# Molecular alloying drives valence change in a van der Waals antiferromagnet

Frédéric Aribot, Laura Voigt, Maja A Dunstan, Wenjie Wan, James N Mcpherson, Mariusz Kubus, Nathan J Yutronkie, Charles J Mcmonagle, Marco Coletta, Anna S Manvell, et al.

► To cite this version:

Frédéric Aribot, Laura Voigt, Maja A Dunstan, Wenjie Wan, James N Mcpherson, et al. Molecular alloying drives valence change in a van der Waals antiferromagnet. Chem, 2025, 11, pp.102557. 10.1016/j.chempr.2025.102557. hal-05062222

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Submitted on 9 May 2025

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Molecular alloying drives valence change
in a van der Waals antiferromagnet

Frédéric Aribot,<fnref n="1" />,<fnref n="12" /> Laura Voigt,<fnref n="1" />,<fnref n="12" /> Maja A. Dunstan,<fnref n="1" /> Wenjie Wan,<fnref n="2" /> James N. McPherson,<fnref n="1" /> Mariusz Kubus,<fnref n="1" />

Nathan J. Yutronkie,<fnref n="3" /> Charles J. McMonagle,<fnref n="4" />,<fnref n="13" /> Marco Coletta,<fnref n="5" /> Anna S. Manvell,<fnref n="1" /> Panagiota Perlepe,<fnref n="6" /> Anton Viborg,<fnref n="1" />

Siena Wong,<fnref n="1" /> Kirstine A. Stampe,<fnref n="2" /> Volodymyr Baran,<fnref n="7" /> Anatoliy Senyshyn,<fnref n="7" /> Sara Talebi Deylamani,<fnref n="8" /> Manh Duc Le,<fnref n="9" />

Helen C. Walker,<fnref n="9" /> Amit Chanda,<fnref n="10" /> Felix Trier,<fnref n="10" /> Nini Pryds,<fnref n="10" /> Fabrice Wilhelm,<fnref n="3" /> Joerg R. Jinschek,<fnref n="8" /> Dawid Pinkowicz,<fnref n="11" />

Michael R. Probert,<fnref n="4" /> Rodolphe Clérac,<fnref n="6" /> Niels B. Christensen,<fnref n="2" /> Euan K. Brechin,<fnref n="5" /> Andrei Rogalev,<fnref n="3" />

and Kasper S. Pedersen<fnref n="1" />

<footnote n="1">¹Department of Chemistry, Technical University of Denmark, Kgs. Lyngby, Denmark</footnote>

<footnote n="2">²Department of Physics, Technical University of Denmark, Kgs. Lyngby, Denmark</footnote>

<footnote n="3">³European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex 9, France</footnote>

<footnote n="4">⁴Chemistry, School of Natural and Environmental Sciences, Newcastle University, Newcastle upon Tyne, UK</footnote>

<footnote n="5">⁵School of Chemistry, The University of Edinburgh, Edinburgh, UK</footnote>

<footnote n="6">⁶Univ. Bordeaux, CNRS, CRPP, UMR 5031, 33600 Pessac, France</footnote>

<footnote n="7">⁷FRM-II, Technical University of Munich, Garching, Germany</footnote>

<footnote n="8">⁸National Centre for Nano Fabrication and Characterization (DTU Nanolab), Technical University of Denmark, Kgs. Lyngby, Denmark</footnote>

<footnote n="9">⁹ISIS Neutron and Muon Source, Rutherford Appleton Lab. Laboratory, Chilton, UK</footnote>

<footnote n="10">¹⁰Department of Energy Conversion and Storage, Technical University of Denmark, Kgs. Lyngby, Denmark</footnote>

<footnote n="11">¹¹Faculty of Chemistry, Jagiellonian University, 30-387 Kraków, Poland</footnote>

<footnote n="12">¹²These authors contributed equally</footnote>

<footnote n="13">¹³Present address: Swiss-Norwegian Beamlines, European Synchrotron Radiation Facility, 71 Avenue des Martyrs, 38000 Grenoble, France</footnote>
Since the discovery of graphene, the science of 2D magnetic materials and their stacked analogs, van der Waals (vdW) crystals, has been rapidly evolving.¹⁻³ Breakthroughs such as 2D long-range and layer-dependent magnetic order in CrI₃ and Cr₂Ge₂Te₆ have made this field one of the most promising avenues of condensed matter research.⁴,⁵ However, modulation of the electronic structure, the spin state, and the ensuing magnetic properties through electron transfer events via external stimuli remains elusive in vdW and related 2D materials. The possibilities for electronic structure engineering in such materials are fundamentally hindered by the simplicity of the chemical composition of all known, entirely inorganic materials candidates. The confluence of inorganic and organic materials in coordination chemistry⁶ hosts the potential to unlock new vistas for property control at the molecular and atomic level in vdW crystals. The control of entangled geometric and electronic structures, central to coordination chemistry, presents a method to tailor vdW crystals with a chemically encoded valence change event, commonly referred to as a valence tautomeric (VT) conversion. These events are well-known in inorganic materials, where a temperature or pressure trigger leads to, for example, metal-insulator transitions,⁷ or the emergence of superconductivity.⁸ Despite such possibilities for reshaping and tuning materials properties, VT phenomena have not yet been observed in vdW crystals. In the vdW antiferromagnet FePSe₃, the application of pressure leads to emergent superconductivity,¹⁰ tied to a localized spin crossover at the Fe(II) ion. Subsequent theoretical studies suggest that the is structural, but hitherto unobtainable, CrPS₃ may be subject to a pressure-induced VT conversion rather than spin crossover, with a concomitant switch from an antiferro- to a ferromagnetically ordered phase.¹¹ Fundamental to the observation of the VT conversion stands the energy alignment of the electron donor and acceptor units. While difficult to control in inorganic materials, where the VT conversion involves the promotion of a localized electron to the conduction band or vice versa, coordination chemistry principles allow tailoring of electron transfer between metal ions,¹² and between metal ions and organic ligands.¹³,¹⁴

The spontaneous electron transfer between Cr(II) and pyrazine (pyz) has been known for decades¹⁵ but has recently been a key factor in the development of layered conductive magnets¹⁶ and magnets with exceptional magnetic hardness and critical temperatures.¹⁷⁻¹⁹ Chemically related systems, with no apparent electron transfer events, are instead electrically insulating antiferromagnets.²⁰,²¹ The dichotomy in properties stems fundamentally from the redox activity in the organic scaffold being actuated or not, and developing a handle on this mechanism provides a novel route to establish stimuli-dependent switching of metal-organic vdW crystals. Here, we demonstrate the tailoring of switchable magnetic and electrical properties of a square-lattice vdW coordination solid using the concept of molecular alloying.

The self-assembly reaction of Cr(thf)₃I₂ (tetrahydrofuran [thf]; Figure S7) with pyz led to the immediate formation of microcrys-

talline Cr(pyz)₂I₂. Single-crystal X-ray diffraction revealed Cr(pyz)₂I₂ to be an ideal tetragonal net of pyz-bridged {trans-CrI₂} units (Figure 1A; Tables S1 and S2). Although the structure is similar to the previously reported orthorhombic analog, Cr(pyz)₂Cl₂,¹⁶ no indications of a crystallographic symmetry lower than tetragonal could be identified for Cr(pyz)₂I₂. The long Jahn-Teller distorted Cr-I bond of 3.0534(9) Å (298 K; cf. Cr-I of 3.0816(4) Å in CrII₂(CH₃CN)₄ and 2.655(2)-2.692(2) in CrIII₃(CH₃CN)₃²² suggests the presence of Cr(II) in Cr(pyz)₂I₂, as opposed to Cr(III) in Cr(pyz)₂Cl₂ (Cr-Cl 2.337(1) Å).¹⁶ The magnetic susceptibility-temperature (χT) product of Cr(pyz)₂I₂ amounts to 2.4 cm³ K mol⁻¹ at 300 K and decreases steadily upon cooling, indicative of dominant antiferromagnetic superexchange interactions between the high-spin Cr(II) centers (Figures 2A and S19). The χ(T) displays a broad maximum around 30 K and below ~25 K χ(T) becomes magnetic field dependent (Figure S20). The magnetic properties are slightly crystallization dependent, with quickly precipitated samples (cf. methods) showing the χ(T) maximum at ~20 K (Figures S19 and S20). Analysis of the powder X-ray diffractograms (Figure S11) reveals a reduction in the average domain size from 0.79(1) to 0.32(8) µm between synthesis methods, suggesting that the minor differences in magnetic properties originate from increased concentration of uncompensated spins at surfaces and defects. The temperature dependence of the specific heat, cₚ, reveals an anomaly at T<sub>Néel</sub> ≈ 25 K, suggesting the onset of a magnetic phase transition (Figure S25). The 4 K powder neutron diffraction pattern shows weak, additional reflections compared with the diffractogram at 60 K (Figure S27), unveiling an antiferromagnetically ordered ground state structure with a (1/2, 1/2, 0) propagation vector and an ordered Cr moment of 1.8(3) µB, oriented along the crystallographic c axis (Figure 1C). The magnetic excitation spectrum, determined using powder time-of-flight inelastic neutron scattering (INS), displays dispersive spin-wave excitations in the energy range of 1-4.5 meV (Figure 3A). The dispersion minima occur at momenta, Q, that are consistent with the propagation vector determined from neutron powder diffraction, most prominently at Q ≈ 0.62 Å⁻¹, corresponding to Q = (1/2, 1/2, 0). The magnetic nature of the excitations is corroborated by their decrease in intensity with increasing Q and by their temperature dependence, which shows the closure of an anisotropy gap and gradual loss of definition of the dispersive features when the temperature exceeds T<sub>Néel</sub> (Figure S28). The salient features of the low-temperature spin-wave spectrum (Figure 3A) can be modeled using a 2D square-lattice Heisenberg model given by the Hamiltonian

$$H = J \sum_{\langle ij \rangle} \vec{S}_i \cdot \vec{S}_j + D \sum_j (S_j^z)^2$$

with a nearest neighbor exchange interaction of J = 0.51(1) meV (J/kB = 5.9(1) K) and Cr(II) single-ion magnetic anisotropy reflected by D = -0.06(2) meV (D/kB = -0.7(2) K, Figure 3B). Independently, J could be estimated from χT(T) using Curély's solution for the classical 2D Heisenberg model (Figure S19), to yield J/kB = 5.3 K, in good agreement with the value obtained from INS spectroscopy.²³
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Crystallographic characterization</figcaption></figure>

(A and B) Views of two 3 × 3 layer fragments of Cr(pyz)₂I₂ from its single-crystal X-ray diffraction structure (298 K) at (A) ambient pressure and at (B) 1.54 GPa.

(C) Unit cell of the magnetic structure of Cr(pyz)₂I₂.

(D and E) Solid thermal ellipsoids (drawn at 50% probability level): 3D electron diffraction (3D-ED) structure of Cr(pyz)₂IBr (D, 50% at 298 K; E, at 100 K), overlaid onto the atomic positions from 3D-ED structures of fast-precipitated Cr(pyz)₂I₂ (0%) and Cr(pyz)₂Br₂ (100%) obtained under identical conditions. Color codes: green, Cr; orange, Br; purple, I; blue, N; black, C. Hydrogen atoms are omitted for clarity. Key bond lengths for all crystallographically characterized materials are provided in Table S4.

Encouraged by the apparent disparity of the electronic and magnetic properties of Cr(pyz)₂I₂ and the previously reported Cr(pyz)₂Cl₂,¹⁶ the analogous Cr(pyz)₂Br₂ was pursued. Attempts to grow single crystals systematically led to the isolation of orthorhombic Cr(pyz)₂Br₂ isomorphous to, and with similar physical properties as, Cr(pyz)₂Cl₂.²⁴ However, the rapid reaction of an acetonitrile-dimethylformamide solution of Cr(thf)₂Br₂ with molten pyz resulted in the formation of a tetragonal phase of Cr(pyz)₂Br₂, isomorphous to Cr(pyz)₂I₂ (Figure S11). Given the submicron crystal sizes, the structure was determined by 3D electron diffraction (3D-ED, Figures S5 and S6; Tables S1 and S3). The Cr-Br bond length (298 K) of 2.52(3) Å is synonymous with a Cr(III) oxidation state assignment. In stark contrast to the behavior of Cr(pyz)₂I₂, the χT product for Cr(pyz)₂Br₂ increases upon cooling from 300 K (Figure 2A), suggesting different electronic structures.

The magnetization at 5 K saturates at 1.7 µB, in agreement with the value found for the previously reported orthorhombic Cr(pyz)₂Cl₂ (1.8 µB),¹⁶ and far from the value for Cr(pyz)₂I₂ of 0.36 µB (Figure S21). The zero-field cooled-field cooled magnetization data diverge below ~30 K (Figure S23), suggesting a magnetic ordering. However, unlike the Cr(pyz)₂Cl₂ parent, Cr(pyz)₂Br₂ displays only a minor hysteretic effect of the field dependence of the magnetization (Figure S22), and alternating

current (ac) susceptibility measurements do not reveal any significant frequency dependence and only a small χ''/χ' ratio not exceeding 2%, revealing a poorly ordered state similar to spin glasses (Figure S24). The X-ray absorption spectra (XAS) acquired around the Cr K-edge energy of Cr(pyz)₂I₂ and Cr(pyz)₂Br₂ demonstrate a ~1 and ~2.5 eV shift of the pre-edge and rising edge, respectively, confirming the higher oxidation state of Cr in Cr(pyz)₂Br₂ (Figures 4 and S29). Single-crystal X-ray diffraction analysis of Cr(pyz)₂I₂ (120 K) and 3D-ED analysis of Cr(pyz)₂Br₂ (100 K) reveal only minor thermal contractions (Figures S3–S6; Tables S1–S3), and the absence of any changes in Cr oxidation state upon cooling is confirmed by XAS at 50 K (Figure S31). This analysis suggests that the critical ligand field strength to actuate valence change resides between Br⁻ and I⁻ in the spectrochemical series. Given the layered nature of the crystals (Figure 1A), application of an isotropic pressure would be expected to compress the crystals predominantly along the stacking c direction, thereby leading to an increase in the axial component of the crystal field. A high-pressure single-crystal X-ray structure of Cr(pyz)₂I₂ was determined at 1.54 GPa and 290 K (Figure 1B; Table S2). The pressure induces a contraction of the interlayer distance of 16% from 6.297(2) Å at ambient pressure to 5.31(9) Å at 1.54 GPa and a phase transition from tetragonal to monoclinic symm. The compression leads to a distortion of the
<figure><img src="image_3.png" /></figure>

coordination sphere with now nonlinear I⁻–Cr–I (172.25(8)°) and slight deviations of the N–Cr–N angles from the 90° found at ambient pressure. By contrast, the bond lengths are virtually unaltered compared with the ambient pressure structure (Cr–I 3.092(2) Å, Cr···Cr 7.0073(3) Å, and Cr–N 2.15(1) Å, 2.16(1) Å), suggesting the conservation of the Jahn–Teller distorted coordination environment and a Cr(II) oxidation state.

To induce a facile VT conversion at ambient pressure, we increased the chemical pressure by substituting I⁻ in Cr(pyz)₂I₂ with the stronger-field ligand Br⁻ to yield mixed-halide Cr(pyz)₂I₂-xBrₓ solid solutions. A similar “molecular alloying” approach was utilized by Kahn to tune the spin crossover temperature in a series of one-dimensional coordination solids over a wide temperature range.²⁵ The coordination self-assembly reaction using a one-to-one ratio of Cr(thf)₂Br₂ and Cr(thf)₃I₂ afforded a microcrystalline powder (termed 50%). Attempts to grow single-crystalline specimens systematically led to polyphasic mixtures, and the use of the “fast” precipitation synthesis route developed for Cr(pyz)₂Br₂ (hereafter 100%) was required to obtain monophasic samples (see methods). Isolation of Cr(pyz)₂I₂ using this method yielded phase-pure microcrystalline powders (hereafter 0%) with 3D-ED crystal structures identical to the X-ray crystal structure obtained through the “slow” precipitation method (Figure 1). For 50%, elemental analysis confirms the formulation Cr(pyz)₂IBr, and powder X-ray diffraction suggests a phase isomorphous to that of the pristine phases (Figures S8–S11; Table S5). The powder

<caption>**Figure 2.** Magnetic properties and electrical conductivity</caption>

(A) Temperature dependence of the susceptibility-temperature product, χT, (μ₀H = 0.1 T, sweeping rate = 1 K min⁻¹) for the pristine Cr(pyz)₂I₂ (0%) and Cr(pyz)₂Br₂ (100%), and the alloys, Cr(pyz)₂I₂-xBrₓ, labeled according to their percental Br content (0%–100%). The data for 25% coincide with those of Cr(pyz)₂I₂.

(B) Temperature cycling between 180 and 90 K for 40% (1.5 K min⁻¹).

(C) Temperature dependence scans of the electrical conductivity for 0%, 40%, 50%, and 100%. Figures S21 and S22 contain additional magnetization data for the series.

X-ray diffractograms reveal no indications of inhomogeneities, which is supported by energy-dispersive X-ray spectroscopic halide analysis of several submicron crystals (Figure S13). The 3D-ED structure at 298 K could be solved as a Cr(pyz)₂X₂ phase with 50% Br and 50% I at the X position of the asymmetric unit (Figures 1D, S1, and S2). No indications of a larger supercell could be found, suggesting either a random distribution of the X⁻ ions or homo-halide {CrX₂} units in 50%. The Cr–X bond lengths in 50% at 298 K (Cr–I: 3.01(8) Å, Cr–Br: 2.81(9) Å) are consistent with the Cr–I bond (3.07(8) Å) found in the 3D-ED structure of pristine Cr(pyz)₂I₂ at 298 K. At 100 K, however, the Cr–X bonds in 50% are considerably shorter (Figure 1E, Cr–I: 2.71(6) Å, Cr–Br: 2.55(7) Å) and much closer to the Cr–Br bonds (2.51(3) Å) found in 100% at 100 K. This analysis demonstrates the presence of Cr(II) (at 298 K) and Cr(III) (at 100 K) in 50%, not existing in the pristine Cr(pyz)₂Br₂ and Cr(pyz)₂I₂ phases, respectively, and indicates the occurrence of a temperature-driven VT conversion in a single nanocrystal grain (Figures S8–S10). Additional Cr(pyz)₂I₂-xBrₓ alloys were synthesized with Br concentrations between 25% and 75%, all of which were found to consist of a single phase by room temperature (RT) powder X-ray diffraction analysis (Figure S11). The evolution of the unit cell parameters from 0% to 100%, as determined from Rietveld refinements, deviates significantly from linearity in the Br percentage (Figure S12) and is ascribed to the concerted effects on unit cell volume reduction followed by an increase in Br concentration and the ensuing increase in Cr(III) concentration (Table S4).

The χT product of 50% at 300 K is close to that of 0%, indicative of a formal Cr(II) oxidation state (Figure 2A). However, upon cooling, the χT product exhibits an abrupt increase at the temperature T₁* = 162 K (Figure 2A; as defined by the inflection point of χT(T)), and upon further cooling, the χT product closely follows that of 100%. Upon reheating, the χT product decreases precipitously at T₁* = 175 K. This hysteretic thermal response of the χT product (at 1 K min⁻¹) is attributed to a first-order phase transition involving a Cr(II) ↔ Cr(III) conversion, with a concerted
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Inelastic neutron scattering</figcaption></figure>

(A) Time-of-flight INS spectrum of polycrystalline, fully deuterated Cr(pyz-d₄)₂I₂ (6 K, incident neutron energy of 19 meV).

(B) Simulated powder INS cross section using the 2D Heisenberg model discussed in the main text.

pyz ⇌ pyz⁻ electron transfer within the pyz scaffold. Differential scanning calorimetry (Figure S26) confirms an exothermic nature of the Cr(II)-pyz to Cr(III)-pyz⁻ conversion with an enthalpy change (−2.8(1) kJ mol⁻¹) comparable to previously established valence conversions in transition metal ion systems.²⁶ The VT conversion is resistant to fatigue, with repeated cycling experiments demonstrating only a ~1% change in the χT values after thirteen cycles (Figure 2B). Across the family of alloys, the values of T₁* and T₂* are strongly sensitive to the Br doping percentage and can be tuned through a ca. 100 K temperature domain (Figure 2A).

To further support the existence of VT conversions, XAS was complementarily recorded at 300 and 50 K for the **75%** and **40%** alloys (Figures 4 and S31–S33). For **40%**, the 300 K spectrum superposes perfectly to that of **0%** (displaying exclusively Cr(II)), while the 50 K spectrum closely resembles that of **100%** (displaying exclusively Cr(III)). Warming back to 300 K leads to a full recovery of the initially recorded 300 K spectrum (Figures 4A and S31). These observations confirm the conclusions drawn from magnetometry and demonstrate a complete VT conversion in **40%**. On the contrary, in **75%**, the 300 K spectrum can be approximated as an average of the spectra of **0%** and **100%**, suggesting the coexistence of Cr(II) and Cr(III) in **75%** at RT (Figures 4B, S29, and S30). Upon cooling to 50 K,

<figure><img src="image_5.png" /><figcaption>**Figure 4.** X-ray absorption spectroscopy</figcaption></figure>

Cr K-edge XAS spectra of 0%, 100%, and 40% (A) and 75% (B).

the spectrum of **75%** overlaps perfectly with the spectrum of **100%**, reflecting the full conversion to the Cr(III) state (Figures 4B and S33). Br K-edge XAS studies corroborate these conclusions. **100%** exhibits a clear pre-edge shoulder (13,488 eV; Figures S34–S37), which, at RT, decreases and eventually vanishes with decreasing Br doping concentration. This spectral feature is assigned to transitions from core 1s states into vacancies in the Br 4p levels, originating from the increased Cr-Br covalency in Cr(III) over Cr(II). Upon cooling to 50 K, the pre-edge feature increases in intensity, as expected for a Cr (II) → Cr(III) VT conversion (Figures S36 and S37).

The Cr(III) oxidation state in **100%** is synonymous with mixed valency in the organic scaffold, formally constituting equal proportions of neutral pyz and pyz⁻. This situation has been at the origin of high electrical conductivity in the related M(pyz)₂Cl₂ (M = Cr, Ti) phases.¹⁶,²⁷ Electrical resistivity measurements reveal **100%** to be a semiconductor with σ<sub>RT</sub> = 25 mS cm⁻¹ (Figure 2C). By contrast, **0%** displays at least eight orders of magnitude lower conductivity (σ<sub>RT</sub> ≈ 5 × 10⁻⁷ mS cm⁻¹), consistent with the inactivation of pyz-scaffold mixed valency. The **40%** and **50%** alloys are also largely insulating at RT (10⁻⁶ and 10⁻⁵ mS cm⁻¹, respectively); however, upon cooling their conductivities increase drastically around the first-order phase transition, which for **40%** results in an increase in σ of five orders of magnitude (Figure 2C). At lower temperatures, σ(T) for **40%** and **50%** reaches and follows that of **100%**, reflecting the similar electronic structures.

The dramatic increase of σ within a narrow temperature window is remarkable and rivals iconic metal–insulator transitions.²⁸ In vdW materials, first-order phase transitions at temperatures relevant to the Cr(pyz)₂I₂-xBrₓ alloys are prevalent and have been extensively studied in transition metal dichalcogenides. However, the associated changes in electrical conductivity in these systems are typically moderate, reaching up to an order of magnitude.²⁹ In the same temperature regime, the vdW antiferromagnet FePSe₃ exhibits pressure-induced variations in σ spanning three orders of magnitude (up to 16 GPa) but with minimal
temperature-driven changes.¹⁰ These comparisons highlight the exceptional transport properties of Cr(pyz)₂I₂-xBrₓ alloys within the vdW materials class. For magnetic properties, thermally driven switchability in magnetic vdW crystals and 2D materials has so far only been demonstrated in molecule-based networks exhibiting spin crossover.³⁰ Such metal ion-centered interconversions of spin states do not lead to any significant modulation of the extended 2D layers' electronic structure. Thus, the primary interest has focused on utilizing the volumetric change associated with the spin crossover to induce strain in vdW heterostructures.³¹ Akin to this mechanism, the electron transfer from Cr(II) to organic ligands with empty π* orbitals was recently found to be controllable by the axial component of the crystal field, which raises the energy of the singly occupied 3d<sub>2</sub>² orbital located along the Jahn-Teller axis.¹⁶,²⁰,³² The Cr(pyz)₂X₂ alloys constitute unique vdW crystal analogs in which molecular switching can be exploited. The crystallographic, spectroscopic, magnetization, and resistivity data univocally support the existence of a complete temperature-driven VT conversion in the Cr(pyz)₂X₂ alloys, while it is completely absent in the pristine Cr(pyz)₂I₂ and Cr(pyz)₂Br₂ materials. Despite the presence of {Cr(pyz)₄I₂} and {Cr(pyz)₄Br₂} entities that should host Cr(II) and Cr(III), respectively, and potential {Cr(pyz)₄IBr} entities in the alloys, the VT conversion happens cooperatively at a first-order phase transition and involves all Cr centers. This behavior where a VT conversion is actuated in solid solutions of otherwise inactive phases is unprecedented among any materials. Furthermore, in materials displaying metal-to-metal electron transfer, the related phenomenon is rare and limited to few Prussian blue analogs, where the occurrence of charge transfer-induced spin transitions was shown to be critically dependent on the *M/M'* ratio.³³,³⁴ While Cr(pyz)₂X₂ are still in their bulk crystalline form, the calculated exfoliation energy of related metal-pyz nets (0.1 J m⁻²) is smaller than those of archetypal vdW crystals such as graphite, MoS₂, and CrI₃ (0.3–0.4 J m⁻²), suggesting that dimensionality reduction in these materials may be feasible.³⁵ This is further supported by recent, initial results on the exfoliation of related Fe(II)-pyz nets.³⁶

In conclusion, this work establishes a new paradigm in the manipulation of metal-organic vdW crystals, where molecular alloying can drive complete VT conversions from parent materials that do not display any VT themselves. The VT event of local, molecular nature is amplified cooperatively and fully reversibly by the 2D net into a transformed electronic structure of the material, with ensuing dramatic changes to the physical properties. This effect is well illustrated by the Cr(pyz)₂I₂-xBrₓ alloys, which demonstrate the largest jumps in electrical conductivity observed for any vdW material at non-cryogenic temperatures. The rigid character of the 2D nets affords hysteretic switching behavior of the magnetic moment triggered by the VT event, with no indication of fatigue. The very concepts of molecular alloying may be transferable also to the entirely inorganic vdW crystals, heralding a new era of responsive 2D materials. Molecule-based frameworks, however, exhibit significantly greater flexibility compared with inorganic materials and, consequently, are anticipated to produce substantial responses to mechanical stress. The close energetic proximity of the distinct valence states suggests that the molecular alloys may be switchable

by gating or strain, implying the potential for next-generation electronic and spintronic applications, where precise control over charge and spin is of paramount importance.

## METHODS

All manipulations and sample preparations took place inside an InertLab glove box (H₂O, O₂ < 0.5 ppm) under inert Ar atmosphere or with Schlenk line techniques, unless otherwise specified. A Puresolv MD 7 solvent purification system was used for the provision of dry and air-free solvents. Elemental analyses were obtained at the Mikroanalytisches Laboratorium Kolbe (Oberhausen, Germany).

### Synthesis of Cr(thf)₃I₂

Cr powder (8.10 g, 156 mmol) was added to degassed, aqueous HCl (57%, 70 mL), which was subsequently heated to 100°C for 2 h. Afterward, the water was evaporated, and the solid product was dried at 160°C under vacuum overnight before placing the dark brown solid (46.2 g) in the glove box. About a quarter of the crude CrI₂ (12 g) was dissolved in thf (ca. 400 mL), and the solution was filtered to remove insoluble impurities. The filtrate was concentrated to 100 mL and placed in the freezer (−30°C) overnight for Cr(thf)₃I₂ to crystallize. The dark supernatant was pipetted off and replaced by about 40 mL fresh, cold (−30°C) thf three times. The supernatant was collected for a second crystallization. Pale-green crystals of Cr(thf)₃I₂ were filtered and washed with a small amount of cold (−30°C) thf and dried under vacuum. Yield 11.2 g (53%). Anal. calcd. (found) for C₁₂H₂₄CrI₂O₃·C, 27.60% (27.97%); H, 4.63% (4.73%); Cr, 9.96% (9.81%); and I, 48.61% (47.89%). The Cr(thf)₃I₂ could also be synthesized in an alternative way: Cr powder (0.41 g, 7.8 mmol) and I₂ (2.0 g, 7.8 mmol) were ground together, placed in an ampoule (V = 21 mL), sealed under vacuum, and heated to 550°C for 50 h. After cooling to RT, the ampoule was opened in the glovebox, and the dark red-brown, crude CrI₂ was stirred in thf (15 mL) for 5 min before being filtered. This process was repeated twice, and the combined filtrates were cooled to −30°C. After one night, a first crop of pale-green crystals of Cr(thf)₃I₂ was isolated by filtration (1.1 g, 2.1 mmol). The filtrate was concentrated *in vacuo* to 15 mL and cooled to −30°C, resulting in a second batch of Cr(thf)₃I₂ crystals (420 mg, 0.8 mmol). Yield 1.5 g (38 %).

### Synthesis of Cr(thf)₂Br₂

CrBr₃ (2.5 g, synthesized as described in Brauer³⁷) was loaded on a quartz boat and placed in a horizontal tube furnace. The furnace was heated to 200°C for 30 min under a dry Ar stream. Afterward, a constant stream of H₂ in He (10%) was introduced, and the furnace was heated to 600°C for 6 h. The reaction was then cooled to RT and quickly transferred to a dried Schlenk flask, evacuated, and back filled with Ar. The light green crude product was transferred to a glove box, where it was dissolved in thf (ca. 400 mL) under slight heating, and the solution was filtered to remove insoluble impurities. The filtrate was concentrated to ca. 100 mL and left in the freezer (−30°C) overnight for Cr(thf)₂Br₂ to crystallize. Pale crystals of Cr(thf)₂Br₂ were
collected by filtration and washed with a small amount of cold thf (–30°C). The filtrate was used for a second crystallization. Yield: 2.2 g (72%). The crystal structure was identified as the same as previously reported, where the compound was obtained as a side product.³⁸ Anal. calcd. (found) for C<sub>8</sub>H<sub>16</sub>CrBr<sub>2</sub>O<sub>2</sub>: C, 26.99% (27.41%); H, 4.53% (4.61%); Cr, 14.61% (14.39%); and Br, 44.89% (44.19%).

## Synthesis of Cr(pyz)<sub>2</sub>I<sub>2</sub>

### "Fast" method (0%)

A boiling solution of Cr(thf)<sub>3</sub>I<sub>2</sub> (52.2 mg, 100 µmol) in acetonitrile (2.5 mL) was added dropwise to a melt (85°C) of pyz (300 mg, 3.7 mmol), yielding a dark gray precipitate almost immediately. The resulting mixture was cooled to RT (5 min), concentrated under vacuum, filtered by suction, and washed with acetonitrile (2 × 2 mL). After drying under vacuum, a dark gray solid could be isolated. Yield: 44 mg (95%). Anal. calcd. (found) for C<sub>8</sub>H<sub>8</sub>Crl<sub>2</sub>N<sub>4</sub>: C, 20.62% (20.53%); H: 1.73% (1.85%); I, 54.47% (54.04%); and N: 12.02% (11.87%). Attempts to conduct the synthesis at 65°C led to the cocrystallization of an uncharacterizable byproduct.

### "Slow" method

High-crystallinity samples, containing single crystals (up to 0.2 mm) suitable for single-crystal X-ray diffraction analysis, were obtained in an ampoule (V = 21 mL), which was filled with Cr(thf)<sub>3</sub>I<sub>2</sub> (0.10 g, 0.19 mmol) and pyz (1.0 g, 12.5 mmol). After sealing under vacuum, the ampoule was, over the course of 5 h, heated to 200°C, kept at this temperature for 80 h, and subsequently cooled to RT over 20 h. The sample contained a mixture of dark gray crystals and microcrystalline powder, which were washed with acetonitrile (20 mL) and isolated by vacuum filtration. Yield: 67 mg (75% based on Cr). The same method was utilized to synthesize the fully deuterated analog Cr(pyz-d<sub>4</sub>)<sub>2</sub>I<sub>2</sub>.

## Synthesis of Cr(pyz)<sub>2</sub>Br<sub>2</sub> (100%)

A warm solution of Cr(thf)<sub>2</sub>Br<sub>2</sub> (35.6 mg, 100 µmol) in acetonitrile (2.5 mL) and dmf (two drops) was added dropwise to a melt (65°C) of pyz (300 mg, 3.7 mmol), yielding a dark precipitate almost immediately. The resulting mixture was cooled to RT over 5 min, filtered under vacuum, and washed with acetonitrile (2 × 2 mL). After drying, a microcrystalline, dark purple solid could be isolated. Yield: 34 mg (92%). Anal. calcd. (found) for C<sub>8</sub>H<sub>8</sub>N<sub>4</sub>CrBr<sub>2</sub>: C, 25.83% (26.78%); H, 2.17% (2.25%); Br, 42.96% (41.69%); and N, 15.06% (15.62%).

## Synthesis of Cr(pyz)<sub>2</sub>I<sub>2</sub> -xBr<sub>x</sub> alloys

For 50%: a warm solution of Cr(thf)<sub>2</sub>Br<sub>2</sub> (17.8 mg, 50 µmol) and Cr(thf)<sub>3</sub>I<sub>2</sub> (26.1 mg, 50 µmol) in acetonitrile (2.5 mL) was added dropwise to a melt (65°C) of pyz (300 mg, 3.7 mmol), yielding a dark precipitate almost immediately. The resulting mixture was cooled to RT, filtered under vacuum, and washed with acetonitrile (2 × 2 mL). After drying, a dark purple microcrystalline solid could be isolated in essentially quantitative yield. Anal. calcd. (found) for C<sub>8</sub>H<sub>8</sub>N<sub>4</sub>CrlBr: C, 22.93% (22.84%); H, 1.92% (1.97%); Br, 19.07% (19.02%); Cr, 12.41% (12.34%); I, 30.29% (30.11%); and N, 13.37% (13.31%). The Cr(pyz)<sub>2</sub>I<sub>2</sub>-xBr<sub>x</sub> alloys were synthesized following an analogous procedure (for 75%, two drops of dimethylformamide aided the dissolution of the precursors, and a reac-

tion temperature of 55°C was used) and obtained with similar yields in the following ratios: 75%: Cr(thf)<sub>2</sub>Br<sub>2</sub> (26.7 mg, 75 µmol) Cr(thf)<sub>3</sub>I<sub>2</sub> (13.0 mg, 25 µmol). Anal. calcd. (found) for C<sub>8</sub>H<sub>8</sub>N<sub>4</sub>Crl<sub>0.5</sub>Br<sub>1.5</sub>: Br, 30.31% (29.89%); I, 16.04% (15.81%); C, 24.30% (24.72%); N, 14.17% (14.41%); and H, 2.04% (2.07%). 60%: Cr(thf)<sub>2</sub>Br<sub>2</sub> (21.3 mg, 60 µmol), Cr(thf)<sub>3</sub>I<sub>2</sub> (21 mg, 40 µmol). Anal. calcd. (found) for C<sub>8</sub>H<sub>8</sub>N<sub>4</sub>Crl<sub>0.8</sub>Br<sub>1.2</sub>: Br, 23.41% (23.28%); I, 24.79% (24.53%). 40%: Cr(thf)<sub>2</sub>Br<sub>2</sub> (14.2 mg, 40 µmol), Cr(thf)<sub>3</sub>I<sub>2</sub> (31.3 mg, 60 µmol). Anal. calcd. (found) for C<sub>8</sub>H<sub>8</sub>N<sub>4</sub>Crl<sub>1.2</sub>Br<sub>0.8</sub>: Br, 14.92% (14.72%); I, 35.55% (35.07%); C, 22.43% (22.94%); N, 13.08% (13.38%); and H, 1.88% (1.93%). 35%: Cr(thf)<sub>2</sub>Br<sub>2</sub> (12.4 mg, 35 µmol), Cr(thf)<sub>3</sub>I<sub>2</sub> (33.9 mg, 65 µmol). Anal. calcd. (found) for C<sub>8</sub>H<sub>8</sub>N<sub>4</sub>Crl<sub>1.3</sub>Br<sub>0.7</sub>: Br, 12.91% (12.48%); I, 38.09% (36.91%); C, 22.19% (23.30%); N, 12.94% (13.59%); and H, 1.86% (1.96%). 25%: Cr(thf)<sub>2</sub>Br<sub>2</sub> (8.9 mg, 25 µmol), Cr(thf)<sub>3</sub>I<sub>2</sub> (39.1 mg, 75 µmol). Anal. calcd. (found) for C<sub>8</sub>H<sub>8</sub>N<sub>4</sub>Crl<sub>1.5</sub>Br<sub>0.5</sub>: Br, 9.03% (8.92); I, 43.02% (42.79).

## X-ray crystallography

Single crystals of Cr(thf)<sub>3</sub>I<sub>2</sub> (120 K) and Cr(pyz)<sub>2</sub>I<sub>2</sub> (120 and 298 K) were immersed in polybutene oil (Sigma-Aldrich, >90%), mounted onto a nylon loop, and then measured under an N<sub>2</sub> stream with a SuperNova Dual Source charge-coupled device (CCD)-diffractometer using Mo Kα radiation. Olex2<sup>39</sup> was used for structure solution with direct methods using the ShelXS structure solution program and the structure was refined by least squares in the version 2018/3 of ShelXL.<sup>40</sup> Non-hydrogen atoms were refined anisotropically. Single-crystal X-ray diffraction of Cr(pyz)<sub>2</sub>I<sub>2</sub> under high pressure was conducted at the School of Chemistry, University of Newcastle (Newcastle, United Kingdom). A single crystal of Cr(pyz)<sub>2</sub>I<sub>2</sub> was selected and loaded into a diamond anvil cell (DAC) with a hydrostatic medium of Daphne oil and pressurized to 1.54 GPa using ruby as a pressure calibrant.<sup>41</sup> The data were then collected on a Bruker D8 Venture laboratory diffractometer using Ag Kα radiation. The phase transition from the ambient to the high-pressure structure was partially destructive, leading to a splitting of the single crystal into multiple domains under pressure. The shading from the DAC and the low symmetry, monoclinic unit cell led to lower than ideal completeness. Every attempt was made to maximize the quality of data obtained. Four major domains were identified, and the data were integrated as a twin using SAINT and TWINABS for the absorption correction. Olex2 was used to solve and refine the structure. The solution was made using ShelXT with the merged HKLF4, and the refinement was carried out using ShelXL and the unmerged HKLF5. It was not possible to refine a fully anisotropic non-hydrogen model for these data. The low completeness in a specific direction is due to the systematic shading from the DAC. This gives little information on the lighter elements of the pyz ring in one direction, leading to unreasonable ADPs in a fully anisotropic refinement. Therefore, it was deemed more appropriate to isotropically refine this whole group, leaving only the heavier elements, Cr and I, as anisotropic.

## Powder X-ray diffraction

Powder X-ray diffraction patterns were obtained at ambient temperature using Cu Kα radiation (λ = 1.5406 Å) in transmission
geometry on a Malvern Panalytical Empyrean powder X-ray diffractometer equipped with a 1Der detector. The powders were packed into borosilicate capillaries (Ø 0.7 mm) within an Ar-filled glovebox, sealed with grease, and flame-sealed outside the glovebox. All Rietveld refinements were performed in the HighScore Plus 5.1.0 program suite.⁴² All structures were refined against the RT Cr(pyz)₂I₂ single-crystal X-ray structure, and the peak profiles were fitted using a pseudo-Voigt peak shape function. All atomic positions were kept fixed, except for the halide positions, which were refined along the c axis (along the Cr–X bond), and the occupancy was changed so the I/Br ratio matched the synthetic ratios. For all refinements, a background function was determined and used. Temperature-dependent powder X-ray diffraction data of 50% were acquired on a Super-nova (Oxford diffraction) single-crystal diffractometer equipped with an Atlas CCD detector.

## Electron crystallography

Continuous rotation 3D-ED data were acquired using a Rigaku XtaLAB Synergy-ED dedicated electron diffractometer, equipped with a Rigaku Oxford Diffraction HyPix detector.⁴³,⁴⁴ Inside an Ar-filled glove box, microcrystalline powders of Cr(pyz)₂I₂ (0%), Cr(pyz)₂IBr (50%), and Cr(pyz)₂Br₂ (100%) were gently ground then loaded onto continuous carbon film transmission electron microscopy (TEM) grids. The grids were transferred into an airtight sample transfer puck, which was then removed from the glovebox and into a Gatan Cold Transfer Station, where it was submerged directly into liq. N₂ and opened. The grids were then mounted on a Gatan ELSA cold sample holder at ~120 K, which was then inserted into the airlock chamber of the electron diffractometer. The sample stage was then warmed gently and held at 175 K for 15 min to ensure any adventitious ice was sublimed off the grid before the sample was fully inserted into the column and cooled to 100 K. Data acquisition was performed using an electron wavelength of 0.0251 Å (200 keV), with continuous rotation of the crystals around a single axis inside an effective 1.9 µm selected area aperture. The sample stage was then warmed to 298 K, and a second set of data was acquired at this temperature for each crystal. At the conclusion of all diffraction experiments, postmortem high-resolution images (at ×15,000 magnification) were acquired. The raw diffraction data were processed using CrysAlisPro, and the structures were solved using ShelXT.⁴⁵ The structures were refined by ShelXL⁴⁰ using a kinematical approximation in the Olex2³⁹ and ShelXLe⁴⁶ graphical user interfaces. Rigid body restraints were applied to the anisotropic displacement parameters, and, where necessary, additional SIMU commands were applied to stabilize overlapping halide atoms. Bonds between the disordered positions of the pyz rings were suppressed using FREE commands, and then H atoms were placed by the appropriate AFIX instruction, using the default settings for neutron structures (which are generally more appropriate for ED). Bonds between disordered halides and symmetry generated (disordered) H atoms were also removed from the connectivity tables using appropriate FREE commands.

## SEM

Energy-dispersive X-ray spectroscopy (EDS) combined with scanning electron microscopy (SEM) was performed to analyze

the elemental composition of the samples. The measurements were conducted using a Quanta FEG 250 Analytical ESEM equipped with an Oxford Instruments 50 mm² X-Max silicon drift detector. Samples were mounted on carbon tape and coated with a 6–8 nm layer of gold to enhance conductivity. Measurements were obtained at an accelerating voltage of 20 kV with a working distance of 8 mm to optimize X-ray signal collection. Data analysis was carried out using AZtec for elemental quantification.

## Magnetization measurements

Direct current (dc) and ac magnetization measurements were performed on polycrystalline samples sealed in Quantum Design powder capsules with a Quantum Design Dynacool Physical Property Measurement System (PPMS). The sample masses were measured with a Mettler Toledo WXTSDU microbalance, and the experimental data were corrected for diamagnetism. Using the VSM option, the susceptibility was measured from 3 to 300 K at $\mu_0H_{dc} = 0.1$ T, and the magnetization versus field was measured up to 9 T. With the ACMS-II option, the ac magnetization was measured with $H_{ac}$ up to 10 Oe and $\nu_{ac}$ up to 10 kHz.

## Heat capacity measurements

Heat capacity was measured using the thermal relaxation technique on a Quantum Design PPMS-9. The sample consisted of a pressed pellet of Cr(pyz)₂I₂ (~7.0 mg), which was molded under a pressure of 2 tons for 1 h. Measurements were performed in zero dc field over a temperature range of 298 to 2 K. To ensure optimal thermal contact between the sample and the platform, a thin layer of Apiezon N grease (<0.1 mm; 0.7 mg) was applied. Additionally, a separate measurement of a small amount of Apiezon N grease (0.7 mg) was conducted to estimate the sample environment correction, which was then subtracted from the total heat capacity of the sample.

## Differential scanning calorimetry

Differential scanning calorimetry was measured in a Netzsch DSC214 Polyma instrument in TA Instruments Tzero aluminium pans with hermetic lids. The compounds were measured in the range 298–100 K with a sweep rate of 10 K min⁻¹. The reference pan was an empty Tzero pan with a hermetic lid sealed under an Ar atmosphere. The pans were filled with 3–7 mg of compound carefully sealed under Ar by crimping with hermetic lids.

## Neutron scattering

Powder diffraction patterns were measured on the high-resolution powder diffractometer SPODI of Heinz Maier-Leibnitz Zentrum (Germany), neutron wavelength $\lambda = 2.5370$ Å. A powder sample of non-deuterated Cr(pyz)₂I₂ was sealed in a V sample holder can. The sample was cooled in a closed-cycle cryostat with a base temperature of 4.5 K. Powder diffraction data were analyzed with MAG2POL.⁴⁷ The INS experiment was performed at the MARI spectrometer at the ISIS Neutron and Muon Source (United Kingdom). A powder sample (3.0 g) of fully deuterated Cr(pyz-d₄)₂I₂ was mounted in an annular geometry inside an aluminum can, which in turn was connected to the cold finger of a closed-cycle refrigerator. The data were obtained in repetition rate multiplication mode with incident neutron
energies $E_i = 8.7, 19$, and 68.7 meV and a chopper frequency of 200 Hz, yielding calculated elastic energy resolutions of 0.28, 0.78, and 3.1 meV, respectively. Data were recorded for sample temperatures of 6, 15, 30, 60, and 100 K. The highest temperature data were employed for subtracting phonon contribution to the total spectrum. The raw data were processed using the Mantid software package,⁴⁸ and the resulting data analyzed in MATLAB using SpinW.⁴⁹

## Electrical conductivity

Temperature-dependent electrical resistivities of Cr(pyz)₂I₂ (0%) Cr(pyz)₂Br₂ (100%), 50%, and 40% were measured with a Keithley 6487 picoammeter, with samples mounted in a PPMS. The samples were placed in a homemade PTFE cell and pressed between two stainless steel screws, giving sample thicknesses in the range of 0.3–0.6 mm. The cell was attached to a Quantum Design dc resistance puck, with each terminal connected to the corresponding screw via Cu wire, and the cell fastened to the puck with thermal varnish, mounted in the PPMS for temperature control. The PPMS was connected to the picoammeter via a custom Aivon BoBoX breakout box and controlled using a homebuilt code in the LabView software. Measurements were obtained with a current limit of 25 mA, excitation voltage of 10 V, and averaged over 10–30 measurements per data point. Temperature sweeps were performed at 1–2 K min⁻¹ to allow for thermalization of the sample.

## X-ray spectroscopy

XAS spectra at the K-edges of Cr and Br were recorded on powder samples in flame-sealed quartz capillaries (Ø 0.9 mm) using total fluorescence yield detection mode at the ID12 beamline at the European Synchrotron (ESRF, Grenoble, France). Spectra were normalized using standard procedures and were corrected for reabsorption effects.

Lighthouse Q-MAT. N.P. acknowledges the support of Novo Nordisk Foundation Challenge Programme 2021: Smart nanomaterials for applications in life science, BIOMAG grant NNF21OC0066526, and the support from the ERC Advanced “NEXUS” grant 101054572. E.K.B. thanks the EPSRC (EP/V010573/1, EP/P025986/1, and EP/N01331X/1) and Leverhulme Trust (RPG-2021-176) for funding. M.R.P. thanks the EPSRC (grant no. EP/W021129/1) for continued support of the diffraction facilities. R.C. and P.P. thank the University of Bordeaux, the Région Nouvelle Aquitaine, Quantum Matter Bordeaux (QMBx), the ANR (HiPer-Magnet project; ANR-20-CE07-0022), the Centre National de la Recherche Scientifique (CNRS), and the Association Française de Magnétisme Moléculaire. D.P. acknowledges the financial support of the National Science Centre Poland within the Opus project 2020/37/B/ST5/02735. The X-ray spectroscopy experiments were performed at the ID12 beamline at the European Synchrotron Radiation Facility (Grenoble, France). Experiments at the ISIS Neutron and Muon Source were supported by beamtime allocation RB1920693 from the Science and Technology Facilities Council. The authors thank the Technical University of Denmark for funding the DTU Electron Crystallography Facility and the Danish Agency for Science, Technology, and Innovation for funding the instrument center “Danscatt.”

## AUTHOR CONTRIBUTIONS

K.S.P. conceived and supervised the project. F.A., L.V., M.A.D., A.V., and S.W. synthesized and chemically characterized the materials. L.V., J.N.M., M.K., F.A., A.S.M., and K.S.P. performed the ambient pressure powder and single-crystal X-ray and electron crystallographic analysis. C.J.M., M.C., M.R.P., and E.K.B. performed the high-pressure crystallographic experiments and analysis. D.P. obtained and analyzed the differential scanning calorimetry data. S.T.D. and J.R.J. obtained and analyzed the SEM data. K.S.P., L.V., F.A., M.A.D., W.W., P.P., and R.C. obtained and analyzed the magnetization data. M.A.D., A.C., F.T., K.S.P., and N.P. obtained and analyzed the resistivity data. P.P. and R.C. obtained the specific heat data. L.V., W.W., M.K., K.A.S., V.B., A.S., M.D.L., H.C.W., and N.B.C. acquired and analyzed the neutron diffraction and time-of-flight neutron scattering data. N.J.Y., F.W., and A.R. recorded and interpreted the X-ray absorption data. The manuscript was written by K.S.P., F.A., M.A.D., and J.N.M. through contributions from all authors. All authors have consented to the publication of the manuscript.

## DECLARATION OF INTERESTS

The authors declare no competing interests.

## ACKNOWLEDGMENTS

K.S.P. thanks the Villum Foundation for a VILLUM Young Investigator grant and a VILLUM Young Investigator+ (42094) grant, the Independent Research Fund Denmark for a DFF-Sapere Aude Starting Grant (no. 0165-00073B), and the Carlsberg Foundation for a research infrastructure grant (no. CF17-0637). A.C. and F.T. acknowledge support by research grant 37338 (SANSIT) from Villum Foundation. This work was supported by the Danish National Committee for Research Infrastructure (NUFI) through the ESS-

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Supplemental Crystallographic Details

3D-ED Structure of Cr(pyz)₂IBr (50%)

3D-ED data were collected for two grains of Cr(pyz)₂IBr (50%), at 100 K and at 298 K for both (Figures S1 and S2).
The best data quality was obtained for "Grain 1," while similar data quality was obtained for "Grain 2," but with only
~80% completeness. Merging the two datasets did not improve the statistics, and so the data were treated
independently, representing duplicate measurements at each temperature.

<figure><img src="image_6.png" /></figure>

**Figure S1.** Electron-diffraction crystal structures of Cr(pyz)₂IBr (**50%**). **a.** ORTEP illustration showing the asymmetric unit of Cr(pyz)₂IBr (**50%**) at 100 K (ellipsoids drawn at 50% probability level). The position of atoms related by symmetry are shown by transparent ball and sticks. **b.** ORTEP illustrations of the asymmetric units from the 3D ED structures of both grains of **50%** at 100 K and 298 K (ellipsoids drawn at 50% probability level). **c.** Views down the *a* axis of two layers from all four 3D-ED structures of **50%**. **d.** A view looking down the *c* axis of the structure of Grain 1 (**50%**) at 100 K. Color code: Cr dark green, I purple, Br orange, O red, C black, H white.
<figure><img src="image_7.png" /><figcaption>**Figure S2.** Postmortem high resolution TEM image of 50% grain 1 (a. total accumulated dose 10.8 e Å⁻²) and grain 2 (b. total accumulated dose 9.21 e Å⁻²) acquired at 298 K. Below the TEM images, unwarped precession images from the 3D ED datasets are shown for each grain at 100 K and at 298 K.</figcaption></figure>

### 3D-ED Structure of Cr(pyz)₂I₂ (0%)

3D-ED data were collected for two grains of Cr(pyz)₂I₂ (0%), at 100 K and at 298 K for both grains (Figures S3 and S4). While both datasets had high completeness, these crystallites were extremely small, and more than one crystalline domain was present in the 1.9 µm selected area during each experiment (which can be seen both in the real-space high resolution TEM images, and the unwarped precession images, Figure S4), resulting in higher than ideal RInt. We searched extensively for high quality single nanocrystallites, but this compound appeared to be the least stable of all the Cr(pyz)₂X₂ materials during sample preparation for 3D ED—which does involve unavoidable exposure to air (~5–10 s) while the sample is transferred from the cold transfer station into the microscope airlock chamber. Nonetheless, the 3D-ED datasets were stable during structure refinement, converging to models that were entirely consistent with the SC-XRD structures of the same material (see Figure S3 and Table S1).
<figure><img src="image_8.png" /><figcaption>**Figure S3.** Electron diffraction crystal structures of Cr(pyz)₂I₂ (0%). **a.** ORTEP illustrations of the asymmetric units from the 3D-ED structures of both grains of 0% at 100 K (97 K for grain 2) and 298 K compared to (ambient pressure) SCXRD structures of Cr(pyz)₂I₂ (ellipsoids drawn at 50% probability level). **b.** Views down the *a* axes of two layers from all four 3D-ED 0% and (ambient pressure) SCXRD structures of Cr(pyz)₂I₂. Colour code: Cr dark green, I purple, O red, C black, H white.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure S4.** Postmortem high resolution TEM image of 0% grain 1 (a: total accumulated dose 9.23 e Å⁻²) and grain 2 (b: total accumulated dose 3.27 e Å⁻²) acquired at 298 K. Below the TEM images, unwarped precession images from the 3D ED datasets are shown for each grain at 100 K and at 298 K.</figcaption></figure>

## 3D-ED Structure of Cr(pyz)₂Br₂ (100%)

3D-ED data for three grains of Cr(pyz)₂Br₂ (100%) were collected at 100 K and 298 K (Figures S5 and S6). The crystalline domains of 100% (Figure S6) were much larger than any of the 50% or 0% samples (see Figures S2 and S4, respectively), and while the quality of the 3D-ED data was generally better for the 100% grains, less complete reciprocal space was surveyed during each experiment. Merging the datasets did not improve the completeness, suggesting all three (large) grains had similar (preferred) orientations on the TEM grid.
a

<figure><img src="image_10.png" /><figcaption>Grain 1:</figcaption></figure>

b

<figure><img src="image_11.png" /><figcaption>Grain 2:</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Grain 2:</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Grain 3:</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Grain 3:</figcaption></figure>

<figure><img src="image_15.png" /></figure>

**Figure S5.** Electron diffraction crystal structures of Cr(pyz)₂Br₂ (100%). a. ORTEP illustrations of the asymmetric units from the 3D-ED structures of the three grains of 100% at 100 K and 298 K (ellipsoids drawn at 50% probability level). b. Views down the a axes of two layers from all six 3D-ED structures of 100%. Colour code: Cr dark green, Br orange, O red, C black, H white.
<figure><img src="image_16.png" /><figcaption>**Figure S6.** Postmortem high resolution TEM image of 100% grain 1 (a: total accumulated dose 3.38 e Å⁻²), grain 2 (b: total accumulated dose 5.41 e Å⁻²) and grain 3 (c: total accumulated dose 2.13 e Å⁻²) acquired at 298 K. Below the TEM images (to the right for c), unwarped precession images from the 3D ED datasets are shown for each grain at 100 K and at 298 K.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Figure S7.** Thermal ellipsoid plot of Cr(thf)₃I₂ (*T* = 120(1) K; 70% probability level; Cr dark green, I purple, O red, C black, H white).</figcaption></figure>
Supplemental X-ray Powder Diffraction Data

<figure><img src="image_18.png" /><figcaption>**Figure S8.** Simulated powder X-ray diffractogram of **50%** at 100 K (light blue) and 298 K (pink) and experimental powder X-ray diffractogram of **50%** at 115 K (blue) and 275 K (red).</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Figure S9.** Powder X-ray diffraction of 50% upon cooling and subsequent heating. Starting at T = 275 K (between 275 and 235 in 20 K increments, and between 235 and 115 K in 10 K increments).</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**Figure S10.** Normalized 2θ shift of the peaks indicated in Figure S9 following the temperature variation from 240 K to 120 K (blue symbols) and back to 240 K (red symbols). Sigmoidal fits are guides for the eyes.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Figure S11.** Rietveld refinements of Cr(pyz)₂l₂-xBrx. Normalized powder X-ray diffractograms (black lines) and corresponding Rietveld refinements (colored lines) of Cr(pyz)₂l₂-xBrx (labeled according to the percentage of Br).</figcaption></figure>

The agreement factors, *R*<sub>wp</sub>, are defined by
$$R_{wp} = \sqrt{\frac{\sum w_i(y_{i,obs} - y_{i,calc})^2}{\sum w_i y_{i,obs}^2}} \times 100\%$$
where $y_{i,calc}$ is the net calculated intensity calculated, $y_{i,obs}$ is observed intensity, and the weight factor, *w*<sub>i</sub>, is 1/ESD<sup>2</sup>.
<figure><img src="image_22.png" /><figcaption>**Figure S12.** Unit cell dimensions of Cr(pyz)₂I₂-xBrx at 298 K. Dependence of the length of the *a* axis and *c* axis (left axis) and the Cr–X bond length (right axis) according to the percentage of Br in Cr(pyz)₂I₂-xBrx (cf. Table S5). Solid lines are guides for the eyes.</figcaption></figure>
Supplemental Energy-Dispersive X-ray Spectroscopy Data

<figure><img src="image_23.png" /><figcaption>**Figure S13.** Energy-dispersive X-ray spectra of 0%, 25%, 35%, 40%, 50%, 60%, 75%, and 100% obtained by SEM-EDS on several sub-micron grains. The main signals from Br, Au coating, I, and Cr are indicated by the colored areas. All spectra were normalized to the Cr signal. Representative SEM images are provided in Figures S14–18.</figcaption></figure>
<figure><img src="image_24.png" /><figcaption>**Figure S14.** Representative SEM image of 35% with indicated EDX acquisition spots.</figcaption></figure>

<figure><img src="image_25.png" /><figcaption>**Figure S15.** Representative SEM image of 40% with indicated EDX acquisition spots.</figcaption></figure>
<figure><img src="image_26.png" /><figcaption>**Figure S16.** Representative SEM image of 50% with indicated EDX acquisition spots.</figcaption></figure>

<figure><img src="image_27.png" /><figcaption>**Figure S17.** Representative SEM image of 60% with indicated EDX acquisition spots.</figcaption></figure>
<figure><img src="image_28.png" /><figcaption>**Figure S18.** Representative SEM image of 75% with indicated EDX acquisition spots.</figcaption></figure>
Supplemental Magnetization Data

<figure><img src="image_29.png" /><figcaption>**Figure S19.** Temperature dependence of the magnetic susceptibility-temperature product, $\chi T (\mu_0 H = 0.1 \text{ T})$, for Cr(pyz)$_2$I$_2$ synthesized by the “fast” (0%) and “slow” methods. Solid lines are calculations ($J = 0.46$ meV, $g = 1.97$ for Cr(pyz)$_2$I$_2$ (slow); $J = 0.30$ meV, $g = 1.94$ for Cr(pyz)$_2$I$_2$ (0%)) using the Curély solution to the classical 2D Heisenberg model, as described in the main text.</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>**Figure S20.** Temperature dependence of the magnetic susceptibility, $\chi = M/\mu_0 H$, at selected magnetic fields for Cr(pyz)$_2$I$_2$ synthesized by the “fast” and “slow” methods.</figcaption></figure>
<figure><img src="image_31.png" /><figcaption>**Figure S21.** Magnetization data for Cr(pyz)₂l₂-xBrx solid solutions. Powder magnetization data obtained on polycrystalline Cr(pyz)₂l₂-xBrx alloys in the magnetic field range 0–9 T.</figcaption></figure>
<figure><img src="image_32.png" /><figcaption>**Figure S22.** Magnetization hysteresis for Cr(pyz)₂Br₂ (100%), 50%, and Cr(pyz)₂I₂ (0%) obtained at 3.0 K.</figcaption></figure>

<figure><img src="image_33.png" /><figcaption>**Figure S23.** Zero-field cooled–field cooled (ZFC–FC, µ₀H = 100 Oe) magnetization data for Cr(pyz)₂Br₂ (100%).</figcaption></figure>
<figure><img src="image_34.png" /><figcaption>**Figure S24.** Alternating current (ac) magnetic susceptibility for Cr(pyz)₂Br₂ (100%) obtained at selected ac frequencies and in the absence of a static (dc) magnetic field.</figcaption></figure>

Supplemental Heat Capacity Data

<figure><img src="image_35.png" /><figcaption>**Figure S25.** Temperature dependence of the specific heat, *c*<sub>p</sub>, of Cr(pyz)₂I₂ ("slow" method) in zero-dc field.</figcaption></figure>
Supplemental Differential Scanning Calorimetry Data

<figure><img src="image_36.png" /><figcaption>**Figure S26.** Differential scanning calorimetry temperature scans of **40%**, **50%**, and **60%** obtained with a temperature ramping speed of 10 K min⁻¹. The enthalpy changes are estimated as -2.1(5) kJ mol⁻¹ (**40%**), -2.8(1) kJ mol⁻¹ (**50%**), and -3.4(5) kJ mol⁻¹ (**60%**).</figcaption></figure>
Supplemental Neutron Scattering Data

<figure><img src="image_37.png" /><figcaption>**Figure S27.** Powder neutron diffractogram of Cr(pyz)₂I₂ ("slow" method) at 4 K (λ = 2.5362 Å) and the Rietveld refinement of the nuclear and magnetic structure. R<sub>F</sub> = 9.7%, R<sub>magnetic</sub> = 50.5%, magnetic moment = 1.8(3) µ<sub>B</sub>. Nuclear structure unit cell: a = 7.1013(1) Å, c = 12.5439(3) Å (l4/mmm space group). The inset shows a zoom of the low-angle region and compares the diffractograms below (4 K) and above the T<sub>Néel</sub> (60 K).</figcaption></figure>
<figure><img src="image_38.png" /><figcaption>**Figure S28.** Temperature dependence of the INS spectra of Cr(pyz-d₄)₂I₂ at *E*ᵢ = 9 meV (a–d), and *E*ᵢ = 19 meV (e–h), after subtraction of the 100 K data (corrected for the Bose occupation factor).</figcaption></figure>
Supplemental X-Ray Absorption Spectroscopy Data

<figure><img src="image_39.png" /><figcaption>**Figure S29.** Pre-edge part of the Cr K-edge XANES spectrum shown in Figure 4 of the main text emphasizing the dipole-forbidden 1s → 3d transitions.</figcaption></figure>
<figure><img src="image_40.png" /><figcaption>**Figure S30.** Normalized XANES spectra at the Cr K-edge of Cr(pyz)₂I₂-xBrx alloys (labeled according to their Br percentage) at 300 K, and a linear combination of the **0%** (50% contribution) and **100%** (50% contribution) spectra.</figcaption></figure>
<figure><img src="image_41.png" /><figcaption>**Figure S31.** Normalized XANES spectra at the Cr K-edge of Cr(pyz)₂I₂-xBrx alloys (labeled according to their Br percentage) at 300 K, cooled down to 50 K and heated back up to 300 K.</figcaption></figure>
<figure><img src="image_42.png" /><figcaption>**Figure S32.** Normalized XANES spectra at the Cr K-edge of Cr(pyz)₂I₂-xBrx alloys (labeled according to their Br percentage) at 300 K.</figcaption></figure>
<figure><img src="image_43.png" /><figcaption>**Figure S33.** Normalized XANES spectra at the Cr K-edge of Cr(pyz)₂I₂-xBrx alloys (labeled according to their Br percentage) at 50 K.</figcaption></figure>
<figure><img src="image_44.png" /><figcaption>**Figure S34.** Normalized XANES spectra at the Br K-edge of Cr(pyz)₂Br₂ (100%) at 300 K (solid line) and 50 K (dashed line)</figcaption></figure>
<figure><img src="image_45.png" /><figcaption>**Figure S35.** Normalized XANES spectra at the Br K-edge of Cr(pyz)₂I₂-xBrx alloys (labeled according to their Br percentage) at 300 K.</figcaption></figure>
<figure><img src="image_46.png" /><figcaption>**Figure S36.** Normalized XANES spectra at the Br K-edge of Cr(pyz)₂Br₂ (100%, purple) at 300 K and of 75% at 300 K (dark blue) and 50 K (light blue).</figcaption></figure>
<figure><img src="image_47.png" /><figcaption>**Figure S37.** Normalized XANES spectra at the Br K-edge of Cr(pyz)₂Br₂ (100%, purple) at 300 K and of 40% at 300 K (dark blue) and 50 K (light blue).</figcaption></figure>
<table><caption>**Table S4.** Selected structural metrics and formal Cr oxidation state.</caption><thead><tr><th>Identification code</th><th colspan="2">Cr(pyz)<sub>2</sub>Br<sub>2</sub> (100%)</th><th colspan="2">Cr(pyz)<sub>2</sub>IBr (50%)</th><th colspan="2">Cr(pyz)<sub>2</sub>I<sub>2</sub> (0%)</th><th>Cr(pyz)<sub>2</sub>Cl<sub>2</sub>*</th></tr><tr><th>Experiment</th><th colspan="2">3D-ED</th><th colspan="2">3D-ED</th><th colspan="2">3D-ED</th><th>SCXRD</th><th>PXRD</th></tr></thead><tbody><tr><td>T/K</td><td>100 K</td><td>298 K</td><td>100 K</td><td>298 K</td><td>100 K</td><td>298 K</td><td>298 K</td><td>298 K</td></tr><tr><td>Oxidation state</td><td>Cr(III)</td><td>Cr(III)</td><td>Cr(III)</td><td>Cr(II)</td><td>Cr(II)</td><td>Cr(II)</td><td>Cr(II)</td><td>Cr(III)</td></tr><tr><td>Cr—Br /Å</td><td>2.51(3)</td><td>2.52(3)</td><td>2.55(7)</td><td>2.81(9)</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Cr—I /Å</td><td>-</td><td>-</td><td>2.71(6)</td><td>3.0(8)</td><td>3.05(7)</td><td>3.07(8)</td><td>3.0424(5)</td><td>-</td></tr><tr><td>Cr—Cl /Å</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>2.337(1)</td></tr><tr><td>Cr—N /Å</td><td>2.073(19)</td><td>2.08(2)</td><td>2.09(3)</td><td>2.15(5)</td><td>2.16(5)</td><td>2.17(5)</td><td>2.149(3)</td><td>2.003(2)</td></tr></tbody></table>

<footnote>* As reported in K. S. Pedersen et al. *Nat. Chem.* **2018**, *10*, 1056.</footnote>
<table><caption>**Table S5.** Unit cell parameters of Cr(pyz)₂I₂-xBrₓ alloys from Rietveld refinements.</caption><thead><tr><th>Br percentage</th><th>Unit Cell</th><th>R<sub>wp</sub> / %</th></tr></thead><tbody><tr><td rowspan="3">0</td><td>a / Å = 7.136(1) V / Å³ = 641.40(6)</td><td rowspan="3">4.46</td></tr><tr><td>b / Å = 7.136(1) α = β = γ = 90°</td></tr><tr><td>c / Å = 12.596(2)</td></tr><tr><td rowspan="3">25</td><td>a / Å = 7.1253(2) V / Å³ = 625.321(5)</td><td rowspan="3">7.08</td></tr><tr><td>b / Å = 7.1253(2) α = β = γ = 90°</td></tr><tr><td>c / Å = 12.3168(5)</td></tr><tr><td rowspan="3">40</td><td>a / Å = 7.1326(2) V / Å³ = 622.037(6)</td><td rowspan="3">7.68</td></tr><tr><td>b / Å = 7.1326(2) α = β = γ = 90°</td></tr><tr><td>c / Å = 12.2270(8)</td></tr><tr><td rowspan="3">50</td><td>a / Å = 7.1392(2) V / Å³ = 620.05(1)</td><td rowspan="3">2.79</td></tr><tr><td>b / Å = 7.1392(2) α = β = γ = 90°</td></tr><tr><td>c / Å = 12.1644(2)</td></tr><tr><td rowspan="3">60</td><td>a / Å = 7.1002(3) V / Å³ = 609.23(1)</td><td rowspan="3">5.37</td></tr><tr><td>b / Å = 7.1002(3) α = β = γ = 90°</td></tr><tr><td>c / Å = 12.071(1)</td></tr><tr><td rowspan="3">75</td><td>a / Å = 6.9567(3) V / Å³ = 554.27(4)</td><td rowspan="3">2.55</td></tr><tr><td>b / Å = 6.9567(3) α = β = γ = 90°</td></tr><tr><td>c / Å = 11.455(2)</td></tr><tr><td rowspan="3">100</td><td>a / Å = 6.9482(3) V / Å³ = 546.06(4)</td><td rowspan="3">9.84</td></tr><tr><td>b / Å = 6.9482(3) α = β = γ = 90°</td></tr><tr><td>c / Å = 11.3107(9)</td></tr></tbody></table>

The agreement factors, R<sub>wp</sub>, are defined by R<sub>wp</sub> = √[Σ w<sub>i</sub>(y<sub>i,obs</sub> - y<sub>i,calc</sub>)² / Σ w<sub>i</sub>y<sub>i,obs</sub>²] × 100%, where y<sub>i,calc</sub> is the net calculated intensity calculated, y<sub>i,obs</sub> is observed intensity, and the weight factor, w<sub>i</sub>, is 1/ESD².