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Synthesis, Structures, and Properties of Core-expanded Azacoronene Analog: A Twisted #-system with Two N-doped Heptagons Kosuke Oki, Masayoshi Takase, Shigeki Mori, Akitoshi Shiotari, Yoshiaki Sugimoto, Keishi Ohara, Tetsuo Okujima, and Hidemitsu Uno J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06079 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 1, 2018
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Journal of the American Chemical Society
Synthesis, Structures, and Properties of Core-expanded Azacoronene Analog: A Twisted π-system with Two N-doped Heptagons Kosuke Oki,† Masayoshi Takase,*,† Shigeki Mori,‡ Akitoshi Shiotari,§ Yoshiaki Sugimoto,§ Keishi Ohara,† Tetsuo Okujima,† and Hidemitsu Uno*,† †
Department of Chemistry and Biology, Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan ‡ Advanced Research Support Center, Ehime University, Matsuyama 790-8577, Japan § Department of Advanced Materials Science, The University of Tokyo, Kashiwa 277-8561, Japan Supporting Information Placeholder ABSTRACT: A core-expanded, pyrrole-fused azacoronene analog containing unusual two N-doped heptagons was obtained from commercially available octafluoronaphthalene and 3,4-diethylpyrrole in two steps as a heteroatom-doped non-planar nanographene. Full fusion with the formation of the tetraazadipleiadiene framework and the longitudinally twisted structure were unambiguously confirmed by singlecrystal X-ray diffraction analysis. The edge-to-edge dihedral angle along the acene moiety was 63°. This electron-rich πsystem showed four reversible oxidation peaks. Despite the non-planar structure, the Hückel aromaticity owing to a peripheral π-conjugation in the dicationic state was concluded from the bond-length alternation and nucleus-independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations.
Non-planar polycyclic aromatic hydrocarbons (PAHs) and their heteroatom-doped analogs (polycyclic heteroaromatic molecules, PHAs) have received much attention in nanocar1,2 bon chemistry recently. Nanographenes containing heptagonal rings in particular have inspired scientists because of their conformational changes and the electronic properties that derive from the curved π-systems, leading to the synthe3a sis of a grossly warped nanographene, a dipleiadiene3h 3 embedded aromatic saddle, and other structures. In addition, it is well known that the introduction of heteroatoms into PAH frameworks drastically affects their electronic na1c ture without modifying the structure. Thus, heteroatom doping in such non-planar PAHs with heptagons is strongly desired to develop PHA chemistry for versatile application. For instance, the on-surface synthesis of an N-doped buckybowl with inverse Stone-Thrower-Wales topology was 4 recently studied. However, the chemical synthesis of heteroatom-doped nanographene with heptagonal rings still remains a challenge owing to harsh conditions in building the curved π-systems associated with the formation of heptagonal rings. A more reasonable method to synthesize N-doped nanographene has been developed and it uses a synthetic route
involving an aromatic nucleophilic substitution (SNAr) reac5 6 tion and subsequent oxidative cyclodehydrogenation. With 6b this method, hexapyrrolohexaazacoronene (HPHAC), a pyrrole-based analog of hexa-peri-hexabenzocoronene (HBC), 6c and related HPHAC-HBC hybrid derivatives were synthesized and characterized. These molecules exhibited electrochromism with NIR-absorption in their oxidized states and altered their aromaticity via reversible multistep oxidation. 6d,e Azacoronene analogs with heptagonal rings and radially 6f π-extended HPHAC have been reported in previous studies. In this study, pentagonal, electron-rich pyrrole was used as a key unit to fabricate curved π-systems containing N-doped heptagons instead of benzenes. Herein, we report the rapid synthesis of a core-expanded azacoronene analog 2 that has two heptagons and eight inner nitrogen atoms (Figure 1). This is the first example of a coreexpanded and structurally well-defined alkyl substituted azacoronene analog. Although the benzene-based 3 has not 7 been synthesized, the fully fused structure of 2 was clearly
Figure 1. (a) Hexa-peri-hexabenzocoronene (HBC) and pyrrole-fused azacoronene (HPHAC) systems. (b) Benzene-based core-expanded molecule 3 and related pyrrole-based analog 2 synthesized in this work.
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confirmed using single-crystal X-ray diffraction analysis, and its physical properties were investigated. The compound 2c was synthesized following our previous 6b,c procedure. Thus far, 3,4-diarylpyrroles such as 3,4-di(4trifluoromethylphenyl)pyrrole, have been used to synthesize 6b-f soluble and stable azacoronenes. In this study, 3,4dialkylpyrrole was chosen to reduce the steric hindrance, thereby facilitating the formation of core-expanded analog. As shown in Scheme 1a, SNAr reaction was conducted using 3,4-diethylpyrrole, which afforded 1,2,3,4,5,6,7,8-octakis(3,4diethylpyrrol-1-yl)naphthalene (OPN, 1c) as a yellow powder with an 80% yield. Similar to previous report on octapyrrol5 ylnaphthalene, the naphthalene skeleton was slightly distorted owing to the steric hindrance between peripheral pyrrolyl groups (Figure S4 in Supporting Information (SI)). The oxidative cyclodehydrogenation of 1c with 2,3-dichloro-5,6dicyano-p-benzoquinone (DDQ) and trifluoromethanesulfonic acid (TfOH) at 0 °C and subsequent treatment with hydrazine resulted in fully fused OPN 2c as a dark brown powder with good yields (79%). Despite the high HOMO level (-3.80 eV, Figure 4b), 2c was successfully isolated as a neutral compound exhibiting high solubility in common organic solvents including hexane (at least 10 mg/mL). Ow1 ing to the D2-symmetric structure in the solution, the H- 13 and C-NMR spectra of 2c showed simple signals in CDCl3 in the presence of hydrazine and D2O to prevent the formation of trace amounts of radical species (Figure S2 in SI).
Scheme 1. (a) Synthesis of naphthalene-based core-expanded azacoronene analog 2 and (2) possible resonance structures of its oxidized species.
Figure 2. (a) ORTEP drawing of 2c with thermal ellipsoids at 50% probability level; Contributions of the disordered solvent molecules have been removed from the diffraction data using SQUEEZE from PLATON software. Disordered atoms and hydrogen atoms are omitted for clarity. (b) Torsion angles of C18-C19-C34-C35 (top) and 9 C42-C11-C26-C27 (bottom). Ethyl groups are omitted for clarity. (c, d) Packing structures of 2c as a space-filing model (P and M isomers are colored in gray and blue, respectively).
Figure 3. Twist-to-twist inversion behavior between twisted (P) and (M) of 2a. The optimized structure and the relative energy values were calculated at the B3LYP/631G(d) level of theory. 3h,8
Single crystals suitable for X-ray crystallography were obtained through vapor diffusion of CH3CN into an ethyl acetate solution of 2c. The crystal of 2c was determined to be P-1 (#2) and contains two crystallographically independent molecules with slightly different skeletons (Figures S5,S7, Table S1 in SI). X-ray diffraction analysis revealed the full fusion between neighboring pyrroles with the formation of an N-
doped dipleiadiene framework and its D2-symmetric twisted structure (one of the two independent structures is shown in Figure 2a). The selected edge-to-edge averaged torsion angles are 63.3° (C18-C19-C34-C35, Figure 2b (top)) 9 and 83.2° (C42-C11-C26-C27, Figure 2b (bottom)). Enantiomers of 2c exist in the packing structures and they were alternately stacked along the c axis and were aligned on the ab 10 plane (Figure 2c,d and Figure S5 in SI). Owing to the twisted structure and peripheral ethyl groups, no π-π interaction between molecules was observed in the crystal state. This explains the high solubility of 2c. To understand the dynamic behavior between the enantiomers, a density functional theory (DFT) calculation was carried out at the B3LYP/6-31G(d)
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Journal of the American Chemical Society level of theory. This DFT study revealed a saddle structure in the transition state (TS) and the inversion barrier was esti-1 mated as 5.9 kcal·mol , which is smaller than the calculated -1 3a bowl inversion barriers for corannulene (10.4 kcal·mol ), -1 2b benzene-fused azacorannulene (15.3 kcal·mol ), and su-1 11 manene (16.3 kcal·mol ) (at B3LYP/6-31G(d)), and greater than the inversion barrier for a recent example of negatively -1 3h curved saddle (1.8 kcal·mol ) (at B3LYP/6-31G) (Figure 3).
The chemical oxidation of 2c with incremental addition of AgPF6 in the CH2Cl2 solution revealed the stepwise formation of cationic species up to trication with drastic changes in absorption in the near-infrared region (Figure 4c and Figures 12 S10-11 in SI). Electron spin resonance (ESR) measurements show broad but flat signals in solutions with mono- and trications (Figure S12 in SI).
Voltammetric experiments were performed to investigate the electrochemical properties of 2c. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measuremen+ ts of 2c (CH2Cl2, rt, Fc/Fc ) revealed four reversible oneox1 electron oxidation steps with the half-wave potentials of E ox2 ox3 ox4 = -0.52 V, E = -0.32 V, E = 0.33 V, and E = 0.60 V (Figure S9 in SI). Similar to the previously reported case of 6b,c ox3 ox2 HPHAC, the difference between E and E (0.65 V) is ox2 ox1 significantly larger than that between E - E (0.20 V) and ox4 ox3 2+ E - E (0.27 V), which implies that dication 2c is stabilized (Scheme 1b). To understand the electronic nature of 2c and its cationic species, their photophysical properties were investigated. As shown in Figure 4a, the absorption spectrum of 2c displays two major peaks at 292 and 338 nm along with weak absorption ranging from 500 to 660 nm. As an indicator of the core expansion and possibly, distortion from planarity, a red shift in the lowest energy transition was clearly observed com6b,c pared to the previous case of HPHAC. Time-dependent DFT (TD-DFT) calculations on this region at the B3LYP/631G+(d) level support their forbidden HOMO → LUMO transition (656.6 nm) and HOMO → LUMO+1 (571.1 nm) transition with a low oscillator strength (f = 0.0280 and 0.0560, respectively) (Figure 4a). Fused OPN 2c exhibits negligible fluorescence in the CH2Cl2 solution.
2+
-
Figure 5. (a) ORTEP drawing of 2c [PF6 ]2 with thermal ellipsoids at 50% probability level. Solvent molecules and disordered atoms are omitted for clarity. (b) Bond and ring labeling for 2c. NICS(0) values in the neutral (black) and dication (blue) states were calculated at the GIAO/HF/6-311+G(d,p)//B3LYP/6-31G(d) level of theory.
Table 1. Selected bond lengths of neutral and dicationic species in the crystal (2c) and optimized (2a) structures
a
2+
Averaged bond values of 2c and 2c are shown with standard deviations calculated using the following equa2 1/2 b 2+ tion: {Σ(xi-) /(n-1)} . Bond lengthes of 2b and 2b calculated at the B3LYP/6-31G(d) level of theory. 2+
Figure 4. (a) UV-vis absorption spectrum of 2c in CH2Cl2 along with the oscillator strengths of 2b (black bars) obtained by TD-DFT calculations at the B3LYP/6-31+G(d) level of theory. (b) Frontier molecular orbitals of 2b calculated at the B3LYP/6-31G(d) level of theory (Isovalue = •+ 2+ 0.02) (c) UV-vis-NIR spectra of 2c (green), 2c (red), •3+ and 2c (blue) obtained by titration of AgPF6. Concen-6 -5 tration of solution: [2c] = 7.5×10 M (for spectra), 7.5×10 M (for picture).
Dication 2c was prepared with AgPF6 to investigate the cationic species. The slow evaporation of the ethyl acetate 2+ 2+ solution of 2c (2c [PF6 ]2) resulted in single crystals. These crystals were stable at room temperature under air for several 2+ days. The molecular structure of 2c was unambiguously determined through X-ray diffraction analysis (Figure 5a). 2+ The space group of 2c is Pc (#7) and two crystallographically independent molecules exist in asymmetric unit cell. The 2+ averaged torsion angles of 2c (C18-C19-C34-C35: 49.2°, C42C11-C26-C27: 73.7°) are smaller than those in the neutral 9 state. The averaged bond-lengths for the pyrrole units of 2+ neutral 2c and dication 2c are summarized in Table 1. Based on the short lengths of Cα-Cβ and the long lengths of Cα-Cα and Cβ-Cβ in the neutral state, neutral 2c exhibits distinct bond-length alternations (BLA) in the pyrrole units. However,
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in the case of dication 2c , the averaged Cα-Cβ bonds are elongated, while the averaged Cα-Cα and Cβ-Cβ bonds are shortened. These results are in good agreement with the optimized structures predicted via DFT calculations. Harmonic oscillator model of aromaticity (HOMA) values also 2+ support decrease in BLA of 2c compared to those of 2c (Table S2 in SI). Then, nucleus-independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations were carried out to determine the aromaticity of 2+ 2 and 2 (Table S3 and Figure S8 in SI). As shown in Figure 5b, the NICS(0) values of ring B, D, and E were positive, 14 while those of A, C, and F were negative in the neutral state. 2+ However, all the values are negative for 2a , indicating an increase in aromaticity in the dicationic state. In addition, the ACID plot clearly demonstrates an amplified current 2+ density of the peripheral pathway for 2a (Figure S8 in SI). These results represent overall Hückel aromaticity through 14 peripheral 30π {(4n+2)π} conjugation in the dicationic state. In summary, a core-expanded, non-planar, pyrrole-fused azacoronene with N-doped heptagonal rings was synthesized in just two steps. The distorted structures of both neutral 2c 2+ and dication 2c [PF6 ]2 were unambiguously determined using X-ray diffraction analysis. In spite of the twisted structure, the overall aromaticity of the N-doped nanographene 2+ was clearly determined for 2c . The synthesis strategy described herein demonstrates the potential use in chemical synthesis of highly expanded and distorted N-doped nanographene by using large PAHs as a core. Such approaches are currently underway in our laboratory.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental procedures and spectroscopic data, results of theoretical calculations and crystallographic data (PDF) 2+ Crystallographic data for 1c, 2c, and 2c (CIF)
AUTHOR INFORMATION Corresponding Author *
[email protected] *
[email protected] ORCID Masayoshi Takase: 0000-0002-9737-9779 Shigeki Mori: 0000-0001-6731-2357 Akitoshi Shiotari: 0000-0002-8059-3752 Tetsuo Okujima: 0000-0002-6552-2606 Hidemitsu Uno: 0000-0003-3597-9434
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This work was supported by Grant-in-Aid for Scientific Research on Innovative Areas “Molecular Architectonics: Orchestration of Single Molecules for Novel Functions” (JP25110003 and JP16H00959) and for Scientific Research (C) (JP16K05698). We thank Dr. Yasushi Honda (HPC systems, Inc.) for the assistance with Reaction plus Pro in TS calculations.
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Journal of the American Chemical Society (14) The DFT calculations suggest antiaromatic nature for tetracation. For details, see Tables S2-3 and Figure S8 in SI.
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Scheme 1. (a) Synthesis of naphthalene-based core-expanded azacoronene analog 2 and (2) possible resonance structures of its oxidized species. 83x95mm (300 x 300 DPI)
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