Crystal Engineering of Tolane Bridged Nitronyl Nitroxide Biradicals


Sep 12, 2014 - with hydrogen bond donor and acceptor functional groups were synthesized. ... Stable neutral organic biradicals are of special interest...
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Crystal Engineering of Tolane Bridged Nitronyl Nitroxide Biradicals: Candidates for Quantum Magnets Prince Ravat, Yulia Borozdina,† Yoshikazu Ito,‡ Volker Enkelmann, and Martin Baumgarten* Max Planck Institute for Polymer Research, Ackermannweg-10, D-55128, Mainz, Germany S Supporting Information *

ABSTRACT: The tolane bridged nitronyl nitroxide biradicals with hydrogen bond donor and acceptor functional groups were synthesized. The magnetic measurements and the DFT calculations were performed to ascertain the influence of the functional groups on inter- and intramolecular magnetic exchange interactions. While upon functionalizing the tolane bridge the intramolecular exchange interactions remained nearly unchanged, the fine-tuning of intermolecular exchange interactions could be achieved by employing the crystal engineering approach.



INTRODUCTION Stable neutral organic biradicals are of special interest because they offer the possibility to tune the magnetic interactions through the appropriate design of a spacer.1 Thus, by considering the topological rules of physics and at the same time employing the advanced methods of synthetic organic chemistry, molecules with predictable magnetic properties can be synthesized.2 Among the biradical family, antiferromagnetically (AF) coupled species can be considered as a source of interacting bosons.3 Consequently, such biradicals can serve as molecular models of a gas of magnetic excitations which can be used for quantum computing application.4 Notably, the vanishing triplet state population at very low temperature (2−4 K) in weakly AF coupled biradicals can be switched into a larger triplet population upon application of an external magnetic field. Therefore, such biradical systems are promising molecular models for studying the phenomena of magneticfield-induced Bose−Einstein condensation in the solid state.5 To observe such phenomena, it is very important to control intra- as well as intermolecular magnetic exchange interactions. The intramolecular magnetic exchange interactions can be tuned by either changing the length of a π-spacer molecule carrying the radical moieties or changing the radical moiety while maintaining the same π-spacer.6 The intermolecular magnetic exchange interactions, which usually operate through hydrogen bond or other short intermolecular contacts, highly depend on the crystal packing and are quite difficult to predict or control.7 To some extent, the intermolecular magnetic exchange interactions can be regulated by employing a crystal engineering approach.8 For instance, introduction of hydrogen bond donor or acceptor (or a combination of both) functional groups in the π-spacer fragment can result in an additional hydrogen bonding, which in turn is advantageous for smooth transmission of magnetic interactions through the lattice. As a result, multidimensional spin-networks can be constructed. © 2014 American Chemical Society

Recently, we have found that the tolane bridged nitronyl nitroxide biradical (NN, Figure 1) undergoes quasi-two-

Figure 1. Functionalized tolane bridged nitronyl nitroxide biradicals.

dimensional magnetic field induced quantum phase transition at 191 mK in the routine laboratory magnetic field up to 10 T.9 These findings demonstrate prospective utilization of weakly AF coupled nitronyl nitroxide biradicals to generate quantum magnets. Continuing our investigations in this direction, the tolane bridged biradicals, decorated with hydrogen bond donor and/or acceptor functional groups, were synthesized (Figure 1) to achieve the fine-tuning of the intermolecular magnetic exchange interactions and to obtain a three-dimensional hydrogen bonded spin−lattice in the crystalline form. Herein we report the synthesis, crystal structure, and magnetic properties of four new nitronyl nitroxide biradicals with a functionalized tolane bridge. In contrast to our previous work,6a−c the focus of the present study was turned toward the structurally similar compounds exhibiting different magnetic behavior. It was expected to achieve such an effect by retaining the intramolecular exchange pathway through the fixed geometry of the tolane bridge, and leaving the intermolecular exchange interactions as the main variable. A functional group is a delicate, yet powerful tool with the potential to alter the Received: July 17, 2014 Revised: September 9, 2014 Published: September 12, 2014 5840

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Scheme 1. General Synthetic Route for the Synthesis of Dialdehyde, Bishydroxylamine, and Nitronyl Nitroxide Derivatives

Figure 2. Crystal packing and hydrogen bond network of NN1.

motif of the crystal packing. It was therefore intriguing to examine the influence of various functional groups on the character and magnitude of the inter- and intramolecular magnetic exchange interactions. The key precursors for the synthesis of nitronyl nitroxide biradicals are dialdehydes.10 The general approach toward the synthesis of tolane bridged dialdehydes relied on Pd(II) catalyzed Sonogashira−Hagihara cross coupling reaction, as shown in Scheme 1. Condensation of dialdehyde with 2,3bis(hydroxylamino)-2,3-dimethylbutane (BHA) gave bishy-

droxylamine in quantitative yield. Oxidation of bishydroxylamines is a gentle process, and must be performed with care to avoid formation of imino nitroxide radicals or mixed species.6a Thus, oxidation with equimolar NaIO4 in an ice bath preferably led to the desired nitronyl nitroxide biradicals.11 It should be noted that during the oxidation of bishydroxylamine NN2b the cyano functional group underwent oxidation to amide.12 The biradicals were characterized by UV−vis, EPR, and single crystal X-ray analysis. Magnetic measurements and DFT calculations were performed to collect information about the 5841

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Figure 3. Crystal packing and hydrogen bond network of NN2.

Figure 4. Crystal packing and hydrogen bond network of NN3.

(Figure S1, Supporting Information). The typical EPR spectra of biradicals consisted of nine well-resolved lines due to the hyperfine coupling (hfc) of two electron spins with four

electronic structure of the biradical species and their magnetic properties. All the biradicals displayed characteristic n−π* transition stemming from nitronyl nitroxide around 620 nm 5842

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Figure 5. (a) Molar magnetic susceptibility, χmol (emu mol−1 Oe−1), as a function of temperature under a magnetic field of 0.1 T. (b) Magnetization as a function of magnetic field at 2 K.

equivalent nitrogen atoms (J ≫ aN) at giso = 2.0068 (Figure S2, Supporting Information).



radical moieties form two different kinds of hydrogen bonding motifs (highlighted with green and yellow strips in Figure 4a). While one of the radical moieties forms a C−H···O hydrogen bond with methyl, the other radical moiety forms a C−H···O hydrogen bond with the methoxy functional group and a phenyl ring. The Br functional group of NN3 is involved in an interlayer C−H···Br hydrogen bond. Moreover, torsion angles of terminal nitronyl nitroxides with a tolane bridge are 22.7 and 13.4° for NN2 and 28.6 and 11.3° for NN3. One of the terminal radical moieties is more in-plane with tolane compared to the other one. According to the crystal structure analysis, we could significantly influence the pattern of the interacting spins in the lattice, and move from two- to three-dimensional order by directing new hydrogen bonding.

CRYSTAL STRUCTURE ANALYSIS

Magnetic interactions highly depend on the geometry and packing of the molecules in the crystal lattice. Therefore, single crystal analysis of the radicals is a vital requirement to understand their magnetic properties. The single crystals were grown by slow diffusion of hexane to a solution of biradical in DCM at room temperature. Good quality single crystals were obtained for the biradicals NN1, NN2, and NN3.13 All the attempts to obtain a single crystal of NN4 failed. The unit cell parameters, space group, and other crystallographic details are listed in crystallographic Table S1 (Supporting Information). Biradical NN1 crystallized in the non-centrosymmetric monoclinic, P21, chiral space group. Crystal structure analysis revealed that NN1 forms a sheet structure. In the sheet, molecules of NN1 recognize each other through C−H···O and C−H···Br hydrogen bonds, forming the planar herringbone pattern in two dimensions (Figure 2a). The >N−O group of the radical moiety in NN1 forms a weak C−H···O hydrogen bond with phenyl and methyl protons of two neighboring molecules (highlighted blue area, Figure 2a). These sheets further stack through π−π interaction and a C−H···O hydrogen bond (Figure 2b). Furthermore, the torsion angles of the terminal nitronyl nitroxides with a tolane bridge are 26.6 and 25.6°, indicating slight twisting of the radical moiety with respect to the tolane bridge. Biradicals NN2 and NN3 were crystallized in monoclinic, Pc, and tetragonal, I41/a, space groups, respectively. Careful structural analysis showed that, while NN1 exhibited only a two-dimensional hydrogen bonded network, interestingly the NN2 and NN3 formed three-dimensional networks (Figures 3 and 4). This is probably due to the more flexible and/or hydrogen bond donor functional groups attached to the tolane bridge in the case of NN2 and NN3. In two dimensions, the molecules of NN2 form a nonplanar herringbone pattern (Figure 3a). The molecules of NN2 recognize each other through π−π stacking and a strong N−H···O hydrogen bond. This plane consisting of molecules arranged in a herringbone fashion further extends in three dimensions through N−H···O and C−H···O hydrogen bonds (Figure 3b). It should be noted that the >N−O group of the radical moiety in NN2 recognizes two neighboring molecules through an amide functional group utilizing a strong N−H···O hydrogen bond, thereby indicating the possible way to transmit magnetic exchange interaction through a strong hydrogen bond. Interestingly, NN3 forms a nonplanar layered structure in two dimensions in which the two



MAGNETIC PROPERTIES

Crystal structure analysis provided the evidence about the possible ways of transmitting magnetic interactions in the lattice. On the basis of crystal structure data, the information about the magnitude of exchange interactions cannot be obtained precisely. Therefore, the magnetic susceptibility and magnetization of a polycrystalline sample of biradicals were measured in the temperature range 2 K ≤ T ≤ 200 K using a SQUID magnetometer to understand the nature and extent of the magnetic exchange interactions prevailing in the synthesized tolane bridged biradicals. As shown in Figure 5a, the molar magnetic susceptibility (χmol) initially increased with the Curie−Weiss behavior at the higher temperature region and decreased at lower temperature with a broad peak mainly caused by intramolecular AF interactions. On further lowering the temperature, χmol abruptly decreases close to zero at 2 K which means the biradicals switch from a thermally populated magnetic spin triplet state to a nonmagnetic spin singlet ground state. All the biradicals exhibited Tmax from 6 to 8 K. The intradimer coupling constant Jintra of R-tolane-R′ was then estimated using an isolated dimer model (H = −2JintraSRSR′).14 The obtained intramolecular exchange interaction values appear in a very narrow range from −3.2 to −4.5 cm−1 (Table 1). Notably, the Jintra for functionalized tolane bridged biradicals NN1, NN2, and NN4 was very close to the nonfunctionalized tolane NN biradical (Table 1). Only in the case of NN3, the Jintra was slightly higher by 1 cm−1 compared to other functionalized tolane bridged biradicals. This very small change in Jintra may be originated from the captodative effect of two different functional groups (electron donor methoxy and electron acceptor bromo) attached to the tolane bridge in NN3.15 These observations led to the inference that the functionalization of the tolane bridge did not influence the 5843

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method suggested by Yamaguchi et al.17 For the molecule with two exchange coupled unpaired electrons, the Heisenberg− Dirac−Van Vleck (HDVV) Hamiltonian is H = −2J12S1S2 (the exchange coupling constant J is negative in the case of AF interaction); S1 and S2 are the spin angular momentum operators. The exchange interaction is J = (E(BS) − E(T))/ (S2(T) − S2(BS)), where E(BS) is the energy of the brokensymmetry (BS) solution, a mixture of singlet and triplet states with SZ = 0 and S2(BS) close to 1, E(T) is the energy of the triplet state with S2(T) close to 2, and S2 are the eigenvalues of the spin operator for these states. Thus, the direct exchange yields J ≈ E(BS) − E(T). All DFT calculations were performed with the Gaussian 09 program package.18 Starting from the most commonly used functional B3LYP for calculating J values, we found an overestimation (Table 2).19 The calculated intramolecular exchange interaction values Jintra(calcd) were much higher than the Jintra(exptl) values obtained from the magnetic susceptibility measurements (Table 1).20 The observed overestimation of the calculated J values using the UB3LYP functional can be attributed to the high spin contamination brought by Hartree−Fock.21 Thus, to avoid the Hartree−Fock spin contamination, we calculated J with the UBLYP functional using the 6-31G(d) basis set. Interestingly, UBLYP/6-31G(d) provided rather accurate results. The calculated exchange interactions were very close to the one obtained from the magnetic susceptibility measurements (Table 1). Additionally, the calculated spin density of the triplet state of biradicals NN1−4 was localized over the radical moiety and functionalization of the tolane bridge did not influence the distribution of the spin density (Figure 6). In conclusion, we have successfully introduced hydrogen bond donor and acceptor functional groups at the tolane bridge to obtain functionalized tolane bridged nitronyl nitroxide biradicals. Although the intramolecular exchange interactions remained similar on functionalizing the tolane bridge, a significant difference in the magnetization was observed upon application of an external magnetic field. Crystal structure analysis revealed that hydrogen bond donor functional groups formed hydrogen bonds directly with radical moieties and thereby increased the intermolecular exchange interactions. Thus, by utilizing the crystal engineering approach, tuning of intermolecular exchange interactions was realized. Furthermore, DFT calculations were also employed to determine the exchange interactions. The calculated values of the intramolecular coupling constant Jintra were well in accordance with the ones obtained from the magnetic susceptibility measurements.

Table 1. Magnetic Properties of Biradicals Tmax (K) NN NN1 NN2 NN3 NN4

6.5 7.5 8.0 6.0

θa (K)

Jintra(exptl)b (cm−1)

Jintra(calcd)c (cm−1)

−5.2 −9.0 −17.7 −3.5

−3.3 −3.6 −3.5 −4.5 −3.2

−6.3 −5.7 −5.5 −7.3 −6.4

d

a

Weiss temperature. bEstimated from the magnetic susceptibility measurements (Figure 5a) using an isolated dimer model (S = 1/2). c Calculated at the UBLYP/6-31G(d) level of DFT (the single crystal geometries were used for calculation; see the text below). dReference 6c.

intramolecular magnetic exchange interactions significantly. The negative Weiss temperature was observed in all the biradicals, indicating the existence of AF intra- and intermolecular magnetic exchange interactions. The magnetization curves of all the biradicals NN1−4 were measured at 2 K to understand the influence of hydrogen bonds on intermolecular exchange interactions. Interestingly, despite having similar intramolecular AF interactions, NN1, NN2, and NN4 showed significant differences in the magnetization under the influence of an applied magnetic field up to 5 T (Figure 5b). This difference can only be attributed to substantial alteration of the intermolecular exchange interactions because the intramolecular exchange interactions were quite similar. The increase of magnetization in the very low-field region below 0.1 T for NN1 can be ascribed to the small amount of monoradical impurity. The NN3 with relatively higher Jintra showed the smallest magnetic field dependence in comparison with the other functionalized tolane bridged biradicals. In the presence of an applied magnetic field, the population of triplet state increases in the order NN4 > NN1 > NN2 > NN3. As observed during the crystal structure analysis, NN2 and NN3 possessing a hydrogen bond donor functional group form hydrogen bonds with a radical moiety, showing a smaller increase in magnetic field induced triplet state population compared with NN1 and NN4. Therefore, the hydrogen bonds play an important role in transmitting intermolecular exchange interactions.



DFT CALCULATIONS The intramolecular exchange interaction energies of the biradical species were also estimated from the broken-symmetry DFT calculations.16 The geometry of biradical NN1−3 was taken from the X-ray diffraction determinations without further optimization. The geometry of NN4 was obtained from the Xray geometry of NN1 by replacing two bromo functional groups by a nitro group and hydrogen. The broken-symmetry approach was employed to elucidate the magnetic properties of the biradical species under study. The exchange coupling constant (J) was calculated by the generalized spin projection Table 2. Summary of DFT Calculations

NN NN1 NN2 NN3 NN4

method

E(triplet) (eV)

S2(triplet)

E(BS) (eV)

S2(BS)

J (cm−1)

UB3LYP/6-31G(d) UBLYP/6-31G(d) UBLYP/6-31G(d) UBLYP/6-31G(d) UBLYP/6-31G(d) UBLYP/6-31G(d)

−43683.7450 −43665.54415 −183593.6877 −48253.3057 −116740.9387 −49229.37964

2.1163 2.0281 2.0274 2.0284 2.0336 2.0314

−43683.74792 −43665.54494 −183593.6884 −48253.30630 −116740.9396 −49229.38044

1.1226 1.0295 1.0287 1.0297 1.0356 1.0323

−23.5 −6.3 −5.7 −5.5 −7.3 −6.4

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Figure 6. Spin density distribution in triplet state of NN1−4.



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ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental procedures, UV−vis and EPR spectra, ORTEP diagram, crystallographic table, and CIF files. This material is available free of charge via the Internet at http:// pubs.acs.org.



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Present Addresses †

Institute of Biochemistry, Ernst-Moritz-Arndt University Greifswald, Felix-Hausdorff-Straße 4, 17487 Greifswald, Germany. ‡ WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■ ■ ■

ACKNOWLEDGMENTS Support from SFB-TR49 and a scholarship for P.R. are gratefully acknowledged. ABBREVIATIONS AF, antiferromagnetic; DFT, density functional theory; BS, broken-symmetry; TLC, thin layer chromatography REFERENCES

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