.B. David, D. Milstein, J. Am. Chem. Soc. 127 (2005) 10840–10841. doi: 10.1021/ja052862b

  7. [7]

     D. Srimani, E. Balaraman, B. Gnanaprakasam, Y.B. David, D. Milstein, Adv. Synth. Catal. 354 (2012) 2403–2406. doi: 10.1002/adsc.201200438

  8. [8]

     E. Fogler, J.A. Garg, P. Hu, et al., Chem. Eur. J. 20 (2014) 15727–15731. doi: 10.1002/chem.201405295

  9. [9]

     J. Cheng, M. Zhu, C. Wang, et al., Chem. Sci. 7 (2016) 4428–4434. doi: 10.1039/C6SC00145A

  10. [10]

      N. Yamamoto, Y. Obora, Y. Ishii, J. Org. Chem. 76 (2011) 2937–2941. doi: 10.1021/jo2003264

  11. [11]

      D. Spasyuk, C. Vicent, D.G. Gusev, J. Am. Chem. Soc. 137 (2015) 3743–3746. doi: 10.1021/ja512389y

  12. [12]

      C. Liu, J. Wang, L. Meng, et al., Angew. Chem. Int. Ed. 50 (2011) 5144–5148. doi: 10.1002/anie.201008073

  13. [13]

      D.S. Mannel, J. King, Y. Preger, et al., ACS Catal. 8 (2018) 1038–1047. doi: 10.1021/acscatal.7b02886

  14. [14]

      D.S. Mannel, M.S. Ahmed, T.W. Root, S.S. Stahl, J. Am. Chem. Soc. 139 (2017) 1690–1698. doi: 10.1021/jacs.6b12722

  15. [15]

      R. Lu, F. Lu, J. Chen, et al., Angew. Chem. Int. Ed. 55 (2016) 249–253. doi: 10.1002/anie.201509149

  16. [16]

      S. Gowrisankar, H. Neumann, M. Beller, Angew. Chem. Int. Ed. 50 (2011) 5139–5143. doi: 10.1002/anie.201008035

  17. [17]

      W.J. Zhou, G.M. Cao, G. Shen, et al., ACS Appl. Mater. Interfaces 50 (2019) 46678–46687.

  18. [18]

      H. Miyamura, T. Yasukawa, S. Kobayashi, Green Chem. 12 (2010) 776–778. doi: 10.1039/b926877d

  19. [19]

      Q. Xiao, Z. Liu, A. Bo, et al., J. Am. Chem. Soc. 137 (2015) 1956–1966. doi: 10.1021/ja511619c

  20. [20]

      S. Rautiainen, O. Simakova, H. Guo, et al., Appl. Catal. A: Gen. 485 (2014) 202–206. doi: 10.1016/j.apcata.2014.08.003

  21. [21]

      L.L. Chng, J. Yang, J.Y. Ying, ChemSusChem 8 (2015) 1916–1925. doi: 10.1002/cssc.201403469

  22. [22]

      S. Lim, S. Kwon, N. Kim, K. Na, Nanomaterials 11 (2021) 1536. doi: 10.3390/nano11061536

  23. [23]

      S.S.R. Gupta, A. Vinu, M.L. Kantam, J. Catal. 389 (2020) 259–269. doi: 10.1016/j.jcat.2020.05.032

  24. [24]

      R.A. Molla, M.A. Iqubal, K. Ghosh, S.M. Islam Kamaluddin, Dalton Trans. 44 (2015) 6546–6559. doi: 10.1039/C4DT03838J

  25. [25]

      X. Liu, J. Wu, Z. Shang, Synth. Commun. 42 (2012) 75–83. doi: 10.1080/00397911.2010.521966

  26. [26]

      M.T. Pope, A. Miiller, Angew. Chem. Int. Ed. 30 (1991) 34–48. doi: 10.1002/anie.199100341

  27. [27]

      J.T. Rhule, C.L. Hill, D.A. Judd, R.F. Schinazi, Chem. Rev. 98 (1998) 327–358. doi: 10.1021/cr960396q

  28. [28]

      Z. Li, Z. Lv, H. Yu, et al., CCS Chem. 4 (2022) 2938–2945. doi: 10.31635/ccschem.021.202101573

  29. [29]

      D. Cheng, B. Li, S. Sun, et al., CCS Chem. 2 (2020) 1649–1658.

  30. [30]

      X. Han, Z. Zhang, T. Zhang, et al., J. Am. Chem. Soc. 136 (2014) 5359–5366. doi: 10.1021/ja412886e

  31. [31]

      Y. Wang, G. Zhuang, J. Zhang, et al., Angew. Chem. Int. Ed. (2022) e202216592.

  32. [32]

      H. Sun, H. Yin, W. Shi, et al., Nano Res. 15 (2022) 3026–3033. doi: 10.1007/s12274-021-3937-3

  33. [33]

      H. Yin, L. Yang, H. Sun, et al., Chin. Chem. Lett. 34 (2023) 107337. doi: 10.1016/j.cclet.2022.03.060

  34. [34]

      C. Venturello, R. D'Aloisio, J.C.J. Bart, M. Ricci, J. Mol. Catal. 32 (1985) 107–110. doi: 10.1016/0304-5102(85)85037-9

  35. [35]

      C. Venturello, E. Alneri, M. Ricci, J. Org. Chem. 48 (1983) 3831–3833. doi: 10.1021/jo00169a052

  36. [36]

      Z. Han, A.M. Bond, C. Zhao, Sci. China Chem. 54 (2011) 1877–1887. doi: 10.1007/s11426-011-4442-4

  37. [37]

      H.L. Li, C. Lian, G.Y. Yang, Sci. China Chem. 65 (2022) 892–897. doi: 10.1007/s11426-022-1215-1

  38. [38]

      C. Venturello, M. Gambaro, Synthesis 4 (1989) 295–297.

  39. [39]

      B. Ten, G. J, I.W.C.E. Arends, R.A. Sheldon, Science 287 (2000) 1636–1639. doi: 10.1126/science.287.5458.1636

  40. [40]

      K. Qin, D. Zang, Y. Wei, Chin. Chem. Lett. 34 (2023) 107999. doi: 10.1016/j.cclet.2022.107999

  41. [41]

      H. Li, M. Yang, Z. Yuan, et al., Chin. Chem. Lett. 33 (2022) 4664–4668. doi: 10.1016/j.cclet.2021.12.081

  42. [42]

      E.V. Chubarova, M.H. Dickman, B. Keita, et al., Angew. Chem. Int. Ed. 47 (2008) 9542–9546. doi: 10.1002/anie.200803527

  43. [43]

      B.K. Chen, X.Q. Huang, B. Wang, et al., Chem. Eur. J. 19 (2013) 4408–4413. doi: 10.1002/chem.201203854

  44. [44]

      B. Nahum, A.M. Khenkin, R. Neumann, J. Am. Chem. Soc. 126 (2004) 10236–10237. doi: 10.1021/ja0493547

  45. [45]

      K. Kamata, K. Yonehara, Y. Nakagawa, K. Uehara, N. Mizuno, Nat. Chem. 2 (2010) 478–483. doi: 10.1038/nchem.648

  46. [46]

      Y. Gu, Q. Li, D. Zang, et al., Angew. Chem. Int. Ed. 60 (2020) 14218–14219.

  47. [47]

      J. Li, D. Zhang, Y. Chi, C. Hu, Catal. Commun. 9 (2008) 1996–1999. doi: 10.1016/j.catcom.2008.03.036

  48. [48]

      J. Li, D. Zhang, Y. Chi, C. Hu, Polyoxometalates 1 (2022) 9140012. doi: 10.26599/POM.2022.9140012

  49. [49]

      L.P. d. Meireles, K.A. d. S. Rocha, I.V. Kozhevnikov, E.V. Gusevskaya, Appl. Catal. A: Gen. 409-410 (2011) 82–86.

  50. [50]

      P. Morin, B. Hamad, G. Sapaly, et al., Appl. Catal. A: Gen. 330 (2007) 69–76. doi: 10.1016/j.apcata.2007.07.011

  51. [51]

      B. Li, W. Ma, C. Han, Micropor. Mesopor. Mater. 156 (2012) 73–79. doi: 10.1016/j.micromeso.2012.02.017

  52. [52]

      E.R.S. Khder, H.M.A. Hassan, M.S.E. Shall, Appl. Catal. A: Gen. 411-412 (2012) 77–86.

  53. [53]

      S.K. Bhorodwaj, D.K. Dutta, Appl. Catal. A: Gen. 378 (2010) 221–226. doi: 10.1016/j.apcata.2010.02.026

  54. [54]

      Z. Weng, N. Ogiwara, T. Kitao, et al., Nanoscale 13 (2021) 18451–18457. doi: 10.1039/D1NR04762K

  55. [55]

      Y. Leng, J. Wang, D. Zhu, et al., Angew. Chem. Int. Ed. 121 (2009) 174–177. doi: 10.1002/ange.200803567

  56. [56]

      Y. Qiao Li, L. Hua, Z. Hou, et al., ChemCatChem 2 (2010) 1165–1170. doi: 10.1002/cctc.201000021

  57. [57]

      R. Gopinath, B.K. Patel, Org. Lett. 2 (2000) 577–579. doi: 10.1021/ol990383+

  58. [58]

      C.B. Vilanculo, M. Jose da Silva, RSC Adv. 11 (2021) 34979–34987. doi: 10.1039/D1RA06718D

  59. [59]

      Y. Qiao, W. Ma, N. Theyssen, C. Chen, Z. Hou, Chem. Rev. 117 (2017) 6881–6928. doi: 10.1021/acs.chemrev.6b00652

  60. [60]

      V. Nair, S. Vellalath, M. Poonoth, R. Mohan, E. Suresh, Org. Lett. 8 (2006) 507–509. doi: 10.1021/ol052926n

  61. [61]

      M. He, J.W. Bode, Org. Lett. 7 (2005) 3131–3134. doi: 10.1021/ol051234w

  62. [62]

      S. Shee, S. Barik, A. Ghosh, A.T. Biju, Org. Lett. 23 (2021) 8039–8044. doi: 10.1021/acs.orglett.1c03059

  63. [63]

      Y. Suzuki, S. Ota, Y. Fukuta, Y. Ueda, M. Sato, J. Org. Chem. 73 (2008) 2420–2423. doi: 10.1021/jo7023569

  64. [64]

      Y. Suzuli, T. Toyota, A. Miyashita, M. Sato, Chem. Pharm. Bull. 54 (2006) 1653–1658. doi: 10.1248/cpb.54.1653

  65. [65]

      V. Nair, B.P. Babu, S. Vellalatha, E. Suresh, Chem. Commun. (2008) 747–749.

  66. [66]

      Y. Suzuki, T. Toyota, F. Imada, M. Sato, A. Miyashita, Chem. Commun. (2003) 1314–1315.

  67. [67]

      V. Nair, S. Vellalath, M. Poonoth, E. Suresh, J. Am. Chem. Soc. 128 (2006) 8736–8737. doi: 10.1021/ja0625677

  68. [68]

      S.S. Sohn, E.L. Rosen, J.W. Bode, J. Am. Chem. Soc. 126 (2004) 14370–14371. doi: 10.1021/ja044714b

  69. [69]

      M.N. Hopkinson, C. Richter, M. Schedler, F. Glorius, Nature 510 (2014) 485–496. doi: 10.1038/nature13384

  70. [70]

      V. Huynh, Chem. Rev. 118 (2018) 9457–9492. doi: 10.1021/acs.chemrev.8b00067

  71. [71]

      N.U.D. Reshi, J.K. Bera, Coord. Chem. Rev. 422 (2020) 213334. doi: 10.1016/j.ccr.2020.213334

  72. [72]

      M. Liu, Zhang Z, H. Liu, Z. Xie, Q. Mei, B. Han, Sci. Adv. 4 (2018), doi: 10.1126/sciadv.aas9319.

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  Scheme 1  Catalytic systems for esterification reaction. (a) Previous catalytic systems. (b) Previous catalytic systems of POM. (c) This work: esterification reaction catalyzed by POV-based molecular catalysts.

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  Figure 1  Crystal structure of compounds 1–6.

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  Figure 2  Different hydrogen bonds of cat. 6.

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  Figure 3  Esterification of aldehydes and alcohols catalyzed by cat. 6. Reaction conditions: Cat. 6 (0.7 mol%), benzaldehyde (1 mmol), CH₃OH (2.0 mL), 30% H₂O₂ (4.0 equiv.) at 50 ℃. The yield of isolated products.

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  Figure 4  The ⁵¹V NMR spectra of cat. 6 before and after reaction.

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  Figure 5  CVs comparison of the cat. 6 (black curve), cat. 6 + H₂O₂ (green curve), cat. 6 + H₂O₂ + PhCHO (green curve). Cyclic voltammetry was carried out under CH₃OH solution with 0.1 mol/L TBAPF₆, 1 × 10⁻⁴ mol/L related analyst, respectively. Scan rate: 100 mV/s.

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  Figure 6  Normalized EPR spectra of DMPO spin adducts.

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  Figure 7  Proposed mechanism for the 6-catalyzed esterification of aldehydes with alcohols.

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  Table 1.  Optimization of reaction conditions.^a

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