Catalytic performance of Keplerate polyoxomolybdates in green epoxidation of alkenes with hydrogen peroxide

Abstract

In this work, the catalytic perspective of three Keplerate polyoxometalates, Mo₇₂M₃₀ (M = V, Cr, Fe) in the epoxidation of various alkenes is considered. The catalytic reactions were performed in water under mild conditions and high to excellent conversions and selectivities were obtained using H₂O₂ as oxidant and Mo₇₂Cr₃₀ as the best catalyst. Keplerate POM catalysts have shown a very good recyclability, up to five times, without any appreciable decreases in catalytic activity and selectivity.

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1. Introduction

Selective oxidation of olefins to epoxides is among the important reactions in the chemical industry and organic chemistry.^1,2 To develop environmentally benign synthetic methodologies, there is a great interest in new and more efficient catalytic oxidations based on low-cost, accessible and environmentally benign catalysts and oxidants.^3–5 The oxidation of olefins with aqueous hydrogen peroxide has received much attention, since H₂O₂ is clean, inexpensive, safe, readily available, has a high content of active oxygen species and gives water as the sole by-product.⁶ Furthermore, using nontoxic solvents such as water, which is safer, cheaper, and more environmentally friendly compared to other solvents in use today, is a major problem in these reactions.⁷

Polyoxometalates (POMs), as a class of metal–oxygen clusters, because of the different structures and the diversity in shapes, charges, compositions, solubility and reactivity showed a lot of applications.^8,9 Increasing in POMs usage are evidently revealed in various literatures in the fields of catalysis, material science, medicine, analytical chemistry and so on.^10–12 In the last decades various types of POMs have drawn wide attention as effective and important catalysts for epoxidation of alkenes.^13–24

Metal oxide based nanocapsules of the type {(Mo^VI)Mo^VI₅}₁₂(linker)₃₀ which called Keplerates was firstly reported by Müller and coworkers.²⁵ Keplerate POMs have attracted considerable interest and indicated a lot of applicability in various fields such as chemical science, molecular physics, magnetochemistry, biological sciences, materials science, and even mathematics.²⁶ However, despite extremely promising potential in different fields of science the catalytic properties of Keplerate POMs in the oxidation of organic compounds really needs to be further investigations.²⁷ For the first time, the catalytic activity of Keplerate POMs in the oxidation of sulfides was reported.²⁸ Selective epoxidation of an aqueous suspension of olefins under mild reaction conditions catalyzed by (NH₄)₄₂[Mo^VI₇₂Mo^V₆₀O₃₇₂ (CH₃COO)₃₀ (H₂O)₇₂] was the other work.^29,30 It was also reported that Mo₇₂M₃₀ is thermodynamically stable to oxidation and contains transition metals (molybdenum and iron) capable of activating various oxidants.²⁸ Furthermore, Mo₇₂M₃₀ have been shown very well stability in various solvents.³¹ In continuation of our current research,³² a novel method for oxidation of organic compounds with hydrogen peroxide as a green oxidant in the presence of Keplerate POMs and especially in sustainable media was developed. In this regard, for the first time, the catalytic epoxidation of different alkenes using 30% H₂O₂ catalyzed by three Keplerate POMs, Na₈K₁₄(VO)₂[{(Mo^VI)Mo^VI₅O₂₁(H₂O)₃}₁₀{(Mo^VI)Mo^VI₅O₂₁(H₂O)₃(SO₄)}₂{V^IVO(H₂O)}₂₀{V^IVO}₁₀({KSO₄}₅)₂]·150H₂O {Mo₇₂V₃₀}, Mo^VI₇₂Fe^III₃₀O₂₅₂(CH₃COO)₁₂{Mo₂O₇(H₂O)}₂{H₂Mo₂O₈(H₂O)}(H₂O)₉₁]·150H₂O {Mo₇₂Fe₃₀} and [{Na(H₂O)₁₂}[Mo^VI₇₂Cr^III₃₀O₂₅₂(CH₃COO)₁₉(H₂O)₉₄}]·120H₂O {Mo₇₂Cr₃₀}, in aqueous media are described (Scheme 1).

Scheme 1 Catalytic epoxidation of alkenes with H₂O₂ catalyzed by Mo₇₂V₃₀, Mo₇₂Cr₃₀ and Mo₇₂Fe₃₀.

2. Results and discussion

In our previous work, the catalytic activity of Keplerate POMs, Mo₇₂M₃₀ (M = V, Cr, Fe), in successfully conversion of different sulfides into the corresponding sulfones under mild reaction conditions with high to excellent yields were showed. Also, the reusability of these catalysts was very good and spectral results showed that the structure of recycled catalyst is preserved and it is stable since catalytic recycling.³³

In this work, at first, the catalytic efficiency of three Keplerate POMs (Mo₇₂Fe₃₀, Mo₇₂Cr₃₀, and Mo₇₂V₃₀) in the epoxidation of olefins was tested using cyclooctene as a model compound. In order to obtain the optimum amounts of Keplerate POM catalysts, the epoxidation of cyclooctene was studied by various amounts of Mo₇₂Cr₃₀. From the results in Fig. 1, by increasing in the catalyst amounts up to 10 μmol of Mo₇₂Cr₃₀ the yields of epoxide was improved. In the presence of higher amounts of Keplerate catalyst the selectivity for epoxide was decreased. In this system, it was also showed that the presence of catalyst is crucial for the oxidation reaction. So, 10 μmol of Mo₇₂Cr₃₀ which provides the highest yields for epoxidation of cyclooctene was chosen as the best amount of catalyst.

Fig. 1 The effect of Mo₇₂Cr₃₀ amounts in the epoxidation of cyclooctene by 30% hydrogen peroxide in water after 2.5 h reaction. Reaction conditions: cyclooctene (1 mmol), H₂O₂ (1 mmol) and catalyst at 50 °C. Yields refer to GC yield.

In the oxidation reaction of cyclooctene by different amounts of H₂O₂, the optimum amount of hydrogen peroxide was obtained in the presence of 10 μmol catalyst after 2.5 h reaction. The results in Fig. 2 show that 1 mmol of 30% H₂O₂ is sufficient for completion of the reaction at request time. Thus, 1 mmol of hydrogen peroxide was chosen as the best amount of oxidant in this catalytic epoxidation system.

Fig. 2 The effect of hydrogen peroxide amounts in the epoxidation of cyclooctene (1 mmol) catalyzed by Mo₇₂Cr₃₀ (10 μmol) at 50 °C after 2.5 h reaction. Yields refer to GC yield.

By the optimal reaction conditions – 10 μmol of catalyst, 1 mmol of 30% H₂O₂, 50 °C temperature, and water as solvent – selective epoxidation of various alkenes with Mo₇₂V₃₀, Mo₇₂Cr₃₀, and Mo₇₂Fe₃₀ as catalyst were investigated (Table 1). As it could be seen in Table 1, epoxidation of styrene was gave styrene oxide as the main product by 98% selectivity (Table 1, entry 4). The catalytic epoxidation of cyclic alkenes such as cyclohexene, cyclooctene, and indene were efficiently proceeded to afford the corresponding epoxides in high to excellent yields (Table 1, entries 1–3, 5). Also, nonactivated aliphatic alkenes such as 1-octene has been converted into the corresponding epoxide selectively (Table 1, entry 6). In 2-cylohexene-1-ol (Table 1, entry 7) C=C double bond was selectively oxidized into corresponding epoxide and hydroxyl functional groups remained intact.

Table 1 Epoxidation of different alkenes with 30% H₂O₂ catalyzed by Mo₇₂M₃₀ (M = V, Fe, Cr) in water^a

Entry	Alkene	Time (h)	Epoxide^b (%)
			Mo72Fe30	Mo72V30	Mo72Cr30
a Reaction conditions: the reactions were run at 50 °C in 3 mL of aqueous suspension of olefin (1 mmol), 10 μmol of Mo72M30 (M = V, Fe, Cr) as catalyst and 1 mmol of 30% H2O2.b Yields refer to isolated yields.c The selectivity of epoxides was 98% and 2% benzaldehyde was also produced.
1		5	10	83	92
2		5	7	63	75
3		2.5	7	86	97
4^c		3	8	85	96
5		6	7	80	91
6		6	6	79	87
7		6.5	6	83	92

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Among three Keplerate POM catalysts in epoxidation of various alkenes under the same reaction conditions, Mo₇₂Cr₃₀ was shown higher catalytic activity than two others. Albeit, Mo₇₂V₃₀ was also demonstrated a better catalytic activity respect to Mo₇₂Fe₃₀.

It was reported that the catalytic activity of POMs could be inversely correlated to the rate of hydrogen peroxide dismutation.³⁴ So, in this work hydrogen peroxide dismutation by Mo₇₂Cr₃₀, Mo₇₂V₃₀, and Mo₇₂Fe₃₀ was also studied. Our results showed that in the presence of Mo₇₂Cr₃₀ decomposing of hydrogen peroxide was performed with lower rate compared to Mo₇₂V₃₀ and also Mo₇₂Fe₃₀. Thus, this is rationale for Mo₇₂Cr₃₀ which shows a better catalytic activity than the two other Keplerate catalysts. From Fig. 3 the inversely correlations between catalytic activity of Mo₇₂M₃₀ (V, Cr, and Fe) catalysts and the rate of hydrogen peroxide decomposition in the absence of substrate could be deduced. It was also demonstrated that Keplerate structure of Mo₇₂Fe₃₀ could be preserved in the presence of a large excess of H₂O₂.³⁰

Fig. 3 H₂O₂ decomposition in the presence of Mo₇₂M₃₀ (V, Cr and Fe) catalysts. Reaction conditions: H₂O₂ (10 mmol), catalysts (100 μmol), water (3 mL) at 50 °C.

The epoxidation reaction of cyclooctene was used for studying the recyclability of these Keplerate POM catalysts. In this regard, the catalysts was simply recovered after each run of the oxidation reaction and using ethyl acetate, as a safe solvent, the organic products were isolated. Aqueous solution of the Keplerate catalyst was recycled for the next round of the reaction without further purification. The reusability of as mentioned POM catalysts, for at least five times, without any significant loss of catalytic activity is one of the advantages in this catalytic system (Table 2).

Table 2 Recycling of Mo₇₂M₃₀ (Cr and V) catalysts in the epoxidation of cyclooctene by H₂O₂ according to the procedure mentioned in Table 1^a

Run	Epoxide (%)
	Mo72Cr30	Mo72V30
a Yields refer to GC yields.
1	97	86
2	97	86
3	97	85
4	95	85
5	95	83

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As it could be seen in Fig. 4, similar XRD patterns for fresh and recycled Mo₇₂Cr₃₀ catalysts in the epoxidation of alkenes were obtained. This stated that the structure of catalyst is preserved and it is stable since catalytic recycling.

Fig. 4 The XRD pattern of Mo₇₂Cr₃₀ fresh (red) and recovered in run 3 (black) and run 5 (pink).

Sustainable oxidation of olefins by H₂O₂ in water as solvent shows lot of potential advantages for this catalytic system in both environmental and economic aspects. The applicability and efficiency of this green system was shown by comparing the catalytic results with some of the reported methods on the epoxidation of cyclohexene. From Table 3, it could be seen that the proposed catalytic system has superior results in comparison to most of the other methods. Additionally, our catalytic system has a very well reusability along with using a nontoxic solvent.

Table 3 Catalytic results for the oxidation of cyclohexene in the presence of different POM catalysts^a

Entry	Catalyst	Solvent	Conv. (yields) (%)	Ref.
a The oxidant for entries 1–5 and 8 was H2O2, F5PhI(TFAc)2 for entry 6 and UHP for entry 7.
1	[γ-SiW10O34]^4−	CH3CN	84 (>99)	17
2	[γ-1,2-H2SiV2W10O40]^4−	CH3CN/t-BuOH	90 (99)	17
3	(Diperoxotungsto)phosphates	C6H6	88 (100)	14
4	[NMe4]3[(MePO3){MePO2(OH)}W6O13(O2)4(OH)2(OH2)]	C6H6	36 (100)	20
5	[π-C5H5N(CH2)15CH3]3[PW4O16]	CHCl3	95 (97)	24
6	[P2W17MnO61]^8−	CH2Cl2/CH3CN	48 (16)	18
7	(CPB)3[PW12O40]	CH3CN	80 (97)	23
8	Mo72Cr30	H2O	92 (>99)	—

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The mechanism of alkenes epoxidation in the presence of POM catalysts using H₂O₂ as oxidant has been extensively investigated by several groups. In this regard, different active species such as Q₂[HPO₄{WO(O₂)₂}₂], [(PO₄){WO(O₂)₂}₄]³⁻, [(PO₄){WO(O₂)₂}₂{WO(O₂)₂(H₂O)}]³⁻, [(PO₃(OH)){WO(O₂)₂}₂]²⁻, and some of the other hydroperoxo intermediates have been proposed.³⁵ In this work, the favor reaction path for the oxidation reaction by Mo₇₂M₃₀ Keplerates could also be as above.

3. Experimental

3.1. General methods and materials

Mo₁₃₂, Mo₇₂V₃₀, Mo₇₂Fe₃₀ and Mo₇₂Cr₃₀ prepared according to the literature methods.³⁶ All common laboratory chemicals were reagent grade, commercially prepared and used without further purification. Elemental analyses for C, S, Cr, Fe, V, Mo and K were carried out on a Perkin-Elmer 7300 DV and Leco CHNS-932 elemental analyzer. Infrared spectra (KBr pellets) were recorded on a JASCO FT/IR-6300 instrument. ¹H NMR spectra were recorded on a Bruker 400 (296 K) using CDCl₃ as solvent and TMS as an internal reference. The oxidation products were quantitatively analyzed by gas chromatography (GC) on a Chrompack CP 9001 instrument using a SIL-5CB column (50 m, 32 mm, 5 μm) and FID detector. UV-vis spectra were recorded on a JASCO V-670 UV-vis spectrophotometer (190–2700 nm). Powder X-ray diffraction (XRD) data were obtained on a D8 Advanced Bruker using Cu Kα radiation (2θ = 5–80°).

3.2. Synthesis of Mo₇₂Cr₃₀

CrCl₃·6H₂O (8.6 g, 32.27 mmol) was added to a solution of Na₂MoO₄·2H₂O (9.0 g, 37.2 mmol) in a H₂O (80 mL)/CH₃COOH (100%; 35 mL) mixture. After stirring for 5 min it was acidified with 1 M HCl (17 mL; resulting pH ∼ 2), and the solution was heated under reflux for 45 min. The cooling of resulting green solution to room temperature gave the pale green precipitate. Elemental analysis: found: C, 2.53; Cr, 8.86; Mo, 38.80. Calc. for Mo₇₂Cr₃₀: C, 2.58; Cr, 8.81; Mo, 38.99%. IR (KBr pellet, cm⁻¹): 1622 (m, H₂O), 1547 (m, COO), 1423 (w, COO), 953 (m, (Mo=O)), 796 (s), 635 (w), 580 (s), 455 (w).

The synthesis method for Mo₇₂Fe₃₀ and Mo₇₂V₃₀ are presented in ESI.† The XRD patterns of these three Keplerate POM are also shown in Fig. S1.†

3.3. General procedure for epoxidation of alkenes

The Keplerate catalyst (10 μmol), an alkenes (1 mmol) and water (3 mL) were added to a glass tube under air. Then, hydrogen peroxide (1 mmol, 30%) was added and the reaction mixture was stirred for appropriate time in a preheated oil bath at 50 °C. The reaction progress was monitored by TLC and GC, and the yield of products was determined by GC analysis using toluene as the internal standard. After completion of the reaction ethyl acetate, 3 × 2 mL was added, and the extraction of organic phase was followed by evaporation of solvent. Higher purification of products was achieved by chromatography over silica gel (eluent: n-hexane/ethyl acetate 80 : 20). For the recyclability reactions, an aqueous solution of catalyst was used for the next runs.

4. Conclusion

In summary, the Keplerate catalysts Mo₇₂M₃₀ (M = V, Cr and Fe) were used as efficient catalysts for epoxidation of various olefins to epoxides at 50 °C in water using 30% H₂O₂. Among three Keplerate POM catalyst, Mo₇₂Cr₃₀ showed the highest conversions and selectivity for epoxidation of alkenes. In this system, Mo₇₂M₃₀ (M = V and Cr) catalysts efficiently reused many times without any loss of activity. Furthermore to the above-mentioned advantageous, the use of nontoxic and eco-friendly reagents, green solvents, and safe workup procedures makes this catalytic methodology appropriate for industrial goals.

Acknowledgements

We are grateful to Prof. Achim Müller and Prof. Hartmut Bögge (the University of Bielefeld) for their helps. Support for this research by the University of Isfahan is acknowledged.

References

1. J. E. Backvall, Modern Oxidation Methods, Wiley-VCH, Weinheim, Germany, 2nd edn, 2010.
2. Z. Guo, B. Liu, Q. Zhang, W. Deng, Y. Wang and Y. Yang, Chem. Soc. Rev., 2014, 43, 3480–3524.
3. A. L. Nuzhdin, D. N. Dybtsev, V. P. Fedin and G. A. Bukhtiyarova, Dalton Trans., 2009, 10481–10485.
4. J. Legros and C. Bolm, Angew. Chem., Int. Ed., 2003, 42, 5487–5489.
5. D. N. Dybtsev, A. L. Nuzhdin, H. Chun, K. P. Bryliakov, E. P. Talsi, V. P. Fedin and K. Kim, Angew. Chem., Int. Ed., 2006, 45, 916–920.
6. K. Kamata, K. Yonehara, Y. Sumida, K. Yamaguchi, S. Hikichi and N. Mizuno, Science, 2003, 300, 964–966.
7. G. T. Brink, I. W. C. E. Arends and R. A. Sheldon, Science, 2000, 287, 1636–1639.
8. N. V. Izarova, M. T. Pope and U. Kortz, Angew. Chem., Int. Ed., 2012, 51, 9492–9510.
9. X. P. Zheng, Y. Lu, H. Zhang, Z. M. Zhang and E. B. Wang, Inorg. Chem. Commun., 2013, 33, 29–32.
10. D. A. Judd, J. H. Nettles, N. Nevins, J. P. Snyder, D. C. Liotta, J. Tang, J. Ermolieff, R. F. Schinazi and C. L. Hill, J. Am. Chem. Soc., 2001, 123, 886–897.
11. C. Y. Sun, S. X. Liu, D. D. Liang, K. Z. Shao, Y. H. Ren and Z. M. Su, J. Am. Chem. Soc., 2009, 131, 1883–1888.
12. D. L. Long, R. Tsunashima and L. Cronin, Angew. Chem., Int. Ed., 2010, 49, 1736–1758.
13. (a) B. Notari, Adv. Catal., 1996, 41, 253–334; (b) M. Klawonn, M. K. Tse, S. Bhor, C. Döbler and M. Beller, J. Mol. Catal. A: Chem., 2004, 218, 13–19; (c) E. G. Ankudey, H. F. Olivo and T. L. Peeples, Green Chem., 2006, 8, 923–926.
14. C. Venturello and R. D'Aloisio, J. Org. Chem., 1988, 53, 1553–1557.
15. Y. Ishii, K. Yamawaki, T. Ura, H. Yamada, T. Yoshida and M. Ogawa, J. Org. Chem., 1988, 53, 3587–3593.
16. R. Noyori, M. Aoki and K. Sato, Chem. Commun., 2003, 1977–1986.
17. (a) K. Kamata, M. Kotani, K. Yamaguchi, S. Hikichi and N. Mizuno, Chem.–Eur. J., 2007, 13, 639–648; (b) Y. Nakagawa, K. Kamata, M. Kotani, K. Yamaguchi and N. Mizuno, Angew. Chem., Int. Ed., 2005, 44, 5136–5141.
18. (a) R. Neumann, Prog. Inorg. Chem., 1998, 47, 317–370; (b) A. Khenkin, D. Kumar, S. Shaik and R. Neumann, J. Am. Chem. Soc., 2006, 128, 15451–15460.
19. L. Salles, J. Y. Piquemal, R. Thouvenot, C. Minot and J. M. Bregeault, J. Mol. Catal. A: Chem., 1997, 117, 375–387.
20. W. P. Griffith, B. C. Parkin, A. J. P. White and D. J. Williams, J. Chem. Soc., Chem. Commun., 1995, 2183–2184.
21. D. C. Duncan, R. C. Chambers, E. Hecht and C. L. Hill, J. Am. Chem. Soc., 1995, 117, 681–691.
22. Z. Xi, N. Zhou, Y. Sun and K. Li, Science, 2001, 292, 1139–1141.
23. Y. Ding, Q. Gao, G. Li, H. Zhang, J. Wang, L. Yan and J. Suo, J. Mol. Catal. A: Chem., 2004, 218, 161–170.
24. J. Gao, Y. Chen, B. Han, Z. Feng, C. Li, N. Zhou, S. Gao and Z. Xi, J. Mol. Catal. A: Chem., 2004, 210, 197–204.
25. (a) A. Müller, P. Kögerler and C. Kuhlmann, Chem. Commun., 1999, 1347–1358; (b) A. Müller, E. Krickemeyer, H. Bögge, M. Schmidtmann and F. Peters, Angew. Chem., Int. Ed., 1998, 110, 3567–3571.
26. (a) A. Müller, A. M. Todea, J. V. Slageren, M. Dressel, H. Bogge, M. Schmidtmann, M. M. Luban, L. Engelhardt and M. Rusu, Angew. Chem., Int. Ed., 2005, 44, 3857–3861; (b) A. Muller, P. Kogerler and A. W. M. Dress, Coord. Chem. Rev., 2001, 222, 193–218; (c) P. Kogerler, B. Tsukerblat and A. Müller, Dalton Trans., 2010, 21–36; (d) A. Müller and P. Gouzerh, Chem. Soc. Rev., 2012, 41, 7431–7463.
27. (a) S. Y. Men'shikov, Y. V. Mishina, Y. Mikushina and V. A. A. Ostroushko, Russ. J. Appl. Chem., 2008, 81, 52–54; (b) R. Canioni, C. Marchal-Roch, N. Leclerc-Laronze, M. Haouas, F. Taulèlle, J. Marrot, S. Paul, C. Lamonier, J.-F. Paul, S. Loridant, J.-M. Millet and E. Cadot, Chem. Commun., 2011, 47, 6413–6415.
28. N. V. Izarova, O. A. Kholdeeva, M. N. Sokolov and V. P. Fedin, Russ. Chem. Bull., Int. Ed., 2009, 58, 134–137.
29. A. Rezaeifard, R. Haddad, M. Jafarpour and M. Hakimi, J. Am. Chem. Soc., 2013, 135, 10036–10039.
30. A. Rezaeifard, R. Haddad, M. Jafarpour and M. Hakimi, ACS Sustainable Chem. Eng., 2014, 2, 942–950.
31. J. Zhang, D. Li, G. Liu, K. J. Glover and T. Liu, J. Am. Chem. Soc., 2009, 131, 15152–15159.
32. (a) M. R. Farsani, B. Yadollahi, H. A. Rudbari, A. Amini, T. Caradoc-Davis and J. R. Price, Inorg. Chem. Commun., 2014, 43, 39–44; (b) M. R. Farsani, F. Jalilian, B. Yadollahi and H. A. Rudbari, Polyhedron, 2014, 76, 102–107; (c) R. Afrasiabi, M. R. Farsani and B. Yadollahi, Tetrahedron Lett., 2014, 55, 3923–3925; (d) M. R. Farsani and B. Yadollahi, J. Mol. Catal. A: Chem., 2014, 392, 8–15; (e) E. Nikbakht, B. Yadollahi and M. R. Farsani, Inorg. Chem. Commun., 2015, 55, 135–138.
33. F. Jalilian, B. Yadollahi, M. R. Farsani, S. Tangestaninejad, H. A. Rudbari and R. Habibi, Catal. Commun., 2015, 66, 107–110.
34. D. Sloboda-Rozner, P. Witte, P. L. Alsters and R. Neumann, Adv. Synth. Catal., 2004, 346, 339–345.
35. (a) L. Salles, A. Catherine, T. Rene, R. Francis, D. Claudine, C. Genevieve, L. Henry, J. Yves and M. B. Jean, Inorg. Chem., 1994, 33, 871–878; (b) T. Sakamoto and C. Pac, Tetrahedron Lett., 2000, 41, 10009–10012; (c) N. M. Gresley, W. P. Griffith, A. C. Laemmel, H. S. Nogueira and B. C. Parkin, J. Mol. Catal., 1997, 117, 185–198; (d) R. Neumann and M. Dahan, J. Am. Chem. Soc., 1998, 120, 11969–11976; (e) T. A. Trubitsina, N. S. Antonova, J. J. Carbó, J. M. Poblet, G. Al-Kadamany, U. Kortz and O. A. Kholdeeva, Eur. J. Inorg. Chem., 2010, 5312–5317; (f) P. Jimenez-Lozano, I. D. Ivanchikova, O. A. Kholdeeva, J. M. Poblet and J. J. Carbo, Chem. Commun., 2012, 48, 9266–9268.
36. (a) A. Müller, S. Sarkar, S. Q. N. Shah, H. Bögge, M. Schmidtmann, Sh. Sarkar, P. Kögerler, B. Hauptfleisch, A. X. Trautwein and V. Schünemann, Angew. Chem., Int. Ed., 1999, 38, 3238–3241; (b) A. M. Todea, A. Merca, H. Bögge, J. V. Slageren, M. Dressel, L. Engelhardt, M. Luban, T. Glaser, M. Henry and A. Müller, Angew. Chem., Int. Ed., 2007, 119, 6218–6222; (c) A. Müller, E. Krickemeyer, H. Bögge, M. Schmidtmann and F. Peters, Angew. Chem., Int. Ed., 1998, 37, 3360–3363; (d) U. Kortz, A. Müller, J. V. Slageren, J. Schnack, N. S. Dalal and M. Dressel, Coord. Chem. Rev., 2009, 253, 2315–2327.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12488c

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