Communication pubs.acs.org/IC
Phthalocyanine-Based Single-Component Molecular Conductor [MnIII(Pc)(CN)]2O Mitsuo Ikeda,† Hiroshi Murakawa,† Masaki Matsuda,‡ and Noriaki Hanasaki*,† †
Department of Physics, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan Graduate School of Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan
‡
S Supporting Information *
component crystals with useful function. In this paper, we report the preparation, crystal structure, IR absorption spectra, energydispersive X-ray spectrometry (Supporting Information), and electrical and magnetic properties of [Mn(Pc)(CN)]2O and discuss the complex’s intrinsic structural disorder and magnetic properties. [Mn(Pc)(CN)]2O was synthesized by an electrocrystallization method in air. An electrocrystallization cell equipped with a glass frit between the two compartments was filled with 60 mL of an acetonitrile solution of TPP[Mn(Pc)(CN)2] (50 mg). A starting simple manganese salt, TPP[Mn(Pc)(CN)2], was prepared by the method reported in ref 12. A constant current of typically 1 μA was applied between two platinum electrodes immersed in the solution for 2−3 weeks at 20 °C. Platelike crystals grew on the anode during the current application and were harvested by filtration. Figure 1a shows the molecular structure of the Pc dinuclear complex [Mn(Pc)(CN)]2O. This molecule has an oxygen atom sandwiched between two [Mn(Pc)(CN)] units. The oxygen atom is believed to come from air. The intramolecular Mn−Mn
ABSTRACT: A new manganese complex, [Mn(Pc)(CN)]2O, was prepared by an electrocrystallization method. This material is a single-component molecular conductor that displays semiconducting behavior with room temperature conductivity of 4.5 × 10−3 S cm−1. Furthermore, we observed negative magnetoresistance at room temperature due to interaction between conduction π electrons and localized d spins. X-ray structural analysis and IR absorption spectroscopy indicated structural disorder. The magnetic susceptibility measurements suggested the unequal spin states of two manganese atoms owing to this structural disorder.
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onducting materials based on phthalocyanine (Pc) are known to form one-dimensional face-to-face stacking conductors [M(Pc)]I (M = Ni, Cu, etc.).1,2 Recently, several studies have reported multidimensional conductors with a slipped-stack structure, such as M(Pc)(CN)2 (M = Co, Fe, etc.),3,4 in which a central metal is coordinated to axial ligands. When the transition metal with a magnetic moment occupies the center of the Pc ring, a strong π−d interaction between the Pc π and d electrons of the central metal occurs.5,6 This interaction is the key to inducing the remarkable transport properties of the molecular conductor. For example, TPP[Fe(Pc)(CN)2]2 (TPP = tetraphenylphosphonium), which is an axially substituted Pcbased conductor, shows a giant negative magnetoresistance owing to the strong π−d interaction.7 Single-component molecular conductors have recently attracted much attention. Kobayashi et al. developed singlecomponent molecular conducting crystals with the dithiolate ligand,8 and other sources of single-component conductors have been reported, such as bis(dithiazolyl) radicals9 and catecholfused tetrathiafulvalene skeletons.10 Moreover, a single-component molecular conductor exhibiting strong π−d interaction has already been reported.11 However, there has been no study on magnetoresistance originating from the π−d interaction in a single-component molecular conductor. In this study, we synthesized an axially ligated manganese phthalocyanine complex, [Mn(Pc)(CN)]2O. This material has both π electrons and localized d spins and exhibits a finite conductivity. It therefore meets the necessary condition to show the magnetoresistance effect attributed to a strong π−d interaction. Because [Mn(Pc)(CN)]2O is the first singlecomponent molecular conductor exhibiting magnetoresistance, this material may be important for the development of single© XXXX American Chemical Society
Figure 1. (a) Molecular structure of [Mn(Pc)(CN)]2O. (b and c) Crystal structure of [Mn(Pc)(CN)]2O. (b) Stacking of molecules along the [111] direction. (c) View from the b axis. Received: March 18, 2016
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DOI: 10.1021/acs.inorgchem.6b00678 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
in which the bond length d(Mn1−CCN) differs from d(Mn2− CCN), as indicated by X-ray structural analysis. The temperature dependence of the electrical resistivity is shown in Figure 2a. The sample crystals exhibit semiconducting behavior. The electrical resistivity in the ab plane is 2.22 × 102 Ω cm at room temperature. The activation energy of conduction is Ea = 0.159 eV in the ab plane and Ea = 0.140 eV along the c axis. Thus, the conducting properties are almost isotropic, reflecting the three-dimensional network of π−π overlap. Figure 2b shows the magnetic-field dependence of the resistivity normalized to the value in a zero magnetic field. A negative magnetoresistance was observed, which was independent of the magnetic-field direction (Figure S3). This negative magnetoresistance may have been caused by the π−d interaction between conduction π electrons of the Pc rings and localized d spins of the central metal manganese. The magnetoresistance observed even at 300 K indicated the existence of a strong π−d interaction, although the magnitude of this magnetoresistance was not very large. The temperature dependence of the magnetic susceptibility of the polycrystalline samples is shown in Figure 3. The magnetic
distance is about 3.5 Å, and the twist angle between the upper and lower Pc rings was determined to be 37°. The oxidation state was estimated from the stoichiometry because the bond lengths have been found to be rather insensitive to the charge on the Pc ring.13 Taking into account the oxidation potentials of CN− and O2−, the charge of the Mn(Pc) unit was evaluated to be 2+. The finite conductivity, observed in the resistivity measurement (vide infra) and shown in Figure 2a, indicated that the π electron contributed
Figure 2. (a) Temperature dependence of the resistivity ρ of the [Mn(Pc)(CN)]2O crystal. The inset shows the Arrhenius plot of the resistivity. (b) Magnetic-field dependence of the normalized resistivity ρ(B)/ρ(0 T).
Figure 3. Temperature dependence of the magnetic susceptibility χ of the polycrystalline [Mn(Pc)(CN)]2O. The inset shows the effective magnetic moment μeff as a function of T.
to conduction. Therefore, the charge of the Pc ring was believed to be 1− because the charge of the Pc ring was 2− in the case of the closed-shell Pc. Also, the effective charge of the central metal manganese was 3+. The crystal structure of [Mn(Pc)(CN)]2O is shown in Figure 1b,c. The crystal data are summarized in Table S1. There was scarcely any π−π overlap between the Pc rings along the a, b, and c axes. On the other hand, the molecules formed a line along the [111] direction, as shown in Figure 1b. Because this compound has tetragonal symmetry, there are four stacking directions equivalent to the [111] direction, and the π−π overlap between the Pc rings forms a three-dimensional network. X-ray structural analysis indicated structural disorder (Figure S1). In this disorder, the manganese atom, which occupies a single site in the average structure, occupies two different positions. The manganese atom with the longer distance to the oxygen atom is called Mn1 [d(Mn1−O) = 1.761(13) Å], and the other is called Mn2 [d(Mn2−O) = 1.747(3) Å]. The length between the Mn1 atom and CN ligand [d(Mn1−CCN) = 2.15(9) Å] is significantly longer than that between the Mn2 atom and CN ligand [d(Mn2−CCN) = 1.83(7) Å]. In the IR absorption spectra of the powder sample (Figure S2), we observed two dips (2170 and 2186 cm−1) originating from the CN stretching vibrations. This peak splitting indicates the existence of two kinds of CN ligands in the different environments. This result is consistent with structural disorder,
susceptibility seems to follow the Curie−Weiss law, while the effective magnetic moment (μeff) increases with temperature and becomes 3.16 μB at 300 K. This value is higher than the calculated value of S = 1 (μeff = 2.83 μB). The increase of μeff may be attributed to the thermally activated magnetic moments of π and d electrons. The contribution of π electrons, which are activated from the possible π−π antiferromagnetic coupling state, can be expressed as a thermally activated paramagnetic term, while the thermally activated magnetic moments of d electrons are attributed to the d-electron configuration in the first excited state (see the Supporting Information). Therefore, the spin value in the ground state was expected to be S = 1 per [Mn(Pc)(CN)]2O. Previous studies have reported that in [Mn(Pc)(CN)2]−12 and [Mn(CN)6]3−14 the manganese ion takes the low-spin S = 1 state in a strong ligand field. If the same situation is applicable to the two manganese atoms in [Mn(Pc)(CN)]2O, μeff should be 4.00 μB. However, this was inconsistent with the experimental results. The discrepancy may have come from the displacement of the axial ligand accompanied by structural disorder. In this structural disorder, the crystal fields on the two manganese sites are different. Because the Mn2 atom has a shorter distance to the oxygen atom or CN ligand, it takes the low-spin S = 1 state similar to those observed in previous studies.12,14 On the other hand, the crystal field by the axial ligands on the Mn1 site is weaker than B
DOI: 10.1021/acs.inorgchem.6b00678 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry that on the Mn2 site. Thus, this ligand field may become the square-planar type because the crystal field given by the macrocyclic ligand (Pc ring) is almost unchanged. In a previous study on [Mn(Pc)(Py)]2O (Py = pyridine)15 in which the crystal field on the manganese site seemed to be a square-planar octahedral due to the weak crystal field from the axial ligand Py, it was suggested that the four electrons paired up in the dyz and dzx levels. Taking this into account, the Mn1 atom probably takes the low-spin state (S = 0) as (dyz)2(dzx)2. By adopting this concept that the two manganese atoms take different spin states owing to structural disorder, we can explain the effective magnetic moment (μeff = 3.16 μB) per [Mn(Pc)(CN)]2O. We succeeded in synthesizing a Pc-based single-component molecular conductor, [Mn(Pc)(CN)]2O, which is the first single-component molecular conductor exhibiting negative magnetoresistance attributed to a strong π−d interaction. Xray structural analysis and IR absorption spectra indicated structural disorder. In addition, the magnetic susceptibility measurement suggested the spin-state inequivalence of two manganese atoms originating from the displacement of the axial ligand accompanied by structural disorder.
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(7) Hanasaki, N.; Matsuda, M.; Tajima, H.; Ohmichi, E.; Osada, T.; Naito, T.; Inabe, T. J. Phys. Soc. Jpn. 2006, 75, 033703. (8) (a) Tanaka, H.; Okano, Y.; Kobayashi, H.; Suzuki, W.; Kobayashi, A. Science 2001, 291, 285−287. (b) Cui, H.; Kobayashi, H.; Ishibashi, S.; Sasa, M.; Iwase, F.; Kato, R.; Kobayashi, A. J. Am. Chem. Soc. 2014, 136, 7619−7622. (9) Mailman, A.; Winter, S. M.; Yu, X.; Robertson, C. M.; Yong, W.; Tse, J. S.; Secco, R. A.; Liu, Z.; Dube, P. A.; Howard, J. A. K.; Oakley, R. T. J. Am. Chem. Soc. 2012, 134, 9886−9889. (10) Ueda, A.; Yamada, S.; Isono, T.; Kamo, H.; Nakao, A.; Kumai, R.; Nakao, H.; Murakami, Y.; Yamamoto, K.; Nishio, Y.; Mori, H. J. Am. Chem. Soc. 2014, 136, 12184−12192. (11) Kobayashi, A.; Zhou, B.; Kobayashi, H. Polyhedron 2011, 30, 3298−3302. (12) Matsuda, M.; Yamaura, J.; Tajima, H.; Inabe, T. Chem. Lett. 2005, 34, 1524−1525. (13) Morimoto, K.; Inabe, T. J. Mater. Chem. 1995, 5, 1749−1752. (14) Buschmann, W. E.; Liable-Sands, L.; Rheingold, A. L.; Miller, J. S. Inorg. Chim. Acta 1999, 284, 175−179. (15) Yamamoto, A.; Phillips, L. K.; Calvin, M. Inorg. Chem. 1968, 7, 847−852.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00678. Experimental details, structural disorder, IR absorption spectra, thermally activated magnetic moment, and energy-dispersive X-ray spectroscopy (PDF) X-ray crystallographic data in CIF format (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank S. Takamuku and Prof. M. Nakano (Osaka University) for their fruitful discussions. This work was supported, in part, by a Grant-In-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS KAKENHI Grant Numbers 24340084 and 25800196).
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REFERENCES
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DOI: 10.1021/acs.inorgchem.6b00678 Inorg. Chem. XXXX, XXX, XXX−XXX
