Placing Metal in the Bowl: Does Rim Alkylation Matter

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Placing Metal in the Bowl: Does Rim Alkylation Matter? Andrey Yu. Rogachev,*,† Shuyang Liu,† Qi Xu,† Jingbai Li,† Zheng Zhou,‡ Sarah N. Spisak,‡ Zheng Wei,‡ and Marina A. Petrukhina*,‡ †

Department of Chemistry, Illinois Institute of Technology, Chicago, Illinois 60616, United States Department of Chemistry, University at Albany, State University of New York, Albany, New York 12222, United States

‡

Organometallics Downloaded from pubs.acs.org by NEW MEXICO STATE UNIV on 01/05/19. For personal use only.

S Supporting Information *

ABSTRACT: In-depth theoretical analysis of the consequences of methylation of the rim sites of corannulene is completed. The full set of derivatives ranging from parent C20H10 to fully substituted C20(CH3)10 has been evaluated, revealing consistent trends along the series. The controlled one-electron chemical reduction of selected methylated corannulenes, such as monomethyl- (C20H9(CH3)), sympentamethyl- (1,3,5,7,9-C20H5(CH3)5), and decamethylcorannulene (C20(CH3)10) has also been investigated. Cesium metal was used as a reducing agent to access endo complexes having cesium ion placed inside the concave cavity of the monoreduced bowls. Two products, [{Cs+(18-crown-6)}{C20H9(CH3)−}] (1) and [{Cs+(18crown-6)}{C20H5(CH3)5−}] (2), have been crystallized in the presence of 18-crown-6 ether and crystallographically characterized to confirm the concave cesium ion coordination. The direct structural comparison of 1 and 2 with the complex of unsubstituted corannulene, [{Cs+(18-crown-6)}{C20H10−}] (3), has been conducted. Furthermore, the nature and strength of metal binding for the series of concave cesium complexes with methyl-substituted corannulene bowls has been evaluated using different theoretical methods.

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prepared by Cs reduction of C20H10 in 2011.13 Using accurate theoretical modeling, we confirmed that the large Cs+ ion favors endo-coordination in contrast to smaller alkali metal ions that clearly prefer binding to the convex (exo-) face of the corannulene bowl. This tendency was later illustrated by several crystallographically characterized examples of Li, Na, K, and Rb salts of monoreduced corannulene,14 all showing convex metal placement. The selective double concave coordination of Cs by the bicorannulenyl dianion15 and two sumanenyl anions16 further reaffirmed the above trend. On the basis of theoretical calculations, the preference for endo cesium ion binding inside the corannulene bowl was estimated to be rather small (2−3 kcal). However, the 5-fold symmetric core of corannulene provides a great platform for the design of rim-substituted analogues with tailored properties and structures.17 Specifically, halogenation of C20H10 can be controlled to afford monobromo-, sym-pentachloro-, and decachlorocorannulenes, which can serve as convenient starting reagents to form the respective alkylated analogues through halogen-substitution reactions.18 As a result, a series of alkylated corannulene ligands with the increasing number of methyl groups bound to the rim has become available.18 It is expected that stepwise alkylation of corannulene provides more electron-rich π-ligands, which could enhance stability of the resulting metal complexes. For the above series, several indepth NMR studies on transition metal coordination and surface migration using the tetra- and pentamethylated corannulene derivatives have been reported.19 One rhodium

INTRODUCTION Bowl-shaped polyaromatic hydrocarbons (PAHs) having two distinctly different aromatic surfaces, convex and concave, constitute a unique and interesting class of π-ligands.1 This family includes a variety of π-bowls with different depths, sizes, and symmetries that range from the smallest corannulene (C20H10)2 and sumanene (C21H12)3 to the deep carbon-rich C50H10 molecule that can also be considered a short nanotube.4 In recent years, multiple synthetic groups have added new examples to this remarkable family of carbon bowls. In 2012, Amsharov et al. synthesized a large C46H18 bowl, which represents more than 75% of the C60-fullerene surface.5 Wu and co-workers prepared several highly curved fragments of fullerenes with extended π-surfaces.6 Miao et al. provided access to novel nonplanar PAHs with negative curvatures.7 Most recently, modular synthetic strategies utilizing bowlshaped synthons have been successfully utilized by the groups of Stuparu8 and Shustova9 in design of novel functional organic materials and metallorganic frameworks. The observed structural modularity of bowl-shaped PAHs reinforces fundamental studies of unique properties associated with their curved and strained molecular structures. Investigations of relative preferences of the convex and concave metal binding of various π-bowls attract special attention of both experimental chemists and theoreticians.10 Controlled concave metal placement in the carbon container is especially appealing due to unique properties and applications of endohedral fullerenes and nanotubes.11 Over the years, sumanene has been shown by Hirao and coworkers to exhibit the concave binding for several transition metals,12 but the first concave complex of corannulene was © XXXX American Chemical Society

Received: November 14, 2018

A

DOI: 10.1021/acs.organomet.8b00837 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. Schematic Representation of Sequential Methylation of the Corannulene Core

Figure 1. Equilibrium geometry configurations for selected methylated corannulenes of the series (PBE0/cc-pVTZ).

complex with η6-coordinated 1,3,5,7,9-pentamethylcorannulene was crystallographically characterized.19c In contrast, only one main group metal complex of 1,2,5,6-tetramethylcorannulene with the highly electrophilic mercury unit, C18F12Hg3, can be mentioned here,20 but no crystalline products from the alkali metal reduction reactions have been reported for any alkyl-substituted corannulene to date. In this work, we performed a combined theoretical− experimental investigation of the binding abilities of the family of methylated corannulenes toward large Cs+ ion. The systematic modifications of corannulene core are based on the addition of 1−10 methyl groups to the rim of the bowl (Scheme 1), and the whole series is used for a comprehensive computational study. Selected methylated corannulenes, namely, C 2 0 H 9 (CH 3 ), 1,3,5,7,9-C 2 0 H 5 (CH 3 ) 5 , and C20(CH3)10, have been tested in controlled chemical reduction reactions. As a result, a detailed computational analysis of the full series of methylated corannulenes in neutral and anionic forms has been completed, followed by the discussion of their complexation abilities with cesium cations with and without complementing crown ether ligand. Two new cesium complexes, [{Cs+(18-crown-6)}{C20H9(CH3)−}] (1) and [{Cs+(18-crown-6)}{C20H5(CH3)5−}] (2), have been isolated in the single-crystalline form and crystallographically characterized, reaffirming cesium concave binding preferences.

distances. All differences fall in a narrow range of 0.02 Å. At the same time, the bowl depth underwent significant flattening from 0.88 Å in the parent C20H10 molecule to 0.64 Å in its decamethylated derivative. Notably, the most pronounced effect of methylation on the bowl depth is observed starting from the C20H10−n(CH3)n systems with n = 5−6 to that with n = 10 (Table 1), which can be attributed to the increasing influence of steric hindrance between the methyl groups. As expected, the flatter molecule shows a lower bowl-to-bowl inversion energy barrier, which is dramatically reduced from +10.03 kcal/mol in C20H10 to +3.58 kcal/mol in C20(CH3)10. Again, the main drop down in energy is observed in the second part of the series (Table 1), thus showing good correlation with trends in geometrical parameters. It is important to note here that the calculated parameters are in good agreement with available experimental data (for instance, E(bowl-to-bowl, C20H10) = 10.2 ± 0.2 kcal/mol21). The addition of donor alkyl group to rim sites of the corannulene core should afford more electron-rich bowls. Indeed, estimation of the electron affinities (EA) for target series clearly follows this trend, showing the largest magnitude for corannulene (−12.87 kcal/mol, EA(C20H10, exp.) = −11.53 kcal/mol22) and the smallest one for decamethylcorannulene (−7.90 kcal/mol). Thus, the replacement of hydrogen atom(s) at the rim of C20H10 by methyl group(s) results in substantial increase of donor ability of the bowl (Table 1). Importantly, all methods (albeit varying in absolute numbers) show exactly the same trend (see results of calculations based on PBE0, xDHPBE0, and B2PLYP-D3 methods in Table 1). This observation makes the conclusions independent of the method selection. Significant difference in electron affinities of the methylated bowls might be indicative of changes in their electronic structures. However, theoretical calculation of 3D charge distribution in these molecules (as illustrated by molecular electrostatic potential maps, Figure 2; for the complete set of MEP maps see the Supporting Information) revealed that electronic density distribution remains essentially the same in the whole series. Only some asymmetry was observed in

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RESULTS AND DISCUSSION Theoretical Analysis of Methylated Corannulenes. Before studying the interactions of cesium cation with a series of methylated corannulene ligands, it is informative to analyze the influence of substitution of the bowl-shaped skeleton on geometrical and electronic structures of neutral bowls. The calculated equilibrium geometry configurations are exemplified in Figure 1; selected geometrical and energetic characteristics are summarized in Table 1. Interestingly, the stepwise replacement of hydrogen atoms with methyl groups at the periphery of the corannulene bowl resulted in only minimal changes in carbon−carbon bond B


1.38 1.38 1.41 1.44 98 0.88 +10.03 −12.87 −9.55 −8.70

0

1

1.38 (1.38) 1.38 1.41 1.44 (1.45) 98 0.87 +9.81 −12.86 −9.55 −8.70

2 1.38 (1.39) 1.38 1.41 1.44 (1.45) 98 0.86 +9.90 −12.05 −8.80 −8.01

3 1.38 (1.39) 1.38 1.41 1.44 (1.45) 98 0.86 +10.06 −11.45 −8.23 −7.62

4 1.38 (1.39) 1.38 1.41 1.44 (1.45) 98 0.86 +10.01 −11.07 −7.91 −7.18

5 1.39 (1.40) 1.37 1.41 1.44 (1.45) 98 0.86 +9.90 −9.92 −6.81 −6.28

n 6 1.39 (1.40) 1.37 1.41 1.44 (1.45) 97 0.81 +8.26 −9.51 −6.56 −5.68

7 1.39 (1.40) 1.37 1.40 1.44 (1.45) 97 0.77 +7.56 −9.29 −6.48 −5.61

8 1.39 (1.40) 1.37 1.41 1.44 (1.45) 97 0.72 +6.69 −8.89 −6.13 −5.09

9 1.39 (1.40) 1.37 1.40 1.44 (1.45) 96 0.66 +3.82 −7.95 −5.32 −4.12

10 (1.40) 1.37 1.40 (1.45) 96 0.64 +3.58 −7.90 −5.26 −3.95

a All data are calculated at the PBE0/cc-pVTZ level, unless otherwise noted. Bond lengths are in Å. Angles and POAV values are in degree, energetic characteristics are in kcal/mol. bGeometrical parameters in parentheses are those for bonds with methyl group attached (averaged). cPOAV, E(bowl-to-bowl), and EA abbreviations are used for π-orbital axis vector, activation energy for a bowl-tobowl inversion, and electron affinity, respectively. The bowl depth is calculated as a distance between the plane defined by five hub carbon atoms and the plane defined by ten rim carbon atoms.

rim spoke hub flank POAVc bowl depth E (bowl-to-bowl)c EA (PBE0)c EA (xDH-PBE0)c EA (B2PLYP-D3)c

parameter

b

Table 1. Selected Geometrical and Energetic Parameters for Methylated Corannulenesa

Organometallics Article

C


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Figure 2. Molecular electrostatic potential maps (MEP) for selected methylated corannulene derivatives of the series (PBE0/cc-pVTZ).

Figure 3. Full ACID isosurfaces (top) and its π-component (bottom) for selected methylated corannulenes of the series (n = 0, 1, 5, 10; PBE0/ccpVTZ) in their neutral (a) and anionic (b) forms. Current density vectors are plotted onto the ACID isosurface to indicate dia- and paratropic ring currents. Applied magnetic field is perpendicular to the 5-membered ring. Bold arrows highlight the direction of ring currents as exemplified for parent corannulene.

systems with nonsymmetrical placement of the methyl substituents (n = 1−4 and 4−9).

At the same time, aromatic behavior of individual rings in these bowl-shaped systems becomes more pronounced upon D


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Organometallics

Table 2. Aromaticity Descriptors Calculated for Selected Methylated Corannulene Derivatives of the Series in Their Neutral and Anionic (in parentheses) Formsa

AIMb system

C20H10

C20H9(CH3)

C20H5(CH3)5

C20(CH3)10

ring

HOMA

A B C D E F A B C D E F A B C D E F A B C D E F

0.889 0.750 0.750 0.750 0.750 0.750 0.890 0.701 0.754 0.746 0.747 0.749 0.898 0.703 0.704 0.703 0.702 0.701 0.946 0.510 0.534 0.549 0.500 0.552

NICS

(0.899) (0.849) (0.649) (0.852) (0.747) (0.760) (0.901) (0.616) (0.854) (0.711) (0.796) (0.837) (0.906) (0.731) (0.821) (0.617) (0.821) (0.713) (0.952) (0.679) (0.630) (0.470) (0.712) (0.395)

9.16 −6.38 −6.38 −6.38 −6.38 −6.38 9.38 −5.63 −6.49 −6.39 −6.20 −6.36 10.30 −5.61 −5.63 −5.63 −5.59 −5.62 9.87 −5.19 −5.19 −5.19 −5.19 −5.19

PDI

(−21.15) (23.30) (26.13) (23.18) (25.17) (25.01) (−18.45) (20.29) (19.45) (21.21) (19.21) (18.48) (−20.78) (19.93) (18.73) (21.25) (18.30) (20.54) (−13.78) (12.39) (11.74) (12.65) (10.92) (13.66)

fuzzy atomic spacec FLU

0.059 0.059 0.059 0.059 0.059

(0.052) (0.046) (0.052) (0.049) (0.049)

0.056 0.060 0.059 0.059 0.059

(0.045) (0.052) (0.048) (0.050) (0.052)

0.057 0.057 0.057 0.057 0.057

(0.048) (0.050) (0.045) (0.050) (0.048)

0.055 0.056 0.056 0.055 0.056

(0.049) (0.048) (0.045) (0.050) (0.043)

0.027 0.014 0.014 0.014 0.014 0.014 0.027 0.015 0.014 0.014 0.014 0.014 0.028 0.014 0.014 0.014 0.014 0.014 0.028 0.014 0.014 0.014 0.015 0.014

PDI

(0.022) (0.010) (0.018) (0.010) (0.014) (0.014) (0.022) (0.018) (0.010) (0.016) (0.012) (0.011) (0.023) (0.014) (0.011) (0.018) (0.011) (0.014) (0.023) (0.011) (0.013) (0.016) (0.010) (0.019)

0.060 0.060 0.060 0.060 0.060

(0.054) (0.049) (0.054) (0.052) (0.052)

0.057 0.061 0.060 0.060 0.061

(0.047) (0.054) (0.051) (0.053) (0.054)

0.058 0.058 0.058 0.058 0.058

(0.050) (0.052) (0.048) (0.052) (0.050)

0.056 0.056 0.056 0.055 0.056

(0.050) (0.050) (0.047) (0.052) (0.045)

FLU 0.038 0.019 0.019 0.019 0.019 0.019 0.038 0.019 0.019 0.019 0.019 0.019 0.020 0.020 0.020 0.020 0.020 0.020 0.039 0.021 0.020 0.020 0.022 0.020

(0.032) (0.015) (0.023) (0.015) (0.019) (0.019) (0.032) (0.023) (0.015) (0.021) (0.018) (0.016) (0.033) (0.019) (0.017) (0.023) (0.017) (0.020) (0.033) (0.018) (0.019) (0.023) (0.017) (0.025)

a n = 0, 1, 5, 10. Anionic data in parentheses. Reference of C−C in HOMA is 1.388 Å and a = 257.7; NICSs are calculated at the PBE0/cc-pVTZ level (in ppm). The reference delocalization index of C−C in Bader’s FLU is 1.3921e from benzene optimized at the PBE0/cc-pVTZ level, and in Becke’s FLU, it is 1.4637e. bAtomic overlap matrix is based on the basin integral in AIM. cAtomic overlap matrix is based on the fuzzy atom space integral over Becke atomic space. For complete sets of data for all systems see the Supporting Information.

Table 3. Selected Geometrical and Energetic Parameters for Monoanions of Methylated Corannulenesa n parameter

0

1

2

3

4

5

6

7

8

9

10

rim spoke hub flank POAVb bowl depth E(bowl-to-bowl)b

1.40 1.39 1.41 1.44 97 0.82 +7.50

1.40 1.39 1.40 1.44 97 0.82 +7.52

1.41 1.39 1.40 1.45 97 0.82 +7.73

1.40 1.39 1.41 1.44 97 0.82 +7.95

1.40 1.38 1.40 1.44 98 0.82 +8.03

1.40 1.39 1.40 1.44 98 0.82 +8.13

1.40 1.39 1.40 1.44 97 0.77 +6.65

1.40 1.39 1.40 1.45 97 0.73 +5.66

1.40 1.39 1.41 1.45 97 0.70 +6.56

1.40 1.38 1.40 1.45 96 0.63 +2.85

1.41 1.39 1.40 1.44 96 0.60 +2.93

a

All data are calculated at the PBE0/cc-pVTZ level, unless otherwise noted. Bond lengths are in Å, angles and POAV values are in degree, energetic characteristics are in kcal/mol. Since due to symmetry anionic corannulene derivatives are Jahn−Teller active systems, average values for all geometrical parameters are presented. bPOAV, E(bowl-to-bowl), and EA abbreviations are used for π-orbital axis vector and activation energy for a bowl-to-bowl inversion, respectively.

methylation. While ACID maps clearly show strong paratropic ring current of the central 5-membered ring (5MR) and diatropic ring currents for 6-membered rings (6MRs) in all

systems (Figure 3), the absolute magnitude (as evaluated by NICS, HOMA, FLU, and other descriptors) shows notable variations (Table 2). For instance, the antiaromaticity of the E


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Organometallics

Figure 4. Spin density distribution (isosurface level 0.003 a.u.) in selected methylated corannulene anions of the series (n = 0, 1, 5, 10; PBE0/ccpVTZ).

observed in all methylated corannulene bowls (Figure 3 and Table 2). Theoretical Analysis of Methylated Corannulenes Adducts with Naked Cs+. We have previously identified the perfect match between the size of corannulene concave cavity and large cesium ion as one of the main driving forces for endo-coordination mode to be the most preferable.13 In contrast, smaller alkali metal cations tend to form exo-bound adducts with bowl-shaped PAHs.14 The X-ray crystallographic characterization of cesium complexes with methylated corannulene derivatives isolated in this work (vide infra) further reinforced this trend. Considering the dramatic influence of the methylation of corannulene skeleton on its bowl depth and donor ability, one could expect significant alteration in stability of the corresponding adducts with Cs+ ions. In order to provide the most reliable results and conclusions, we have used two alternative energy decomposition schemes, namely, Natural Energy Decomposition Analysis (NBO-NEDA23), realized within the framework of the NBO approach, and Energy Decomposition Analysis (EDA24), developed by Morokuma and by Ziegler and Rauk. Analysis of the distances between the center of 5MR and cesium cation in target adducts revealed that they show only small (ca. 0.06 Å) variations, with tendency to decrease in magnitude along with increased level of methylation from C20H10 to C20(CH3)10. This finding shows a minimal influence of the bowl methylation on the strength of cation−π interactions. Indeed, subsequent investigation of the bonding energy, as the quantity that describes such strength also helps to evaluate thermodynamic stability of products, clearly showed only minimal dependence on the nature of the bowlshaped ligand. The bonding energy was found to fall in a very narrow range of ∼1 kcal/mol for the whole series (Table 4). Analysis of the spin density distribution in all cesium adducts revealed a complete electron transfer from cesium to the bowl (Figure 5). Therefore, the resulting products are best described as [{Cs+}{C20H10−n(CH3)n−}]. Analysis of the bonding energy and its components was also in agreement with this conclusion. As expected, both decomposition schemes show domination (>70%) of the electrostatic or ionic contribution (EEL in NBONEDA and ΔEelstat in EDA schemes, respectively) to the bonding (Table 4), whereas the orbital or covalent part (ECT and ΔEorb, respectively) covers only ∼20%. Interestingly, the variations of both terms, covalent and ionic, are within a small range of 3−4 kcal/mol (Table 4), thus showing an insignificant effect of methylation on the product stability. Both decomposition schemes, although they have very different origins and utilize different methodologies, exhibit a very consistent behavior in the description of bonding energy and

central 5MR experiences small but notable strengthening, when going from unsubstituted corannulene (NICS = 9.16 ppm) to its decamethylated derivative (NICS = 9.86 ppm). In contrast, the aromatic behavior of 6MRs becomes weaker (−6.37 ppm and −5.19 ppm, respectively). Importantly, all aromaticity descriptors show consistent behavior and the same trends (Table 2). Theoretical Analysis of Monoanions of Methylated Corannulenes. For the next step, the monoanions of the methylated corannulene series have been investigated. Adding one electron to the bowl core showed only a minor influence on carbon−carbon bond lengths, accompanied by slight elongation of the rim and spoke bonds (Table 3). This can be explained by the nature of LUMO (accommodating an extra electron) having antibonding character with respect to these two bonds. At the same time, the formation of monoanionic species resulted in the bowl flattening by 0.04− 0.06 Å. As expected, the curvature reduction results in lowering the energy barrier for the bowl-to-bowl inversion by ∼3 kcal/ mol. Notably, all observed trends, as functions of methylation, remained exactly the same as for neutral bowls (Table 1). As it follows from spin density distribution maps (Figure 4), a single unpaired electron is delocalized over the whole πsurface of the bowl-shaped monoanions. The topology of this distribution clearly reflects those of the highest unoccupied molecular orbital of the neutral methylated corannulenes (see the Supporting Information for details). No significant influence of the methylation degree on the spin density distribution is observed. The most dramatic changes in the electronic structure of the bowl-shaped corannulene anions, as compared with those of their neutral counterparts, were found in the aromatic behavior of individual 5MR and 6MRs. The ACID maps clearly reveal that aromaticity of these systems is inverted. For instance, the central 5MR exhibits strong diamagnetic ring current in all monoanionic species, in contrast to paramagnetic one in the parent neutral systems (Figure 3). At the same time, 6MRs, previously known to exhibit diamagnetic ring currents, unambiguously show paramagnetic ones after adding one electron to the neutral polyaromatic system. Subsequent analysis of quantification of aromaticity of different rings with help of various descriptors (Table 2) provided a strong support for this conclusion. For instance, the antiaromatic behavior of the central 5MR in neutral corannulene (NICS = 9.16 ppm) is transformed into strong aromatic one (NICS = −21.15 ppm). In opposite, the aromatic character of 6MR in uncharged bowls becomes significantly antiaromatic in nature in monoanionic ones (NICS = −6.38 and 23.30 ppm in C20H100 and C20H101−, respectively). The same trends are F


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2.88 −102.16 −118.24 −18.34 34.84 −101.74 −103.41 −93.89 (74.53%) −32.09 (25.47%) 22.58 0.33 −103.08 2.89 −101.67 −118.41 −18.86 35.27 −101.99 −103.51 −93.95 74.62%) −31.96 (26.38%) 22.40 0.80 −102.71 ΔEPauli ΔEprep −De

ΔEorb

9 8

2.90 −101.73 −118.08 −19.33 35.62 −101.78 −103.20 −93.55 (74.60%) −31.85 (25.40%) 22.21 0.61 −102.59 2.91 −101.78 −118.35 −20.09 36.58 −101.86 −103.15 −93.52 (74.61%) −31.82 (25.39%) 22.19 0.58 −102.57

7 6

2.91 −101.62 −117.78 −20.16 36.37 −101.57 −102.72 −93.16 (74.82%) −31.36 (25.18%) 21.80 0.48 −102.24 2.92 −101.30 −117.29 −20.41 36.44 −101.26 −102.28 −92.69 (74.85%) −31.15 (25.15%) 21.56 0.47 −101.81

5 4

2.92 −101.37 −117.12 −20.34 36.13 −101.32 −102.11 −92.88 (75.24%) −30.57 (24.76%) 21.33 0.45 −101.66 2.93 −101.29 −116.77 −20.24 35.78 −101.23 −101.89 −92.93 (75.59%) −30.01 (24.41%) 21.05 0.40 −101.49

3 2

2.93 −101.22 −116.42 −20.11 35.43 −101.10 −101.57 −93.06 (76.05%) −29.31 (23.95%) 20.81 0.44 −101.13 2.93 −101.17 −116.15 −20.01 35.09 −101.07 −101.40 −93.20 (76.43%) −28.74 (23.57%) 20.54 0.46 −100.94

1

its components in target cesium products. This observation provides additional reliability for the results and conclusions produced in this study. X-ray Crystallographic Study. Three selected methylated corannulene derivatives with increased level of substitution from C20H9(CH3) to C20H5(CH3)5 and C20(CH3)10 have been investigated in chemical reactions with metallic cesium. The target complexes were prepared by the time-controlled reduction of the corannulene derivatives with Cs in THF (see the Supporting Information for more details). All reactions were accompanied by a quick color change to green, which is characteristic of the first reduction stage. The addition of 18-crown-6 ether was used to facilitate crystallization of the resulting cesium products of the monoreduced bowls. The X-ray quality crystals of [{Cs+(18-crown-6)}{C20H9(CH3)−}] (1) and [{Cs+(18-crown-6)}{C20H5(CH3)5−}] (2) were isolated in moderate 30−40% yields by slow diffusion of hexanes into THF solutions. The UV−vis and 1H NMR spectra of 1 and 2 are consistent with those of the previously reported cesium complex of unsubstituted corannulene monoanion, [{Cs+(18-crown-6)}{C20H10−}] (3),13 which was used below for comparison. Although the crystalline products from the reduction reactions of C10(CH3)10 have been isolated multiple times (see the Supporting Information for 4), the crystals were always nondiffracting and that precluded their crystallographic analysis. The X-ray diffraction studies of 1 and 2 showed the endo placement of the large Cs+ ion, capped by 18-crown-6 ether, inside the concave cavity of the monoreduced bowls (Figure 6). In both 1 and 2, the cesium cation sits almost exactly above the central 5MR of the corannulene bowl (Figure 7). Similar to complex 3, a large Cs+ ion is pulled out the of the coordinated 18-crown-6 ether molecules with the Cs···O bond length distances in 1 and 2 (Table 5) being close to those previously reported for some [Cs(18-crown-6)]+ containing salts.13,25 The cation−π distance is often considered as one of the major structural parameters to gauge the strength of the interaction. A linear relationship between the cation−π distance and interaction energy has been observed in the case of planar aromatic compounds.26 The isolated cesium products provide an interesting set to test if such correlations persist for bowl-shaped PAHs. However, considering the distance between the Cs ion and the centroid of 5MR, no general correlation was observed for 1−3. The Cs···Cη5centroid distances vary in a small range for two independent C20H5(CH3)5− monoanions in the asymmetric unit of 2 (average 3.321(20) Å) and are close to those in 1 (3.309(5) Å) and 3 (3.285(3) Å, Table 5). From Table 1 it can be clearly seen that alkylation of C20H10−n(CH3)n bowls with n = 1−5 shows minimal effect on curvature. The same trend is observed for the “naked” monoanions C20H10−, C20H9(CH3)− and C20H5(CH3)5− (Table 3), but when considering the X-ray structural data (Table 5), a curvature reduction upon increased rim methylation and cesium complexation is seen on going from 1 (0.856(6) Å) to 2 (0.808(18) Å). This prompted us to look into the effect of secondary metal ion coordination. Theoretical Study of the Influence of Coordinated Crown Ether. Since in all experimentally determined X-ray structures the cesium cation is surrounded by a capping crown ether ligand, we performed comprehensive theoretical

2.94 −101.02 −115.69 −19.92 34.75 −100.85 −101.03 −93.23 (76.87%) −28.05 (21.13%) 20.25 0.33 −100.70

0 parameter

n

Table 4. Selected Geometrical (Å) and Energetic Characteristics (kcal/mol) for Adducts of Methylated Corannulenes with Cs+

Cs−centroid NBO-NEDA Ebonding EEL ECT ECO Etot EDA-NOCV ΔEint ΔEelstat

10

Organometallics

G


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Organometallics

Figure 5. Spin density distribution (isosurface level 0.003 au) in adducts of selected methylated corannulene anions of the series (n = 0, 1, 5, 10; PBE0/cc-pVTZ) with naked Cs+.

and weak interactions was performed for [{Cs+(18-crown6)}{C20H10−}] (3). Calculated parameters are collected in Figure 8 (for numerical data see the Supporting Information). The interaction between Cs + and crown ether is characterized by presence of six bond critical points (BCPs) of (3, −1) type, which correspond to six Cs−O bonds (Figure 8, right). Topological parameters such as Laplacian (∇2ρb), total energy density as well as its component, kinetic (G(rb)) and potential energy (V(rb)) densities, and binding degree parameter (I(rb)), calculated for these BCPs (see the Supporting Information) unambiguously indicate the closedshell type of interactions between a metal center and O atoms of crown ether ligands. This type of interactions accompanied by relatively small values of electron density is usually associated with ionic character of the bond, whereas negative value of Laplacian together with high value of electron density is attributed to a distinct covalent character (so-called shared interaction, see Figure 9). In accordance with Espinosa’s evaluation scheme,29 energies of the Cs−O bonds in [{Cs+(18crown-6)}{C20H10−}] (3) are within the range of 1.00−2.00 kcal/mol. The interaction between corannulene bowl and Cs+ is represented by two BCPs (Figure 8). Combination of calculated topological parameters (Table S3) clearly indicates the closed-shell type of interaction with energy estimated to be ca. −2 kcal/mol. Subsequent investigation of topology of electron density and its gradient maps (Figure 9) revealed an extended network of weak intramolecular C−H···π interactions. Consistent with previous findings, while being individually weak, such interactions, when playing together, could result in significant

Figure 6. Molecular structures of 1 (left) and 2 (right), ball-and-stick models. H atoms of 18-crown-6 are omitted for clarity.

Figure 7. Space-filling models of 1 (left) and 2 (right).

investigation of such complexes for the whole series of methylated corannulenes. Previously, it was found that a net of weak interactions such as C−H···π, C−H···O, and O···π can be effectively used for stabilization of earlier unknown or rarely observed coordination modes in adducts of alkali metals with curved27 and planar14d polyaromatic ligands. Such interactions can be quantified by topological analysis of electron density ρb using tools of Quantum Theory of Atoms In Molecules (QTAIM)28 approach. Topological Analysis of [{Cs+(18-crown-6)}{C20H10−}]. At the initial step, the full topological analysis of electron density Table 5. Key Distances in 1, 2, and 3 (Å)

2a

313

Cs−C20H5(CH3)5

Cs−C20H10

1 Cs−C20H9(CH3) Cs···O

3.1130(13)−3.2139(12)

Cs···Cη5

3.430(6)−3.631(6)

centroid Cs···Cη6 centroid exp. bowl depth

3.309(6) 3.430(6)−3.862(6) 3.342(6) 0.856(6)

2.995(17)−3.247(18) 3.125(8)−3.389(10) 3.383(11)−3.691(11) 3.395(20)−3.593(20) 3.350(11), 3.291(20) 3.383(11)−3.768(11), 3.395(20)−3.851(20) 3.291(11), 3.304(20) 0.808(18) avg

3.139(2)−3.216(2) 3.424(3)−3.573(3) 3.285(3) 3.424(3)−3.916(3) 3.375(3) 0.846(3)

a

There are two independent pentamethylcorannulene cores in the asymmetric unit. H


Article

Organometallics

Figure 8. Bond critical points along with bond paths for [{Cs+(18-crown-6)}{C20H10−}]. h1−h7, m1−m2, and p1 denote C−H···π, Cs···C, and O···π bonding contacts, respectively. Ring and cage critical points are omitted for clarity.

Figure 9. Gradient vector of electron density maps (top) and corresponding 2D intersections of Laplacian (bottom) for the Cs···O bonding contacts (a), Cs···C bonding contacts (b), C−H···π interactions (c), and O···π interaction (d) in the adduct [{Cs+(18-crown-6)}{C20H10−}]. For the Laplacian, blue lines define positive regions and red lines define negative ones.

Table 6. Selected Energetic Characteristics (Interaction Energies Eint and Total Bonding Energy Ebonding) of [{Cs+(18-crown6)}{C20H10−n(CH3)n−}] (n = 0−10) systems (kcal/mol, PBE0) n parameter

0

1

2

3

4

5

6

7

8

9

10

Einta Ebonding

−4.36 −73.03

−3.78 −72.75

−4.27 −72.99

−4.33 −73.34

−3.65 −73.12

−3.18 −72.77

−3.02 −72.55

−2.28 −71.67

−2.33 −73.48

−3.25 −73.69

−3.55 −74.77

a

Interaction energies are calculated within a framework of topological QTAIM approach.

energetic preferences of a specific coordination mode.25,26 Evaluation of their energetics in [{Cs + (18-crown-6)}{C20H10−}] showed that these interactions are within a range from −0.38 kcal/mol to −0.77 kcal/mol. However, their total contribution was calculated to be −4.36 kcal/mol. Cesium Adducts with Methylated Corannulenes. Investigation of adducts of cesium cation capped by crown ether ligand with methylated corannulenes revealed essentially the same topology of bonding as that observed in the corannulene analogue 3 (see the Supporting Information for numerical data). For instance, a set of six Cs···O interactions was found in all adducts of the series. The energy density parameters confirmed their pure closed-shell nature with notable positive values of binding degree I(rb). The energy of stabilizing C−H···π intramolecular contacts shows some correlation with number of methyl groups in bowl-shaped ligands (Table 6). The largest stabilizing effect

was found for the system with parent corannulene. Subsequent addition of CH3 group(s) to the bowl resulted in a notable decrease of interaction energy, reaching its minimum for system with n = 7. Surprisingly, further replacement of hydrogen atoms by methyl groups led to increase of the Eint in magnitude (Table 6). The observed trend can be explained by geometric transformations of methylated bowls along the series. Adding methyl groups to the corannulene rim naturally increases steric effects, thus weakening C−H···π bonding interactions in the resulting adducts. At the same time, starting with n = 8, the bowl core undergoes the most pronounced flattening (Table 1). The later allows better access to the πsurface and, consequently, results in strengthening of C−H···π interactions. The competition between these opposite events seems to play a crucial role in stabilization of weak interactions in the final products. I


Article

Organometallics Total Thermodynamic Stability of [{Cs+(18-crown-6)}{C20H10−n(CH3)n−}] Adducts. In the final step, we evaluated the thermodynamic stability of [ {Cs + (18- crown-6)}{C20H10−n(CH3)n−}] (n = 0−10) complexes, using the total bonding energy (Ebonding, Table 6). Interestingly, the results clearly revealed that Ebonding follows exactly the same trend as Eint, calculated by QTAIM approach. For systems with n = 0− 7, increasing the level of methylation of the corannulene core led to decrease in stability of corresponding adducts (from −73.03 kcal/mol in [{Cs+(18-crown-6)}{C20H10−}] to −71.67 kcal/mol in [{Cs+(18-crown-6)}{C20H3(CH3)7−}]). At the same time, starting from [{Cs+(18-crown-6)}{C20H2(CH3)8−}], further addition of CH3-groups to the bowl resulted in notable stabilization of the complexes, as evidenced by bonding energy (−73.48, −73.69, and −74.77 kcal/mol for systems with n = 8, 9, and 10, respectively). This excellent correlation between the collective strength of C−H···π interactions and total thermodynamic stability of corresponding adducts shows the importance of weak interactions in the formation of large supramolecular aggregates. Interestingly, this trend is different from what was observed for respective adducts with naked Cs+ ions (Table 4), demonstrating the role of secondary interactions in such systems. Specifically, the number of methyl groups in C20H10−n(CH3)n linearly correlates with the bonding energy, such as the higher methylation degree of the ligand corresponds to larger Ebonding of the products (Table 4).

molecules, exhibit strong aromaticity after addition of one electron to the bowl core. Subsequent analysis of complexes of methylated corannulenes with cesium revealed complete electron transfer from metal center to the bowl. Thus, the corresponding products can be best described as [{Cs+}{C20H10−n(CH3)n−}]. Analysis of energetics of interactions in terms of two alternative energy decomposition schemes (EDA and NEDA) showed that the bonding between the metal cation and π-systems is expectedly dominated by electrostatic or ionic component (Table 4). Interestingly, all variations of both terms, covalent and ionic, are within a very narrow range of 3−4 kcal/mol. This finding clearly indicates the minimal effect of methylation on the stability of final products. For the last step, the influence of the secondary ligand, 18crown-6 ether, was analyzed. The general trend in stability of the corresponding adducts remains essentially the same as in the systems formed by naked Cs+ cation (Tables 4 and 5). Subsequent application of the topological theory (QTAIM) allowed us to quantify the contributions from the net of weak C−H···π interactions. It was found that such a network plays an important role in stabilization of the final products. Importantly, all theoretical conclusions are supported by crystallographically confirmed examples, stemming from controlled one-electron chemical reduction of selected methylated corannulene derivatives. An agreement between theoretical and experimental results provides a key justification for trends observed.

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CONCLUDING REMARKS The first comprehensive theoretical−experimental study of the influence of methylation on the structures and reactivity of corannulene bowl toward complexation with Cs+ has been accomplished. The full set of derivatives ranging from C20H10 to the fully substituted C20(CH3)10 bowl was considered in their neutral and monoanionic forms. This study was followed by in-depth investigation of complexation of methylated bowlshaped π-ligands with large cesium ion. The influence of the secondary 18-crown-6 ligand was also considered and quantified. Importantly, different theoretical approaches/ techniques used in this study have consistently shown the same trends, thus making results and conclusions methodindependent. First, it was observed that methylation of corannulene bowl results in minimal changes in bond length distribution. The most pronounced geometrical changes are observed for the bowl depth−adding more methyl groups to the periphery of corannulene leads to notable flattening of the bowl. Importantly, these trends are found for both neutral and monoanionic forms of target bowl-shaped species (Tables 1 and 3). In general, adding one electron to the neutral corannulene derivative makes the system more flat and flexible, as illustrated by the calculated barriers of the bowl-to-bowl transitions. All such changes can be interpreted as a consequence of completely delocalized electronic structure of methylated corannulene molecules. At the same time, reduction of the methylated corannulenes results in dramatic changes of their aromaticity. In the case of monoanionic systems, the aromatic behavior of all rings of corannulene core is inverted compared with neutral counterparts (Table 2). Sixmembered rings, previously found to be aromatic, in monoreduced systems show strong antiaromaticity. In contrast, five-membered rings, which are antiaromatic in neutral

EXPERIMENTAL SECTION

Materials and Methods. All manipulations were carried out using break-and-seal30 and glovebox techniques under an atmosphere of argon. Solvents (THF and hexanes) were dried over Na/ benzophenone and distilled prior to use. THF-d8 was dried over NaK2 alloy and vacuum-transferred. Cesium and 18-crown-6 ether were purchased from Sigma-Aldrich. Monomethylcorannulene (C20H9(CH3)), sym-pentamethylcorannulene ((C20H5(CH3)5), and decamethylcorannulene ((C20(CH3)10) were prepared18b as described previously and sublimed at 150, 180, and 185 °C, respectively, prior to use. The 1H NMR spectra were measured on a Bruker AC-400 spectrometer at 400 MHz and were referenced to the resonances of the corresponding solvent used. All shifts are reported in ppm. The UV−vis spectra were recorded on a PerkinElmer Lambda 35 spectrometer. The extreme air- and moisture-sensitive natures of 1 and 2 prevented obtaining of elemental analysis data. Preparation of [{Cs+(18-crown-6)}{C20H9(CH3)}−] (1). THF (2.0 mL) was added to a flask containing monomethylcorannulene (5 mg, 0.0189 mmol), 18-crown-6 (6 mg, 0.0225 mmol), and Cs metal (∼5 equiv). The color changed from pale yellow to dark green within 3 min. The reaction was stirred at room temperature for 10 min. The reaction was stopped before the formation of {C20H9(CH3)}2− was observed, which is indicative by the appearance of purple color. The suspension was filtered, and the green filtrate was layered with hexanes (1.3 mL) and placed at 10 °C. After 2 weeks, dark blockshaped crystals were present in low yield (30%). 1H NMR (400 MHz, THF-d8, 20 °C, ppm): δ = 4.90. 1H NMR (400 MHz, THF-d8, −60 °C, ppm): δ = 5.04. UV/vis (THF): λmax 443, 609 (sh), 655, 795 nm. Preparation of [{Cs+(18-crown-6)}{C20H5(CH3)5}−] (2). THF (1.5 mL) was added to a flask containing sym-pentamethylcorannulene (3 mg, 0.009 mmol), 18-crown-6 (3 mg, 0.011 mmol), and Cs metal (∼5 equiv). The reaction was stirred at room temperature for 5 min. The color changed from pale yellow to dark green within 1 min. The reaction was stopped before the formation of {C20H5(CH3)5}2− was observed, which is indicative by the appearance of purple color. The suspension was filtered, and the green filtrate was layered with hexanes (1.2 mL) and placed at 10 °C. After 1 week, dark blockJ


Article

Organometallics shaped crystals were present in moderate yield (40%). 1H NMR (400 MHz, THF-d8, 20 °C, ppm): δ = 3.87. 1H NMR (400 MHz, THF-d8, −20 °C, ppm): δ = 3.91. UV/vis (THF): λmax 473, 546, 693 nm. Preparation of [{Cs+(18-crown-6)}{C20(CH3)10}−] (4). THF (2.0 mL) was added to a flask containing decamethylcorannulene (5.0 mg, 0.013 mmol), 18-crown-6 (3.4 mg, 0.013 mmol), and Cs metal (∼5 equiv). The color changed from pale yellow to dark green within 4 min. The reaction was stirred at room temperature for 7 min. The reaction was stopped before the formation of {C20(CH3)10}2− was observed, which is indicative by the appearance of purple color. The suspension was filtered, and the green filtrate was layered with hexanes (1.5 mL) and placed at 10 °C. After 2 weeks, dark needleshaped crystals were present in low yield (31%). 1H NMR (400 MHz, THF-d8, 20 °C, ppm): δ = 3.85. 1H NMR (400 MHz, THF-d8, −40 °C, ppm): δ = 4.01. UV/vis (THF): λmax 493, 655(sh), 705 nm. Crystal Structure Determination and Refinement. For 1, data collection was performed on a Bruker D8 VENTURE X-ray diffractometer with PHOTON 100 CMOS detector and Mo-target X-ray tube (λ = 0.71073 Å) at T = 100(2) K. For 2, data collection was performed on a Bruker D8 VENTURE X-ray diffractometer with PHOTON 100 CMOS detector and mirror-monochromated Cu Kα radiation (λ = 1.54178 Å) at T = 100(2) K. In both cases, data were corrected for absorption effects using the empirical method SADABS.31 The structures were solved by direct methods and refined using the Bruker SHELXTL (version 6.14) software package.32 All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were included in idealized positions for structure factor calculations. In 1, the monomethylcorannulene core is disordered over two orientations, which were treated with an occupancy ratio of 0.57:0.43. Both orientations were refined with anisotropic thermal parameters using the following restraints: the anisotropic displacement parameters of two atoms in the direction of bond between them were restrained to be equal within an uncertainty value of 0.01 Å2 and to have the same Uij components within the standard uncertainty value of 0.04 Å2. There are two independent molecules in the asymmetric unit in 2. The sym-pentamethylcorannulene core coordinated to Cs2 is disordered over two orientations, which were treated with an occupancy ratio of 0.54:0.46. In the second case, the 18-crown-6 molecule coordinated to Cs1 is disordered over two orientations, which were treated with an occupancy ratio of 0.49:0.51. Both orientations were refined with anisotropic thermal parameters using the following restraints: The anisotropic displacement parameters of two atoms in the direction of bond between them were restrained to be equal within an uncertainty value of 0.01 Å2 and to have the same Uij components within the standard uncertainty value of 0.04 Å2. Several atoms were constrained to have same anisotropic displacement parameters. Crystallographic data for 2 and 3 are listed in Table S1. The molecular structures of 1 and 2 are shown in Figures S10 and S11, respectively. The solid state packing in 1 and 2 is shown in Figures S12 and S13. Calculations Details. Geometry optimizations were performed at the DFT level of theory with the help of PBE033 hybrid correlationexchange functional. All light atoms (C, H, O) were described by the correlation-consistent basis sets of triple-ζ quality (cc-pVTZ), whereas a triple-ζ def2-TZVP one was used for cesium (accompanied by effective core potential). In all cases, no symmetry restrictions were applied. All calculated structures correspond to local minima (no imaginary frequencies) on the corresponding potential energy surfaces, as determined by calculation of the full Hessian matrix, followed by estimation of frequencies in the harmonic approximation. All these calculations were performed using the Firefly program (version 8.1.0).34 NBO and NBO-EDA. Converged wave functions were then used to investigate details of electronic structure within the framework of natural bond orbitals (NBO) technique.35 The donor−acceptor interactions are quantified by examining all possible interactions between filled (donor) Lewis-type NBOs and empty (acceptor) nonLewis NBOs and by evaluating their energetic importance using the second-order perturbation theory in the NBO basis. Since these

interactions lead to the loss of occupancy from localized NBOs of an idealized Lewis structure to empty non-Lewis orbitals (and thus to deviations from an idealized Lewis structure description), they are referred to as delocalization corrections to the zeroth-order natural Lewis structure. For each donor NBO (i) and acceptor NBO (j) the stabilization energy, E(2), associated with delocalization i → j is estimated as ΔEi(2) → j = −2

⟨σi|F |̂ σj⟩2 εj − εi *

where F̂ is an effective orbital Hamiltonian (Fock or Kohn−Sham operator), εi = ⟨σi|F̂ |σi⟩ and εj = ⟨σj*|F̂ |σj*⟩ are orbital energies for donor and acceptor NBOs, respectively. Such an analysis was accompanied by the Natural Energy Decomposition Analysis (NEDA).16,19 The total bonding energy, ΔEtot, was partitioned into three terms: EEL for electrical interaction, ECT for charge transfer, and ECO for core repulsion. All these computations were performed with help of NBO (version 6.0) program.35 Energy Decomposition Analysis (EDA). The bonding between naked Cs + and methylated corannulenyl anion was further investigated by the alternative energy decomposition analysis (EDA) developed by Morokuma and by Ziegler and Rauk.24 For this purpose, single-point calculations were performed by the ADF program package36 with the same functional (PBE0). All atoms were described by uncontracted Slater-type orbitals (STOs) with TZ2P quality as basis functions.37 An auxiliary set of s, p, d, and f STOs was used to fit the molecular densities and to represent the Coulomb and exchange potentials accurately in each SCF cycle.38 Scalar relativistic effects have been taken into account by ZORA. Within an EDA approach, the interaction energy Eint can be divided into three main components, ΔEint = ΔEelstat + ΔEPauli + ΔEorb. The term ΔEelstat corresponds to the quasi-classical electrostatic interaction between the unperturbed charge distributions of the prepared atoms, very often associated with ionic contribution. The ΔEPauli term accounts for the Pauli repulsion from the superposition of the unperturbed electron density of the isolated fragments. The ΔEorb term evaluates the energy change from the charge transfer and polarization effects, mostly considered as covalent part of the bonding.39 The very good review of most widely used energy decomposition schemes was recently published by Skylaris et al.40 In our previous works, as an indicator of stability we used the bond dissociation energy, De, where −De = ΔE for the reaction fragments ↔ molecule. The same set of single-point calculations was carried out using recently developed double-hybrid DFT functionals B2PLYP-D341 (in combination with empirical dispersion correction from Grimme et al.42 and RIJCOSX43 acceleration scheme) and xDH-PBE0.44 The superior performance of double-hybrid functionals in terms of energetics is achieved by adding perturbation theory component. Selected functionals were proved to provide high accuracy in energy estimations, very close to that of CCSD(T) method. Calculations at the xDH-PBE0/cc-pVTZ level of theory were performed with Firefly program, whereas B2PLYP-D3/TZVP calculations were done using ORCA software. Topological analysis of electron density (based on converged PBE0/cc-pVTZ(C,H,O)/def2-TZVP(+ECP)(Cs) wave function) was performed within QTAIM approach using AIMALL program package.45 Energetics of bonding contacts was carried out with help of Espinosa’s correlation scheme.29 The presence of all BCPs was confirmed by analysis of maps of electron density gradient, in which such point can be identified as the point where all Ñ r(r) trajectories terminate. 2D maps of Laplacian (second derivative of electron density with respect to coordinates) were also analyzed in order to properly assign the nature of interactions as well as unambiguously localize charge accumulation and charge depletion regions. Aromaticity Descriptors. Using PBE0/cc-pVTZ-optimized geometries, a set of theoretical descriptors/indexes of aromaticity was calculated. This set includes: (i) structure-based Harmonic Oscillator Model of Aromaticity (HOMA, as defined by Kruszewski and Krygowski),46 (ii) Nuclear Independent Chemical Shift (NICS, K


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Organometallics introduced by von R. Schleyer et al.),47 and (iii) descriptors based on topological Quantum Theory of Atom in Molecule (QTAIM)28 approach such as Para-Delocalized Index (PDI)48 and Aromatic Fluctuation Index (FLU).49 In the latter, two types of atomic spaces were tested, namely, AIM, using Bader’s atomic basin definition,50 and Fuzzy atomic space, using Becke atomic space definition.51 The correlation between the results calculated in fuzzy atomic space and in AIM atomic space was previously reported to be excellent.52 All QTAIM calculations were carried out by Multiwfn 3.3.7 program.53 Calculations of NICS values were performed using Gauge Independent Atomic Orbitals (GIAO) approach with help of Gaussian 09 program54 at the PBE0/cc-pVTZ level of theory. The set of descriptors was augmented by detailed consideration of magnetic induced ring current on target systems using Anisotropy of the Induced Current Density (ACID) approach.55 The applied magnetic field is perpendicular to the five-membered ring. To obtain induced current vectors and plot map, ACID 2.0.0 program uses the current density tensors, calculated by Continuous Set of Gauge Transformations (CSGT) method56 implemented in Gaussian 09 package.

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Nanotubes: Designed Synthesis, Unusual Reactions, and Coordination Chemistry; John Wiley & Sons, 2012; pp 413. (d) Wu, Y.-T.; Siegel, J. S. Synthesis, Structures, and Physical Properties of Aromatic Molecular-Bowl Hydrocarbons. Top. Curr. Chem. 2014, 349, 63−120. (2) (a) Janata, J.; Gendell, J.; Ling, C.-Y.; Barth, W. E.; Backes, L.; Mark, H. B.; Lawton, R. G. Concerning the Anion and Cation Radicals of Corannulene. J. Am. Chem. Soc. 1967, 89, 3056−3058. (b) Mehta, G.; Panda, G. A New Synthesis of Corannulene. Tetrahedron Lett. 1997, 38, 2145−2148. (c) Scott, L. T.; Cheng, P.C.; Hashemi, M. M.; Bratcher, M. S.; Meyer, D. T.; Warren, H. B. Corannulene. A Three-Step Synthesis. J. Am. Chem. Soc. 1997, 119, 10963−10968. (d) Sygula, A.; Xu, G.; Marcinow, Z.; Rabideau, P. W. ″Buckybowls″-Introducing Curvature by Solution Phase Synthesis. Tetrahedron 2001, 57, 3637−3644. (e) Petrukhina, M. A.; Andreini, K. W.; Mack, J.; Scott, L. T. X-ray Quality Geometries of Geodesic Polyarenes from Theoretical Calculations: What Levels of Theory Are Reliable? J. Org. Chem. 2005, 70, 5713−5716. (f) Butterfield, A. M.; Gilomen, B.; Siegel, J. S. Kilogram-Scale Production of Corannulene. Org. Process Res. Dev. 2012, 16, 664−676. (3) (a) Sakurai, H.; Daiko, T.; Hirao, T. A Synthesis of Sumanene, a Fullerene Fragment. Science 2003, 301, 1878. (b) Sakurai, H.; Daiko, T.; Sakane, H.; Amaya, T.; Hirao, T. Structural Elucidation of Sumanene and Generation of Its Benzylic Anions. J. Am. Chem. Soc. 2005, 127, 11580−11581. (c) Amaya, T.; Nakata, T.; Hirao, T. Synthesis of Highly Strained π-Bowls from Sumanene. J. Am. Chem. Soc. 2009, 131, 10810−10811. (d) Amaya, T.; Hirao, T. A Molecular Bowl Sumanene. Chem. Commun. 2011, 47, 10524−10535. (e) Amaya, T.; Hirao, T. Chemistry of Sumanene. Chem. Rec. 2015, 15, 310−321. (4) (a) Scott, L. T.; Jackson, E. A.; Zhang, Q.; Steinberg, B. D.; Bancu, M.; Li, B. A Short, Rigid, Structurally Pure Carbon Nanotube by Stepwise Chemical Synthesis. J. Am. Chem. Soc. 2012, 134, 107− 110. (b) Liu, B.; Liu, J.; Li, H.-B.; Bhola, R.; Jackson, E. A.; Scott, L. T.; Page, A.; Irle, S.; Morokuma, K.; Zhou, C. Nearly Exclusive Growth of Small Diameter Semiconducting Single-Wall Carbon Nanotubes from Organic Chemistry Synthetic End-Cap Molecules. Nano Lett. 2015, 15, 586−595. (5) (a) Amsharov, K. Yu.; Kabdulov, M. A.; Jansen, M. Facile BuckyBowl Synthesis by Regiospecific Cove-Region Closure by HF Elimination. Angew. Chem., Int. Ed. 2012, 51, 4594−4597. (b) Amsharov, K. Rational Synthesis of Fullerenes, Buckybowls, and Single-Walled Carbon Nanotubes by a Surface-Assisted Approach. Adv. Polym. Sci. 2017, 278, 127−145. (6) (a) Wu, T.-C.; Hsin, H.-J.; Kuo, M.-Y.; Li, C.-H.; Wu, Y.-T. Synthesis and Structural Analysis of a Highly Curved Buckybowl Containing Corannulene and Sumanene Fragments. J. Am. Chem. Soc. 2011, 133, 16319−16321. (b) Wu, T.-C.; Chen, M.-K.; Lee, Y.-W.; Kuo, M.-Y.; Wu, Y.-T. Bowl-Shaped Fragments of C70 or Higher Fullerenes: Synthesis, Structural Analysis, and Inversion Dynamics. Angew. Chem., Int. Ed. 2013, 52, 1289−1293. (c) Chen, M.-K.; Hsin, H.-J.; Wu, T.-C.; Kang, B.-Y.; Lee, Y.-W.; Kuo, M.-Y.; Wu, Y.-T. Highly Curved Bowl-Shaped Fragments of Fullerenes: Synthesis, Structural Analysis, and Physical Properties. Chem. - Eur. J. 2014, 20, 598−608. (d) Wu, Y.-T.; Wu, T.-C.; Chen, M.-K.; Hsin, H.-J. Synthetic and Structural Considerations on Highly Curved BowlShaped Fragments of Fullerenes. Pure Appl. Chem. 2014, 86, 539− 544. (7) (a) Cheung, K. Y.; Chan, C. K.; Liu, Z.; Miao, Q. A Twisted Nanographene Consisting of 96 Carbon Atoms. Angew. Chem., Int. Ed. 2017, 56, 9003−9007. (b) Pun, S. H.; Chan, C. K.; Luo, J.; Liu, Z.; Miao, Q. Dipleiadiene-Embedded Aromatic Saddle Consisting of 86 Carbon Atoms. Angew. Chem., Int. Ed. 2018, 57, 1581−1586. (c) Li, C.; Yang, Y.; Miao, Q. Recent Progress in Chemistry of Multiple Helicenes. Chem. - Asian J. 2018, 13, 884−894. (d) Pun, S. H.; Miao, Q. Toward Negatively Curved Carbons. Acc. Chem. Res. 2018, 51, 1630−1642. (8) (a) Li, J.; Terec, A.; Wang, Y.; Joshi, H.; Lu, Y.; Sun, H.; Stuparu, M. C. π-Conjugated Discrete Oligomers Containing Planar and Nonplanar Aromatic Motifs. J. Am. Chem. Soc. 2017, 139, 3089−3094.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00837. Cartesian coordinates for the calculated structures (XYZ) Computational results: MEP maps, ACID maps, spin density maps, and complete set of aromaticity descriptors for all systems; X-ray structural details, UV−vis and NMR spectroscopy data (PDF) Accession Codes

CCDC 1876533 and 1876534 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +1 312 567 3151. (A.Yu.R.). *E-mail: [email protected]. Fax: +1 518 442 3462. Phone: +1 518 442 4406. (M.A.P.). ORCID

Marina A. Petrukhina: 0000-0003-0221-7900 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of this work from the National Science Foundation (CHE-1608628 and CHE-1337594, M.A.P) and from Illinois Institute of Technology (start-up funds, A.Yu.R) as well as partial support from the 381688-FSU/ChemRing/ DOD-DOTC (A.Yu.R) is gratefully acknowledged.

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REFERENCES

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