Chemistry and catalysis advances in organometallic chemistry and catalysis
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Figure 33.10 Multibranched amorphous carbon nanostructure showing a spherical core with darker contrast (marked by an arrow), from where the lobes emerge. Reprinted from Reference 27c. Copyright 2005, with permission from Elsevier.
454 THE ROLE OF ORGANOMETALLIC COMPLEXES IN THE SYNTHESIS OF SHAPED CARBON MATERIALS (a) (b)
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Figure 33.11 Bamboo structures of CNTs obtained from the pyrolysis of toluene and ferrocene in the presence of 8% diethylamine [22]. Reprinted from Reference 22a. Copyright 2010, with permission from Elsevier. (XPS). The presence of nitrogen also leads to the modified chemical behavior of the SCMs. Typically, nitrogen addition can also be achieved by using a N atom which is part of a ligand attached to the catalyst. The synthesis of bamboo-shaped tubular structures is usually performed using the CVD method. The synthesis can also be achieved by using a volatile C source that contains N (e.g., acetonitrile) [23]. Other common N sources include pyridine, melamine, benzylamine, etc. [6, 23]. Bamboo shapes were recently produced by a nebulized floating catalyst method using a mixture of toluene and nitrogen- containing reagents (e.g., aniline, benzylamine) by changing the ratio of the reactants [22]. This is a suitable method for controlling the bamboo compartments inside the tube axis as well as their lengths and diameters. Vertically aligned bamboo structures were recently grown on iron NPs deposited on silicon substrates by thermal decomposition of methane/ammonia and acetylene/ammonia mixtures at 900–1100 ◦ C [39]. CNTs doped with a range of N concentrations of 0–10 at% were prepared via a floating catalyst CVD method using FcH, NH 3 , or pyridine and xylene [39]. The pyridinic-like fraction selectively increased from 0.0 to 4.5 at% as the N content in the reaction increased [39]. At higher N contents, the tubes are highly constricted. A recent study reported the formation of bamboo nanostructures with different morphologies from turpentine oil by the aerosol route at 1000 ◦ C. FcH was used as a catalyst source and sulfur as a promoter [40]. These structures have sharp tips, bamboo shapes, open ends, hemispherical caps, pipe-like morphology, and metal particles trapped inside the wide hollow cores.
CONCLUSION 455 33.6.5 Coiled Carbons A beautiful example of the synthesis of coiled carbons made by using Fe(CO) 5 has been reported by Hou et al. [30]. In that study, the authors found that coiling was influenced by the carbon source (toluene, pyridine). Surprisingly no N-doped coils were observed when pyridine was used as reactant. Many types of helical carbons are known and are formed by the floating catalyst route either by the catalytic pyrolysis of polyethylene or through the use of tin and indium catalysts [30].
Recent studies on the crystallization behavior of amorphous CNTs annealed at high temperatures demonstrated that the microstructure of the CNTs (prepared by the floating catalyst route) have an amorphous character [27]. The tubes have finite dimensions and tube-like shapes, and their crystallization behavior is found to be completely different from bulk amorphous carbons. This unique type of SCM can also be made in a confined space at autogenic pressure and at a low temperature using FcH as a catalyst, benzene as carbon source, thiophene as additive, and H 2 as the atmosphere [27]. The self-catalyzed decomposition of FcH/benzene at a low temperature ( <210 ◦ C) gave long, amorphous CNT bundles and nanoribbons. This is the first paper to report nanotube bundles and nanoribbons composed of pure amorphous carbon. 33.6.7 Spherical Carbons CSs (Fig. 33.5) are often observed when attempts to make CNTs are performed at high temperatures. It is not clear whether the catalyst plays any role in their synthesis [26] (Fig. 33.12). However, hollow carbon spheres (HCSs) do require the interaction of a catalyst as a template. HCSs with smooth single shells, deformed single shells, double shells, and N-doped shells have been prepared using silica templates and FcH/benzene mixtures via the CVD process. The morphology of the HCSs was found to depend on the CVD reaction time and temperature. Results showed that relatively large silica spheres favored the formation of HCSs with a very smooth surface. However, a short CVD time led to a thin carbon shell and deformed HCSs [25]. Spherical carbons (solid and hollow) with a spongy appearance and controlled sizes were produced by CVD from camphor and FcH. The spheres were seen to be fused and interconnected [25]. Core–shell carbons can also be produced by floating catalysts. For example, a picric-acid-detonation-induced pyrolysis of FcH or Fe(CO) 5 gave carbon-encapsulated Fe NPs (5–20 nm) [29]. The reaction is characterized by a self-heating and extremely fast process. Tubular structures are formed at a high C/Fe ratio in this reaction. The pyrolysis of metallocenes such as FcH, cobaltocene, or nickelocene is known to yield CNTs and metal-filled onion-like structures [21]. The wall thickness (diameter) is controlled by the FcH content. Carbons onions have been synthesized in a CVD reactor at a temperature of 900
◦ C using Fe 3 (CO)
12 as the catalyst under an Ar/O 2 atmosphere [29]. 33.6.8 Other Shapes Leaf-like carbon sheets were obtained by the pyrolysis of dichloromethane and FcH in an autoclave at 300–600 ◦ C [41]. The formation process was studied by observing the product evolution utilizing the real-time imaging capabilities of emission SEM. The study also reported the synthesis of different carbon nanostructures by the chlorination of FcH at different temperature conditions; the products included amorphous CNTs, open-ended branches, and carbon nanobags [41] (Fig. 33.13). A straightforward method for the preparation of novel CNTs/iron nanoparticle hybrids, amorphous carbon, T- or Y- junctions with some degree of shape control was reported [42, 43]. This was done by either the thermolysis CTP/FcH mixtures in a CVD reactor [42] or FcH/thiophene mixtures in a closed steel vessel [43]. In summary, FcH and Fe(CO) 5 have been shown to produce a wide range of carbons with different structures. In addition to the reaction temperatures and gas flow rates, which play important roles in the formation of different carbon nanostructures, the Fe/carbon ratios, the amount of Fe used, and even the use of additives and metal cocatalysts are also important synthesis parameters.
The objective of this chapter was not to give a comprehensive review of organometallic catalysts that had been used to make structured carbon materials or a discussion of the range of structured carbon materials that have been formed from
456 THE ROLE OF ORGANOMETALLIC COMPLEXES IN THE SYNTHESIS OF SHAPED CARBON MATERIALS 0 400 450 500 550 Diameter of CMBs (nm) 600 650 700 5 10
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Figure 33.12 FE-SEM images of the products of the pyrolysis of CTP–ferrocene mixtures with ferrocene (a) 0, (b) 5, (c) 10, and (d) 20 wt% in the reaction zone. Reprinted from Reference 42. Copyright 2007, with permission from Elsevier. 5 μm 1 μm 5 μm 2 μm 2 μm 2 μm 1 μm (a) (b)
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(g) Figure 33.13 FSEM images of leaf structures grown from FcH at different temperatures. Reprinted from Reference 40. Copyright 2010, with permission from Elsevier.
REFERENCES 457 organometallic catalysts. Rather, the objective was to indicate the role that organometallic complexes can play and have played in making “carbon materials with shape”. This type of study is still in its infancy. Much effort will be required in future to generate appropriate catalysts from metal organometallic precursors that will lead to an understanding of the mode of C–C growth on the metal particles.
The authors wish to thank the University of the Witwatersrand, the DST/NRF Centre of Excellence in Strong Materials, and the NRF for financial support. They also wish to acknowledge the many postgraduate and postdoctoral fellows in the group who have assisted in the studies that have led to this paper. REFERENCES 1. Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318 , 162. 2. Kr¨atschmer, W.; Fostiropoulos, K.; Huffman, D. R. Chem. Phys. Lett. 1990. 170 , 167. 3. Iijima, S. Nature 1991, 354 , 56. 4. Davis, W. R.; Slawson, R. J.; Rigby, G. R. Nature 1953, 171 , 756. 5. Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6 , 183. 6. (a) Ewels, C. P.; Glerup, M. J. Nanosci. Nanotechnol. 2005, 5 , 1345 (b) Nxumalo, E. N.; Coville, N. J. Materials 2010, 3 , 2141; (c) Ayala, P.; Arenal, R.; R¨ummeli, M.; Rubio, A.; Pichler, T. Carbon 2010, 48 (3), 575. 7. (a) Shaikjee, A.; Coville, N. J. Carbon 2012, 50 , 1099–1110; (b) Shaikjee, A.; Coville, N. J. Carbon 2012, 50 , 3376. 8. (a) Nyamori, V. O.; Mhlanga, S. D.; Coville, N. J. J. Organomet. Chem., 2008, 693 , 2205; (b) Govindaraj, A.; Rao, C. N. R., Pure Appl. Chem. 2002, 74 , 1571; (c) A. Huczko, Appl. Phys. A Mater. Sci. Process. 2002, 74 , 617; (d) Mondal, K.; Coville, N. J. Encyclopedia of Nanoscience and Nanotechnology; Nalwa H. S., ed., 2011, Vol. 14; p 161, American Scientific Publishers, USA. 9. Tessonier, J. P.; Su, D. S. ChemSusChem. 2011, 4 , 824. 10. Nikolaev, P. J. Nanosci. Nanotechnol. 2004, 4 , 307. 11. (a) Deshmukh, A. A.; Mhlanga, S. D.; Coville, N. J. Mater. Sci. Eng. 2010, 70 , 1; (b) Coville, N. J.; Mhlanga, S. D.; Nxumalo, E. N.; Shaikjee, A. S. Afr. J. Sci. 2011, 107 , 44. 12. (a) Ajima, K.; Yudasaka, M.; Murakami, T.; Maigne, A.; Shiba, K.; Iijima, S. Mol. Pharm. 2005, 2 , 475; (b) Ma, Y.; Hu, Z.; Huo, K.; Lu, Y.; Hu, Y.; Liu, Y.; Hu, J.; Chen, Y. Carbon 2005, 43 , 1667. 13. (a) Kawaguchi, M.; Nozaki, K.; Motojima, S.; Iwanaga, H. J. Cryst. Growth. 1992, 118 , 309; (b) Fejes, D.; Hernadi, K. Materials 2010, 3 , 2618; (c) Shaikjee, A.; Coville, N. J. J. Adv. Res. 2012, 3 , 195. 14. Hanson, P. L.; Wagner, J. B.; Helveg, S.; Rostrup-Nielsen, J. R.; Clausen, B. S.; Topsøe, H. Science 2002, 295 , 2053. 15. Tibbetts, G. G.; Bernardo, C. A.; Gorkiewicz, D. W.; Alig, R. L. Carbon 1994, 32 , 569. 16. Young, D. J.; Zhang, J.; Geers, C.; Schutze, M. Mater. Corros. 2011, 62 , 7. 17. Helveg, S.; Sehested, J.; Rostrup-Nielsen, J. R. Catal. Today 2011, 178 , 42. 18. (a) Liu, J.; Lam, J. W. Y.; Tang, B. Z. Chem. Rev. 2009, 109 , 5799; (b) Plata, D. L.; Meshot, E. R.; Reddy, C. M.; Hart, A. J.; Gschwend, P. M. ACS Nano 2010, 4 , 7185. 19. Cao, A. Y.; Zhang, X. F.; Wei, J. Q.; Li, Y. H.; Xu, C. L.; Liang, J.; Wu, D. H.; Wei, B. Q. J. Phys. Chem. B 2001, 105 , 11937. 20. (a) Tibbetts, G. G.; Gorkiewicz, D. W. Carbon 2009, 31 , 809; (b) Barreiro, A.; Kramberger, C.; Rummeli, M. H.; Gruneis, A.; Grimm, D.; Hampel, S.; Gemming, T.; Buchner, B.; Bachtold, A.; Pichler, T. Carbon 2007, 45 , 55; (c) Barreiro, A.; Hampel, S.; Rummeli, M. H.; Kramberger, C.; Gruneis, A.; Biedermann, K.; Leonhardt, A.; Gemming, T.; Buchner, B.; Bachtold, A.; Pichler, T.
21. (a) Nyamori, V. O.; Coville, N. J. Organomet. Chem. 2007, 26 , 4083; (b) Hou, H.; Schaper, A. K.; Weller, F.; Greiner, A. Chem. Mater. 2002, 14 , 3990; (c) Liu, H.; Cheng, G.; Zheng, R.; Liang, C. Diamond Relat. Mat. 2008, 17 , 313; (d) Andrews, R.; Jacques, D.; Rao, A. M.; Derbyshire, F.; Qian, D.; Fan, X.; Dickey, E. C.; Chen, J. Chem. Phys. Lett. 1999, 303 , 467; (e) Mohlala, M. S.; Liu, X.-Y.; Coville, N. J. J. Organomet. Chem. 2006, 691 , 4768; (f) Merchan-Merchan, W.; Saveliev, A. V.; Kennedy, L.; Jimenez, W. C. Prog. Energy Combust. Sci. 2010, 36 , 696; (g) Dichiara, A.; Bai, J. Diamond Relat. Mater. 2012, 29 , 52; (h) Atiyah, M. R.; Awang Biak, D. R.; Ahmadun, F.; Ahamad, I. S.; Mohd Yasin, F.; Mohamed Yusoff, H. J. Mater. Sci. Technol., 2011, 27 , 296. 22. (a) Nxumalo, E. N.; Letsoala, P. G.; Cele, L. M.; Coville, N. J. J. Organomet. Chem. 2010, 695 , 2596; (b) Han, W.; Kohler-Redlich, P.; Seeger, T.; Ernst, F.; Ruhle, M.; Grobert, N.; Hsu, W.-K.; Chang, B.-H.; Zhu, Y.-Q.; Kroto, H. W.; Walton, D. R. M.; Terrones, M.; Terrones, H. Appl. Phys. Lett. 2000, 77 , 1807; (c) Nxumalo, E. N.; Nyamori, V. O.; Coville N. J. J. Organomet. Chem. 2008, 693 , 2942. 458 THE ROLE OF ORGANOMETALLIC COMPLEXES IN THE SYNTHESIS OF SHAPED CARBON MATERIALS 23. (a) Yadav, R. M.; Dobal, P. S.; Shripathi, T.; Katiyar, R. S.; Srivastava, O. N.; Nanoscale Res. Lett. 2009, 4 , 197; (b) Villalpando-Paez, F.; Zamudio, A.; Elias, A. L.; Son, H.; Barros, E. B.; Chou, S. G.; Kim, Y. A.; Muramatsu, H.; Hayashi, T.; Kong, J.; Terrones, H.; Dresselhaus, G.; Endo, M.; Terrones, M.; Dresselhaus, M. S. Chem. Phys. Lett. 2006, 424 , 345; (c) Terrones, M.; Grobert, N.; Terrones, H. Carbon 2002, 40 , 1665; (d)Cruz-Silva, E.; Cullen, D. A.; Gu, L; Romo-Herrera, J. M.; Mu˜noz-Sandoval, E.; L´opez-Ur´ıas, F.; Sumpter, B. G.; Meunier, V.; Charlier, J.-C.; Smith, D. J.; Terrones, H.; Terrones, M. ACS Nano 2008, 2 , 441. 24. (a) Ci, L.; Li, Y.; Wei, B.; Liang, J.; Xu, C.; Wu, D. Carbon 2000, 38 , 1933; (b) Kato, T.; Kusakabe, K.; Morooka, S. J. Mat. Sci. Lett. 1992, 11 , 674. 25. (a) Su, F.; Zhao, X. S.; Wang, Y.; Wang, L.; Lee, J. Y. J. Mater. Chem. 2006, 16 , 44139; (b) Sharon, M.; Mukhopadhyay, K.; Yase, K.; Iijima, S.; Ando, Y.; Zhao, X. Carbon 1998, 36 , 507; (c) Zou, G.; Yu, D.; Lu, J.; Wang, D.; Jiang, C.; Qian, Y. Solid State
26. Liu, X.-Y.; Huang, B.-C.; Coville, N. J. Carbon 2002, 40 , 2791. 27. (a) Ci, L.; Wei, B.; Xu, C.; Liang, J.; Wu, D.; Xie, S.; Zhou, W.; Li, Y.; Liu, Z.; Tang, D. J. Cryst. Growth 2001, 233 , 823; (b) Luo, T.; Chen, L.; Bao, K.; Yu, W.; Qian, Y. Carbon 2006, 44 , 2844; (c) Urones-Garrote, E.; ´ Avila-Brande, D.; Katcho, N. A.; G´omez-Herrero, A.; Landa-C´anovas, A. R.; Otero-D´ıaz, L. C. Carbon 2005, 43 , 978. 28. Deepak, F. L.; Govindaraj, A.; Rao, C. N. R. Chem. Phys. Lett. 2001, 345 , 5. 29. (a) Lu, Y.; Zhu, Z.; Liu, Z. Carbon 2005, 43 , 369; (b) Schnitzler, M. C.; Oliveira, M. M.; Ugarte, D.; Zarbin, A. J. G. Chem. Phys. Lett. 2003, 381 , 541; (c) Sano, N.; Akazawa, H.; Kikuchi, T.; Kanki, T. Carbon 2003, 41 , 2159. 30. (a) Hou, H.; Jun, Z.; Weller, F.; Greiner, A. Chem. Mater. 2003, 15 , 3170; (b) Daraio, C.; Nesterenko, V. F.; Jin, S. J. Appl. Phys. 2006, 100 , 0643091; (c) Kong, Q.; Zhang, J. Polym. Degrad. Stab. 2007, 92 , 2005; (d) Wang, W.; Yang, K.; Gaillard, J.; Bandaru, P. R.; Rao, A. M. Adv. Mater. 2008, 20 , 179. 31. (a) Moisala, A.; Nasibulin, A. G.; Brown, D. P.; Jiang, H.; Khriachtchev, L.; Kauppinen, E. I. Chem. Eng. Sci. 2006, 61 , 4393. (b) Anisimov, A. S.; Nasibulin, A. G.; Jiang, H.; Launois, P.; Cambedouzou, J.; Shandakov, S. D.; Kauppinen, E. I. Carbon 2010, 48 , 380. 32. Awasthi, K.; Kumar, R.; Raghubanshi, H.; Awasthi, S.; Pandey, R.; Singh, D.; Yadav, T. P.; Srivastava, O. N. Bull. Mat. Sci. 2011, 34 , 607. 33. (a) Zaj´ıckov´a, L.; Jaˇsek, O.; Synek, P.; Eli´aˇs, M.; Kudrle, V.; Kadlec´ıkov´a, M.; Breza, J.; Hanzl´ıkov´a, R. Appl. Surf. Sci. 2009, 255 , 5421; (b) Kuo, C.-S.; Bai, A.; Huang, C.-M.; Li, Y.-Y.; Hu, C.-C.; Chen, C.-C. Carbon 2005, 43 , 2760; (c) Castillejos E.; Bachiller- Baez, B.; P´erez-Cadenas, M.; Gallegos-Suarez, E.; Rodr´ıguez-Ramos, I.; Guerrero-Ruiz, A.; Tamargo-Martinez, K.; Martinez-Alonso, A.; Tasc´on, J. M. D. J. Alloys Comp. 2012, 536S , S460; (d) Harris, J. D.; Raffaelle, R. P.; Gennett, T.; Landie, B. J.; Hepp, A. F. J.
, 116 , 369; (e) Edgar, K., Spencer, J. L. Curr. Appl. Phys. 2004, 4 , 121. 34. (a) Dumitrache, F.; Morjan, I.; Alexandrescu, R.; Ciupina, V.; Prodan G.; Voicu, I.; Fleaca, C.; Albu, L.; Savoiu, M.; Sandu, I.; Popovici, E.; Soare, I. Appl. Surf. Sci. 2005, 247 , 25; (b) Dumitrache, F.; Morjan, I.; Alexandrescu, R.; Morjan, R. E.; Voicu, I.; Sandu, I.; Soare, I.; Ploscaru, M.; Fleaca, C.; Ciupina, V.; Prodan, G.; Rand, B.; Brydson, R.; Westwood, A. Diamond Relat. Mater.
35. Bai, S.; Li, F.; Yang, Q. H.; Cheng, H. M.; Bai, J. B. Chem. Phys. Lett. 2003, 376 , 83. 36. (a) Horvath, Z. E.; Kertesz, K.; Petho, L.; Koos, A. A.; Tapaszto, L.; Vertesy, Z.; Osvath, Z.; Darabont, A.; Nemes-Incze, P.; Sarkozi, Z.; Biro, L. P. Curr. Appl. Phys. 2006, 6 , 135; (b) Mohlala, M. S.; Liu, X.-Y.; Robinson, J. M.; Coville, N. J. Organomet. Chem. 2005, 24 , 972. 37. Satishkumar, B. C.; Thomas, P. J.; Govindaraj, A.; Rao, C. N. R. Appl. Phys. Lett. 2000, 77 , 2530. 38. Rao, C. N. R.; Govindaraj, A. Acc. Chem. Res. 2002, 35 , 998. 39. (a) Jang, J. W.; Lee, C. E.; Lyu, S. C.; Lee, T. J.; Lee, C. J. Appl. Phys. Lett. 2004, 84 , 2877; (b) Maldonado, S.; Morin, S.; Stevenson, K. J. Carbon 2006, 44 , 1429. 40. Ghosh, P.; Soga, T.; Ghosh, K.; Afre R.A.; Jimbo, T.; Ando, Y. J. Non-Crys. Solids 2008, 354 , 4101. 41. Ju, Z.; Wang, T.; Wang, L.; Xing, Z.; Xu, L.; Qian, Y. Carbon 2010, 48 , 3420. 42. Liu, X.; Yang, Y.; Ji, W.; Liu, H.; Zhang, C.; Xu, B. Mater. Chem. Phys. 2007, 104 , 320. 43. Liu, S.; Wehmschulte, R. J. Carbon 2005, 43 , 1550.
34 METAL CATALYSIS IN FULLERENE CHEMISTRY Salvatore Filippone, Enrique E. Maroto, and ´ Angel Mart´ın-Domenech
Nazario Mart´ın* Departamento de Qu´ımica Org´anica I, Universidad Complutense, Madrid, Spain; IMDEA Nanoscience, Madrid, Spain 34.1 INTRODUCTION TO FULLERENES The discovery of the soccer-ball-shaped C 60 molecule as the third allotrope of carbon by Robert F. Curl, Sir Harold W. Kroto, and the late Richard E. Smalley in 1985 brought a new and unprecedented nanoform of molecular carbon, unlike the reticular diamond and graphite allotropes, and emerged as a new material exhibiting unique properties [1]. Soon afterwards, a major breakthrough in fullerene science occurred in 1990, when Wolfgang Kr¨atschmer and Donald Huffman, two astrophysicists, prepared fullerene C 60 from a carbon arc in multigram amounts, thus paving the way to the chemical functionalization of fullerenes and, therefore, to the synthesis of new and sophisticated fullerene architectures. It was in 1991 that Iijima achieved another major development in fullerene science with the discovery of multiwalled [2] and single-walled [3, 4] carbon nanotubes. More recently, another intriguing carbon-based material, named graphene [5], the thinnest and strongest material reported so far and formed by one-atom-thick flat sheet of carbon, has provoked great excitement in the scientific community due to the singular properties they exhibit (Fig. 34.1). Important landmarks in carbon nanostructure science occurred in 1996 when fullerenes’ discoverers were awarded the Nobel Prize in Chemistry and in 2010 when Andr´e K. Geim and Konstantin S. Novoselov received the Nobel Prize in Physics for the discovery of graphenes. However, the figure of carbon nanoforms is significantly larger than mentioned above, and other less explored nanoforms such as nanohorns, nanoonions, nanotorus, nanobuds, nanocups, and peapods. are only some of the possible presentations of carbon whose properties and chemical reactivity are quite unknown so far [6]. Furthermore, fullerenes have been skillfully combined with other elements in their inner space, affording the large and fruitful family of endohedral fullerenes or endofullerenes, which contain an atom, molecule, or cluster in their inner cavity [7]. An intriguing question, however, is, why among the many possible cages that can be formed with carbon atoms, that containing 60 atoms is the favored one? Furthermore, since all fullerenes C n are constituted by hexagons (n ≥ 20 with the exception of n = 22) and pentagons (12 for all fullerene cages, which are responsible for the curved geometry), why, among the 1812 possible isomers for 60 carbon atoms, was only the icosahedral symmetry I h C 60 molecule (soccer-ball shape) formed?
These intriguing questions were answered by Kroto, who proposed that the local strain increases with the number of bonds shared by two pentagons (pentalene unit), thus affording less stable molecules. This rule was coined as the “isolated pentagon rule” (IPR), which states that all pentagons must be surrounded by hexagons, thus forming the corannulene moiety [8].
First Edition. Edited by Armando J. L. Pombeiro. © 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.
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