crystal engineering of tolane bridged nitronyl nitroxide biradicals: candidates for quantum magnets

Crystal Engineering of Tolane Bridged Nitronyl Nitroxide Biradicals:Candidates for Quantum MagnetsPrince 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 biradicalswith hydrogen bond donor and acceptor functional groupswere synthesized. The magnetic measurements and the DFTcalculations were performed to ascertain the influence of thefunctional groups on inter- and intramolecular magneticexchange interactions. While upon functionalizing the tolanebridge the intramolecular exchange interactions remainednearly unchanged, the fine-tuning of intermolecular exchangeinteractions could be achieved by employing the crystalengineering approach.

■ INTRODUCTION

Stable neutral organic biradicals are of special interest becausethey offer the possibility to tune the magnetic interactionsthrough the appropriate design of a spacer.1 Thus, byconsidering the topological rules of physics and at the sametime employing the advanced methods of synthetic organicchemistry, molecules with predictable magnetic properties canbe synthesized.2 Among the biradical family, antiferromagneti-cally (AF) coupled species can be considered as a source ofinteracting bosons.3 Consequently, such biradicals can serve asmolecular models of a gas of magnetic excitations which can beused for quantum computing application.4 Notably, thevanishing triplet state population at very low temperature(2−4 K) in weakly AF coupled biradicals can be switched into alarger triplet population upon application of an externalmagnetic field. Therefore, such biradical systems are promisingmolecular models for studying the phenomena of magnetic-field-induced Bose−Einstein condensation in the solid state.5

To observe such phenomena, it is very important to controlintra- as well as intermolecular magnetic exchange interactions.The intramolecular magnetic exchange interactions can betuned by either changing the length of a π-spacer moleculecarrying the radical moieties or changing the radical moietywhile maintaining the same π-spacer.6 The intermolecularmagnetic exchange interactions, which usually operate throughhydrogen bond or other short intermolecular contacts, highlydepend on the crystal packing and are quite difficult to predictor control.7 To some extent, the intermolecular magneticexchange interactions can be regulated by employing a crystalengineering approach.8 For instance, introduction of hydrogenbond donor or acceptor (or a combination of both) functionalgroups in the π-spacer fragment can result in an additionalhydrogen bonding, which in turn is advantageous for smoothtransmission of magnetic interactions through the lattice. As aresult, multidimensional spin-networks can be constructed.

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

dimensional magnetic field induced quantum phase transitionat 191 mK in the routine laboratory magnetic field up to 10 T.9

These findings demonstrate prospective utilization of weaklyAF coupled nitronyl nitroxide biradicals to generate quantummagnets. Continuing our investigations in this direction, thetolane bridged biradicals, decorated with hydrogen bond donorand/or acceptor functional groups, were synthesized (Figure 1)to achieve the fine-tuning of the intermolecular magneticexchange interactions and to obtain a three-dimensionalhydrogen bonded spin−lattice in the crystalline form.Herein we report the synthesis, crystal structure, and

magnetic properties of four new nitronyl nitroxide biradicalswith a functionalized tolane bridge. In contrast to our previouswork,6a−c the focus of the present study was turned toward thestructurally similar compounds exhibiting different magneticbehavior. It was expected to achieve such an effect by retainingthe intramolecular exchange pathway through the fixedgeometry of the tolane bridge, and leaving the intermolecularexchange interactions as the main variable. A functional groupis a delicate, yet powerful tool with the potential to alter the

Received: July 17, 2014Revised: September 9, 2014Published: September 12, 2014

Figure 1. Functionalized tolane bridged nitronyl nitroxide biradicals.

Article

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© 2014 American Chemical Society 5840 dx.doi.org/10.1021/cg5010787 | Cryst. Growth Des. 2014, 14, 5840−5846

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motif of the crystal packing. It was therefore intriguing toexamine the influence of various functional groups on thecharacter and magnitude of the inter- and intramolecularmagnetic exchange interactions.The key precursors for the synthesis of nitronyl nitroxide

biradicals are dialdehydes.10 The general approach toward thesynthesis of tolane bridged dialdehydes relied on Pd(II)catalyzed Sonogashira−Hagihara cross coupling reaction, asshown in Scheme 1. Condensation of dialdehyde with 2,3-bis(hydroxylamino)-2,3-dimethylbutane (BHA) gave bishy-

droxylamine in quantitative yield. Oxidation of bishydroxyl-amines is a gentle process, and must be performed with care toavoid formation of imino nitroxide radicals or mixed species.6a

Thus, oxidation with equimolar NaIO4 in an ice bath preferablyled to the desired nitronyl nitroxide biradicals.11 It should benoted that during the oxidation of bishydroxylamine NN2b thecyano functional group underwent oxidation to amide.12 Thebiradicals were characterized by UV−vis, EPR, and singlecrystal X-ray analysis. Magnetic measurements and DFTcalculations were performed to collect information about the

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.

Crystal Growth & Design Article

dx.doi.org/10.1021/cg5010787 | Cryst. Growth Des. 2014, 14, 5840−58465841

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

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





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

■ CRYSTAL STRUCTURE ANALYSIS

Magnetic interactions highly depend on the geometry andpacking of the molecules in the crystal lattice. Therefore, singlecrystal analysis of the radicals is a vital requirement tounderstand their magnetic properties. The single crystals weregrown by slow diffusion of hexane to a solution of biradical inDCM at room temperature. Good quality single crystals wereobtained for the biradicals NN1, NN2, and NN3.13 All theattempts to obtain a single crystal of NN4 failed. The unit cellparameters, space group, and other crystallographic details arelisted in crystallographic Table S1 (Supporting Information).Biradical NN1 crystallized in the non-centrosymmetricmonoclinic, P21, chiral space group. Crystal structure analysisrevealed that NN1 forms a sheet structure. In the sheet,molecules of NN1 recognize each other through C−H···O andC−H···Br hydrogen bonds, forming the planar herringbonepattern in two dimensions (Figure 2a). The >N−O group ofthe radical moiety in NN1 forms a weak C−H···O hydrogenbond with phenyl and methyl protons of two neighboringmolecules (highlighted blue area, Figure 2a). These sheetsfurther stack through π−π interaction and a C−H···O hydrogenbond (Figure 2b). Furthermore, the torsion angles of theterminal nitronyl nitroxides with a tolane bridge are 26.6 and25.6°, indicating slight twisting of the radical moiety withrespect to the tolane bridge.Biradicals NN2 and NN3 were crystallized in monoclinic, Pc,

and tetragonal, I41/a, space groups, respectively. Carefulstructural analysis showed that, while NN1 exhibited only atwo-dimensional hydrogen bonded network, interestingly theNN2 and NN3 formed three-dimensional networks (Figures 3and 4). This is probably due to the more flexible and/orhydrogen bond donor functional groups attached to the tolanebridge in the case of NN2 and NN3. In two dimensions, themolecules of NN2 form a nonplanar herringbone pattern(Figure 3a). The molecules of NN2 recognize each otherthrough π−π stacking and a strong N−H···O hydrogen bond.This plane consisting of molecules arranged in a herringbonefashion further extends in three dimensions through N−H···Oand C−H···O hydrogen bonds (Figure 3b). It should be notedthat the >N−O group of the radical moiety in NN2 recognizestwo neighboring molecules through an amide functional grouputilizing a strong N−H···O hydrogen bond, thereby indicatingthe possible way to transmit magnetic exchange interactionthrough a strong hydrogen bond. Interestingly, NN3 forms anonplanar layered structure in two dimensions in which the two

radical moieties form two different kinds of hydrogen bondingmotifs (highlighted with green and yellow strips in Figure 4a).While one of the radical moieties forms a C−H···O hydrogenbond with methyl, the other radical moiety forms a C−H···Ohydrogen bond with the methoxy functional group and aphenyl ring. The Br functional group of NN3 is involved in aninterlayer C−H···Br hydrogen bond. Moreover, torsion anglesof terminal nitronyl nitroxides with a tolane bridge are 22.7 and13.4° for NN2 and 28.6 and 11.3° for NN3. One of theterminal radical moieties is more in-plane with tolane comparedto the other one. According to the crystal structure analysis, wecould significantly influence the pattern of the interacting spinsin the lattice, and move from two- to three-dimensional orderby directing new hydrogen bonding.

■ MAGNETIC PROPERTIES

Crystal structure analysis provided the evidence about thepossible ways of transmitting magnetic interactions in thelattice. On the basis of crystal structure data, the informationabout the magnitude of exchange interactions cannot beobtained precisely. Therefore, the magnetic susceptibility andmagnetization of a polycrystalline sample of biradicals weremeasured in the temperature range 2 K ≤ T ≤ 200 K using aSQUID magnetometer to understand the nature and extent ofthe magnetic exchange interactions prevailing in the synthe-sized tolane bridged biradicals. As shown in Figure 5a, themolar magnetic susceptibility (χmol) initially increased with theCurie−Weiss behavior at the higher temperature region anddecreased at lower temperature with a broad peak mainlycaused by intramolecular AF interactions. On further loweringthe temperature, χmol abruptly decreases close to zero at 2 Kwhich means the biradicals switch from a thermally populatedmagnetic spin triplet state to a nonmagnetic spin singlet groundstate. All the biradicals exhibited Tmax from 6 to 8 K. Theintradimer coupling constant Jintra of R-tolane-R′ was thenestimated using an isolated dimer model (H = −2JintraSRSR′).

14

The obtained intramolecular exchange interaction values appearin a very narrow range from −3.2 to −4.5 cm−1 (Table 1).Notably, the Jintra for functionalized tolane bridged biradicalsNN1, NN2, and NN4 was very close to the nonfunctionalizedtolane NN biradical (Table 1). Only in the case of NN3, theJintra was slightly higher by 1 cm−1 compared to otherfunctionalized tolane bridged biradicals. This very small changein Jintra may be originated from the captodative effect of twodifferent functional groups (electron donor methoxy andelectron acceptor bromo) attached to the tolane bridge inNN3.15 These observations led to the inference that thefunctionalization of the tolane bridge did not influence the

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.



intramolecular magnetic exchange interactions significantly.The negative Weiss temperature was observed in all thebiradicals, indicating the existence of AF intra- andintermolecular magnetic exchange interactions.The magnetization curves of all the biradicals NN1−4 were

measured at 2 K to understand the influence of hydrogen bondson intermolecular exchange interactions. Interestingly, despitehaving similar intramolecular AF interactions, NN1, NN2, andNN4 showed significant differences in the magnetization underthe influence of an applied magnetic field up to 5 T (Figure5b). This difference can only be attributed to substantialalteration of the intermolecular exchange interactions becausethe intramolecular exchange interactions were quite similar.The increase of magnetization in the very low-field regionbelow 0.1 T for NN1 can be ascribed to the small amount ofmonoradical impurity. The NN3 with relatively higher Jintrashowed the smallest magnetic field dependence in comparisonwith the other functionalized tolane bridged biradicals. In thepresence of an applied magnetic field, the population of tripletstate increases in the order NN4 > NN1 > NN2 > NN3. Asobserved during the crystal structure analysis, NN2 and NN3possessing a hydrogen bond donor functional group formhydrogen bonds with a radical moiety, showing a smallerincrease in magnetic field induced triplet state populationcompared with NN1 and NN4. Therefore, the hydrogen bondsplay an important role in transmitting intermolecular exchangeinteractions.

■ DFT CALCULATIONSThe intramolecular exchange interaction energies of thebiradical species were also estimated from the broken-symmetryDFT calculations.16 The geometry of biradical NN1−3 wastaken from the X-ray diffraction determinations without furtheroptimization. The geometry of NN4 was obtained from the X-ray geometry of NN1 by replacing two bromo functionalgroups by a nitro group and hydrogen. The broken-symmetryapproach was employed to elucidate the magnetic properties ofthe biradical species under study. The exchange couplingconstant (J) was calculated by the generalized spin projection

method suggested by Yamaguchi et al.17 For the molecule withtwo exchange coupled unpaired electrons, the Heisenberg−Dirac−Van Vleck (HDVV) Hamiltonian is H = −2J12S1S2 (theexchange coupling constant J is negative in the case of AFinteraction); S1 and S2 are the spin angular momentumoperators. The exchange interaction is J = (E(BS) − E(T))/(S2(T) − S2(BS)), where E(BS) is the energy of the broken-symmetry (BS) solution, a mixture of singlet and triplet stateswith SZ = 0 and S2(BS) close to 1, E(T) is the energy of thetriplet state with S2(T) close to 2, and S2 are the eigenvalues ofthe spin operator for these states. Thus, the direct exchangeyields J ≈ E(BS) − E(T).All DFT calculations were performed with the Gaussian 09

program package.18 Starting from the most commonly usedfunctional B3LYP for calculating J values, we found anoverestimation (Table 2).19 The calculated intramolecularexchange interaction values Jintra(calcd) were much higherthan the Jintra(exptl) values obtained from the magneticsusceptibility measurements (Table 1).20 The observed over-estimation of the calculated J values using the UB3LYPfunctional can be attributed to the high spin contaminationbrought by Hartree−Fock.21 Thus, to avoid the Hartree−Fockspin contamination, we calculated J with the UBLYP functionalusing the 6-31G(d) basis set. Interestingly, UBLYP/6-31G(d)provided rather accurate results. The calculated exchangeinteractions were very close to the one obtained from themagnetic susceptibility measurements (Table 1). Additionally,the calculated spin density of the triplet state of biradicalsNN1−4 was localized over the radical moiety and functional-ization of the tolane bridge did not influence the distribution ofthe spin density (Figure 6).In conclusion, we have successfully introduced hydrogen

bond donor and acceptor functional groups at the tolane bridgeto obtain functionalized tolane bridged nitronyl nitroxidebiradicals. Although the intramolecular exchange interactionsremained similar on functionalizing the tolane bridge, asignificant difference in the magnetization was observed uponapplication of an external magnetic field. Crystal structureanalysis revealed that hydrogen bond donor functional groupsformed hydrogen bonds directly with radical moieties andthereby increased the intermolecular exchange interactions.Thus, by utilizing the crystal engineering approach, tuning ofintermolecular exchange interactions was realized. Furthermore,DFT calculations were also employed to determine theexchange interactions. The calculated values of the intra-molecular coupling constant Jintra were well in accordance withthe ones obtained from the magnetic susceptibility measure-ments.

Table 1. Magnetic Properties of Biradicals

Tmax (K) θa (K) Jintra(exptl)b (cm−1) Jintra(calcd)

c (cm−1)

NN −3.3d −6.3NN1 6.5 −5.2 −3.6 −5.7NN2 7.5 −9.0 −3.5 −5.5NN3 8.0 −17.7 −4.5 −7.3NN4 6.0 −3.5 −3.2 −6.4

aWeiss temperature. bEstimated from the magnetic susceptibilitymeasurements (Figure 5a) using an isolated dimer model (S = 1/2).cCalculated at the UBLYP/6-31G(d) level of DFT (the single crystalgeometries were used for calculation; see the text below). dReference6c.

Table 2. Summary of DFT Calculations

method E(triplet) (eV) S2(triplet) E(BS) (eV) S2(BS) J (cm−1)

NN UB3LYP/6-31G(d) −43683.7450 2.1163 −43683.74792 1.1226 −23.5UBLYP/6-31G(d) −43665.54415 2.0281 −43665.54494 1.0295 −6.3

NN1 UBLYP/6-31G(d) −183593.6877 2.0274 −183593.6884 1.0287 −5.7NN2 UBLYP/6-31G(d) −48253.3057 2.0284 −48253.30630 1.0297 −5.5NN3 UBLYP/6-31G(d) −116740.9387 2.0336 −116740.9396 1.0356 −7.3NN4 UBLYP/6-31G(d) −49229.37964 2.0314 −49229.38044 1.0323 −6.4



■ ASSOCIATED CONTENT*S Supporting InformationDetailed experimental procedures, UV−vis and EPR spectra,ORTEP diagram, crystallographic table, and CIF files. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] Addresses†Institute of Biochemistry, Ernst-Moritz-Arndt UniversityGreifswald, Felix-Hausdorff-Straße 4, 17487 Greifswald,Germany.‡WPI Advanced Institute for Materials Research, TohokuUniversity, Sendai 980-8577, JapanAuthor ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSSupport from SFB-TR49 and a scholarship for P.R. aregratefully acknowledged.

■ ABBREVIATIONSAF, antiferromagnetic; DFT, density functional theory; BS,broken-symmetry; TLC, thin layer chromatography

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



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crystal engineering of tolane bridged nitronyl nitroxide biradicals: candidates for quantum magnets

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