CRYSTAL GROWTH & DESIGN
Formation of a One-Dimensional Stacking Structure of π-Conjugated Nitroxyl Radical Bearing a 1,2,5-Thiadiazole Ring and Its Magnetic Property
2005 VOL. 5, NO. 2 413-417
Masaru Yao, Satoshi Asakura, Masahiro Abe, Hidenari Inoue, and Naoki Yoshioka* Department of Applied Chemistry, Faculty of Science and Technology, Keio University, Yokohama 223-8522, Japan Received November 28, 2004;
Revised Manuscript Received January 1, 2005
ABSTRACT: A novel stable organic radical, 7,7-diphenyl-6,7-dihydro[1,2,5]thiadiazolo[3,4-f]quinoline-6-oxyl (1), forms a one-dimensional columnar structure consisting of slipped π-stacks, which is in contrast to dimer formation of the isoelectronic benzologue, 3,3-diphenyl-3,4-dihydrobenzo[f]quinoline-4-oxyl (2). Magnetic susceptibility of 1 obeyed an antiferromagnetic chain model with J ) -47 cm-1, while 2 behaved as an isolated doublet spin state even in the dimer. Research interest in fused ring systems in the field of molecule-based magnets and molecular conductors has developed in two separate, albeit, closely related directions.1,2 In the first one, synthetic chemists continue to design new spin centers taking advantage of the delocalization effect through a π-conjugation system based on its chemical stability. In the second one, emphasis is on the electronic interaction through an intra- and/or intermolecular π-conjugation system. As a typical example, the chemistry of phenalenyl3,4 and thiazyl5-7 radicals have successfully developed both experimentally and theoretically. To realize molecular self-assembly with a long-range magnetic interaction induced by π-π stacking, we focused on the aromatic N-oxyl, 2,2-diphenyl-1,2-dihydroquinoline1-oxyl (DPQN) originally synthesized by Colonna et al.8 (Chart 1).
Scheme 1
Chart 1
DPQN is chemically stable both in the solution and in the solid states; however, it behaves as an isolated doublet spin center in the crystal.9 Two phenyl groups at the 2-position and the fused coplanar structure could synergistically contribute to the prevention of direct close contact between the N-O moieties, which usually causes a strong antiferromagnetic interaction often observed in less hindered N-tert-butylphenyl nitroxyl radicals. Our attention has centered on the chemical modification of benzo ring to provide the role of an extended magnetic interaction pathway. It is favorable to design multipoint close contacts between the SOMOs, because the intermolecular magnetic interaction is described by the sum of the spin-density product of nearby atoms belonging to neighboring molecules as suggested by McConnell.10 According to this strategy, we have synthesized a novel benzologue of DPQN, 2,2-diphenyl-1,2-dihydrobenzo[g]quinoline-1-oxyl,9 and found the formation of an edge-to-edge dimer in the crystal with a ferromagnetic interaction obeying the Bleaney-Bowers equation with 2J ) +4.6 cm-1, which can be rationalized by the bifurcated edge-to-edge contacts between the R-spin site (nitroxyl O atom) and the β-spin site (aryl-H atoms).9 * To whom correspondence
[email protected].
should
be
addressed.
E-mail:
Figure 1. (a) Change in color tones caused by ring extension. (b) UV-vis spectra of DPQN, 1 and 2 in CH2Cl2.
To extend the magnetic interaction from the isolated dimer to an infinite system, we decided to introduce an -NSNlinkage into the DPQN system, which often causes a drastic
10.1021/cg049599v CCC: $30.25 © 2005 American Chemical Society Published on Web 01/26/2005
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Figure 2. ORTEP drawings (a, b) and crystal structures (c, d) of 1 and 2 (50% ellipsoids). Hydrogen atoms in the crystal structures are omitted for clarity.
perturbation in the molecular arrangement by the formation of a π-π stacked column,11,12 by polymeric intermolecular N‚‚‚S close contact,11,12 and by the complexation to metal ions.13 In the present paper, the synthesis of a novel π-conjugated nitroxyl radical bearing a thiadiazole ring, 7,7diphenyl-6,7-dihydro[1,2,5]thiadiazolo[3,4-f]quinoline-6oxyl (1), and its crystal structure and magnetic properties were clarified. Moreover, the magnetostructural correlation of 1 was compared to the isoelectronic 3,3-diphenyl-3,4dihydrobenzo[f]quinoline-4-oxyl (2), which was originally synthesized by Colonna14 to elucidate the effect of the -NSN- linkage introduction. 1 was obtained via a six-step synthesis as described in Scheme 1. First, [1,2,5]thiadiazolo[3,4-f]quinoline-6-oxide 3 was derived from the commercially available 6-nitoroquinoline in four steps using Sharma’s method.15,16 The following introduction of a phenyl ring at 2-position was performed by the reaction with phenylmagnesium bromide. The subsequent reaction with phenylmagnesium bromide gave an air-sensitive 6-hydroxyl derivative 5. The atmospheric oxidation of 5 gave a brown-colored solid of 1. 2 was synthesized by the modified route based on original report.14 1 exhibited a remarkable color difference in solution, while conventional N-oxyl radicals generally exhibit a monotonic red-yellow color in solution.17 DPQN exhibited a red color in the solution similar to conventional N-oxyls;
however, 1 and 2 displayed brown and green colors, respectively. The apparent color difference of 1 and 2 is due to a spectral change in the visible region. To obtain further information on the electronic structure of these radicals, the UV-vis spectra were recorded for 1 and 2 in CH2Cl2 (Figure 1). All the bands of 1 and 2 exhibited a red shift compared with DPQN, which could be attributed to the effect of the ring extension. The intense bands observed around the near-ultraviolet region are attributed to π-π* transitions, and some weak bands in the visible region are dominantly assigned to the n-π* transitions of the radical centers. In particular, the n-π* band of 1 reaches the nearIR region of around 800 nm. X-ray crystallographic analyses clarified the molecular structures and the alignment of 1 and 2 in the crystal.18-21 Figure 2 shows their ORTEP drawings and crystal structures. As were expected from the chemical structure, the angles between the N-O group and the adjacent aromatic rings are very small due to the characteristic fused structure (1: 5.82°, 2: 1.16°). The observed coplanarity could lead to the large delocalization of unpaired electron over the whole π-conjugated systems. In the crystal, 1 formed a one-dimensional columnar structure consisting of slipped π-stacks along the c-axis in the crystal. The intermolecular short distances are 3.632(3) Å for C(3)‚‚‚ S(1′), 3.452(4) Å for C(3)‚‚‚N(3′), 3.640(4) Å for C(4)‚‚‚C(6′), 3.520(4) Å for C(5)‚‚‚C(7′), and 3.682(3) Å for S(1)‚‚‚C(8′). The S‚‚‚S or S‚‚‚N contact, which is often seen in thiadiazole
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Crystal Growth & Design, Vol. 5, No. 2, 2005 415 Table 1. Calculated Spin Density Distributionsa of Quinoline Skeleton of 1 and 2
compound 1
Figure 3. ESR spectra of 1 in benzene solution at room temperature. (a) Experimental; (b) simulation.
derivatives, was not observed in the crystal of 1. On the other hand, 2 formed a centrosymmetric dimer, in which benzo[f]quinoline units locate an offset π-stacked arrangement. The intermolecular short C-C distances are 3.612(2) Å for C(5)‚‚‚C(7′) and 3.521(2) Å for C(9)‚‚‚C(13′). ESR measurement was performed to investigate the spin distribution over the π-conjugated systems of 1 and 2. The spectra of 1 and 2 exhibited very complicated patterns due to the presence of several nonequivalent nuclei. Figure 3 shows the spectrum of 1 in dilute benzene solution at room temperature. Generally, the dialkyl N-oxyl radicals exhibit hfccs of ca. 1.4-1.5 mT for the N-oxyl nitrogen atoms. These values
a
compound 2
site
spin density
site
spin density
O1 N1 C1 C2 C3 C4 C5 C6 C7 C8 C9 N2 N3 S1
0.520690 0.270269 -0.033867 0.136380 -0.088703 0.249791 -0.068522 0.051075 -0.081573 0.075299 -0.149141 0.097163 -0.052718 0.024010
O1 N1 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13
0.464433 0.322068 -0.024551 0.103459 -0.068546 0.211472 -0.069317 0.085134 0.076532 0.090633 -0.135347 0.068295 -0.045388 0.076847 -0.050994
UB3LYP/6-31G(d).
indicate that the spin density is localized on the N-O group.22 A computer simulation23 of the spectrum of 1 and 2 gave small aN values of 0.81 and 0.93 mT for their N-O sites, respectively, confirming the delocalization of the unpaired electron at the π-conjugated systems. In addition, small but appreciable aN values for the thiadiazolo nitrogen atoms were observed for 1 (0.13, 0.06 mT), which also support the delocalization of the unpaired electron in the π-conjugated systems.
Figure 4. SOMOs and spin density distribution of 1 and 2. Computations were performed on the unrestricted B3LYP/6-31G*. Positive spin densities are shown in blue and negative ones are in green.
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Figure 6. Overlapping of quinoline skeleton of 1 (a) and 2 (b).
Figure 5. Temperature dependence of χm (O) and χmT (4) for 1 (a) and 2 (b). The solid lines correspond to the calculated curves.
In addition to ESR measurements, we performed a DFT calculation to reveal the spin density distributions using the GAUSSIAN 03 program package.24 For the computation, the popular unrestricted B3LYP hybrid functional25,26 with the standard split valence 6-31G* basis set was used on the coordinates obtained by the X-ray analysis. Figure 4 shows the calculated SOMOs and spin density distributions for 1 and 2. The calculated spin densities are summarized in Table 1. Over -NSN- linkage, a positive spin density is observed on S1 and N2 sites and a negative one on the N3 site. The spin density ratio between N2 and N1 well coincides with above aN ratio. The magnetic properties of the isolated radicals 1 and 2 were measured with a SQUID magnetometer using microcrystalline samples at 1.8-300 K. The temperature dependences of χm and χmT for 1 and 2 are shown in Figure 5. The χmT values of 2 were virtually constant, suggesting that 2 has a nearly isolated doublet state. 2 formed a π-stacked dimer structure in the crystal; however, the observed distances of close contacts are greater than the sum of the van der Waals radii (C‚‚‚C: 3.40 Å). In addition, these contacting atoms are β-spin sites, which also weaken the intermolecular interaction, since the interaction between the β-spin sites is weaker than that of the R-spin sites (Figure 6).27 On the contrary, the χmT values of 1 gradually decreased as the temperature was lowered, and the χm of 1 increased
and had a broad maximum at around 40 K, indicating a strong intermolecular antiferromagnetic interaction (Figure 5). This behavior could be fitted to a one-dimensional antiferromagnetic chain model (Bonner-Fisher model28) with J ) -47 cm-1. In the columnar structure, the nearest O(1)‚‚‚O(1′) distance is 9.37 Å. There seems to be almost no direct interaction between the N-O sites.29 Therefore, the interaction was understood to be due to the close contact of the delocalized spins. The observed close contacts are longer than the sum of the van der Waals radii; however, there are some close contacts between the atoms carrying an R-spin: C(4)‚‚‚C(6′), and C(8)‚‚‚S(1′) (Figures 2 and 6). These contacts are thought to be the main path for the observed intermolecular antiferromagnetic interaction. To obtain a theoretical insight into the observed antiferromagnetic interaction, a DFT calculation was performed on the dimeric coordinate of 1 extracted from its column structure. The J value was estimated from the equation suggested by Yamaguchi et al.30 The computation at UB3LYP/6-31G* and UBLYP/6-31G* levels gave J values of -14 and -32 cm-1, respectively, and qualitatively reproduced the experimentally obtained negative J value of -47 cm-1, thus supporting the antiferromagnetic interaction through the π-π stacking. The functional dependency might be due to limitations of the DFT method in the present case. In summary, a novel aromatic N-oxyl radical bearing thiadiazole unit 1 was prepared and then magnetically characterized in connection with the isoelectronic benzologue 2. ESR measurements and DFT calculations revealed that 1 and 2 have delocalized unpaired electrons on their π-conjugated systems. In the crystal, 1 was assembled in a columnar structure by a π-π interaction, and 2 formed a dimeric structure. In the SQUID measurement, 2 exhibited a paramagnetic behavior; however, 1 exhibited an antiferromagnetic behavior, which is best fitted by the 1D
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chain model with J ) -47 cm-1. DFT calculations for 1 reproduced a negative J value, supporting the antiferromagnetic interaction through the π-π stacking. Within the column, the delocalized unpaired electron plays an important role in the strong antiferromagnetic interaction. Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research (B) 15310094, Exploratory Research 116651070, from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. Financial support from the Keio Gijuku Academic Development Funds is also acknowledged. M. Y. gratefully acknowledges the Grant-in-Aid for the 21st Century COE program “KEIO Life Conjugate Chemistry” from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. Supporting Information Available: Experimental procedures, characterization data, and crystallographic data of 1 and 2. These materials are available free of charge via the Internet at http://pubs.acs.org.
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(17) Rozantsev, E. G.; Sholle, V. D. Synthesis 1971, 190. (18) Crystallographic parameters of 1: Formula C21H14N3OS, M ) 356.42, Crystal system orthorhombic, Space group Pbca, a ) 12.256(5) Å, b ) 33.236(11) Å, c ) 8.387(2) Å, V ) 3416.1(20) Å3, Z ) 8, D )1.386 g cm-3, R (I > 2σ(I)) )0.049, Rw (all data) ) 0.137, S ) 0.98. Crystallographic parameters of 2: Formula C25H18NO, M ) 348.42, Crystal system monoclinic, Space group P21/c, a ) 11.173(1) Å, b ) 12.536(2) Å, c ) 13.026(1) Å, β ) 90.916(9)°, V ) 1824.3(4) Å3, Z ) 4, D ) 1.268 g cm-3, R (I > 2σ(I)) )0.044, Rw (all data) ) 0.128, S ) 1.07. (19) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.; Giacovazzo, C.; Guagliardi, A.; Polidori, G. J. Appl. Crystallogr. 1994, 27, 435. (20) Sheldrich, G. M. SHELXL-97: Program for the Refinement of Crystal Structure; University of Go¨ttingen, Germany, 1997. (21) teXsan Single-Crystal Structure Analysis Software, Version 1.11; MSC: The Woodlands, TX (Rigaku, Tokyo, Japan). (22) Aurich, H. G.; Hahn, K.; Stork, K.; Weiss, W. Tetrahedron Lett. 1977, 33, 969. (23) EPR spectrum simulation was performed using PEST WinSIM (authored by D. Duling, National Institute of Environmental Health Sciences, 1996). (24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; R. Martin, L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.04; Gaussian, Inc.: Pittsburgh, PA, 2003. (25) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (26) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (27) (a) Dietz, F.; Tyutyulkov, N.; Baumgarten, M. J. Phys. Chem. B 1998, 102, 3912, (b) Takano, Y.; Taniguchi, T.; Isobe, H.; Kubo, T.; Morita, T.; Yamamoto, K.; Nakasuji, K.; Takui, T.; Yamaguchi, K. J. Am. Chem. Soc. 2003, 124, 11122. (28) Bonner, J. C.; Fisher, M. E. Phys. Rev. A 1964, 135, 640. (29) Kawakami, T.; Yamanaka, S.; Mori, W.; Yamaguchi, K.; Kajiwara, A.; Kamachi, M. Chem. Phys. Lett. 1995, 235, 414. (30) Yamanaka, S.; Kawakami, T.; Nagao, H.; Yamaguchi, K. Chem. Phys. Lett. 1994, 231, 25.
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