VOLUME 15, NUMBER 15 © Copyright 2003 by the American Chemical Society
Communications Magnetism of Conjugated Organic Nitroxides: Structural Scaffolding and Exchange Pathways
Scheme 1. Synthesis of β-BDA-NIT (1)
Lora M. Field and Paul M. Lahti* Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003 Received April 2, 2003 Revised Manuscript Received May 23, 2003 The role of hydrogen atom interactions in molecular magnets has been an area of substantial interest in both inorganic and organic materials. Hydrogen bonds have been suggested as direct electronic exchange linkers, based both on computational studies1 and experimental2 investigations. In some cases, hydrogen bonds act purely as molecular assembly interactions, even when it is plausible that they act as exchange linkers.3 It is therefore not entirely clear when hydrogen bonds have an electronic role in molecular magnets, and when a purely structural role. In this study, we report a new radical, β-4-(N-tert-butyl-N-aminoxyl)-1,1′-biphenyl-3′,5′dicarboxylic acid (β-BDA-NIT, 1, NIT ) nitroxide), with hydrogen-bonded interactions of both types. (1) Zhang, J.; Wang, R.; Baumgarten, M. Mol. Cryst. Liq. Cryst. 1997, 306, 705. (2) (a) Sugawara, T.; Matsushita, M. M.; Izuoka, A.; Wada, N.; Takeda, N.; Ishikawa, M. J. Chem. Soc., Chem. Commun. 1994, 1723. (b) Sugawara, T.; Izuoka, A. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1997, 305, 41. (c) Sugawara, T.; Nakazaki, J.; Matsushita, M. M. In Magnetic Properties of Organic Materials; Lahti, P. M., Ed.; Marcel Dekker: New York, 1999; Chapter 26, p 540. (d) Cirujeda, J.; Ochando, L. E.; Amigo´, J. M.; Rovira, C.; Rius, J.; Veciana, J. Angew. Chem., Int. Ed. Engl. 1995, 34, 55. (e) Cirujeda, J.; Herna`ndez-Gasio´, E.; Panthou, F. L.-F.; Laugier, J.; Mas, M.; Molins, E.; Rovira, C.; Novoa, J. J.; Rey, P.; Veciana, J. Mol. Cryst. Liq. Cryst. 1995, 271, 1. (f) Cirujeda, J.; Herna`ndez-Gasio´, E.; Rovira, C.; Stanger, J.-L.; Turek, P.; Veciana, J. J. Mater. Chem. 1995, 5, 243. (3) Ferrer, J. R.; Lahti, P. M.; George, C.; Oliete, P.; Julier, M.; Palacio, F. Chem. Mater. 2001, 13 (7), 2447.
Radical 1 was synthesized as shown in Scheme 1 by coupling of protected hydroxylamine 2 with dimethyl 3-iodoisophthalate 3, followed by appropriate deprotection and oxidation. The syntheses of intermediates 2-6 are described in the Supporting Information. Compound 1 is a red solid that appears indefinitely stable when stored in the dark.4 Two crystallographic allotropes have been identified to date, a triclinic P1 h form (R-1) and an (4) Characterization of 1: small red needles, decomposes without melting upon heating. Anal. Calcd for C18H18N1O5: C, 65.84; H, 5.53; N, 4.27. Found: C, 65.59; H, 5.48; N, 4.13. ESR (benzene): aN ) 12.02 (nitroxide N), aH(aromatic) ) 2.12 (2H) and 0.93 (2H) gauss. For β-1 crystallography: red needle, molecular formula ) C18H18NO5, M ) 328.344; orthorhombic, Pbca, a ) 16.4458(3) Å, b ) 6.7793(1) Å, c ) 29.6942(6) Å, V ) 3310.64(10) Å3 , Z ) 8, m ) 0.097 mm-1, T ) 293 K, data/parameters ) 2886/219, converging to R1 ) 0.0637, wR2 ) 0.1525 (on 1803 reflections, I > 2σ(I) observed data); R1 ) 0.1078, wR2 ) 0.1761 (all data), residual electron density: 0.267 e/Å3. See the Supporting Information for crystallographic coordinates and other information. CCDC ref # 204608.
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Figure 1. Crystal packing scheme for β-1. For the stick scheme, red lies above black, which lies above blue. Dotted lines show the (H′)C′(9)‚‚‚O(1)N(1) close contacts (see also Figure 3, A‚‚‚A′ contacts).
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Figure 3. Aryl NIT(NO)‚‚‚‚NIT(tert-butyl) and CH‚‚‚‚ON(NIT) contacts along the b-axis, shown as A‚‚‚A (dashed lines) and A‚‚‚A′ (arrows) in the picture. An ORTEP diagram with crystallographic number is shown in the lower right, approximately in the orientation of an “A”-molecule. Each molecule is part of a separate hydrogen-bonded chain in Figure 1.
coupling constant and θ is a mean field constant. Other constants have their usual meanings, while coefficients A-F are given in the supporting material from ref 5b. An excellent fit to the experimental data was obtained with C0 ) 0.348 emu‚K/Oe‚mol, J/k ) (-)3.86 K, and θ ) -0.38 K, as shown in Figure 2. A simple spin-pairing model,6 for example, gives not nearly so good a fit (see Supporting Information).
χ) Figure 2. Plot of paramagnetic susceptibility (χ) versus absolute temperature. The solid line shows the fit to a Bonner-Fisher chain model (eq 1) as described in the text. The inset shows a Curie-Weiss plot of reciprocal magnetic susceptibility versus temperature, for which a solid line is a linear fit to the T > 100 K data.
orthorhombic Pbca form (β-1). Allotrope R-1 has smaller Ph-NIT bond torsion, 4.5° and 30.9° for the two crystallographically distinct molecules of the R-form versus 37.3° for the β-form. We have only isolated very small amounts of R-1, and typically obtained the β-1 phase. Both allotropes form extended 1-D hydrogenbonded chains based on the 1,3-phenylenedicarboxylic acid units, with NIT groups interdigitated between the hydrogen-bonded chains (Figure 1). We carried out dc magnetic susceptibility studies of β-1 at various temperatures. The results are shown in Figure 2 with a paramagnetic susceptibility (χ) versus T plot and a Curie-Weiss plot of 1/χ versus T. The latter plot is highly linear down to low temperatures, with a slope of 3.026 Oe‚mol/emu and a Weiss constant of θ ) -0.48 K for the T > 100 K data. We found the best fit of the χ(T) data using a 1-D Bonner-Fisher antiferromagnetically exchange coupled chain model for S ) 1/2 spin carriers, with inclusion of a mean field term.5 This model is given by eq 1, where J/k is the exchange
(
4C0
)
A + Bx + Cx2 [T - θ] 1 + Dx + Ex2 + Fx3
x )| J/kT|; C0 ) Ng2µB2S(S + 1)/3k
(1)
Solution state ESR hyperfine analysis shows no resolvable spin density delocalization from the NIT onto the 1,3-phenylenedicarboxylic acid ring, so we assume that the carboxylic acid-linked ribbons are not electronically important in the magnetic behavior of 1. With this exclusion, we sought chain-type motifs in the crystal structure to match the exchange behavior. The closest through-space approach between NIT units is r[O(1)‚‚ ‚N(1′)] ) 4.584(4) Å, along the c-axis, in a geometry that greatly limits orbital overlap. There is no close π-stacking between aromatic rings with significant spin density. The Ph-NIT b-axis stacks are strongly offset, with an interplane distance of about 5.5 Å. There is a chain of intramolecular contacts from NIT NO units to NIT tert-butyl methyl groups along the b-axis, N(1)O(1)‚‚‚HC(1′)-C(4′)-N(1′)O(1′)‚‚‚, where r[O(1)‚‚‚C(1′)] ) 3.160(4) Å (Figure 3, A‚‚‚A contacts). But if this were the dominant spin polarization exchange pathway, the net coupling between NIT units would likely be ferromagnetic, not AFM as observed (see Supporting Information). Also, due to the very small spin populations (5) (a) Bonner, J. C.; Fisher, M. E. Phys. Rev. A 1964, 135, 650. (b) Bonner, J. C. Ph.D. Dissertation, University of London, UK, 1968. (6) Bleaney, B.; Bowers, K. D. Proc. R. Soc. London, A 1952, 214.
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on the tert-butyl groups,7 the A‚‚‚A contacts should yield an extremely weak exchange pathway. A more plausible exchange pathway connects NIT units from different hydrogen-bonded chains into a spiral chain N(1)O(1)‚‚‚H(9′)C(9′)-C(10′)-C(5′)-N(1′)O(1′)‚‚‚ (Figure 1 and Figure 3 A‚‚‚A′ contacts). The NIT N(1)O(1) and neighboring m-H(9)C(9) groups are in close proximity, with r[O(1)‚‚‚m-C(9′)] ) 3.715(4) Å. Assuming that the H(9)C(9) meta proton has the qualitative spin polarization shown in Figure 1, this path should yield AFM exchange, as observed. The proton meta to the NIT in 1 has an experimental hyperfine coupling constant of 0.9 G, corresponding to a spin density of about (-)4% on the meta carbon. UB3LYP/6-31G* model computations8,9 for the Ph-NIT ring of β-1 predict the proton hyperfine coupling to be 0.7-0.8 G, in good accord with experiment. The computed qualitative spin polarization pattern agrees with Figure 1 (Supporting Information). The N(1)-O(1) singly occupied molecular orbital points of 1 right at the meta H(9′)C(9′) bond in the neighboring molecule of the exchange chain of Figure 1, providing a good overlap mechanism to enhance the A‚‚‚A′ AFM interaction shown in Figures 1 and 3. On (7) Kreilick, R. W. J. Chem. Phys. 1966, 45, 1922. (8) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (9) Computations were carried out with: Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; HeadGordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.3; Gaussian, Inc.: Pittsburgh, PA, 1998.
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the basis of these contacts, the significant experimental spin density on H(9)C(9), and the analysis of Figure 1, we feel that this exchange mechanism is most consistent with the observed 1-D AFM chain behavior. Although alternative exchange mechanisms such as throughspace coupling of spin density sites that are not in close contact cannot definitively be ruled out in β-1, the observed behavior is most consistent with the proposed mechanism and magneto-structurally reasonable. This analysis of β-BDA-NIT magnetism exemplifies multiple types of close hydrogen-bond-like contacts, where some act as scaffolding alone, and others as important electronic exchange transmission pathways. Hopefully, such magneto-structural correlations will help to identify electronically predictable structure elements in organic and inorganic/organic hybrid materials that incorporate similar10 contacts. Acknowledgment. This work was supported by the National Science Foundation (CHE 0109094). Magnetic measurements were obtained at the UMass-Amherst Nanomagnetics Characterization Facility (NSF CTS0116498). The authors thank A. Chandrasekaran of the UMass-Amherst X-ray Structural Characterization Center (NSF CHE-9974648) for crystallographic analyses and G. Dabkowski of the UMass-Amherst Microanalytical Laboratory for elemental analyses. Supporting Information Available: Details of syntheses for 1-6, ESR spectroscopy, selected details of crystallographic and magnetic analyses, and computed spin density information for 1 (CIF and PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM034225V (10) For example, as noted by a reviewer, see similar AFM chain behavior of (Me3N-TEMPO)+ PF6-) described in Aonuma, S.; Casellas, H.; Faulmann, C.; de Bonneval, B. G.; Malfant, I.; Cassoux, P.; Lacroix, P. G.; Hosokoshic, Y.; Inoue, K. J. Mater. Chem. 2001, 11, 337.
