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Open AccessCCS ChemistryRESEARCH ARTICLE1 May 2023

S-Shaped Fused Azacorannulene Dimer: Structural and Redox Properties

    CCS Chem. 2023, 5, 1108–1117
    https://doi.org/10.31635/ccschem.022.202202165

    Because of their unique structural and chemical properties, buckybowl molecules have attracted considerable attention in a wide range of scientific disciplines. The importance and utility of buckybowl molecules significantly increases once they acquire larger π-surface area and/or heteroatoms. The fusion of buckybowl molecules has emerged as a new strategy to extend the π-surface of polycyclic aromatic compounds; however, the π-extension of heteroatom-embedded buckybowls by the fusion strategy is still rare. Here we report the synthesis and propeties of a fused azacorannulene dimer bearing a C62N2 core ( 1a), which can also be regarded as a double aza[5]helicene. Due to the steric repulsion between two azapentabenzocorannulene moieties, this molecule shows a rigid S-shaped structure where the two azacorannulene bowls face in opposite directions. Stepwise chemical oxidation of 1a resulted in the formation of the corresponding radical cation ( 1a ·+) and dication ( 1a 2+), providing an important insight into their aromaticity. The fusion of heteroatom-embedded buckybowls provides a powerful way to synthesize π-extended polycyclic aromatic molecules.

    Introduction

    Buckybowls are bowl-shaped polycyclic aromatic molecules that are regarded as fragments of fullerenes and typically bear pentagons and hexagons arranged to satisfy the isolated-pentagon rule (IPR)a,1,2 Due to their distinct architectural structure and unique properties, buckybowl molecules have been applied to a wide range of scientific disciplines,3–6 including organic electronics7–10 and supramolecular chemistry.11–14 Since the appeal of buckybowls to the scientific community will largely increase if they acquire larger π-surface area, significant effort has been devoted to the extension of buckybowls’ π-surface.15–17 In general, the π-extension of buckybowls, such as corannulene and sumanene, can be achieved by functionalization (introduction of substituents) followed by cyclization.3–6,15–17 Another effective method for buckybowl π-extension is the fusion of two or more buckybowls to form a single fully conjugated entity, resulting in more π-extended fullerene fragments. Since the first report in 1994,18 various fused corannulenes19–24 have been synthesized and investigated (Figure 1a).

    Figure 1

    Figure 1 | (a) Selected examples of fused buckybowls. (b) Fused heteroatom-containing buckybowl dimers.

    Despite the significant progress in π-extended buckybowls, the π-extension by the fusion strategy of heteroatom-embedded buckybowls has not emerged until quite recently (Figure 1b).25,26 In 2021, Zhang et al.27 reported the synthesis of compound A, which can be regarded as a fused dimer of heteroatom-containing subtriangulenes. It is worth noting that boat-shaped fused dimers of non-IPR fullerene fragments28,29 and fused dimers of “isomeric fullerene fragments”30,31 have also been reported. During our continuous investigations of the synthesis of nitrogen-embedded buckybowls,32,33 we successfully synthesized fused azacorannulene dimer B as a fusion of two azacorannulenes linked with a pyrene unit.34 Herein we report the synthesis and structure of novel fused azacorannulene dimer 1, which can be viewed as two azacorannulenes fused with one benzene ring. Unlike the other large π-surface heteroatom-embedded buckybowls in Figure 1b which possess a bowl/boat-shaped conformation, azacorannulene dimer 1 has an S-shaped molecule with two bowls curved in opposite directions due to its double aza[5]helicene structure. Moreover, azacorannulene dimer 1 undergoes a well-defined multiple oxidation to generate an open-shell radical cation and a closed-shell dication, which is rarely observed in the other heteroatom-doped buckybowls.27,35

    Experimental Methods

    Synthesis of azacorannulene dimer 1a

    To a mixture of compound 4 (60 mg, 0.035 mmol), Pd(OAc)2 (23 mg, 0.10 mmol), and t-Bu2MePH·BF4 (77 mg, 0.31 mmol) in a 25-mL Schlenk tube were added 1,8-diazabicyclo[5.4.0]undec-7-ene (0.75 mL) and N,N-dimethylformamide (3.0 mL) via a syringe. After nitrogen gas was bubbled into the solution for 10 min, the mixture was stirred for 22 h at 160 °C. After cooling to room temperature and dilution with toluene (5 mL), the mixture was washed with water (3 × 5 mL) and dried over sodium sulfate. After the filtration, the mixture was evaporated in vacuo. The crude product was purified by silica gel column chromatography and eluted with hexane/dichloromethane (19/1) to obtain compound 1a as a dark brown solid (11.2 mg, 9.0 μmol, 26%). Full experimental details can be found in the Supporting Information.

    Results and Discussion

    Scheme 1 illustrates the synthetic strategy of fused azacorannulene dimer 1a, which bears eight t-butyl substituents. Toward its efficient synthesis, bifunctional iminium salt 2 was prepared by a similar procedure in a literature report ( Supporting Information Figure S1).36,37 A 1,3-dipolar cycloaddition of azomethine ylides formed in situ from iminium salt 2 with a dipolarophile, 2,2′,6-tribromo-di-4-t-butylphenylethyne ( 3), followed by oxidation by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), was conducted to form the fused pyrrole compound 4 in total 35% yield. Subsequently, palladium-catalyzed intramolecular cyclization was conducted to afford an azacorannulene dimer 1a in 26% yield. The formation of 1a was unambiguously confirmed by nuclear magnetic resonance (NMR) analysis and mass spectrometry as well as X-ray diffraction analysis (vide infra). It is worth noting that 1a is found to be more sensitive to oxygen in the solution state than azapentabenzocorannulene (APBC),32 and thus for long periods of storage needs to be kept under an inert atmosphere.

    Scheme 1

    Scheme 1 | Synthetic route to fused azacorannulene dimer 1a.

    The single crystal of 1a suitable for X-ray crystallographic analysis was obtained from the slow evaporation of dichloromethane solution under an argon atmosphere (Figure 2).b The X-ray analysis shows that 1a has an S-shaped wavy structure22,38 as two bowl-shaped APBC units facing in opposing directions (Figure 2a). This is similar to a fused corannulene dimer24 but different from the previously reported compound B, which shows a boat-shaped structure.27,32,33 The bowl depths were determined by measuring the perpendicular distance between the mean plane of the central pyrrole ring (N1–C1–C2–C3–C4) and four carbon atoms (C8, C14, C20, and C26) at the edge to be 1.84, 1.89, 1.61, and 2.01 Å, respectively (Figure 2b). The bowl-depth distances are deeper than those of APBC (1.38–1.73 Å)32 due to the presence of the [5]helicene units and comparable to that of compound B (1.65–2.06 Å). The dihedral angle of two terminal benzene rings in the central aza[5]helicene units was determined to be 58.1° (Figure 2c), which is larger than that of 1-aza[5]helicene (45.7°39 and 44.0°40) and even the parent [5]helicenes (46.4°41 and 52.8°42). This is partly due to the repulsion between the t-butyl groups at the C8 and C26 positions. In the unit cell of the crystal structure, two molecules of 1a are packed in a manner such that one t-butyl group is located in the concave face of the APBC moiety of another molecule partly via C–H…π interactions (Figure 2d). No π-π stacking was observed due to the presence of eight bulky t-butyl groups around the π-core.

    Figure 2

    Figure 2 | (a) Oak Ridge thermal ellipsoid plot structure of 1a with thermal ellipsoids at 50% probability. Hydrogen atoms and t-butyl group are omitted. (b) Side view and bowl depths of 1a. (c) Dihedral angle of the aza[5]helicene moiety. (d) Partial packing structure of 1a (stick model) with one t-butyl group shown using a transparent space-filling model.

    The molecular orbitals of 1 were analyzed by density functional theory (DFT) calculations at the B3LYP/6-31G(d) level of theory (Figure 3). The highest occupied molecular orbital (HOMO) was found to be distributed all over the molecule but with little to no contribution to the nitrogen atoms. Meanwhile, the lowest unoccupied molecular orbitals (LUMOs) showed comparably little molecular orbital (MO) lobes on the nitrogen atoms but an overall distribution that was largely concentrated around the center of the molecule and less on its peripheral atoms. Overall, these results are consistent with the MO distribution of compound B, which also showed little contribution of the nitrogen atom in the HOMOs and LUMOs. The HOMO–LUMO gap was determined to be 2.46 eV. This value is smaller than APBC (3.22 eV)32 and larger than compound B (2.20 eV),34 which is attributed to the size of the π-surfaces.43

    Figure 3

    Figure 3 | Kohn–Sham molecular orbitals (from HOMO−1 to LUMO+1) of 1 calculated at the B3LYP/6-31G(d) level of theory.

    Conformational analysis of 1 with DFT calculation revealed the presence of four possible conformers, depending on the syn/anti relationship of bowl direction and the butterfly/twist conformation (Figure 4). Among the four conformers, the anti-butterfly structure is more stable by 6.3–14.8 kcal/mol than the others, which reasonably explains that only the anti-butterfly structure was detected in the X-ray analysis (Figure 2). The transition states between these conformers were also calculated. The activation barriers of bowl inversion and helicene flipping from the most stable anti-butterfly structure were determined to be 19.4 and 21.8 kcal/mol, respectively. Considering that 1a has four tert-butyl groups at the 2, 11, 14, and 23 positions, these results indicate that 1a has a more rigid structure than APBC32 and compound B.34 However, it is difficult to confirm this conformational behavior experimentally because clear NMR signals can only be observed in a mixture of CDCl3/CS2, which prevents NMR measurements at higher temperatures.

    Figure 4

    Figure 4 | Interconversion pathways of 1 calculated at the B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d) level of theory. Blue highlights indicate a transition state that involves a helicene flipping of the [5]helicene moiety, while pink highlights indicate a transition state that involves a bowl inversion of the azacorannulene unit.

    To investigate the redox properties of 1a, cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements were carried out in dichloromethane, resulting in the observation of three chemically reversible oxidation waves (Figure 5). The peak-to-peak separation values (0.105 and 0.080 V) compared with that of ferrocene (0.107 V) indicate that the first oxidation wave (Eox1 = 0.05 V) and the second oxidation wave (Eox2 = 0.39 V) represent single-electron oxidation. Meanwhile, the final oxidation wave (Eox3 = 0.70 V) has a slightly larger peak-to-peak separation (0.137 V), which is most likely caused by a slow rate of heterogenous electron transfer.44 Given the fact that a corannulene molecule fused with two π-extended pyrroles undergoes four-step single-electron oxidation at E1/2 = 0.04, 0.20, 0.49, and 0.62 V,45 the oxidation peak at 0.70 V is also consistent with a single-electron transfer (based on its peak currents), with any additional processes occurring at more positive values, possibly beyond the solvent/electrolyte potential window. These results are significantly different in comparison to the cyclic voltammogram of compound B, which shows two overlapped quasi-reversible oxidation waves at E1/2 = 0.16 and 0.25 V.

    Figure 5

    Figure 5 | CV (red) and DPV (blue) of 1a (0.10 mM) measured under inert atmosphere with Bu4NPF6 (0.10 M) in dichloromethane using Ag/Ag+ as the reference electrode, Pt as the working electrode, and Pt wire as the counter electrode.

    Encouraged by the reversible oxidation waves observed in the CV analyses, stepwise chemical oxidation of 1a was performed under an inert atmosphere (Scheme 2). The addition of 1.0 equiv of AgSbF6 as an oxidant to a solution of 1a resulted in the formation of a radical cation 1a ·+·SbF6−, which was characterized by absorption spectroscopy and electron-spin resonance (ESR) analysis (vide infra). Meanwhile, the addition of 2.0 equiv of AgSbF6 resulted in the formation of the dication 1a2+·2SbF6−, which was successfully characterized by absorption spectroscopy and X-ray diffraction analysis. These cationic species, 1a ·+ and 1a 2+, can be converted back into their original neutral species using triethylamine as a reductant ( Supporting Information Figure S14c). The structural analysis of the dication, 1a 2+·2SbF6−, clearly indicates that the there are two counter anions of SbF6− per 1a 2+ placed in the crystal lattice (Scheme 2) and that the original S-shaped structure was retained due to its rigid structure of 1a. Notably, this is in contrast to the previous heteroatom-containing buckybowls, which underwent large structural change from curved to planar upon oxidation.27,35 The bowl-depth distances of 1a 2+ (the perpendicular distance between the mean plane of the central pyrrole ring and carbon atoms C8, C14, C20, and C26) are determined to be 1.96, 1.20, 1.51, and 1.91 Å, respectively, which are slightly shallower than neutral 1a (1.61–2.01 Å). This implies that the ring strain of the bowl structure of 1a is released by oxidation, which is commonly observed in other buckybowl molecules.24,35

    Scheme 2

    Scheme 2 | Stepwise oxidation to form radical cation 1a ·+·SbF6− and dication 1a 2+·2SbF6−

    The changes from neutral 1a to radical cation 1a ·+ and dication 1a 2+ can also be monitored by a UV–vis–near-infrared (NIR) spectroscopy (see Figure 6). First, the purple line in this figure represents the UV–vis absorption spectrum of neutral 1a in dichloromethane, which exhibits broad absorption at 300–900 nm due to its extended π-surface. To investigate the origin of the absorption, time-dependent DFT (TD-DFT) calculation of 1 was performed at the B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d) level of theory ( Supporting Information Figure S25 and Table S3). The absorption bands were simulated to be at λ = 434, 486, 545, and 638 nm, which corresponds to the HOMO−1→LUMO+1, HOMO−2→LUMO as well as HOMO→LUMO+2, HOMO→LUMO+1, and HOMO→LUMO transitions, respectively. Then, titration experiments were carried out using four different kinds of oxidants, namely silver hexafluoroantimonate (AgSbF6: E° = 1.01 V vs Fc/Fc+48), trifluoroacetic acid (TFA), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ: E° = 0.51 V46), and tris(4-bromophenyl)ammoniumyl hexachloroantimonate (BAHA: E° = 1.16 V46). The addition of AgSbF6 into a diluted solution of 1a in dichloromethane resulted in a drastic change in the UV/vis absorption spectra (Figure 6a). After adding 1.0 equiv of AgSbF6, a broad absorption band in the NIR region at λmax= ca. 1770 nm (green line) was observed. This could be attributed to the formation of the radical species, 1a ·+, which has a narrow HOMO–LUMO gap. A TD-DFT calculation of 1 ·+ also supported that this absorption band is derived from the transition from HOMO(204B) to LUMO(205B) ( Supporting Information Figures S18 and S26 and Table S4). Furthermore, the addition of more equivalents of AgSbF6 caused the change where the broad NIR absorption gradually decreased, and a strong absorption band at λmax = ca. 1030 nm (red line) newly appeared. The new absorption is due to the generation of dicationic species, 1a 2+, which can also be supported by TD-DFT calculations ( Supporting Information Figure S27 and Table S5). The titration experiment was also conducted with TFA as an oxidant (Figure 6b). In this case, the addition of TFA caused the appearance of a broad NIR absorption at λmax = ca. 1770 nm. Even after the addition of excess amounts of TFA; however, an absorption band at λmax = ca. 1030 nm was not observed. These results clearly indicate that the oxidizing ability of TFA is high enough to promote one-electron oxidation but is not enough to generate the dication species. In the trial of other oxidants, it was found that BAHA showed a similar behavior to AgSbF6 that can form both the radical cation and dication ( Supporting Information Figure S14a) and that DDQ shows a similar behavior to TFA that promotes one-electron oxidation to generate only the radical cation ( Supporting Information Figure S14b). Through the investigation on UV–vis–NIR spectroscopy, we were also able to examine the stability of compound 1a, 1a ·+·SbF6−, and 1a 2+·2SbF6− in the solution state ( Supporting Information Figure S15). Interestingly, it was determined that the oxidised species ( 1a ·+ and 1a 2+) are relatively stable under ambient air for up to an hour. It is worth noting that neither 1a, 1a ·+, nor 1a 2+ is fluorescent. This signifies the difference in optical properties from compound B, which show a fluorescence quantum yield of ΦF = 0.31.34

    Figure 6

    Figure 6 | UV–vis–NIR spectra of 1a titrated with (a) AgSbF6 (0–3.0 equiv) and (b) TFA (0–300 equiv) in dichloromethane (5.0 × 10−4 M).

    The formation of radical cation 1a ·+ was further confirmed by ESR measurements. Compound 1a ·+·SbF6− gives a distinct signal without a hyperfine splitting pattern at g = 2.002 in both the solution (in toluene) and solid state. The solid-state variable-temperature ESR in Figure 7 shows that the signal intensity reduces marginally at higher temperatures, indicating the presence of a typical monoradical species. Meanwhile, the ESR analysis of a solution of 1a with AgSbF6 (2.5 equiv) exhibits a very weak signal at g = 2.002 ( Supporting Information Figure S16). However, further investigation indicates that xthe weak signal is attributed to the contamination of the radical cation as an impurity, which indicates that the dication 1a 2+·2SbF6− is a closed-shell species. This is in agreement with the results of DFT calculations at the B3LYP/6-31G level of theory, showing that the closed-shell singlet state of the dication is more stable than the triplet diradical state (10.8 kcal/mol) and the open-shell singlet diradical state could not be determined ( Supporting Information Table S2). The closed-shell singlet state of dication 1a 2+·2SbF6− is also supported by the CV experiments that indicated that the first and second one-electron transfer reactions are separated by >0.4 V, showing that there is strong electronic communication between both halves of the molecule.

    Figure 7

    Figure 7 | Variable-temperature ESR spectra of monoradical cation 1a ·+ and the linear fitting plot of I against 1/T (I, maximum intensity; T, temperature).

    The aromaticity of compounds 1, 1 ·+, and 1 2+ was evaluated by the nucleus-independent chemical shift (NICS) and anisotropy of the induced current density (ACID) analyses at the B3LYP/6-31G(d) level of theory (Figure 8). First, the NICS(0) simulation stipulates large negative values at the inner pyrrole cores (−17.5 ppm) and the four peripheral benzene rings (−9.9 to −8.5 ppm) (Figure 8a). The central benzene ring also shows a value of −7.7 ppm, which is comparable to those of the other peripheral benzene rings. These results disclose compound 1 as having an analogous aromatic nature as that of its parent monomer, APBC (Figure 8b, left).32 The ACID analysis provides further insight on the aromaticity of compound 1 (Figure 8c, left). The plot clearly shows clockwise diatropic ring currents around the two pyrrole rings and the central benzene ring, indicating a substantial degree of six π-electron conjugation. Meanwhile, a clockwise 50π ring current travelling over the molecule (shown with a solid red arrow) can be seen, apart from the 6π ring currents on the two peripheral benzene rings which are directly linked to the nitrogen atoms of the pyrrole cores (shown with a dashed pink line). The ACID analysis corroborates the aromatic nature of compound 1, inferred by the NICS analysis. In the NICS(0) of 1 ·+ and 1 2+, significant changes compared to neutral 1 were observed in the central benzene and two pyrrole rings: Upon oxidation, the values of the central benzene ring of −7.7 increased to −0.8 and 4.9 while those of the pyrrole ring changed from −17.5 to −1.3 and 14.3. This indicates that oxidation mainly affects the central 1,4-dipyrrolylbenzene to illuminate the contribution of the quinodimethane structure (Figure 8b, right), which is clearly seen in the ACID plot of 1 2+ (Figure 8c, right).

    Figure 8

    Figure 8 | (a) NICS(0) values for 1 (black), 1 ·+ (blue), and 1 2+ (red) calculated at the B3LYP/6-31G(d) level of theory. Values with a change difference of more than 5 are shown in boldface. (b) Bond alternation pattern of 1 and 1 2+ (c) ACID plots of 1 and 1 2+.

    Conclusion

    We have successfully designed and synthesized the fully conjugated azacorannulene dimer 1a and investigated its structural and chemical properties. The fusion of two azacorannulene moieties with a benzene ring resulted in an antifolded buckybowl structure featuring two of its curved segments bending in an opposite direction, forming a wavy S-shaped structure as opposed to typical bowl/boat shapes. Upon CV, compound 1a exhibited three chemically reversible oxidation peaks, consistent with three one-electron oxidation processes. The chemical oxidation of 1a by TFA or DDQ resulted in the formation of the corresponding radical cation ( 1a ·+). In contrast, the oxidation with stronger oxidants such as AgSbF6 and BAHA promotes two-electron oxidation that produces dication species ( 1a 2+), which has a closed-shell state as determined by ESR analysis. Last but not least, the strategy of buckybowl fusion provides a powerful way to design π-extension of heteroatom-embedded buckybowls.

    Footnotes

    a Although there is debate over this terminology, we refer to the bowl-shaped polycyclic aromatic molecules that exhibit a fragment structure of fullerenes that satisfy the IPR as a “buckybowl.”

    b CCDC 2151023 ( 1a) and 2151024 ( 1a 2+·2SbF6−) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif

    Supporting Information

    Supporting Information is available and includes synthesis and characterization of compounds; NMR, MS, UV–vis–NIR, and ESR spectra; X-ray crystallographic data; and theoretical calculations.

    Conflict of Interest

    There is no conflict of interest to report.

    Funding Information

    This work was supported by Nanyang Technological University.

    Acknowledgments

    Computing resources from the NTU High Performance Computing Team are gratefully acknowledged. We thank Dr. Yongxin Li (NTU) for his assistance with the X-ray diffraction analysis.

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