Tetraaza[14]- and Octaaza[18]paracyclophane: Synthesis and

Article pubs.acs.org/joc

Tetraaza[14]- and Octaaza[18]paracyclophane: Synthesis and Characterization of Their Neutral and Cationic States Daisuke Sakamaki,* Akihiro Ito,* Yusuke Tsutsui, and Shu Seki* Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan S Supporting Information *

ABSTRACT: Two kinds of aza[1n]paracyclophanes, tetraaza[14]paracyclophane (P4) and octaaza[18]paracyclophane (P8), were synthesized as the smallest and the largest monodisperse macrocyclic oligomers of polyaniline ever made. Herein we report the electronic nature of the cationic species of these two macrocycles with different ring size. By combining ESR spectroscopy and DFT calculations it was suggested that P4·+ was classified as delocalized class III or poised on the class II/III borderline while P8·+ was regarded as a localized class II mixed-valence system. We successfully isolated the dication of P4 as a stable dicationic salt P42+· 2[SbF6]−, and the structure of P42+ was determined by X-ray crystal analysis. Variable-temperature NMR measurements for P42+·2[SbF6]− unequivocally showed that P42+ was a 22π electron system with a singlet ground state. The supercharged hexacation of P8 was also isolated as P86+·6[SbCl6]−, and X-ray crystal analysis revealed that P86+ includes one SbCl6− anion in its macrocyclic cavity.

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INTRODUCTION Conjugated macrocycles with well-defined structures have been of special interest for a long time,1−4 because they are linked to many important concepts in chemistry, such as aromaticity in extended π-systems,5,6 molecular recognition,7,8 and dynamics of charge or energy in the circularly confined systems as artificial models of photosynthesis.9,10 Recently, the chemistry of cyclophenylenes11,12 has attracted renewed attention triggered by the syntheses of cyclo-para-phenylenes (CPPs) in the 2000s13−15 as scaffolds for carbon-based materials. Hetera[1n]cyclophanes, which are heteroatom-bridged cyclophenylenes,16,17 are also a fascinating class of macrocyclic compounds not only due to their host−guest properties similar to methylene-bridged [1n]cyclophanes18,19 but also from their various electronic properties depending on the nature of the bridging heteroatoms. Among the hetera[1n]cyclophanes, macrocyclic oligoarylamines, i.e., aza[1n]cyclophanes, are of interest because of their multiredox activity derived from the introduction of electron-rich amino groups.20−22 In the last two decades, various kinds of aza[1n]cyclophanes have been reported23−35 associated with the rapid progress of the palladium-catalyzed aryl amination reaction (Buchwald− Hartwig reaction).36,37 In particular, aza[1n]paracyclophanes, which could be regarded as monodisperse macrocyclic oligomers of polyaniline, are a special class of model systems to study the behavior of charges and spins in the circularly confined polyanilines (Figure 1). However, there have been no studies on aza[1n]paracyclophanes other than hexaaza[16]paracyclophanes.31,32 P6 was proved to be a very good electron donor with the first oxidation potential of −0.28 V (vs © 2017 American Chemical Society

Figure 1. Structures of N-anisyl-substituted aza[1n]paracyclophanes (n = 4, 6, 8).

ferrocene/ferrocenium), and the generated radical spin by oneelectron oxidation was equally distributed over the six nitrogen atoms on the ESR time scale.31 From a geometric perspective, hexaaza[16]paracyclophanes have geometrically most relaxed structures. Meanwhile, the other aza[1n]paracyclophanes (n ≠ 6) are also intriguing compounds because of the expected strained structures resulting from geometrical demands. In particular, aza[1n]paracyclophanes with a smaller ring size (n ≤ 5) are challenging synthetic targets, and in this context, we aimed at the synthesis of a tetraaza[14]paracyclophane with one anisyl group on every nitrogen atom (P4). Although the various tetraaza[14]cyclophanes with different substitution pattern, i.e., m,m,m,m,23−25 m,p,m,p,26−28 o,m,o,m,29 o,p,o,p,30 m,m,m,p,29 and m,m,m,o,29 have already been synthesized, there has been no report of the successful synthesis of tetraaza[14 ]Received: September 26, 2017 Published: November 15, 2017 13348

DOI: 10.1021/acs.joc.7b02437 J. Org. Chem. 2017, 82, 13348−13358

Article

The Journal of Organic Chemistry Scheme 1. Synthetic Route for P4 and P8

paracyclophanes because of its expected largest strain energy among tetraaza[14]cyclophane families, and therefore, the investigation of their structural and electronic nature is still a fascinating topic as an analogue of the smallest cyclic oligoaniline. Furthermore, we also decided to synthesize a larger octaaza[18]paracyclophane (P8) as the largest aza[1n]paracyclophane at present, because of interest in its expected conformational flexibility and possible application as a redoxactive host molecule.

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RESULTS AND DISCUSSION

Synthesis. Toward a synthesis of P4 and P8, we chose a one-pot Buchwald−Hartwig coupling reaction between dibromo-substituted monoamine (3) and a triamine 2 (Scheme 1). After the coupling reaction between 2 and 3, insoluble polymeric products were removed by filtration. The filtrate was evaporated and then purified by conventional silica gel column chromatography, and then P4 and P8 were successfully obtained in yields of 0.3% and 10%, respectively. The low yield of P4 could be attributed to the expected ring strain due to the small cyclic structure. The structures of P4 and P8 were fully characterized by 1H and 13C NMR and high-resolution mass spectrometry (HRMS). Both compounds showed only one singlet 1H signal and two 13C signals for the protons and carbons in the macrocyclic moiety, showing their highly symmetrical aza[1n]paracyclophane structures unequivocally (Figures S3−8). P4 and P8 showed good solubility in common organic solvents such as dichloromethane, toluene, and tetrahydrofuran, in contrast to the limited solubility of P6 in these solvents. Crystal Structure of P4. Single crystals of P4 suitable for X-ray crystal analysis were obtained, and the structure of P4 was clearly determined as shown in Figure 2. This is the first example of X-ray structure analysis of aza[1n]paracyclophanes. The crystals of P4 belong to the monoclinic space group P21/c. The macrocyclic skeleton of P4 takes quasi-D2d symmetry, and the four nitrogen atoms form the edge of a parallelogram close to a square. As is often observed in smaller cyclophanes such as [2.2]paracyclophane, the four phenyl rings in the macrocyclic

Figure 2. X-ray structure of P4. Hydrogen atoms are omitted for clarity. For the two side views, hydrogen atoms and anisyl groups are omitted for clarity. Thermal ellipsoids are set at 50% probability.

moiety are slightly bent. However, the average bending angle is only 5.4°, which is smaller than the value of [3.3]paracyclophane of 6.4°.38 On the other hand, the four C− N−C angles in the macrocyclic structure were reduced considerably to 111.5−113.4° from the typical C−N−C angle of aromatic amines (about 120°), suggesting that the ring strain is compensated by decreasing these C−N−C angles. To evaluate a degree of quinoidal deformation of each benzene ring in the macrocyclic structure, we introduced the bondlength-alternation (BLA) parameter (see SI). The values of BLA were close to zero for all four rings in the macrocyclic structure. Each benzene ring in the macrocyclic structure is tilted toward the plane formed by the four nitrogen atoms (averaged tilt angle = 48°) in the crystal state. These rings are presumed to rotate or flip rapidly in solution according to the results of the NMR measurements. Single crystals of neutral P8 suitable for X-ray crystal analysis could not be obtained due to its amorphous nature. 13349


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Figure 3. Relative energy levels of the frontier Kohn−Sham molecular orbitals for P4′, P6′, and P8′ at the B3LYP/6-31G* level. Blue arrows represents the major transitions expected by TD-DFT calculations at the B3LYP/6-31G* level.

DFT Calculations. In order to investigate the electronic structures of P4 and P8, we performed the DFT calculations at the B3LYP/6-31G* level on the model compounds P4′ and P8′, in which all methoxy groups were replaced by hydrogen atoms. All-N-phenyl-substituted hexaaza[16]paracyclophane (P6′) was also calculated as a reference. In structural optimization of azacyclophanes, all optimized geometries are confirmed as respective energy minima by frequency analysis. The optimized structure of P4′ had D2d symmetry, being in good accordance with the crystal structure (Figure S22). The optimized structure of P8′ had a tub-shaped geometry with D2d symmetry in analogy with cyclooctatetraene (Figure S23). The Kohn−Sham MOs near the frontier levels are shown in Figure 3. The energy levels of HOMO and LUMO of P8′ were almost the same as those of P6′, and both the HOMO and the LUMO of P4′ were lowered by about 0.4 eV compared with those of P6′ and P8′. Consequently, the HOMO−LUMO gaps of these three azacyclophanes were about the same values. The lowering of the energies of the frontier MOs in P4′ could be explained by its strained structure; the lowering of the HOMO could be attributed to the tilted benzene rings in the macrocyclic structure toward the molecular plane (51°), which decrease effective conjugation among the benzene rings and the amino nitrogen, and the lowering of the LUMO could be attributed to the decreased aromaticity of the slightly bent benzene rings in the macrocyclic structure similar to the case of smaller CPPs.39 In all these compounds, the HOMO−LUMO (S1 ← S0) transitions were symmetry forbidden in the most stable geometries. Photophysical Properties. The results of UV−vis absorption and fluorescence spectra are given in Figure 4 for P4, P6, and P8 in toluene at room temperature as well as the photophysical parameters summarized in Table 1. The absorption and fluorescence spectral shapes of P8 were very similar to those of P6. In striking contrast to P8, P4 exhibited

Figure 4. (a) Absorption and emission spectra of P4 (light blue), P6 (blue), and P8 (green) in toluene at room temperature (λex = 300 nm). (b) Photograph of solutions of P4, P6, and P8 in toluene under UV light (365 nm).

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case for smaller CPPs.41,42 Furthermore, P4, P6, and P8 showed bright phosphorescence with long lifetimes enough to observe delayed luminescences by the naked eye in 2methyltetrahydrofuran glass at 77 K (see SI). The phosphorescence emission maxima (λP) were blue shifted with increasing ring size similar to the fluorescence spectra (Table 1 and Figure S19). Electrochemistry. The redox properties of P4 and P8 were evaluated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in dichloromethane containing 0.1 M nBu4NBF4 as supporting electrolyte at room temperature. As shown in Figure 5a, P4 showed two reversible oxidation steps followed by an irreversible one. Judging from the peak current of the DPV (Figure 5c), these three oxidation steps could be attributed to the three successive one-electron oxidation processes. The irreversibility of the third oxidation and the absence of the fourth oxidation suggest the instability of the triand tetracationic states of P4. This is in contrast to all-N-anisylsubstituted tetraaza[14]m,p,m,p-cyclophane showing four reversible one-electron oxidation steps.26,27 This is due to the difficulty in forming the quinoidal phenylenediamine (PD) structures in the higher oxidation states of P4 due to the sterical demand. As shown in Figure 5b and 5d, P8 showed six successive one-electron oxidation steps followed by one simultaneous two-electron oxidation step, showing that P8 can be oxidized up to the octacation. All redox waves were unchanged upon multisweep scans. The oxidation potentials [Eox vs Fc0/+] of P4 and P8 and of previously reported P631 are summarized in Table 2. The first oxidation potential of P4 was higher than that of P6 by 0.20 V, whereas the first oxidation potential of P8 was almost the same as that of P6. The results of the electrochemical measurements and the DFT calculations

Table 1. Photophysical Parameters of P4, P6, and P8 upon Photoexcitation at λex = 300 nm

P4 P6 P8

λabsa (nm)

λFa (nm)

ΦFa

Stokes shifta (cm−1)

λPb (nm)

ΦPb

324 348 348

482 434 427

0.17 0.07 0.07

10 117 5694 5316

511 487 476

0.80 0.76 0.94

a

In toluene solutions at room temperature. bIn glassy matrices of 2methyltetrahydrofuran at 77 K.

blue-shifted absorption by 2129 cm−1 and red-shifted emission by 2672 cm−1. The Stokes shift of P4 (10117 cm−1) was remarkably larger than those of P6 and P8 (5694 and 5316 cm−1). Furthermore, the fluorescence quantum efficiency of P4 was higher (0.17) than that of P6 and P8 (0.07), which was clearly identified by the naked eye as shown in Figure 4b; whereas P6 and P8 exhibit weak blue emission, P4 exhibits bright sky-blue emission in toluene solutions. In general, oligoarylamines without explicit fluorophores are nonemissive or very weakly emissive compounds showing shorter wavelength emissions in the visible region (purple or blue), and hence, the bright and relatively low-energy emission of P4 was quite unusual. To address the excited state dynamics of P4, P6, and P8, the fluorescent lifetime (τF) was measured in toluene at room temperature (Figure S18). All of these fluorescence decay profiles were well fitted by single-exponential functions. The fluorescent lifetime of P4 (8.2 ns) was significantly longer than those of P6 (1.3 ns) and P8 (2.0 ns). The trend of the ring size dependence of the fluorescence properties was similar to that of CPPs,40,41 and the reason for the red-shifted emission of P4 could also be attributed to the large structural deformation in the excited state, allowing emission from the S1 state as is the

Figure 5. Cyclic voltammograms of (a) P4 and (b) P8 and differential pulse voltammograms of (c) P4 and (d) P8 measured in CH2Cl2 containing 0.1 M nBu4NBF4 at 298 K. 13351


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The Journal of Organic Chemistry Table 2. Oxidation Potentials (V versus Fc0/+) of P4, P6, and P8 Determined by Measurements and Digital Simulations of DPV43 in CH2Cl2 (0.1 M nBu4NBF4) E1 P4 P6 P8 a

−0.08 −0.28 −0.30

E2 0.08 −0.17 −0.20

E3 0.69 0.20 −0.01

E4

E5

E6

E7

0.45 0.16

0.72b 0.37

0.53

0.76b

grew with an isosbestic point at 0.97 eV (1277 nm) (Figure 6b). On oxidation of neutral P8 to P8·+, a low-energy band at 0.80 eV (1550 nm) and a band at 2.75 eV (451 nm) were enhanced with the reduction of a band at 3.59 eV (345 nm, Figure 7a). The oxidation to dication of P8 resulted in a hypsochromic shift of the low-energy band to 0.90 eV (1377 nm) (Figure 7b). On further oxidation from P82+, this lowest energy band continued to grow with a further hypsochromic shift and reached a maximum at 1.05 eV (1180 nm) (Figure 7c−e). The trend of the spectral change during the electrochemical oxidation of P8 was similar to that of octaaza[1 8 ] m,p,p,p,m,p,p,p-cyclophane.34 In the higher oxidized states of both P4 and P8, no apparent absorption bands corresponding to the dicationic quinoidal PD structures (about 1.6 eV)46 were observed similar to the cases of oligoarylamines having consecutive para-PD moieties.31,34,44 ESR Spectroscopy of P4·+ and P8·+. The ESR spectra of P4·+ and P8·+ were measured in CH2Cl2 solutions to investigate the spin distributions in the macrocyclic structures of P4 and P8 in their monocationic states. The radical cations were generated by chemical oxidation by 0.5 equiv of tris(4bromophenyl)aminium hexachloroantimonate (Magic Blue)47 in order to generate the monocations selectively. As shown in Figure 8a, P4·+ showed a spectrum with a multiplet hyperfine structure. This spectrum was reproduced by digital simulation considering the presence of four equivalent nitrogen nuclei with a hyperfine coupling constant of |aN| = 0.290 mT. For P8·+, the observed ESR spectrum was broad and structureless due to the hyperfine interactions from a number of nuclei (Figure 8b). The observed spectrum was tentatively simulated by assuming the existence of eight equivalent nitrogen nuclei with a hyperfine coupling constant of |aN| = 0.145 mT, which is one-half of the value used for P4·+. The unchanged spectral shapes over the temperature range from 213 to 293 K suggest that the spin is equally delocalized over the eight nitrogen atoms within the ESR time scale. Theoretical Calculations of P4·+ and P8·+. The structural and electronic properties of P4·+ and P8·+ were further studied by theoretical calculations. The structural optimization at the B3LYP/6-31G* level gave D2d symmetrical structures with the fully delocalized spin and charge distributions for both of P4·+ and P8·+ (Figures S24 and S28). However, the lowest electronic transition energies calculated by the TD-DFT method (0.97 eV for P4·+ and 0.47 eV for P8·+) were poorly matched with the experimental values (0.87 eV for P4·+ and 0.80 eV for P8·+), particularly for P8·+ (Figures S27 and S31). Recently, Kaupp and co-workers reported that the calculation with the B3LYP functional tends to overestimate the degree of charge and spin delocalization in organic mixed-valence systems.48 According to their results, we employed the calculation condition recommended by Kaupp and co-workers using the BLYP-based hybrid functional with 35% exact exchange and the SVP basis sets and the CPCM method49 to include solvent effect of dichloromethane. The structure optimizations of both P4·+ and P8·+ were started from the two different starting geometries, symmetrical (D2d) and asymmetrical (C1) structures. As a result, the C1 structures with asymmetrical charge and spin distributions were predicted to be more stable than the symmetrical structures for both P4·+ and P8·+ (Figures S26 and S30). The energy differences between the asymmetrical and the symmetrical structures, which corresponds to the energy barrier to the intramolecular charge hopping, were 0.27 and 8.95 kJ

a

b

Irreversible oxidation process. Quasi-two-electron transfer process.

(Figure 3) confirmed the quantum chemically derived deeper HOMO level of P4 resulting from the strained structure. Spectroelectrochemistry. To obtain information on the electronic structures of oxidized states of P4 and P8, we measured the UV−vis−NIR absorption spectra in CH2Cl2 solutions during the stepwise electrochemical oxidation using an optically transparent mesh electrode. The spectral change from the neutral P4 to P4·+ is shown in Figure 6a. As the

Figure 6. UV−vis−NIR spectra of the stepwise electrochemical oxidation of (a) P4 to P4·+ and (b) P4·+ to P42+ in CH2Cl2 with 0.1 M nBu4NBF4 at 298 K.

oxidation of neutral P4 proceeds, the absorption at 3.83 eV (324 nm), which corresponds to an absorption band of neutral triarylamines, decreased and new bands appeared at 0.87 (1425 nm), 2.34 (529 nm), 2.89 (429 nm), and 3.24 eV (383 nm) with an isosbestic point at 3.44 eV (363 nm). The lowest energy absorption band in the NIR region is characteristic to the monocation of the oligoarylamine with a consecutive PD skeleton.44,45 Upon further oxidation to the dicationic state, the intensity of the lowest energy band at 0.87 V gradually decreased and a new absorption band at 1.19 eV (1042 nm) 13352


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Figure 7. UV−vis−NIR spectra of the stepwise electrochemical oxidation of (a) P8 to P8·+, (b) P8·+ to P82+, (c) P82+ to P83+, and (d) P83+ to P84+, and (e) further oxidation from P84+ in CH2Cl2 with 0.1 M nBu4NBF4 at 298 K.

mol−1 for P4·+ and P8·+, respectively. This result suggests that the intramolecular charge/spin transport in P4·+ is nearly barrierless, while that in P8·+ requires a larger activation energy. The calculated lowest electronic transition energies of P4·+ and P8·+ were improved (0.90 eV for P4·+ and 0.72 eV for P8·+) by using the Kaupp’s calculation condition and the resulting asymmetrical structures (Figures S27 and S31). While the EPR data did not allow us to clearly distinguish whether the observed spin density distribution is static or dynamic, the theoretical consideration suggests that P4·+ is a delocalized class III mixed-valence (MV) compound or has class II/III borderline behavior, while P8·+ is a localized class II MV compound.50−54 Structures and Electronic Structures of Polycationic States. Encouraged by the multiredox activity of P4 and P8, we tried to isolate the polycations of P4 and P8 as crystalline salts. Single crystals of the dication salt of P4 were successfully obtained by slow diffusion of ethyl acetate into a solution of P42+·2[SbF6]− in CH2Cl2, which was prepared by chemical

oxidation of P4 using 2 equiv of AgSbF6. The crystals of P42+· 2[SbF6]− belong to the monoclinic space group P21/m. Figure 9 shows the crystal structure of the dication of P4 and the packing structure. In the crystal, one P42+ molecule is sandwiched by two SbF6− anions as shown in Figure 9c. In the dication of P4, the configuration of the four nitrogen atoms became closer to a square than that in the crystal structure of neutral P4. Although the standard deviations of the bond lengths in the macrocyclic structures are larger, it can be safely said that there is no apparent quinoidal deformation of PD units in the macrocycle of P42+. Judging from this crystal structure, the two positive charges are thought to be shared among the four nitrogen atoms and four benzene rings in the macrocyclic skeleton. These four benzene rings are still tilted toward the plane of the four nitrogen atoms with an average angle of 41°, also indicating the absence of planar quinoidal PD units. The oxidation of P4 with Magic Blue did not give good crystals suitable for X-ray analysis probably due to the larger size of the SbCl6− anion for P4. 13353


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other hand, five singlet signals gradually grew with decreasing temperature, suggesting the singlet ground state of P42+. In the spectrum at 183 K, considering that the peak at 3.98 ppm corresponds to the protons of four methoxy groups (12H), the remaining four peaks were attributed to the four sets of protons on phenylene units (each 8H). The existence of four peaks of phenylene protons indicates that the inner and outer protons in the macrocyclic moiety were observed as two different peaks. Interestingly, one peak of phenylene protons is abnormally shifted to higher field (1.31 ppm). This peak could be ascribed to the inner protons of the macrocycle, and the very large highfield shift of this peak suggests the existence of a diatropic ring current in the macrocyclic structure of P42+ as a 22π electron system. The DFT-calculated 1H NMR chemical shifts at the B3LYP/6-31G* level support the observed NMR spectrum; the chemical shift of the inner protons were predicted to be 0.68 ppm, whereas the outer peak was predicted to be at 8.34 ppm (Table S5). The solution of P42+·2[SbF6]− in dichloromethane showed a weak ESR signal at 293 K, and the signal intensity decreased upon cooling, also showing its singlet ground state (Figure S20). These observations indicate that P42+ can be regarded as a rare example of charge-delocalized square-type doubly charged mixed-valence systems.55 Judging from the results of electrochemistry, P8 can undergo different seven-step multicharged states, and therefore, we tried to isolate the single crystals of cationic P8 by varying the equivalents of oxidant (Magic Blue). Surprisingly, only the hexacation salt P86+·6[SbCl6]− was obtained as single crystals belonging to the tetragonal space group I4̅ suitable for X-ray analysis (Figure 11). To the best of our knowledge, this is the first example of the supercharged cation salt of oligoarylamines whose structure was elucidated by X-ray crystal analysis. The single crystals of P86+·6[SbCl6]− were also obtained even in the batch treated with four equivalents of oxidant, suggesting the charge disproportionation in the solution and the favored packing structure of P86+·6[SbCl6]−. We found some kinds of crystals having different shape and cell parameters in the same batch probably due to the charge disproportionation, but the quality of the crystals other than P86+·6[SbCl6]− was too low for X-ray crystal analysis. In a unit cell of the crystal, there exists one crystallographically independent P86+ molecule and three kinds of SbCl6− anions (Figure 11c). The macrocyclic skeleton of P86+ takes a tub-shaped geometry close to D2d symmetry. Interestingly, one SbCl6− anion is encapsulated inside the macrocyclic cavity of P86+. The other anions exist in the external spaces surrounded by P86+ cations. The encapsulated SbCl6− anion in the cavity of P86+ is disordered in two equivalent sites near the center of P86+ in a 1:1 ratio. The space-filling representation shows that the size and shape of the cavity inside P86+ fit closely with the SbCl6− anion (Figure 11b). This result exhibits the availability of azacyclophanes with larger ring size as redox-active host materials, which can accommodate counteranions within their cavity.56−59 We carried out 1H NMR and ESR measurement using the powder sample of P82+·2[SbCl6]− obtained by reprecipitation of the solution of P8 treated with 2 equiv of Magic Blue because the crystallization of P82+·2[SbCl6]− was unsuccessful. The 1H NMR spectra showed no signals from 293 to 183 K, showing the existence of paramagnetic species even at 183 K. The CH2Cl2 solution of P82+ showed a clear ESR signal with no hyperfine structures, and the signal intensity decreased with decreasing temperature (Figure S21). These results suggest that P82+ also has a singlet ground state but the strength of the

Figure 8. ESR spectrum of (a) P4·+ and (b) P8·+ in CH2Cl2 at 293 K. Simulated spectra are drawn with a broken line.

Figure 9. (a) ORTEP representation of P42+·2[SbF6]−. (b and c) Unit cell of the crystal of P42+·2[SbF6]−. Hydrogen atoms and solvent molecules are omitted for clarity. Thermal ellipsoids are set at 50% probability.

Following the success of the crystal analysis of P42+, we measured the variable-temperature (VT) 1H NMR spectra of P42+ using the solution of the obtained crystalline sample. The solution of P42+·2[SbF6]− in dichloromethane-d6 showed no signals above 273 K, showing the contribution of the paramagnetic diradical state of P42+ (Figure 10). On the 13354


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Figure 10. Variable-temperature 1H NMR spectra of P42+·2[SbF6]− in dichloromethane-d6. Peak marked with an asterisk is the solvent residual peak.

the highly symmetric structure of neutral P4. From the electrochemical measurements, it was shown that P4 and P8 could be reversibly oxidized up to the dication and the octacation, respectively. Photophysical properties of P4 were substantially different from those of P8 and other aromatic amines, probably reflecting the large structural relaxation in the excited state of P4 due to the ring strain derived from the small ring size. The ESR measurements for P4 ·+ and P8 ·+ demonstrated the full delocalization of the generated spins over the entire macrocycles on the ESR time scale. However, the DFT computations suggested that P4·+ is classified into delocalized class III or poised on the class II/III borderline, while P8·+ is regarded as localized class II. The dication of P4 and the supercharged hexacation of P8 were isolated as stable salts, and their structures were confirmed by X-ray crystallography. The 1H NMR measurements of P42+ at low temperature exhibited the existence of a diatropic ring current in the macrocyclic circuit of P42+ as a 22π electron system. In the single crystal of P86+·6[SbCl6]−, P86+ encapsulated one SbCl6− anion in its macrocyclic cavity, and this is the first example of the azacyclophane which was proved to include its counteranion in the oxidized state. This result opens the way toward the application of azacyclophanes as redox-active host materials whose encapsulation/release behavior is controlled by a redox stimulus. More importantly, because of the capacity for supercharging of P8, aza[1n]paracyclophanes with large ring sizes may serve as charge storage as a cathode of rechargeable

antiferromagnetic interaction between the two radical spins is weaker than in P42+. The relative energies of the different spin states of P42+ and P82+ were evaluated by DFT calculations for the model compounds P4′2+ and P8′2+ at the (U)B3LYP/6-31G* level. For P4′2+, the closed-shell singlet state was predicted to be the most stable state, which is lower than the triplet state by 21.1 kJ mol−1. The calculation with the broken-symmetry (BS) method60 for P4′2+ converged to the same result as the spinrestricted calculation, suggesting the closed-shell ground state of P4′2+. On the other hand, for P8′2+, the open-shell singlet state calculated with the BS method was estimated to be the ground state, which was lower than the triplet state by only 6.4 kJ mol−1. The calculated spin density map of the open-shell singlet state of P8′2+ clearly shows that each spin is confined within one-half of the macrocyclic moiety of P8′2+. Such a charge and spin density separation is in marked contrast with the charge-delocalized P42+. These results indicate that the strength of the antiferromagnetic coupling between the two radical spins becomes weaker by forming a biradicaloid structure due to the larger ring size of P82+.61

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CONCLUSIONS In this work, we newly synthesized two kinds of aza[1n]paracyclophanes, tetraaza[14]paracyclophane (P4) and octaaza[18]paracyclophane (P8). X-ray single-crystal analysis revealed 13355


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Figure 11. (a) ORTEP representation and (b) CPK model of P86+·6[SbCl6]−. (c) Unit cell of the crystal of P86+·6[SbCl6]− (Each symmetrically independent component is shown in a different color; P86+ is colored pink, SbCl6− are colored red, blue, and green). Hydrogen atoms and solvent molecules are omitted for clarity. Thermal ellipsoids are set at 50% probability. monochromator was adjusted to 200 μm corresponding to a wavelength resolution of 12 nm. Fluorescence decay was obtained by averaging a 20 nm wavelength range around their peak tops (P4, 470−490 nm; P6 and P8, 430−450 nm). Synthesis. 2: A mixture of p-anisidine (2.00 g, 16.2 mmol), 4bromo-N-4-bromophenyl)N-(4-methoxyphenyl)aniline 3 (1.73 g, 4.0 mmol), [(t-Bu)3P]HBF4 (0.071 g, 0.25 mmol), Pd2(dba)3 (0.111 g, 0.12 mmol), and NaOt-Bu (1.15 g, 12.0 mmol) in toluene (50 mL) was refluxed with stirring under a nitrogen atmosphere for 3 h. The reaction mixture was cooled down to room temperature and washed with water. The organic layer was dried over Na2SO4, and the solvent was removed by rotary evaporation. The crude product was filtered through a silica gel plug and washed with methanol to afford 2 (1.61 g, 78%) as white solid. mp 183−185 °C; 1H NMR (400 MHz, DMSOd6) δ = 7.70 (s, 2H), 6.98 (d, J = 9.27 Hz, 4H), 6.88−6.80 (m, 16H), 3.71 (s, 3H), 3.70 (s, 6H); 13C NMR (100 MHz, DMSO-d6) δ = 154.3, 153.4, 141.8, 140.1, 139.9, 137.1, 124.7, 123.7, 119.4, 116.7, 114.7, 114.6, 55.2 (two peaks are overlapped); HRMS (ESI) m/z calcd for C33H31N3O3 517.2360 [M]+; found 517.2350. P4 and P8: Toluene (100 mL) was added to 3 (0.878 g, 2.03 mmol), [(t-Bu)3P]HBF4 (0.022 g, 0.077 mmol), Pd2(dba)3 (0.034 g, 0.037 mmol), and NaOt-Bu (0.292 g, 3.04 mmol) in a flask equipped with a dropping funnel which was charged with toluene (100 mL) and 2 (0.522 g, 1.09 mmol), and the toluene solution was stirred under a nitrogen atmosphere at 100 °C for a while. The suspension in the dropping funnel was gradually added to the solution in the flask for 3 h, and then the reaction mixture was stirred for 16 h. The reaction mixture was filtered through Celite, and the filtrate was washed with brine. The organic layer was dried over Na2SO4, and the solvent was removed by rotary evaporation. The crude product was chromatographed on silica gel (ethyl acetate/hexane as eluent). First, P4 was eluted with ethyl acetate/hexane = 1:9 to afford P4 as a faint yellow powder (2.5 mg, 0.3%), and then P8 was eluted with ethyl acetate/ hexane = 1:1 to afford P8 as a faint yellow powder (84.8 mg, 10%). P4: mp > 300 °C; 1H NMR (400 MHz, CD2Cl2) δ = 6.97 (d, J = 9.27 Hz, 8H), 6.83 (s, 16H), 6.80 (d, J = 9.27 Hz, 8H), 3.75 (s, 12H); 13C NMR (100 MHz, CD2Cl2) δ = 154.0, 148.2, 141.3, 129.1, 119.0, 115.0,

battery electrodes. The present results not only provide deeper insight into the ring size dependency on the electronic properties of aza[1n]paracyclophanes but also open new applications of this unique class of redox-active macrocyclic compounds.

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EXPERIMENTAL SECTION

General Information. All purchased reagents were of standard quality and used without further purification. Column chromatography was performed with silica gel (Kanto Chemical Co., Inc., silica gel 60N, spherical neutral). 1H and 13C NMR spectra were recorded by a JEOL JNM-AL400 FT-NMR spectrometer. In the NMR measurements of P4 and P8, a trace amount of hydrazine was added to avoid signal broadening caused by generation of radical cations in CD2Cl2. Lowtemperature 1H NMR spectra were recorded by a JEOL JNMECZ500R/S1 FT-NMR spectrometer. Low- and high-resolution electrospray ionization (ESI) mass spectra (MS) were obtained on a Thermo Fisher Scientific Exactive mass Fourier-transform orbitrap spectrometer. Matrix-assisted-laser-desorption/ionization (MALDI) MS were obtained on a Bruker Ultraflex III MALDI-TOF mass spectrometer with dithranol as a matrix. UV−vis−NIR absorption spectra were obtained with a JASCO V-570 spectrometer. ESR spectra were recorded with a JEOL JES-FA-200 X-band spectrometer. All DFT calculations in this study were carried out by using the Gaussian 09 program package (Revision D.01). Fluorescence spectra were recorded on an absolute PL quantum yield measurement system (HAMAMATSU Quantaurus-QY). Fluorescence lifetimes were measured from the time-resolved photoluminescence measurement as follows. Tripled harmonic 355 nm picosecond pulse train (∼7 ps pulse width, 1 kHz repetition) was picked up from a 85 MHz Nd:YVO4 laser (PL2250, EKSPLA, ∼12 mW) and used as an excitation source. Fluorescence from toluene solutions of each compounds was monitored by the monochromator (C5094, HAMAMATSU), the streak scope (C4334, HAMAMATSU) equipped with microchannel plate (MCP), and the digital delay generator (DG645, Stanford Research Systems). The slit width of the 13356


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The Journal of Organic Chemistry 56.1; HRMS (ESI) m/z calcd for C52H44N4O4 788.3347 [M]+; found 788.3357. P8: mp > 300 °C; 1H NMR (400 MHz, CD2Cl2) δ = 7.04 (d, J = 8.78 Hz, 16H), 6.88 (s, 32H), 6.81 (d, J = 8.78 Hz, 16H), 3.76 (s, 24H); 13C NMR (100 MHz, CD2Cl2) δ = 156.3, 143.4, 126.6, 124.4, 119.7, 115.1, 56.0; HRMS (ESI) m/z calcd for C104H88N8O8 1576.6728 [M]+; found 1576.6720. X-ray Crystallography. The single crystals of P4 were obtained by slow evaporation of a mixed solution (CH2Cl2 and methanol). The single crystals of P42+·2[SbF6]− and P86+·6[SbCl6]− were obtained by slow diffusion of ethyl acetate (for P42+·2[SbF6]−) or hexane (for P86+·6[SbCl6]−) into a CH2Cl2 solution of the cation salts. Data collections were performed on a Rigaku Saturn70 CCD diffractometer with Mo Kα radiation at 143 K. Hydrogen atoms were restrained to ride on the atom to which they are bonded. All calculations were performed by using CrystalStructure crystallographic software package,62 except for refinement, which was performed by using SHELXL97.63

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(9) Kim, P.; Park, K. H.; Kim, W.; Tamachi, T.; Iyoda, M.; Kim, D. J. Phys. Chem. Lett. 2015, 6, 451−456. (10) (a) Nakamura, Y.; Aratani, N.; Osuka, A. Chem. Soc. Rev. 2007, 36, 831−845. (b) Aratani, N.; Kim, D.; Osuka, A. Acc. Chem. Res. 2009, 42, 1922−1934. (11) Lewis, S. E. Chem. Soc. Rev. 2015, 44, 2221−2304. (12) Xue, J. Y.; Ikemoto, K.; Takahashi, N.; Izumi, T.; Taka, H.; Kita, H.; Sato, S.; Isobe, H. J. Org. Chem. 2014, 79, 9735−9739. (13) Jasti, R.; Bhattacharjee, J.; Neaton, J. B.; Bertozzi, C. R. J. Am. Chem. Soc. 2008, 130, 17646−17647. (14) Takaba, H.; Omachi, H.; Yamamoto, Y.; Bouffard, J.; Itami, K. Angew. Chem., Int. Ed. 2009, 48, 6112−6116. (15) Yamago, S.; Watanabe, Y.; Iwamoto, T. Angew. Chem., Int. Ed. 2010, 49, 757−759. (16) Tsue, H.; Ishibashi, K.; Tamura, R. Top Heterocycl. Chem. 2008, 17, 73−96. (17) König, B.; Fonseca, M. H. Eur. J. Inorg. Chem. 2000, 2000, 2303−2310. (18) (a) Gutsche, C. D. Calixarenes;Stoddart, J. F., Ed.; Royal Society of Chemistry: Cambridge, 1989. (b) Gutsche, C. D. Calixarenes Revisited; Stoddart, J. F., Ed.; Royal Society of Chemistry: Cambridge, 1998. (19) Ogoshi, T.; Yamagishi, T.; Nakamoto, Y. Chem. Rev. 2016, 116, 7937−8002. (20) Ito, A.; Tanaka, K. Pure Appl. Chem. 2010, 82, 979−989. (21) Bujak, P.; Kulszewicz-Bajer, I.; Zagorska, M.; Maurel, V.; Wielgus, I.; Pron, A. Chem. Soc. Rev. 2013, 42, 8895−8999. (22) Ito, A. J. Mater. Chem. C 2016, 4, 4614−4625. (23) Ito, A.; Ono, Y.; Tanaka, K. New J. Chem. 1998, 22, 779−781. (24) Bushby, R. J.; Kilner, C. A.; Taylor, N.; Vale, M. E. Tetrahedron 2007, 63, 11458−11466. (25) Ito, A.; Inoue, S.; Hirao, Y.; Furukawa, K.; Kato, T.; Tanaka, K. Chem. Commun. 2008, 3242−3244. (26) Ito, A.; Ono, Y.; Tanaka, K. Angew. Chem., Int. Ed. 2000, 39, 1072−1075. (27) Hauck, S. I.; Lakshmi, K. V.; Hartwig, J. F. Org. Lett. 1999, 1, 2057−2060. (28) Kurata, R.; Sakamaki, D.; Ito, A. Org. Lett. 2017, 19, 3115−3118. (29) Haddoub, R.; Touil, M.; Raimundo, J.-M.; Siri, O. Org. Lett. 2010, 12, 2722−2725. (30) Sakamaki, D.; Ito, A.; Matsumoto, T.; Tanaka, K. RSC Adv. 2014, 4, 39476−39483. (31) Ito, A.; Yokoyama, Y.; Aihara, R.; Fukui, K.; Eguchi, S.; Shizu, K.; Sato, T.; Tanaka, K. Angew. Chem., Int. Ed. 2010, 49, 8205−8208. (32) Yang, T.-F.; Chiu, K. Y.; Cheng, H. C.; Lee, Y. W.; Kuo, M. Y.; Su, Y. O. J. Org. Chem. 2012, 77, 8627−8633. (33) Kulszewicz-Bajer, I.; Maurel, V.; Gambarelli, S.; Wielgus, I.; Djurado, D. Phys. Chem. Chem. Phys. 2009, 11, 1362−1368. (34) Sakamaki, D.; Ito, A.; Furukawa, K.; Kato, T.; Tanaka, K. J. Org. Chem. 2013, 78, 2947−2956. (35) Panagopoulos, A. M.; Zeller, M.; Becker, D. P. J. Org. Chem. 2010, 75, 7887−7892. (36) Ruiz-Castillo, P.; Buchwald, S. L. Chem. Rev. 2016, 116, 12564− 12649. (37) Hartwig, J. F. Acc. Chem. Res. 2008, 41, 1534−1544. (38) Cram, D. J.; Cram, J. M. Acc. Chem. Res. 1971, 4, 204−213. (39) Iwamoto, T.; Watanabe, Y.; Sakamoto, Y.; Suzuki, T.; Yamago, S. J. Am. Chem. Soc. 2011, 133, 8354−8361. (40) Segawa, Y.; Fukazawa, A.; Matsuura, S.; Omachi, H.; Yamaguchi, S.; Irle, S.; Itami, K. Org. Biomol. Chem. 2012, 10, 5979−5984. (41) Fujitsuka, M.; Cho, D. W.; Iwamoto, T.; Yamago, S.; Majima, T. Phys. Chem. Chem. Phys. 2012, 14, 14585−14588. (42) Camacho, C.; Niehaus, T. A.; Itami, K.; Irle, S. Chem. Sci. 2013, 4, 187−195. (43) Huang, W.; Henderson, T. L. E.; Bond, A. M.; Oldham, K. B. Anal. Chim. Acta 1995, 304, 1−15. (44) Ito, A.; Sakamaki, D.; Ichikawa, Y.; Tanaka, K. Chem. Mater. 2011, 23, 841−850.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b02437. Crystallographic data for P4 (CIF) Crystallographic data for P42+·2[SbF6]− (CIF) Crystallographic data for P86+·6[SbCl6]− (CIF) NMR, MS, ESR, DFT calculations, and X-ray structural data (PDF) (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] ORCID

Daisuke Sakamaki: 0000-0001-6503-1607 Akihiro Ito: 0000-0002-8698-0032 Shu Seki: 0000-0001-7851-4405 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for KAKENHI (17H04874) from the Japan Society for the Promotion of Science (JSPS) and a Grant-in-Aid for Scientific Research on Innovative Areas “π-System Figuration” (26102011) and “New Polymeric Materials Based on Element-Blocks” (15H00734). Theoretical calculations were performed using the Supercomputer System of Research Center for Computational Science in Okazaki (Japan).

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Tetraaza[14]- and Octaaza[18]paracyclophane: Synthesis and

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