Benzoic acid and phenol derivatives of nitronyl nitroxide biradical as building blocks of organic...

Appl. Magn. Reson. 19, 45-57 (2000) AppliedMagnetic Resonance© Springer-Verlag 2000Printed in Austria

Benzoic Acid and Phenol Derivativesof Nitronyl Nitroxide Biradical as Building Blocks

of Organic Molecule-Based Ferrimagnets

K. Kamiyama, D. Shiomi, S. Hase, M. Nishizawa, K. Sato, M. Kozaki,K. Okada, and T. Takui

Departments of Materials Science and Chemistry, Graduate School of Science,Osaka City University, Osaka, Japan

Received October 31, 1999; revised December 9, 1999

Abstract. A crystal-engineering approach to organic ferrimagnets is reported. Coulombic energy be-tween an anionic biradical with S = I and a cationic monoradical with S = 1/2 can be utilized as adriving force of cocrystallization of open-shell molecules with different spin quantum numbers, lead-ing to organic salt ferrimagnets. In this study, 3,5-substituted phenol and benzoic acid derivatives ofnitronyl nitroxide biradicals were synthesized as an ionizable S = 1 component of organic salt ferri-magnets. The molecular ground states of the biradicals in the neutral state were examined by con-tinuous wave electron spin resonance (ESR) spectroscopy and static paramagnetic susceptibility mea-surements in the solid state. The molecular ground state of the phenol derivative was found to betriplet (S = 1) with the singlet-triplet energy gap of AE/k5 25 K, indicating that the biradical canbe a building block of organic salt ferrimagnetics. The benzoic acid derivative was found to have asinglet (S = 0) ground state (4E/k5 ? — 5 K), exemplifying that meta-(3,5)-linkage of unpaired elec-trons in it-aromatic rings does not necessarily give a triplet ground state for heteroatomic-substitutedit conjugation. The molecular ground states of the biradicals determined in the ESR experiments wereconfirmed by the susceptibility in the solid state.

1 Introduction

Molecule-based magnetics has been attracting considerable interests in recentyears. Many kinds of organic crystals or organic-metal complexes exhibiting in-termolecular magnetic interactions have been synthesized [1, 2]. Genuinely or-ganic ferrimagnets, however, have not been documented yet. Considering thetendency for organic open-shell molecules to have antiferromagnetic intermolecu-lar interactions, ferrimagnetic spin alignment, i.e., antiparallel spin alignmentbetween neighboring molecules with different magnetic moments is one of thefacile approaches to organic ferromagnets. Only one model compound of organicferrimagnets was synthesized in 1994 [3]. The crystal structure of the model com-pound consists of an alternating chain of a monoradical with S = 1/2 and abiradical with S = 1 [3]. The model compound exhibited no phase transition to

46

K. Kamiyama et al.

OH

O \ O+

N / N

1

COOH

N / N

2

Fig. 1. Phenol and benzoic acid derivative of nitronyl nitroxide biradical.

ferrimagnetic long-range ordered state. In our previous papers [4, 5], we reportedthe continuous wave (cw) and pulsed electron spin resonance (ESR) spectra ofthe model compound and numerical calculations of a spin Hamiltonian for theS = 1/2 and S = 1 alternating chain. We proposed a formation of singlet (S = 0)pairs between the monoradical and the biradical molecules as a possible mecha-nism for suppressing ferrimagnetic spin alignment. In order to test the mecha-nism and to obtain a guiding principle for constructing organic molecule-basedferrimagnets, molecular complexes with various crystal structures and intermo-lecular interactions should be studied in view of the magneto-structural relation-ship in the heterospin molecular systems. In general, cocrystallization of distinctmolecules, e.g., an S = 1/2 monoradical and an S = 1 biradical, in the asymmetricunit of a lattice involves a decrease in entropy that must be compensated by afurther decrease in potential energy in the crystal lattice. We have proposed anovel crystal-engineering approach which is necessary for crystallizing organicopen-shell molecular complexes in a controllable manner [6]: Coulombic energyin ionic salts can be utilized as a promising driving force of cocrystallization,when a biradical anion and a monoradical cation are combined in a crystal.

In this study, we deal with 3,5-substituted phenol 1 and benzoic acid 2 de-rivatives of nitronyl nitroxide biradicals (Fig. 1) as precursors of anionic com-ponents in the above strategy referred to as "organic salt ferrimagnets ". Meta-or 1,3,5-benzene linkage of unpaired electrons in it-aromatic homoatomic sys-tems such as hydrocarbons of it conjugation usually affords a high-spin groundstate [7], or a triplet ground state for 1,3-disubstituted benzene derivatives [8].Only a few biradicals with functional groups, e.g., an ionizable group, have beenfound so far, which are chemically stable enough to investigate magnetic prop-erties in crystalline solid states. The high-spin ground state would be, however,affected by heteroatomic substitution in the it conjugation [9, 10]. The molecu-lar ground states of 1 and 2 are examined by ESR spectroscopy. The crystalstructure and the magnetic susceptibilities of 1 and 2 in crystalline solid statesare invoked to support the conclusions from ESR experiments.

2 Experimental

The biradicals 1 and 2 were obtained by the coupling of 3,5-diformylphenol or3,5-diformyl benzoic acid with 2,3-bis(hydroxyamino)-2,3-dimetylbutane followed

Ionizable Biradicals for Organic Ferrimagnet 47

by oxidation with sodium periodate [11]. The diformyl compounds were preparedaccording to literature procedures [12, 13]. Recrystallization from acetone solu-tions gave microcrystals of I and 2.

The ESR spectra were measured for the glassy solution of methanol and theglassy solution of methanol plus 2-methyltetrahydrofuran (1:1) with a Bruker X-band spectrometer ESP300. The spectral simulation for allowed transitions wascarried out with SimFonia (Bruker) on the basis of the second-order perturba-tion treatment. The perturbation treatment of a spin Hamiltonian was enough toreproduce observed spectra satisfactorily. The magnetic susceptibility was mea-sured on a Quantum Design SQUID magnetometer MPMS2 with the applied fieldof 0.1 T. X-ray diffraction data for 1 was collected on a Rigaku four-circle dif-fratometer AFC7R. The structure was solved by direct methods with the pro-gram package "Texsan" by Molecular Structure Corporation.

3 Results and Discussion

3.1 Forbidden ESR Transitions

The temperature dependence of Am s = 2 transition intensity was measured in theglassy solution in order to examine the magnetic properties of the isolated mol-ecule. The integrated signal intensity of the Ams = 2 transitions is displayed asa function of temperature in Figs. 2 and 3. The peak-to-peak height of the sig-nals are also plotted in the figures, which correspond to the integrated intensity.The spectra are shown in the inset. In order to avoid saturation effects, the power

Fig. 2. Temperature dependence of the Am, = 2 transition intensity for compound 1. The closed sym-bols denote the observed intensity for the glassy solution of methanol plus 2-methyltetrahydrofuran(1:1) (a), while open symbols are for the glassy solution of methanol (b). The circles and trianglesrepresent the integrated intensity and the peak-to-peak height, respectively. The solid and dashed linesrepresent the calculated intensity using Eq. (1) with AE/kB = — 10 and 26 K, respectively. In the in-

set are shown the observed spectra of the Am, = 2 transitions.

48 K. Kamiyama et al.

Fig. 3. Temperature dependence of the Am, = 2 transition intensity for compound 2. The closed sym-bols denote the observed intensity for the glassy solution of methanol plus 2-methyltetrahydrofuran(1:1) (a), while open symbols are for the glassy solution of methanol (b). The circles and trianglesrepresent the integrated intensity and the peak-to-peak height, respectively. The solid and dashed linesrepresent the intensity calculated with Eq. (1) with AE/kB = —10 and —5 K, respectively. In the inset

are shown the observed spectra of the Am, = 2 transitions.

of the microwave applied in the measurements was kept such that the signalintensity was proportional to the square root of the power. The lineshape of theAm, = 2 spectra remained from 2.9 up to 50 K. The observed data were ana-lyzed assuming the thermal equilibrium between the singlet (S = 0) and the triplet(S = 1) states

I0c T[3 s

+exp^ AEJJ , (1)

where 4E is the energy gap between the triplet state and the singlet state

AE = E(S = 0) — E(S = 1) . (2)

For the glassy solution of methanol plus 2-methyltetrahydrofuran (1:1), the ob-served temperature dependence for the biradical 1 was represented by assumingthe singlet ground state with AE/kB = —10 K. For the methanol solution, on theother hand, the observed data were fitted to Eq. (1) with AE/kB = 26 K, contra-dicting the results of the mixed solvent (Fig. 2).

For the biradical 2, the temperature dependence of the signal intensity wasfound to depend on the solvent used. The integrated signal intensity of theAms = 2 transitions observed in the glassy solution of methanol plus 2-methyl-tetrahydrofuran (1:1) was fitted to Eq. (1) by assuming the singlet ground statewith 4E/kB = —10 K. The spectra from the methanol solution gave the singletground state with t1Elk B = —5.0 K (Fig. 3). Although the sign of the intramo-


lecular exchange interaction is not dependent on the solvent, as in the case ofthe phenol derivative, the possibility of the underestimation of ESR intensity inthe less polar solvent is not excluded for the benzoic acid biradical 2.

3.2 Allowed ESR Transitions

For both 1 and 2 in the glassy solution of methanol plus 2-methyltetrahydrofuran(1:1), the lineshape of the Am, = I transitions was distorted below 15 K. Theweak signal in the central field region is assigned to a monoradical (S = 1/2)impurity. The observed spectra indicate a mixture of dispersion and absorptioncomponents of the ESR transition, suggesting a spin-lattice relaxation anomalysuch as a rapid passage effect [14]. The distortion was not removed even at lower

Fig. 4. ESR spectra of the Am, = 1 transitions in the glassy solution of methanol plus 2-methyl-tetrahydrofuran (1:1) measured with various levels of microwave power. a Compound 1 (7.3 K),

b compound 2 (12.1 K).


Fig. 5. ESR spectra of the Am = 1 transitions in the glassy solution of methanol plus 2-methyl-tetrahydrofuran (1:1) measured with various frequencies of the field modulation (0.1 mT). a Com-pound 1 measured at 7.3 K with 1 µW of microwave power, b compound 2 measured at 12.1 K

with 1 .tW of microwave power.

power of the applied microwave as shown in Fig. 4. Lowering of the frequencyin field modulation improved the lineshapes as given in Fig. 5, but the distor-tion at low temperatures was not completely removed.

With methanol as a protic solvent of higher polarity, the lineshape distortion dueto the spin-lattice relaxation anomaly was removed as depicted in Fig. 6. The sig-nals from the monoradical (S = 1/2) impurity, however, were intensified. From theseresults of the Am. = 1 spectra, the contradiction of the intramolecular exchange in-teractions is attributable to the anomaly in the spin-lattice relaxation. Since theAm = 2 transitions are not affected by the monoradical impurity, the singlet-tripletenergy gaps obtained from the methanol solutions are reliable.


Fig. 6. ESR spectra of the Am, = 1 transitions in the glassy solution of methanol measured withvarious levels of microwave power. a Compound 1 (5.1 K), b compound 2 (2.9 K).

As the temperature increased, the lineshape distortion was diminished. Asshown in Fig. 7, the Am s = I spectra with least distortion were recorded in theglassy solutions of methanol plus 2-methyltetrahydrofuran (1:1) at 50 K both for1 and 2. The observed spectra were simulated by the spin Hamiltonian

c = gav pB S-B + D(S — S(S + 1) /3)+ E(SX — Sy) , (3)

where D and E are the zero-field splitting parameters and is the g-factoraveraged for the three principal directions. The best-fit parameters were obtainedas D/hcj = 8.21 • 10 -3 cm- ', lE/hcl — 0 cm - ', gay = 2.0066 for 1 and D/hcj =8.23 • 10 - ' cm- ', lE/hcl — 0 cm - ', ga„ = 2.0066 for 2. These values are similar

52

K. Kamiyama et al.

Fig. 7. Observed and simulated ESR spectra of Am, = 1 transition in the glassy solution of metha-nol plus 2-methyltetrahydrofuran (1:1) at 50 K. a Compound 1, the microwave frequency is 9.46579GHz and the amplitude of 100 kHz field modulation is 0.1 mT. b Compound 2, the microwave fre-

quency is 9.47329 GHz and the amplitude of the 12.5 kHz field modulation is 0.1 mT.

to those of m-phenylene bis nitronyl nitroxide biradical (ID/hc = 8.30 • 10 -3

cm - ', IE/hcj — 0 cm- ', g = 2.006) [8, 10]. This shows that the substitution ofthe hydroxyl group and the carboxyl group into the phenyl ring of the biradi-cals does not affect the spin delocalization to a significant degree.

3.3 X-ray Crystal Structure of the Phenol Biradical 1

Magnetic susceptibility of 1 in a crystalline solid state will be analyzed belowto estimate the intra- and intermolecular exchange interaction. As a basis of thediscussion on the solid-state magnetic properties, the crystal structure is solvedfor 1. The crystal structure of 1 is displayed in Fig. 8. The crystal data are listedin Table 1. The crystal belongs to a monoclinic system with the space group ofP2,/n. The unit cell contains four molecules, one of which is crystallographicallyindependent. As is shown in Fig. 8, the unit cell contains dimers in which thetwo molecules are related by an inversion center. The shortest intermolecular dis-tance of 3.34 A is found between the nitroxide oxygen of one molecule and the


Fig. 8. Crystal structure of compound 1 projected onto the be plane. The methyl groups andhydrogen atoms of phenyl rings are omitted for clarity.

nitroxide nitrogen of the other molecule within the dimer. In nitronyl nitroxideradicals, unpaired electrons are basically localized on the nitroxide (0-N) groups[15]. The intermolecular shortest distance in the dimer is expected to have adominant contribution to the intermolecular magnetic interaction: the interdimerinteractions are negligible to the first approximation.

Table 1. Lattice parameters of 1.

Formula C20H28N4O5

Formula weight 404.465Crystal system monoclinicSpace group P2,/n

a 10.410(9) Ab 10.814(1) Ac 18.502(8) Afi 100.096(5)°V 2050.6(2) A3

R 0.043Z 4


3.4 Magnetic Susceptibilityof the Phenol Biradical I in the Solid State

The temperature dependence of magnetic susceptibility xp of 1 measured forrandomly oriented microcrystals is depicted in the xp T vs. T plot in Fig. 9. Thestructural features described above suggest that the magnetic behavior of 1 isapproximated as an assembly of the isolated dimers. The dimer consisting of fourelectron spins is described by a model Hamiltonian of a four-spin cluster

cam= —2J1 (S 1 •S2 + S 3 •S4) — 2J2(S,•S 3 + S 2 S4) (4)

where J, is the intramolecular exchange interaction and J 2 is the intermolecularexchange interaction. The Hamiltonian Eq. (4) retains the inversion symmetry asexpected from the crystal structure. The model is schematically illustrated in Fig.9. The Hamiltonian is diagonalized to give energy eigenvalues e, (i = 1, 2, ...,6) for each spin state; one S = 2 state, three S = 1 states and two S = 0 states:

c(S = 2) = —J, — Jz , (Sa)

E(S= 1) =J,— J2 , (5b)

6(S = 1) = —J, + J2 , (5c)

s(S= 1)=J,+J2 , (5d)

s(S = 0) = J, + J, — 2 J; — J,J2 + Jz , (5e)

E(s =0) =J + J2 + 2 J; -J,J2 +J2 . (5f)

1.0

0.9

0.80E

0.7

Es 0.6

I-rz° 0.5

0.4

03

S, Ji $

J2 ,,.-J2

$g Ji $4

0 50 100 150 200

T (K)

Fig. 9. zP T-T plot for compound 1. The circles represent the observed values. The solid line is cal-culated by the four-spin-cluster model (Eq. (4)) with J1 /k5 = 12.0 K, J2/kB = — 1.0 K and g = 2.006.The inset shows the four-spin-cluster model, where J, and J 2 are the intramolecular and intermo-

lecular exchange interactions, respectively.


The xp T value was calculated by assuming the Boltzmann distribution amongthe spin states with the eigenvalues obtained above

6

2 1(Si (Si + 1)(2S; + l)exp(—s, /kBT))

xpT — N B,=1,=16 (6)

((2S, + l)exp(—s; / kB T))

The best fit to the observed data was obtained with the adjustable param-eters J,/kg = 12.0 K, J2/kB = — 1.0 K, and g = 2.006. The singlet-triplet energygap of 2J,lk5 = 24.0 K from the magnetic susceptibility in the solid state is inagreement with that from the ESR measurements in the methanol glassy solu-tion (AE/kB = 26 K). It is concluded that the ground state of 1 is a triplet withthe energy gap of about 25 K.

3.5 Magnetic Susceptibilityof the Benzoic Acid Biradical 2 in the Solid State

The X T vs. T plot for randomly oriented microcrystals of 2 is depicted in Fig.10. The xp T value was calculated by the modified Bleaney-Bowers equation

X 2Nng2 1(7)

p kB (T — 8) 3 + exp(-2J/kBT) '

where J is the intramolecular exchange interaction defined by

ems= —2JS,•S 2 , (S, = S2 = 1/2) . (8)

0.8

0.7

0.6

E 0.5

E 0.4

0.3

0.2

0.1

0.00 50 100 150 200

T (K)

Fig. 10. XP T-T plot for compound 2. The circles are the observed values. The solid line is calculatedby the modified Bleaney-Bowers model (Eq. (7)) with J/k i, = —2.2 K, 8 = — 5.5 K and g = 2.006.


The parameter 0 is the Weiss constant, which is an estimate of the intermo-lecular interaction in the mean-field approximation. The best fit was obtainedwith J/kB = — 2.2 K, 0= — 5.5 K, g = 2.006. The singlet-triplet energy gap of2J/kB = — 4.4 K from the magnetic susceptibility in the solid state is in agree-ment with that from the ESR measurements in the methanol glassy solution(AElkg = — 5.0 K). It should be noted that the intramolecular interaction 2J/kB =

—4.4 K is not entirely justified, because it is comparable to the intermolecularmean-field of 0= —5.5 K. The mean field is meaningless at low temperaturesT — 101. The above analysis gives only an estimate of the intramolecular ex-change interaction. The experimental results of the ESR and the susceptibilitymeasurements suggest that the ground state of 2 is singlet and the singlet-trip-let energy gap is AE/k8 —5.0 K. The benzoic acid biradical 2 is the firstexample of a ground-singlet biradical in the nitronyl nitroxide derivatives withmeta-phenylene structures.

4 Conclusion

As the building block of purely organic ferrimagnetics, two kinds of nitronylnitroxide biradicals were synthesized and magnetic properties were examined. Inthe glassy solution ESR measurements, the spectral distortion of the Am, = 1transitions due to the spin-lattice relaxation anomaly was observed. The distor-tion was removed by using a protic solvent of higher polarity, although the de-composition of the biradical was accelerated. From the results of the Am, = 2measurements and magnetic susceptibility, the ground state was determined tobe triplet (AE/kB 25 K) for the phenol derivative and to be singlet (AE/kg

—5.0 K) for the benzoic acid derivative. In the case of heteroatomic is conjuga-tion systems such as pyridine, the high-spin preference in the ground state isnot always expected for the meta- linkage of unpaired electrons [9, 10]. There-fore, the difference of spin multiplicity of the ground state between molecule 1(AE/k8 25 K) and molecule 2 (AE/kB —5.0 K) seems to be related to thesubstituent with the heteroatoms, hydroxyl group and carboxyl group in the metaposition.

Ionization of the both biradicals is now in progress. The perturbation causedby the ionization of the substituents will afford more information on the mag-netic properties in the in conjugated system.

Acknowledgements

This work has been supported by Grants-in Aid from the Ministry of Educa-tion, Science, Sports and Culture, Japan. D.S. and K.S. acknowledge Grants-in-Aid for Encouragements of Young Scientists (No. 07740468, 07740553,08740462, 09740528, and 10740275) from the Ministry of Education, Science,Sports and Culture, Japan.


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Authors' address: Takeji Takui, Department of Chemistry, Graduate School of Science, Osaka CityUniversity, Sugimoto 3-3-138, Sumiyoshi-ku, 558-8585 Osaka, Japan

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