Tuning Magnetic Dimensionality via Metal‐Ion Dilution in a Polyoxometalate‐Templated 2D Framework

ABSTRACT

The rational design of superparamagnetic molecular magnets remains a central challenge in molecular materials chemistry. Herein, we report a silicotungstate‐templated, 2D cyanoferrate coordination polymer and its four isostructural Zn²⁺‐diluted analogs, {(SiW₁₂O₄₀)@[TpFe(CN)₃]₂Fe_{3‐ x} Zn _x (MeOH)₆∙6MeOH} _n (Tp = Tri(pyrazolyl)borate; 1–5, x = 0–3). The neutral hybrid layers adopt a cyanide‐bridged honeycomb topology, with (SiW₁₂O₄₀)^4– anions located in the cavities and interlayer metal‐to‐metal separations exceeding 10 Å. Magnetic study revealed that 1 (x = 0) behaves as a metamagnet with an ordering temperature of 15.5 K, featuring intralayer ferromagnetic and interlayer antiferromagnetic coupling. Remarkably, Zn²⁺ doping disrupts the 2D magnetic layer, triggering a dimensional crossover in magnetism. As a result, compounds 2 (x = 0.41) and 3 (x = 0.93) exhibit slow relaxation of the magnetization, suggesting a superparamagnetic behavior, with effective energy barriers of 40.9 and 46.3 K, respectively. This transition from 2D to lower‐dimensional magnetism arises from a fragmentation of the honeycomb network into isolated spin chains (clusters) via metal‐ion dilution. Such a metal‐doping‐induced “top‐down” transformation of magnetic dimensionality provides a novel alternative to conventional bottom‐up synthetic strategies for engineering low‐dimensional magnetic materials.

Diamagnetic Zn²⁺ doping drives a magnetic dimensional crossover in a robust polyoxometalate‐templated 2D cyanoferrate, transforming antiferromagnetically coupled ferromagnetic layers into low‐dimensional spin chains and clusters with superparamagnetic relaxation.

1. Introduction

The exploration of two‐dimensional (2D) van der Waals (vdW) magnets is a rapidly advancing field, driven by their potential applications in spintronic devices such as tunneling magnetoresistance components, magnetic circular/linear dichroism platforms, and anomalous Hall effect‐based systems [1, 2, 3, 4, 5]. These materials hold promise for next‐generation computing, ultrahigh‐density data storage, and flexible magnetic technologies. A fundamental obstacle, however, is the Mermin–Wagner theorem, which precludes long‐range magnetic order in isotropic 2D Heisenberg systems at finite temperatures due to thermal fluctuations [6]. This limitation can be overcome by introducing magnetic anisotropy, as demonstrated by prototypical inorganic 2D magnets like CrI₃ (Ising‐type) and Cr₂Ge₂Te₆ (anisotropic Heisenberg‐type) [7, 8, 9, 10, 11, 12].

Beyond these inorganic magnets, molecule‐based magnets, assembled from molecular building blocks via coordination chemistry, offer unparalleled chemical tunability for designing low‐dimensional magnetic materials with tailored properties [13, 14, 15, 16, 17]. A prime example is the family of 2D vdW magnets, [M(bimX)₂] (M = Fe, Co, Mn, Zn; HbimX = substituted benzimidazole), developed by Coronado and co‐workers, where systematic ligand modification enables precise control over magnetic properties and enhanced stability [18, 19]. This study underscores the versatility of molecular engineering in accessing targeted magnetic states.

A distinct advantage of molecular systems is their capacity to emulate low‐dimensional behavior through deliberate structural design, particularly via a modular building‐block approach in cyanide‐bridged assemblies [20, 21, 22]. This methodology provides precise synthetic control and supports bottom‐up assembly pathways, enabling the fabrication of architectures that undergo dimensional crossover from one‐dimensional (1D) chains towards extended 2D networks [23, 24, 25]. Tricyanoiron(III) complexes with strong magnetic anisotropy are known as versatile building blocks for 0D and 1D structures with tunable magnetic properties, including slow magnetization relaxation and photo‐magnetism [26, 27, 28, 29, 30]. However, the bulky Tp^R ligand usually prevents the formation of higher‐dimensional frameworks. Despite this limitation, we recently showed that polyoxometalates (POMs) can induce extended structures through combined electrostatic and supramolecular interactions, leading to quasi‐1D assemblies with limited lateral width [25, 31]. Additionally, POMs’ natural affinity for metal–oxygen bonding promotes diverse hybrid architectures, suggesting that covalently linking POM clusters to cyanometallate units could enable rational dimensional control and improve structural stability [32].

Furthermore, diamagnetic dilution of high‐dimensional molecular magnetic materials provides a powerful route to investigate dimensional crossover phenomena in magnetism and to ultimately achieve targeted magnetic behavior [33, 34, 35, 36, 37]. A notable example is the metal‐formate perovskite [CH₃NH₃][Mn_xZn_1‐x(HCOO)₃] where gradual substitution of magnetic Mn²⁺ by nonmagnetic Zn²⁺ quenches long‐range spin‐canted antiferromagnetism and induces a transition to a paramagnetic state [38]. In certain low‐dimensional systems, dilution or doping can also modify the effective magnetic domains. For instance, studies on depleted spin‐ladder systems reveal that nonmagnetic defects can create emergent spin islands or introduce new characteristic magnetic length scales [39, 40, 41]. Similarly, Zn doping in an inorganic honeycomb magnetic layer has been shown to suppress long‐range antiferromagnetic order and foster short‐range glassy magnetic states [42]. However, examples in molecule‐based magnetic frameworks where metal‐ion dilution induces low‐dimensional magnetic fragments accompanied by slow magnetic relaxation remain limited.

Herein, we report the construction of a novel family of POM‐templated 2D vdW crystals, {(SiW₁₂O₄₀)@[TpFe(CN)₃]₂Fe_3‐xZn_x(MeOH)₆·6MeOH}_n (1–5, x = 0–3), featuring a honeycomb topology with an interlayer spacing of 12.8 Å and interlayer metal–metal separations exceeding 10 Å. Diamagnetic Zn(II) doping induces a pronounced magnetic dimensional crossover. The undoped parent compound 1 (x = 0) exhibits long‐range metamagnetism with a critical temperature Tc = 15.5 K, while diluted counterparts 2 and 3 (x = 0.41, 0.93) display superparamagnetic properties with effective energy barriers (Ueff) up to 46.3 K. This study demonstrates a successful top‐down strategy for realizing quasi‐superparamagnets, using dilution to reduce magnetic dimensionality.

2. Results and Discussion

2.1. General Remarks

Compounds 1–5 were synthesized via a three‐layer diffusion method, consisting of a top methanolic layer of M^II(OTf)₂ (1: M = Fe; 2–4: M = Fe/Zn mixtures; 5: M = Zn), a middle methanol buffer layer, and a bottom methanol/water solution containing Li[(Tp)Fe^III(CN)₃] and H₄SiW₁₂O₄₀. High yields of dark red, hexagonal, plate‐like crystals were obtained in a week (Scheme 1; see sections in Supporting Information). These crystals exhibited exceptional purity and reproducibility, highlighting the stability and robustness of the resulting 2D framework. It should be noted that replacing H₄SiW₁₂O₄₀ with H₃PMo₁₂O₄₀ yielded a cyanide‐bridged wide tape structure formed with (PMo₁₂O₄₀)³⁻ acting solely as a counterion [25]. For complexes 2–4, inductively coupled plasma mass spectrometry (ICP‐MS) gave Fe/Zn ratios of 1:0.09, 1:0.23, and 1:0.87, respectively. Using the formula {Fe^III ₂Fe^II _{3‐ x} Zn^II _x}, the corresponding x values were calculated to be 0.41, 0.93, and 2.33. These values are in reasonable agreement with the initial precursor ratios (Fe²⁺/Zn²⁺ = 5:1, 2:1, 1:4), which would give theoretical x values of 0.5, 1.0, and 2.4, respectively (Table S4). Moreover, these results are consistent with those inferred from magnetic measurements (see below).

SCHEME 1.

Templated assembly of the 2D {(SiW₁₂O₄₀)@[TpFe(CN)₃]₂Fe₃(MeOH)₆∙6MeOH}_n (1) layer and its metal‐ion dilution (Zn‐doping) induced magnetic dimension transition.

Powder x‐ray diffraction (PXRD) patterns for compounds 1–5 were in close agreement with the simulated pattern of 1 from the single‐crystal x‐ray diffraction (SCXRD) data (Figure S1). Thermogravimetric analysis (TGA) further revealed outstanding thermal stability: lattice and coordinating solvent molecules were released between 170°C– and 210°C, followed by weight loss that can be tentatively attributed to the pyrolysis of pyrazole ligands, while the metal–cyanide–POM framework remained intact up to 800°C (Figure S2).

2.2. Crystal Structures Description

Single crystal x‐ray diffraction (SCXRD) analysis revealed that compounds 1 and 5 are isostructural and both crystallize in the trigonal $R\overline{3}$ space group. The asymmetric unit of 1 comprises two methanol molecules (one for coordinating, the other as lattice solvent), one‐third of a [(Tp)Fe^III(CN)₃]^– (Fe1) unit, one‐half of a M(II) ion (i.e., Fe₂ in 1), and one‐sixth of a (SiW₁₂O₄₀)^4– anion (Figure S4), forming an extended cyanide‐bridged 2D honeycomb network in the ab plane with the polyoxoanions occupying the hexagonal cavities through Fe‐O‐W linkages (Figure 1b). Each [(Tp)Fe^III(CN)₃]^– unit functions as a tritopic linker, coordinating to three distinct M(II) centers through its three cyanide ligands. Conversely, each M(II) center adopts a characteristic trans‐[FeO₄(Ncyanide)₂] octahedral geometry, serving as a linear connector between two [(Tp)Fe^III(CN)₃]^– units to propagate the extended layer structure. As depicted in Figure 1a, the hexagonal unit contains six [(Tp)Fe^III(CN)₃]^– nodes at the vertices and six edge‐bridging M(II) ions, with a central (SiW₁₂O₄₀)^4– polyoxoanion that coordinates to all six M(II) centers through its equatorial oxygen atoms. The layers exhibit a well‐defined thickness of approximately 12 Å, with the six M(II) ions in each hexagon forming a planar arrangement that is symmetrically flanked by three [(Tp)Fe^III(CN)₃]^– units above and three below, resulting in a distinctive chair conformation of the hexagonal rings (Figure 1c).

FIGURE 1.

Crystal structure of 1. (a) Top view of the (SiW₁₂O₄₀)@[(Tp)Fe(CN)₃]₆Fe₆] unit; the central (SiW₁₂O₄₀)⁴⁻ coordinates to six Fe^II ions. (b) Top view of the honeycomb‐like 2D layer with (SiW₁₂O₄₀)⁴⁻ templates. (c) Side view of the (SiW₁₂O₄₀)@[TpFe(CN)₃]₆Fe₆] unit reveals a chair conformation, the (SiW₁₂O₄₀)⁴⁻ is shown in space‐filling mode. Note that six FeII ions of the hexagon define a plane that bisects the 2D layer, and half of the [(Tp)Fe(CN)₃]⁻ units are on the top and the other half are on the bottom of the plane; (d) Packing diagram of 1 along the crystallographic a axis. Hydrogen atoms are omitted for clarity. Color Code: Fe^III, orange; Fe^II, green; N, blue; C, grey; O, red; W, cyan; Si, dark red.

For 1, the Fe1–C and Fe1–N bond lengths are 1.913(17) and 1.979(15) Å, respectively. The Fe₂ center adopts a distorted octahedral geometry, binding to two cyanide nitrogen atoms (Fe–N_cyanide = 2.123(15) Å), two POM oxygen atoms (Fe–O_POM = 2.163(12) Å), and two methanol oxygen atoms (Fe–O_MeOH = 2.100(12) Å). As a reference to Fe2, the Zn1 in 5 also adopts a distorted octahedral geometry. Comparative analysis with the Zn analog 5 shows that Zn1 maintains a similar distorted octahedral geometry. The Zn–N_cyanide distance [2.045(13) Å] is significantly shorter than the corresponding Fe–N_cyanide bond, while the Zn–O_POM [2.166(10) Å] and Zn–O_MeOH [2.088(11) Å] distances remain comparable to their Fe(II)‐counterparts, indicative of a contraction of the 2D layer in the Zn analog. For the Fe^III–C–N–Fe^II linkages, the Fe^III–C–N angles are close to 180°, while the Fe^II–N–C angles are significantly bent with an angle of 162.5°.

The interlayer spacing in this layered material is approximately 12.8 Å, with interlayer B–H···O–W hydrogen‐bonding interactions (Figure S5). The shortest interlayer distance between adjacent paramagnetic centers is 10.9 Å, precluding significant direct interlayer magnetic exchanges.

3. Magnetic Properties

3.1. General Magnetic Properties for 1–5

Variable‐temperature magnetic susceptibility measurements for complexes 1–5 (per [Fe^III ₂Fe^II _{3‐ x} Zn^II _x]) were conducted under a direct current (dc) field of 1 kOe (Figure 2a). The room‐temperature χT products decrease progressively from 1 to 5, with values of 14.61, 12.38, 10.30, 4.48, and 1.35 cm³ mol⁻¹ K, respectively. Using 5 ([Fe^III ₂Zn^II ₃]) as a reference, the Lande g _Fe(III) parameter for the low‐spin (LS) Fe(III) centers (S_Fe(III) = 1/2) was calculated to be 2.68. Subsequent analysis of 1 gave g _Fe(II) = 2.43 for the high‐spin (HS) Fe(II) centers (S_Fe(II) = 2). The large g values indicate considerable orbital contributions. These magnetic estimates are consistent with the ICP‐MS‐derived compositions.

FIGURE 2.

General magnetic measurements for 1–5. (a) Variable‐temperature magnetic susceptibility data of 1–5 collected at an applied dc field of 1 kOe; (b) FCM/ZFCM plots of 1–5 at 2–20K at an applied dc field of 50 Oe; (c) Magnetic hysteresis loops of 1–5 at 2 K.

Upon cooling, the χT products for compounds 1–3 increase steadily and reaching maxima of 223.2 cm³ mol^{− 1} K (1, @12.5 K), 119.4 cm³ mol^{− 1} K (2, @8.8 K), and 68.8 cm³ mol^{− 1} K (3, @6.4 K), followed by a decrease to ∼40 cm³ mol^{− 1} K at 2 K. These increases indicate dominate ferromagnetic (F) coupling between the cyanide‐bridged Fe(III) and Fe(II) ions. For 1, a Curie–Weiss fit of the susceptibility data above 100 K gave the Curie constant C = 14.35 and the Weiss constant θ = +5.3 K, further supporting predominant ferromagnetic coupling (Figure S6). The subsequent decreases are likely attributed to interlayer antiferromagnetic (AF) coupling and zero‐field splitting effects. In contrast, compound 4 shows a slight decrease in χT upon cooling, followed by a moderate increase to a maximum of 9.21 cm³·mol⁻¹·K at 4.6 K, after which it decreases again. Compound 5 displays typical paramagnetic behavior with χT decreasing gradually to 1.06 cm³ mol⁻¹ K at 2 K.

Isothermal field‐dependent magnetization (M vs. H) measurements for compounds 1–5 were performed at 2 K (Figure S8). For 1, the M‐H plot exhibited a characteristic sigmoidal profile: the magnetization increased gradually at low fields, underwent a sharp transition, and then rose more gradually at higher fields without reaching full saturation, attaining a value of 8.8 Nβ at 70 kOe, far below the expected value (> 14 Nβ) for a ferromagnetically coupled [Fe^{III, LS} ₂Fe^{II, HS} ₃] system, indicating strong magnetic anisotropy. The sigmoidal profile suggests a metamagnetic behavior, in which an AF ground state due to the interlayer AF interactions undergoes a field‐induced transition to a ferromagnetic phase. To further examine this field‐induced magnetic transition, isothermal magnetization curves of 1 were measured at 2–16 K (Figure 3a). Upon increasing the temperature, the abrupt magnetization change becomes progressively less pronounced, and the curves gradually evolve into a smoother field‐dependent response, which can be attributed to thermal fluctuations that weaken the antiferromagnetic ground state and broaden the field‐induced metamagnetic transition. A similar sigmoidal shape was observed for compounds 2 and 3, though their transitions occurred at lower applied fields. Compound 4 displayed a rapid initial increase in magnetization with substantially reduced values. In contrast, compound 5 exhibited a magnetization curve characteristic of a paramagnetic system.

FIGURE 3.

The meta‐magnetism of 1. (a) Isothermal curves of magnetization of 1 at 2–16 K; (b) Field‐cooled‐magnetization (FCM) curves of 1 show a clear field dependency and shift to lower temperatures; (c) Magnetic hysteresis loops of 1 at 2–11 K.

Field‐cooled and zero‐field‐cooled magnetization (FCM / ZFCM) measurements for complexes 1–5 were performed under a dc field of 50 Oe (Figure 2b and Figures S11–S15). The FCM curves for 1–3 display distinct and sharp peaks at 15.5, 11.4, and 7.2 K, respectively. These temperatures can be assigned as the Néel temperatures (TN), an assignment supported by zero‐dc‐field ac susceptibility measurements. For example, complex 1 shows a distinct peak in the in‐phase component (χ′) at 15.5 K with no corresponding out‐of‐phase signal (χ″), as shown in Figure S16. No frequency‐dependent behavior was observed. Below TN, the ZFC curves diverge slightly from the FC curves at approximately 11 K for 1 and 8 K for 2, indicating possible magnetic irreversibility. Compound 3 exhibits a more pronounced divergence around 4 K. The bifurcation between field‑cooled and zero‑field‑cooled curves below the ordering temperature reflects magnetic irreversibility arising from domain wall pinning or competing exchange interactions, which trap the system in a history‑dependent nonequilibrium state. In contrast, 4 shows no peak, only a small ZFC/FC separation near 2.6 K, while 5 remains paramagnetic across the measured range.

Supporting these observations, magnetic hysteresis loops were measured at 2 K using a field sweep rate of 20 Oe s^{− 1}. As Fe^{2 +} ions were progressively replaced by Zn^{2 +} from 1 to 4, the hysteresis loops narrowed systematically. The corresponding coercive fields decrease progressively, measuring 4.0, 3.1, 1.25, and 0.07 kOe for 1–4, respectively (Figure 2c). For compound 4, the very narrow loop opening should not be interpreted as evidence for long‐range magnetic ordering. Instead, it likely reflects weak magnetic irreversibility associated with slow relaxation of residual finite‐size magnetic fragments or uncompensated spins in the highly diluted framework. This interpretation is supported by the weak frequency‐dependent ac susceptibility response observed at low temperature for compound 4 (Figure S21).

3.2. The Magnetic Nature of 1

The magnetic behavior of compound 1 is consistent with that of an anisotropic metamagnet. This assignment is supported by the presence of long‐range antiferromagnetic order below 15.5 K and by the field‐dependent magnetization curves discussed above, which show a hysteretic field‐induced transition from a low‐magnetization antiferromagnetic state to a high‐magnetization field‐polarized state. Further evidence is provided by additional FCM measurements, which display a sharp peak that disappears when the applied field exceeds 700 Oe (Figure 3b).

This behavior originates from the material's distinct structural architecture. Strong ferromagnetic (F) coupling between cyanide‐bridged Fe(III) and Fe(II) centers generates robust ferromagnetic layers, which are weakly coupled antiferromagnetically along the c axis, most likely via dipole–dipole interactions. This weak interlayer coupling stabilizes an overall AF ground state. Upon application of a moderate external field, the interlayer AF interactions are overcome, leading to alignment of the ferromagnetic layers and triggering the metamagnetic transition.

Despite its antiferromagnetic ground state, compound 1 exhibits pronounced magnetic hysteresis at low temperatures, with coercive fields persisting up to 11 K (Figure 3c). This hysteresis arises from strong effective intralayer magnetic coupling combined with significant magnetic anisotropy associated with the anisotropic Fe^II and Fe^III ions, which together create an energy barrier for moment reorientation. Step‐like features observed near zero field are attributed to field‐driven disruption of the weak interlayer AF coupling.

The evolution of this behavior is summarized in a magnetic phase diagram constructed from temperature‐dependent critical fields (Hc) extracted from dM/dH analyses (Figure S17). The diagram clearly delineates the antiferromagnetic, field‐induced ordered, and paramagnetic regimes, reinforcing the metamagnetic description. Overall, the magnetic properties of compound 1 reflect the interplay between intralayer ferromagnetism, weak interlayer antiferromagnetism, and strong magnetic anisotropy, resulting in a hysteretic, field‐induced transition characteristic of an anisotropic metamagnet.

3.3. Slow Relaxation Dynamics of 2 and 3

Compounds 2 and 3 exhibit clear slow magnetic relaxation, in contrast to compound 1. Variable‐temperature ac susceptibility measurements under a zero dc field reveal strong frequency dependence in χ′ and χ″ at temperatures well below their respective Néel temperatures (Figure S18 and Figure 4a). The Mydosh parameters (φ = 0.11 for 2 and 0.14 for 3) fall in the superparamagnetic‐like range, excluding spin‐glass behavior. For 2, the relaxation times extracted from the χ″ peaks follow a single thermally activated (Arrhenius) process, giving an energy barrier Ueff/kB = 40.9 K and a pre‐exponential factor τ₀ = 2.1 × 10⁻⁹ s. The dynamics of 3 are more complex. Although its median relaxation also obeys an Arrhenius law (Ueff/kB = 46.3 K and τ₀ = 3.8 × 10⁻¹¹ s), Cole–Cole plot analysis shows a broad distribution of relaxation times (α = 0.75–0.86), indicating more heterogeneous dynamics.

FIGURE 4.

Alternating‐current molar magnetic susceptibilities for 3. (a) Temperature dependence of the in‐phase (χ′) and out‐of‐phase (χ″) parts in zero dc field; (b) ZFCM and FCM plots of 3 at 2–20K at an applied dc field of 50 Oe; (c) Cole‐Cole plots; (d) The ln(χT) versus 1/T plot for 3, where χ is the molar component of the ac susceptibility at 1 Hz, Hac = 3 Oe and Hdc = 0; (e) Plot of ln(τ) versus T⁻¹, where the relaxation times (τ) were extracted from the fittings of Cole–Cole plots, and the solid lines correspond to the linear fit according to the Arrhenius law.

In Arrhenius analysis, τ₀ relates to the apparent attempt time for magnetization reversal and can be influenced by spin–phonon coupling, local magnetic anisotropy, damping, and the local magnetic environment. For 3, the larger α values indicate multiple local relaxation environments. Under such conditions, the fitted smaller τ₀ value may also be affected by the distribution of relaxation pathways and by correlations between U_eff and τ₀. Therefore, the smaller τ₀ of 3 should not be overinterpreted as evidence of a fundamentally different microscopic mechanism; rather, it reflects a more distributed, disorder‐influenced relaxation process.

It should be noted that the present data do not establish a genuine SCM phase [43]. As the Zn‐doping fraction increases, the AF ordering temperature progressively decreases, indicating gradual suppression of three‐dimensional magnetic correlations. Nevertheless, the nearly frequency‐independent maximum in χ′ at higher temperatures (11.4 K for 2, 7.2 K for 3) suggests a substantial static magnetic contribution remains, most likely associated with residual AF or metamagnetic correlations inherited from the parent 2D network. By contrast, the frequency‐dependent χ″ signal emerging at lower temperatures is assigned to a secondary dynamic contribution from low‐dimensional magnetic fragments or clusters generated by statistical replacement of Fe²⁺ by Zn²⁺. The relatively weak χ″ signal compared with the dominant χ′ response further supports the heterogeneous nature of the relaxation process: this small χ″/χ′ ratio indicates that only a fraction of the magnetic centers participate in the slow relaxation, while the remaining spins contribute mainly to the static bulk magnetic response. Such behavior is fully consistent with a spatially heterogeneous magnetic state created by statistical Zn dilution, in which low‐dimensional relaxing fragments coexist with residual extended magnetic correlations. Thus, compounds 2 and 3 are best described as spatially heterogeneous systems showing SCM‐like or superparamagnetic‐like slow relaxation, rather than as genuine homogeneous SCMs.

Further analysis supports the interpretation of compound 3 as a magnetically chain‐like magnetic fragment. For an anisotropic Heisenberg or Ising‐like chain (Glauber model), the χT product is expected to increase exponentially upon cooling, following the relationship: χT/Ceff  =  exp(Δξ/kBT), where Δ_ξ is the correlation energy required to create a domain wall. As shown in Figure 4d, a linear region in the ln(χT) versus 1/T plot is observed between 9 and 15 K, yielding Δ_ξ/k_B = 26.0 K. The estimated crossover temperature to the finite‐size regime is T* ≈ 7.9 K. Using the relation Ueff  =  ΔA  +  Δξ, the anisotropy energy (Δ_A) is calculated as 20.3 K. The derived ratio |D/J| = 4Δ_A/Δ_ξ = 3.1 (> 4/3), which exceeds the threshold of 4/3, indicating that the system is best described by an Ising model. Although the ac susceptibility measurements suggest that compound 4 does not exhibit long‐range magnetic order but instead displays only slow magnetic relaxation, a detailed investigation is beyond the scope of the present study.

4. Discussions

4.1. POM Templating

From a synthetic perspective, modular approaches employing tailored building blocks have proven highly effective for constructing cyanometallate frameworks [22]. The polyoxometalate (POM)‐templated strategy presented here offers a complementary and powerful route for structural tuning. In this series, the silicotungstate anion, (SiW₁₂O₄₀)⁴⁻, plays two critical roles. First, it acts as a structural template, guiding the assembly of cyanometallate layers through geometric and charge matching. This templating effect is unique; substituting (SiW₁₂O₄₀)⁴⁻ with the geometrically analogous (PMo₁₂O₄₀)³⁻ yields a cyanide‐bridged tape‐like compound where the POM merely functions as a counterion. Second, the POM contributes to structural stability by forming O–Fe coordination bonds and occupying intralayer cavities, functioning as a “brick‐and‐mortar” stabilizer. This dual role facilitates the formation of robust, high‐symmetry 2D hybrid layers, establishing a new paradigm in cyanometallate synthesis.

4.2. From Metamagnetism to Superparamagnetism

In terms of magnetic properties, Coulon et al. demonstrated that slow magnetization relaxation in single‐chain magnet (SCM) systems can persist even within antiferromagnetically ordered phases [43]. Our findings provide a compelling extension of this concept. Specifically, the superparamagnetic behavior in compounds 2 and 3 emerges from the diamagnetic Zn(II)‐doping of the 2D metamagnet of 1. The evolution of magnetic properties across this series is particularly illuminating. While diamagnetic doping in conventional 3D magnets typically suppresses the ordering temperature—an effect also observed here—the emergence of SCM‐like slow relaxation in such a context is rare. This underscores the potential significance of the parent 2D layered architecture, where substantial interlayer separation ensures the magnetic isolation of individual sheets (Figure 5).

FIGURE 5.

Schematic illustration of possible dilution‐induced magnetic fragmentation in the {(SiW₁₂O₄₀)@[TpFe(CN)₃]₂Fe_3‐xZn_x(MeOH)₆∙6MeOH}_n series. Increasing Zn^II substitution progressively interrupts the magnetic connectivity of the parent 2D honeycomb layer, producing finite low‐dimensional magnetic fragments that may contribute to the observed slow magnetic relaxation. The highlighted fragments are intended only to illustrate possible magnetic connectivity after dilution and should not be regarded as experimentally determined Fe/Zn arrangements.

The employed Zn‐dilution strategy is unlikely to generate a perfectly uniform magnetic environment. This is supported by the Cole–Cole analysis of compound 3, in which the relatively large α values (0.75–0.86, Table S5) indicate a broad distribution of relaxation times. Such a distribution is consistent with a spatially heterogeneous magnetic state, in which low‐dimensional relaxing fragments or clusters coexist with residual extended magnetic correlations. The different ionic radii and coordination preferences of Fe^II and Zn^II, as suggested by the variations in Fe–N(cyanide) and Zn–N(cyanide) bond lengths in 1 and 5, may influence the local magnetic connectivity after Zn substitution. However, the exact Fe/Zn distribution cannot be resolved from the present crystallographic data, because Fe and Zn cannot be reliably distinguished at the mixed divalent metal sites. Therefore, Figure 5 is presented only as a schematic illustration of possible dilution‐induced magnetic fragmentation rather than an experimentally determined Fe/Zn arrangement. Future local structural studies, such as EXAFS or diffuse scattering, would be valuable for clarifying possible short‐range Fe/Zn correlations.

5. Conclusion

In summary, we have developed a family of two‐dimensional solid solutions with the general formula {(SiW₁₂O₄₀)@[TpFe(CN)₃]₂Fe_{3‐ x} Zn _x (MeOH)₆·6MeOH} _n (x = 0–3) through a polyoxometalate‐templated strategy coupled with controlled metal‐ion doping. Parent compound 1 exhibits pronounced metamagnetic behavior with an ordering temperature of 15.5 K, arising from intralayer ferromagnetic and interlayer antiferromagnetic interactions. Gradual substitution of Fe^{2 +} by diamagnetic Zn^{2 +} disrupts the magnetic connectivity of the 2D layers, inducing a distinct dimensional crossover in magnetic properties. Compounds 2 and 3 display slow magnetic relaxation characteristic of superparamagnetic systems, with effective energy barriers of 40.9 and 46.3 K, respectively. This metal‐ion‐dilution‐induced transformation—from two‐dimensional metamagnetism to low‐dimensional superparamagnetism—establishes a rare top‐down route for constructing low‐dimensional magnetic materials. More broadly, this approach provides a general framework for tuning dimensionality and magnetic behavior in 2D systems, paving the way for the rational design of next‐generation molecular spintronic materials.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: smll74577‐sup‐0001‐SuppMat.docx.

Data Availability Statement

Deposition No. 2517447 (For 1), and 2517459 (For 5) contain the supplementary crystallographic data for this paper These data are provided free of charge by The Joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures Service.