Numerous highly curved fragments of C60 and unique subunits of C70 were synthesized under mild conditions using metal-catalyzed protocols. According to X-ray crystallographic analyses, highly curved fragments of C60 have a maximum π-orbital axis vector (POAV) pyramidalization angle of up to 12.9 °, whereas distinctive fragments of C70, analogous to the tube portion of the rugby-shaped buckyball, are less curved. Among the eight buckybowls studied herein, five form polar crystals. Depending on the molecular geometry, the inversion dynamics of buckybowls involves either a planar or an S-shaped (non-planar) transition structure.
Corannulene (1)  and sumanene (2)  are elemental bowl-shaped subunits of C60. The curvature is caused by incorporation of five-membered rings into a sheet of benzenoids, consistent with Euler’s rule . These bowl-shaped compounds, so-called buckybowls or π-bowls, can be extended to form fullerenes  or carbon nanotubes (CNTs) . Buckybowls have interesting physical properties, and have (potential) applications as electro-optical organic materials such as liquid crystals and organic semiconductors . Another potential application of buckybowls in organic synthesis is that they are suitable precursors of “customized” CNTs . Some CNTs exhibit greater electronic conductivity than copper wire , but to date their pure form cannot be acquired in needed amounts [5b,c].
Recently, we have successfully prepared numerous buckybowls 3–5 [9–11]. Bowls 3 and 4 are highly curved fragments of C60, and they likely are suitable starting materials for constructing the smallest corannulene-based CNT (C40H10) . In contrast, 5 can map on the tube portion of C70. Additionally, 5a is also a fragment of numerous higher fullerenes, including C76, C78 , and C84 . Herein, a short review on the synthesis, structural analysis, and physical properties of these highly curved buckybowls is reported.
High inherent strain makes synthesis of highly curved buckybowls challenging. Most are prepared using a strategy that involves extension of the backbone of a smaller bowl and/or the use of high-temperature flash vacuum pyrolysis (FVP) as a synthetic tool [1d]. Successively increasing the curvature may facilitate the formation of highly curved buckybowls from a small bowl under mild synthetic conditions. Examples of such approaches include preparations of pentaindenocorannulene  and trinaphthosumanene  from corannulene and sumanene, respectively; however, many steps are required to prepare the latter two small π-bowls. Circumtrindene is directly obtained from decacyclene under FVP conditions . High-temperature conditions markedly limit the range of functional groups and potentially cause thermal rearrangement of their molecular framework . In contrast to these conventional methods, attempts were made to prepare highly curved buckybowls from planar precursors through the palladium-catalyzed intramolecular arylation (cyclization) under mild reaction conditions. Similar synthetic approaches have been applied for preparations of less curved buckybowls such as as-indaceno[3,2,1,8,7,6-pqrstuv]picenes , dibenzocorannulenes [18, 20] and others . Taking advantage of the palladium-catalyzed protocol and carefully studying reaction conditions made the desired compounds obtainable.
Methylene-bridged buckybowl 3a was prepared straightforwardly by rhodium-catalyzed [(2+2)+2] cycloaddition of 1,8-bis(2,6-dichlorophenylethynyl)naphthalene 6a with 2-butyne to yield fluoranthene 7a , which was subjected to palladium-catalyzed cyclization to give a mixture of 3a and 10a (ratio 71:29) in 28 % yield . Cyclopenta-annulated buckybowl 3b was obtained similarly using 1,2-dihydro-5,6-bis(2,6-dichlorophenylethynyl)naphthalene (6b) as the starting material. Presumably because of the increased curvature, the cyclization of 7b was inefficient (3b:10b = 39:61; 18 % yield).
Buckybowl 4a was first prepared in 0.14 % yield from 7,12-bis(2-bromophenyl)benzo[k]fluoranthene using FVP at 1100 °C . Conversely, our protocol allowed 4 to be generated under mild conditions. The palladium-catalyzed annulation of 1,8-bis(2,6-dichlorophenylethynyl)naphthalene 6a with iodobenzene gave benzo[k]fluoranthene 8a , which was converted into 4a in 31 % yield by palladium-catalyzed cyclization. Cyclopenta-annulated buckybowl 4b was obtained similarly, but the yield for the palladium-catalyzed cyclization of 8b was only 5 %. This unsatisfactory result was likely caused by the increased curvature of 4b.
Buckybowls 5a and 5b were prepared by palladium-catalyzed cyclization of naphtho[1,2-k]cyclopenta[cd]fluoranthene derivatives 9, which were obtained by rhodium-catalyzed [(2+2)+2] cycloaddition of diynes 6 with acenaphthylene , followed by aromatization using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Both buckybowls 5a and 5b were obtained in low yields (ca. 10 %), mainly because of their low solubility in common organic solvents. Some product was irreversibly lost during chromatography.
The structures of buckybowls 3, 4, 5, and 10 have been characterized by X-ray crystallography, and their curvature was determined by analyzing the bowl depth of the corannulene fragment (Table 1). The bowl depths followed the order 4 > 3 > 5 > 10, and all exceeded significantly that of corannulene (0.87 Å) . Bowls 5, which map on the tube portion of C70, are less curved than bowls 3 and 4 due to the lower density of the five-member rings. Cyclopenta-annulated buckybowls 3b, 4b, and 5b have 0.10–0.14 Å deeper bowl depths than their corresponding parent compounds 3a, 4a, and 5a, respectively. Compound 4b is the deepest bowl with a depth of 1.34 Å. Additionally, the bowl depths of the sumanene segment in 4a (1.48 Å) and 4b (1.50 Å) markedly exceed that of sumanene (1.11 Å) . The geometric calculations at the B3LYP/cc-pVDZ level are highly consistent with X-ray structural data.
|X-ray structural dataa||Inversion dynamicsd||References|
|Bowl depth (Å)||POAV (deg)||Path||Barrier (kcal/mol)|
|1||0.87||8.2||I||ca. 10||[26, 32]|
|3a||1.19||12.8, 10.0, 8.9||I||56.2||[9, 11]|
|3b||1.32||12.9, 10.9, 10.9||I||84.4|||
|4a||1.24 [1.48]||12.8, 10.0, 9.0||II||124.3||[9, 10]|
|4b||1.34 [1.50]||12.8, 10.8, 10.6||II||135.1|||
|5a||1.07||10.8, 9.2, 9.1||II||79.8|||
|5b||1.21||11.4, 10.9, 10.4||II||84.3|||
|1.07||10.9, 9.1, 8.9|
|10ab||1.03||11.9, 9.4, 9.0, 8.3, 9.6||–||24.1|||
|1.04||11.7, 9.8, 9.5, 8.9, 8.5|
|10b||1.19||12.0, 10.4, 10.9, 10.3, 7.2||–||–|||
|2||1.11||9.0c||I||ca. 20||[27, 33]|
aThe values were obtained by averaging symmetry-related data. Bowl depth and the POAV pyramidalization angles were determined from a corannulene core and its hub carbon atoms, respectively. The bowl depth for the sumanene segment is shown in the square bracket. bThere are two molecules in the asymmetric unit. cThe POAV pyramidalization angle was determined from the hub six carbons. d. Theoretical studies were calculated at B3LYP/cc-pVDZ level.
The curvature of these buckybowls was also analyzed by the POAV (π-orbital axis vector) pyramidalization angle . The POAV pyramidalization angle is highest at the hub carbon atoms of the corannulene core. Buckybowls 3 and 4 have a maximum POAV pyramidalization angle of approximately 12.8 °, exceeding that of C60 (11.6 °). To the best of our knowledge, 3, 4 and pentaindenocorannulene (12.7 °) are the most curved bowl-shaped fragments of fullerenes. Unlike in previous reports, the cyclopenta-annulation in buckybowls 3 or 4 does not increase the maximum POAV pyramidalization angle. This finding may suggest that the maximum POAV pyramidalization angle reaches its highest value, and the increase in curvature reflects only on an increase in bowl depth. This conclusion can be confirmed by analyzing carbon nanotube C50H10, whose corannulenyl end cap has a very deep bowl depth (1.54 Å), but with a slightly smaller POAV angle (12.3 °) than 3 or 4 . In contrast to 3 and 4, the shallower bowls 5 have lower bowl depths and smaller POAV angles.
The curvature of buckybowls causes their solid-state packing to become highly interesting and complex. The most notable packing characteristic of π-bowls is that they can form bowl-in-bowl stacks and all columns are oriented in the same direction. The resulting polar crystals benefit potential applications as organic materials with high electron mobility (organic semiconductors) [6c], for piezoelectricity or pyroelectricity , or for generating second harmonics (nonlinear optoelectronics) . The factors that are required to produce polar crystals are not yet well understood [1e, 31]. Among the eight buckybowls investigated, 3a, 4a, 4b, 5a, and 10a form polar crystals; however, these molecules slip from side to side within each stack (Table 2). The non-perfect bowl-in-bowl stacks are likely the result of compromise between the intrastack and interstack interactions guided by π/π stacking and CH···π hydrogen bonding. A slipping angle is defined as that between the stacking axis and the axis normal to the molecule, and a large value should be caused by interstack interactions. Interestingly, 4a, 4b, and 5a crystallize with the orthorhombic space group Cmc21, and they have similar packing patterns. Buckybowl 5a is less curved and has a larger π-surface than 4a and 4b, resulting in larger π/π surface overlaps and a smaller slipping angle. The structures of methylene-bridged bowls 3a and 10a have the monoclinic space group C1c1 and the orthorhombic space group Pna21, respectively. Their methylene protons take part in strong intermolecular CH···π interactions within a columnar stack.
Unlike corannulene (ca. 10 kcal/mol)  and sumanene (around 20 kcal/mol) , buckybowls 3–5 have very high inversion barriers, which cannot be measured using common NMR techniques. Although attempts have been made to conduct a variable-temperature NMR study of 3a or a 2D EXSY experiment of the dideutero-substituted compound D2-3a, they have been unsuccessful due to instrumental limitations. This suggests that the bowl-to-bowl inversion barrier of 3a should exceed 35 kcal/mol . Accordingly, the inversion mechanisms and barriers of buckybowls 3–5 were analyzed theoretically using DFT calculations at the B3LYP/cc-pVDZ level and the pseudo intrinsic reaction coordinate (pseudo-IRC). Like corannulene (1) [1e, 32] and sumanene (2) , the bowl-to-bowl inversion of the class 3 proceeds via a planar transition state (route I in Scheme 1), whereas the inversion dynamics of buckybowls 4 and 5 involve an S-shaped (non-planar) transition structure (route II in Scheme 1). In contrast to 3, the buckybowls 4 and 5 are longitudinally long, and they have a greater preference for route II with a lower inversion barrier than that through route I. Among the eight buckybowls studied, 4b (135.1 kcal/mol) and 3a (56.2 kcal/mol) have the highest and lowest inversion barriers, respectively. Within a compound class, peri-annelation increases the height of the inversion barrier. Finally, buckybowls 3, 4, and 5 can be regarded as static bowls at room temperature.
Bowls 3/4 and 5 have absorptions less than and greater than 450 nm, respectively. An absorption band in the region of 450–500 nm plays a critical role in distinguishing between C70 and C60 . Although buckybowls 3, 4, and 5 can map onto C70, theoretical studies indicated that bowls 5 have more π-conjugation and a smaller HOMO/LUMO band gap than 3/4. The photoluminescence of C60 and C70 is very weak because of the highly efficient intersystem crossing . The photoluminescence of 4 is also very weak, whereas 3 and 5 both have strong fluorescence at approximately 416/436 and 528 nm, respectively.
Mono-substituted buckybowl 4c, which was prepared similar to 4a, exhibits “bowl chirality” due to its three-dimensional geometry . The chiral resolution of racemic 4c was performed by HPLC using a chiral column and an eluent system composed of methanol and 2-propanol. Although two well-resolved peaks were observed, chiral resolution in useful amounts is impractical due to the very low solubility of 4c in the eluent system.
Simple synthetic approaches for preparations of highly curved buckybowls from easily obtained planar precursors under mild reaction conditions have been developed. Depending on molecular geometry, the inversion dynamics of buckybowls involves either a planar or an S-shaped (non-planar) transition structure. Buckybowls are suitable starting materials for the smallest corannulene-based CNT (C40H10), and “customized” CNTs. The electron mobility of polar buckybowl crystals, and the surface chemistry of buckybowls on metal surfaces are currently under examination.
This work was supported by the National Science Council of Taiwan (NSC 101-2628-M-006-002-MY3).
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