Cross-linked Mg–porphyrin polymer as an efficient heterogeneous photocatalyst for the cycloaddition of aziridines with CO₂ under ambient conditions

Abstract

The conversion of CO₂ to value-added chemical products is one of the hottest scientific topics that is receiving considerable interest. Oxazolidinone, a product derived from CO₂, has received widespread attention due to its potential applications in the preparation of natural products. Here, we report a new heterogeneous polymer-supported magnesium–porphyrin as a photocatalyst for the conversion of CO₂ and aziridines to oxazolidinones under ambient conditions (room temperature and 1 atmosphere CO₂ pressure). We further studied the scope of oxazolidinone synthesis using various aziridines, and the catalyst was successfully reused several times.

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Introduction

Carbon dioxide (CO₂) is one of the greenhouse gases, and the significant increase in its concentration in the atmosphere, due to the continuous burning of fossil fuels, is considered one of the main culprits of global climate change.^1–4 Hence, the capture and utilization of carbon dioxide has become one of the important research topics of this century.^5–13 Over the past decade, numerous studies have been conducted to convert CO₂ into high-value-added products via the formation of new C–N, C–O, and C–C bonds.^14,15

Oxazolidinones are important five-membered heterocycles that act as intermediates in organic chemistry and are integral to a class of antimicrobial agents.^16–22 As traditional methods for synthesising oxazolidinones involve the use of toxic or reactive intermediates, greater attention has been paid to the catalytic coupling of CO₂ and aziridines.^23,24 A significant focus has been placed on converting CO₂ and aziridines to oxazolidinones using various homogeneous catalysts, such as metal complexes, N-heterocyclic compounds, ionic liquids, and so on.^25–35 However, most of these catalysts operate at high temperatures or high pressures and/or require high catalyst loadings. In contrast, heterogeneous catalysts offer reusability and easy separation of the products.^36–43

Considerable progress has been made in the growth of homogenous catalysts for the conversion of CO₂ and aziridines to oxazolidinones; however, compared to homogenous catalysts, heterogeneous catalysts have received far less attention for the same reaction, possibly due to difficulties involved in immobilization of homogenous systems.^44–46 Recently, we reported homogeneous Mg–porphyrin as an effective photocatalyst for the conversion of CO₂ into oxazolidinones and cyclic carbonates.⁴⁷ Although, our protocol was highly efficient, reusing the catalyst was relatively difficult. Motivated by the use of homogenous Mg–porphyrin as an active photocatalyst, we developed a heterogeneous sustainable catalytic system using Mg–porphyrin for the conversion of CO₂ to oxazolidinones under mild conditions.

Results and discussion

The porphyrin with four styrene units (Por-M) was prepared by reacting meso-tetra(p-hydroxyphenyl)porphyrin with 4-vinylbenzyl bromide in the presence of a base (K₂CO₃). The styrene-substituted monomer Por-M was polymerized in 1,2-dichloroethane (DCE) using 2,2′- azobis(isobutyronitrile) (AIBN) as an initiator (Scheme 1). As Por-M has four styrene units, it acts as a cross-linker by itself, and the polymer thus formed was insoluble in all organic solvents. The monomer and the polymer were characterized using different techniques. The ¹H NMR of the monomer showed the expected AMX pattern between 5.30 and 6.85 ppm. Por-M was further characterized using high-resolution mass spectrometry (HRMS). The cross-linked polymer of Por-M was characterized using ¹³C cross-polarization magic angle spinning (CPMAS). The ¹³C CPMAS of Por-CP showed peaks ranging from 28 ppm to 167 ppm. The peaks in the δ 166–155 ppm range represent the porphyrin skeleton, δ 155–108 ppm refers to the benzene ring, and δ 83–66 ppm is assigned to the –CH₂–O– linker carbons. Additional peaks that appeared in the 61–26 ppm range compared to the monomer are assignable to the (–CH₂–CH–) linker (Fig. 1). Por-CP was subjected to metalation using MgCl₂ in DMF at reflux conditions for 24 h. After filtration, the solid was subjected to Soxhlet extraction to obtain MgPor-CP in 81% yield. The ¹³C CPMAS of MgPor-CP showed similar features to those of Por-CP. We also prepared ZnPor-CP for comparative studies. MgPor-M was synthesised, and the UV-vis absorption spectra of Por-M and MgPor-M recorded in dichloromethane (CH₂Cl₂) are depicted in Fig. 2. Por-M showed a strong Soret band at 422 nm and four Q bands at 518, 555, 594, and 651 nm. The formation of MgPor-M was indicated by the observation of a red-shifted Soret band at 428 nm and the reduction of Q bands to two. MgPor-M, upon excitation at the absorption maximum, revealed a strong emission band at 617 nm. We performed TD-DFT calculations at the B3LYP level of theory and 6-31G(d,p) basis functions to correlate with the UV-visible spectra of aziridine-bound Mg–porphyrin. For simplicity, we used the Mg–porphyrin with only one phenyl group at the meso-position for calculations. The theoretical UV-visible spectrum for the aziridine-coordinated Mg–porphyrin indicated a strong peak at 389 nm and weak peaks at 559 nm and 561 nm. The data pertaining to the theoretical studies are presented in the supporting information (Tables S1 and S2†).

Scheme 1 Synthesis of MgPor-CP.

Fig. 1 Comparison of ¹³C NMR of Por-M (top) and ¹³C CPMAS of polymer, Por-CP (bottom).

Fig. 2 Absorption and emission spectra of Por-M and MgPor-M in CH₂Cl₂ at 10⁻⁵ M concentration.

We studied the morphology of the synthesized MgPor-CP using different techniques. As shown in Fig. 3a, the scanning electron microscope (SEM) reveals that MgPor-CP consists of relatively agglomerated sub-micron spheres. A high-resolution transmission electron microscope shows the presence of irregular pores in the polymer (Fig. 3b). Elemental mapping using energy-dispersive X-ray (EDX) on large areas confirms that magnesium is homogeneously present on the support (Fig. 4).

Fig. 3 (a) SEM image (500 nm), (b) TEM image: (i) 200 nm, (ii) 50 nm.

Fig. 4 Elemental distribution in MgPor-CP determined by TEM-EDX: (I) magnesium, (II) carbon, (III) nitrogen, (IV) oxygen, and (V) original image.

Elemental analysis of MgPor-CP using ICP-OES reveals a magnesium loading of 0.42 mmol g⁻¹, while the ZnPor-CP resulted in a zinc loading of 0.46 mmol g⁻¹. In both instances, the experimental values are lower than the theoretical values, which suggests that the metalation is partial. Thermogravimetric analysis (TGA) of the Por-CP and MgPor-CP in N₂ flow shows a decomposition temperature as high as 340 °C (Fig. S3†). This result indicates that MgPor-CP has excellent thermal stability, which is very important for any catalyst used in organic transformations.

As the morphological studies of MgPor-CP reveal the presence of pores, we investigated MgPor-CP using nitrogen adsorption analysis at 77.3 K and CO₂ adsorption analysis at 298 K (Fig. S3†). N₂ adsorption analysis of MgPor-CP showed a type I adsorption isotherm, which is characteristic of microporous materials. The average pore radius was found to be 0.4 nm, and the surface area observed was 197 m² g⁻¹. Furthermore, the kinetic diameter of CO₂ is approximately 0.33 nanometers (nm), which is smaller than the pore radius of the material. These pores will help effectively capture and concentrate CO₂ to enhance catalytic activity. The CO₂ adsorption isotherm of MgPor-CP displayed reversible adsorption capacity (Fig. S3†).

With the synthesized MgPor-CP in hand, we screened its ability as a photocatalyst for the synthesis of oxazolidinones. From our earlier studies using Mg–porphyrin as a homogenous catalyst, we noted that TBAB serves as a better co-catalyst for the production of oxazolidinones.⁴⁷ Hence, we investigated the conversion of 1-butyl-2-phenyl-aziridine under 1 bar CO₂ pressure using 0.1 mol% of MgPor-CP and 1.0 mol% of TBAB as the catalyst and co-catalyst using blue LED irradiation. Under the specified conditions, a yield of 49% of 3-butyl-5-phenyl-2-oxazolidinone was observed (Table 1, entry 1). With an increase in catalyst and co-catalyst loading, the yield of the oxazolidinone also increased (Table 1, entries 2 and 3). Further increasing the reaction time to 30 and 36 h produced yields of 90% and 97%, respectively (Table 1, entries 4,5). Among the different salts tested, such as TBAI, TBAC, TBAB, TBAAc, and NaI, TBAB exhibited the best activity (Table 1, entries 7–10). A decrease in the catalyst : co-catalyst ratio resulted in a decrease in oxazolidinone yield (Table 1, entry 6). The use of purple light, instead of blue light led to a minor decrease in yield (Table 1, entry 12). The product yield decreased when the reaction was performed without light (Table 1, entry 11). The catalyst alone was not able to catalyse the reaction; however, 20% of the product was produced when the reaction was executed using only the co-catalyst (Table 1, entries 13 and 14). ZnPor-CP was also screened as a catalyst under identical experimental conditions, showing lower activity compared to the magnesium catalyst (Table 1, entry 15 and Fig. S4†). When Por-CP (same weight as MgPor-CP) was used instead of MgPor-CP, 22% of the product was formed (Table 1, entry 16). In the absence of both the catalyst and light, the yield for the reaction was found to be 20% (Table 1, entry 17).

Table 1 MgPor-CP-catalysed CO₂ conversion of aziridine to oxazolidinone^a

Entry	Catalyst (mol%)	Co-catalyst (mol%)	Light	Time (h)	Yield^b (%)
a All reactions were performed at room temperature. b Yield is based on ^1H NMR analysis (1,3,5-trimethyl benzene as internal standard in CDCl3). c ZnPor-CP is used as a catalyst. d Por-CP 6.4 mg was used. (TBAC - Tetrabutylammonium chloride, TBAB - tetrabutylammonium bromide, TBAI - tetrabutylammonium iodide, TBAAc - tetrabutylammonium acetate, NaI - sodium iodide).
1	0.1	TBAB (1)	Blue	24	49
2	0.15	TBAB (1.5)	Blue	24	65
3	0.2	TBAB (2)	Blue	24	81
4	0.2	TBAB (2)	Blue	30	90
5	0.2	TBAB (2)	Blue	36	97
6	0.2	TBAB (1.4)	Blue	36	87
7	0.2	TBAC (2)	Blue	36	74
8	0.2	TBAI (2)	Blue	36	38
9	0.2	TBAAc (2)	Blue	36	66
10	0.2	NaI (2)	Blue	36	32
11	0.2	TBAB (2)	Dark	36	54
12	0.2	TBAB (2)	Purple	36	95
13	0.2	—	Blue	36	0
14	—	TBAB (2)	Blue	36	20
15^c	0.2(Zn)	TBAB (2)	Blue	36	84
16^d	Por-CP	TBAB (2)	Blue	36	22
17	—	TBAB (2)	Dark	36	20

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With the optimized conditions in hand, the scope of various aziridines was screened. Various N-alkyl phenyl aziridines, such as N-hexyl, N-butyl, N-propyl, N-isopropyl, N-ethyl, and N-methyl phenyl aziridines, yielded oxazolidinones (Table 2) in good to excellent yields (96% (1a), 96% (1b), 95% (1c), 94% (1d), 53% (1e), and 87% (1f)). N-Alkyl (p-tolyl) aziridines like N–CH₃, N–C₂H₅, N–C₃H₇, N–C₄H₉, and N–C₆H₁₃-substituted (p-tolyl) aziridines produced the respective oxazolidinones in excellent yields (96% (1g), 96% (1h), 95% (1i), 93% (1j), 85% (1k)). 1-(2-Methoxyethyl)-2-phenylaziridine, 1-(2-methoxyethyl)-2-(p-tolyl) aziridine and 1-(2-methoxyethyl)-2-(4-bromophenyl) aziridine also produced the corresponding oxazolidinones with yields of 94%, 92%, and 92% (Table 2, 1l–1n). A decrease in yield was observed in some instances, especially for 1f, 1k, and 1e, which may be due to an increase in chain length and branching of the alkyl group present in the nitrogen (R₂). The reaction of 2-(4-bromophenyl)-1-ethylaziridine and 2-(4-bromophenyl)-1-butylaziridine with CO₂ under the reaction conditions produced good yields of 95% and 93% (Table 2, 1o and 1p) for the corresponding oxazolidinones. Benzyl derivatives, such as 1-benzyl-2-phenylaziridine and 1-[(4- methoxyphenyl) methyl]-2-phenylaziridine, yielded the respective oxazolidinones, 1q and 1r (Table 2), in moderate yields of 85% and 83%, likely due to the bulkiness of the benzyl derivative. The solid N-tosylaziridine dissolved in 0.2 mL of CH₂Cl₂ was converted to its respective oxazolidinone with a low yield of 42% (Table 2, 1s), probably due to dilution factors and the steric effect of the “tosyl” group.

Table 2 Scope of cycloaddition of aziridines and CO₂^a,b

a Reaction conditions: blue LED light, CO₂ (1 bar), solvent-free, catalyst (0.2 mol%), TBAB (2 mol%), 36 h, room temperature. b Isolated yields. c Reaction conditions: blue LED light, CO₂ (1 bar), 0.2 mL CH₂Cl₂, catalyst (0.2 mol%), TBAB (2 mol%), 36 h, room temperature.

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To reveal the importance of continuous illumination with blue LED light, controlled experiments were performed. Initially, the reaction mixtures were stirred under blue LED for 2 h; subsequently, the reactions were stirred in the dark for an additional 6 h, 14 h, 22 h, 28 h, and 36 h. These reactions yielded substantially lower results compared to the optimised conditions, proving that continuous illumination is required for better conversion.

The recyclability of MgPor-CP was tested under the optimized conditions by recovering the used catalyst from the reaction mixture, washing it with methanol and acetone, and drying it under a high vacuum. In each recycling, fresh aziridine and TBAB were added to continue the next cycle. Notably, after recycling five times (Fig. 5), the heterogeneous catalyst MgPor-CP retained its activity. The reused catalyst was characterized by SEM, TEM, ICP-OES, and TEM-EDX (Fig. 6 and 7). Both SEM and TEM images show the structural integrity after the recycling experiment. The Mg content from the ICP-OES was found to be 0.40 mmol g⁻¹, suggesting that there was no leaching of Mg under the experimental conditions. Furthermore, we also analysed the filtrate for any trace amount of magnesium presence in the reaction mixture; however, it was undetectable in the filtrate, further supporting that under the experimental conditions, MgPor-CP is highly stable. A comparison with the homogeneous catalyst reported for the oxazolidinone synthesis showed that the activity of the present system (turnover number is 485) is inferior to that of the homogeneous system (turnover number is 840).⁴⁷ However, the thermal stability of the MgPor-CP is superior to that of the homogeneous system and also has the advantage of reusability, as it can be separated from the reaction mixture by simple filtration.

Fig. 5 Recyclability of MgPor-CP. Reaction conditions: 1-butyl-2-phenyl-aziridine (2 mL, 11.12 mmol), MgPor-CP (53.3 mg), TBAB (72 mg), an initial pressure of 1 bar, RT, 36 h.

Fig. 6 (a) SEM image (500 nm), (b) TEM image (500 nm).

Fig. 7 Elemental distribution in reused MgPor-CP determined by TEM-EDX: (a) magnesium, (b) carbon, (c) nitrogen, (d) oxygen, (e) and overlay (f) for the polymer.

The EPR spectroscopy study with the catalyst and substrate in the presence of light confirmed the non-involvement of radical species in this process. Additionally, the reaction of 1-butyl-2-phenylaziridine in the presence of TEMPO (2 mol%) under the optimized conditions yielded 73% of the desired product, indicating that the reaction does not progress through free radical formation. Based on previous reports, a probable catalytic cycle for the cycloaddition of aziridines with CO₂ is shown in Scheme 2.^48–50 As shown in the scheme, the catalytic cycle starts with the coordination of aziridines to the magnesium centre in its excited state (intermediate (I) to intermediate (II)). The next step involves the nucleophilic attack on the more-hindered side of the activated aziridine by the bromide ion of the co-catalyst, forming the ring-opened species (III). The final step pertains to CO₂ insertion into the ring-opened aziridine, followed by an intramolecular nucleophilic reaction to generate the respective product and restore the excited state of MgPor-CP.

Scheme 2 Probable mechanism for the cycloaddition of CO₂ with aziridines under blue LED light.

Conclusion

A porphyrin-based cross-linked polymer (Por-CP) was successfully synthesized from a newly developed porphyrin monomer. The metalation of the cross-linked polymer with Mg and Zn resulted in the formation of the desired MgPor-CP and ZnPor-CP catalysts. With TBAB as a nucleophilic additive, both catalysts are active as heterogeneous photocatalysts. The cycloaddition of aziridines with CO₂ was examined with MgPor-CP, which is more active than ZnPor-CP, leading to the formation of oxazolidinones under ambient conditions. In particular, the heterogeneous photocatalyst MgPor-CP was recycled multiple times without any loss of activity.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

KV thanks the Department of Atomic Energy (DAE) (RIN 4002) and the Science and Engineering Research Board (SERB) (CRG/2023/000085) for financial support. SKM thank CSIR for the research fellowship.

References

1. B. Grignard, S. Gennen, C. Jérôme, A. W. Kleij and C. Detrembleur, Chem. Soc. Rev., 2019, 48, 4466–4514.
2. R. Luo, M. Chen, F. Zhou, J. Zhan, Q. Deng, Y. Yu, Y. Zhang, W. Xu and Y. Fang, J. Mater. Chem. A, 2021, 9, 25731–25749.
3. G. Rothenberg, Sustain. Chem. Clim. Action, 2023, 2, 100012.
4. Z. Zhang, Y. Zheng, L. Qian, D. Luo, H. Dou, G. Wen, A. Yu and Z. Chen, Adv. Mater., 2022, 34, 2201547.
5. E. S. Sanz-Pérez, C. R. Murdock, S. A. Didas and C. W. Jones, Chem. Rev., 2016, 116, 11840–11876.
6. M. Aresta and A. Dibenedetto, Dalton Trans., 2007, 2975.
7. J. Godin, W. Liu, S. Ren and C. C. Xu, J. Environ. Chem. Eng., 2021, 9, 105644.
8. C. Kim, C.-J. Yoo, H.-S. Oh, B. K. Min and U. Lee, J. CO2 Util., 2022, 65, 102239.
9. G. Lutzweiler, Y. Zhang and B. Louis, Curr. Opin. Green Sustainable Chem., 2022, 37, 100679.
10. J. E. Gómez and A. W. Kleij, in Adv. Organomet. Chem., Elsevier, 2019, vol. 71, pp. 175–226.
11. H. V. Babu and K. Muralidharan, Dalton Trans., 2013, 42, 1238–1248.
12. S. Ghosh, T. S. Khan, A. Ghosh, A. H. Chowdhury, M. A. Haider, A. Khan and S. M. Islam, ACS Sustainable Chem. Eng., 2020, 8, 5495–5513.
13. L. J. R. Nunes, Environments, 2023, 10, 66.
14. P. Brunel, J. Monot, C. E. Kefalidis, L. Maron, B. Martin-Vaca and D. Bourissou, ACS Catal., 2017, 7, 2652–2660.
15. X. Liu, X. Li and L. He, Eur. J. Org. Chem., 2019, 2019, 2437–2447.
16. A. Bhushan, N. J. Martucci, O. B. Usta and M. L. Yarmush, Expert Opin. Drug Metab. Toxicol., 2016, 12, 475–477.
17. N. Pandit, R. K. Singla and B. Shrivastava, Int. J. Med. Chem., 2012, 2012, 1–24.
18. C. Roger, J. A. Roberts and L. Muller, Clin. Pharmacokinet., 2018, 57, 559–575.
19. A. Zahedi Bialvaei, M. Rahbar, M. Yousefi, M. Asgharzadeh and H. Samadi Kafil, J. Antimicrob. Chemother., 2017, 72, 354–364.
20. D. McBride, T. Krekel, K. Hsueh and M. J. Durkin, Expert Opin. Drug Metab. Toxicol., 2017, 13, 331–337.
21. F. Moureau, J. Wouters, D. Vercauteren, S. Collin, G. Evrard, F. Durant, F. Ducrey, J. Koenig and F. Jarreau, Eur. J. Med. Chem., 1992, 27, 939–948.
22. C. Foti, A. Piperno, A. Scala and O. Giuffrè, Molecules, 2021, 26, 4280.
23. F. Sun, E. V. Van Der Eycken and H. Feng, Adv. Synth. Catal., 2021, 363, 5168–5195.
24. D. Carminati, E. Gallo, C. Damiano, A. Caselli and D. Intrieri, Eur. J. Inorg. Chem., 2018, 2018, 5258–5262.
25. A. Liu, Y. Dang, H. Zhou, J. Zhang and X. Lu, ChemCatChem, 2018, 10, 2686–2692.
26. V. B. Saptal and B. M. Bhanage, ChemSusChem, 2016, 9, 1980–1985.
27. F. Zhou, S.-L. Xie, X.-T. Gao, R. Zhang, C.-H. Wang, G.-Q. Yin and J. Zhou, Green Chem., 2017, 19, 3908–3915.
28. X.-R. Tian, Y. Shi, S.-L. Hou, Y. Ma and B. Zhao, Inorg. Chem., 2021, 60, 15383–15389.
29. A. C. Kathalikkattil, R. Roshan, J. Tharun, R. Babu, G.-S. Jeong, D.-W. Kim, S. J. Cho and D.-W. Park, Chem. Commun., 2016, 52, 280–283.
30. C.-S. Cao, Y. Shi, H. Xu and B. Zhao, Dalton Trans., 2018, 47, 4545–4553.
31. X. Kang, L. Yao, Z. Jiao and B. Zhao, Chem. – Asian J., 2019, 14, 3668–3674.
32. Y. Jiang, T. Hu, L. Yu and Y. Ding, Nanoscale Adv., 2021, 3, 4079–4088.
33. M. D. G. Billacura, R. D. Lewis, N. Bricklebank, A. Hamilton and C. J. Whiteoak, Adv. Synth. Catal., 2023, 365, 3129–3137.
34. K. Fujita, A. Fujii, J. Sato, S. Onozawa and H. Yasuda, Tetrahedron Lett., 2016, 57, 1282–1284.
35. Z.-Z. Yang, L.-N. He, S.-Y. Peng and A.-H. Liu, Green Chem., 2010, 12, 1850.
36. R. A. Sheldon and R. S. Downing, Appl. Catal. A: Gen., 1999, 189, 163–183.
37. H. S. Whang, J. Lim, M. S. Choi, J. Lee and H. Lee, BMC Chem. Eng., 2019, 1, 9.
38. Z. Dai, Q. Sun, X. Liu, C. Bian, Q. Wu, S. Pan, L. Wang, X. Meng, F. Deng and F.-S. Xiao, J. Catal., 2016, 338, 202–209.
39. R. Das, S. S. Manna, B. Pathak and C. M. Nagaraja, ACS Appl. Mater. Interfaces, 2022, 14, 33285–33296.
40. W. Wang, C. Li, J. Jin, L. Yan and Y. Ding, Dalton Trans., 2018, 47, 13135–13141.
41. S. Jayakumar, H. Li, L. Tao, C. Li, L. Liu, J. Chen and Q. Yang, ACS Sustainable Chem. Eng., 2018, 6, 9237–9245.
42. D. Wang, D. Wang, P. Yan, Z. Li, N. Wang and J. Li, Ind. Eng. Chem. Res., 2024, 63, 13469–13479.
43. L. Xu, Y. Wang, Z. Sun, Z. Chen, G. Zhao, F. E. Kühn, W.-G. Jia, R. Yun and R. Zhong, Inorg. Chem., 2024, 63, 1828–1839.
44. A. C. Kathalikkattil, R. Roshan, J. Tharun, R. Babu, G.-S. Jeong, D.-W. Kim, S. J. Cho and D.-W. Park, Chem. Commun., 2016, 52, 280–283.
45. C.-S. Cao, Y. Shi, H. Xu and B. Zhao, Dalton Trans., 2018, 47, 4545–4553.
46. (a) X. Kang, L. Yao, Z. Jiao and B. Zhao, Chem. – Asian J., 2019, 14, 3668–3674; (b) X.-R. Tian, Y. Shi, S.-L. Hou, Y. Ma and B. Zhao, Inorg. Chem., 2021, 60, 15383–15389.
47. S. K. Meher, P. Nayak, S. Dhala, S. Tripathy and K. Venkatasubbaiah, Catal. Sci. Technol., 2024, 14, 3125–3130.
48. C. Damiano, P. Sonzini, M. Cavalleri, G. Manca and E. Gallo, Inorg. Chim. Acta, 2022, 540, 121065.
49. P. Nayak, A. C. Murali, V. R. Velpuri, V. Chandrasekhar and K. Venkatasubbaiah, Adv. Synth. Catal., 2023, 365, 230–237.
50. P. Nayak, A. C. Murali, P. K. Pal, U. D. Priyakumar, V. Chandrasekhar and K. Venkatasubbaiah, Inorg. Chem., 2022, 61, 14511–14516.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5cy00089k

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