Substituent effects on paratropicity and diatropicity in π-extended hexapyrrolohexaazacoronene

Abstract

Research into the application of antiaromatic compounds as molecular materials is an attractive strategy in the development of electronic materials. Antiaromatic compounds have traditionally been considered to be unstable, and thus, the creation of stable antiaromatic compounds has been sought in the field of organic chemistry. Recently, some studies have been reported on the synthesis, isolation, and elucidation of the physical properties of compounds with stability and definitive antiaromatic properties. In general, antiaromatic compounds are considered to be more susceptible to substituents due to their inherently narrow HOMO–LUMO gap compared to aromatic compounds. However, there have been no studies examining substituent effects in antiaromatic compounds. In this study, we have developed a synthetic method to introduce various substituents into π-extended hexapyrrolohexaazacoronene (homoHPHAC⁺), one of the stable and clearly antiaromatic compounds, and investigated the substituent effects on the optical, redox, and geometrical properties and paratropicity of a series of compounds. In addition, the properties of the two-electron oxidized form, homoHPHAC³⁺, were investigated. Control of electronic properties by introducing substituents into antiaromatic compounds provides a new design guideline for molecular materials.

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Introduction

Aromaticity is an important concept in chemistry. When a molecule has cyclic π-conjugation with (4n + 2) π-electrons, it becomes aromatic and is more stable compared to acyclic π-conjugated molecules.^1–3 Therefore, such compounds are widely used in our surroundings, including in medicine, plastics, dyes, and organic electronics. A molecule with 4n π-electrons, on the other hand, becomes antiaromatic; such molecules are known to decompose or undergo structural change to give compounds that have lost their antiaromatic properties. Recent studies have revealed that antiaromatic compounds exhibit optical absorption in the near-infrared region,⁴ high charge transport properties,⁵ and amphoteric redox properties,⁶ but there are still limited examples of such compounds.

Several research groups, including ours, have studied the synthesis and characterisation of hexapyrrolohexaazacoronene (HPHAC), a nitrogen-containing polycyclic aromatic compound, using pyrroles.^7–10 Composed of electron-rich pyrroles, HPHACs are easily oxidised, and their dications exhibit aromaticity based on macrocyclic conjugation, although their antiaromatic tetracations have not yet been successfully isolated and characterized (Fig. 1a). In general, their properties can be switched between ‘aromatic’ and ‘antiaromatic’ via redox, as their aromaticity depends on the number of π-electrons. With this in mind, homoHPHAC, a π-extended analogue of HPHAC with an embedded methine carbon, was synthesized, and its antiaromatic properties as a cation and aromatic properties as a trication were revealed (Fig. 1b).⁹ Because homoHPHACs are originally monocations and the delocalization of cations/π-electrons over the π-system is a major factor in the antiaromaticity and stability, we hypothesized that the introduction of substituents might be effective for controlling the optical and redox properties as well as paratropicity of homoHPHAC⁺.³ Furthermore, because antiaromatic compounds have an intrinsically narrow HOMO–LUMO gap, the investigation of substituent effects, which are usually not great for aromatic compounds,^3c is important for applications such as organic electronic materials (OEMs). To the best of our knowledge, no studies have examined the substituent effects in antiaromatic compounds.¹¹

Fig. 1 (a) Chemical structures of HPHAC 1 and its cationic species. (b) Resonance structures of antiaromatic homoHPHAC 3⁺ and aromatic 3³⁺. (c) HomoHPHAC 3a⁺–3e⁺ with the various substituents studied in this work.

In this study, we synthesized homoHPHAC monocations and trications with various substituents and investigated their effects on the optical, redox, and geometrical properties and para(dia)tropicity of a series of homoHPHAC mono(tri)cations. Friedel–Crafts-type reactions were used to enable the introduction of various substituents.

Results and discussion

Synthesis of monocations 3c+, 3d+, and 3e⁺

Three substituents were chosen to emphasize the electronic contribution to homoHPHAC 3c–3e; the electron-donating 4-dimethylaminophenyl group, the electron-withdrawing pentafluorophenyl group, and the 4-tert-butylphenyl group, which exhibits intermediate properties (Fig. 1c). In the case of 3c⁺, it is also interesting to see how the dimethylamino group affects the resonance structure of the oxidised species. The previous research has shown that σ-bond formation may occur at the meso-position of homoHPHAC⁺ and at the para-position of the aryl substituent in their neutral radical states.^9a Therefore, aryl substituents with a functional group at the para-position were introduced at the meso-position for kinetic protection.

To increase the utility of the previous synthetic method,^9a which facilitates the introduction of various functional groups, a Friedel–Crafts-type reaction was investigated using commercially available benzoyl chlorides and partially unfused HPHAC (secoHPHAC 2) (Scheme 1). The reaction was carried out in 1,2-dichloroethane under reflux conditions in the presence of BF₃·OEt₂ as the Lewis acid; the solution of the reaction mixture changed from yellow to brown. After treatment with an aqueous solution of KPF₆, homoHPHAC 3c⁺[PF₆⁻], 3d⁺[PF₆⁻], and 3e⁺[PF₆⁻] were obtained in 58%, 42%, and 16% yield, respectively. The yields are lower compared to the previous Vilsmeier-type method (88% for 3b⁺[PF₆⁻]).^9a The target compounds were identified using ¹H NMR, ¹³C NMR, and high-resolution MS (Fig. S3a–g and S5a–c in the ESI†). In addition, the crystal structures of all compounds were successfully revealed.

Scheme 1 Synthesis of homoHPHAC 3c⁺–3e⁺ and 3c³⁺–3e³⁺ from secoHPHAC 2 and ring labeling for the NICS calculations.

Crystal structures of 3c⁺, 3d⁺, and 3e⁺

Single crystals were obtained by vapour diffusion of hexane into mixed solutions of anisole and acetone (3c⁺), ethylacetate (3d⁺), or chloroform (3e⁺) as SbF₆⁻ salts after treatment with an aqueous solution of NaSbF₆. X-ray crystallography analysis of 3c⁺[SbF₆⁻], 3d⁺[SbF₆⁻], and 3e⁺[SbF₆⁻] revealed the target structures containing a seven-membered ring with a methyne linkage (Fig. 2a–c and S6a–c in the ESI†). As observed for previously reported 3b⁺[SbF₆⁻],^9a the structures are all slightly twisted due to steric repulsion between the aryl and ethyl substituents. The quinoidal resonance structure in the H pyrrole ring is observed as the double-bondedness of C^β–C^β and the single-bondedness of C^α–C^β, which differ from the bond lengths in the F and G rings with a typical pyrrole framework (Fig. S6d in the ESI†). This result suggests a macrocyclic conjugation of 3⁺, as depicted in Scheme 1. Indeed, the averaged values of harmonic oscillator model of aromaticity (HOMA)¹² in the six pyrrole moieties are calculated to be 0.76 (3c⁺), 0.78 (3d⁺), and 0.69 (3e⁺), which are sufficiently smaller than the reported typical pyrrole with values of 0.85–0.93 (Fig. S6e in the ESI†).^12b

Fig. 2 Crystal structures (top and side views) of (a) 3c⁺[SbF₆⁻], (b) 3d⁺[SbF₆⁻], and (c) 3e⁺[SbF₆⁻]. Thermal ellipsoids are scaled to 50% probability. Solvent molecules and disordered atoms with lower occupancy are omitted for clarity. ESP plots of (d) 3c⁺, (e) 3d⁺, and (f) 3e⁺ calculated at the B3LYP/6-31G(d) level of theory, in which the Et groups have been replaced with Me groups to simplify the calculations.

To discuss the electronic contribution of the aryl substituent, the torsion angles between the mean planes of the aryl substituent and the homoHPHAC skeleton are important.¹³ The values are 46.80° (3c⁺), 39.17° (3d⁺), and 45.00° (3e⁺), which are roughly similar to that of 3b⁺ (52.95°) in the crystal state. These results indicate that all the aryl substituents in 3c⁺–3e⁺ likely influence the π-electron system of homoHPHAC⁺ by an inductive effect, not a resonance effect.

NMR spectroscopy

The ¹H NMR spectra of secoHPHAC 2 and homoHPHAC 3c⁺–3e⁺are shown in Fig. 3a. Disappearance of the signals at the α-position of the pyrroles, H^α, was observed upon cyclisation. Despite being a monocation, the signals for the ethyl groups of 3c⁺–3e⁺ were clearly observed at higher field relative to those of 2, among which the signals of 3e⁺ were observed at the highest field. This is strong evidence of a paratropic ring current, and the paratropicity of 3e⁺ appears to be the strongest of the three compounds. For a more detailed analysis, ¹H–¹H COSY and NOESY measurements were performed to assign the well-resolved signals of 3e⁺ (Fig. S4c and d in the ESI†). The methylene signals next to the meso-position, which are marked with a red dot (labeled as 1 in red), shifted significantly to higher field than the other methylene signals, due to the ring current effect of the aryl substituents in close proximity. On the other hand, the methyl signals of 3e⁺ next to the meso-position, which are marked with a blue dot (labeled as 1 in blue), were observed at relatively low field, which possibly indicates the existence of C–H⋯F interactions.¹⁴ The detailed assignments of 3e⁺ are shown in an enlarged view of the spectrum (Fig. 3b). Except for the signals mentioned above that reflect their local environment, the signals of the ethyl groups close to the aryl group were observed on the lower-field side, while those far from the aryl group appeared on the higher-field side. Although the signals are mainly observed on the higher-field side due to the strong paratropicity of 3e⁺, this result reflects the inductive effect, i.e., electron-withdrawing effect, of the pentafluorophenyl group. The electrostatic surface potential (ESP) plots support this tendency; the ethyl groups close to the meso-position are covered in blue (less surface potential) and the far sides in red (Fig. 2d–f). Notably, there appears to be a correlation between the strength of the electron-withdrawing ability of the substituents and the shifts to higher field, i.e., paratropicity.

Fig. 3 Comparison of the ¹H NMR spectra of (a) secoHPHAC 2 (in CDCl₃) and homoHPHAC 3c⁺–3e⁺ (in acetone-d₆), and (b) an enlarged view of the spectrum of 3e⁺.

DFT evaluation of aromaticity

Aromaticity and its conceptual counterpart, antiaromaticity are not exactly defined, and thus, should be evaluated from multiple perspectives. The magnitude of (anti)aromaticity is typically estimated in terms of criteria such as energetic,¹⁵ geometric,¹² magnetic,^16–18 and electronic¹⁹ aspects, which are usually correlated with each other. Since the aryl substituents affect the paratropic ring current of homoHPHACs⁺, nucleus-independent chemical shift (NICS)^16,17 and anisotropy of the induced current density (ACID)¹⁸ calculations were performed for the optimized structures. For comparison, NICS calculation of secoHPHAC 2 was also performed. Table 1 summarizes the averaged NICS(±1)_zz values calculated at the GIAO/HF/6-311+G(d,p)//B3LYP/6-31G(d) level of theory. Except for 3c⁺, the values at the B, C, D, and E rings are drastically increased upon cyclisation, while those of the A, F, G, and H rings become less negative, strongly indicating global paratropic ring current. It should be pointed out that the positive changes compared to the values of secoHPHAC 2 are the most evident for 3e⁺ among the four compounds including pristine homoHPHAC 3a⁺. Again, there appears to be a correlation between the strength of the electron-withdrawing ability of the substituents and the global paratropicity, as anticipated from the ¹H NMR spectra. The ACID plots also support the presence of a global counterclockwise paratropic ring current in 3c⁺–3e⁺, in which 3e⁺ is most pronounced. The central benzene moieties possess a clockwise ring current as observed in the previous system (Fig. S7 in the ESI†).^9,20

Table 1 The averaged NICS(±1)_zz values of secoHPHAC 2, homoHPHAC monocation 3a⁺,^9a3c⁺–3e⁺ and trications 3a³⁺,^9a3c³⁺ and 3e³⁺, in which the Et groups were replaced with H atoms to simplify the calculations (GIAO/HF/6-311+G(d,p)//B3LYP/6-31G(d) level)

	A	B	C	D	E	F	G	H
2	−4.8	−1.9	−3.7	−4.0	—	−1.5	−2.4	−4.6
3a^+	−5.1	−1.8	3.8	3.8	−2.3	−1.9	−2.3	−1.7
3c^+	−29.7	3.0	−3.6	−3.0	3.2	−22.5	−27.5	−17.0
3d^+	−10.3	31.4	26.2	21.7	32.5	−17.4	−19.0	−6.0
3e^+	7.2	57.6	47.1	37.0	50.3	−15.7	−15.1	3.1
3a^3+	−1.5	−2.6	−2.6	−4.1	−5.4	−5.4	−3.7	−2.8
3c^3+	−30.0	3.0	−3.0	−3.6	−3.2	−17.0	−27.5	−22.5
3e^3+	−59.6	−42.5	−45.3	−36.4	−29.8	−27.1	−38.2	−31.6

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Optical properties

The absorption spectra of 3c⁺–3e⁺ are shown in Fig. 4a. As observed previously,^9a the characteristic absorptions for the antiaromatic compounds were confirmed to be broad, weak absorptions in the NIR region around 750 to 1800 nm. Time-dependent DFT (TD-DFT) calculations at the B3LYP/6-31G+(d) level of theory indicate small oscillator strengths (f) for the HOMO → LUMO transition (3c⁺: 1337 nm, f = 0.0291, 3d⁺: 1388 nm, f = 0.0288, and 3e⁺: 1756 nm, f = 0.0251) (Fig. S9a–c in the ESI†). The differences in the colour of the solutions are well illustrated by the absorption spectra in the visible light range (Fig. 4b and c). As the electron-withdrawing ability of the substituent increases, the spectrum shifts towards longer wavelength (496 nm (3c⁺), 502 nm (3d⁺), and 530 nm (3e⁺)). Dimethylamino-substituted 3c⁺ possesses two distinct absorption maxima in the visible region. The TD-DFT calculations suggest that the absorption around 550–750 nm consists of HOMO − 3–HOMO − 1 → LUMO transitions, but only HOMO − 2 and HOMO − 1 of 3c⁺ have large orbitals on the aryl group. Other orbitals related to the absorption around 550–750 nm extended to the homoHPHAC skeletons (Fig. S9a–c and S10b–d and Table S9 in the ESI†), indicating a charge transfer (CT) transition from the dimethylaminophenyl group to the homoHPHAC skeleton in 3c⁺. In fact, positive solvatochromism was observed mainly for the second peak at around 584 nm in the cyclohexane, benzene, chloroform, methanol, and dimethyl sulfoxide solutions, which was also supported by a relatively large dipole moment of 5.95 D calculated at the B3LYP/6-31G(d) level of theory (Fig. S8a and b in the ESI†).

Fig. 4 (a) Absorption spectra of 3c⁺–3e⁺ in CH₂Cl₂, (b) enlarged view of the spectra in the visible light region, and (c) photographs of the solutions ([3c⁺–3e⁺] = 2.1 × 10⁻⁵ M in CH₂Cl₂).

Electrochemistry

The cyclic and differential pulse voltammograms (CV and DPV) of 3c⁺–3e⁺ were measured in CH₂Cl₂ (Fig. 5). Amphoteric redox behavior was clearly observed with the small HOMO–LUMO gaps of 0.92 eV (3c⁺), 0.94 eV (3d⁺), and 0.81 eV (3e⁺), showing two reversible oxidation steps and one reversible reduction step. The reduction waves of 3a⁺ and 3b⁺ were irreversible,^9a which proved that kinetic protection of both the meso-position of homoHPHAC⁺ and the para-position of the aryl substituent were important to prevent side reactions such as σ-dimer formation between two radical species. Attempts to isolate a one-electron reductant, i.e., neutral radical, have not been successful to date due to the extremely low oxidation potentials. Reflecting the strong electron-withdrawing ability of the pentafluorophenyl group, 3e⁺ has the lowest LUMO level, which also contributes to the higher oxidation potentials compared with those of 3c⁺ and 3d⁺. Notably, the difference between the first and second oxidation potentials of 3c⁺ (0.16 eV) is much smaller than those of 3a⁺ (0.32 eV),^9a3b⁺ (0.32 eV),^9a3d⁺ (0.33 eV), and 3e⁺ (0.33 eV). Therefore, chemical preparation of trication 3c³⁺ and 3e³⁺ was then conducted to investigate their electronic state and diatropicity.

Fig. 5 CV and DPV of 3c⁺–3e⁺ containing 0.1 M ⁿBu₄NPF₆ in CH₂Cl₂ at a scan rate of 100 mV s⁻¹ ([3c⁺–3e⁺] = 1.0 mM).

Preparation and diatropicity of trications 3c³⁺ and 3e³⁺

Trications 3c³⁺ and 3e³⁺ were prepared almost quantitatively in CH₂Cl₂ using AgSbF₆ and NOSbF₆ as the oxidant, respectively (Scheme 1). The change of the absorption spectra was drastic in the oxidation states, as was observed for previous systems (Fig. S9d and e in the ESI†). The isolated compounds were definitively identified using ¹H NMR and ¹³C NMR. As shown in Fig. 6, the ¹H NMR signals of all the ethyl protons of 3c³⁺ and 3e³⁺ appeared at lower field compared with those of 3c⁺ and 3e⁺. This result is thought to be not only due to the change in their oxidation state from monocationic to tricationic, but also switching of their aromaticity from antiaromatic to aromatic.²¹ The low-field shifts for 3e³⁺ are qualitatively more pronounced than those of 3c³⁺. Furthermore, it should be noted that even the signal of the dimethylamino group of 3c⁺, which is located far from the homoHPHAC π-system, is significantly shifted from 2.49 ppm to 3.74 ppm upon oxidation, suggesting the localisation of the cationic charge on the dimethylamino group. The coupling constant of the aryl substituent supports this prediction; the J values of the two doublets in 3c⁺ are 8.7 Hz, which is a typical coupling constant value for para-disubstituted benzene, whereas those in 3c³⁺ are 10.8 Hz, which is in the range for a cis-olefinic one (Fig. S3h in the ESI†). Therefore, 3c³⁺ is considered to dominantly adopt a quinomethane resonance form with two cations over the homoHPHAC π-system and a monocation on the dimethylamino group. The ESP plots clearly show the difference between 3c³⁺ and 3e³⁺ (Fig. 7d).

Fig. 6 ¹H NMR spectra of 3c³⁺ in acetone-d₆ and 3e³⁺ in CD₂Cl₂.

Fig. 7 (a) Crystal structures (top and side views) of 3c³⁺[SbF₆⁻]₃. Thermal ellipsoids are scaled to 50% probability. Solvent molecules and disordered atoms are omitted for clarity. (b) Selected bond lengths (Å) and (c) HOMA values of the crystal structure of 3c³⁺[SbF₆⁻]₃. (d) ESP plots of 3c³⁺ and 3e³⁺ calculated at the B3LYP/6-31G(d) level of theory, in which the Et groups are replaced with Me groups to simplify the calculations.

For comparison of the diatropicity of the trications, NICS(±1)_zz values were calculated for 3c³⁺ and 3e³⁺ (Table 1). HomoHPHAC 3e³⁺ shows large negative values at all points, as observed for 3a³⁺ and 3b³⁺.^9a On the other hand, much less-negative and nearly-zero values are calculated in the case of 3c³⁺, especially for the B, C, D, and E rings. For aromatic compounds, the NMR signals of the inner core provide the most direct evidence due to the shifts toward higher field based on the diatropic ring current. In the present case, the ¹³C NMR signals are important to evaluate the aromaticity in terms of magnetic properties. Three signals of the central benzene moieties were observed in the high-field region at 99.3, 98.1, and 92.7 ppm for 3e³⁺, indicating global diatropicity based on the 22π-cyclic conjugation as depicted in Scheme 1, whereas those for 3c³⁺ were observed at relatively lower field compared with 3e³⁺ (107.2, 103.8, and 92.6 ppm). In the ACID plots, there is a compensation between the differently oriented currents on the outer rim versus those on the individual inner rings (Fig. S7b in the ESI†). In 3e³⁺, the intensity on the outer rim of the homoHPHAC skeleton appears stronger. On the other hand, that on the outer rim is close to those on the inner rings in 3c³⁺, leading to the much less global diatropicity.²⁰

Crystal structure of 3c³⁺

Fortunately, single crystals of 3e³⁺ were obtained by vapour diffusion of hexane into its chloroform and anisole mixed solution. The molecular structure of 3e³⁺[SbF₆⁻]₃ was determined by X-ray diffraction analysis (Fig. 7a and S6f in the ESI†). The most significant difference from the structure of monocation 3e⁺[SbF₆⁻] is that the aryl substituent is largely bent away from coplanarity with the homoHPHAC skeleton. The angle between the two mean planes is 134.38°. The bond lengths of the dimethylaminophenyl moiety confirm the quinomethane resonance form, as suggested by the coupling constant in the ¹H NMR spectrum. The bonds labelled as “i”, “iii”, and “v” (i: 1.331 Å, iii: 1.345 and 1.350 Å, v: 1.397 Å) are predominantly shorter than the bonds “ii” and “iv” (ii: 1.431 and 1.432 Å, iv: 1.438 and 1.448 Å) in the crystal structure (Fig. 7b).²² Because of the double bondedness of the “v” bond (although it is rather long for a C=C double bond due to the steric hindrance of the adjacent ethyl substituents), sufficient homo(cyclic)conjugation at the meso-position is not able to form on homoHPHAC³⁺, leading to the lower global paratropicity of 3e³⁺. The HOMA value of 0.43 at the aryl substituent also shows decreased aromaticity (Fig. 7c). In agreement with the NICS calculations shown in Table 1, the aromaticity of the F ring is the smallest among the F, G, and H rings.

Conclusions

We developed versatility in the synthesis of homoHPHAC via Friedel–Crafts-type reaction of secoHPHAC and commercially available benzoyl chlorides, which enabled the investigation of substituent effects on antiaromatic compounds. The NMR and DFT calculations show that the global paratropicity of 3e⁺ with an electron-withdrawing pentafluorophenyl group substituent is the strongest among the compounds reported here, while the global diatropicity of 3e³⁺ is the strongest. These results indicate that delocalisation of cations/π-electrons enhanced by the substituent promotes cyclic conjugation, leading to strong paratropicity and diatropicity for the monocation and trication, respectively. The effect of a substituent on the electronic structure is essentially the same, regardless of whether it is aromatic or antiaromatic. However, because antiaromatic compounds have a high-lying HOMO and low-lying LUMO and thus have a small HOMO–LUMO gap, the effects appear to be relatively larger in antiaromatic compounds. The present study will provide new design guide for organic electronic materials, which should merit further research on antiaromatic compounds.

Data availability

Experimental procedures, NMR and MS spectra, and computational details and results are available in the ESI.† Details of the crystal structures have been deposited in the Cambridge Crystallographic Data Centre with CCDC 2232450–2232453 for 3c⁺, 3d⁺, 3e⁺, and 3c³⁺, respectively.

Author contributions

M. T. and H. U. designed the project. T. T. performed most of the experimental works except for X-ray measurement and analysis. S. M., K. O., and H. U. performed X-ray measurement and analysis. M. T. and T. T. prepared the overall manuscript. All authors, including K. O., S. M., and H. U., contributed by commenting on the manuscript. M. T. and H. U. supervised the overall research.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by JSPS KAKENHI (JP20H02725, JP19K05422, and JP23H03964). M. T. acknowledges the Takahashi Industrial and Economic Research Foundation and the Tokyo Ohka Foundation for the Promotion of Science and Technology for financial support.

Notes and references

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22. Due to the disorder of 4-dimethylaminophenyl group in the crystal structure of 3c⁺[SbF₆⁻], direct comparison of their bond lengths is avoided.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2232450–2232453. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2sc07037e

‡ Current address: Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan.

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