Deciphering cation-driven structure–property correlations in 0D hybrid ruthenium halide perovskites

Abstract

Hybrid organic–inorganic halides have emerged as a structurally tunable class of materials that allow simultaneous modulation of optical properties and magnetic interactions. By varying the A-site cation with organic amines of different carbon chain lengths, a series of zero-dimensional (0D) hybrid ruthenium halide perovskites, (EDA)₂RuCl₆·Cl·H₂O (1), (PDA)₂RuCl₆·2Cl·3H₂O·(H₃O) (2) and (BDA)₂RuCl₆·2Cl·H₂O·(H₃O) (3) (where EDA = ethylenediamine, PDA = 1,3-diaminopropane and BDA = 1,4-diaminobutane) with absorption edges (1.95–1.92 eV), have been assembled. Thermogravimetric analysis of compounds 1–3 reveals that the choice of organic diamine cations and lattice water content enable tuning of thermal stability in hybrid ruthenium halides, linking the amine structure to decomposition temperature. Low temperature magnetization behaviours of compounds 1 and 2 demonstrate typical J = 1/2 paramagnetism, while all compounds 1–3 exhibit broad deep-blue emission spectra, owing to the isolated single ion [RuCl₆]³⁻ octahedra. These findings establish structure–property correlations, highlighting the interplay of the A-site cation in structural modulation, photophysical properties and magnetic behaviour in a new family of 0D hybrid ruthenium halides.

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Introduction

Hybrid organic–inorganic halide perovskites (HOIPs) have emerged as a versatile class of crystalline materials over the past decade, revolutionizing the field of optoelectronics. Their exotic properties have enabled rapid progress in photovoltaic devices,¹ photonic lasers,² light-emitting diodes (LEDs),³ photodetectors⁴ and spintronics applications.⁵ Their unique structure, combining an inorganic framework with organic templating cations in the conventional ABX₃ perovskite lattice, enables functional advantages such as tunable bandgaps,¹ strong spin–orbit couplings (SOCs),⁵ long carrier diffusion lengths,⁶ low trap densities,⁷ high photoluminescence yields,⁸ large absorption coefficients⁹ and high dielectric screening.¹⁰ While these attributes have driven significant growth in optoelectronics, HOIPs also offer opportunities in relatively less explored areas, particularly magnetism, where their compositional flexibility and solution processability enable new avenues for design.

Magnetism in HOIPs is especially compelling because it is highly sensitive to structural dimensionality, lattice distortions and chemical substitution. Even subtle variations in crystal symmetry or cation composition can markedly influence exchange interactions and magnetic anisotropy, leading to diverse and tunable magnetic ground states.¹¹ Recent advances have revealed rich phenomena in low-dimensional halide systems, including the modulation of spin states in transition metal halide monolayers,¹² spin frustration,¹³ magnetic anisotropy¹⁴ and even chiral ferromagnetism.¹⁵ These features distinguish halide perovskites from more traditional magnetic oxides such as garnets, spinels and oxide perovskites, where cooperative magnetism is comparatively well-established.¹⁶ The structural tunability of HOIPs, combined with the ability to integrate spin functionality into optoelectronic platforms, underscores their potential as model systems for exploring magneto-optical effects and spin–orbit-driven magnetism.¹⁷

Particularly intriguing are spin-1/2 transition metal-halides containing 4d electrons, where geometric frustration and pronounced spin–orbit coupling (SOC) intersect. A notable example is α-RuCl₃, where the Ru³⁺ (t_2g⁵) configuration coupled with SOC gives rise to a proximate Kitaev quantum spin liquid state, a topic of significant current interest for quantum information applications.^18,19 Inspired by this, considerable research advancement has been directed towards ruthenium-based halide perovskites, where the interplay of SOC, lattice distortions and vacancy ordering in Ru(III) and Ru(IV) compounds can stabilize unconventional magnetic states. Despite the structural richness of inorganic Ru halides such as K₂RuCl₆, their hybrid analogues remain scarce.²⁰ Studies on compounds such as (NH₄)₂RuCl₆ and (CH₃NH₃)₂RuCl₆ revealed temperature-dependent magnetic responses and single-ion behaviour,²¹ while studies on alkali–metal ruthenates highlight the role of A-site cation size and halide variation in governing magnetic exchange pathways.²²

It is worth mentioning that the magnetic properties of HOIPs are highly sensitive to their dimensionality, as variations in the connectivity between magnetic ions can substantially alter the strength and nature of magnetic coupling within the lattice.²³ Unlike three-dimensional (3D) and two-dimensional (2D) HOIPs, magnetic coupling in one-dimensional (1D) and zero-dimensional (0D) structures is found to be weakened owing to the reduced connectivity.¹¹ Moreover, with reduced dimensionality, the A-site in HOIPs experiences progressively fewer size limitations, enabling a wider variety of feasible compositional combinations.²³ As such, along with phase and composition, variation in dimensionality strongly influences the magnetic behviour of HOIPs, resulting in the eventual observation of various exotic behaviours with changes in magnetic anisotropy and exchange mechanisms.¹¹ Among the properties, electrical and optical ones stand out to be promising in achieving multifunctional material design.^11,24,25 Interestingly, isolated metal octahedra governing the magnetic properties of HOIPs are also central to the emergent optical behaviour in 0D molecular perovskites.^26,27 In 0D hybrid halides, electronically discrete [MX₆] clusters act as molecular emitters which give rise to a strong exciton confinement, large exciton binding energies and self-trapped exciton (STE) emission.^28–30 This phenomenon is well illustrated in 0D Pb and Sn based frameworks, in which excitation remains localized to single [MX₆] octahedra, giving rise to characteristic LMCT-derived luminescence and ultrafast relaxation dynamics.^31–35 Hence, 0D molecular perovskites are becoming a versatile platform for exploring exciton dynamics and multifunctional optoelectronic behaviours.^36–38 Since spin-1/2 transition-metal halides containing 4d electrons are already at the forefront of research, extending this focus toward 0D hybrid 4d transition-metal halides represents promising progression. In particular, Ru-based hybrid halides with strong spin–orbit coupling (SOC)^21,22 and intrinsic magnetic anisotropy offer an excellent opportunity to explore the interplay of magnetism and optical activity within molecularly confined systems.

Driven by these observations, we have synthesized and structurally characterized three novel ruthenium-based halide perovskites (EDA)₂RuCl₆·Cl·H₂O (1), (PDA)₂RuCl₆·2Cl·3H₂O·(H₃O) (2) and (BDA)₂RuCl₆·2Cl·H₂O·(H₃O) (3) (where EDA = ethylenediamine, PDA = 1,3-diaminopropane and BDA = 1,4-diaminobutane) via a hydrothermal method. The influences of A-site cation identity and structural dimensionality on optical absorption edges, thermal resilience and magnetic properties are systematically explored. These compounds serve as model systems to elucidate structure–property relationships in hybrid ruthenium halides and provide valuable insight into design principles governing magnetism and broaden the landscape of HOIPs for next-generation spin–orbit coupled functional materials.

Results and discussion

The hydrothermal reactions of RuCl₃ with respective alkyldiamines viz. EDA, PDA and BDA in aqueous HCl solution result in single crystals of ruthenium halide perovskites 1–3 with the general formula (A)₂RuCl₆·xCl·yH₂O (A = EDA (1), x = 1, y = 1; PDA (2), x = 2, y = 4; and BDA (3), x = 2, y = 2). The detailed synthetic procedures are provided in the Experimental section. The resulting red crystals of 1–3 were isolated by washing, followed by drying (refer to the Experimental section for a detailed procedure). The crystal formation and phase purity were verified using powder X-ray diffraction (PXRD) measurements. The experimental PXRD patterns of 1–3 compared with simulated single-crystal X-ray diffraction patterns exhibit identical features, which confirms the high phase purity of the as-synthesized crystals (Fig. S2–S4). However, in the case of compound 3, minor impurities were observed, which are attributed to the unreacted RuCl₃ in bulk synthesis.³⁹ The SEM images of compounds 1–3 (Fig. S5) show a block crystal morphology of large, smooth and well-faceted crystals to rod-like and fragmented crystals with rough surfaces, respectively.

Structural descriptions of 1–3

The formation of the desired structures of 1–3 was confirmed by single-crystal X-ray diffraction (Sc-XRD) analysis. The single-crystal structures of compounds 1–3 are depicted in Fig. 1, which reveal distinct isolated [RuCl₆] octahedra. Crystallographic data and relevant structure refinement information for 1–3 are presented in Table S1, while the details of the selected bond lengths and angles are provided in Tables S2–S4.

Fig. 1 Crystal structures of compounds 1 (a and d), 2 (b and e) and 3 (c and f) at room temperature along the a-axis (a, b, and e) and c-axis (c, d, and f). Lattice solvent molecules are omitted for clarity.

Compound 1 adopts a monoclinic P2₁/m space group at room temperature (298 K). It features alternating inorganic and organic layers where the [RuCl₆]³⁻ octahedra are structurally isolated and lack connectivity along both in-plane and out-of-plane axes. The A-site cation EDA occupies a single-layered interspace between octahedra. The structure exhibits a vacancy between adjacent RuCl₆ units, which leads to minimal orbital overlap and low dimensionality. The bond distortion index (Δd) and angle variance (σ²) were calculated to be 4.66 × 10⁻⁶ and 0.347, respectively, indicating a slight octahedral distortion from the ideal octahedral geometry (Fig. 2a, d and Table S6). Compound 2, on the other hand, crystallizes in the I2/m space group with PDA as the organic spacer. The [RuCl₆]³⁻ octahedra remain isolated, yet arranged with enhanced symmetry and tighter stacking compared to those of 1. The symmetrically arranged bilayer motif leads to shorter inter-octahedral distances and dense hydrogen bonding networks.⁴⁰2 displays the lowest distortion parameters in the series (Δd = 1.19 × 10⁻⁹ and σ² = 0.149) (Fig. 2b, e and Table S6), suggesting a nearly ideal octahedral geometry. Compound 3, having the longest chain organic spacer, BDA of the synthesized series, crystallizes in the triclinic P1̄ space group. The increased organic chain length introduces greater structural flexibility, which results in displaced stacking and higher octahedral distortion (Δd = 5.51 × 10⁻⁶ and σ² = 1.261) (Fig. 2c, f and Table S6).

Fig. 2 (a–f) Bond distortion index (Δd) and bond angle variance (σ²) of Ru octahedra for compounds 1 (a and d), 2 (b and e) and 3 (c and f), respectively; (g) summary of Δd and σ² values for compounds 1–3; (h–j) amine chain length and nearest Ru⋯Ru distances of compounds 1–3, respectively.

The structural analysis suggests that both the length and nature of the organic A-site cation directly modulate octahedral symmetry and interlayer interactions across the series. The bond distortion index (Δd) and bond angle variance (σ²) increase from 2 (Δd = 1.19 × 10⁻⁹ and σ² = 0.149) to 3 (Δd = 5.51 × 10⁻⁶ and σ² = 1.261), indicating increased deviation from the ideal octahedral geometry (Table S7). Although all compounds contain isolated octahedra, all three structures are different from each other. The length and adaptability of the diammonium organic spacers influence these variations in the structures. Compound 1, which is templated by EDA, demonstrates a 0D configuration, and is stabilized via hydrogen bonding interactions between N–H⋯Cl and water molecules. The crystal structure of 1 somewhat resembles a Dion–Jacobson (DJ) type 2D perovskite in the sense that the octahedra in successive layers lie above one another. However, in the structures of 2 and 3, the octahedra are very strongly tilted relative to the upper and lower layers, unlike 1. This structural motif of 1 bears similarity to that observed in hybrid vacancy-ordered double perovskites such as (NH₄)₂PtI₆ and (MA)₂PtI₆ where the small organic cations facilitate discrete [PtI₆]²⁻ octahedra within a K₂PtCl₆-type framework.^41,42 The nearest Ru⋯Ru distance in 2 and 3 becomes 7.16 Å and 7.49 Å, respectively, compared to the nearest Ru⋯Ru distance of 7.14 Å in 1. The typical Ru–Cl bond distances of 2 and 3 match that of 1 (ca. 2.37 Å), further indicating that the ruthenium centre of all compounds is trivalent as reported for a variety of ruthenium halides.

It is worth noting that Ru is prone to oxidative changes, allowing it to exist in different oxidation states under ambient conditions. To probe the oxidation state in the resulting complexes, X-ray photoelectron spectroscopy (XPS) was employed to determine the valence states of the Ru centres in 1–3 (Fig. 3). The peaks corresponding to the binding energies of ≈463 eV (3p_3/2) and ≈485 eV (3p_1/2) are attributed to the trivalent oxidation state of ruthenium in 1–3, respectively (Fig. S7 and Table S5).

Fig. 3 XPS survey spectra and core level XPS spectra of Ru 3d and 3p states of 1 (a–c), 2 (d–f) and 3 (g–i).

Thermal behaviour

To elucidate how variations in diamine chain length and rigidity affect the structural integrity of hybrid ruthenium halides, a comprehensive thermogravimetric study has been performed to correlate the organic cation structure with thermal stability. The TGA profiles of compounds 1–3 reveal a two-step decomposition pathway in which lattice water is released at comparatively low temperatures, followed by the concerted loss of organic diamine ligands and chloride units at higher temperatures (Fig. S8–S10; for details, refer to the SI). Compound 1 shows an initial 3.80% loss at 105–150 °C (calcd: 3.66%) due to the release of one lattice water molecule, followed by 55.29% loss at 255–330 °C (calcd: 53.31%) from the release of two EDA molecules and two Cl₂ units (T_d = 285 °C). The remaining weight of 26.80% (calcd 27.08%) at 600 °C is in good agreement with the formation of RuO₂ (Fig. S8). Compound 2 displays an 11.98% loss at 132–185 °C (calcd: 11.82%) corresponding to four lattice water molecules, and then 49.10% loss at 230–315 °C (calcd: 47.62%) corresponding to two PDA molecules and two Cl₂ units (T_d = 251 °C). As anticipated, the final residue of 23.06% (calcd 21.84%) obtained upon heating to 600 °C is consistent with the formation of RuO₂ (Fig. S9). Compound 3 exhibits 2.60% loss at 60–85 °C (calcd: 2.99%) due to the release of one lattice water molecule, followed by 57.62% loss at 245–335 °C (calcd: 55.91%) due to the release of one water molecule, two BDA molecules and two Cl₂ units (T_d = 276 °C) (Fig. S10). The remaining 21.85% of residue matches the theoretical 22.14% for RuO₂. The formation of RuO₂ residues in 1–3 was further ascertained with PXRD analysis (Fig. S11–S13), which closely matches the PXRD patterns of RuO₂.⁴³

The markedly lower decomposition temperatures (251–285 °C) compared to that of the robust all-inorganic analogue K₂RuCl₆ (>450 °C) highlight the pronounced influence of organic amine cations on thermal resilience.²⁰ This study shows that the carbon-chain length and rigidity of diamine cations play a decisive role in governing the packing, hydrogen-bonding networks and water retention of hybrid ruthenium halide frameworks, which in turn dictate their thermal stability. Short, flexible diamines (EDA) afford relatively high decomposition temperatures compared to longer or more rigid diamines (PDA and BDA), reflecting differences in lattice cohesion and ligand release profiles. This correlation establishes that the judicious selection of amine cations offers a viable strategy to modulate the thermal resilience of hybrid metal-halide materials.

Optical behaviour

The presence of various diamines viz., EDA, PDA and BDA at the A-site of the crystal structure warrants investigation of the cation driven optical behaviours in 1–3. UV-visible absorption spectra of all three compounds 1–3 exhibit broad ligand-to-metal charge transfer (LMCT) bands, characteristic of isolated [RuCl₆] octahedra (Fig. 4a). Each of them displays four absorption bands within 225–650 nm, typical of d⁵ (Ru³⁺) octahedral configurations. The diffuse reflectance data of 1–3 were converted into pseudo-absorbance data using the Kubelka–Munk equation: F(R) = α/S = (1 − R)²/(2R), where R is the reflectance and α and S are the absorption and scattering coefficients, respectively. The estimated absorption edges estimated using Kubelka–Munk transformed plots, depicted in Fig. 4d–f, follow the trend: 1.95 eV (1) > 1.94 eV (2) > 1.92 eV (3) (Table S8). Interestingly, this trend of decreasing absorption edge with increasing A-site cation size (EDA < PDA < BDA) contrasts with conventional expectations for low-dimensional halide perovskites, where larger organic cations often introduce dielectric confinement and increase absorption edges.²² This inverse trend in 1–3 is attributed to several interdependent factors like increasing distortion of the RuCl₆ octahedra, which may modulate the ligand-field splitting and lower the LMCT transition energy.⁴⁴ Also the stronger N–H⋯Cl hydrogen bonding in compounds 2 and 3 may induce local lattice strain in the crystal structure.^45,46 The enhanced dielectric screening provided by the longer-chain organic cations may also reduce the Coulomb binding energy of excited states.^47,48 These structural and dielectric effects outweigh any confinement-induced absorption edge widening, which results in red-shifted absorption as the organic spacer length increases.^49,50

Fig. 4 (a–c) UV-visible spectra of compounds 1–3. (d–f) Diffuse reflectance spectra of 1–3 transformed using the Kubelka–Munk transformation F(R) and plotted as a function of photon energy.

The photoluminescence (PL) excitation and PL emission spectra at room temperature were recorded to characterize the photophysical properties of compounds 1–3 (Fig. 5). All three materials show similar PL properties with broad emission bands upon excitation. Compound 1, when excited (λ_ex) at 296 nm (4.19 eV), shows a broad emission from 300 nm (4.13 eV) to 575 nm (2.15 eV) having a maximum λ_em around 353 nm (3.51 eV). Compounds 2 and 3 also show λ_ex at 293 and 296 nm, respectively, with λ_em around 352 and 351 nm, respectively (Table 1). All three compounds show large Stokes shifts of 0.66–0.71 eV along with significant bandwidths. The broad emission bandwidths and significant Stokes shifts observed in compounds 1–3 indicate that after photo-excitation the excited states undergo considerable structural relaxation and are strongly coupled to phonon modes.³³ The emissive wavelength location of the compounds is provided by the CIE chromaticity coordinates, which are calculated to be (0.15, 0.11), (0.14, 0.11) and (0.15, 0.12) for 1–3, respectively. These deep blue-light CIE coordinates indicate that these compounds can be applied in the display and lighting field.⁵¹ These results align well with recent reports on 0D In and Sb halides, where the structural isolation, halide coordination and octahedral distortion influence the broadband self-trapped exciton emission and Stokes shifts.^52–55 Similarly, in our Ru³⁺ system, localized LMCT transitions within [RuCl₆]³⁻ octahedra and strong spin–orbit coupling rationalise the observed PL properties.

Fig. 5 Excitation and emission spectra of 1 (a), 2 (b) and 3 (c), respectively; chromaticity coordinate diagram of 1 (d), 2 (e) and 3 (f), respectively.

Table 1 Data parameters of PL spectra for compounds 1–3

Compound	λ ex	λ em	Stokes shift
(EDA)2RuCl6·Cl·H2O (1)	296 nm (4.19 eV)	353 nm (3.51 eV)	0.68 eV
(PDA)2RuCl6·2Cl·3H2O·(H3O) (2)	293 nm (4.23 eV)	352 nm (3.52 eV)	0.71 eV
(BDA)2RuCl6·2Cl·H2O·(H3O) (3)	296 nm (4.19 eV)	351 nm (3.53 eV)	0.66 eV

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Magnetic behaviour

Low temperature magnetization studies on polycrystalline samples of 1 and 2 were performed under a constant static field of 1000 Oe in the temperature range of 2–300 K and are depicted in Fig. 6a–d. Fig. 6a–c portray the variation of the magnetic susceptibility curves and thermal dependence of χ_MT for 1 and 2. For 1, the susceptibility data exhibit a traditional behaviour, showing a significant increase in the magnetization at low temperature. The high temperature susceptibility data (50–300 K) were fit to the Curie–Weiss law, χ = C/(T − θ_CW) + χ₀, where χ₀ is the temperature independent susceptibility. The fit yielded a Curie constant C = 0.40 emu Oe⁻¹ K mol⁻¹ and a Weiss temperature θ_CW = −2.99 K (Fig. 6a and Table 2). Isothermal field dependence of the magnetization experiments was performed between 0 and 7 T for 1 and 2 at a temperature of 2 K (Fig. 6d). The effective magnetic moment 1.78μ_B closely matches the theoretical spin-only value of 1.73μ_B expected for a low-spin d⁵ ion (S = 1/2), confirming a J = 1/2 ground state influenced by spin–orbit coupling (SOC).⁵⁶

Fig. 6 (a) Temperature dependent magnetic susceptibility curve of 1. The inset graph is the dχ/dT curve of 1, (b) temperature dependent magnetic susceptibility curve of 2. The inset graph is the dχ/dT curve, (c) thermal dependence of the χ_MT products, and (d) field dependence of the magnetization for 1 and 2.

Table 2 Data parameters of CW fit for compounds 1 and –2

Compound	m eff (BM)	θ CW (K)	χ 0 (emu mol^−1 Oe^−1)
(EDA)2RuCl6·Cl·H2O (1)	1.78	−2.99	−2.52 × 10^−4
(PDA)2RuCl6·2Cl·3H2O·(H3O) (2)	1.80	0.83	3.39 × 10^−5

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The dχ/dT curve displays a discontinuity with a dip around 3.92 K (Fig. 6a inset graph), which can be considered as the onset of antiferromagnetic ordering temperature. The χ_MT product (Fig. 6c) remains nearly constant, supporting localized single-ion behaviour. The isothermal M(H) curve at 2 K (Fig. 6d and S14) is nonlinear, further supporting the presence of weak AFM interactions. These results align with previous studies on Ru-based halide compounds, particularly those involving Ru³⁺ (d⁵). Lu et al.²¹ showed that Ru³⁺ compounds with isolated [RuCl₆]³⁻ octahedra, such as (HMA)₄RuCl₆·Cl, (HGly)₄RuCl₆·Cl, and (HGly)₃RuCl₆·2H₂O, exhibit μ_eff values of around 1.74–1.76 (Table S9) μ_B and nearly zero Weiss temperatures, indicating robust J = 1/2 behaviour under strong SOC.

Compound 2 shows a similar magnetic behaviour to compound 1. Although it also follows paramagnetism, the Weiss temperature θ_CW = 0.83 K and Curie constant C = 0.41 emu Oe⁻¹ K mol⁻¹ (Fig. 6b and Table 1) give an effective moment of μ_eff = 1.80μ_B, an expected value for a non-magnetic J = 1/2 ground state of a Ru³⁺ (d⁵) ion. Importantly, the dχ/dT curve shows a sudden dip at ∼2 K (Fig. 6b inset graph); however, since the temperature window does not extend lower, the curve could not be followed further to confirm whether it recovers at lower temperatures to exhibit any long-range ordering. This behaviour suggests very weak or negligible AFM ordering. The χT product is nearly temperature-independent. The M(H) isotherm at 2 K is almost linear with the field, consistent with 1 (Fig. 6d and S15). Despite our efforts to perform magnetic characterization of 3, the presence of minor impurities and its pronounced sensitivity to air and moisture restricted the low-temperature magnetic measurements of 3.

Experimental section

Materials and methods

Ethylenediamine dihydrochloride (EDA) (Sigma Aldrich), 1,3-diaminopropane (PDA) (TCI), 1,4-diaminobutane (BDA) (TCI), RuCl₃ (Sigma Aldrich), and 37 wt% HCl in H₂O (Merck) were purchased from commercial sources and used as received. All compounds were synthesized hydrothermally in a 23 mL Teflon-lined autoclave. The crystals were separated by filtration, washed several times with ethanol and dried under vacuum.

Synthesis of (EDA)₂RuCl₆·Cl·H₂O (1). Ethylenediamine dihydrochloride (133 mg, 1.0 mmol), RuCl₃ (103 mg, 0.5 mmol) and 37 wt% aqueous HCl (3.0 mL) were taken in a 23 mL Teflon vial. The Teflon vial containing the reaction mixture was closed and placed in a stainless-steel autoclave, and heated in a programmable oven at 160 °C for 48 h. The autoclave was cooled to room temperature to obtain dark red crystals. The crystals were filtered out of the acid mother liquor, washed with ethanol, and dried in a vacuum at 55 °C for 25 minutes. Yield: 202 mg, 68%, based on Ru.

Synthesis of (PDA)₂RuCl₆·2Cl·3H₂O·(H₃O) (2). 1,3-Diaminopropane (74 mg, 1.0 mmol), RuCl₃ (103 mg, 0.5 mmol) and 37 wt% aqueous HCl (3.0 mL) were taken in a 23 mL Teflon vial. The Teflon vial containing the reaction mixture was closed and placed in a stainless-steel autoclave, and heated in a programmable oven at 160 °C for 48 h. The autoclave was cooled to room temperature to obtain dark red crystals. The crystals were filtered out of the acid mother liquor, washed with ethanol, and dried in a vacuum at 55 °C for 25 minutes. Yield: 168 mg, 46%, based on Ru.

Synthesis of (BDA)₂RuCl₆·2Cl·H₂O·(H₃O) (3). 1,4-Diaminobutane (88 mg, 1.0 mmol), RuCl₃ (103 mg, 0.5 mmol) and 37 wt% aqueous HCl (3.0 mL) were taken in a 23 mL Teflon vial. The Teflon vial containing the reaction mixture was closed and placed in a stainless-steel autoclave, and heated in a programmable oven at 160 °C for 48 h. The autoclave was cooled to room temperature to obtain dark red crystals. The crystals were filtered out of the acid mother liquor, washed with ethanol, and dried in a vacuum at 55 °C for 25 minutes. Yield: 180 mg, 50%, based on Ru.

Single-crystal X-ray diffraction and refinement

Suitable single-crystals of 1–3 were selected under a microscope, covered with protective oil and mounted on a 50 µm loop. Data collection was performed on a Bruker D8 Quest X-ray diffractometer (Bruker AXS Inc., Madison, WI, USA) using Mo-Kα radiation (λ = 0.71073 Å). SCXRD data were acquired at 300 K. The data were collected using ω scans, with the collection performed using APEX2,⁵⁷ cell refinement carried out with SMART, data reduction performed using SAINT^57,58 and experimental absorption correction performed with SADABS.⁵⁹ The structure was solved by direct methods (SHELXS-2018), refinement was achieved by full-matrix least squares on F² using the SHELXL-2018 program suite,⁶⁰ and the graphical user interface (GUI) ShelXle was used.⁶¹ All amine molecules were refined as follows: hydrogen atoms of the ammonium groups were positioned geometrically (N–H = 0.89 Å) and refined using a riding model (AFIX 137) with U_iso(H) = 1.5U_eq(O). Hydrogen atoms of the carbon groups of the diamines were positioned geometrically (C–H = 0.97 Å) and refined using a riding model (AFIX 23) with U_iso(H) = 1.2U_eq(C). The structures were visualized using VESTA.

Powder X-ray diffraction

Single crystals of 1–3 were ground into powder in an agate mortar and pestle and the powder diffraction data were obtained on a Bruker D8 Advance diffractometer equipped with a Cu–Kα X-ray source (wavelength = 1.54056 Å). The experimental PXRD patterns were compared with the patterns simulated from the CIFs from single-crystal X-ray diffraction data in order to verify the phase purity of the bulk samples.

X-ray photoelectron spectroscopy (XPS)

The chemical compositions and the chemical environments of compounds 1–3 were further ascertained using X-ray photoelectron spectroscopy. The XPS samples were prepared by grinding crystals of each sample into fine powder. Measurements of samples 1–3 were performed using a Thermo Fisher Scientific, K-Alpha model. The data were fitted using Origin Pro 2024 software.

Infrared spectroscopy

A PerkinElmer spectrophotometer was used to record the Fourier transform infrared (FTIR) spectra of the compounds in the range of 4000 to 450 cm⁻¹.

Scanning electron microscopy (SEM)

SEM images of the morphology and energy-dispersive X-ray spectra were obtained using a JEOL model JSM-7900F. An acceleration voltage of 20 kV and an acceleration current of 38 μA were used. The SEM images of the hybrid perovskites show a crystalline morphology with sizes in microns.

Thermogravimetric analysis (TGA)

TGA of all compounds was carried out under continuous flow of nitrogen gas (flow rate; 25 mL per minute) using a TGA instrument. The samples (10–11 mg) were heated in aluminium crucibles at a temperature ramp rate of 10 °C min⁻¹.

Optical measurements

For powder absorption, powder samples were first obtained by grinding single crystals using a mortar and pestle. Linear optical absorption spectra were obtained by performing optical diffuse reflectance measurements using a Shimadzu UV-1800 spectrometer operating in the 1200–200 nm region at room temperature. BaSO₄ was used as the reference of 100% reflectance and to dilute powder samples for all measurements.

Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded using a Jobin Yvon Fluoromax⁺ spectrofluorometer (HORIBA Company) equipped with a xenon lamp in the range of 200 to 750 nm.

Magnetic measurements

Magnetic susceptibility was measured using a Quantum Design MPMS-3 superconducting quantum interference device (SQUID-VSM) magnetometer equipped with a pulse tube cooler, which eliminates the need for liquid cryogens. Powdered samples of 1 and 2 were mounted in plastic caps on a brass rod. Zero-field cooled and field cooled measurements were carried out on warming from 2 K to 300 K under a constant field of 1000 Oe. The magnetic susceptibility χ versus temperature (χT) was transformed into the effective magnetic moment using the Curie's law

Conclusions

In summary, we have designed and structurally characterized three new hybrid ruthenium halide perovskites namely (EDA)₂RuCl₆·Cl·H₂O (1), (PDA)₂RuCl₆·2Cl·3H₂O·(H₃O) (2) and (BDA)₂RuCl₆·2Cl·H₂O·(H₃O) (3) employing a hydrothermal synthesis method. The X-ray diffraction studies of 1–3 suggest the absence of octahedral connectivity, showcasing isolated RuCl₆ octahedra within the framework. TGA reveals that the thermal stability of hybrid ruthenium halides is strongly governed by the nature of the organic diamine cations, with the lattice water content and cation rigidity synergistically modulating decomposition temperatures, offering a tunable route to engineer stability in hybrid metal halide frameworks. Exploration of UV-visible spectroscopy analysis revealed absorption edges ranging from 1.95 to 1.92 eV, attributed to varied amine chain lengths (EDA → PDA → BDA). The PL studies also support their 0D nature along with their potential applications in deep blue LEDs. Low temperature magnetic measurements of 1 and 2 with a low-spin J = 1/2 state reveal excellent agreement with the Curie–Weiss behaviour. These findings highlight the robustness of single-ion physics in 1 and 2 with isolated Ru³⁺ octahedra. This study expands the functional landscape of hybrid perovskite halides beyond conventional optoelectronic applications, showcasing their potential in magneto-optoelectronic materials.

Author contributions

H. P. S.: carried out the experiments, data analysis and curation and wrote the draft of the manuscript. A. L.: data analysis and curation and review of the manuscript. K. A.: data analysis and curation. B. D.: solid UV-visible analysis. A. T.: data analysis and curation and review of the manuscript. S. K. D. and B. G. H.: single crystal X-ray diffraction measurement and review and editing. P. P. M.: project design, conceptualization, methodology, validation, data curation, manuscript writing and revision, supervision, and project administration.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

Data supporting this article are available in the supplementary information (SI). Supplementary information: all experimental data: FT-IR, P-XRD, SEM, XPS, TGA and Sc-XRD data for compounds 1–3. See DOI: https://doi.org/10.1039/d5dt02797g.

CCDC 2463264–2463266 contain the supplementary crystallographic data for this paper.^62a–c

Acknowledgements

The authors sincerely thank the DST for financial support under the DST-PURSE project (SR/PURSE/2022/143 (C)) and DST-FIST project (SR/FST/CS-I/2020/152). We acknowledge STIC Cochin, CSIR-NEIST, IIT Guwahati, SIF BITS Pilani and CSIC Dibrugarh University for various analytical support. PPM thanks the Central Instrumentation Facility (CIF) at IISER Bhopal for providing access to the SQUID-VSM. The authors gratefully acknowledge the unwavering support and sacrifice of PPM's mother, Late Bichitra Borah, and HPS's father, Late Probin Saikia, whose guidance and dedication continue to inspire this work.

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62. (a) CCDC 2463264: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2np74r; (b) CCDC 2463265: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2np75s; (c) CCDC 2463266: Experimental Crystal Structure Determination, 2026,  DOI:10.5517/ccdc.csd.cc2np76t.

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