A Stable π-Expanded o-Quinodimethane via the Photochemical Dearomative Cycloaddition of Corannulene with an Isolable Dialkylsilylene

Abstract

A stable π-expanded o-quinodimethane derivative (2) was synthesized by photochemical dearomative cycloaddition of corannulene with an isolable dialkylsilylene (1) and isolated as a dark blue solid. Compound 2 adopts a very flat bowl shape in contrast to parent corannulene. Structural and spectroscopic characteristics, redox properties, and computational study suggest that 2 has a small but significant diradical character (y₀ = 0.11). One-electron reduction of 2 provides the corresponding radical anion as an isolable salt.

1. Introduction

o-Quinodimethanes (oQDMs) are important reactive dienes to construct benzo-fused cyclic skeletons by cycloaddition, and their properties have received wide attention [1,2,3,4,5]. Recent sophisticated synthetic methods and molecular design provide stable oQDM derivatives, and their electronic structure, involving diradical character due to the narrow HOMO-LUMO gap, has been revealed. oQDMs A reported by Suzuki exhibit redox systems associating large structural change from nonaromatic oQDM to dications (Figure 1a) [6,7]. Escudié has demonstrated that the cycloaddition of a reactive Ge=C doubly bonded compound [Mes₂Ge=CR₂ (CR₂ = fluorenylidene)] with 1,4-naphthoquinone yields stable oQDM B [8,9]. Tobe and Shimizu have developed the chemistry of indeno [2,1-a]fluorene derivatives C and D as stable oQDMs and their characteristic electronic character [10,11,12]. These pioneering works contribute deeper understanding of polycyclic aromatic hydrocarbons (PAHs) and the relationship between structures and their diradical character [13]. Nevertheless, the number of stable oQDMs is still limited and new synthetic methods are highly desired [14,15,16,17].

Silylenes (divalent silicon species) undergo dearomative cycloaddition with aromatic compounds, producing silicon-containing seven-membered rings [18,19,20,21,22,23,24,25,26,27,28,29,30]. This type of reaction, a silicon version of Büchner ring expansion, becomes a powerful tool for constructing nonaromatic π-electron conjugated systems from readily available aromatic compounds in short steps. We have studied the dearomative cycloadditions of benzenes, azulenes, diaryl ketones, and related compounds using an isolable silylene (1) [31] and properties of the products, non-benzenoid π-electron conjugated systems [32,33,34,35,36]. For instance, 1 reacts with benzenes under visible light irradiation (λ > 420 nm) to afford silepins (silacycloheptatrienes) E–H (Figure 1b) [32]. The formation mechanism of E via the singlet excited state of 1 was theoretically investigated [37]. Based on these previous reports, we envisaged the dearomative cycloaddition of corannulene, a bowl-shaped nonplanar hydrocarbon with C_5v symmetry [38,39], with 1 to afford benzo[6,7]fluorantheno[1,10-cde]silepin derivative 2 that has a π-expanded oQDM structure. We report herein the detailed synthesis of 2 and its structure, properties, reaction with O₂, and one-electron reduction (Figure 1c).

2. Results and Discussion

2.1. Synthesis and Structure

When a hexane solution of corannulene and 1.1 equiv of 1 (λ_max = 440 nm) was irradiated by LED light (λ_max = 448 nm) at −20 °C, the color of the solution turned from yellow to intense blue within a minute. Further irradiation for 1 h at −20 °C was conducted to complete the reaction. After the recrystallization of the crude product from a saturated hexane solution, cycloadduct 2 was obtained in 57% yield as an air-sensitive dark-blue powder (Figure 1c). The structure of 2 was confirmed by a combination of NMR spectroscopy, MS spectrometry, single crystal X-ray diffraction (sc-XRD) study, and elemental analysis. According to the previous reports on the related reactions of silylenes [18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36], the formation of 2 would proceed via (1 + 2) cycloaddition (cheletropic reaction) to afford silanorcaradiene 2′ and the subsequent electrocyclic ring-opening reaction.

The molecular structure of 2 obtained by sc-XRD is displayed in Figure 2a. The π-electron system of 2 is almost planar. The bowl depth (d_bowl), defined as the distance from the center of the hub carbon atoms (C16−C20) to the mean planes of the carbon atoms at the rim positions (C3, C4, C6, C7, C9, C10, C12, C13), is only 0.099 Å for 2. The d_bowl of 2 is much smaller than that of corannulene (d_bowl = 0.87 Å). Introducing a seven-membered ring reduces the strain of the corannulene core. Generally, silepins adopt a boat conformation characterized by fold angles θ and φ (Figure 2b). The fold angles of 2 (θ = 17.0° and φ = 4.9°) are smaller than those of silepin E (33.0° and 24.5°) [32], which would result from the fused polycyclic aromatic ring system. Intermolecular π-stacking was found to form a dimeric structure (Figure 2c); the shortest distance between the mean planes of C(sp²) atoms is 3.45 Å. As shown in Figure 2d, the C(sp²)–C(sp²) bond lengths of 2 support the silepin structure, but small bond alternation is found around aromatic rings; the C1–C2/C14–C15 and C16–C20 bond lengths [1.373(3)/1.381(3) and 1.419(3) Å] of 2 are longer than those of the corresponding parts of silepin E [1.335(3)/1.346(4) and 1.332(3) Å], while C2–C16/C14–C20 bonds [1.428(3)/1.431(3) Å] are shortened compared to those of E [1.453(4)/1.445(5) Å]. The harmonic oscillator model of aromaticity (HOMA) indices [40,41] for rings A and D (0.77 and 0.80) exhibit their aromatic character, but their degrees are smaller than those for rings B and C (0.95 and 0.95). The HOMA indices of oQDM units of 2 are larger than those of ring a in C (0.65) and rings b and c in D (0.71 and 0.81) [10,11]. These structural characteristics indicate that 2 represents a resonance hybrid of a silepin as π-expanded oQDM 2a and open-shell diradical 2b due to the recovery of two Clar’s sextet (Figure 2e) [42,43,44]. ¹H NMR spectrum of 2 exhibits a diamagnetic character of 2. ¹H NMR spectrum of 2 in C₆D₆ shows sharp two singlet peaks (0.20 and 1.98 ppm) with a ratio of 36:4 in the aliphatic region due to four trimethylsilyl groups and the methylene protons on the silacyclopentane, which indicates a facile boat-to-boat inversion in NMR time scale. In the aromatic region of the ¹H NMR spectrum of 2, four doublet signals and a sharp singlet signal appeared with a ratio of 2:2:2:2:2. The singlet signal due to protons on C1/C15 atoms was located at 6.56 ppm, but in ¹³C NMR, the tertiary C1/C15 signals at 132.6 ppm is considerably broadened compared to the other signals (Figure S2). The observed signal broadening may be due to the contribution of the open-shell character of 2b.

2.2. UV-vis Absorption Spectrum and Electronic Structure

The Uv-vis spectrum of 2 in hexane exhibits four absorption bands associated with vibronic structures (Figure 3a). Characteristic absorption bands in the visible region (band-I: 673 nm and band-II: 502 nm) are not found in silepins E–H or corannulene. To obtain insight into the electronic structure of 2, we carried out TD-DFT calculations for the optimized structure of 2 (2_opt) at the B3PW91/6-31+G(d,p)//B3LYP-D3/6-31G(d) level of theory, and the frontier orbitals of the optimized structure of 2_opt are shown in Figure 3b. Based on the band positions, band-I is assignable to the HOMO–LUMO transition and band-II is attributed to overlapped HOMO-1–LUMO and HOMO-2–LUMO transitions. The HOMO and LUMO are π- and π*-orbitals delocalized on the π-system and are mainly located on the silepin moiety (Figure 3b). The estimated HOMO-LUMO gap of 2.18 eV is relatively small due to the π-expanded o-QDM structure of 2. HOMO-1 and HOMO-2 are also delocalized over the rings A–D. As the structural characteristics and narrow HOMO–LUMO gap suggest a contribution of diradical character, we conducted further calculations to estimate the diradical index (y₀) using Yamaguchi’s method at the LC-(U)BLYP/6-311G(d) level of theory [45,46,47]. Based on the natural orbital occupation number (NOON) of HONO (1.608) and LUNO (0.392) of the broken symmetry state of 2 (2_BS), the approximately spin projected diradical index y₀ is 0.11. Thus, the obtained y₀ value of 2 is much smaller than those of indeno[2,1-a]fluorene derivatives C (0.33) and D (0.63) [10,11]. The small but significant singlet diradical character contributed to the electronic structure of 2. The triplet state of 2 is 40.7 kJ mol⁻¹ above the broken-symmetry singlet state. The contribution of the triplet state to the UV-vis spectrum would be negligible.

2.3. Decomposition of 2 in Air

Corannulene and silepins E-H are storable in the air, while 2 was readily decomposed to a mixture containing dicarbonyl compound 3. Even though bulky four trimethylsilyl groups protect the silepin moiety of 2, facile Si–C bond cleavage occurred via oxidation. Compound 3 was isolated in 25% yield as a yellow solid by the brief exposure of O₂ gas in THF at room temperature for 10 min (Scheme 1). Although the yield should be improved, the selective C=C bond cleavage at the rim position of corannulene to convert benzo[ghi]fluoranthene derivative is achieved in two steps without harsh conditions [48,49]. A proposed formation mechanism is drawn at the bottom of Scheme 1. As several endoperoxides are reported as the product of the reactions of oQDM with oxygen [9,10,12,50], endoperoxide 3a forms as an initial intermediate in this case. Then, a homolytic O–O bond cleavage of 3a furnishes diradical 3b, and the subsequent Si–C bond cleavage to form a formyl group affords 3c. Finally, forming the strong C=O bond drives a facile 1,2-hydrogen shift of 3c to give 3. The molecular structure of 3 was unequivocally determined by sc-XRD (Figure 4).

2.4. Cyclic Voltammetry and Chemical One-Electron Reduction

Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) of compound 2 at room temperature in THF exhibit three reduction quasi-reversible waves at −1.47, −2.13, and −2.39 V (vs. Fc/Fc⁺), respectively (Figure 5). The three-step reduction wave indicates a redox system among neutral, radical anion, dianion, and radical trianion states. The first reduction wave became reversible when the sweep direction was reversed at ca. −1.9 V (Figure S10a) and is positively shifted by 1.01 V compared to that of corannulene (−2.48 V, see Figure S10b).

The electrochemical behavior prompted us to conduct the chemical one-electron reduction of 2; 1.3 equiv of KC₈ was subjected to 2 in the presence of 1 equiv [2.2.2]cryptand in THF for 3 h. The residue did not show significant ¹H NMR signals in THF-d₈, suggesting the formation of radical anion 2^•−. Recrystallization of the residue with a 1:3 mixed solvent of hexane and THF gave radical anion salt [K(222-crypt)]⁺2^•− in 58% isolated yield (Figure 6a). The structure was unequivocally determined by sc-XRD and EPR spectroscopy. In the solid state, [K(222-crypt)]⁺2^•− exists as a solvent-separated ion pair (Figure 6b), and 2^•− has a planar benzo[ghi]fluoranthene skeleton. Although the poor quality of the data hampers a detailed discussion on the bond length change upon one-electron reduction of 2, no intermolecular π-stacking in contrast to the neutral state was found in the solid state.

The ESR spectrum of [K(222-crypt)]⁺2^•− in THF at room temperature showed a multiline signal (g = 2.0028, Figure 7a), which was reproduced by computer simulation using three hyperfine coupling constants (hfccs) due to three types of each two hydrogen atoms [a(¹H^1/15) = 0.655 mT, a(H^3/13) = a(¹H^7/9) = 0.202 mT]. (Figure 7b). The isotropic Fermi contact terms obtained by DFT calculation at the UB3PW91/6-31G(d) level of theory of 2^•− were 0.87 mT for H^1/15, 0.27 mT for H^3/13, 0.25 mT for H^7/9, respectively, which support the above assignment of hfccs and the structure of 2^•− in solution. The SOMO of 2^•− that corresponds to the LUMO of 2 is delocalized over the carbon π-system, and the spin density plot of 2^•− (Figure 7c) displays that the largest orbital coefficient is on the two carbon atoms adjacent to the silicon atom.

3. Materials and Methods

3.1. General

All reactions treating air- and moisture-sensitive compounds were carried out under argon and nitrogen atmosphere using a high-vacuum line, standard Schlenk techniques, or a glove box, as well as dry and oxygen-free solvents. ¹H (500 MHz), ¹³C (126 MHz), and ²⁹Si (99 MHz) NMR spectra were recorded on a Bruker Avance III 500 FT NMR spectrometer (Bruker Japan, Yokohama, Japan). The ¹H NMR chemical shifts were referenced to residual ¹H of the solvent: C₆D₆ (¹H δ 7.16), CDCl₃ (¹H δ 7.26). The ¹³C and ²⁹Si NMR chemical shifts were relative to SiMe₄ (δ 0.0) in ppm. Sampling of air-sensitive compounds was carried out using a VAC NEXUS 100027-type glove box (Vacuum Atmospheres Co., Hawthorne, CA, USA). UV-vis spectra were recorded on a JASCO V-660 spectrometer (JASCO, Tokyo, Japan). ESR spectra were recorded on a JEOL X330 ESR spectrometer (JEOL, Tokyo Japan). Photochemical reaction was carried out using a LED light source device (Techno Sigma PER-448 AMP-N1, Okayama, Japan). Cyclic voltammograms (CV) and differential pulse voltammograms (DPV) were recorded on an ECstat-301WL electrochemical analyzer (EC Frontier, Kyoto, Japan). Mass spectra were recorded on a Bruker Daltonics SolariX 9.4T (Bruker Japan, Yokohama, Japan). Melting point was measured using an SRS OptiMelt MPA100 (Stanford Research Systems, Sunnyvale, CA, USA). Elemental analysis was performed with a J-SCIENCE LAB JM-11 (J-SCIENCE, Kyoto, Japan).

3.2. Materials

Dry and degassed hexane and THF were prepared using a VAC 103991 solvent purifier (Vacuum Atmospheres Co., Hawthorne, CA, USA). C₆D₆ and CH₂Cl₂ were degassed by freeze–pump–thaw cycles (three times) and dried over molecular sieves 4 Å. Corannulene and [2.2.2]cryptand were commercially available and used without further purification. 2,2,5,5-Tetrakis(trimethylsilyl)-1-silacyclopentane-1,1-diyl (1) was prepared according to procedure described in the literature [31].

3.3. Synthesis of 2′,2′,5′,5′-Tetrakis(trimethylsilyl)spiro[benzo[6,7]fluorantheno[1,10-cde]silepine-2,1′-silolane] (2)

In a Schlenk tube (50 mL) equipped with a magnetic stir bar and a glass tube insert having a glass joint and an LED for internal irradiation, corannulene (16 mg, 62 μmol) and 1.1 equiv of silylene 1 (25 mg, 68 μmol) were dissolved in hexane (ca. 5 mL). When the reaction mixture was stirred with irradiation of an LED light source (λ_max = 448 nm) at −20 °C, the color of the solution turned yellow of 1 to dark blue of 2 within 1 min. The irradiation was continued for 1 h at −20 °C to complete the reaction. This reaction was performed three times in the same condition. Total amounts of corannulene and 1 are 46 mg (168 μmol) and 75 mg (204 μmol), respectively. After the resulting mixtures were combined and then concentrated in vacuo, the crude product was washed with acetonitrile. Recrystallization of the obtained crude product from hexane at −35 °C produced 2 as dark blue crystals in 57% yield (60 mg, 97 μmol). The color change during the photoirradiation was recorded in Video S1 in the Supplementary Materials. In this video, photoirradiation was conducted at room temperature to clarify the color change.

2—dark blue crystal; mp 218–220 °C; ¹H NMR (500 MHz, C₆D₆, 298 K, δ) 0.29 (s, 36H, SiMe₃), 2.14 (s, 4H, CH₂), 6.56 (s, 2H, H¹), 6.89 (d, J = 18.8 Hz, 2H, H^3/4), 6.91 (d, J = 18.8 Hz, 2H, H^3/4), 7.29 (d, J = 8.4 Hz, 2H, H^6/7), 7.55 (d, J = 8.4 Hz, 2H, H^6/7); ¹³C{¹H} NMR (126 MHz, C₆D₆, 298 K, δ) 5.3 (SiMe₃), 23.4 (C), 35.2 (CH₂), 126.0 (C^3/4H), 126.1 (C^6/7H), 126.4 (C), 128.1 (C), 128.2 (C^6/7H), 130.7 (C), 132.4 (C), 132.6 (broad singlet, C¹H), 136.7 (C), 136.9 (C^3/4H), 143.3 (C); ²⁹Si{¹H} NMR (99 MHz, C₆D₆, 298 K, δ) 2.3 (SiMe₃), 9.6 (Si); UV-vis (hexane, rt) λ_max, nm (ε): 673 (1.33 × 10³), 502 (2.17 × 10³), 362 (1.45 × 10⁴), 290 (3.32 × 10⁴); HRMS-APCI (m/z): [M + H]⁺ calcd for C₃₆H₅₁Si₅, 623.2837; found 623.2832; Anal. Calcd for C₃₆H₅₀Si₅: C, 68.38; H, 8.09%. Found: C, 68.76; H, 8.22%.

3.4. Synthesis of (2,2,5,5-Tetrakis(trimethylsilyl)silolane-1-carbonyl)benzo[ghi]fluoranthene-5-carbaldehyde (3)

In a recovery flask (50 mL), compound 2 (19 mg, 30 μmol) in THF (ca. 5 mL) was placed under O₂ atmosphere for 10 min with stirring. The color of the solution turned from dark blue to yellow. The volatiles were removed in vacuo and the residue was washed with hexane to produce compound 3 as a yellow powder in 25% yield (5 mg, 7.6 μmol).

3—a yellow powder; mp 167–169 °C; ¹H NMR (500 MHz, CDCl₃, 297 K, δ) 0.20 (s, 18H, SiMe₃), 0.26 (s, 18H, SiMe₃), 1.97–2.23 (m, 4H, CH₂), 5.30 (s, 1H, SiH), 7.88–7.95 (m, 4H, overlapped signals), 7.97 (d, J = 8.2 Hz, 1H), 8.10 (d, J = 8.2 Hz, 1H), 8.11 (d, J = 8.2 Hz, 1H), 8.48 (d, J = 8.2 Hz, 1H), 10.72 (s, 1H); ¹³C{¹H} NMR (126 MHz, CDCl₃, 298 K, δ) 2.1 (SiMe₃), 3.8 (SiMe₃), 7.0 (C), 33.7 (CH₂), 126.7 (CH), 126.8 (CH), 127.3 (CH), 127.4 (CH) 127.5 (CH), 127.9 (C), 128.0 (CH), 128.5 (CH), 130.2 (C), 130.5 (C), 132.5 (C), 132.8 (CH), 134.0 (C), 134.4 (C), 134.9 (C), 135.1 (C), 138.0 (C), 140.0 (C), 192.8 (CHO), 234.4 (SiC=O); ²⁹Si{¹H} NMR (99 MHz, CDCl₃, 297 K, δ) –3.7 (Si), 3.8 (SiMe₃), 5.0 (SiMe₃); MS (EI, 70 eV) m/z (%): 654 (25, M⁺), 639 (4, [M–Me]⁺), 373 (2), 299 (1), 73 (100); Anal. Calcd for C₃₆H₅₀Si₅O₂: C, 65.99; H, 7.69%. Found: C, 65.74; H, 7.77%.

3.5. One-Electron Reduction of 2

In a Schlenk tube (50 mL) equipped with a magnetic stir bar, 2 (15 mg, 24 μmol), [2.2.2]cryptand (9 mg, 24 μmol), and KC₈ (4 mg, 30 μmol) were dissolved in THF (ca. 2 mL). The color of solution turned to reddish brown during stirring at room temperature for 3 h. After the filtration of insoluble materials, volatiles were removed in vacuo. Recrystallization of thus obtained crude from a mixed solution of THF and hexane with a 1:1 ratio afforded [K(222-crypt)]⁺2^•− as a brown solid in 58% yield (15 mg, 14 μmol).

[K(222-crypt)]⁺2^•−: a brown solid; mp 149–153 °C; Anal. Calcd for C₅₄H₈₆KN₂O₆Si₅ C, 62.44; H, 8.34%. N, 2.70%. Found: C, 59.39; H, 8.04; N, 2.41%. Elemental analysis of [K(222-crypt)]⁺2^•− did not provide satisfactory results due to the extremely air-sensitive nature of [K(222-crypt)]⁺2^•−, even in the solid state. The pyrophoric nature of solution of [K(222-crypt)]⁺2^•− prevented further analysis such as MS spectrometry.

3.6. X-Ray Crystallographic Analysis

Single crystal-X-ray diffraction analysis (sc-XRD) was conducted using a Bruker AXS APEX II CCD diffractometer (Bruker Japan, Yokohama, Japan) with graphite monochromated Mo-Kα radiation. Single crystals, suitable for X-ray diffraction study, were obtained by recrystallization under the following conditions: from hexane at −35 °C for 2, THF at room temperature 3, and hexane and THF (3:1) at −35 °C for [K(222-crypt)]⁺2^•−. The single crystals for data collection coated by Apiezon^® grease were mounted on a glass fiber and transferred to the cold nitrogen gas stream of the diffractometer. An empirical absorption correction based on the multiple measurements of equivalent reflections was applied using a program (SADABS, version 2.03) and the structures were solved by direct methods and refined by full-matrix least squares against F² using all data (SHELXL-2019) [51,52]. Molecular structure was analyzed by Yadokari-XG software (version 2007.5.12) [53].

Crystal Data of 2 (CCDC 2419073): C₃₆H₅₀Si₅; FW 623.21; 100 K; monoclinic; space group P2₁/n, a = 9.8555(6) Å, b = 22.0832(13) Å, c = 15.9673(10) Å, β = 93.013(2)°, V = 3470.3(4) ų, Z = 4, D_calc = 1.193 Mg/m³, R1 = 0.0530 (I > 2σ(I)), wR2 = 0.1253 (all data), GOF = 1.026.

Crystal Data of 3 (CCDC 2419074): C₃₆H₅₀O₂Si₅; FW 655.21; 100 K; Triclinic; space group P-1, a = 9.2036(8) Å, b = 11.5218(10) Å, c = 36.978(3) Å, α = 95.551(2)°, β = 90.183(2)°, γ = 113.327(2)°, V = 3580.1(5) ų, Z = 4, D_calc = 1.216 Mg/m³, R1 = 0.0591 (I > 2σ(I)), wR2 = 0.1333 (all data), GOF = 1.008.

Crystal Data of [K(222-crypt)]⁺2^•− (CCDC 2419075): C₅₄H₈₅KN₂O₆Si₅; FW 1037.78; 100 K; Monoclinic; space group P2₁/n, a = 10.4531(17) Å, b = 33.929(5) Å, c = 16.349(3) Å, β = 92.229(5)°, V = 5794.1(16) ų, Z = 4, D_calc = 1.190 Mg/m³, R1 = 0.1030 (I > 2σ(I)), wR2 = 0.2304 (all data), GOF = 1.013.

3.7. Theoretical Calculations

All theoretical calculations were performed using Gaussian 09 [54] and GRRM 14 [55,56,57,58,59,60] programs. Geometry optimization and frequency analysis of 2 and 2^•− were carried out at the (U)B3PW91-D3/6-31G(d) level of theory. No imaginary frequencies were found in the optimized structures. Atomic coordinates of the optimized structures are listed in the Supplemental Materials. Forty excited states of 2 calculated at the B3PW91/PCM(hexane)/6-31+G(d,p) level of theory.

4. Conclusions

π-expanded silepin 2 was synthesized by photochemical reaction of silylene 1 and corannulene. Compound 2 has an oQDM structure with a small but significant singlet diradical character (y₀ = 0.11). The UV-visible absorption spectrum of 2 shows broad absorption bands in the 400–700 nm region due to the narrow HOMO-LUMO gap (2.18 eV) estimated by computational studies. Both the HOMO and LUMO of 2 have large orbital coefficients on the two carbon atoms adjacent to the central silicon atom. Compound 2 reacted with O₂, giving acylsilane 3 via facile Si–C bond cleavage. The electrochemical behavior of 2 was revealed by CV and radical anion 2^•− was synthesized by one-electron reduction of 2 with KC₈ and isolated as [K(222-crypt)]⁺2^•−. The presented synthetic method would offer potential access to unprecedented π-expanded quinoids and diradicaloids.