Synthesis and magnetic properties of nitronyl nitroxide radicals carrying a purine ring

Polyhedron 20 (2001) 1151– 1155 www.elsevier.nl/locate/poly

Synthesis and magnetic properties of nitronyl nitroxide radicals carrying a purine ring Hideaki Nagashima, Naoki Yoshioka *, Hidenari Inoue Department of Applied Chemistry, Faculty of Science and Technology, Keio Uni6ersity, 3 -14 -1 Hiyoshi, Kohoku-ku, Yokohama 223 -8522, Japan Received 17 September 2000; accepted 2 November 2000

Abstract 2-(Purin-8-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxyl-3-oxide (Pu-NN) was designed and synthesized. Two crystalline phases (a- and b-phases) exhibiting different IR spectral patterns at the wNH region were obtained depending on the crystallizing condition. The temperature dependences of the magnetic susceptibility of these two phases followed Curie– Weiss law with q= − 5.7 K (a-phase) and q= − 0.3 K (b-phase) implying intermolecular antiferromagnetic interaction, which were quite different from that of parent benzimidazole derivative, 2-(benzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H- imidazolyl-1oxyl-3-oxide that formed a 1-D ferromagnetic chain induced by an intermolecular hydrogen bonding. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Molecule-based magnetism; Hydrogen bond; Nitronyl nitroxide; Purine; Magnetic interaction

1. Introduction There has been much interest in the study of molecule-based magnetism in recent years [1,2]. A number of organic free radicals have been magnetically characterized, and their magneto-structural correlation has been widely investigated. Especially since the discovery of the first organic ferromagnet [3], tens of purely organic crystals with bulk ferromagnetism have been successively reported, and a suitable arrangement

Fig. 1. Molecular structures of radicals. * Corresponding author. Fax: +81-45-566-1551. E-mail address: [email protected] (N. Yoshioka).

of magnetic orbital for realizing a ferromagnetic spin alignment has been elucidated. Organic ferromagnets so far reported, however, have been fortuitously discovered, and the methodology for controlling the spatial position of open-shell molecules in the crystal lattice has not been established yet. In the field of supramolecular chemistry, hydrogen bonding has played an important role for such purposes, because it is strong in energy and its directional property is better understood than many other types of noncovalent bonding. As these advantages of hydrogen bonding are aimed at crystal engineering, various NN (= 4,4,5,5-tetramethyl4,5-dihydro-1H-imidazolyl-1-oxyl-3-oxide) derivatives with heterocycles having –NH sites such as triazole [4], N-protonated pyridine [5], and uracil [6] have been reported as well as with hydroxylated phenyl rings [7]. Diazoles, i.e. imidazole and pyrazole, have both a proton donating –NH site and accepting –N site, are seemed to be an excellent building block for molecular self-assemblies [8,9]. Recently, Lahti and co-workers attached the benzimidazole ring to t-butylnitroxide to give a stable radical solid exhibiting an antiferromagnetic interaction [10]. We also have reported stable radical derivatives having an imidazole ring at the 2-position of the NN group (Fig. 1) [11]. Both Im-NN

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Fig. 2. Synthetic route of Pu-NN.

and BIm-NN formed chain structures, and showed magnetic interaction rationalized by intermolecular SOMO –SOMO interaction based on McConnell’s theory [12]. In this paper, we describe the synthesis and magnetic characterization of the NN derivative carrying a purine ring (Fig. 1). Purine can be regarded as not only one of diaza analog of benzimidazole, but also an essential component of naturally occurring materials such as adenine and guanine in DNA. These characteristics of purine derivatives promote us to synthesize this new NN derivative.

2 days at room temperature. The white precipitate was filtered, washed with methanol and dichloromethane, and dried under vacuum to give the white powder (0.78 g, 27%). 1H NMR spectrum indicated the existence of two isomers (a and b, a/b=ca. 3/2, by 1H NMR signal integration) due to the proton exchange between the imidazole rings. 1H NMR (DMSO-d6, 300 MHz) lH ppm: 9.04 (a), 8.98 (b) (1H, 2s, Ar– H), 8.90 (b), 8.85 (a) (1H, 2s, Ar–H), 8.07 (b), 8.02 (a) (2H, 2s, – OH), 4.91 (b), 4.86 (a) (1H, 2s, –CH –), 1.13, 1.08 (12H, 2s, –CH3). Anal. Found: C, 51.69; H, 6.54; N, 30.14. Calc. for C12H18N6O2: C, 51.79; H, 6.52; N, 30.20%. M.p. (dec.): 208–210°C.

2. Experimental

2.2.2. 2 -(Purin-8 -yl) -4,4,5,5 -tetramethyl-4,5 -dihydro1H-imidazolyl-1 -oxyl-3 -oxide (Pu-NN) The above intermediate (0.78 g, 3.13 mmol) was oxidized with an aqueous solution of sodium periodate at 0°C followed by extraction using dichloromethane and dried over anhydrous sodium sulfate. The solution was evaporated and the crude product was purified by silica gel column chromatography with ethyl acetate as an eluent (0.29 g, 38%). ESR (dichloromethane): aN = 0.74 mT, g = 2.0069. FAB-MS: observed (M+1)+ = 276 (calc. for C12H15N6O2 = 275.29). UV–Vis (dichloromethane) umax (nm) (m, M − 1 cm − 1): 307 (2.56× 104), 319 (2.29× 104), 372 (1.39×104), 613 (1.58× 104), 672 (1.39×102). Two different phases were obtained depending on the solvents used for crystallization. a-phase (from methanol solution, light green powder). IR (neat): 3165 cm − 1 (wNH). Anal. Found: C, 52.23; H, 5.54; N, 30.24. Calc. for C12H15N6O2: C, 52.36; H, 5.49; N, 30.53%. M.p. (dec.): 227°C. b-phase (by slow evaporation of a dichloromethane–benzene solution, a fine needle-shaped green crystal). IR (neat): 3233 cm − 1 (wNH). Anal. Found: C, 52.16; H, 5.45; N, 30.28. Calc. for C12H15N6O2: C, 52.36; H, 5.49; N, 30.53%. M.p. (dec.): 222°C.

2.1. Materials and instruments Purine-8-carbaldehyde and 2,3-bis(hydroxyamino)2,3-dimethylbutane were prepared as previously described [13,14]. Other reagents were commercially available, and used without further purification. IR spectra were recorded on a BIO-RAD FTS-65 with a RAS apparatus. The magnetic susceptibilities were measured using a Quantum Design MPMS-5 SQUID susceptometer working at the field strength of 0.5 T in the temperature range of 1.8– 300 K. ESR spectrum was recorded on a JEOL JES-RE3X X-band (9.4 GHz) spectrometer. 1H NMR spectra were obtained on a JEOL JNM-LA300 spectrometer. UV–Vis spectra were obtained on a JASCO V-550 spectrophotometer. MS spectrum was measured by a JEOL JMSGCmate mass spectrometer.

2.2. Synthesis of compounds A novel NN derivative, 2-(purin-8-yl)-4,4,5,5-tetramethy - 4,5 - dihydro - 1H - imidazolyl - 1 - oxyl - 3 - oxide, was synthesized from the corresponding aldehyde as shown in Fig. 2 following the method of Ullman and co-workers [15].

2.2.1. 1,3 -Dihydroxy-2 -(purin-8 -yl) 4,4,5,5 -tetramethylimidazolidine A methanol solution (50 ml) of purine-8-carbaldehyde (1.93 g, 11.6 mmol) and 2,3-bis(hydroxyamino)2,3-dimethylbutane (2.31 g, 15.6 mmol) was stirred for

3. Results and discussion

3.1. Molecular design Our basic concept for molecular design is shown in Fig. 3. The hydrogen bonding pattern can be mainly controlled by the steric effect of substituents R and/or

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R% and the subtle change in intermolecular distance can be affected by the electronic effect of these substituents. Imidazolyl-NN derivatives are expected to have two basic motifs due to the competition between the formations of A1···D versus A2···D. In the first case, an intermolecular hydrogen bond between imidazole rings results in a polymeric chain structure linked by NH···N bonding. Im-NN formed this type of chain structure and exhibited an antiferromagnetic interaction that was explained by the close contact of NO moieties between hydrogen bonded chains. The second case occurs when the intermolecular – NH···ON – hydrogen bond was preferred rather than the former. BIm-NN formed the latter chain structure, and revealed a ferromagnetic interaction explained by the overlap of SOMOs on the adjacent NNs that favored parallel spin alignment on the basis of McConnell’s theory. Purine has a combined structure of imidazole and pyrimidine, and this structure causes the corrugation of charge distribution compared to benzimidazole. By these chemical modifications of benzimidazole ring, the electronic effect on hydrogen motif and magnetic interaction might be examined.

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Table 1 Atomic charge distribution of benzimidazole and its analogs calculated by the MNDO method

3.2. Charge distribution of parent heterocycles Charge density distributions of benzimidazole and its analogs are summarized in Table 1. Charge densities of H(3) and N(4) of purine are more positive and less negative than those of benzimidazole, respectively, because of the p-deficiency of pyrimidine ring. The magnitude of acidic pKa values which were experimentally evaluated corresponds to these electronic effects (Table 2, [19]). From the computational results listed in Table 1, N(1) and N(2) might act as proton acceptor sites as well as N(4). The electronic corrugation at the six-membered ring might also induce stacking. Such molecular stacking was reported by Rey and co-workers on NN derivatives substituted by pyrimidine ring [16].

3.3. IR spectra

Fig. 3. Molecular design and hydrogen bonding pattern of NN with imidazole ring.

Hydrogen bonding patterns could be qualitatively analyzed by IR spectra in the stretching mode of NH site. Im-NN exhibited very broad and complex band

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around 2200–3200 cm − 1 corresponding to the hydrogen bonded N– H stretching vibration, while BIm-NN showed a single strong band at 3160 cm − 1 [17]. Pu-NN shows two different IR patterns according to the solvents used for crystallization (Fig. 4). The band of a-phase appears at 3165 cm − 1 whose shape was similar to that of BIm-NN, and this implies that the –NH site participates in the intermolecular hydrogen bonding. On the other hand, b-phase shows very narrow absorption peak at 3233 cm − 1. The IR patterns are quite different from each other, which exhibit different hydrogen bonding patterns in solid crystals.

Table 2 Acidic pKa values of benzimidazole and its analogs

Fig. 5. Temperature dependence of mT ( ) and  m− 1 () for Pu-NN (a-phase). Solid lines correspond to calculated curves (see text).

Fig. 6. Temperature dependence of mT ( ) and  m− 1 () for Pu-NN (b-phase). Solid line corresponds to calculated curve (see text).

3.4. Magnetic properties

Fig. 4. IR spectra of radicals (neat).

The magnetic data for Pu-NN as thermal variation of reciprocal molar susceptibility,  m− 1 and mT product are shown in Figs. 5 and 6, respectively. With regard to BIm-NN, the formation of a high-spin molecular assembly was observed at low temperature, which could be interpreted in terms of the 1-D Heisenberg ferromagnetic chain model [11] with J= + 12 cm − 1. The magnetic data of a- and b-phases of Pu-NN are enormously different from that of BIm-NN. The  m− 1 value of a-phase Pu-NN was fitted to Curie–Weiss law, m =C/ (T− q) with the Curie constant of 0.375 emu K mol − 1 and the Weiss constant of −5.7 K. The mT at 300 K is 0.368 emu K mol − 1, which is a little smaller than that of an isolated monoradical. Below approximately 110 K, the value decreases rapidly. Temporally, we

H. Nagashima et al. / Polyhedron 20 (2001) 1151–1155

analysed the magnetic data using the 1-D Heisenberg antiferromagnetic chain model [18], m =

NAg 2v 2B 0.25 + 0.074975x + 0.075235x 2 kBT 1.0+ 0.9931x +0.172135x 2 +0.757825x 3

with x = J /kBT. A good fit was obtained for J = − 5.6 cm − 1 using least-square fitting. On the other hand, the  m− 1 value of b-phase Pu-NN was fit to the Curie – Weiss law with a Curie constant of 0.356 emu K mol − 1 and a Weiss constant of − 0.3 K. The mT at 300 K is 0.356 emu K mol − 1, remains almost constant over temperature range measured, and we could not observe notable magnetic interaction. Unfortunately, we could not obtain a crystal suitable for X-ray analysis for both phases due to the difference in solvent solubility between Pu-NN and BIm-NN. The replacement of hydrocarbon unit by nitrogen atoms largely affects the hydrogen bonding motif. As a result, Pu-NN exhibits quite a different magnetic behavior compared to that of BIm-NN. Further investigations to give single crystals suitable for X-ray analysis are in progress.

Acknowledgements This work was supported by the Grant-in-Aid for Scientific Research (No. 3022392) from the Ministry of Education, Science, Sports and Culture of the Japanese Government.

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