Cesium Cyano-Bridged CoIIâMV (M = Mo and W) Layered

Article pubs.acs.org/crystal

Cesium Cyano-Bridged CoII−MV (M = Mo and W) Layered Frameworks Exhibiting High Thermal Durability and Metamagnetism Koji Nakabayashi,† Szymon Chorazy,† Daisuke Takahashi,† Takaaki Kinoshita,† Barbara Sieklucka,‡ and Shin-ichi Ohkoshi*,†,§ †

Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Cracow, Poland § CREST, Japan Science and Technology (JST), K’s Gobancho, 7 Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan ‡

S Supporting Information *

ABSTRACT: Two-dimensional cesium bimetal cyano-bridged assemblies CsI4CoII[MoV(CN)8]Cl3 (CsCoMo) and CsI4CoII[WV(CN)8]Cl3 (CsCoW) were synthesized. The negatively charged and solvent-free {CoII[MV(CN)8]Cl3}4− (M = Mo, W) coordination layers are separated by Cs+ ions. Themogravimetric measurements show that these compounds reveal high thermal durability up to 523 K (250 °C), which is due to the absence of solvent molecules in their crystal structures. The magnetic measurements show that CsCoMo and CsCoW are metamagnets showing the field-induced transition from an antiferromagnetic phase with Néel temperature of 25 K to a ferromagnetic phase, which is observed at high critical magnetic field of 24 kOe at 1.8 K. These originate from antiferromagnetic interactions between ferromagnetically coupled cyano-bridged CoII−MV layers, and the contribution from single-ion anisotropy of CoII.

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INTRODUCTION The studies on functional magnetic materials are one of the most interesting topics in the field of coordination chemistry and materials science. Cyano-bridged metal assemblies can potentially reveal a high Curie temperature,1 and show various unique magnetic functionalities, such as ferroelectricityferromagnetism,2a,b nonlinear magneto-optical effects,2c,d humidity-sensitive magnetism,3a−c gas adsorption,3d,e proton conductive magnetism,3f and photoinduced magnetization.4 Among cyano-bridged bimetal assemblies, magnetic octacyanometalate-based frameworks recently draw much attention. The building blocks, octacyanometalates [M(CN)8]n− (M = Mo, W, and Nb), have an advantage in constructing zerodimensional (0-D), 1-D, 2-D, and 3-D structures5 due to their flexibility between coordination geometries such as a dodecahedron, a bicapped trigonal prism, or a square antiprism.5a,6 From the viewpoints of the preparation of highTC molecule-based magnets, [M(CN)8]n‑ ions prefer high coordination numbers and preserve large superexchange interactions due to their diffuse 4d or 5d orbitals. Up to date, many octacyanometalate-based assemblies exhibiting a high Curie temperature5b,6j and functionalities such as photoinduced magnetization,4a,7 alcohol-sensitive magnetism,3e and luminescence8 have been reported. Additionally, octacyanometalatebased materials recently provided new opto-magnetic functionalities, e.g., light-induced spin-crossover magnetization in Fe2[Nb(CN)8]·(4-pyridinealdoxime)8·2H2O,9 and 90-degree © 2014 American Chemical Society

optical switching of magnetization-induced second harmonic generation in Fe2[Nb(CN)8]·(4-bromopyridine)8·2H2O chiral magnet.10 To improve these magnetic functionalities, thermal durability is important. In this work, we draw our attention to thermal durability of octacyanometalate-based assemblies. We present the syntheses, crystal structures, optical, thermogravimetric, and magnetic properties of cyano-bridged Co−W bimetallic assemblies, CsI4CoII[MV(CN)8]Cl3 (M = Mo, W), which show high thermal durability up to 250 °C and metamagnetic behavior.

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EXPERIMENTAL SECTION

Syntheses. Cs4CoII[MoV(CN)8]Cl3 (CsCoMo). The single crystals were obtained by using a diffusion method. CoIICl2·6H2O (0.13 mol dm−3, 18.8 mg, 0.08 mmol) together with CsCl (0.23 mol dm−3, 24.2 mg, 0.14 mmol) were dissolved in a minimal amount of water (0.6 cm3). Then 21 cm3 of acetone was slowly added, and the change of color from pink to blue, followed by the precipitation of light blue powder, was observed. The resulting suspension was shortly mixed and left undisturbed for a few minutes (when the whole precipitation was on the bottom of a crystallization vessel). Then concentrated aqueous solution of Cs3[MoV(CN)8]·2H2O11 (0.12 mol dm−3, 53.2 mg, 0.07 mmol, 0.6 cm3) was extremely carefully introduced on the bottom which results in the orange aqueous layer clearly separated from the blue acetone phase situated above. Such mixture was tightly closed and Received: August 22, 2014 Revised: September 20, 2014 Published: September 23, 2014 6093

dx.doi.org/10.1021/cg501250p | Cryst. Growth Des. 2014, 14, 6093−6100

Crystal Growth & Design

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Figure 1. Crystal structure of CsCoMo: (a) the asymmetric unit shown by 80% probability thermal ellipsoids (b) the projection along a axis, and (c) the view within ac plane. The green, purple, yellow, red, gray, and light blue represent Mo, Co, Cs, Cl, C, and N atoms, respectively. Cs4CoII[WV(CN)8]Cl3 (CsCoW). The synthetic procedure of the single crystals was completely the same as described for CsCoMo with the exception of using Cs3[WV(CN)8]·2H2O11 (0.12 mol dm−3, 59.5 mg, 0.07 mmol, 0.6 cm3) instead of analogous MoV salt. A large amount of dark blue plate-type single-crystals of CsCoW were collected from almost colorless solution after 4 days, filtrated and airdried. This crystalline product of CsCoW was obtained with high yield (78 mg, 89%), and it was reasonably stable during exposition to the air. Found: Cs, 48.71%; Co, 5.42%; W, 16.90%; C, 8.88%; H, 0.12%; N, 10.18%. Calculated for Cs4Co1W1Cl3C8N8 (M = 1089 g·mol−1): Cs, 48.82%; Co, 5.41%; W, 16.88%; C, 8.82%; H, 0%; N, 10.29%. IR

left in the dark for crystallization. Dark green platelet single-crystals of CsCoMo appeared after a few days, but the significant amount of yellow and grayish amorphous byproducts were also observed. Therefore, the pure crystalline product of CsCoMo was carefully separated by repeated decantation, filtrated, and air-dried (48 mg, 60%). The obtained sample was fairly stable during exposition to the air. Found: Cs, 52.85%; Co, 6.11%; Mo, 9.52%; C, 9.98%; H, 0.22%; N, 11.43%. Calculated for Cs4Co1Mo1Cl3C8N8 (M = 1001 g·mol−1): Cs, 53.11%; Co, 5.89%; Mo, 9.58%; C, 9.60%; H, 0%; N, 11.19%. IR (KBr): 2137, 2143, and 2147 cm−1 for terminal cyanides; 2170, 2175, and 2178 cm−1 for cyanide bridges.5 6094



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Figure 2. UV−vis−NIR diffuse reflectance spectra in the 300−1100 nm range together with photos of the polycrystalline samples of (a) CsCoMo and (b) CsCoW. Green and blue colored lines are experimental results, and black lines represent the sum of the respective bands shown by dotted lines. The colors of powder samples are presented in the insets. (KBr): 2139, 2146, and 2150 cm−1 for terminal cyanides; 2173, 2178, and 2181 cm−1 for cyanide bridges. Note that the elemental analyses for both CsCoMo and CsCoW revealed a very small but non-negligible amount of hydrogen which is not expected from their chemical formula. This can be explained by the presence of a small amount of water from the air and the absorber of the analyzer or the general moisture sensitivity of the reported compounds. Such amount of water gives the values of 0.22% and 0.12% for CsCoMo and CsCoW, respectively, which are below the experimental detection limit of 0.3% for hydrogen. Crystal Structure Determination. For the single-crystal X-ray structural analysis of CsCoMo, a single crystal of 0.04 × 0.03 × 0.01 mm3 was mounted on a Micro Mounts. The measurement at 90(2) K was conducted using the synchrotron radiation (λ = 0.68890 Å) at the Photon Factory-Advanced Ring for Pulse X-rays (PF-AR) of the High Energy Accelerator Research Organization (KEK). Data integration and scaling were performed with the HKL/HKL-2000 program.12 For the single crystal X-ray structural analysis of CsCoW, a single crystal of 0.08 × 0.04 × 0.02 mm3 was used. The measurement at 93(2) K was performed on a Rigaku RAXIS RAPID imaging plate area detector with graphite monochromated Mo−Kα radiation. The data processes were performed using the Crystal Structure crystallographic software package. Both crystal structures were solved by a direct method and refined by a full-matrix least-squares technique using SHELXL-97.13 All atoms were refined anisotropically. Methods. Elemental analysis of Cs, Co, and W for the prepared crystals were measured by Agilent 7700 Series inductively coupled plasma mass spectroscopy (ICP-MS), and those of C, H and N were determined by standard microanalytical methods. IR spectrum was recorded on a JASCO FTIR-4100 spectrometer in the 4000−400 cm−1 region on KBr discs. The UV−vis diffuse reflectance spectra were measured by a Shimadzu UV-3100 spectrometer on the powder samples mixed with barium sulfate. Thermogravimetric measurement was conducted by RIGAKU Thermo Plus TG8120 in the 30° − 450 °C range at a heating rate of 2 °C min−1. Powder XRD diffractograms were measured on Rigaku RINT 2100 with Cu Kα radiation at 293(2)K. Magnetic measurements were carried out by Quantum Design MPMS-5 and PPMS superconducting quantum interference device (SQUID) magnetometers. The magnetic susceptibility data were corrected for the diamagnetic contribution using Pascal constants.14 Continuous Shape Measure Analyses for coordination spheres of eight-coordinated [MoV(CN)8]3− and [WV(CN)8]3− complexes were performed using SHAPE software version 2.0.15

CsCoMo consists of cyano-bridged two-dimensional layers with Cs+ ions located between them (Figure 1; also see Tables S1−S4 and Figures S3−S4 in the Supporting Information). The layers are composed of four-metallic {Co2Mo2} squares (5.4 × 5.4 Å) and eight-metallic nearly rectangular {Co4Mo4} rings (∼15 × 5.4 Å), both built by CoII and MoV centers alternately connected through CN− bridges (Figure 1b). Smaller fourmetallic units are combined by the common edges with four different eight-metallic neighbors. The whole 2-D coordination layer is built only by one symmetrically independent CoII−MoV pair (Co1−Mo1, Figure 1a). Three cyanides of [Mo1 V (CN) 8 ] 3− units form molecular bridges to the neighboring CoII, when the five remaining are terminal. Their geometry is very close to the ideal square antiprism (SAPR-8) (Supporting Information Tables S5 and S6). Co1 center coordinates three N atoms of cyanides in mer-configuration, and three chlorides, resulting in the six-coordinated [Co1IICl3(NC)3]4− complex of a slightly distorted octahedral geometry. The coordination of two different anionic ligands to CoII centers (octacyanide metalloligand and inorganic chloride ligand) causes that {CoII[MoV(CN)8]Cl3}4− layers of CsCoMo reveal high negative charge of −4 for CoII−MoV unit, which is unique among cyano-bridged bimetal assemblies. To get the neutral character of the whole compound, the asymmetric unit of CsCoMo is completed by four Cs+ cations (Figure 1a). These alkaline ions are mainly distributed in the interlayer space, but one forth of them were found within the plane of coordination layers, in the channels limited by large {Co4Mo4} rings (Figure 1c). The three-dimensional supramolecular network of CsCoMo is maintained by interactions between positive Cs+ ions and external parts of negatively charged coordination layers (Supporting Information Figure S3 and Table S3). The most significant interactions for Cs+ ions are observed with chlorides coordinated to CoII, and N atoms of terminal cyanides. The shortest Cs+···Cl and Cs···N contacts are 3.4 Å (Cs4···Cl2) and 3.0 Å (Cs3···N8), respectively, while the average values are within the range 3.5−3.6 Å for Cs+···Cl, and 3.4−3.7 Å for Cs+···N distances. Additionally, the ion−π interactions between Cs+ and π-bonds of bridging cyanides, with short Cs+···πCN contacts within the 3.6−4.2 Å range, costabilize the layered structure. Because of the ion−ion and ion−π interactions, the average distances between neighboring Cs+ ions are rather long (4.8−5.0 Å). However, several relatively short Cs+···Cs+ contacts of 4.2−4.5 Å are present in the crystal structure, especially for interlayer Cs3 center

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RESULTS AND DISCUSSION Crystal Structures. The single crystal X-ray structural analysis shows that CsCoMo has the monoclinic system with a P21/n space group (a = 9.330(2) Å, b = 13.642(3) Å, c = 16.734(3) Å, β = 95.92(3)°, V = 2118.5(7) Å3, Z = 4). 6095



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potential between MoIV/V (E°1/2 = −0.56 V vs Ag/AgCl) and WIV/V (E°1/2 = −0.81 V vs Ag/AgCl), which affects the energy of MMCT process.16,20 It is noteworthy that optical CoIIHS → MoV MMCT observed in CsCoMo, which is an analogue of broadly investigated CoIIHS → WV transition,5b,7c,21 is clearly shown for the first time. Thermal Durability. The thermogravimetric analyses of CsCoMo and CsCoW are shown in Figure 3. Up to 523 K

(Supporting Information Figure S4, Table S4). The presence of Cs+ ions between layers leads to the significant separation of coordination layers with the shortest interlayer Co1−Mo1 distance of about 8 Å. The crystal structure of CsCoW (monoclinic, P21/n space group, a = 9.346(2) Å, b = 13.677(2) Å, c = 16.773(3) Å, β = 95.956(3)°, V = 2132.4(6) Å3, Z = 4) is identical to those presented for CsCoMo (Supporting Information Tables S1−S4 and Figures S2−S4). Only differences are observed in metric parameters of bond lengths as [WV(CN)8]3− is larger than [MoV(CN)8]3− unit which results in a slight expansion of the whole structure (Supporting Information Tables S1 and S2). The short contacts between Cs+ ions and external atoms of {CoII[MV(CN)8]Cl3}4− (M = Mo, W) layers as well as Cs···Cs distances are also slightly longer in CsCoW with the average difference of about 0.005 Å (Supporting Information Tables S3 and S4). It is noteworthy that the crystal structures of CsCoMo and CsCoW do not reveal any solvent molecules although water and acetone were used in the synthetic procedure. Instead of this, heavy atoms of Cs+ ions are embedded between coordination layers which leads to high density as calculated from the results of single-crystal X-ray structural analysis (Supporting Information Table S1). Powder X-ray diffraction patterns for both CsCoMo and CsCoW are presented in Supporting Information Figure S1. The respective calculated patterns fit well the experimental ones indicating that structural models obtained from single-crystal X-ray structural analyses are valid for bulk samples used in other physical measurements. Optical Properties. In the UV−vis−NIR diffuse reflectance spectrum (Figure 2), CsCoMo exhibits a strong band in the UV region with a maximum at 380 nm overlapping with broad visible absorption with a maximum around 680 nm. Deconvolution of the spectrum gave four main components with maxima at 380 (peak 1), 680 (peak 2), 750 (peak 3), and 835 nm (peak 4). Peak 1 can be assigned to ligand-to-metal charge transfer (LMCT) from CN− to MoV, that is to three main LMCT transitions from E”(2B1) ground state to E′(2A2), E′(2E), and E”(2E) excited states.16 Peak 2, covering almost the whole visible range, is connected with metal-to-metal charge transfer (MMCT) from high-spin (HS) CoII to MoV.17 Much weaker peaks 3 and 4 are related to d-d transitions in CoIIHS centers [4T1g(4F) → 4T1g(4P) for peak 3; 4T1g(4F) → 4A2g(4F) for peak 4].18 The relatively low energy of MMCT and d-d transitions is the consequence of chlorides coordinated to CoII, which are rather weak-field ligands leading to the small crystalfield splitting parameter for octahedral CoIIHS units.19 The absorption spectrum of CsCoW is comparable to those observed for CsCoMo. Deconvolution of the spectrum gave four main components with maxima at 345 (peak 1, LMCT),16 600 (peak 2, MMCT),17,20 730 (peak 3, d-d transition of 4 T1g(4F) → 4T1g(4P)), and 810 nm (peak 4, d-d transition of 4 T1g(4F) → 4A2g(4F)).18 The chloride coordination contributes to lowering the energies of MMCT and d-d transitions. The main difference between CsCoMo and CsCoW, resulting in a different color of the sample, is the large red shift of two main bands assigned to LMCT from CN− to MoV (peak 1) and MMCT from CoIIHS to MoV (peak 2). The red shift of peak 1 is in a good agreement of the general weaker splitting of metal center excited states in the cyanides ligand field when going from electron richer 5d W5+ ion to electron poorer 4d Mo5+ ion.16,19 The difference between CsCoW and CsCoMo is also connected with the difference of redox

Figure 3. Results of thermogravimetric analysis for (a) CsCoMo (green line) and (b) CsCoW (blue lines) in the 27−420 °C range under a scan rate of 2 °C per minute.

(250 °C), the samples do not show any significant weight loss indicating the complete stability of these compounds during exposition to such temperatures. Later, the decrease in the weight between 250 and 400 °C is connected with the loss of five terminal cyanides. The total losses of 12.8% for CsCoMo and 12.1% for CsCoW are in a good agreement with the theoretical values of 13.1% for CsCoMo and 12.0% for CsCoW, respectively, calculated from the decrease of the molar mass after the removal of terminal cyanides.22 The observed high thermal stability for CsCoMo and CsCoW is due to absence of solvent molecules in the crystal structures.23 Magnetic Properties. Magnetic characteristics of CsCoMo and CsCoW are presented in Figures 4−7 and Supporting Information Figure S5−S7. The value of magnetic susceptibility−temperature product (χMT) for CsCoMo at room temperature is 3.1 K cm3 mol−1, which corresponds well to the expected value of 3.1 K cm3 mol−1 for an uncoupled CoIIMoV unit (assuming SMo = 1/2, gMo = 2.0, SCo = 3/2, and gCo = 2.4) (Figure 4a). Upon cooling, the χMT value (H = 5 kOe) increases slowly between 300 and 80 K, and then sharply below 80 K. The maximum of 7.0 K cm3 mol−1 is observed at T = 25.9 K. Later, the χMT abruptly decreases to almost 0 K cm3 mol−1 at 1.8 K. 6096



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Figure 4. Temperature dependences of χMT (H = 5 kOe) for (a) CsCoMo and (b) CsCoW.

Figure 5. Field-cooled magnetization curves of (a) CsCoMo and (b) CsCoW for the indicated magnetic fields from 5 to 30 kOe (top). Plots of the χM values at 10 K vs the magnetic fields (bottom).

obtained from χM−1 versus T curve obeying the Curie−Weiss law in the 200−300 K range (Supporting Information Figure S5). Further decrease in χMT(T) plot, with almost vanishing of a magnetic signal at lowest temperatures, suggest the presence of interlayer antiferromagnetic interactions leading to an antiferromagnetic ordering between ferromagnetic layers, with

The increase of χMT upon cooling indicates the presence of CoII−MoV ferromagnetic magnetic coupling within cyanobridged coordination layers which is consistent with other known molecular systems based on CoII centers and octacyanometallates.5b This is also in a good agreement with the positive value of the Weiss constant, θCsCoMo = 48 K, 6097



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Figure 6. Field dependences of magnetization at T = 1.8 K for (a) CsCoMo and (b) CsCoW (top), and the field dependences of its first derivative (bottom).

interactions.21 As a result, the antiparallel magnetic moments of neigbouring layers remain in the same direction as the increasing applied magnetic field, and an intermediate spinflop phase with canted magnetic moments is not observed.25b,c Further proof of strong magnetic anisotropy in CsCoMo is an appearance of the hysteresis loop (Figure 7). The critical magnetic fields of metamagnetic transition gradually decrease with the increase of temperature, which was shown for a series of M versus H isotherms performed at temperatures ranging from 1.8 to 40 K (Supporting Information Figure S6). The width of hysteresis loops also decreases with increasing temperature and finally disappears above 18 K. Plotting the critical field HC as a function of temperature leads to the magnetic phase diagram, fully presenting the magnetic behavior of CsCoMo (Supporting Information Figure S7). CsCoW shows similar magnetic properties to CsCoMo (Figures 4−7 and Supporting Information Figures S5−S7). The χMT value at room temperature is 3.2 K cm3 mol−1, corresponding well to the expected value of 3.1 K cm3 mol−1 for an uncoupled CoIIWV unit assuming SW = 1/2, gW = 2.0, SCo = 3/2, and gCo = 2.4. The χMT value reaches the maximum of 8.2 K cm3 mol−1 at 25.5 K and decreases to almost 0 K cm3 mol−1 at 1.8 K (Figure 4b). The χM−1versus T curve obeying the Curie−Weiss law in the 200−300 K range gives the Weiss constant of θCsCoW = 55 K (Supporting Information Figure S5). The positive value of the Weiss constant indicates the CoII−WV ferromagnetic magnetic coupling within cyano-bridged coordination layers. The observed decrease in χMT(T) plot at low temperatures suggests interlayer antiferromagnetic interactions between the ferromagnetic layers. In addition, the field-cooled magnetization curves for several magnetic fields reveal the Néel

the expected cancellation of magnetic moments. This interpretation is also supported by the field-cooled magnetization curve for H = 5 kOe, revealing the sharp maximum at TN = 25 K, which determines the critical Néel temperature of the phase transition to the antiferromagnetically ordered state (Figures 4a and 5a). However, upon increasing the strength of the applied magnetic field, the maximum of the M = f(H) curves shifts toward lower temperatures, and finally disappears above H = 28 kOe, showing a saturation of the magnetization (Figure 5a). This indicates the appearance of field-induced transition from an antiferromagnetic state existing at lower magnetic fields to a ferromagnetic state, which is characteristic for metamagnets. More precise analysis of metamagnetic behavior of CsCoMo was conducted using a series of isothermal magnetization versus applied field curves (Figures 6 and 7). The M−H plot at T = 1.8 K presents a sigmoidal shape with a critical magnetic field (HC) found from the maximum in the field dependence of the first derivative of magnetization (Figure 6). Upon increasing the applied field, the magnetization increases first almost linearly, and later abruptly reaching 3.1 Nβ. This values are in a good agreement with theoretical 3.2 Nβ, calculated for a ferromagnetic state with all MoV spins (S = 1/2, g = 2.0) aligned parallel to all CoII magnetic moments (Seff = 1/2, gaverage = 13/3).24 The sigmoidal shape of M = f(H) curve is characteristic of a metamagnetic behavior resulting from the transition from an antiferromagnetic to a field-induced ferromagnetic phase. The value of critical magnetic field of HC = 24 kOe is considerably high among cyano-bridged assemblies,25a,d indicating a significant magnetic anisotropy coming from single-ion anisotropy of CoII centers, which is many times stronger than the interlayer antiferromagnetic 6098



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transition to an antiferromagnetic state, coming from interlayer antiferromagnetic interaction below a critical temperature of TN = 25 K. Besides, below 18 K, the field-induced transition to the ferromagnetic state is observed, which indicates the metamagnetic behavior. This transition occurs at high magnetic field of 24 kOe at 2 K, which is related to the strong magnetic anisotropy connected with single-ion anisotropy of CoII centers. Such thermally durable magnets with many alkali cations have a potential in showing an ionic conductivity at high temperatures over 200 °C. In future, we will try to synthesize the analogues with other alkali cations A4{CoII[MV(CN)8]Cl3} (A = alkali cation, M = Mo, W), and measure the ionic conductivity.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the single-crystal X-ray structural analyses, powder Xray diffraction patterns, additional magnetic characteristics, and crystallographic CIF data for CsCoMo (CCDC 1020778) and CsCoW (CCDC 1020779). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-3-5841-4331. Fax: +81-3-3812-1896. E-mail: [email protected]. Website: http://www.chem.s.utokyo.ac.jp/users/ssphys/english/index.html. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The presented research was supported partly by the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Agency (JST), Advanced Photon Science Alliance (APSA) from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), Global Science course from MEXT, the Cryogenic Research Center, The University of Tokyo, the Center for Nano Lithography & Analysis, The University of Tokyo, supported by MEXT, the Photon Factory Program Advisory Committee (2012G187, 2014G050), and the Polish National Science Centre within the Research Project DEC-2011/01/B/ ST5/00716.

Figure 7. Magnetic hysteresis loops recorded between −90 and 90 kOe at T = 1.8 K for (a) CsCoMo and (b) CsCoW.

temperature of 25 K, and a field-induced transition from an antiferromagnetic state to a ferromagnetic state (Figure 5b). The M−H plot at T = 1.8 K gives a sigmoidal shape with a hysteresis loop and a critical magnetic field of HC = 24 kOe. The width of hysteresis loop and the critical magnetic field gradually decrease with the increase temperature from 1.8 to 40 K (Figures 6 and 7 and Supporting Information Figure S6). The saturated magnetization value is 3.2 Nβ. This value is in an excellent agreement with theoretical 3.2 Nβ, calculated for a ferromagnetic state with all WV spins (S = 1/2, g = 2.0) aligned parallel to all CoII magnetic moments (Seff = 1/2, gaverage = 13/ 3).24 Such a M = f(H) curve is characteristic of a metamagnetic behavior. The high value of HC = 24 kOe and the existence of hysteresis loop indicate significant magnetic anisotropy coming from single-ion anisotropy of CoII centers (Figure 7b). The HC−T plot gives the magnetic phase diagram of CsCoW (Supporting Information Figure S7), showing the metamagnetic behavior.

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REFERENCES

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CONCLUSION The high thermally durable metamagnets Cs4{CoII[MV(CN)8]Cl3} (M = Mo, W) were synthesized. These compounds have the layered structure with many Cs+ ions between the layers, and exhibit thermal stability up to 523 K (250 °C) because of the absence of solvent molecules in the crystal structure. The magnetic studies show cyano-mediated CoIIHS−MV (M = Mo, W) ferromagnetic coupling within the 2-D layers, and the phase 6099



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Cesium Cyano-Bridged CoIIâMV (M = Mo and W) Layered

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