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A NiII–WV(CN)8 layer magnet showing metamagnetic behavior

  • Shintaro Akagi ORCID logo , Junhao Wang ORCID logo , Kenta Imoto ORCID logo , Kunal Kumar ORCID logo , Shin-ichi Ohkoshi ORCID logo and Hiroko Tokoro ORCID logo EMAIL logo
Published/Copyright: July 7, 2023
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Pure and Applied Chemistry
From the journal Pure and Applied Chemistry Volume 95 Issue 6

Abstract

By crystal engineering of molecular magnets, unique magnetic functionalities can be intentionally designed. In this work, we synthesized a novel two-dimensional cyanido-bridged coordination network, [NiII(pz)4]{[NiII(pz)3][WV(CN)8]}2·3.5H2O (NiW, pz = pyrazole), which exhibits metamagnetic behavior. NiW has wavy coordination layers created by two differently sized molecular square ladders and exhibits strong intralayer ferromagnetic interactions. Nevertheless, due to the relatively short interlayer distance, NiW shows spontaneous antiferromagnetic ordering below a Néel temperature of 21 K. By applying an external magnetic field, such antiferromagnet can be converted into a ferromagnet with a coercive field of 600 Oe at 2 K, elucidating the metamagnetic behavior of NiW.

Keywords: Crystal structures; inorganic chemistry; magnetic properties; Mary L. Good

Introduction

Molecular magnets made of magnetic metal ions and organic ligand/radicals are at the forefront of research because they can provide unique magnetic functional features through careful crystal engineering that traditional magnets such as metals and metal oxides cannot [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. Among multiple kinds of molecular magnets, those based on octacyanidometallates, [M(CN)8] (M = Nb, Mo, W, Re), have drawn great attention as they can be readily designed by integrating suitable organic ligands, magnetic metal ions, and [M(CN)8] (M = Nb, Mo, W, Re) building blocks, and various [M(CN)8]-based molecular magnets can be acquired ranging from 0 dimensional (0D) coordination aggregate to 1–3D polymeric systems. Lower dimensional 0 or 1D molecular magnets can exhibit single-molecule or single-chain magnet behavior that are promising for next-generation high-density magnetic recording materials [21], [22], [23], [24], [25], whereas higher dimensional 2 or 3D ones can exhibit advanced ferro/ferrimagnetic features that interact with various electrical, optical, and magneto functioanlites [26], [27], [28], [29], [30], [31]. In 2D complexes with long-range magnetic ordering, magnetic interactions between layers become relevant when the interlayer distance is sufficiently small. Due to the relatively weak nature of these interlayer interactions, metamagnetism may occur, resulting in a significant change in magnetization in response to external stimuli such as a magnetic field [32], [33], [34], [35], [36]. In this study, we created a two-dimensional nickel–octasyanidotungstate complex [NiII(pz)4]{[NiII(pz)3][WV(CN)8]}2·3.5H2O (NiW, pz = pyrazole) and assessed its crystal structure and magnetic properties. The complex comprises two-dimensional layers with intralayer ferromagnetic interactions; however, due to the relatively short interlayer distance, NiW spontaneously ordered antiferromagnetically with a Néel temperature (T N) of 21 K. The antiferromagnetic interactions between the layers can be easily canceled by an external magnetic field above 400 Oe, and such spin-flip transitions imply metamagnetic behavior. Here, the comprehensive synthesis, crystal structure, and magnetic properties are presented.

Results and discussion

Crystal structure

Needle-shaped yellow crystals of NiW were formed by slow diffusion of aqueous solutions containing Cs3[WV(CN)8], NiIICl2, and pz ligand. After preliminary characterization using thermogravimetric analysis and IR spectroscopy (Figs. S1 and S2), we performed a single-crystal X-ray structural analysis, which revealed that NiW crystallizes in an orthorhombic Pbcn space group (a = 29.0676(9), b = 10.4430(3), c = 20.7787(7)), and consists of wavy cyanido-bridged coordination layers with water solvent molecules in between (Figs. 1, S3, Tables S1 and S2). The asymmetric unit of NiW shown in Fig. S3 suggests that the layers contain a single type of [WV(CN)8] moiety that processes a distorted dodecahedral coordination geometry, whereas the NiII are divided into two crystallographic sites, Ni1 coordinating to four pz and two bridging cyanide ligands giving [Ni1II(pz)4(NC)2] complex moiety and Ni2 coordinating to three pz and three bridging CN ligands giving [Ni2II(pz)3(NC)3] complex moiety (Fig. 1a). The Ni and W moieties are then polymerized by bridging cyanide ligands, resulting in the formation of wavy coordination layers that extend in the ab crystallographic plane. When viewed from the c axis, the coordination skeleton of the layers has a ladder-shaped topology consisting of two types of squares, a smaller four-metal-membered {[NiII(pz)3][WV(CN)8]}2 square and a larger eight-metal-membered {[NiII(pz)3][WV(CN)8][NiII(pz)4][WV(CN)8]}2 square (Fig. 1b). The smaller squares are made up of two Ni2 atoms and two W1 atoms, whereas the larger squares are made up of two Ni1 atoms, two Ni2 atoms, and four W1 atoms. As a result, the smaller and larger ladders are made up of different sized squares. Along the wave propagational a axis, the crest and trough of the layer are formed by smaller ladders of four-membered squares, whereas the slopes of the waves are formed by larger ladders of eight-membered squares, and they share {[NiII(pz)3][WV(CN)8]} n chain edges along the b axis (Fig. 1b, c).

Fig. 1: Crystal structure of NiW: (a) Local coordination environment of NiII and WV complex fragments, and crystal packing views along the (b) c axis and (c) b axis. C, N, Ni, W, O, and H atoms are presented in gray, purple, green, orange, blue, and light gray, respectively, except that bridging cyanide ligands are presented in light orange to highlight the coordination skeleton.
Fig. 1:

Crystal structure of NiW: (a) Local coordination environment of NiII and WV complex fragments, and crystal packing views along the (b) c axis and (c) b axis. C, N, Ni, W, O, and H atoms are presented in gray, purple, green, orange, blue, and light gray, respectively, except that bridging cyanide ligands are presented in light orange to highlight the coordination skeleton.

The distances between Ni1 and W1 atoms, as well as between Ni2 and W1 atoms within the layers are 5.36 Å and 5.35 Å, respectively, via the cyanide groups. The intermetallic distances between the layers, including W1–W1, W1–Ni1, W1–Ni2, Ni1–Ni1, and Ni2–Ni2 are 10.98 Å, 11.05 Å, 10.07 Å, 11.29 Å, and 11.43 Å, respectively, and these relatively short interlayer distances accounts for the metamagnetic behaviors found in NiW (see below). The determined crystal structure model and purity of the bulk sample were validated by powder-XRD patterns with Rietveld analysis (Fig. S4).

Magnetic properties

Magnetization measurements on polycrystalline samples were carried out. Temperature (T) dependence of the product of T and magnetic susceptibility (χ M), χ M T, was measured in the 2–300 K range under an external magnetic field (H ex) of 1000 Oe (Fig. S5). The observed χ M T value at 300 K was 5.1 K cm3 mol−1, which was slightly larger than the calculated spin-only value of 4.4 K cm3 mol−1 for a {NiII 3WV 2} molecular unit, considering the spin values of Ni (S = 1, g = 2.2) and W (S = 1/2, g = 2.0) due to orbital contributions. As the temperature was reduced from 300 K, the χ M T value increased gradually up to 50 K and then abruptly increased below around 25 K, reaching a maximum of 301 K cm3 mol−1 at 19.1 K. As the temperature was reduced further, the χ M T value dropped sharply, eventually reaching 42.8 K cm3 mol−1 at 2 K. The corresponding χ M −1 vs. T plot was fitted using the Curie–Weiss law in the 150–300 K range, giving a Curie constant (C) of 4.2 K cm3 mol−1, which is in good agreement with the calculated values of 4.4 K cm3 mol−1 for Ni (S = 1, g = 2.2) and W (S = 1/2, g = 2) (Fig. S6). A positive Weiss temperature (θ) of 54 K was also extrapolated, indicating a ferromagnetic interaction between WV and NiII via cyanide ligands.

To further investigate the spontaneous ordering characteristics of NiW, we conducted the field cooled magnetization (FCM) curve under H ex = 100 Oe (Fig. 2a). A peak corresponding to the transition from a paramagnetic state to an antiferromagnetic state was observed at 21 K, indicating that NiW exhibits antiferromagnetic spontaneous ordering with a Neel temperature (T N ) of 21 K, which occurs between the ferromagnetic layers (Fig. 2a). Figure S7 demonstrates such intralayer and interlayer magnetic interactions found in NiW. However, such interlayer antiferromagnetic properties are only found at small H ex. When the external magnetic field was increased above 400 Oe, the magnetization curve resembled that of a ferromagnet, indicating a metamagnetic behavior in NiW (Fig. 2b). Such field-induced metamagnetic transition is also revealed by the M–H curve collected at 2 K (Fig. 3). It was discovered during the initial magnetization process that the magnetization of NiW increased gradually at first, but then increased much faster above the critical H ex of approximately 400 Oe, indicating the field-induced spin-flip transition at 400 Oe (Fig. 3a). Further increasing the H ex up to 5 T yielded a saturation magnetization value of 8.8 µ B, which is in good agreement with the theoretical value of 8.6 µ B for the parallel aligned spins of NiII and WV (Fig. 3b). After the virgin magnetization, a magnetic hysteresis concerning the ferromagnet characteristics was also observed, and the coercive field is approximately 600 Oe (Fig. 3b inset).

Fig. 2: Temperature dependences of magnetization of the polycllistalline NiW sample. (a) FCM, RM, and zFCM curves of the polycrystalline NiW sample under an external field of 100 Oe. (b) FCM curves of the polycrystalline NiW sample under various external fields from 100 to 500 Oe.
Fig. 2:

Temperature dependences of magnetization of the polycllistalline NiW sample. (a) FCM, RM, and zFCM curves of the polycrystalline NiW sample under an external field of 100 Oe. (b) FCM curves of the polycrystalline NiW sample under various external fields from 100 to 500 Oe.

Fig. 3: Field dependence of magnetization of the polycllistalline NiW sample. (a) Initial magnetization curve of NiW at 2 K. (b) Field dependences of magnetization of NiW at 2 K (inset: Enlarged view of (b) in −4000 to 4000 Oe range).
Fig. 3:

Field dependence of magnetization of the polycllistalline NiW sample. (a) Initial magnetization curve of NiW at 2 K. (b) Field dependences of magnetization of NiW at 2 K (inset: Enlarged view of (b) in −4000 to 4000 Oe range).

Conclusions

In this study, we created a new octacyanidometallte-based coordination compound, [NiII(pz)4]{[NiII(pz)3][WV(CN)8]}2·3.5H2O (NiW, pz = pyrazole), and assessed its crystal structure and magnetic properties. NiW comprises wavy coordination layers made up of two differently sized molecular squares with respective ladder-shaped topologies. NiW exhibits ferromagnetic interactions between NiII and WV within the layers via the cyanide groups; however, due to the relatively short interlayer distance, spontaneous ordering was found to be antiferromagnetic with a Néel temperature of 21 K. Such interlayer antiferromagnetic interactions can be overcome by external magnetic fields above 400 Oe, indicating the metamagnetic behavior of NiW.

Corresponding author: Hiroko Tokoro, Department of Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan; and Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, e-mail:

Article note: A special issue of PAC in honor of Dr. Mary L. Good (1931–2019), a leader and pioneer in the field of inorganic chemistry. In addition to a distinguished career in academia, industry, and government.

Funding source: University of Tsukuba

Award Identifier / Grant number: Tsukuba Basic Research Support Program (Type S)

Funding source: Yazaki Memorial Foundation for Science and Technology

Funding source: Japan Science and Technology Agency

Award Identifier / Grant number: FOREST Program (JPMJFR213Q)

Funding source: Japan Society for the Promotion of Science

Award Identifier / Grant number: Grant-in-Aid for Scientific Research (A) (20H00369

Award Identifier / Grant number: Grant-in-Aid for Scientific Research (B) (22H02046

Funding source: Japan Society for the Promotion of Science

Award Identifier / Grant number: Unassigned

Funding source: University of Tokyo

Award Identifier / Grant number: Unassigned

Funding source: Japan Science and Technology Agency

Award Identifier / Grant number: SPRING (JPMJSP2124)

Acknowledgment

We acknowledge the Cryogenic Research Center and Center for Nano Lithography & Analysis at The University of Tokyo, which are supported by MEXT.

  1. Research funding: This research was supported in part by the Japan Science and Technology FOREST Program (JPMJFR213Q), Japan Society for the Promotion of Science Grant-in-Aid for Scientific Research (B) (22H02046), Grant-in-Aid for Scientific Research (A) (20H00369), the Yazaki Memorial Foundation for Science and Technology, the Tsukuba Basic Research Support Program (Type S), and the JST SPRING program (JPMJSP2124).

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Supplementary Material

This article contains supplementary material (https://doi.org/10.1515/pac-2023-0303).

Published Online: 2023-07-07
Published in Print: 2023-06-27

© 2023 IUPAC & De Gruyter

Articles in the same Issue

  1. Frontmatter
  2. In this issue
  3. Editorial
  4. Special issue in honour of Dr. Mary Lowe Good
  5. Special topic papers
  6. Metal ion-assisted supramolecular gelation
  7. Synthesis and optical properties of phosphorus doped ZnO: X-ray absorption, X-ray emission, and X-ray excited optical luminescence studies
  8. A facile preparation of graphene hydrogel-supported bimetallic RuM (M: Co, Ni, Cu) nanoparticles as catalysts in the hydrogen generation from ammonia borane
  9. How to get deeper insights into the optical properties of lanthanide systems: a computational protocol from ligand to complexes
  10. Heuristic algorithms for understanding chemistry via simple quantities
  11. A NiII–WV(CN)8 layer magnet showing metamagnetic behavior
  12. Transition metal complexes for electrochromic and electrofluorochromic devices
  13. Dispersion control by using a bulky surfactant medium in the LB films for the enhancement of linearly polarized luminescence of Eu complexes
https://doi.org/10.1515/pac-2023-0303
Keywords for this article
Crystal structures; inorganic chemistry; magnetic properties; Mary L. Good

Articles in the same Issue

  1. Frontmatter
  2. In this issue
  3. Editorial
  4. Special issue in honour of Dr. Mary Lowe Good
  5. Special topic papers
  6. Metal ion-assisted supramolecular gelation
  7. Synthesis and optical properties of phosphorus doped ZnO: X-ray absorption, X-ray emission, and X-ray excited optical luminescence studies
  8. A facile preparation of graphene hydrogel-supported bimetallic RuM (M: Co, Ni, Cu) nanoparticles as catalysts in the hydrogen generation from ammonia borane
  9. How to get deeper insights into the optical properties of lanthanide systems: a computational protocol from ligand to complexes
  10. Heuristic algorithms for understanding chemistry via simple quantities
  11. A NiII–WV(CN)8 layer magnet showing metamagnetic behavior
  12. Transition metal complexes for electrochromic and electrofluorochromic devices
  13. Dispersion control by using a bulky surfactant medium in the LB films for the enhancement of linearly polarized luminescence of Eu complexes
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