Letter pubs.acs.org/OrgLett
Isolable Triradical Trication of Hexaaza[16]paracyclophane with Embedded 9,10-Anthrylenes: A Frustrated Three-Spin System Ryohei Kurata,† Daisuke Sakamaki,*,† Masashi Uebe,† Mariko Kinoshita,† Tetsuo Iwanaga,*,‡ Takashi Matsumoto,§ and Akihiro Ito*,† †
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan Department of Chemistry, Faculty of Science, Okayama University of Science, 1-1 Ridaicho, Kita-ku, Okayama 700-0005, Japan § X-ray Research Laboratory, Rigaku Corporation, Matsubara-cho 3-9-12, Akishima, Tokyo 196-8666, Japan ‡
S Supporting Information *
ABSTRACT: A new derivative of hexaaza[16]paracyclophane in which pphenylenes are alternately replaced by 9,10-anthrylenes was prepared to investigate the impact on overall π-conjugation as well as conformational change of the macrocycle. The charge and spin distribution for one-electron and three-electron oxidation of the macrocycle was elucidated by means of electrochemical, spectroelectrochemical, EPR spectroscopic, and SQUID magnetometric methods. In particular, the triradical trication was successfully isolated as an air-stable salt, and moreover, its structure was disclosed by X-ray analysis. The triradical trication was characterized as a spin-frustrated three-spin system with the antiferromagnetic exchange interaction (J/kB ≃ − 74 K). elocalization or localization of charge/spin in πconjugated molecular systems has been a fascinating problem in fundamental chemistry as well as materials chemistry.1 In particular, comparative studies between the corresponding doped linear and cyclic oligomers as models for conducting polymers have attracted much attention as exemplified by the studies on oligothiophenes2,3 and oligoparaphenylenes.4,5 In 2010, we reported the first synthesis of hexaaza[16]paracyclophane 1, as a cyclic hexamer of polyaniline, in which all six nitrogen centers are connected with pphenylene units (Figure 1). The mono-oxidized species of 1 showed a multiplet hyperfine-structured ESR spectrum that is indicative of delocalization of the spin and charge over the entire macrocyclic molecular backbone,6 whereas the spin and charge localization were confirmed for the radical cations of the corresponding linear oligoanilines.7 In addition, the multiredox activity and shape-persistency of (poly)macrocyclic oligoaryl-
D
amines are expected to expand the possibility of hole- and spincontaining scaffolds for molecule-based electronics.8 Herein, we report a new derivative of hexaaza[16]paracyclophane, 2a and 2b (Figure 1), in which p-phenylenes are alternately replaced by 9,10-anthrylenes as a modifier for overall conformational change of the macrocycles.9 The introduction of bulky arene units should modulate the πconjugation through the macrocyclic molecular systems, whereby quasi-degenerate HOMO and HOMO-1 can be realized for 2a (Figure S1). Such a substantial change in the electronic structure resulted in the generation of the triradical trication of 2a. The syntheses of 2a and 2b were readily achieved starting from the corresponding 9,10-diaminoanthracene and pdibromobenzene by using a Buchwald−Hartwig reaction10,11 and were obtained as a red solid in yields of 14% and 17.5%, respectively.12 X-ray crystallographic analysis of 2a13 revealed that (i) the six nitrogen centers were almost in the same plane and (ii) three phenylene units were slightly inclined at 23−35°, whereas 9,10-anthrylenes tilted 75−82° from the macrocyclic plane (Figure 2a), suggesting some degree of segmentation of π-conjugation through the macrocyclic backbone of 2a, as compared with that of 1. In addition, no noticeable bond alternation was discerned about the N−C bonds along the macrocycle, though N−Cphenylene bonds tend to be slightly shortened as compared to N−Canthrylene bonds, in conjunction with the difference in tilted angles of phenylenes and
Figure 1. Structures of hexaaza[16]paracyclophane 1 and its derivatives 2a and 2b. © XXXX American Chemical Society
Received: July 9, 2017
A
DOI: 10.1021/acs.orglett.7b02088 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
Table 1. Oxidation Potentials (V vs Fc0/+) and Potential Differences (ΔE1−2) of 1−4 in CH2Cl2 at 298 K a
1 2a 2b 3d 4e
E1
E2
−0.28 −0.10 +0.07 −0.13 +0.19
−0.17 +0.05b +0.19b +0.35 +0.25
E3 +0.20 +0.58c +0.64c
E4 +0.45
ΔE1−2
E5 b
+0.72
0.11 0.15 0.12 0.48 0.06
a
From ref 6. bQuasi-two-electron oxidation. cQuasi-three-electron oxidation. dFrom ref 15. eFrom ref 16b.
of 1 (E1 = −0.28 V) and was close to the value of N,N,N′,N′tetraanisyl-p-phenylenediamine (3) rather than that of 9,10bis(dianisylamino)anthracene (4). This observation again indicates that the π-conjugation through the macrocyclic backbone of 2a is weakened by the introduction of 9,10anthrylenes: the macrocycle 2a can be regarded as a molecular system bearing three p-phenylenediamine (PD) moieties weakly segmented by three anthrylene units. Nevertheless, the relatively large splitting between the first and second oxidation potentials (ΔE1−2 = 0.15 V) strongly suggests that the generated radical cation of 2a can be still stabilized by the charge/spin delocalization over the macrocyclic backbone. Further insight into the charge distribution over the oxidized species 2•+ was provided by monitoring the absorption spectral change during electrochemical oxidation in CH2Cl2 (Figure 3
Figure 3. UV−vis−NIR absorption spectra of the stepwise electrochemical oxidation of 2a in CH2Cl2 (at 1 × 10−4 M) with 0.1 M nBu4NBF4 at 298 K: (a) 2a (in red) to 2a•+ (in blue); (b) 2a•+ (in red) to 2a3+ (in blue). Figure 2. ORTEP drawings and selected bond lengths (in Å) and dihedral angles of (a) 2a and (b) 2a3+·3[B(C6F5)4]−. Thermal ellipsoids are set at the 50% probability level. Hydrogen atoms and C6F5 groups in [B(C6F5)4]− are omitted for clarity; boron, nitrogen, and oxygen atoms are colored in green, blue, and red, respectively. Cyclohexane molecules as crystallization solvent of 2a in the crystal packing are colored in gray. B(C6F5)4− as counteranion of 2a3+ in the crystal packing are also colored in gray, and fluorine atoms are omitted for clarity. Note that the dihedral angles with two values are ascribed to the non-coplanarity between the adjacent two NC3 planes in 2a3+.
for 2a and Figure S4 for 2b). In the first oxidation process of 2a, two bands appeared in the near IR region [0.60 eV (2070 nm) and 1.17 eV (1060 nm)] and continued to grow up until the oxidation to 2a•+ had been completed (Figure 3a). It should be noted here that the next lowest energy band at 1.17 eV closely resembles the lowest energy band observed for the radical cation of 317 and can be considered as the chargeresonance (CR) intervalence band (IV) originating from the charge-delocalized semiquinone radical cation of the PD moiety.18 More interestingly, the lowest energy band at 0.60 eV is attributable to the IV charge transfer (IVCT) band between two nitrogen centers in the 9,10-diaminoanthracene moiety, judging from a close similarity to the IVCT band observed for the radical cation of 4 (Figure S5).16 This indicates that the generated charge is dynamically delocalized over the three PD moieties through 9,10-anthrylenes as bridging units. During the next oxidation process from 2a•+ to 2a3+, a continuous increase in absorbance for the CR IV band with a slight hypsochromic shift [1.29 eV (961 nm)] was detected, and the intensity reached up to almost three times that of 2a•+, showing that all of the PD moieties have positively
anthrylenes. The similar structural features are also confirmed for the unsubstituted macrocycle 2b (Figure S2).13 To investigate the effect of introduction of 9,10-anthrylenes, the oxidation potentials of 2a and 2b were examined electrochemically in CH2Cl2 using nBu4NBF4 as an electrolyte at room temperature. As shown in Table 1 and Figure S3, three reversible redox couples were observed for both 2a and 2b, and these redox processes were estimated to correspond to oneelectron, quasi-two-electron, and quasi-three-electron transfer processes,14 respectively, indicating the macrocycle 2 is oxidizable up to hexacation. The first oxidation potential of 2a was shifted to a more positive potential as compared to that B
DOI: 10.1021/acs.orglett.7b02088 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters charged semiquinoidal structures (Figure 3b). In contrast, the lowest energy IVCT band decreased and finally disappeared simply because the charge-transferable redox sites disappeared due to oxidation of all three PD moieties, indicating that three charged semiquinoid PD moieties are annularly being connected by the highly twisted 9,10-anthrylenes. The EPR spectrum (Figure S6) of the radical cation 2a•+ generated by treating 2a with 1 equiv of Ag[B(C6F5)4]19 in CH2Cl2 exhibited a multiplet hyperfine structure, and the spectrum was simulated only by assuming the following hyperfine coupling constants: |aN| = 0.173 mT (6 N) and | aH(p‑phenylene)| = 0.02 mT (12H). The contribution from the unresolved hydrogen nuclei was incorporated in the line width (0.22 mT), thus suggesting the dynamic spin delocalization over the entire molecule on the EPR time scale in spite of some degree of segmentation of π-conjugation through the macrocyclic backbone in 2a•+ due to the highly twisted 9,10anthrylenes. This observation is consistent with the result of the spectroelectrochemical measurements (Figure 3a). Note that the shape of the spectrum did not change within the measured temperature range, suggesting a low barrier to thermally activated intra-annular spin transfer (Figure S7). Unfortunately, we could not obtain any single crystals or analytically pure powder samples for the monocation salt of 2a. In contrast to 2a•+, we were able to isolate crystalline material of the trication salt, 2a3+·3[B(C6F5)4]−, from the oxidation of 2a with 3 equiv of Ag[B(C6F5)4] in CH2Cl2, and moreover, the isolated salts were stable under ambient conditions and can be stored under aerobic conditions at room temperature.20 In addition, the absorption spectrum of 2a3+·3[B(C6F5)4]− in CH2Cl2 solution was in good agreement with that of the electrochemically generated tricationic species of 2a (Figure S9). As expected, the X-ray structural analysis revealed the clear-cut quinoidal deformation of all the PD moieties of 2a3+ (Figure 2b):13 the averaged bond-length alternation (BLA) in phenylene rings for 2a3+ is distinct (0.07 Å), whereas it is 0.01 Å for 2a; in addition, the averaged bond length of N−Cphenylene for 2a3+ is shortened by 0.035 Å, as compared to that for 2a (Tables S4 and S5). In contrast, significant changes were not observed in 9,10-anthrylenes for both 2a and 2a3+. As a result, the macrocyclic molecular skeleton of 2a3+ turned out to be distorted, while the six nitrogen atoms in 2a are almost coplanar with a slight deviation (Figure 2a). Note that two [B(C6F5)4]− counterions were located above and below the tricationic macrocycle, whereas the remaining counterion was located far apart to some extent from 2a3+. Taken together, three positive charges of 2a3+ are equally pinned down to the three PD moieties, and moreover, each charge is delocalized within each PD moiety. This picture was also verified by the DFT-computed spin density distribution of 2a3+ (Figure S10). Judging from the molecular packing in 2a3+·3[B(C6F5)4]− (Figure 2b and Figure S11), adjacent charged macrocycles can be considered to be well separated by the counterions and crystal solvent molecules, thereby suggesting weak intermolecular magnetic interactions, as revealed by the weak Weiss constant (θ = −0.27 K). To address the intramolecular magnetic interaction between the generated three spin-centers in 2a3+, the temperature dependence of the molar magnetic susceptibility (χM) for analytically pure samples was measured by using the SQUID magnetometer. As shown in Figure 4a, the χMT value gradually decreased with decreasing temperature and reached a nearly constant value close to a theoretical value
Figure 4. (a) Plot of χMT versus T for 2a3+·3[B(C6F5)4]− at 0.1 T. The solid curve represents the best theoretical fit. (b) Isosceles triangle three-spin interacting model for 2a3+ and the energy level diagram for the two doublet states and the one quartet state.
(0.375 emu K mol−1) for a single spin-1/2 per macrocyclic molecular unit at around 30 K, indicating intramolecular antiferromagnetic interaction for 2a3+. The X-ray structural analysis suggested an isosceles triangular three-spin model, where three magnetically interacting spins-1/2 are located on the apexes of an isosceles triangle with two different exchange interactions J1 and J2, based on the spin Hamiltonian: H = −2J1(S1·S2 + S1·S3) − 2J2(S2·S3). In this model, temperature dependence of the χMT value can be represented as eq 1,21 where θ denotes the Weiss constant which is associated with weak intermolecular interactions ⎛ 10 + exp(−Δ1/kBT ) + exp(−Δ2 /kBT ) ⎞ T χM T = 0.375⎜ ⎟ ⎝ 2 + exp(−Δ1/kBT ) + exp(−Δ2 /kBT ) ⎠ T − θ (1)
where Δ1 = 3J1 and Δ2 = J1 + 2J2 correspond to the energy differences between the two competing doublet states and the quartet state (Figure 4b).22 From the fitting to the experimental data, the best-fitted parameters were determined: Δ1/kB ≅ Δ2/ kB = −223 K23 and θ = −0.27 K. This means that the present three-spin system can be virtually considered as a regular trianglular spin system where J1/kB = J2/kB ≡ J/kB ≃ − 74 K, and moreover, the doublet ground state is doubly degenerate, thereby indicating the present three-spin system is typically spin-frustrated, in conjunction with rare examples in organic polyradicals.24 In summary, we have investigated the spin and charge distributions for the radical cation and the triradical trication of the highly twisted anthrylene-embedded hexaaza[1 6 ]paracyclophane. The electrochemical, spectroelectrochemical, and EPR studies revealed the dynamically delocalized spin and charge distribution for the radical cation, despite somewhat segmentation of π-conjugation through the macrocyclic backbone by the 9,10-anthrylenes. Furthermore, we have succeeded in isolation of the stable triradical trication salt of the C
DOI: 10.1021/acs.orglett.7b02088 Org. Lett. XXXX, XXX, XXX−XXX
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(5) (a) Kayahara, E.; Kouyama, T.; Kato, T.; Takaya, H.; Yasuda, N.; Yamago, S. Angew. Chem., Int. Ed. 2013, 52, 13722−13726. (b) Golder, M. R.; Wong, B. M.; Jasti, R. Chem. Sci. 2013, 4, 4285−4291. (6) Ito, A.; Yokoyama, Y.; Aihara, R.; Fukui, K.; Eguchi, S.; Shizu, K.; Sato, T.; Tanaka, K. Angew. Chem., Int. Ed. 2010, 49, 8205−8208. (7) Grossmann, B.; Heinze, J.; Moll, T.; Palivan, C.; Ivan, S.; Gescheidt, G. J. Phys. Chem. B 2004, 108, 4669−4672. (8) (a) Ito, A.; Tanaka, K. Pure Appl. Chem. 2010, 82, 979−989. (b) Ito, A. J. Mater. Chem. C 2016, 4, 4614−4625. (9) (a) Toyota, S. Chem. Lett. 2011, 40, 12−18. (b) Yoshizawa, M.; Klosterman, J. K. Chem. Soc. Rev. 2014, 43, 1885−1898. (c) Kurata, R.; Sakamaki, D.; Ito, A. Org. Lett. 2017, 19, 3115−3118. (10) Ruiz-Castillo, P.; Buchwald, S. L. Chem. Rev. 2016, 116, 12564− 12649. (11) (a) Hartwig, J. F. Nature 2008, 455, 314−322. (b) Hartwig, J. F. Acc. Chem. Res. 2008, 41, 1534−1544. (12) The same reaction between N,N′-dianisyl-p-phenylenediamine and 9,10-dibromoanthracene resulted in a low yield (5%) of 2a. Synthetic procedures for 2a and 2b are described in the Supporting Information. From the mass spectra of crude reaction mixtures, we observed the formation of small quantity of cyclic dimer, tetramer, and pentamer, but we could not isolate them. The red color of neutral species, 2a and 2b, is in contrast to the pale yellow color of 1; this red color can be ascribed to the introduction of 9,10-anthrylenes simply because of the similar red color of bis(dianisylamino)anthracene (4). (13) CCDC 1545929 (2a), 1551588 (2b), and 1545930 (2a3+· 3[B(C6F5)4]−) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. (14) The simulated DPV curve of 2a (Figure S3a) was well fitted to the observed one by using the following oxidation potentials [vs Fc0/+ (ne)]: E1 = − 0.10 V (1e), E2 = + 0.03 V (1e), E3 = + 0.07 V (1e), E4 = + 0.51 V (1e), E5 = + 0.57 V (1e), and E6 = + 0.61 V (1e). (15) Ito, A.; Inoue, S.; Hirao, Y.; Furukawa, K.; Kato, T.; Tanaka, K. Chem. Commun. 2008, 3242−3244. (16) (a) Lambert, C.; Risko, C.; Coropceanu, V.; Schelter, J.; Amthor, S.; Gruhn, N. E.; Durivage, J. C.; Brédas, J.-L. J. Am. Chem. Soc. 2005, 127, 8508−8516. (b) Uebe, M.; Kato, T.; Tanaka, K.; Ito, A. Chem. - Eur. J. 2016, 22, 18923−18931. (17) Lambert, C.; Nöll, G. J. Am. Chem. Soc. 1999, 121, 8434−8442. (18) (a) Szeghalmi, A. V.; Erdmann, M.; Engel, V.; Schmitt, M.; Amthor, S.; Kriegisch, V.; Nöll, G.; Stahl, R.; Lambert, C.; Leusser, D.; Stalke, D.; Zabel, M.; Popp, J. J. Am. Chem. Soc. 2004, 126, 7834− 7835. (b) Hirao, Y.; Ito, A.; Tanaka, K. J. Phys. Chem. A 2007, 111, 2951−2956. (c) Ito, A.; Sakamaki, D.; Ichikawa, Y.; Tanaka, K. Chem. Mater. 2011, 23, 841−851. (19) Kuprat, M.; Lehmann, M.; Schulz, A.; Villinger, A. Organometallics 2010, 29, 1421−1427. (20) We could not obtain any single crystals or analytically pure powder samples for the dication salt of 2a. We measured the EPR spectrum of 2a after addition of 2 equiv of oxidizing reagents; unfortunately, definitive fine-structured spectra characteristic of a spintriplet species for 2a2+ were not observed in a frozen CH2Cl2 matrix at 123 K (Figure S8). (21) Catala, L.; Le Moigne, J.; Gruber, N.; Novoa, J. J.; Rabu, P.; Belorizky, E.; Turek, P. Chem. - Eur. J. 2005, 11, 2440−2454. (22) Sinn, E. Coord. Chem. Rev. 1970, 5, 313−347. (23) The DFT-computed doublet-quartet energy difference (ΔED−Q/ kB) of −209 K (B3LYP/6-31G*) was in good agreement with the experimental value of −223 K. (24) (a) Awaga, K.; Okuno, T.; Yamaguchi, A.; Hasegawa, M.; Inabe, T.; Maruyama, Y.; Wada, N. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 3975−3981. (b) Fujita, J.; Tanaka, M.; Suemune, H.; Koga, N.; Matsuda, K.; Iwamura, H. J. Am. Chem. Soc. 1996, 118, 9347− 9351. (c) Itoh, T.; Matsuda, K.; Iwamura, H.; Hori, K. J. Am. Chem. Soc. 2000, 122, 2567−2576. (d) Wu, Y.; Krzyaniak, M. D.; Stoddart, J. F.; Wasielewski, M. R. J. Am. Chem. Soc. 2017, 139, 2948−2951.
macrocycle, in which three segmented PD-based radical spins were found to be mutually antiferromagnetically coupled with J/kB ≃ − 74 K, leading to a typical spin-frustrated three-spin system. The present findings can lead to a new strategy of designing multispin molecular systems by the structural modulation of π-conjugation over the macrocyclic backbone.
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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.7b02088. Crystallographic data for 2a (CIF) Crystallographic data for 2b (CIF) Crystallographic data for 2a3+ ([B(C6F5)4]− salt) (CIF) Synthetic details, NMR, UV−vis−NIR, EPR, and DFT calculations and X-ray structural data for 2a, 2b, and 2a3+ trication salt (PDF)
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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 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partly supported by a Grant-in-aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks (No. 2401)” (JSPS KAKENHI Grant No. JP15H00734, A.I.) and by a Grant-in-Aid for Young Scientists (B) (JSPS KAKENHI Grant No. 16K17874, T.I.). We are grateful to Dr. Hiroyasu Sato (Rigaku Corp.) for the useful suggestion of the X-ray analysis. Elemental analyses were performed by Center for Organic Elemental Microanalysis, Kyoto University. Numerical calculations were partly performed at the Research Center for Computational Science in Okazaki (Japan).
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REFERENCES
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DOI: 10.1021/acs.orglett.7b02088 Org. Lett. XXXX, XXX, XXX−XXX
