HETEROCYCLES

Paper | Regular issue | Vol. 87, No. 10, 2013, pp. 2031-2036
Received, 5th August, 2013, Accepted, 2nd September, 2013, Published online, 13th September, 2013.
DOI: 10.3987/COM-13-12800

■ INTENTIONAL SYNTHESIS OF BINARY RADICAL THAT BEARING NITRONYL AND IMINO NITROXIDES: X-RAY ANALYSIS AND MAGNETIC MEASUREMENT

Fumiyasu Iwahori,* Takayuki Suzuki, and Kosuke Kato

Department of Chemistry, College of Humanities and Sciences, Nihon University, Sakurajousui, Setagaya-Ku, Tokyo 156-8550, Japan

Abstract

We have synthesized new binary radical that bearing both nitronyl and imino nitroxides with the aim of enabling to control the occupancy of oxygen atom in the nitronyl nitroxide crystal. X-ray analysis, ESR study and magnetic measurement revealed that the tuning of occupancy of the oxygen atom in the crystal might make an effect on the magnetic property of nitronyl nitroxides.

INTRODUCTION
To date nitronyl nitroxides and imino nitroxides have been received much attention because of their stabilities and conveniences of the preparations and applications.1,2 The nitronyl nitroxide derivatives are usually synthesized in two steps from aldehydes and 2,3-bis(hydroxyamino)-2,3-dimethylbutane. The first step is the cyclization reaction to form imidazolidine ring of precursor that followed by the oxidation reaction to give nitronyl nitroxides. When using bis-aldehyde as the starting reactant, bis(nitronyl nitroxide) radicals can be obtained. The series of imino nitroxides are easily prepared by treatment of related nitronyl nitroxides with nitrogen oxide. During the workup of the oxidation reaction of bis(nitronyl nitroxide) precursor, the minor product as well as target biradical is sometimes separated out. This minor product is a binary radical that bearing both nitronyl and imino nitroxide substituents. There are many experimental and theoretical studies on bis(nitronyl nitroxide) and/or bis(imino nitroxide) molecules,1,2 but limited papers have described about binary radicals consists of both nitronyl and imino nitroxides.1,3,4 The intermolecular magnetic interaction is very significant factor for the magnetism of the molecular crystals. The magnetic property of the crystalline sample of nitroxide-based radical is sometimes determined by the intermolecular magnetic interaction that closely related to the stacking pattern of neighboring N-O parts. We addressed our attention to the intentional synthesis of binary radical with the aim of enabling to control the oxygen atom occupancy, and expected to find a difference in the magnetic property of nitronyl nitroxide crystal. To our knowledge, only Turek and co-workers reported the magnetic properties of binary radicals that bearing nitronyl and imino nitroxides in detail,4 but those radicals are recognized as unexpected side products even in their reports. In this paper, we would like to fix our attention on the intentional synthesis, structural and magnetic studies of binary radical 1O3 (Scheme 1).

RESULTS AND DISCUSSION
Binary radical 1O3 was synthesized by stepwise cyclization method as shown in Scheme 1 (Details are in Experimental section). Structures and abbreviations of reference biradicals (1O2 and 1O4) that obtained by simultaneous cyclization reaction are also depicted in Scheme 1.

The absorption spectra of CHCl3 solution of three biradicals (1O2, 1O3 and 1O4) are shown in Figure 1. For 1O4 and 1O2, typical absorption maxima of nitronyl nitroxide (λmax ≈ 610 nm) and imino nitroxide (λmax ≈ 450 nm) were observed. In contrast, spectrum of 1O3 shows broad absorption band in visible range without clear absorption maximum. Owing to this broad absorption band, the color of the solution of 1O3 is black, while those of 1O2 and 1O4 are orange and blue.

The existence of two isosbestic points at 410 and 530 nm indicates that the electronic states of two radical substituents in 1O3 are independent of each other. ESR spectrum of the toluene solution of 1O3 (1.7 × 10–4 M) that degassed by freeze-pump-thaw cycle was measured at room temperature. The observed and simulated ESR spectra are shown in Figure 2. Taking into account of hyperfine coupling of four nitrogen atoms of nitronyl nitroxide and imino nitroxide, observed spectrum was reproduced by using hyperfine

coupling constants of aN1 = aN2 = 0.367 mT (nitronyl nitroxide fragment), aN3 = 0.450 mT and aN4 = 0.208 mT (imino nitroxide fragment).
X-Ray crystallographic analyses of three radicals were carried out at 200 K.5(a) Although the crystallographic data of 1O4 has already reported,7 we individually measured its crystal structure to make better condition of comparison of the parameters. Unexpectedly, the crystallographic data of 1O2 has not been reported, and therefore we prepared the single crystal and determined the crystal structure. Their cell parameters are listed in Table 1, and the molecular structure of 1O3 is shown in Figure 3.

All biradical molecules have symmetry centers in the crystals. The cell parameters of 1O3 are similar to those of 1O4 rather than 1O2. The crystal packing manner of 1O3 seems to identical with that of 1O4, but the occupancy of the oxygen atom is very different from each other. Of course, the occupancies of all oxygen atoms in 1O4 are unity, but those found in 1O3 by least square refinement are 0.944(6) Figure 4. Intermolecular short contacts. Distances (Å) are (a) 3.45, (b) 4.36, (c) 3.62, (d) 3.36, (e) 4.68, (f) 4.77for O(1) and 0.568(6) for O(2), respectively. Based on this result, the sum of oxygen occupancies par one molecule of 1O3 determined to 3.02. This value is consistent with the molecular structure of 1O3.

Torsion angles between the mean planes of p-phenylene and imidazoline rings are 17.8˚ (1O2), 33.3˚ (1O3) and 33.4˚ (1O4), respectively. Surprisingly, the torsion angle found in 1O3 is almost identical to that of 1O4 in spite of the difference in the oxygen occupancies. This result predicts that the intramolecular magnetic exchange of 1O3 and 1O4 would be similar magnitude.2c The short contact distances between O(2) and intermolecular neighboring heterocycles are shown in Figure 4. This X-ray result of intermolecular contact led us to expect to find a difference in the intermolecular magnetic interaction.

Additionally, we have succeeded in determining unambiguous molecular structure of 1O3 with the aid of the coordination compound. Bis(hexafluoroacetylacetonato)- manganese(II) complex of 1O3 was prepared6 for unambiguous analysis of the molecular structure and the X-Ray result is shown in Figure 5. Unlike the authentic biradical, no disorder of oxygen atom was found in Mn(II) complex. The coordination bond was formed between Mn(II) and Figure 5. Mn complex of 1O3oxygen of nitronyl nitroxide, not imino nitroxide fragment.5b

The magnetic moment of crystalline 1O3 was recorded in 0.5 T of applied magnetic field by using SQUID magnetometer. The temperature dependence of magnetic susceptibility (χm) and χmT was depicted in Figure 6. As decreasing temperature, χmT value gradually decreased and approached to zero. This behavior indicates that antiferromagnetic magnetic interaction is predominant in the crystal of 1O3. Upon cooling the temperature, χm increased and reached maximum at 32 K, then recorded a minimum at 6 K and turned to rising again. The theoretical fitting of χmT curve by using of Bleaney-Bowers model revealed that there is exchange interaction of J/kB = –26 K (for H = –2J S1S2). This result is very different from that of 1O4 reported by Gatteschi and his coworkers.7 In their report, χm of 1O4 goes through a maximum at 65 K. Although the X-ray result predicted that the intramolecular exchange interaction of 1O3 and 1O4 would be similar, the magnetic measurement revealed that J/kB found in 1O3 is almost half of the Figure 6. Magnetic property of 1O3reported value7 of 1O4. Our result revealed that the tuning of the occupancy of oxygen atom in nitronyl nitroxide crystal make an effect on the magnetic properties in low temperature region.

In conclusion, we have successfully synthesized nitronyl and imino nitroxide binary biradical. The oxygen occupancies in the crystal of 1O3 were determined to 0.944 and 0.568 by X-ray study. The lowered occupancy of oxygen atom in nitronyl nitroxide affected magnetic behavior of biradical crystal. Our result revealed that the occupancy control of oxygen in nitronyl nitroxide gives a remarkable difference in the magnetic property. It seems like an important clue to designing and tuning the intermolecular magnetic interaction of nitroxide radicals.

EXPERIMENTAL
Synthesis of 2: 1.48 g (10 mmol) of 2,3-bis(hydroxyamino)-2,3-dimethylbutane and 4.82 g (36 mmol) of terephthalaldehyde was dissolved in 230 mL of benzene and refluxed 3 h. The solution was cooled to room temperature and precipitation was filtered off, then dried under reduced pressure. 920 mg of 2 (34.8%) was obtained as white powder. 1H NMR (270 MHz, DMSO-d6) δ (ppm): 1.06-1.09 (d, 12H), 4.60 (s, 1H), 7.36-7.90 (dd, 4H), 10.01 (s, 1H).
Synthesis of 3: To the solution of 529 mg (2.0 mmol) of 2 in 30 mL of MeOH, SeO2 (22mg, 0.2 mmol) was added and refluxed 1.5 h. The solution was cooled, then filtered through the Celite filter and the filtrate was evaporated under reduced pressure. After the silica-gel column chromatography (Rf = 0.57, CHCl3:MeOH = 7:1), 326 g of 3 (66.1 %) was obtained. 1H NMR (270 MHz, DMSO-d6) δ (ppm): 1.14 (br, 12H), 7.93-7.96 (d, 4H), 10.04 (s, 1H).
Synthesis of 4: In 7 mL of MeOH, 150 mg (0.61 mmol) of 3 and 108 mg (0.73 mmol) of 2,3-bis(hydroxyamino)-2,3-dimethylbutane were dissolved and refluxed 1 day. The solvent was removed under reduced pressure and the residue was treated by silica-gel column chromatography (Rf = 0.60, CHCl3:MeOH = 3:1). White solid of radical precursor, 4 was obtained (121 mg, 52.9%). 1H NMR (270 MHz, DMSO-d6) δ (ppm): 0.98-1.24 (m, 24H), 4.55 (s, 1H), 7.50-7.89 (d, br, 4H).
Synthesis of 1O3: To the ice-cooled CHCl3 (10 mL) solution of 4 (50.0 mg, 0.13 mmol), aqueous NaIO4 (83 mg, 0.39 mmol) was added and stirred 1 h at 0 ˚C. After the usual workup, crude product was purified by silica-gel column chromatography (Rf = 0.37, CHCl3:MeOH = 20:1) and black solid of biradical 1O3 (29 mg, 60.6%) was obtained. IR(KBr): ν(N-O) = 1363 cm-1.
Preparation of single crystals: An X-ray-quality single crystal was prepared by recrystallization. The vapor of n-hexane was slowly introduced into CHCl3 solution of 1O3 and black block crystal of 1O3 was obtained after 2 days. Other reference radicals, 1O2 (imino nitroxide biradical) and 1O4 (nitronyl nitroxide biradical) were also synthesized by the methods similar to those in literatures.1,7 The single crystals of 1O2 and 1O4 are prepared by similar procedure to that used for 1O3. For the recrystallization of 1O2, n-pentane was used as poor solvent instead of n-hexane.

ACKNOWLEDGEMENTS
This research was financially supported by the Japan Ministry of Education, Culture, Sports, Science and Technology-Supported Program for the Strategic Research Foundation at Private Universities, 2009-2013 (No. S0901022). We thank Professors Akiko Kobayashi, Hayao Kobayashi and Biao Zhou for the SQUID measurement.

References

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