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An effective retro- of M3N@C80

Cite this: Chem. Commun., 2013, (M = Sc, Lu, Ho) metallofulleropyrrolidines† 49, 10489 Bo Wu,a Taishan Wang,a Zhuxia Zhang,a Yongqiang Feng,a Lihua Gan,b Li Jianga Received 15th August 2013, and Chunru Wang*a Accepted 17th September 2013

DOI: 10.1039/c3cc46247a

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Sc3N@C80 fulleropyrrolidines are typical metallofullerene deriva- Echegoyen et al. proposed the isolation of metallofullerenes by an 16 tives which were generated by the reaction of Sc3N@C80 with electrochemical method. In this communication, we report N-trityloxazolidinone. Here we report an effective retro-reaction of a reversible cycloaddition reaction for TNT metallofullerenes,

Sc3N@C80 fulleropyrrolidine by using 3-chloroperoxybenzoic acid which is considered to be a promising separation technique for

to give pristine Sc3N@C80 in a high yield. This technique is metallofullerenes.

expected to be used for separating metallofullerenes (M3N@C80) It is well known that the 1,3-dipolar cycloaddition reaction from hollow without HPLC. between (metallo)fullerenes and azomethine ylides (Prato reac- tion) is widely used to prepare various (metallo)fulleropyr- Endohedral metallofullerenes (EMFs) are novel nanomaterials rolidines.17–19 Moreover, the retro-cycloaddition reactions of with stable nesting structures and unique electronic proper- fulleropyrrolidines and metallofulleropyrrolidines are also of ties.1–4 In this family, tri-metallic nitride template (TNT) cluster- great concern as the cycloaddition reversibility is related to 5–7 21 fullerenes, such as Sc3N@C80,Gd3N@C80,andLu3N@C80,have their yield optimization and recycling of metallofullerenes. attracted special attention due to their stable structures and high However, so far the reported retro-cycloaddition results have yields of production, and have found wide applications in medical always been achieved under special experimental conditions science,8 electronics,9 photochemistry,10 and materials science.11 such as metal-catalyzation,21 electrochemical redox,22 and micro The arc-discharging method is usually employed to produce wave elimination.23 In this report, we present a convenient and

the TNT clusterfullerenes, which are always accompanied with efficient way to realize the retro-cycloaddition of M3N@C80 (M = 12 Published on 17 September 2013. Downloaded by Institute of Chemistry, CAS 06/12/2013 02:30:00. hollow fullerenes (C60,C70, etc.) and other metallofullerenes. Sc, Ho, Lu) metallofulleropyrrolidines using 3-chloroperoxy- Therefore, to separate and purify TNT clusterfullerenes in a benzoic acid (MCPBA) under moderate conditions.

convenient way remains a big challenge in their application. In As shown in Fig. 1, Sc3N@C80 fulleropyrrolidine was synthe- 20 the past few decades, besides the expensive laboratory-used high- sized as reported previously. Briefly, Sc3N@C80 was heated performance liquid chromatography (HPLC) method, several with N-ethylglycine and paraformaldehyde at 120 1Cin non-HPLC approaches have been explored to isolate TNT cluster- o-dichlorobenzene (o-DCB) to give a metallofulleropyrrolidine 13 14 fullerenes. For example, Stevenson et al. adopted the cyclo- in a yield of 50%. C60 and C70 fulleropyrrolidines were also pentadiene (CPD) immobilized Merrifield resin as a reactive prepared for comparison. HPLC was employed to analyse the support for hollow fullerenes in a flash chromatographic column yields of products. Typically, these (metallo)fulleropyrrolidines to separate TNT clusterfullerenes from empty fullerenes. Soon were easily isolated by silica column chromatography. after that, they developed another non-chromatographic method named ‘‘Stir and Filter Approach’’ (SAFA)15 to purify metallofuller- enes by using cyclopentadienyl and amino functionalized silica.

a Key Laboratory of Molecular Nanostructure and Nanotechnology, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China. E-mail: [email protected]; Fax: +86-10-62652120; Tel: +86-10-62652120 b School of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715, China † Electronic supplementary information (ESI) available: Experimental details, HPLC chromatograms, MALDI-TOF spectra and theoretical calculations. See

DOI: 10.1039/c3cc46247a Fig. 1 Prato reaction and retro-cycloaddition of Sc3N@Ih-C80.

This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 10489--10491 10489 View Article Online Communication ChemComm

Fig. 2 HPLC profiles of the retro-cycloaddition of Sc3N@C80 fulleropyrrolidine for different reaction times. HPLC conditions: a 10 Â 250 mm Buckyprep column, 12 mL minÀ1 flow rate with .

Fig. 3 HPLC profiles of the retro- of Ho N@C and Lu N@C Table 1 The retro-cycloaddition of Sc N@C fulleropyrrolidine in the presence 3 80 3 80 3 80 fulleropyrrolidines. Black lines represent the metallofulleropyrrolidine reactants, of MCPBA at 40 1C for different reaction times and red lines denote the recovered TNT clusterfullerenes.

Recovered Residual Sc3N@C80 a a Time (min) Sc3N@Ih-C80 (%) fulleropyrrolidine (%) derivatives, as the retro-reaction of [5,6] and [6,6] monoadducts 20 54.9 39.9 of Ho N@C took 30 and 15 minutes, respectively, to completely 25 82.6 15.1 3 80 30 86.1 0.3 convert into the pristine Ho3N@C80. Obviously, the retro- 40 68.1 0.0 cycloaddition of M3N@C80 (M = Sc, Ho, Lu) metallofulleropyr- 60 49.4 0.0 rolidines using MCPBA can be extended to be a universal a Determined by peak areas of HPLC profiles. technique for TNT clusterfullerenes.

However, for hollow fullerenes C60 and C70, no retro-reaction of their corresponding fulleropyrrolidines was observed under

The retro-reaction of Sc3N@C80 fulleropyrrolidine was similar experimental conditions (see ESI†). Instead, C60 and C70 achieved by adding MCPBA at 40 1C under stirring in o-DCB. pyrrolidine-N-oxides were produced using MCPBA as reported As shown in Fig. 2 and listed in Table 1, it was observed that in previous literature.24 In our experiments, as shown in Fig. S3

Sc3N@C80 fulleropyrrolidine was recovered to Sc3N@Ih-C80 and S4 (ESI†), the nitrogen oxides of C60 and C70 fullero-

Published on 17 September 2013. Downloaded by Institute of Chemistry, CAS 06/12/2013 02:30:00. quickly, and after 30 minutes, the recovered Sc3N@Ih-C80 pyrrolidines were detected by MALDI-TOF mass spectroscopy reached a maximum yield of 86.11%. Further prolonging the and HPLC. However, the nitrogen oxide of Sc3N@C80 fullero-

reaction time would lead to a significant loss of Sc3N@Ih-C80 pyrrolidine was not detected but obtained the parent Sc3N@Ih-C80. due to the oxidation of metallofullerenes. In comparison, a This phenomenon may be attributed to the unstable nitrogen oxide 21 previously reported retro-cycloaddition technique for Sc3N@C80 of Sc3N@C80 fulleropyrrolidine. Theoretical calculations were then fulleropyrrolidine requires it to be refluxed in o-DCB solution with employed to analyze the reason why the Prato products of hollow maleic anhydride and copper triflate, and required a reaction time fullerenes and TNT clusterfullerenes show different behaviors in of as long as 8–18 hours. Therefore, the current retro-reaction of the retro-cycloaddition. Ab initio calculations of the partial natural

Sc3N@C80 fulleropyrrolidine is a fast and convenient process, bond orbital (NBO) charge distributions of three Prato derivatives which would significantly enhance its utility in the chemistry of reveal that the nitrogen atom of pyrrolidines has the most metallofullerenes. negative charge at ca. À0.5 eV in each molecule (see Fig. S6, The current retro-reaction technique is not only used for ESI†). Therefore, in the retro-reactions the peroxide in MCPBA

Sc3N@C80 fulleropyrrolidine, but also for other TNT cluster- will attack the N atom of pyrrolidines to form a nitrogen oxide

derivatives, such as Ho3N@C80 and Lu3N@C80 Prato group in the (metallo)fulleropyrrolidines. And also, the reason for

products. M3N@C80 (M = Ho, Lu) metallofulleropyrrolidines the instability of nitrogen oxide of Sc3N@C80 fulleropyrrolidine were prepared and purified by a similar procedure to that of needs further theoretical calculations.

Sc3N@C80 fulleropyrrolidine. In their retro-cycloaddition reac- The different behaviors of C60 fulleropyrrolidine, C70 fullero- tions, after adding MCPBA into the o-DCB solutions of M3N@C80 pyrrolidine, and TNT metallofulleropyrrolidines for this type of (M = Ho, Lu) metallofulleropyrrolidines at 40 1C, the pristine retro-cycloaddition are expected to lead to the development of a

Ho3N@C80 and Lu3N@C80 were also successfully obtained, as new technique for separating TNT clusterfullerenes without the shown in Fig. 3. Fascinatingly, the retro-reaction time of metallo- use of the expensive HPLC method. For the sake of simplicity, fulleropyrrolidines would depend on the regioisomers of the we prepared a model of fullerene and metallofullerene mixture

10490 Chem. Commun., 2013, 49, 10489--10491 This journal is c The Royal Society of Chemistry 2013 View Article Online ChemComm Communication

Notes and references

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Fig. 4 Separation of Sc3N@Ih-C80 from a mixture of Sc3N@Ih-C80,C60 and C70 8 P. P. Fatouros, F. D. Corwin, Z.-J. Chen, W. C. Broaddus, J. L. Tatum, solution after a Prato reaction and a retro-cycloaddition reaction process. B. Kettenmann, Z. Ge, H. W. Gibson, J. L. Russ and A. P. Leonard, Radiology, 2006, 240, 756–764. 9 L. Feng, M. Rudolf, S. Wolfrum, A. Troeger, Z. Slanina, T. Akasaka, S. Nagase, N. Martı´n, T. Ameri, C. J. Brabec and D. M. Guldi, J. Am. Chem. Soc., 2012, 134, 12190–12197. solution in o-DCB containing C60,C70 and Sc3N@Ih-C80 in a 10 R. B. Ross, C. M. Cardona, D. M. Guldi, S. G. Sankaranarayanan, mass ratio of 1 : 0.8 : 0.4 (C60 :C70 :Sc3N@Ih-C80). The mixture M. O. Reese, N. Kopidakis, J. Peet, B. Walker, G. C. Bazan, E. Van solution was heated at 120 1CtogetherwithN-ethylglycine and Keuren, B. C. Holloway and M. Drees, Nat. Mater., 2009, 8, 208–212. paraformaldehyde for 3 h. Then, the products were subjected 11 M. Wolf, K. H. Mu¨ller, Y. Skourski, D. Eckert, P. Georgi, M. Krause and L. Dunsch, Angew. Chem., Int. Ed., 2005, 44, 3306–3309. to silica column chromatography and eluted by toluene, and 12 H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl and R. E. Smalley, the (metallo)fulleropyrrolidines with longer retention time were Nature, 1985, 318, 162–163. separated out easily. 13 K. Akiyama, T. Hamano, Y. Nakanishi, E. Takeuchi, S. Noda, Z. Wang, S. Kubuki and H. Shinohara, J. Am. Chem. Soc., 2012, Afterwards, the obtained (metallo)fulleropyrrolidines were 134, 9762–9767. heated at 40 1CinthepresenceofMCPBAino-DCB for 30 min. 14 S. Stevenson, K. Harich, H. Yu, R. R. Stephen, D. Heaps, C. Coumbe Then the products were again separated by silica gel column and J. P. Phillips, J. Am. Chem. Soc., 2006, 128, 8829–8835. 15 S. Stevenson, M. A. Mackey, C. E. Coumbe, J. P. Phillips, B. Elliott chromatography. Pure Sc3N@Ih-C80 was isolated from C60 fullero- and L. Echegoyen, J. Am. Chem. Soc., 2007, 129, 6072–6073. pyrrolidine and C70 fulleropyrrolidine successfully. As shown in 16 B. Elliott, L. Yu and L. Echegoyen, J. Am. Chem. Soc., 2005, 127, Fig. 4, HPLC results suggest that high purity of Sc N@I -C is 10885–10888. 3 h 80 17 (a) M. Maggini, G. Scorrano and M. Prato, J. Am. Chem. Soc., 1993, obtained after the final retro-cycloaddition reaction. By comparing 115, 9798–9799; (b) S. R. Wilson and Q. Lu, J. Org. Chem., 1995, 60, with the starting Sc3N@Ih-C80, about 55% of Sc3N@Ih-C80 was 6496–6498. recovered in this separation procedure. 18 N. Crivillers, Y. Takano, Y. Matsumoto, J. Casado-Montenegro, M. Mas-Torrent, C. Rovira, T. Akasaka and J. Veciana, Chem. Com- In conclusion, although both fullerenes and TNT clusterfuller- mun., 2013, 49, 8145–8147. enes can form fulleropyrrolidines and metallofulleropyrrolidines 19 P. Salice, M. Mauri, M. Castellino, M. De Marco, A. Bianchi, A. Virga, via the Prato reaction, they behave differently in retro-cycloaddition A. Tagliaferro, R. Simonutti and E. Menna, Chem. Commun., 2013, 49, 8048–8050. Published on 17 September 2013. Downloaded by Institute of Chemistry, CAS 06/12/2013 02:30:00. reactions using MCPBA as an oxidant, i.e., pristine M3N@C80 20 C. M. Cardona, A. Kitaygorodskiy, A. Ortiz, M. A´. Herranz and (M = Sc, Ho, Lu) metallofullerenes were obtained after this type L. Echegoyen, J. Org. Chem., 2005, 70, 5092–5097. of retro-addition reaction, but C and C were not recovered. 21 N. Martı´n, M. Altable, S. Filippone, A. Martı´n-Domenech, 60 70 L. Echegoyen and C. M. Cardona, Angew. Chem., Int. Ed., 2006, 45, Based on these differences, a non-HPLC technique was devel- 110–114. oped to isolate TNT clusterfullerenes from hollow fullerenes 22 O. Lukoyanova, C. M. Cardona, M. Altable, S. Filippone, A´. Martı´n with a high yield, which would be a promising separation Domenech, N. Martı´n and L. Echegoyen, Angew. Chem., Int. Ed., 2006, 45, 7430–7433. method for metallofullerenes. 23 I. Guryanov, A. M. Lo´pez, M. Carraro, T. Da Ros, G. Scorrano, We thank the National Basic Research Program (2012CB932900), M. Maggini, M. Prato and M. Bonchio, Chem. Commun., 2009, the National Natural Science Foundation of China (21121063, 3940–3942. 24 (a) P. Brough, C. Klumpp, A. Bianco, S. Campidelli and M. Prato, 21203205, 51002102), NSAF (11179006), and the China Postdoctoral J. Org. Chem., 2006, 71, 2014–2020; (b) N. Abramova, A. Ginzburg Science Foundation (201104153). and V. Sokolov, Russ. Chem. Bull., 2010, 59, 1964–1966.

This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 10489--10491 10491