Multifunctional Compound Combining Conductivity and Single

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Multifunctional Compound Combining Conductivity and SingleMolecule Magnetism in the Same Temperature Range Nataliya D. Kushch,*,† Lev I. Buravov,† Pavel P. Kushch,† Gennadii V. Shilov,† Hideki Yamochi,‡,§ Manabu Ishikawa,§ Akihiro Otsuka,‡,§ Alexander A. Shakin,⊥ Olga V. Maximova,⊥,∥ Olga S. Volkova,⊥,∥ Alexander N. Vasiliev,⊥,∥,¶ and Eduard B. Yagubskii*,† †

Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka 142432, Russia Division of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan § Research Center for Low Temperature and Materials Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan ⊥ National University of Science and Technology “MISiS”, Moscow 119049, Russia ∥ Lomonosov Moscow State University, Moscow 119991, Russia ¶ National Research South Ural State University, Chelyabinsk 454080, Russia ‡

S Supporting Information *

characteristic for SMMs the frequency-dependent alternatingcurrent (ac) magnetic susceptibility at low temperatures.14,15 Another somewhat wider group of conductive SMMs refers to metal complex conductors based on radical-anion metal dithiolene complexes with certain cation SMMs as counterions.16−18 Very recently, the conducting molecular nanomagnet of DyIII with partially charged tetracyanoquinodimethane radicals was synthesized.19 However, in all of these compounds, conductivity and single-molecule magnetism exist in different temperature ranges: SMM properties appear at low (liquidhelium) temperatures, whereas conductivity exists at room temperature or at best in the liquid-nitrogen temperature range. The purpose of the present study is to synthesize a conductive SMM that would retain conductivity down to liquid-helium temperature, where one can expect the manifestation of special magnetic properties associated with single-molecule magnetism. To solve this problem, the oxygen analogue of BEDT-TTF, bis(ethylenedioxo)tetrathiafulvalene (BEDO; Scheme S1), is chosen as a donor in this work. The choice of the BEDO donor is due to the fact that it forms radical-cation salts with counterions of different geometry and size that retain metallic-type conductivity down to liquid-helium temperature. Unlike BEDT-TTF and its other derivatives, in the case of BEDO, the crystal packing of the conducting layers is determined primarily by the nature of BEDO itself, and the counterions have practically no effect on the packing of the radical-cation layers.20,21 BEDO layers in salt crystals have a so-called β″-type packing, where the BEDO radical cations stack to form twodimensional layers because of numerous short side-by-side contacts between the heteroatoms and C−H···O intermolecular hydrogen bonds along the face-to-face stacking direction. A consequence of this packing is the formation of wide twodimensional electronic bands in BEDO salts, which leads to stabilization of the metallic state down to liquid-helium temperatures.20,21 As a counterion in the BEDO salt, we used a hexafluoride complex of ReIV, [ReF6]2−. Recently, it has been

ABSTRACT: We report the first highly conducting single-molecule magnet, (BEDO) 4 [ReF 6 ]·6H 2 O [1; BEDO = bis(ethylenedioxo)tetrathiafulvalene], whose conductivity and single-molecule magnetism coexist in the same temperature range. The compound was synthesized by BEDO electrocrystallization in the presence of (Ph4P)2[ReF6]·2H2O and characterized by crystallography and measurements of the conductivity and alternating-current magnetic susceptibility.

M

ultifunctionality is one of the most attractive directions in the chemistry of modern materials.1 Among multifunctional materials, the compounds combining electrical conductivity and magnetism in the same crystal lattice are objectives of intensive study in the past decades.2−4 This interest is associated with possible synergy of these properties, which may lead to novel phenomena. The majority of research in this field has focused on the family of quasi-two-dimensional (super)conductors based on the radical cation salts of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and its derivatives with various kinds of paramagnetic metal complex anions.2−4 In such materials, conductivity is associated with mobile electrons in organic layers, whereas magnetism usually originates from localized spins of transition-metal ions in insulating counterion layers.5−10 The search for suitable organic donors and magnetic counterions is one of the main strategic tasks in the synthesis of multifunctional organic conductors and superconductors based on radical-cation salts. Among the possible magnetic counterions, the molecular nanomagnets, also called single-molecule magnets (SMMs), attract great attention because they exhibit unique magnetic properties at the molecular level, such as slow relaxation of magnetization, blocking, and quantum tunneling of magnetization (QTM), which can be used to design spintronic devices.11−13 There have been only two successful attempts to synthesize conductive radical-cation salts based on organic donors (BEDT-TTF and TTF) with anionic complexes of rare earths (Dy), showing © XXXX American Chemical Society

Received: December 14, 2017

A

DOI: 10.1021/acs.inorgchem.7b03152 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Å. There are shortened contacts of the type F···O (2.763 Å), F··· C (3.011 and 3.123 Å), and F···H−C (2.462 and 2.492 Å) between these F atoms and BEDO molecules from the cation layers. The temperature dependence of the resistance of β″-1 crystals shows nonmonotonic behavior, changing little in the temperature range 300−4.2 K (Figure 2). In the context of this

found that this anion in (PPh4)2[ReF6]·2H2O displays SMM properties below 5 K: the application of a small permanent magnetic field gives rise to clear peaks in the out-of-phase component of the ac susceptibility, χ″, characteristic of slow dynamics of magnetization.22 Unlike most SMMs based on mononuclear complexes that exhibit negative uniaxial magnetic anisotropy D < 0, the [ReF6]2− anion has easy-plane-type anisotropy D > 0. Currently, the [ReF6] module is the solely unique example of a mononuclear 5d complex, which shows magnetization relaxation while possessing easy-plane anisotropy. Herein we report the first highly conducting SMM, (BEDO)4[ReF6]·6H2O (1), whose conductivity and single-molecule magnetism coexist in the same temperature range. The compound was synthesized by BEDO electrocrystallization in the presence of (Ph4P)2[ReF6]·2H2O (see the Supporting Information) and characterized by crystallography at 100 K and measurements of the conductivity and ac magnetic susceptibility. A triclinic lattice of 1 (Table S1) contains two crystallographically independent BEDO molecules (A and B) occupying the general positions (Figures S1 and S2). The compound has a layered structure in which conducting radical-cation layers alternate along the c axis with the 2D polymeric anion layers composed of [ReF6]2− and water molecules (Figure S2). The BEDO layers have the β″ type of molecular packing and are composed of parallel BEDO stacks, each built from independent BEDO molecules (A and B stacks; Figure 1). Interplanar

Figure 2. Temperature dependence of the relative resistance of the β″-1 crystal measured in the plane of radical-cation layers. The inset shows the low-temperature part of the R(T) curve.

Communication, it is important that the conductivity at 4.2 K remains fairly high, close to the conductivity at room temperature (σ300 K = 40−60 Ω−1 cm−1). The nonmonotonic character of the conductivity is probably due to the influence of disorder in the anionic layer on the conductivity of the crystals, which can lead to localization of the electronic states. The structure of the crystals was solved and refined at 100 K. Analysis of the structure showed the presence of disorder in the anionic layer: in two of the three crystallographically independent water molecules, one H atom is disordered in two positions with a probability of 50% (see section 3.1 and Figure S1). Attempts were made to carry out X-ray experiments in the temperature interval 200−300 K, but poor accuracy of the experimental data impeded the obtainment of quality structural information (section 3.1). Nevertheless, at a qualitative level, the cationic part of the crystal structure at 210 K remains practically unchanged compared to 100 K, and all of the changes occur in the anionic layer, in particular, water molecules become disordered. Apparently, the change in the character of the disorder with decreasing temperature affects the conductivity behavior. The existence of disorder in the anionic layers is characteristic of β″-BEDO salts.20,23 The weak conjugation of cationic and anionic sublattices is a distinctive peculiarity of these salts. As a result, anionic layers are often disordered or have their own periodicity, differing from the periodicity of the radical cation of the layers (the formation of superlattices).20,23 Despite the presence of disorder in the anion system, BEDO salts with β″-type packing of radical-cation salts usually exhibit a temperature dependence of the metallic-type conductivity over a wide temperature range.20,23−25 The degree of disorder affects the magnitude of the drop of resistance with decreasing temperature and often leads to a slight increase of the resistance at T < 20 K.20,23 A weak low-temperature resistance growth is also observed in the crystals β″-(BEDO)4[ReF6]·6H2O (Figure 2, inset). The growth of the resistance in BEDO salts at low temperatures is probably associated with localization effects due

Figure 1. Projection of the radical-cation layer on the ab plane. Dashed lines indicate shortened C−H···O, S···S, and S···O contacts. Color designation of atoms in the BEDO molecule: S, yellow; O, red; C, gray; H, white.

distances in the A and B stacks are 3.418 and 3.458 Å, respectively. The central CC bonds of two independent BEDO molecules are practically identical, 1.365(8) and 1.369(9) Å. This indicates identical charge states of the two radical cations, BEDO0.5+, according to the chemical formula. There are several shortened C−H···O contacts within each donor stack and numerous S···S (3.294−3.571 Å) and S···O (3.025−3.487 Å) contacts between the adjacent donor stacks in the layer (Figure 1 and Table S2). The anion layers present an endless 2D polymer network formed by [ReF6]2− anions and water molecules interlinked by hydrogen bonds (Figure S3). Only four F atoms in the octahedral environment of Re are involved in the formation of the polymeric anion network with Re−F bond lengths equal to ≅1.95 and ≅1.96 Å. The bond length of Re with two other F atoms is 1.917 B

DOI: 10.1021/acs.inorgchem.7b03152 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

relaxation processes occurring in complex 1. Cole−Cole fittings by the generalized Debye model gave distribution of the relaxation time α (Table S3).11,29 The obtained α values are in the range of 0.11−0.21 (from 4.0 to 2.0 K), which suggests a relatively narrow distribution of the relaxation time. The efficient energy barrier for 1 refers to the high-temperature data (4.0−3.0 K) considering only a thermally activated (Orbach) process (Figure S7). Below 3.0 K, the plot of ln(τ) versus T−1 shows clear curvature. This behavior suggests the coexistence of multiple relaxation processes. Probably, extending the experiment to lower temperatures (T < 2 K) would show “saturation” in the ln(τ) versus 1/T plot due to QTM. In summary, the first conductive SMM, 1, in which conductivity and single-molecule magnetism coexist in the same temperature range was synthesized and characterized structurally. We have performed a study of the conductivity and SMM behavior of 1. It has been shown that the complex retains high conductivity down to 4.2 K and displays slow magnetic relaxation, indicating SMM behavior. Our result opens a synthetic route to new highly conductive SMMs at liquid-helium temperatures based on radical-cation salts.

to the quantum interference of the carriers in the presence of elastic scattering processes.26,27 This effect is characteristic for the systems with disorder. In order to test whether the anion of [ReF6]2− as a counterion in the conductive radical-cation BEDO salt retains the properties of SMM, we investigated the ac magnetic susceptibility of 1 because the frequency dependences of its in-phase (χ′) and outof-phase (χ″) components provide information about the character of the magnetization relaxation.11,12 Figures S4 and S5 and 3 show the frequency dependences of the out-of-phase

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03152.

Figure 3. Frequency dependences of the out-of phase (χ″) ac susceptibility for β″-1 at temperatures of 2.0−4.0 K in a dc field of 2700 Oe (dots, experiment; red lines, fitting by the generalized Debye model with the parameters listed in Table S3).

Experimental details, crystallographic data, and additional structural and magnetic figures and table (PDF)

and in-phase components of the ac susceptibility of complex 1 at different temperatures (2.0−4.0 K) in a zero direct-current (dc) field and 2700 Oe, respectively. In a zero dc field, the frequency dependences of χ″ are observed; however, the positions of the peak maxima lie in the frequency range inaccessible for our magnetometer (Figure S4). The application of an optimal dc field of 2700 Oe (Figure S6) gives rise to clear peaks in the outof-phase component (Figure 3), which shift toward higher frequencies with increasing temperature, as in the case of the starting complex (Ph4P)2[ReF6]2−·2H2O in a dc field of 2500 Oe.22 Because of the presence of maxima in the χ″ versus ν plots (at different temperatures), the effective energy barrier (Ueff) and relaxation time (τ0) were determined from the out-of-phase ac susceptibility data using an Arrhenius equation,11 τ(T) = τ0 exp[Ueff/(kbT)], where τ = 1/2πνac (Figure S7). Best fits to the Arrhenius equation gave Ueff = 19 K and τ0 = 5 × 10−7 s. The values of Ueff and τ0 for (BEDO)4[ReF6]·6H2O differ markedly from those for (PPh4)2[ReF6]·2H2O and [Zn(1-vinylimidazole)4(ReF6)]∝ (28.3 K, 9.6 × 10−9 s and 29.6 K, 4.7 × 10−10 s, respectively), which are close to each other. In contrast to these rhenium complexes, in which the lengths of the Re−F bonds in the [ReF6]2− octahedron are close (1.95−1.97 Å), in complex 1, the lengths of the Re−F bonds in the equatorial and axial planes differ markedly (1.95−1.96 vs 1.917 Å). Recently, the Mills group has demonstrated that magnetic relaxation in SMMs is effected by coupling of the electronic states to the lattice vibrational modes.28 Perhaps in 1, the shortened Re−F bonds and/or hydrogen bonding implicated in the disorder could provide vibrational modes to relax the spin more readily than in (PPh4)2[ReF6]·2H2O and [Zn(1-vinylimidazole)4(ReF6)]∝ complexes. The plots of χ″ versus χ′ known as the Cole− Cole29 or Argand plots are shown in Figure S8 as evidence of

Accession Codes

CCDC 1577789 contains the supplementary crystallographic data for this paper. 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.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Nataliya D. Kushch: 0000-0002-9691-7037 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research (Project 17-03-00167 for N.D.K., L.I.B., E.B.Y., and O.V.M.) and JSPS KAKENHI Grant JP26288035 (to H.Y.). Support by the Ministry of Education and Science of the Russian Federation in the framework of the Increase Competitiveness Program of NUST “MISiS” through Grants K2-2016-084 and κ2-2017-024 and by Acts 211 of the Government of Russian Federation under Contracts 02.A03.21.0004 and 02.A03.21.0011 is acknowledged by O.S.V., O.V.M., and A.N.V. The authors thank V. A. Kopotkov and P. A. Lizyakina for assistance in this work. C

DOI: 10.1021/acs.inorgchem.7b03152 Inorg. Chem. XXXX, XXX, XXX−XXX

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(21) Ishiguro, T.; Yamaji, K.; Saito, G. Organic superconductors, 2nd ed.; Fulde, P., Ed.; Springer: New York, 1997. (22) Pedersen, K. S.; Sigrist, M.; Sorensen, M. A.; Barra, A.-L.; Weyhermüller, T.; Piligkos, S.; Thuesen, C. A.; Vinum, M. G.; Mutka, H.; Weihe, H.; Clerac, R.; Bendix, J. [ReF6]2−: A Robust Module for the Design of Molecule-Based Magnetic Materials. Angew. Chem., Int. Ed. 2014, 53, 1351−1354. (23) Prokhorova, T. G.; Simonov, S. V.; Khasanov, S. S.; Zorina, L. V.; Buravov, L. I.; Shibaeva, R. P.; Yagubskii, E. B.; Morgunov, R. B.; Foltynowicz, D.; Swietlik, R. Bifunctional molecular metals based on BEDO-TTF radical cation salts with paramagnetic [MIII(CN)6]3‑ anions, M = Fe, Cr, (Fe0.5Co0.5). Synth. Met. 2008, 158, 749−757. (24) Yamochi, H.; Horiuchi, S.; Saito, G.; Kusunoki, M.; Sakaguchi, K.; Kikuchi, T.; Sato, S. Strong tendency of BEDO-TTF to produce organic metals. Synth. Met. 1993, 56, 2096−2101. (25) Yamochi, H. Oxygen Analogues of TTFs. In TTF Chemistry − Fundamentals and Applications of Tetrathiafulvalene; Yamada, J., Sugimoto, T., Eds.; Kodansha/Springer: Tokyo, 2004; pp 83−118. (26) Altshuler, B. L.; Aronov, A. G. Electron-electron interaction in disordered conductors. In Electron−Electron Interactions in Disordered Systems; Efros, A. L., Pollak, M., Eds.; Elsevier: Amsterdam, North Holland, 1985. (27) Gantmakher, V. F. Electrons and Disorder in Solids; Claredon Press: Oxford, U.K., 2005. (28) Goodwin, C. A. P.; Ortu, F.; Reta, D.; Chilton, N. F.; Mills, D. P. Molecular magnetic hysteresis at 60 K in dysprosocenium. Nature 2017, 548, 439−445. (29) Cole, K. S.; Cole, R. H. Dispersion and Absorption in Dielectrics I. Alternating Current Characteristics. J. Chem. Phys. 1941, 9, 341−352.

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DOI: 10.1021/acs.inorgchem.7b03152 Inorg. Chem. XXXX, XXX, XXX−XXX