Article pubs.acs.org/crystal
Planar Biphenyl-Bridged Biradicals as Building Blocks for the Design of Quantum Magnets Evgeny A. Mostovich,†,# Yulia Borozdina,† Volker Enkelmann,† Katarina Remović-Langer,‡ Bernd Wolf,‡ Michael Lang,*,‡ and Martin Baumgarten*,† †
Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany Physikalisches Institut, J.W. Goethe-Universität, Max-von-Laue Strasse 1, D-60438 Frankfurt(M), Germany
‡
S Supporting Information *
ABSTRACT: We have synthesized and investigated a new biphenyl-4,4′bis(nitronyl nitroxide) radical with intermediately strong antiferromagnetic interactions. This organic biradical belongs to a family of materials that can be used as a building block for the design of new quantum magnets. For quantum magnetism, special attention has been paid to coupled S = 1/2 dimer compounds, which when placed in a magnetic field, can be used as model systems for interacting boson gases. Short contacts between the oxygen atoms of the nitronyl nitroxide units and the hydrogen atoms of the benzene rings stabilize a surprisingly planar geometry of the biphenyl spacer and are responsible for a small magnetic interdimer coupling. The strength of the antiferromagnetic intradimer coupling constant J/kB = −14.0 ± 0.9 K, fitting the experimental SQUID-data using an isolated-dimer model. The deviations from the isolated-dimer model are attributed to a small interdimer coupling J′/kB, on the order of 1 K, consistent with the crystal structure.
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INTRODUCTION Organic biradicals have for a long time fascinated chemists and physicists.1 It was the control of the electronic properties of these molecules, plus their function as building blocks or components for molecular electronics and molecular magnets, which brought them into focus.2 Although biradicals with conjugated p-phenylene bridging usually give a singlet ground state,3 there are also some examples with m-phenylene bridges, which have been found in the low spin ground state.4 In order to adjust the character and size of the spin exchange energy (J) in biradicals, the geometries, topologies, and heteroatom substitutions can be modified to act as setscrews, allowing for a broad variation and fine-tuning of the spin exchange energy.5 In order to further handle such biradicals, they should be stable under ambient conditions for in-depth characterization and supramolecular ordering. In that respect, nitronyl and imino nitroxides are excellent candidates for adopting them as building blocks.6 They have widely been used for the construction of new magnetic materials with different prospective properties.6−8 Thus, a promising synthetic approach would be to combine the nitronyl nitroxide spins through a p-phenylene bridge, in order to obtain a novel antiferromagnetically coupled biradical. These biradicals, properly designed, may serve as building blocks for new quantum magnets for studying fundamental aspects of interacting quantum particles.9 In fact, by exposing closely spaced pairs of spin S = 1/2 entities (dimers) to a magnetic field, strong enough to close the singlet−triplet gap of the isolated dimer, a gas of delocalized bosonic excitations can be generated. Depending on the dimensionality of the dimer−dimer interaction, phenomena such as Bose−Einstein condensation of magnons9 for threedimensional systems or Luttinger-liquid behavior10 in quasi© 2011 American Chemical Society
one-dimensional arrays can be studied. The comparatively weak intradimer interactions, the absence of any orbital contribution to the magnetism, and the weak dimer−dimer interactions always present through the hydrogen atoms11 (and references cited therein) in the crystal structure make organic biradicals very promising candidates for studying interacting quantum particles under well-controlled conditions. In a previous study, we investigated nitronyl-nitroxide-based biradicals, bridged by tolane-derived linkers,12 and found a magnetic field-induced quantum phase transition13 (unpublished results) at temperatures below 0.3 K and magnetic fields 7.8 T ≤ B ≤ 10.2 T While these observations prove the use of nitronyl-nitroxide biradicals for generating quantum magnets, extensions of this concept should be explored. In particular, there is a need for tuning the materials’ parameter in a way such that the B-induced phase transition can be accessed by standard experimental conditions, that is, not too low temperatures and magnetic fields below about 10 T. Here we describe the synthesis, X-ray structure, and magnetic properties of 4,4′-bis(1-oxyl-3-oxy-4,4,5,5-tetramethyl-4H,5Himidazolin-2-yl)biphenyl in solution and in the solid state. The strength of the antiferromagnetic intradimer coupling constant is determined to be J/kB = −14.0 ± 0.9 K. This value, together with an interdimer coupling J′/kB on the order of 1 K, implies a B-induced transition for field distinctly above 20 T, depending on the topology of the magnetic couplings. Fields of this size in Received: April 25, 2011 Revised: November 4, 2011 Published: December 1, 2011 54
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purified by TLC (SiO2, CH2Cl2-MeOH, 40:1, Rf = 0.6) yielding bluegreen crystalline solid. Yield 50 mg (39%), UV/vis (λmax, lg ε) CHCl3: 601 (3.09), 383 (4.40), 324 (4.72) nm. Anal. Calcd for C26H32N4O4 (464.60): C, 67.22; H, 6.94; N, 12.06. Found C, 66.77; H, 6.98; N, 12.12.
combination with the small magnetization density of the material make an experimental investigation very difficult.
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EXPERIMENTAL SECTION
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All the reagents were used as received. 2,3-Bis(hydroxylamino)-2,3dimethylbutane14 and 4-bromo-(1,3-dioxalan-2-yl)benzene15 were prepared according to the literature from commercially available 2,3dinitro-2,3-dimethylbutane. Electron paramagnetic resonance (EPR) spectra were recorded in diluted and oxygen-free solutions of toluene, with a concentration of 10−4 M unless otherwise stated, by using a Bruker X-band spectrometer ESP300E, equipped with an NMR gaussmeter (Bruker ER035), and a frequency counter (Bruker ER 041 XK). The g-factor corrections were obtained by using either PNT (g = 2.0026) or DPPH radicals (g = 2.0037) as EPR standards. UV/vis spectra were recorded in toluene and chloroform solution with a Perkin-Elmer spectrometer (UV/vis/NIR Lambda 900) by using a 1 cm optical-path quartz cell at room temperature. 1H and 13C NMR spectra were recorded on a Bruker AMX 250 spectrometer. Mass spectra were obtained on a FD-MS, VG Instruments ZAB-2 mass spectrometer. C, H, N elemental analyses were carried out by the Chemical Service Center of the Novosibirsk Institute of Organic Chemistry (Varieo EL). Melting points were measured on a Büchi B-545 apparatus (uncorrected) using open-ended capillaries. The X-ray crystallographic data were collected on a Nonius Kappa CCD diffractometer equipped with graphite monochromator and with Mo−Kα (λ = 0.71073 Å) radiation. The structures were solved by a direct method and refined by a full-matrix least-squares procedure. The magnetic susceptibility was measured in the temperature range between 2−200 K and in magnetic fields of B = 0.1 T and B = 1 T, using a Quantum Design superconducting quantum interference device (SQUID) magnetometer. The magnetization data were taken at a fixed temperature of 10 K and fields up to 5 T. Synthesis of Biphenyl-4,4′-dicarbaldehyde (2). The mixture of 4-bromo-(1,3-dioxalan-2-yl)benzene 1 (4.0 g, 18.4 mmol) with magnesium turnings (0.22 g, 9.2 mmol) was refluxed in dry THF for 24 h until all magnesium was consumed. Then a solution of Ni(PPh3)2Cl2 (80 mg, 0.7 mol % of 1) in benzene (10 mL) was added and refluxing was continued for 72 h. The resultant solution was mixed with saturated NH4Cl (20 mL), vigorously stirred for 12 h at rt, and then extracted with CH2Cl2 (3 × 40 mL). The combined organic phases were dried under MgSO4, and the solvents were removed under reduced pressure. The residual oil was purified chromatographically (SiO2, EtOAc/ pentane, 1:1). The main fraction containing 2 was combined and evaporated, yielding white transparent crystals. Yield: 1.39 g (72%). 1H NMR (CDCl3, 250.13 MHz) δ: 10.06 (s, 2H, −CHO), 7.99 (d, 4H, Ar−H, 3JHH = 8.20 Hz), 7.76 (d, 4H, Ar−H, 3JHH = 8.20 Hz). MS-FD (70 eV, DMSO): M-H 210.7 (calc. MW 210.1). Synthesis of 2,2′-(Biphenyl-4,4′-diyl)bis(4,4,5,5-tetramethylimidazolidine-1,3-diol) (3). A mixture of 2 (80 mg, 0.4 mmol) and 2,3-bis(hydroxylamino)-2,3-dimethylbutane (0.130 g, 0.9 mmol) in toluene (10 mL) was heated near reflux under argon atmosphere for 3 h, and then the residue was filtered off, washed carefully with toluene, and then dried in air, yielding white tranparent crystals. Yield: 175 mg (100%), mp =180−183 °C. 1H NMR (DMSO-d6, 250.13 MHz) δ: 1.11 (s, 12H, CH3), 1.13 (s, 12H, CH3), 4.59 (s, 2H, N−CH-N), 7.42−7.76 (m, 8H, CH biph), 7.83 (s, 4H, N−OH). 13C NMR (DMSO-d6, 62.9 MHz) δ: 17.6 and 24.8 (8C, CH3), 66.5 (4C, C(CH3)2), 90.4 (2C, N− CH−N), 126.3 and 129.5 (8C, CH, biph), 139.8 and 141.4 (2C, Cipso, biph). MS-FD (70 eV, DMSO): M-H 470.1 (calc. MW 470.6). Anal. Calcd. for C26H38N4O4 (470.60) C, 66.36; H, 8.14; N, 11.91; Found C, 66.66; H, 8.19; N, 12.04. Synthesis of 4,4′-Bis(1-oxyl-3-oxy-4,4,5,5-tetramethyl4H,5H-imidazolin-2-yl)biphenyl (4). The solution of 3 (125 mg, 0.27 mmol) in CH2Cl2−H2O mixture (50 mL, 1:1) was stirred vigorously at room temperature for 10 min to form a suspension. A solution of NaIO4 (115 mg, 0.54 mmol) in water (5 mL) was added drop by drop to this mixture. After 5 min, the characteristic blue color appeared, the mixture was stirred further for 30 min, and the organic layer was separated, washed with water, dried under MgSO4, filtered off, and evaporated under reduced pressure. The obtained solid was
RESULTS AND DISCUSSION The preparation of nitronyl and imino nitroxide biradicals usually follows the standard protocol via a dialdehyde precursor, which is then condensed with 2,3-bis(hydroxylamino)-2,3-dimethylbutane (BHA) to the bisimidazolidine moiety, followed by oxidation with NaIO4 to the nitronyl nitroxide or imino nitroxide radical (Scheme 1).16 The Kumada-Tamao method17 was successfully Scheme 1. Synthetic Procedure for Biradical 4a
a
(a) Ethylene glycol, toluene, reflux, 70%. (b) 1. Mg (0.5 equiv), THF, reflux, 24 h; 2. Ni(PPh3)2Cl2 (0.7 mol %), reflux, 72 h, 72%. (c) 2,3-Bis(hydroxylamino)-2,3-dimethylbutane, PhCH3, reflux, 100%. (d) NaIO4, CH2Cl2−H2O, 39%.
utilized for the synthesis of 4,4′-biphenyldicarbaldehyde 2. In this vein, double excess of 4-bromo-(1,3-dioxalan-2-yl)benzene 1 was reacted with 1 equiv of magnesium in the presence of 0.7 mol % of Ni(PPh3)2Cl2 in THF at reflux to promote the homocoupling product 1. Quenching of the reaction mixture with aqueous NH4Cl led to removal of the 1,3-dioxolane protecting groups and formation of the target dialdehyde 2 in 72% yield. Condensation of the dialdehyde 2 with BHA and oxidation of the corresponding bis(imidazolidine) derivative 3 resulted in the bis(nitronyl nitroxide) 4, which was isolated by thin layer chromatography in 39% yield. The UV−vis absorption spectra were recorded in toluene and CHCl3 as shown in Figure 1. They contain two main features, one band in the 300−400 nm range and one around 600 nm, as found for many nitronyl nitroxide biradicals. The optical absorptions show maxima at 324 nm for the biphenyl moiety (π−π* transition) with large extinction and distinct shoulder at 383 nm (ε = 18−25 × 103 M−1 cm−1), together with a weak absorption in the visible region around 600 nm (ε = 900−1200 M−1 cm−1), the latter originating from the two nitronyl 55
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and c = 11.4935(5) Å.18 The N−O bonds have lengths of 1.266 and 1.268 Å, which are typical for nitronyl nitroxides (Table 1). Table 1. Selected Bond Lengths for Biradical 4 experimental
geometry optimizationa
1.266 1.268 1.445 1.494 1.358 1.353
1.276 1.276 1.462 1.481 1.364 1.364
O(1)−N(1) N(2)−O(2) C(7)−C(4) C(1)-C′(1) N(1)−C(7) N(2)−C(7) a
Figure 1. UV−vis absorption spectra of 4 with concentration of 2.1 × 10−5 M (inset: 2.1 × 10−4 M) in CHCl3 and in toluene (dashed line) solution.
B3LYP, 6-31G*.
The torsional angles between the benzene ring and the ONCNO group of nitronyl nitroxide are 30.8° N(2)−C(7)− C(4)−C(5) and 30.0° N(1)−C(7)−C(4)−C(3), respectively (Table 2). Each radical molecule forms 12 short contacts and
nitroxides (n−π* transitions). The difference in the absorption features between toluene and the slightly more polar chloroform mainly consists of higher extinction for the chloroform solution by roughly one-third compared to toluene, while nearly no shift of the maxima or their partially resolved vibrational splitting (shoulders) was observed. The ESR spectrum was recorded in toluene solution (Figure 2). At room temperature, it is represented by well-resolved 9 lines
Table 2. Selected Dihedral Angles for Biradical 4 C(2)−C(1)−C′(1)−C′(6) N(2)−C(7)−C(4)−C(5) N(1)−C(7)−C(4)−C(3) a
experimental
geometry optimizationa
0.9 30.8 30.0
35 11 11
B3LYP, 6-31G*.
coordinates with six neighbored molecules. According to the obtained X-ray structures (Figure 4), the oxygen atom O(2) coordinates with two hydrogen atoms H(21) and H(61) of biphenyl from another molecule with relatively short H-bond contacts of 2.473 and 2.508 Å, respectively. These van der Waals interactions lead to a surprisingly planar orientation of the two neighboring biphenyl rings. The torsional angle between the planes of the two benzene rings is 2σI), wR2 = 0.0571 (all data), GOF = 0.857. CCDC-816629 contains supplementary crystallographic data for this paper. (19) (a) Shoji, M.; Koizumi, K.; Kitagawa, Y.; Kawakami, T.; Yamanaka, S.; Okumura, M.; Yamaguchi, K. Chem. Phys. Lett. 2006, 432, 343−347. (b) Soda, T.; Kitagawa, Y.; Onishi, T.; Takano, Y.; Shigeta, Y.; Nagao, H.; Yoshioka, Y.; Yamaguchi, K. Chem. Phys. Lett. 2000, 319, 223−230.
(c) Yamaguchi, K.; Jensen, F.; Dorigo, A.; Houk, K. N. Chem. Phys. Lett. 1988, 149, 537−542. (20) Kahn, O. Molecular Magnetism; VCH: Weinheim-New York, 1993. (21) Johnston, D. C.; Kremer, R. K.; Troyer, M.; Wang, X.; Klümper, A.; Bud’ko, S. L.; Panchula, A. F.; Canfield, P. C. Phys. Rev. B 2000, 61, 9558. (22) Remović-Langer, K.; Haussühl, E.; Wiehl, L.; Wolf, B.; Sauli, F.; Hasselmann, N.; Kopietz, P.; Lang, M. J. Phys.: Condens. Matter 2009, 21, 185013.
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