Metamagnetic Behavior Observed in Purely Organic 5-Azaindol-2-yl Nitronyl Nitroxide Brick-Wall Architecture Hideaki Nagashima, Shin’ya Fujita, Hidenari Inoue, and Naoki Yoshioka* Department of Applied Chemistry, Faculty of Science and Technology, Keio University, Kohoku-ku, Yokohama 223-8522, Japan Received July 7, 2003;
CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 1 19-21
Revised Manuscript Received August 27, 2003
ABSTRACT: A novel stable organic radical, 2-(5-azaindol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazoline-1-oxyl-3oxide (3), forms a two-dimensional brick-wall architecture based on hydrogen bonding and π-stacking. Low-temperature magnetization curves revealed an S-shaped dependence on the applied field below ca. 3.0 K, which could be interpreted as metamagnetic behavior. The supramolecular assembling process using noncovalent bonds such as hydrogen bonds or π-stacks has been recognized as a high-potential technique to construct functional molecular materials.1 As a successful example, we have reported the hydrogen-bonded benzimidazol-2-yl nitronyl nitroxide (1) self-assembly, which exhibits a strong intermolecular ferromagnetic interaction.2 In the crystal of 1, the NH proton donor site plays a very important role in forming the characteristic branched hydrogen bond, which enables 1 to have a close contact between NN units. This result prompted us to start the systematic study of nitronyl nitroxide derivatives having the NHCC(NO)NO supramolecular synthon. During the course of our investigation on azaindol-2-yl nitronyl nitroxide derivatives having such a synthon,3 we found that 5-azaindol-2-yl nitronyl nitroxide (3) formed a two-dimensional brick-wall architecture based on hydrogen bonds and π-stacks. Although tens of metal complexes forming a brick-wall architecture have been characterized structurally4 as well as magnetically,5 there are still only a few purely organic examples of this architecture,6 and 3 is the first example of a purely organic radical that forms a brick-wall architecture based on distinct intermolecular forces. In addition, its magnetic property is peculiar, which could be interpreted as metamagnetic behavior. Here, we describe the synthesis, crystal structure, and magnetic property of the novel nitronyl nitroxide 3.
The synthetic route of 3 is described in Scheme 1 (see Supporting Information for more details). Acetal 5 was prepared by the Sonogashira cross-coupling of 4-amino-3iodopyridine7 with propargylaldehyde diethylacetal using a palladium catalyst to yield 4, followed by the intramolecular cyclization of 4 under a strong basic condition using KOtBu.8 Compound 5 was converted to the corresponding aldehyde 6 by hydrolysis. Condensation of the corresponding aldehyde 6 with 2,3-bis(hydroxyamino)-2,3-dimethylbutane,9 followed by chemical oxidation with NaIO4, yielded a nitronyl nitroxide 3 as reported by Ullman et al.10 A single crystal of 3 suitable for X-ray diffraction analysis was prepared by slow evaporation from a dichloromethane/ n-hexane solution. * To whom correspondence should be addressed. Tel: +81-45-566-1585. Fax: +81-45-566-1551. E-mail:
[email protected].
Figure 1. ORTEP drawing and atomic numbering of the molecular structure of 3.
Compound 3 crystallizes in the space group P21/a with four molecules in a unit cell.11 Figure 1 shows the ORTEP drawing of 3. The dihedral angle between the best planes of the pyrrole ring and the ONCNO moiety is 6.98(7)°. When compared to the corresponding angle between the imidazole ring and the ONCNO moiety of 24.3° for 1, such high coplanarity is the common characteristic for azaindol2-yl nitronyl nitroxide derivatives (e.g., the corresponding dihedral angles of 2 are 3.3(2) and 5.8(2)°). This characteristic is explained by the concommitance of the intramolecular hydrogen bonds at C-H‚‚‚O-N and N-H‚‚‚O-N. The pyridine-type imine character of N(6) leads to the slightly shorter distances of N(6)-C(18) and N(6)-C(19) (1.320(2) and 1.356(3) Å, respectively) as compared to those of other C-C bonds. Crystal packing viewed on the ab plane and along the a axis is shown in Figure 2. A short intermolecular distance between N(5)‚‚‚O(1)* of 2.867(2) Å is found, corresponding to an intermolecular hydrogen bond (symmetry code: (*) x + 1/2, -y + 1/2, z). The hydrogen-bonding motif is repeated to form a noticeable chain structure along the a axis. These hydrogen bonds trigger rather short O(2)‚‚‚O(1)* contacts of 3.269(2) Å. However, the orbital overlap between SOMOs is expected to be small by taking into account the large dihedral angle between the neighboring ONCNO moieties of 63.0(2)°. Between the hydrogen-bonded chains, a relatively close intermolecular contact is observed, corresponding to π-stacking between the 5-azaindole rings and ONCNO moieties (symmetry code: (**) -x + 2, -y + 1, -z + 2). Close contacts within 3.8 Å are summarized in Table S1 (see Supporting Information). Although the N(6) atom could become a proton acceptor site, it does not participate in the intermolecular hydrogen bonding. Calculations show that the 5-azaindole ring has the highest dipole moment among the azaindoles,12 so the introduction of an N-atom at the 5-position of the indole ring seems to make 3 more easier to π-stack rather than to form a hydrogen bond. The N(H)‚‚‚O hydrogen bond and π-stack
10.1021/cg0341205 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/15/2003
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Crystal Growth & Design, Vol. 4, No. 1, 2004 Scheme 1.
Communications Synthetic Route of 3
Figure 3. χmT - T plot of 3 measured at 0.5 T. Inset shows χm T plots measured at 0.5 T (b) and 3.0 T (O).
Figure 2. Crystal packing of 3 viewed on the ab plane (a) and viewed along the a axis (b). All hydrogen atoms except for H5 are omitted. In the case of (a) C8-C13 are also omitted for clarity. Blue dotted lines represent intermolecular hydrogen bonding, and black dotted lines highlight the brick-wall architecture. The black rectangles highlight the brick-wall piling along the c axis.
form a two-dimensional brick-wall architecture on the ab plane (Figure 2a) in the crystal. The layers pile along the c axis while facing the four methyl groups of nitronyl nitroxide units (Figure 2b). Because no contact between the ONCNO moieties within 4.2 Å is observed between the layers, a strong interlayer magnetic interaction caused by direct orbital overlap of SOMO would not be expected. The temperature dependence of the magnetic susceptibility was measured for a polycrystalline sample of 3 in the temperature range of 1.8-300 K. Figure 3 shows the χmT - T plot measured under the applied field of 0.5 T.
Figure 4. Low-temperature magnetization curves of 3 measured at 1.8 K (b), 2.0 K (O), 3.0 K (4), and 4.0 K (0).
The χmT value slightly increases when the temperature decreases to ca. 30 K, showing the occurrence of a very weak but noticeable ferromagnetic interaction. The magnetic data from 30 to 100 K obey the Curie-Weiss law with a Curie constant of 0.367 emu K mol-1 and positive Weiss constant of +0.6 K, showing that the intralayer interaction is ferromagnetic. Below ca. 20 K, however, the χmT value suddenly decreases and reaches 0.102 emu K mol-1 at 1.8 K, suggesting an interlayer antiferromagnetic interaction. The χm - T plot (Figure 3, inset) shows a cusp around 3.0 K, which disappears under the high external magnetic field of 3.0 T. This indicates that a transition from the antifer-
Communications romagnetic to ferromagnetic state occurs under the adequately large applied field below 3.0 K. Figure 4 shows the low-temperature magnetization curves at 1.8, 2.0, 3.0, and 4.0 K. At 1.8 and 2.0 K, we can clearly confirm the S-shape dependence on the magnetic field. The observed behavior is quite similar to those of the Cu2(OH)3(nCmH2m+1COO) (m ) 0, 1) layered complexes13 or {[Cu(mal)(0.5pyz)]‚H2O}n,14 which was reported as metamagnetic behavior. Consequently, we found that 3 revealed metamagnetic behavior below ca. 3.0 K. It is very difficult to clearly discuss the nature of the intralayer ferromagnetic interaction because the magnitude of the ferromagnetic interaction is rather weak. The UB3LYP/EPR-II single point calculation for the experimental geometry of 3 shows that a small negative spin density (-0.001) and a large positive spin density (+0.345) are induced on H(5) and O(1), respectively (Table S2, see Supporting Information). The hydrogen bond between the NH proton and the nitroxide O-atom might propagate the ferromagnetic interaction because of the close contact between the atoms on which the opposite sign of the spin densities are induced that causes a ferromagnetic interaction.15 On the other hand, each layer is separated by facing the four methyl groups of the nitronyl nitroxide unit. The occurrence of an interlayer antiferromagnetic interaction is understandable by taking into account the negative spins polarized on the H-atoms of the methyl groups due to the close contact between the atoms on which the same sign of the spin densities are induced that causes the antiferromagnetic interaction. In summary, we have synthesized a curious nitronyl nitroxide brick-wall network based on hydrogen bonding and π-stacking. The magnetic measurements show metamagnetic behavior, which is plausible by taking into account the layered crystal structure. Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research (B) 15310094 from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. H.N. gratefully acknowledges the financial support from Keio University (Keio Leading-Edge Laboratory of Science and Technology) and a 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: Syntheses of 3-6, Table S1 (close atomic contact observed in π-stacked dimer within 3.8 Å), Table S2 (calculated spin densities of 3), and an X-ray crystallographic file of 3 in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.
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