Magnetic Dimensional Crossover from Two- to Three-Dimensional

Article pubs.acs.org/crystal

Magnetic Dimensional Crossover from Two- to Three-Dimensional Heisenberg Magnetism in a Cu−W Cyano-Bridged Bimetal Assembly Ryo Yamada,† Hiroko Tokoro,†,‡ Noriaki Ozaki,† and Shin-ichi Ohkoshi*,†,§ †

Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku Tokyo 113-0033, Japan NEXT, JSPS, 8 Ichibancho, Chiyoda-ku, Tokyo 102-8472, Japan § CREST, JST, K’s Gobancho, 7 Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan ‡

S Supporting Information *

ABSTRACT: In this work, we synthesized a cyano-bridged Cu−W bimetal assembly, [CuII(pyrimidine)2]4[CuII(H2O)2]2[WV(CN)8]4·4H2O (1), which has a monoclinic crystal structure (P21/n space group, a = 15.7365(3) Å, b = 21.1555(4) Å, c = 27.1871(5) Å, β = 91.8630(7)°, and Z = 4). In this compound, Cu and W sites form two-dimensional (2D) layers along the ab plane, while the other Cu sites are bridged between the 2-D layers, constructing a threedimensional (3-D) structure. The magnetic susceptibility measurement showed that ferromagnetic interaction operates in the magnetic spins of the present compound. The fieldcooled-magnetization (FCM) curve indicates that the magnetization gradually increases in the temperature range of ca. 40−8 K, and the spontaneous magnetization appears at a Curie temperature of 8 K. To understand the anomalous magnetization increase in the temperature range of ca. 40−8 K, we measured the magnetic heat capacity (Cmag). The Cmag vs T plots have a broad peak around 18 K and a sharp peak at 8 K. Such a type of Cmag vs T plots indicates a dimensional crossover from a 2-D to a 3-D Heisenberg magnetic model. This is because 1 has a pseudo 2-D network structure; that is, the magnitude of the intralayer superexchange interaction is much larger than that of the interlayer superexchange interaction. Such a magnetic dimensional crossover is a rare and intriguing issue in the field of magnetic substances.

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INTRODUCTION Whether a magnetic phase transition occurs depends on the dimensionality of the spatial coordination of the magnetic spins linked by a spin exchange, that is, one-, two-, and threedimensional (1-D, 2-D, and 3-D) magnetic spin network. In addition to spatial dimensionalities, magnetic behavior is controlled by the dimensionality of magnetic anisotropy (Table 1).1 For example, (i) when magnetic anisotropy occurs in one direction (corresponding to a magnetic easy axis), the magnetic system is called an Ising model, (ii) when the magnetic anisotropy occurs along one plane (corresponding to

a magnetic easy plane), the system is called a XY model, and (iii) when the magnetic anisotropy is isotropically spread 3-D, the system is called a Heisenberg model. The magnetic cyano-bridged bimetal assembly enables various spatially dimensional networks, that is, a high spin cluster,2 1-D magnetic chain,3 2-D magnetic layer,4 and 3-D magnetic network,5 to be synthesized by choosing the transition metal ions and organic ligands. In octacyanometalate [M(CN)8]n− (M = Mo, W) based magnets, magnetic network structures with various dimensionalities have been reported. In the present work, we synthesized a new cyano-bridged Cu−W bimetal assembly, [Cu II (pyrimidine) 2 ] 4 [Cu II (H 2 O) 2 ] 2 [WV(CN)8]4·4H2O (1), and observed a magnetic dimensional crossover from a 2-D Heisenberg model to a 3-D Heisenberg model. Such an observation of magnetic dimensional crossover is very rare in usual magnetic materials.

Table 1. Whether a Magnetic Phase Transition Can Occur (○) or not (×) Depends on the Dimensionality of the Spatial Coordination of the Magnetic Spins Linked by a Spin Exchange magnetic dimensionalitya

1-D

2-D

3-D

Ising model XY model Heisenberg model

× × ×

○ × ×

○ ○ ○

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Received: December 27, 2011 Revised: February 25, 2012 Published: February 29, 2012

a

Magnetic dimensionality means the dimensionality of the spin exchange. © 2012 American Chemical Society

EXPERIMENTAL SECTION

Synthesis. Single crystals of 1 were prepared by reacting a Cs3[W(CN)8]·2H2O (1.2 × 10−2 mol dm−3) aqueous solution and a

2013

dx.doi.org/10.1021/cg201710s | Cryst. Growth Des. 2012, 12, 2013−2017

Crystal Growth & Design

Article

mixed aqueous solution of CuCl2·2H2O (1.8 × 10−2 mol dm−3) and pyrimidine (4.8 × 10−2 mol dm−3) in an H-tube by the diffusion method. Elemental analyses by inductively coupled plasma mass spectroscopy and standard microanalytical methods confirmed that the for mula of 1 was [Cu I I (pyrimidine) 2 ] 4 [Cu I I (H 2 O) 2 ] 2 [WV(CN)8]4·4H2O: Calculated: Cu, 13.95; W, 26.89; C, 28.11; H, 1.77; N, 24.59%: Found: Cu, 13.76; W, 26.93; C, 28.04; H, 1.88; N, 24.73%. The infrared (IR) spectrum showed CN stretching peaks at 2200 and 2161 cm−1, which were assigned to the CN stretching peaks of the bridged CN group (WV−CN−CuII) and the nonbridged CN group (WV−CN), respectively. Characterization. X-ray structural analysis was conducted by mounting a single crystal on a glass fiber. The measurement was performed on a Rigaku RAXIS RAPID imaging plate area detector with graphite monochromated Mo Kα radiation. Data were collected at 296 ± 1 K. The crystal structure was solved by a direct method and refined by a full-matrix least-squares technique using SHELXL-97. All calculations were performed using a crystal structure crystallographic software package.6 All atoms were refined anisotropically. The hydrogen atoms of pyrimidine were refined using the riding model. Crystallographic data for the structure reported in this paper have been deposited with Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-856449. The magnetic properties were measured using a Quantum Design magnetic properties measurement system (MPMS) superconducting quantum interference device (SQUID) magnetometer, while the heat capacity measurement was conducted by a relaxation method using a Quantum Design physical property measurement system (PPMS).

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RESULTS AND DISCUSSION Crystal Structures. X-ray single crystal analysis indicates that the crystal structure of 1 is monoclinic, P21/n space group (a = 15.7365(3) Å, b = 21.1555(4) Å, c = 27.1871(5) Å, β = 91.8630(7)°, and Z = 4) (Table 2). Figures 1a, S1, and S2 show Table 2. Crystallographic and Refinement Data for 1 compound

1

formula formula weight crystal system space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Z calculated density/g cm−3 temperature/K absorption coefficient μ/cm−1 number of measured reflections number of independent reflections number of refined parameters GOF on F2 R1 [I > 2σ(I)] wR2

C64H48Cu6N48O8W4 2734.16 monoclinic P21/n 15.7365(3) 21.1555(4) 27.1871(5) 90.0000 91.8630(7) 90.0000 9046.2(3) 4 1.949 296(1) 6.520 20656 14065 1174 1.032 0.0450 0.0965

Figure 1. Crystal structure of [CuII(pyrimidine)2]4[CuII(H2O)2]2[WV(CN)8]4·4H2O. (a) Asymmetric unit. Red, blue, light gray, gray, and sky blue balls represent Cu, W, C, N, and O, respectively. Hydrogen atoms are omitted for clarity. (b) View along the a axis. Gray sticks represent the framework containing C and N atoms. (c) View along the a axis. Yellow balls represent pores.

nitrogen atoms of the cyano group and two nitrogen atoms of the pyrimidine molecules. The occupancies of Cu1, Cu4, and Cu5 are one, whereas those of Cu2 and Cu3 are 0.5. Cu6 and Cu7, which have occupancies of one, are coordinated by two nitrogen atoms of the cyano group, two nitrogen atoms of the pyrimidine molecules, and two oxygen atoms of the ligand water molecules. The thermal ellipsoids on the nonbridged pyrimidine units are sizable, indicating some disorder. The coordination geometries of the W sites are bicapped trigonal prism (C2v). The number of free cyano groups coordinated to

the asymmetric unit and the coordination environments around Cu and W. ORTEP diagram of asymmetric unit with thermal ellipsoids is shown in Figure S3. 1 contains seven Cu sites (Cu1−Cu7) and four W sites (W1−W4). The coordination geometries of all the Cu sites are pseudo-octahedron (D4h) due to Jahn−Teller distortion. Cu1−Cu5 are coordinated by four 2014



Article

W1, W2, W3, and W4 are 2, 3, 3, and 4, respectively. Cu (Cu1−Cu5) and W (W1−W4) form 2-D layers along the ab plane, while the other Cu (Cu6 and Cu7) sites bridge the layers to construct a 3-D structure (Figures 1b and S4). The intralayer Cu−W distances (average: 5.34 Å) are shorter than the interlayer Cu−W distances (average: 5.64 Å). The pore (diameter = ca. 7.5 Å) in the crystal is occupied by two water molecules (Figure 1c). The water molecules in the pores are connected to the nitrogen atoms of the free cyano groups of [W(CN)8] through hydrogen bonding (H−O−H ··· NC−) (Figure S5). These water molecules and hydrogen bonding are considered to play an important role to produce the pores. Magnetic Properties. The molar magnetic susceptibility (χM) of the present compound was measured. The χM−1 vs temperature (T) plots indicate the χM−1 value linearly decreases from 300 K to ca. 60 K as T decreases, and then the χM−1 value slowly decreases to zero (Figure 2a). The shape of the χM−1 vs

Figure 3. (a) FCM (red circles), ZFCM (black circles), and RM (white circles) curves of [Cu II (pyrimidine) 2 ] 4 [Cu II (H 2 O) 2 ] 2 [WV(CN)8]4·4H2O under an external field of 10 Oe. (b) Enlarged illustration of FCM, ZFCM, and RM curves.

increases in the temperature region of ca. 40−8 K and abruptly increases at around 8 K. In the RM curve, magnetization disappears at 8 K with increasing T, indicating that 1 is a bulk magnet with a Curie temperature (TC) of 8 K. However, the gradual increase in the magnetizations observed in the FCM and ZFCM curves in the temperature region of ca. 40−8 K suggests an anomalous behavior occurs above TC in 1. Heat Capacity. To understand the gradual increase in the FCM in the temperature region of ca. 40−8 K, the heat capacity was measured using PPMS. Figure 4a shows the heat capacity (Cp) vs T plots during the cooling process. As temperature decreases, the Cp value monotonically decreases, and a peak is observed at 8 K. The Cp in a ferromagnetic substance can be expressed by the sum of the heat capacity due to lattice vibration (Clat), short-range magnetic ordering (Cmag‑short), and long-range magnetic ordering (Cmag‑long) as

Figure 2. (a) χM−1 vs T plots of [CuII(pyrimidine)2]4[CuII(H2O)2]2[WV(CN)8]4·4H2O in an external field of 5000 Oe during the cooling process. Black circles and red line represent measured plots and the fitting curve based on Curie−Weiss law. (b) χMT vs T plots in an external field of 5000 Oe during the cooling process.

C p = Clat + Cmag ‐ short + C mag ‐ long

(1) 3

4

where Clat and Cmag‑short are approximately Clat = aT + bT + cT5 + dT6 and Cmag‑short =AT −2.7,8 Here, the Cp vs T plots are fitted by the term of Clat + Cmag‑short in eq 1 in the range of 60− 20 K because Cmag‑long is negligible in this temperature region. Consequently, the magnetic heat capacity (Cmag ≡ Cmag‑short + Cmag‑long) is extracted (Figure 4b). Its fitting parameters are a = 3.75 × 10−2 J K−4 mol−1, b = −1.45 × 10−3 J K−5 mol−1, c = 2.15 × 10−5 J K−6 mol−1, d = −1.15 × 10−7 J K−7 mol−1, and A = 1.33 × 104 J K mol−1. The Cmag vs T plots have a broad peak around 18 K and a sharp peak at 8 K. The sharp peak agrees with the TC value observed in the magnetic measurement. Before the discussion of the broad peak, let us evaluate thermodynamical parameters for the magnetic term. Using the obtained Cmag vs T plots between 60 and 2 K, the magnetic transition enthalpy (= ∫ Cmag dT) and magnetic transition

T plots indicates that 1 has a ferromagnetic interaction. The Weiss temperature (θ), which is obtained by an extrapolation line in the temperature range between 300 and 60 K, is +27 K. The product of the χM and T vs T plots indicate that the χMT value at 300 K is 3.85 K cm3 mol−1, which is consistent with the expected spin only value of 3.75 K cm3 mol−1 for six CuII (S = 1/2) and four WV (S = 1/2) and the value rapidly increases at low T (Figure 2b). These types of χMT−T plots also support the fact that the present compound has a ferromagnetic coupling. Figure 3 shows the field-cooled-magnetization (FCM) curve, zero-field-cooled magnetization (ZFCM) curve, and remanent magnetization (RM) curve in the temperature region of 40−2 K. In the FCM and ZFCM curves, magnetization gradually 2015



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the other hand, a magnetic phase transition is observed in the 3-D Heisenberg model and divergence in Cmag is observed at TC. The observed Cmag vs T plots of 1 seem to be a sum of these two models. Additionally, Cmag vs T curves for other models, that is, 1-D Heisenberg, 1-D XY, 1-D Ising, and 2-D XY models, are quite different from that of 1 (Figures S6 and S7). Thus, we concluded that 1 exhibits a magnetic dimensional crossover from the 2-D Heisenberg model to the 3-D Heisenberg model. One reason for the dimensional crossover from the 2-D to 3D Heisenberg model is 1 has a pseudo 2-D network structure; that is, the 2-D cyano-bridged Cu−W layers are linked by other Cu atoms. The magnitude of the intralayer superexchange interaction is much larger than that of the interlayer superexchange interaction. Hence, the intralayer superexchange interaction operates, inducing a 2-D Heisenberg type of magnetic behavior, while the interlayer superexchange interaction is added at TC, inducing a 3-D Heisenberg type of magnetic behavior.

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CONCLUSION In this work, we demonstrate the crystal structure, magnetic properties, and heat capacity of [Cu II (pyrimidine) 2 ] 4 [CuII(H2O)2]2[WV(CN)8]4·4H2O (1). 1 displays a magnetic dimensional crossover from a 2-D Heisenberg model to a 3-D Heisenberg model. This phenomenon is attributed to the pseudo 2-D structure in this system. The observation of such a magnetic dimensional crossover10,11 is very rare in typical magnetic materials such as magnetic metal oxides and magnetic metal alloys. Hence, the magnetic dimensional crossover is due to the ability of octacyanometalate-based magnets to control network structure. In fact, in addition to the present system, it is known that (tetrenH5)0.8CuII4[WV(CN)8]4·7.2H2O, which has a 2-D lattice framework and an in-plane magnetic anisotropy, demonstrates the 2-D XY model.12 The architecture of the coordination network in octacyano-bridged bimetal assemblies will be useful for understanding the relationship between a magnetic dimensional crossover and crystal structure.

Figure 4. (a) Heat capacity Cp (black line) and calculated lattice vibration Clat (red line) vs T plots of [CuII(pyrimidine)2]4[CuII(H2O)2]2[WV(CN)8]4·4H2O at a zero external magnetic field during the cooling process. (b) Plots of magnetic heat capacity Cmag (white circles) and calculated short-range order Cmag‑short (red line) vs T at a zero external field during the cooling process.

entropy ΔSmag (= ∫ Cmag d lnT) are estimated as ΔHmag = 1.07 kJ mol−1 and ΔSmag = 57.5 J K−1 mol−1. The theoretical ΔSmag value is given by the spin multiplicity for a given formula as ΔSmag = Rln (2S + 1) where R is the gas constant and S is the spin quantum number. Because 1 is composed of six CuII (S = 1/2) and four WV (S = 1/2), the theoretical ΔSmag is R ln 210 = 57.6 J K−1 mol−1. This theoretical ΔSmag agrees well with the experimental ΔSmag value. Figure 5 schematically illustrates the Cmag vs T curves in the 2-D Heisenberg model and the 3-D Heisenberg model.9 In the case of the 2-D Heisenberg model, a magnetic phase transition cannot occur and only a heat capacity hump is observed. On

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ASSOCIATED CONTENT

S Supporting Information *

Atomic coordination, local structures of the Cu and W sites, crystal structures, ORTEP diagram of asymmetric unit, hydrogen-bonding in [CuII(pyrimidine)2]4[CuII(H2O)2]2[WV(CN)8]4·4H2O, and schematic illustrations of magnetic heat capacity of 2-D and 1-D theoretical models. X-ray crystallographic information files (CIF) are available for this compound. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-3-5841-4331. Fax: +81-3-3812-1896. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Dr. Shu Tanaka (The University of Tokyo) for very helpful advice and Asuka Namai and Kosuke Nakagawa (The University of Tokyo) for the careful proofreading. The

Figure 5. Schematic illustration of heat capacity of the 2-D Heisenberg ferromagnetic quadratic lattice (blue line) and 3-D Heisenberg ferromagnetic cubic lattice (red line) based on ref 9. 2016



Article

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present research was supported in part by the CREST program of JST, a Grant-in-Aid for Young Scientists (S) and the NEXT program from JSPS, the Global COE Program “Chemistry Innovation through Cooperation of Science and Engineering”, the Photon Frontier Network Program from MEXT, and the Asahi Glass Foundation.

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Magnetic Dimensional Crossover from Two- to Three-Dimensional

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