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Ferro- and antiferromagnetic interactions of cyano-substituted thioaminyl radicals
Polyhedron 20 (2001) 1355– 1357 www.elsevier.nl/locate/poly
Ferro- and antiferromagnetic interactions of cyano-substituted thioaminyl radicals Masaaki Nakatsuji a, Yozo Miura a,*, Yoshio Teki b b
a Department of Applied Chemistry, Faculty of Engineering, Osaka City Uni6ersity, Sumiyoshi-ku, Osaka 558 -8585, Japan Department of Material Science, Graduate School of Science, Osaka City Uni6ersity, Sumiyoshi-ku, Osaka 558 -8585, Japan
Received 17 September 2000; accepted 18 October 2000
Abstract Four kinds of thioaminyls, N-(arylthio)-2-ethoxycarbonyl- (1), N-(arylthio)-2-acetyl- (2), N-(arylthio)-2-cyano- (3), and N-(arylthio)-2-fluoro-4,6-diarylphenylaminyls (4) were prepared. Thioaminyls 3 were quite persistent and could be isolated as radical crystals. For the isolated three radicals (3a, 3b and 3d) the magnetic susceptibility measurements were carried out using a SQUID magnetometer in the temperature range 1.8–300 K. The magnetic interactions of 3b and 3d were antiferromagnetic, and that of 3a was ferromagnetic. Analysis of the molT versus T plots of 3a with the one-dimensional regular Heisenberg model gave 2J/kB = +11.2 K. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Thioaminyl; ESR; SQUID; Ferromagnetic interaction
1. Introduction Organic molecule-based magnetism has attracted considerable interest since the discovery of the purely organic ferromagnet in 1991 [1]. Since the discovery, a variety of stable free radical crystals have been studied and many organic ferromagnets have been found [2]. However, the most radicals showing ferromagnetism are nitroxides or nitronyl nitroxides. Since those radicals have the localized unpaired electron spin, their Curie temperatures (Tc) are very low. To rise Tc, quest of new stable free radicals having the delocalized p-spin system is necessary. Thioaminyls are interesting as the radicals having an extensively delocalized p-spin system [3]. We have prepared a variety of thioaminyls, including N-(arylthio)2,4,6-triarylphenylaminyls and isolated them as stable radical crystals. Their magnetic susceptibility measurements have shown that six thioaminyls couple with the ferromagnetic interactions of 2J/kB =3.6 – 28 K. In the present study N-(arylthio)-2-ethoxycarbonyl(1), N-(arylthio)-2-acetyl- (2), N-(arylthio)-2-cyano- (3)
and N-(arylthio)-2-fluoro-4,6-diarylphenylaminyls (4) were investigated and radicals 3a– d could be isolated as radical crystals. Herein we report isolation, ESR spectra, and magnetic characterization of 3, along with the ESR spectra of 1, 2 and 4.
* Corresponding author. 0277-5387/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 5 3 8 7 ( 0 1 ) 0 0 6 1 8 - 0
Scheme 1.
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M. Nakatsuji et al. / Polyhedron 20 (2001) 1355–1357
3.2. The ESR spectra
Fig. 1. molT vs. T plots for 3a. The solid line is the theoretical one.
2. Experimental Radical precursors N-(arylthio)-2-ethoxycarbonyl(5), N-(arylthio)-2-acetyl- (6), N-(arylthio)-2-cyano- (7), and N-(arylthio)-2-fluoro-4,6-diarylanilines (8) were prepared according to Scheme 1. Treatment of 2-substituted 4,6-diarylanilines with arenesulfenyl chlorides in dry ether or anhydrous THF in the presence of Et3N gave 5 –8 in 37–52%. Radicals 1 – 4 were generated by oxidation of the corresponding precursors with PbO2, and radicals 3a – d were isolated in the following procedures. A benzene solution of 7 was treated with PbO2 and filtrated. The solvent was removed by freeze-drying, and the remaining crystalline residue was crystallized from EtOH or EtOH– EtOAc to give 3a –d in 37–47% yields. The magnetic susceptibility measurements were carried out using polycrystalline samples in the temperature range 1.8– 300 K with a SQUID magnetometer (Quantum Design MPMS2).
3. Results and discussion
3.1. Preparation and isolation of radicals 1 – 4 Precursors 5 –8 were prepared according to Scheme 1. Oxidation of 5– 8 was carried out with PbO2 in benzene. When PbO2 was added to a stirred solution of 5, 6 or 7, the solution immediately turned dark green and the solution gave an intense ESR signal. The characteristic dark green color was constant at room temperature over a long period under the atmospheric conditions, indicating that 1 – 3 are very persistent. This observation prompted us to isolate them, and isolation was tried. Although 1a, 1b and 2 could not be isolated, 3a–d were successfully isolated in 37– 47% yields. In the IR spectra of 3a–d no absorption peak due to stretching vibration of NH was observed, and the elemental analyses agreed with the calculations. In the case of 8, the solution turned dark green upon addition of PbO2, but the dark green color immediately disappeared, indicating 4 is very unstable.
The ESR spectra of 1–4 were recorded at room temperature using benzene as the solvent. In the cases of 1– 3, the ESR spectra were split into a broad 1:1:1 triplet with hyperfine coupling constants of 0.844–0.908 mT and no hyperfine splittings due to protons were observed. We further prepared 4- and 6-diphenyl deuterated thioaminyl 3b–d according to the same procedure as for the corresponding nondeuterated radical. The spectrum was split into a 1:1:1 triplet, and each component of the triplet was further split into an incomplete 1:3:3:1 quartet. Computer simulation of this spectrum gave aN = 0.853, aH = 0.130 (2H), and 0.085 mT (1H). The protons giving a hfc constant of 0.130 mT are the anilino meta protons, and the proton giving a hfc constant of 0.085 mT is the phenylthiyl ortho proton. The ESR spectrum of 4 was split into a 1:2:2:1 quartet by the interaction with N and F. Computer simulation of this spectrum gave aN = 0.873 and aF = 0.980 mT.
3.3. Magnetic properties of radicals 3a, 3b and 3d The magnetic properties of isolated 3a, 3b, and 3d were investigated using polycrystalline samples in the temperature range 1.8–300 K with a SQUID magnetometer. The diamagnetic corrections were made by using Pascal’s sum rule. The spin concentrations of 3a, 3b and 3d were determined to be 95–100% by ESR and SQUID. Fig. 1 shows the molT versus T plots for 3a. The molT value is constant (0.376 emu K mol − 1) above 100 K, and increased with decreasing temperature below 100 K. After giving a maximum at ca. 6 K, it is decreased with decreasing temperature. The molT versus T plots were analyzed in terms of the one-dimensional regular Heisenberg model[4] (Eq. (1)) and the modified Bleaney–Bower model. Although the latter model gave no satisfactory fit with the experimental results in the whole temperature region, the former model gave the satisfactory fit in the high temperature region, giving + 11.2 K as the intrachain ferromagnetic interaction (2J/kB). Thus, this magnetic behavior shows that the intrachain interaction is ferromagnetic and the interchain interaction is antiferromagnetic. H= −2JS Si Sj
(1)
Radical 3b showed a strong antiferromagnetic interaction. Fig. 2 shows the molT versus T plots for 3b. The molT value is already decreased with decreasing temperature in the high temperature region, and at 1.8 K the molT value is nearly 0 emu K mol − 1. The molT versus T plots were analyzed in terms of the alternating linear chain model (Eq. (2)), giving 2J/kB = −200 K and h= 0.87. This large antiferromagnetic interaction is
M. Nakatsuji et al. / Polyhedron 20 (2001) 1355–1357
1357
4. Conclusion
Fig. 2. molT vs. T plots for 3b. The solid line is the theoretical one.
probably the result of the large SOMO– SOMO overlap between the neighboring molecules. H = − 2JS(S2i − 1S2i +hS2i S2i + 1)
(2)
Radical 3d also showed a strong antiferromagnetic interaction. The molT versus T plots were analyzed in terms of the dimer model (Eq. (3)), giving 2J/kB = −57 K. mol =
NA 2g 2v 2B 1 2Wmol 3+ exp( −2J/kT) 3kT
(3)
.
Four kinds of thioaminyls having an ethoxycarbonyl, acetyl, cyano, or fluoro at the anilino ortho position were generated. Four cyano-substituted thioaminyls (3a–d) were isolated as radical crystals. The ESR spectra showed that the unpaired electron is delocalized over the p-system. The magnetic susceptibility measurements for three isolated thioaminyls showed that, although the magnetic interactions of 3b and 3d were antiferromagnetic, that of 3a was ferromagnetic. Analysis of its susceptibility date with the one-dimensional regular Heisenberg model gave 2J/kB = + 11.2 K.
References [1] M. Kinoshita, P. Turek, M. Tamura, K. Nozawa, D. Shiomi, Y. Nakazawa, M. Ishikawa, M. Takahashi, K. Awaga, T. Inabe, Y. Maruyama, Chem. Lett. (1991) 1225. [2] P.M. Lahti (Ed.), Magnetic Properties of Organic Materials, Marcel Dekker, New York, 1999. [3] Y. Miura, Recent Research Developments in Organic Chemistry, vol. 2, Part II, Transworld Research Network, Trivandrum, India, 1998, p. 251. [4] J.C. Bomner, M.E. Fisher, Phys. Rev. A 135 (1964) 640.
Ferro- and antiferromagnetic interactions of cyano-substituted thioaminyl radicals Masaaki Nakatsuji a, Yozo Miura a,*, Yoshio Teki b b
a Department of Applied Chemistry, Faculty of Engineering, Osaka City Uni6ersity, Sumiyoshi-ku, Osaka 558 -8585, Japan Department of Material Science, Graduate School of Science, Osaka City Uni6ersity, Sumiyoshi-ku, Osaka 558 -8585, Japan
Received 17 September 2000; accepted 18 October 2000
Abstract Four kinds of thioaminyls, N-(arylthio)-2-ethoxycarbonyl- (1), N-(arylthio)-2-acetyl- (2), N-(arylthio)-2-cyano- (3), and N-(arylthio)-2-fluoro-4,6-diarylphenylaminyls (4) were prepared. Thioaminyls 3 were quite persistent and could be isolated as radical crystals. For the isolated three radicals (3a, 3b and 3d) the magnetic susceptibility measurements were carried out using a SQUID magnetometer in the temperature range 1.8–300 K. The magnetic interactions of 3b and 3d were antiferromagnetic, and that of 3a was ferromagnetic. Analysis of the molT versus T plots of 3a with the one-dimensional regular Heisenberg model gave 2J/kB = +11.2 K. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Thioaminyl; ESR; SQUID; Ferromagnetic interaction
1. Introduction Organic molecule-based magnetism has attracted considerable interest since the discovery of the purely organic ferromagnet in 1991 [1]. Since the discovery, a variety of stable free radical crystals have been studied and many organic ferromagnets have been found [2]. However, the most radicals showing ferromagnetism are nitroxides or nitronyl nitroxides. Since those radicals have the localized unpaired electron spin, their Curie temperatures (Tc) are very low. To rise Tc, quest of new stable free radicals having the delocalized p-spin system is necessary. Thioaminyls are interesting as the radicals having an extensively delocalized p-spin system [3]. We have prepared a variety of thioaminyls, including N-(arylthio)2,4,6-triarylphenylaminyls and isolated them as stable radical crystals. Their magnetic susceptibility measurements have shown that six thioaminyls couple with the ferromagnetic interactions of 2J/kB =3.6 – 28 K. In the present study N-(arylthio)-2-ethoxycarbonyl(1), N-(arylthio)-2-acetyl- (2), N-(arylthio)-2-cyano- (3)
and N-(arylthio)-2-fluoro-4,6-diarylphenylaminyls (4) were investigated and radicals 3a– d could be isolated as radical crystals. Herein we report isolation, ESR spectra, and magnetic characterization of 3, along with the ESR spectra of 1, 2 and 4.
* Corresponding author. 0277-5387/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 5 3 8 7 ( 0 1 ) 0 0 6 1 8 - 0
Scheme 1.
1356
M. Nakatsuji et al. / Polyhedron 20 (2001) 1355–1357
3.2. The ESR spectra
Fig. 1. molT vs. T plots for 3a. The solid line is the theoretical one.
2. Experimental Radical precursors N-(arylthio)-2-ethoxycarbonyl(5), N-(arylthio)-2-acetyl- (6), N-(arylthio)-2-cyano- (7), and N-(arylthio)-2-fluoro-4,6-diarylanilines (8) were prepared according to Scheme 1. Treatment of 2-substituted 4,6-diarylanilines with arenesulfenyl chlorides in dry ether or anhydrous THF in the presence of Et3N gave 5 –8 in 37–52%. Radicals 1 – 4 were generated by oxidation of the corresponding precursors with PbO2, and radicals 3a – d were isolated in the following procedures. A benzene solution of 7 was treated with PbO2 and filtrated. The solvent was removed by freeze-drying, and the remaining crystalline residue was crystallized from EtOH or EtOH– EtOAc to give 3a –d in 37–47% yields. The magnetic susceptibility measurements were carried out using polycrystalline samples in the temperature range 1.8– 300 K with a SQUID magnetometer (Quantum Design MPMS2).
3. Results and discussion
3.1. Preparation and isolation of radicals 1 – 4 Precursors 5 –8 were prepared according to Scheme 1. Oxidation of 5– 8 was carried out with PbO2 in benzene. When PbO2 was added to a stirred solution of 5, 6 or 7, the solution immediately turned dark green and the solution gave an intense ESR signal. The characteristic dark green color was constant at room temperature over a long period under the atmospheric conditions, indicating that 1 – 3 are very persistent. This observation prompted us to isolate them, and isolation was tried. Although 1a, 1b and 2 could not be isolated, 3a–d were successfully isolated in 37– 47% yields. In the IR spectra of 3a–d no absorption peak due to stretching vibration of NH was observed, and the elemental analyses agreed with the calculations. In the case of 8, the solution turned dark green upon addition of PbO2, but the dark green color immediately disappeared, indicating 4 is very unstable.
The ESR spectra of 1–4 were recorded at room temperature using benzene as the solvent. In the cases of 1– 3, the ESR spectra were split into a broad 1:1:1 triplet with hyperfine coupling constants of 0.844–0.908 mT and no hyperfine splittings due to protons were observed. We further prepared 4- and 6-diphenyl deuterated thioaminyl 3b–d according to the same procedure as for the corresponding nondeuterated radical. The spectrum was split into a 1:1:1 triplet, and each component of the triplet was further split into an incomplete 1:3:3:1 quartet. Computer simulation of this spectrum gave aN = 0.853, aH = 0.130 (2H), and 0.085 mT (1H). The protons giving a hfc constant of 0.130 mT are the anilino meta protons, and the proton giving a hfc constant of 0.085 mT is the phenylthiyl ortho proton. The ESR spectrum of 4 was split into a 1:2:2:1 quartet by the interaction with N and F. Computer simulation of this spectrum gave aN = 0.873 and aF = 0.980 mT.
3.3. Magnetic properties of radicals 3a, 3b and 3d The magnetic properties of isolated 3a, 3b, and 3d were investigated using polycrystalline samples in the temperature range 1.8–300 K with a SQUID magnetometer. The diamagnetic corrections were made by using Pascal’s sum rule. The spin concentrations of 3a, 3b and 3d were determined to be 95–100% by ESR and SQUID. Fig. 1 shows the molT versus T plots for 3a. The molT value is constant (0.376 emu K mol − 1) above 100 K, and increased with decreasing temperature below 100 K. After giving a maximum at ca. 6 K, it is decreased with decreasing temperature. The molT versus T plots were analyzed in terms of the one-dimensional regular Heisenberg model[4] (Eq. (1)) and the modified Bleaney–Bower model. Although the latter model gave no satisfactory fit with the experimental results in the whole temperature region, the former model gave the satisfactory fit in the high temperature region, giving + 11.2 K as the intrachain ferromagnetic interaction (2J/kB). Thus, this magnetic behavior shows that the intrachain interaction is ferromagnetic and the interchain interaction is antiferromagnetic. H= −2JS Si Sj
(1)
Radical 3b showed a strong antiferromagnetic interaction. Fig. 2 shows the molT versus T plots for 3b. The molT value is already decreased with decreasing temperature in the high temperature region, and at 1.8 K the molT value is nearly 0 emu K mol − 1. The molT versus T plots were analyzed in terms of the alternating linear chain model (Eq. (2)), giving 2J/kB = −200 K and h= 0.87. This large antiferromagnetic interaction is
M. Nakatsuji et al. / Polyhedron 20 (2001) 1355–1357
1357
4. Conclusion
Fig. 2. molT vs. T plots for 3b. The solid line is the theoretical one.
probably the result of the large SOMO– SOMO overlap between the neighboring molecules. H = − 2JS(S2i − 1S2i +hS2i S2i + 1)
(2)
Radical 3d also showed a strong antiferromagnetic interaction. The molT versus T plots were analyzed in terms of the dimer model (Eq. (3)), giving 2J/kB = −57 K. mol =
NA 2g 2v 2B 1 2Wmol 3+ exp( −2J/kT) 3kT
(3)
.
Four kinds of thioaminyls having an ethoxycarbonyl, acetyl, cyano, or fluoro at the anilino ortho position were generated. Four cyano-substituted thioaminyls (3a–d) were isolated as radical crystals. The ESR spectra showed that the unpaired electron is delocalized over the p-system. The magnetic susceptibility measurements for three isolated thioaminyls showed that, although the magnetic interactions of 3b and 3d were antiferromagnetic, that of 3a was ferromagnetic. Analysis of its susceptibility date with the one-dimensional regular Heisenberg model gave 2J/kB = + 11.2 K.
References [1] M. Kinoshita, P. Turek, M. Tamura, K. Nozawa, D. Shiomi, Y. Nakazawa, M. Ishikawa, M. Takahashi, K. Awaga, T. Inabe, Y. Maruyama, Chem. Lett. (1991) 1225. [2] P.M. Lahti (Ed.), Magnetic Properties of Organic Materials, Marcel Dekker, New York, 1999. [3] Y. Miura, Recent Research Developments in Organic Chemistry, vol. 2, Part II, Transworld Research Network, Trivandrum, India, 1998, p. 251. [4] J.C. Bomner, M.E. Fisher, Phys. Rev. A 135 (1964) 640.