Letter pubs.acs.org/OrgLett
Synthesis of Ring-Fused Pyridinium Salts by Intramolecular Nucleophilic Aromatic Substitution Reaction and Their Optoelectronic Properties Yuki Asanuma,† Hiroshi Eguchi,† Hiroki Nishiyama,‡ Ikuyoshi Tomita,‡ and Shinsuke Inagi*,‡ †
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan ‡ Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan S Supporting Information *
ABSTRACT: The synthesis of nitrogen cation-doped polycyclic aromatic hydrocarbons (PAHs) having a variety of counteranions is reported via the trimethylsilyl (TMS)promoted intramolecular aromatic nucleophilic substitution (SNAr) reaction of fluoroarenes and pyridine groups. The electrochemical properties and optical properties of the obtained nitrogen cation-doped PAHs were studied in detail, clarifying that they have low-lying LUMO levels and good emission properties derived from the incorporation of planar N-arylpyridinium moieties.
P
trimethylsilyl trifluoromethanesulfonate (TMS-OTf) in acetonitrile (MeCN), the intramolecular SNAr cyclization successfully proceeded to give ring-fused pyridinium salt 2a in a high yield (entry 1 in Table 1). Such TMS-promoted SNAr reaction
olycyclic aromatic hydrocarbons (PAHs) have been widely studied due to their unique properties including stacking behavior arising from the nature of planar molecules.1 Heteroatom-doping can tune the electronic state of the corresponding backbone PAHs and thus drastically alter their optoelectronic properties.2 Among them, PAH derivatives containing nitrogen cations exhibit interesting optical properties and aggregation behavior.3,4 However, the synthetic protocols of such nitrogen cation-doped PAHs are generally not straightforward. One of the basic skeletons, N-arylpyridinium salt, has been synthesized by a Zincke reaction,5 pyrylium salt method,6 anodic substitution reaction,7 and nucleophilic aromatic substitution reaction (SNAr);8,9 however, the obtained N-arylpyridinium salts were highly twisted between a pyridinium ring and an aryl group. Therefore, the development of a synthetic procedure for planar N-arylpyridinium salts is worth investigation. Our approach toward nitrogen cation-doped fused aromatic systems in this research is to employ intramolecular SNAr cyclization, in which nucleophilic pyridine attacks the ipsoposition of a pentafluorophenyl group, followed by the elimination of a fluoride ion to give a triphenylene-like skeleton. It has been known that trimethylsilyl reagents can trap eliminated halide ions to exchange in situ counteranions of generated ring-fused pyridinium salts.9 First, 1-(2-pyridyl)-2-pentafluorophenylbenzene 1 was prepared by the treatment of 2-(2-bromophenyl)pyridine with nbutyllithium, followed by the nucleophilic reaction with an excess amount of hexafluorobenzene (see Supporting Information). When 1 was heated at 50 °C in the presence of © XXXX American Chemical Society
Table 1. Synthesis of Ring-Fused Pyridinium Salts 2 via Intramolecular SNAr Reaction
a
entry
X source
product
yield (%)a
1 2 3 4 5
TMS-OTf − TMS-Cl TMS-Br TMS-I
2a 2b 2c 2d 2e
81 0b 89 84 99
Isolated yield. bDetermined by 19F NMR.
of fluoroarenes was found to be very effective in this case. Without a silyl reagent, the SNAr reaction might proceed, but unstable product 2b immediately decomposed (entry 2).10 The use of other silyl reagents such as TMS-Cl, TMS-Br, and TMS-I also afforded the corresponding pyridinium salts 2c−2e with Cl−, Br−, and I− as a counteranion, respectively (entries 3−5). Received: February 27, 2017
A
DOI: 10.1021/acs.orglett.7b00590 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters The structure of 2a was characterized by NMR, HRMS, and single crystal X-ray analysis. Interestingly, the mixing of pyridine and hexafluorobenzene under the same conditions did not provide the corresponding pyridinium salt; rather a higher temperature was required for the progress of the reaction. This means that the intramolecular approach accelerated the SNAr reaction. Pyridinium 2a was soluble to some extent in polar solvents such as methanol, dimethyl sulfoxide (DMSO), MeCN, and acetone. Compounds 2c−2e, having a halide ion as a counterion, also showed good solubility in the solvents above. In addition, 2c having a chloride ion was soluble in water. The crystal structure of 2a exhibited that the relatively planar triphenylene-like structure incorporates a pyridinium moiety along with a CF3SO3− ion (TfO−) (Figure 1). In general, a
Figure 2. Cyclic voltammograms of 2a and 1 (1 mM) in 0.1 M Bu4NOTf/MeCN using a Pt working electrode at a scan rate of 100 mV/s.
respectively.12 The LUMO level was decreased due to the pyridinium formation by the ring-fusing reaction. In the calculated molecular orbital diagram in 2a by density functional theory (DFT), the LUMO is located at the pyridinium moiety, whose contribution to the electron-accepting behavior is large (Figure S1). The singly occupied molecular orbital (SOMO) of the radical state of 2a was also calculated to be located mainly on the 4- and 6-positions of the pyridine ring (Figure S2). This suggests that a one-electron-reduced form of 2a may be unstable and reactive to undergoing dimerization, which is consistent with the CV results of 2a exhibiting a small oxidation peak at 0.17 V vs SCE derived from the oxidation of the dimerized product (Figure 2).13,14 Next, the optical properties of pyridinium 2a were studied. The UV−vis spectrum of 2a in a MeCN solution exhibited two absorption bands at around 250 and 350 nm, which is markedly different from the spectrum of neutral precursor 1 (Figure 3).
Figure 1. (a) ORTEP drawing (50% probability for thermal ellipsoids) and (b,c) packing within the crystal of 2a. (d) Electrostatic potential map of 2a calculated by B3LYP/6-31G(d).
torsion angle of two aromatics in N-arylpyridinium is close to 90°;9b however, that of the pyridine ring and the perfluorophenyl moiety in 2a was 20°. The ring-fusing resulted in affording the relatively planar N-arylpyridinium salt. Thus, the pyridinium moiety can participate in the conjugation of the triphenylene backbone and affect its optoelectronic properties. The estimated bond lengths of each ring suggest that aromaticity of the center ring is lower compared to the other three rings, as discussed with the average bond length of other fused ring systems.11 The ring-fused pyridinium moiety in 2a formed a stacking pair with ca. 3.2 Å of each distance between the pyridinium and benzene rings by a cationic−π interaction, as observed in the crystal structures of other ring-fused Narylpyridinium salts.4 The electrostatic potential map of pyridinium 2 indicates the strong electronegativity of the nitrogen cation and thus supports the possible interaction between two molecules as discussed above (Figure 1). The electrochemical properties of pyridinium 2a and its precursor 1 was investigated by cyclic voltammetry in 0.1 M tetrabutylammonium trifluoromethanesulfonate (Bu4NOTf)/ MeCN using a platinum (Pt) working electrode (Figure 2). The cyclic voltammogram of 1 showed a reduction wave with its cathodic peak potential (Epc) at −1.11 V vs SCE. However, it was quasi-reversible to show the corresponding small anodic wave. In sharp contrast, the voltamogram of 2a exhibited an irreversible cathodic wave (Epc = −0.70 V vs SCE). From these data, the lowest unoccupied molecular orbital (LUMO) levels of 1 and 2a were estimated to be −3.29 and −3.70 eV,
Figure 3. UV−vis and PL spectra of 2a and 1 (10−5 M) measured in MeCN.
The structured red-edge absorption pattern is found in the spectra of other ring-fused pyridinium derivatives.4,6 The absorption energy remains in a similar manner regardless of the solvent such as DMSO and dichloromethane (DCM) (Figure S4); therefore, this seems to be related to a vibronic effect. In the case of 2e in DCM, the red-edge band appeared at a longer wavelength region with a higher extinction coefficient (Figure S4). Such specific solvatochromism of pyridinium salts in halogenated solvents was similarly reported by Ooyama et al.15 B
DOI: 10.1021/acs.orglett.7b00590 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters When excited in a MeCN solution, 2a exhibited a strong emission peak at 428 nm (ΦF = 0.59),16 whereas the precursor 1 did not emit at all (Figure 3). The optical properties of 1 and 2a−2e are summarized in Table S1. From comparison of the photoluminescence (PL) behavior of 2a and precursor 1, it was observed that ring-fused pyridinium 2a efficiently emitted avoiding an energy loss by the rotation of the adjacent aromatic rings. Pyridinium 2e containing an iodide ion showed a lower quantum yield, probably due to the influence of the intersystem crossing due to a heavy atom effect.17 Another possible mechanism which can influence the quantum yield is excited state-electron transfer from the pyridinium moiety to the easily oxidizable iodide ion. To investigate the scope of this intramolecular SNAr cyclization, we next examined the cyclization reaction of another derivative 3 possessing two pyridine moieties (Table 2). The first trial under the optimized conditions established in
Figure 4. Cyclic voltammogram of 4a and 3 (1 mM) in 0.1 M Bu4NOTf/MeCN using a Pt working electrode at a scan rate of 100 mV/s.
5). The LUMO diagram calculated by DFT is located on the pyridinium moieties as shown in Figure S7. The PL of 4a
Table 2. Synthesis of Ring-Fused Bispyridinium Salts 4 via Intramolecular SNAr Reaction
entry
X source
solvent
temp (°C)
reaction time (h)
product
yield (%)a
1 2 3 4 5 6 7
TMS-OTf TMS-OTf TMS-OTf − TMS-Cl TMS-Br TMS-I
MeCN DMSO DMSO DMSO DMSO DMSO DMSO
50 50 90 90 90 65 65
24 24 24 24 24 48 48
4a 4a 4a 4b 4c 4d 4e
0b 73b 92 0b 85 84 79
a
Figure 5. UV−vis and PL spectra of 4a and 3 (10−5 M) measured in MeCN.
Isolated yield. bDetermined by 19F NMR.
Table 1 resulted in the complete recovery of the starting material (entry 1). The use of DMSO as a solvent instead of MeCN was effective to give desired product 4a in a good yield (entry 2). When reacted at higher temperature (90 °C), 4a was obtained in 92% yield (entry 3). Without a silyl reagent, the corresponding product (4b) was not obtained due to its instability and high reactivity (entry 4). The other series of 4 with chloride (4c), bromide (4d), and iodide (4e) were successfully obtained in good yields (entries 5−7). The double cyclization proceeded in a regioselective manner, evidenced by the single crystal X-ray analysis of 4a as shown in Figure S5. In the crystal structure of 4a, the staircase-like stacking was observed with sharing one phenylpyridinium moiety for each stack (distance: ca. 3.4 Å) (Figure S5). The cyclic voltammogram of 4a measured in Bu4NOTf/ MeCN showed two one-electron reduction waves (Epc1 = −0.53 V and Epc2 = −1.19 V vs SCE) derived from the reduction of the two pyridinium rings, but they were irreversible (Figure 4). The LUMO level of 4a was estimated to be −3.87 eV, significantly lower than that of 2a, indicating that two pyridinium rings are electronically communicating through the fluoroaromatic center.18 In the UV−vis spectrum of 4a in a MeCN solution, absorption bands appeared at longer wavelength regions compared to those observed in 2a (Figure
showed a strong emission peak at 452 nm (ΦF = 0.62). The optical properties of 4a−4e are summarized in Table S2. In conclusion, we have successfully synthesized ring-fused pyridinium salts with various counteranions via the intramolecular SNAr reaction of a fluoroarene derivative. The TMSpromoted SNAr reaction was effective to obtain ring-fused products in good yields. The optoelectronic properties of the ring-fused pyridinium salts 2 were highly improved compared to precursor 1, showing lower LUMO levels and stronger PL in solutions. Further expansion of the π-system (4) was realized using bispyridine derivative 3 as a precursor for the SNAr reaction, resulted in lowering the LUMO level and tuning the emission wavelength compared to 2. In addition, the remaining fluoroaryl moieties of the products would be modified by sequential SNAr reactions with nucleophiles. Further studies on the synthesis of extended π-systems using the promising intramolecular SNAr reaction are now underway in our group.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b00590. C
DOI: 10.1021/acs.orglett.7b00590 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
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(13) Potentiostatic electrolysis of 2a at − 0.90 V vs SCE for 1 F/mol in 0.1 M Bu4NOTf/MeCN was carried out to trace the fate of its radical form, but a complex mixture was obtained. (14) The anodic scan of 2a under the same conditions did not show an anodic peak around 0.17 V (see Figure S3), indicating that the anodic peak observed in Figure 2 is derived from a reductive coupling product. (15) Ooyama, Y.; Asada, R.; Inoue, S.; Komaguchi, K.; Imae, I.; Harima, Y. New J. Chem. 2009, 33, 2311−2316. (16) The value of ΦF is a relative quantum yield compared to that of quinine sulfate (ΦF = 0.55) as a standard. (17) Zhao, C. F.; Gvishi, R.; Narang, U.; Ruland, G.; Prasad, P. N. J. Phys. Chem. 1996, 100, 4526−4532. (18) When the cathodic sweep of 4a was returned at − 0.6 V vs SCE, an anodic peak derived from a dimerized product was observed similarly to the case of 2a (see Figure S6).
Experimental procedures, analytical data, and copies of the 1H, 13C, and 19F NMR spectra for all new products (1, 2a−2e, 3, and 4a−4e) (PDF) Single-crystal X-ray data for 2a and 4a (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shinsuke Inagi: 0000-0002-9867-1210 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by JSPS KAKENHI Grant Numbers JP26708013, JP15H00724. We thank Prof. Kazuhiro Chiba at Tokyo University of Agriculture and Technology for NMR analysis.
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REFERENCES
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DOI: 10.1021/acs.orglett.7b00590 Org. Lett. XXXX, XXX, XXX−XXX
