Crystal structure and physical properties of a magnetic molecular conductor (EDO-TTFVODS)2FeCl4

Synthetic Metals 160 (2010) 2413–2416

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Crystal structure and physical properties of a magnetic molecular conductor (EDO-TTFVODS)2 FeCl4 Xunwen Xiao a,∗ , Jianghua Fang a , Jin Zhou a , Haoqi Gao a , Hideki Fujiwara b , Toyonari Sugimoto b,∗ a b

Department of Chemical Engineering, Ningbo University of Technology, Cuibai Road 89, Ningbo 315016, China Department of Chemistry, Graduate School of Science, Osaka Prefecture University, Japan

a r t i c l e

i n f o

Article history: Received 15 April 2010 Received in revised form 8 September 2010 Accepted 17 September 2010 Available online 15 October 2010 Keyword: Bent donor molecule Magnetic molecular conductor Metallic behavior Antiferromagnetic interaction ␲–d interaction

a b s t r a c t Crystal structure, and electrical conducting and magnetic properties of a radical cation salt of EDO-TTFVODS with magnetic FeCl4 − ion, (EDO-TTFVODS)2 FeCl4 (EDO-TTFVODS = ethylenedioxytetrathiafulvalenoquinone-1,3-diselenolemethide) are reported. In this salt, there are two independent donor molecules formed two different layers A and B, and the counter FeCl4 − ions layer is sandwiched between two donor layers A and B along the b-axis. The donor molecules form the one-dimensional columns along the a-axis in both donor layers. This salt shows high conductivity at room temperature ( RT = 25 S cm−1 ) and a metallic behavior down to ca. 80 K, where a metal–insulator transition however occurs. The magnetic susceptibility obeys a Curie–Weiss law (Curie constant C = 4.42 emu K mol−1 and Weiss temperature  = −1.5 K), without any magnetic ordering down to 1.8 K. This result suggests the weak antiferromagnetic interaction between the d spins of FeCl4 − ions. © 2010 Elsevier B.V. All rights reserved.

1. Introduction In the crystals of cation radical salts of ␲-electron donor molecules with magnetic transition metal counter anions, the donor molecules and counter anions usually form alternating stacks, which are responsible for their electrical conductivities and magnetisms [1–3], respectively. In the past decade, numerous efforts have been made to bring about strong magnetic exchange interactions between the conduction ␲-electrons on donor molecules and localized d-spins on counter anions in magnetic molecular conductors. These materials have the potential to be utilized as a key component for developing a new type of molecular electronics, so-called “spin electronics or spintronics [4,5]. Up to date, most of the magnetic molecular conductors are designed and synthesized on the basis of the straight ␲electron donor molecules, while only a few cation radical salts of bis(ethylenedithio)tetraselenafulvalene have reached the goal of coexisting metallic and antiferromagnetic ordered states [6–8]. On the other hand, the bent donor molecule is one of the successful system to afford the antiferromagnetic molecular metals [9–11]. We focus on the exploration of new cation radical salts of bent donor molecules with magnetic counter anions, for example, FeX4 − (X = Br, Cl). In the resulting cation radical salts, the conduction ␲-electrons are spin-polarized by the magnetic layers due to the strong ␲–d interaction between conduction ␲-electrons and

∗ Corresponding author. Fax: +86 574 87081240. E-mail addresses: [email protected] (X. Xiao), [email protected] (T. Sugimoto). 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.09.019

localized d spins. Therefore, these materials are regarded as the prospective candidates for the organic spintronics. We have recently reported a radical cation salt of ethylenedioxytetrathiafulvalenoquinone-1,3-diselenolemethide (EDO-TTFVODS, 1) (see Chart 1) with FeBr4 − ion, (EDOTTFVODS)2 FeBr4 (DCE)0.5 (DCE = 1,2-dichloroethane) [11], which is an antiferromagnetic molecular metal. However, the aiming ferromagnetic ordering has not yet been achieved. So we intend to use another counteranion of FeCl4 − ion to get an unprecedented ferromagnetic metal. Herein, we report the preparation, crystal structure and physical properties of the FeCl4 − salt of 1, 12 FeCl4 . 2. Experiment 2.1. Preparation of crystals of 1 and 12 • FeCl4 The donor molecule, 1 was synthesized according to the literature [11], and the single crystal was obtained by slow evaporation of the CS2 /hexane solution. The cation radical salt 12 • FeCl4 was prepared by electrochemical oxidation of 1 in the presence of NBu4 FeCl4 in chlorobenzene/ethanol (9:1, v/v) under a constant current of 0.1 ␮A. After 10 days, the black needle-like single crystals of 12 FeCl4 formed on the platinum rod. 2.2. X-ray crystallography The single crystal diffraction data were collected at room temperature on a Rigaku R-axis Rapid IP area detector. The crystal structures were solved by a direct method of SHELXS-97 [12a], and

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Chart 1. The chemical structure of EDO-TTFVODS (1).

refined by a full-matrix least-squares method on F2 by means of SHELXL-97 [12b]. All non-hydrogen atoms were refined anisotropically. Positional parameters of hydrogen atoms were calculated under a fixed C–H bond length of 1.00 A˚ with sp2 or sp3 configuration of the bonding carbon atoms. In the refinement procedures, isotropic temperature factors with the magnitudes of 1.2 times to those of the equivalent temperature factors of the bonding carbon atoms were applied for hydrogen atoms. Table 1 summarizes the details about the data collection and structure refinement. The temperature-dependent electrical resistance of 12 FeCl4 was measured along the long axis of single crystal by a standard fourprobe technique in the temperature range of 300–4.2 K. The gold wires with the diameter of 15 ␮m were attached to the crystal using the conducting graphite paste. The temperature dependence of magnetic susceptibility (obs ) of 12 FeCl4 was measured on the polycrystalline samples under an applied field of 1 kOe with a SQUID magnetometer (MPMS XL, Quantum Design). The paramagnetic susceptibility (p ) was obtained by subtracting the diamagnetic contribution calculated by a Pascal method from obs . 3. Results and discussion The donor molecule, 1 crystallizes in the orthorhombic space group Pna21 with one molecule crystallographically unique. Fig. 1 depicts the molecular structure of 1, the bond distances and angles in the expected range. The central skeletal moiety of 1 shows a boat conformation (see Fig. 1b), which is the typical molecular confirmation for the neutral TTF derivatives [13,14] In the crystal structure, the molecules are stacked in the head-to-head mode to form the one-dimensional columns along the c-axis as shown in Fig. 2. The molecules are uniformly arranged in each column and ˚ which the interplanar distance of molecular mean planes is 3.65 A, is very close to the sum of van der Waals radii of two sulfur atoms ˚ [15], indicating the relatively strong overlap between the (3.60 A) ␲-orbitals of the neighboring molecules. The long axes of the cen-

Fig. 1. Molecular structure of 1: (a) top view and (b) side view.

tral skeletons of molecules show two orientations in the ab-plane: one is almost parallel to the a + b direction and the other is approximately perpendicular to this direction. There are atomic short ˚ S2–C2, 3.45 A) ˚ between the two neighcontacts (S2–Se2, 3.66 A; boring molecules whose molecular long axes are parallel to each other, while there is no atomic contact between the two neighboring molecules showing the orthogonal orientations. This indicates that the molecular columns are slightly dimerized. The cation radical salt, 12 • FeCl4 crystallized in the monoclinic space group Cm and the asymmetric unit contains two donor molecules and one counter FeCl4 − ion. The two independent donor molecules formed two different layers A and B, and the counter FeCl4 − ions layer is sandwiched between two donor layers A and B along the b-axis as shown in Fig. 3a. The donor molecules form the one-dimensional columns along the a-axis in both donor layers. The donor molecules in layer A are ordered and uniformly packed.

Table 1 Crystallographical data of 1 and 12 FeCl4 .

Chemical formula Formula weight Crystal dimension (mm3 ) Crystal system Space group Irradiation a (Å) b (Å) c (Å) ˛ (◦ ) ˇ (◦ )  (◦ ) V (Å3 ) Z Dcalc (g cm−3 ) F(000) No. of unique reflns. No. of used reflns. No. of params. R (I > 2(I)) wR2 (all data)

1

12 FeCl4

C11 H6 O3 S4 Se2 472.32 0.80 × 0.05 × 0.03 Orthorhombic Pna21 Mo K␣ 18.66(2) 19.38(2) 4.073(3) 90.00 90.00 90.00 1473(2) 4 2.130 912 3289 1853 182 0.023 0.027

C22 H12 Cl4 FeO6 S8 Se4 1142.32 0.07 × 0.06 × 0.01 Monoclinic Cm Mo K␣ 7.149(2) 73.10(2) 6.827(1) 90 103.680(3) 90 3467(1) 4 2.188 2200 6499 2666 438 0.1005 0.3047

Fig. 2. Crystal structure of 1 projected along the c-axis. The dashed lines indicate the atomic contacts shorter than the sum of their van der Waals radii [15].

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Fig. 3. (a) Crystal structure of 12 FeCl4 projected along the a-axis, and (b) the stacking motif of donor layer A with the scheme of the intermolecular atomic contacts (dashed lines), where the numeric data indicate the distance in the unit of angstrom (Å).

Fig. 4. Temperature dependences of (a) normalized resistivity (/rt ) and (b) production of magnetic susceptibility and temperature (T) of 12 FeCl4 . The red circle and blue lines in (b) indicate the observed and theoretical T values, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

There are several atomic contacts between the two neighboring donor molecules in this layer as shown in Fig. 3b. However, the donor molecules in layer B show the positional disorder, therefore it is difficult to define the intermolecular atomic contacts. It is known that the electrons are strongly scattered or localized by the disorder in the conduction layer of the low-dimensional system [16]. In the present system, the disorder in layer B may cause the increase of resistivity in the low temperature region as that reported in the organic superconductor of (BO)2 ReO4 (H2 O) [BO = bis(ethylenedioxy)tetrathiafulvalene] [17]. In fact, this salt shows the enhancement of resistivity below 80 K as will be discussed later. There is no short atomic contact between the donor molecules and the FeCl4 − ions and between the neighboring FeCl4 − ions. The shortest distances between Se–Cl and Cl–Cl are 4.73 and ˚ respectively, which are fairly longer than the correspond4.08 A, ing van der Waals distances. These structural features suggest very weak d–d interaction between the d spins of FeCl4 − ions and also very weak ␲–d interaction between the conduction ␲-electrons of the donor molecules and the d spins of FeCl4 − ions. Electrical resistivity () of 12 FeCl4 was measured down to 4.2 K along the long axis of the single crystal, which is parallel to the donor molecular face-to-face stacking direction. This salt shows fairly good conductivity at room temperature conductivity ( RT = 25 S cm−1 ), and a metallic behavior is observed down to 80 K as shown in Fig. 4a. This electronic transport property is in good agreement with its stacking structure, the approximately uniform conduction columns [18]. Below 80 K, an upturn of resistivity is observed, which is ascribable to the disorder of donor column (layer B) as mentioned above. The magnetic susceptibility of 12 FeCl4 was measured on the polycrystalline sample in the temperature range of 1.9–300 K. Fig. 4b depicts the temperature dependent magnetic susceptibility of this salt, which obeys the Curie–Weiss law (p = C/(T − ), where C is the Curie constant and  is the Weiss temperature). The best fitting parameters are C = 4.42 emu K mol−1 and  = −1.5 K. The C

value is very close to that calculated for the Fe(III) (S = 5/2) d-spin of an FeCl4 − ion. The small and negative  value suggests very weak and antiferromagnetic interaction between the d spins of FeCl4 − ions. 4. Conclusion In summary, we synthesized and solved the crystal structure of bent donor molecule 1 and its cation radical salt with FeCl4 − ion. This donor molecule shows the strong tendency to form the columnar stacks either in the neutral or cation radical states, that make it a useful building block to prepare the low-dimensional magnetic molecular conductors. The cation radical salt 12 FeCl4 shows the relatively high conductivity at room temperature and metallic behavior down to cryogen temperature. Although the magnetic FeCl4 − ions in 12 FeCl4 show the weak antiferromagnetic interaction, it is possible to prepare the ferromagnetic metal on the basis of 1 by replacement of the magnetic counter ions. Synthesis of the other cation radical salts with this donor molecule is under going. Supporting information Crystallographic data for the structures 1 and 12 • FeCl4 in this paper have been deposited with the Cambridge Crystallographic Data Centre under deposition numbers 791,845 and 791,846, respectively. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data request/cif, or by emailing data [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. Acknowledgements This work was supported by the National Natural Science Foundation of China (20902051), Foundation of the Education

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