2-[2,6-Diisopropylphenyl]-4-phenyl-5H-5,9b[1′,2′]-benzonaphtho[1,2-b]pyrrol-2-ium Tetrafluoroborate

Abstract

A novel α,β-unsaturated iminium salt (3) incorporated into a rigid dibenzobarrelene backbone was synthesized by heating N-(anthracen-9-ylmethyl)-2,6-diisopropylaniline (2) and 3-phenyl-2-propynal in THF in the presence of excess amounts of magnesium sulfate and 0.5 equivalents of an HBF₄-Et₂O complex. The molecular structure of 3 was characterized unambiguously by NMR spectroscopy and single-crystal X-ray diffraction (SCXRD) analyses. Compound 3 exhibits yellow luminescence in CH₂Cl₂ (λ_em = 516 nm) and in the solid state (λ_em = 517 nm) with relatively high to moderate quantum yields (Φ_F(CH2Cl2) = 0.63; Φ_F(solid) = 0.34).

1. Introduction

The condensation of carbonyl compounds such as aldehydes or ketones with a primary amine typically results in an equilibrium where a substantial amount of imine is present. In contrast, when a secondary amine is employed, the aldehyde may condense to form an iminium ion, which, owing to the absence of a deprotonation step, can only be isolated as a salt of a strong acid [1,2]. For example, cyanine derivatives containing α,β-unsaturated iminium moieties are currently widely used not only as functional materials, such as dyes for optical recording media [3,4], but also as bio-imaging dyes [5,6,7]. However, the synthesis of an iminium salt that emits strongly both in solution and in the solid state is extremely rare [8]. Meanwhile, we are conducting an investigation into the synthesis of a series of 1,4-diaryl-1,3-butadiene derivatives that are incorporated into a rigid dibenzobarrelene backbone, wherein both components share each ethene unit and the 1 position of the 1,3-butadiene unit is linked to the bridgehead of the dibenzobarrelene by a main group element (Figure 1) [9,10,11,12,13]. These derivatives exhibit high fluorescence efficiency in solution and in the solid state due to the conformationally fixed 1,4-diaryl-1,3-butadiene fluorophore and the sterically bulky dibenzobarrelene moiety, which prevent intramolecular interactions. Herein, we present the synthesis, structure, and photophysical properties of an α,β-unsaturated iminium salt fixed in a dibenzobarrelene skeleton.

2. Results and Discussion

N-(Anthracen-9-ylmethyl)-2,6-diisopropylaniline (2) was prepared as yellow crystals in 79% yield by the reduction of the starting imine N-(9-anthracenylmethylene)-2,6-bis(1-methylethyl)benzenamine (1) [14] with LiAlH₄ in THF at 0 °C (Scheme 1). The target iminium salt (3) was successfully synthesized as yellow crystals in 31% yield by refluxing a THF solution of 3-phenyl-2-propynal [15] and the amine (2) in the presence of excess amounts of magnesium sulfate and 0.5 equivalents of an HBF₄-Et₂O complex (Scheme 1, Path A). The structure of 3 was characterized through the following spectroscopic analyses: In the ¹H NMR spectrum of 3 in CDCl₃, the methine and methylene protons at the bridgehead positions were observed as a singlet signal at 6.08 and 5.49 ppm, respectively. The methyl protons of the isopropyl groups on the Dip group showed doublet signals at 1.26 and 1.44 ppm, while a septet signal due to the methine proton resonated at 2.67 ppm. The iminium proton (9.20 ppm) appeared in a characteristic low-field region, which is comparable to that of the related five-membered iminium salt (9.48 ppm) [16]. In the ¹³C{¹H} NMR spectrum of 3, the iminium carbon is observed at 172.1 ppm. Furthermore, the ¹¹B{¹H} NMR spectrum of 3 displayed a signal at −1.14 ppm, suggesting that the [BF₄]⁻ anion exists as a free counter anion in solution.

Single crystals of 3 were obtained by slow evaporation of its CH₂Cl₂ solution at room temperature, and the molecular structure was determined by a single-crystal X-ray diffraction (SCXRD) analysis. The iminium salt (3) crystallizes in the monoclinic space group P2₁/n with one H₂O molecule and [BF₄]⁻ anion in the asymmetric unit. Given that 3 was recrystallized under atmospheric conditions, it is probable that the H₂O molecule was incorporated into the unit cell of 3 during crystallization. The ORTEP is shown in Figure 2, and the relevant geometrical parameters are summarized in

Table 1. Selected bond lengths [Å] and bond angles [°].

Bond Lengths	[Å]	Bond Angles	[°]
C4–C5	1.355(2)	N1–C3–C4	111.67(16)
C3–C4	1.409(2)	C3–C4–C1	107.34(15)
C3–N1	1.308(2)	C3–C4–C5	135.48(16)
C2–N1	1.484(2)	C1–C4–C5	117.10(15)
C1–C2	1.521(2)	C4–C5–C18	110.06(15)
C1–C4	1.547(3)	C31–C5–C4	128.92(16)
H3…O1	2.1845(17)	C3–N1–C19–C20	−77.0(2)
B1…N1	5.194(2)	C4–C5–C31–C32	−153.38(17)
		N1–C3–C4–C5	179.34(18)

. The distance between H3…O1 is 2.1845(17) Å, which is shorter than the sum of the van der Waals radii of the two atoms (2.60 Å) [17], indicating the presence of hydrogen bonding in the unit cell. In contrast to the related 2H-pyrrolium salts [18,19,20,21,22,23], it is exceptional for the H₂O molecule to engage in hydrogen bonding with the C–H bond of the iminium fragment. This is likely due to the steric hindrance surrounding the iminium fragment and the highly acidic iminium proton, which serves as a donor to the H₂O molecule. The intermolecular hydrogen bonds involving lattice water molecules and BF₄⁻ ions were also confirmed in the crystal packing of 3. The non-bonded OH…F distances (H35*…F4 and H36*…F4) are 1.88(2) and 2.12(2), which can be regarded as a relatively strong interaction [24]. The distance between the nitrogen atom (N1) in the iminium moiety and the counter anion (B1…N1) is 5.194(2) Å, which is considerably longer than the sum of the van der Waals radii of the two atoms (3.84 Å) [17], suggesting that the iminium moiety exists as a free cation in the crystalline state. The C=N [1.308(2) Å], C–N [1.484(2) Å], and C–C bond lengths [1.409(2)–1.547(3) Å] in the five-membered ring are similar to those of the reported 2H-pyrrolium salts [18,19,20,21,22,23]. Furthermore, the dihedral angle of the conjugated moiety comprising N1–C3–C4–C5 is almost 180° as a result of its incorporation into a rigid framework. The torsion angle (C4–C5–C31–C32) between the alkenyl moiety and the benzene ring attached to the C5 carbon is −153.38(17)°, which falls within the range of values observed in related dibenzobarrelene-incorporated 1,3-butadiene derivatives [8], thus forming an effective conjugated system. In contrast, the dihedral angle (C3–N1–C19–C20) comprising the benzene ring in the Dip group and the imine moiety is −77.0(2)°, which is nearly perpendicular due to the bulkiness of the Dip group.

The formation of 3 can be explained by the intramolecular Diels–Alder reaction of iminium intermediate 4. To verify this supposition, we attempted the direct synthesis of 4. Upon reacting 2 with 3-phenyl-2-propynal in the presence of an excess amount of magnesium sulfate and an HBF₄-Et₂O complex at 0 °C in Et₂O, the corresponding (Z)-4 was obtained in an almost quantitative yield as red crystals (Scheme 1, Path B). In the ¹H NMR spectrum of (Z)-4, the iminium proton was observed at 9.86 ppm, which showed a somewhat down-field shift compared to that of 3 (9.20 ppm). The molecular structure of (Z)-4 was finally confirmed by SCXRD (see Supplementary Materials). Subsequently, the intramolecular cyclization reaction, accompanied by the isomerization from (Z)-4 to (E)-4, proceeded under refluxing conditions in THF, resulting in the formation of the corresponding 3 in a 25% yield.

Interestingly, the iminium salt (3) exhibited yellow fluorescence in CH₂Cl₂ solution and in the solid state. The UV–visible absorption spectrum of 3 in CH₂Cl₂ shows two absorption bands, with the longest absorption maximum (λ_abs) at 423 nm and a molar absorption coefficient ε of 9820 M⁻¹ cm⁻¹ (Figure 3a,

Table 2. Photophysical data for 3 ^a.

λabs [nm]	ε [M−1cm−1]	λem (CH2Cl2) [nm]	Stokes Shift [cm−1] (nm)	ΦF (CH2Cl2) b	λem (solid) [nm]	ΦF (Solid) b
423	9820	516	4300 (152)	0.63	517	0.34

^a 1 × 10⁻⁵ M in CH₂Cl₂ at room temperature under argon. ^b The absolute fluorescence quantum yields were determined using a calibrated integrating sphere system.

). On the other hand, the fluorescence spectrum of 3 in CH₂Cl₂ has a maximum wavelength (λ_em) of 516 nm with a Stokes shift of 4300 cm⁻¹ (152 nm). We reported the Stokes shifts of the 1,3-butadiene derivatives incorporated into the dibenzobarrelene skeleton in the range of 4700–6100 cm⁻¹ [9], which are larger than those of 3. This result suggests that the structure change in the excited state of 3 is small compared with those derivatives. The fluorescence quantum yield Φ_F has a relatively high value of 0.63. Furthermore, the λ_em of 3 in the solid state was observed at 517 nm, which is almost the same as that in the CH₂Cl₂ solution. The Φ_F of 3 in the solid state was 0.34, which was significantly lower than that in the solution.

3. Materials and Methods

3.1. General Considerations

Unless otherwise noted, all experiments were carried out under an argon atmosphere using standard Schlenk-line techniques. The ¹H and ¹³C spectra were recorded on a Bruker AVANCE-400 (400 and 101 MHz, respectively) or a Bruker AVANCE-500 (500 and 126 MHz, respectively) spectrometer using CDCl₃ as the solvent at room temperature. The ¹¹B and ¹⁹F NMR spectra were recorded on a Bruker AVANCE-500 (160 and 471 MHz, respectively) spectrometer using CDCl₃ as the solvent at room temperature. The UV–vis spectra were recorded on a U-1900 spectrophotometer (HITACHI Co., Ltd., Hitachi, Japan). The fluorescence spectra were recorded on FP-6600 and FP-6300 spectrofluorometers (JASCO Corp., Tokyo, Japan). The absolute photoluminescence quantum yields were measured using the calibrated integrating sphere system C10027 (Hamamatsu Photonics Co. Ltd., Hamamatsu City, Japan). The elemental analyses were carried out at the Comprehensive Analysis Center for Science, Saitama University. The thermogravimetry (TG) analysis was carried out using an NETZSCH analyzer, the STA2500, as follows: under a helium atmosphere with a temperature range of 25–300 °C and a heating rate of 10 °C /min. All melting points were determined on a Mel-Temp capillary tube apparatus and are uncorrected. All solvents were dried over 4A molecular sieves or a potassium mirror before use. All materials were obtained from commercial suppliers and used without further purification, except for N-(9-anthracenylmethylene)-2,6-bis(1-methylethyl)benzenamine (1) [14] and 3-phenyl-2-propynal [15], which were prepared according to their respective literature procedures.

3.2. Synthesis of N-(Anthracen-9-ylmethyl)-2,6-diisopropylaniline (2)

A total of 321 mg of LiAlH₄ (8.47 mmol) was added to a solution of 1 (2.05 g; 5.64 mmol) in THF (50 mL) at 0 °C. Subsequently, the temperature was raised to room temperature over a period of 4 h. After cooling the solution to 0 °C, 1 M NaOH aq. was added. The suspension was filtered through a pad of Celite^®. The mixture was extracted with Et₂O, and the extract was washed with water, dried over anhydrous Na₂SO₄, and evaporated to dryness. The residue was washed with a small amount of hexane (ca. 10 mL) and dried under reduced pressure to provide pure 2 (1.63 g; 79%) as yellow crystals. Mp. 147–148 °C. ¹H NMR (400 MHz; 25 °C; CDCl₃): δ = 1.16 (d, J = 6.8 Hz, 12H), 3.25 (sept, J = 6.8 Hz, 2H), 3.32 (s, 1H), 5.04 (s, 2H), 7.09–7.13 (m, 3H, Ar), 7.46–7.51 (m, 4H, Ar), 8.03 (d, J = 8.4 Hz, 2H, Ar), 8.30 (d, J = 8.4 Hz, 2H, Ar), 8.45 (s, 1H, Ar). ¹³C{¹H} NMR (101 MHz, 25 °C, CDCl₃): δ = 24.2 (2 CH₃), 28.0 (CH), 48.1 (CH₂), 123.58 (CH), 123.68 (CH), 124.4 (CH), 126.0 (CH), 127.5 (CH), 129.0 (CH), 130.2 (C), 131.6 (2 C), 141.8 (C), 144.1 (C). Anal. Calcd. For C₂₇H₂₈N: C, 88.48; H, 7.77; N, 3.82. Found: C, 88.22; H, 7.68; N, 3.77.

3.3. Synthesis of 2-[2,6-Diisopropylphenyl]-4-phenyl-5h-5,9b[1′,2′]-benzonaphtho[1,2-b]pyrrol-2-ium Tetrafluoroborate (3)

A complex of HBF₄-Et₂O (0.0204 M; 0.13 mL; 0.75 mmol) was added dropwise to a solution of 3-phenyl-2-propynal (0.179 mL; 193 mg; 1.51 mmol), 2 (554.4 mg; 1.51 mmol), and MgSO₄ (110 mg) in anhydrous THF (70 mL) at 0 °C. The mixture was heated under reflux overnight. The suspension was filtered through a pad of Celite^®. After evaporation, the residue was reprecipitated from CH₂Cl₂/benzene to provide pure 3 as yellow crystals (428 mg; 31%). Mp. 276–278 °C (dec.). ¹H NMR (400 MHz; 25 °C; CDCl₃): δ = 1.26 (d, J = 6.5 Hz, 6H), 1.44 (d, J = 6.5 Hz, 6H), 2.67 (sept, J = 6.5 Hz, 2H), 5.49 (s, 2H, CH₂), 6.08 (s, 1H, CH), 7.21–7.29 (m, 4H, Ar), 7.38 (d, J = 7.5 Hz, 4H, Ar), 7.56–7.64 (m, 6H, Ar), 7.79 (d, J = 7.5 Hz, 2H, Ar), 9.20 (s, 1H). ¹³C{¹H} NMR (126 MHz, 25 °C, CDCl₃): δ = 19.2 (CH₃), 24.3 (CH₃), 24.8 (CH₃), 27.3 (CH), 29.1 (CH₃), 29.7 (CH), 58.3 (CH), 58.9 (C), 61.0 (CH₂), 121.8 (CH), 125.3 (CH), 125.5 (CH), 127.2 (CH), 127.4 (CH), 127.6 (CH), 127.8 (CH), 128.1 (CH), 128.6 (CH), 130.3 (CH), 132.2 (CH), 132.9 (CH), 133.3 (CH), 133.8 (CH), 141.0 (C), 142.2 (C), 143.3 (C), 143.4 (C), 167.1 (CH), 172.1 (C). ¹¹B{¹H} NMR (160 MHz, 25 °C, CDCl₃): δ = −1.14 (s). ¹⁹F{¹H} NMR (471 MHz, 25 °C, CDCl₃): δ = −153.1 (s).

3.4. Synthesis of (Z)-Anthracenyl-9-methyl-(2,6-diisopropylphenyl)-(3-phenylprop-2-ynylidene)ammonium Tetrafluoroborate [(Z)-4]

A complex of HBF₄-Et₂O (0.0204 M, 0.07 mL, 0.41 mmol) was added dropwise to a solution of 3-phenyl-2-propynal (0.096 mL; 105.6 mg; 0.82 mmol), 2 (299.5 mg; 0.82 mmol), and MgSO₄ (60 mg) in anhydrous Et₂O (30 mL) at 0 °C. The mixture was stirred for 4 h at 0 °C. The suspension was filtered through a pad of Celite^®. After evaporation, the residue was washed with hexane (ca. 10 mL) to provide pure (Z)-4 as red crystals (753 mg; 97%). Mp. 150–151 °C (dec.). ¹H NMR (500 MHz; 25 °C; CDCl₃): δ = 0.45 (d, J = 6.7 Hz, 6H), 0.98 (d, J = 6.7 Hz, 6H), 2.39 (sept, J = 6.7 Hz, 2H), 6.50 (s, 2H), 7.11–7.12 (m, 2H, Ar), 7.20–7.21 (m, 2H, Ar), 7.32–7.35 (m, 2H, Ar), 7.42–7.55 (m, 6H, Ar), 7.87 (d, J = 7.7 Hz, 2H, Ar), 8.00 (d, J = 7.7 Hz, 2H, Ar), 8.53 (s, 1H, Ar), 9.86 (s, 1H). ¹³C{¹H} NMR (126 MHz, 25 °C, CDCl₃): δ = 22.4 (CH₃), 25.5 (CH), 28.7 (CH), 59.9 (CH₂), 85.3 (C), 117.4 (C), 118.7 (C), 121.9 (CH), 124.3 (C), 125.2 (CH), 125.4 (CH), 128.3 (CH), 129.3 (CH), 129.6 (CH), 131.3 (C), 131.7 (CH), 131.79 (CH), 131.83 (CH), 134.4 (CH), 135.0 (C), 143.7 (C), 159.0 (CH). ¹¹B{¹H} NMR (CDCl₃, 160 MHz): δ = −0.49 (s). ¹⁹F{¹H} NMR (CDCl₃, 471 MHz): δ = −152.0 (s).

3.5. SCXRD Analysis of 3

Yellow single crystals of 3 were grown by slow evaporation of its CH₂Cl₂ solution at room temperature. The intensity data were collected at 100 K on a Bruker SMART APEX II diffractometer employing graphite-monochromated MoKα radiation (λ = 0.71073 Å). The structure was solved by direct methods (SHELXT) [25] and refined by full-matrix least-squares procedures on F² for all reflections (SHELXL) [26]. The hydrogen atoms were located by assuming ideal geometry and included in the structure calculations without further refinement of the parameters.

The crystal data for C₃₆H₃₆BF₄NO (3·H₂O) are as follows: M = 585.47 g mol⁻¹, monoclinic, P2₁/n, a = 10.5927(15), b = 16.799(2), c = 17.106(2) Å, β = 100.067(2)°, V = 2997.2(7) ų, Z = 4, D_x = 1.297 g cm⁻³, F(000) = 1232, and μ = 0.094 mm⁻¹. CCDC’s deposition number is 2236656.

4. Conclusions

We have demonstrated the synthesis and characterization of a novel α,β-unsaturated iminium salt (3) incorporated into a rigid dibenzobarrelene skeleton. In CH₂Cl₂ solution and in the solid state, 3 showed yellow emission with relatively high to moderate quantum yields. Further applications using 3 are currently being investigated in our laboratory.