Low-Temperature Ordering in the Cluster Compound (Bi₈)Tl[AlCl₄]₃

Abstract

The reaction of Bi, BiCl₃, and TlCl in the ionic liquid [BMIm]Cl·4AlCl₃ (BMIm = 1-n-butyl-3-methylimidazolium) at 180 °C yielded air-sensitive black crystals of (Bi₈)Tl[AlCl₄]₃. X-ray diffraction on single crystals at room temperature revealed a structure containing ${}_{\infty }{}^1{\left[\mathrm{Tl}\right(\mathrm{AlCl}}_4{)}_3]{}^{2-}$ strands separated by isolated Bi₈²⁺ square antiprisms. The thallium(I) ion is coordinated by twelve Cl⁻ ions of six [AlCl₄]⁻ groups, resulting in a chain of face-sharing [TlCl₁₂]¹¹⁻ icosahedra. The Bi₈²⁺ polycation is disordered, simulating a threefold axis through its center and overall hexagonal symmetry (space group P6₃/m). Slowly cooling the crystals to 170 K resulted in increased order in the Bi₈ cluster orientations. An ordered structure model in a supercell with a’ = 2a, b’ = 2b, c’ = 3c and the space group P6₅ was refined. The structure resembles a hexagonal perovskite, with complex groups in place of simple ions.

1. Introduction

The crystal structure of bismuth monochloride (Bi₆Cl₇), reported by Corbett and Hershaft in 1962 [1], was the first example of a solid containing a homonuclear bismuth polycation. Ever since then, a number of compounds have been synthesized, showing bismuth clusters of various sizes and shapes either isolated [1,2,3,4,5,6,7] or coordinating to electron-rich transition metals like Au, Cu, or Ru [8,9,10,11,12,13]. These cationic clusters typically form in bismuth-rich halide melts. In many cases, an additional Lewis acid—mostly MX₃ with M = Al, Ga and X = Cl, Br, I—was used in the synthesis, resulting in the crystallization of salts, including the corresponding [MX₄]⁻ anions [14]. The weakly coordinating nature and the high symmetry of these tetrahedral anions enable thermally activated reorientation of the polyhedral bismuth cations. Since the symmetry of the coordination environment of the bismuth polycation is typically higher than its point group, (almost) energetically equivalent orientations can be adopted. Therefore, orientational disorder is a common phenomenon in such structures [2,6,9,10,12,15,16,17]. This ranges from resolvable disorder on a few alternative positions [12] to the formation of plastic crystals, where the superposition of orientations in time and space creates an averaged electron density resembling a hollow sphere [17].

Herein we report the ionic liquid (IL) [18] based synthesis and structure determination of the bismuth cluster compound (Bi₈)Tl[AlCl₄]₃ (1). It shows hexagonal pseudosymmetry at room temperature through a dynamic rotational disorder of Bi₈²⁺ polycations. Upon slowly cooling the crystals to 170 K, the cluster orientations freeze into a long-range ordered superstructure with an enlarged unit cell of lower symmetry.

2. Results and Discussion

2.1. Synthesis and Role of the Ionic Liquid

The reaction of stoichiometric amounts of Bi, BiCl₃, and TlCl in the Lewis-acidic ionic liquid [BMIm]Cl∙xAlCl₃ (BMIm = 1-n-butyl-3-methylimidazolium; 3 ≤ x ≤ 5) at 180 °C resulted in shiny black, air-sensitive, needle-shaped crystals of (Bi₈)Tl[AlCl₄]₃ (1). The compound did not form when the same mixture of starting materials was heated in pristine AlCl₃. Obviously, the IL is necessary for product formation. Moreover, the amount of AlCl₃ used for the formation of the IL plays an important role, as had been observed in previous cases [19]. Empirically, we found that three to five equivalents of AlCl₃ per [BMIm]Cl are needed for the successful synthesis of (Bi₈)Tl[AlCl₄]₃. The Lewis acidity of the reaction medium as well as the precipitation of solids depend on complex, temperature- and concentration-dependent equilibria including Cl⁻, AlCl₃, and Lewis acid–base adducts [Al_nCl_3n+1]⁻ (n = 1–3).

2.2. Crystal Structure of (Bi₈)Tl[AlCl₄]₃ at Room Temperature

X-ray diffraction on a single crystal revealed that octabismuth(2+)-thallium(I)-tris[tetrachloridoaluminate(III)] crystallizes in the hexagonal space group P6₃/m (no. 176) with two formula units per unit cell and lattice parameters a = 1303.3(1) pm and c = 1039.9(1) pm at room temperature. Atomic parameters and interatomic distances are listed in Tables S1 and S2 of the Supplementary Materials. The crystal structure (Figure 1) is composed of homonuclear, square antiprismatic Bi₈²⁺ polycations, and tetrahedral [AlCl₄]⁻ ions that are coordinating thallium(I) cations, to form ${}_{\infty }{}^1{\left[\mathrm{Tl}\right(\mathrm{AlCl}}_4{)}_3]{}^{2-}$ strands extending along [001].

The thallium(I) ion on the 6₃ screw axis (point symmetry $\overline{3}$) is surrounded by twelve chloride ions with distances of 337.1(3) pm and 373.8(3) pm, which fall in the range of Tl–Cl distances found in Tl[AlCl₄] (332.0–389.3 ppm) [20]. Six nearer chloride ions form a trigonal antiprism that is expanded to a distorted icosahedron by six more distant chloride ions (Figure 2a). The Tl···Tl distance of 519.9(1) pm suggests no direct interaction between these atoms.

The center of gravity of the Bi₈²⁺ polycation is located on the threefold rotation axis (point symmetry $\overline{6}$). This site symmetry does not comply with the D_4d symmetry of a square antiprism and thus gives rise to orientational disorder. Extensive structure refinements in space groups with lower symmetry (down to P112₁), including (multiple) twinning, did not resolve the apparent disorder (see Supplementary Materials) and no indications for a larger unit cell were found. We thus used space-group symmetry P6₃/m to describe the disordered room temperature structure.

The Bi₈²⁺ polycation, an arachno-cluster according to modified Wade’s rules [21], is slightly distorted, showing Bi–Bi distances of 302.5(2) to 307.1(2) pm (average 305.0(2) pm) along the square faces and 305.0(3) to 315.9(2) pm (average 309.6(3) pm) between them. The average distances are slightly shorter than those reported by Beck et al. for (Bi₈)[AlCl₄]₂ (309 and 311 pm) [7], which can be attributed to the strong librations that are insufficiently modeled by the harmonic approximations for individually vibrating atoms.

All orientations of the Bi₈²⁺ polycations are equivalent in energy because their anionic surrounding obviously obeys high point group symmetry. A voluminous distorted snub cube—one of the 13 Archimedean solids [22]—consisting of 6 square and 32 triangular faces is formed from 24 chloride ions (Figure 2b). The Bi–Cl distances range from 326(1) to 414(1) pm. This coordination polyhedron is rather uncommon and has, so far, been reported for only a few large organic molecules [23] and intermetallic phases [24,25,26], and was predicted as the shape of the hypothetical B₂₄H₂₄²⁻ cluster [27].

The average Al–Cl distance of 211.8(6) pm is about 2 pm shorter than the one found in compounds like Li[AlCl₄], Na[AlCl₄], or K[AlCl₄] [28,29,30]. It shows almost no deviation from the mean distance, which is unusual compared to the abovementioned structures or the Al–Cl distances in (Bi₈)[AlCl₄]₂, where at least one distance differs significantly or the [AlCl₄]⁻ tetrahedra are distorted completely [7].

Furthermore, the room temperature structure of (Bi₈)Tl[AlCl₄]₃ can be described by the hexagonal perovskite structure type, first observed in BaNiO₃, where each [AlCl₄]⁻ ion takes the position of the oxide ion, the Ni⁴⁺ ion is replaced by the Tl⁺ ion, and the center of the Bi₈²⁺ polycation is located on the Ba²⁺ position (Figure S1) [31,32]. (Bi₈)Tl[AlCl₄]₃ fits that structure type with a Goldschmidt tolerance factor [33] of about 0.86, using a simple geometric approximation for the radial dimensions of [AlCl₄]⁻ (r_x = 211.8(1) pm) and Bi₈²⁺ (r_A = 254(8) pm), alongside the ionic radius for Tl⁺ in twelvefold coordination (r_M = 170 pm) [34]. To the best of our knowledge, (Bi₈)Tl[AlCl₄]₃ presents the first example of a perovskite structure containing two complex ions.

2.3. Low-Temperature Structure of (Bi₈)Tl[AlCl₄]₃

To decrease the librations and vibrations in the structure, a single crystal was slowly cooled to 170(1) K over a period of 20 h. Additional weak reflections in its diffraction pattern indicated a supercell with a’ = 2a, b’ = 2b, and c’ = 3c (Figure 3). Furthermore, low-temperature DSC measurements were conducted to determine the ordering temperature. The cooling curve presented no discernable signal, probably due to the nature of the ordering process, where the rotation continuously slows down without any spontaneous lock-in transitions that would be visible in the heat flow. However, the heating curve revealed a weak endothermic signal at around 200 K which indicated the simultaneous start of rotation of all the clusters (Figure S2).

The structure refinement revealed a largely, but not fully, ordered structure with the chiral hexagonal space group P6₅ (no. 170), which is a subgroup of index 24 of the room temperature space group P6₃/m (Scheme S1). The symmetry reduction is accompanied by the formation of inversion twins (t2 transition) as well as antiphase domains (k4 and k3 transitions). The lattice parameters at 170(1) K are a = b = 2611.1(2) pm and c = 3122.2(2) pm. The unit cell comprises 24 formula units of (Bi₈)Tl[AlCl₄]₃. Atomic parameters and interatomic distances are listed in Tables S1 and S2 of the Supplementary Materials.

As can be expected, the basic structure remains the same, but the different orientations of the Bi₈²⁺ polycations are no longer superimposed along the c direction (Figure 4). The 6₅ screw axis runs through the thallium ions with x ≈ y ≈ 0, while there are 2₁ screw axes through the ${}_{\infty }{}^1{\left[\mathrm{Tl}\right(\mathrm{AlCl}}_4)]{}^{2-}$ strands with x and/or y ≈ 0.5. In each column along [001], the three orientations of the Bi₈²⁺ polycations alternate periodically. They still match local mirror symmetry perpendicular to the c-axis, which is, however, not global because of the mismatch with the chiral axis.

Compared to the room temperature structure determination, the distribution of interatomic distances in the bismuth polycations is narrower, with 304.9(1) to 308.9(1) pm (average 306.9(2) pm) along the square faces and 302.2(1) to 310.4(1) pm (average 306.6(3) pm) between them. Although the ${}_{\infty }{}^1{\left[\mathrm{Tl}\right(\mathrm{AlCl}}_4)]{}^{2-}$ strands remain virtually unchanged and, above all, preserve the pseudosymmetry of the room temperature structure, the [AlCl₄]⁻ tetrahedra accommodate the polycation ordering. The Tl–Cl distances now range from 325.8(1) to 366.7(1) pm, showing the expected temperature-induced shortening. The coordination of chloride ions around the bismuth polycations changed considerably. The previously applied description of a snub cube of 24 chloride ions around each cluster would only be applicable if the Bi–Cl distances for attractive interactions were to be extended to a distance of 446.4(1) pm, which corresponds to an increase of over 30 pm compared to the structure at room temperature. Instead, a smaller number of chloride ions in closer proximity to the polycations dominate, which seems to be key to the long-range order.

3. Materials and Methods

3.1. Synthesis Results and Role of the Ionic Liquid

All compounds were handled in an argon-filled glove box (M. Braun; p(O₂)/p⁰ < 1 ppm, p(H₂O)/p⁰ < 1 ppm). The reactions were carried out in silica ampoules with a length of 120 mm and a diameter of 14 mm. The synthesis took place in the ionic liquid [BMIm]Cl·4AlCl₃, which acted as solvent and reactant. The ampoule was loaded with 23.9 mg TlCl (99%, Sigma Aldrich, St. Louis, MO, USA), 153.1 mg Bi (99.9%, abcr, treated twice with H₂ at 220 °C), 21.3 mg BiCl₃ (98%, Alfa Aesar, Karlsruhe, Germany, sublimed three times), 150.4 mg [BMIm]Cl (98%, Sigma-Aldrich, dried under vacuum at 100 °C), and 451.7 mg AlCl₃ (sublimed three times). The evacuated and sealed ampoule was heated at 180 °C for 60 h. The ionic liquid turned black at this temperature. Before cooling the mixture to room temperature at ΔT/t = −6 K/h, the ampoule was tilted to enable separation of already precipitated byproducts. Black crystals of (Bi₈)Tl[AlCl₄]₃ were obtained alongside colorless Tl[AlCl₄], black (Bi₈)[AlCl₄]₂, red (Bi₅)[AlCl₄]₃, and colorless AlCl₃ in a roughly estimated yield of 40 to 50 wt %. The crystals of 1 could easily be mechanically separated from the byproducts, resulting in a “pure” product with residual IL sticking to the surface, causing a high background in the PXRD analysis (Figure S3). Alternatively, the product was washed three times with dry dichloromethane (DCM), resulting in a mixture whose PXRD diffractogram contained mostly 1 alongside small amounts of (Bi₈)[AlCl₄]₂ and a compound we have, so far, been unable to identify (Figure S4) whose reflections were only observed in samples that were treated with DCM. Apparently, the product(s) react with the solvent, forming an impurity in the course of the washing process. Single crystals of 1 were selected for diffraction experiments under inert conditions. The crystals were attached to the tip of glass capillaries (Hilgenberg, Malsfeld, Germany, 0.2 mm inner diameter) with water-free silicon grease (KEL-F^®) and encased with another capillary (Hilgenberg, 0.3 mm inner diameter). Both capillaries were molten together on an incandescent Kanthal wire and sealed with picein wax. For powder samples, a capillary (0.3 mm diameter) was filled with powder and sealed using the same procedure.

3.2. EDS Analysis

Energy-dispersive X-ray spectroscopy (EDS) was performed using a SU8020 SEM (Hitachi, Krefeld, Germany) equipped with a silicon drift detector (SDD) X-Max^N (Oxford Cryosystem, Oxford, UK) to check the chemical composition of the crystals. However, several issues impeded the interpretation of the measured data. Since the samples could not be polished, because of their high sensitivity to moisture, the crystal surfaces were not flat, which made finding a suitable surface for the measurement difficult. Furthermore, the [AlCl₄]⁻ ions partially decompose in the high-energy electron beam (U_a = 30 kV) that is necessary to activate bismuth for this measurement [13]. We thus focused on the ratio between thallium and bismuth. Regarding these values, we were able to support the ratio Tl/Bi = 1:8 within the error of the method. Calculated/experimental Tl/Bi/Al/Cl (atom %) in (Bi₈)Tl[AlCl₄]₃ = 4.2:33.3:12.5:50/1.6(3):17(3):30(3):51.6(7).

3.3. X-ray Crystal Structure Determination

Single-crystal X-ray diffraction was performed on a four-circle Kappa APEX II CCD diffractometer (Bruker, Karlsruhe, Germany) with a graphite (002) monochromator and a CCD detector at 296(1) and 170(2) K. Mo Kα radiation (λ = 71.073 pm) was used. Numerical absorption corrections based on an optimized crystal description were applied [35]. The initial structure solution was performed with JANA2006 [36,37]. The structure models were refined with SHELXL against F_o² [38,39].

3.3.1. Crystallographic Data for (Bi₈)Tl[AlCl₄]₃ at 296(1) K

Hexagonal; space group P6₃/m (no. 176); a = 1303.3(1) pm, c = 1039.8(1) pm, V = 1529.7(2) × 10⁶ pm³; Z = 2; ρ_calcd. = 5.173 g·cm⁻³; μ(Mo Kα) = 52.2 mm⁻¹; 2θ_max = 50°, −15 ≤ h ≤ 16, −16 ≤ k ≤ 15, −12 ≤ l ≤ 12; 18520 measured, 1077 unique reflections, R_int = 0.047, R_σ = 0.023; 63 parameters, R₁[799 F_o > 4σ(F_o)] = 0.039, wR₂(all F_o²) = 0.095, GooF = 1.04, min/max residual electron density: −2.10/2.25 e × 10⁻⁶ pm⁻³. Every bismuth position is occupied to one-third of its full occupancy, corresponding to the superposition of three orientations of the Bi₈²⁺ polycation. For atomic parameters, see Table S2 of the Supplementary Materials.

3.3.2. Crystallographic Data for (Bi₈)Tl[AlCl₄]₃ at 170(2) K

Hexagonal; space group P6₅ (no. 170); a = 2611.1(2) pm, c = 3122.2(2) pm; V = 18,433.3(2) × 10⁶ pm³; Z = 24; ρ_calcd. = 5.151 g·cm⁻³; μ(Mo Kα) = 52.0 mm⁻¹; 2θ_max = 52.2°, −31 ≤ h ≤ 31, −31 ≤ k ≤ 30, −37 ≤ l ≤ 37; 209738 measured, 21762 unique reflections, R_int = 0.169, R_σ = 0.178; 637 parameters, R₁[6278 F_o > 4σ(F_o)] = 0.049, wR₂(all F_o²) = 0.058, GooF = 0.932, min/max residual electron density: −2.70/2.07 e × 10⁻⁶ pm⁻³. 450 restraints: Three cluster positions show multiple orientations due to an incomplete process of ordering. The first and second positions were described as two clusters with 75.4% and 73.4% probability of occurrence for the main orientation, while the third one is split in three orientations with 54.5%, 23.8%, and 21.7% probability of occurrence, respectively. The Bi–Bi distances were restrained to be equal between equipositional bismuth atoms of each cluster with a 0.2 pm margin of error. Equal atomic displacement parameters were used for corresponding atoms of the [AlCl₄]⁻ anions, as well as several disordered bismuth atoms. For atomic parameters, see Table S3 of the Supplementary Materials.

Further details of the crystal structure determinations are available from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (Germany), E-mail: [email protected], upon quoting the depository numbers CSD-1898840 (296 K) and 1898841 (170 K).

3.4. Differential Scanning Calorimetry

In an argon-filled glove box (MBraun UNIlab, Garching, Germany), 20.73 mg of the sample was loaded and sealed in an Al container (40 µL). Heat-flux differential scanning calorimetry (DSC) was performed using a DSC 204 F1 Phoenix (Netzsch, Selb, Germany) in the temperature range from 109 to 283 K, with heating and cooling rates of 2 K/min under a dry N₂ atmosphere. The characteristic temperature derived from the DSC curve was an extrapolated onset temperature.

4. Conclusions

The new bismuth-rich metalate (Bi₈)Tl[AlCl₄]₃ consists of ${}_{\infty }{}^1{\left[\mathrm{Tl}\right(\mathrm{AlCl}}_4\left)\right]{}^{2-}$ strands and isolated Bi₈²⁺ polycations. The latter shows dynamic disorder at room temperature but order upon slow cooling to 170 K. Similar dynamics had been observed in other compounds with isolated polyhedral ions [2,6,9,10,12,15,16,17]. However, in most of those cases, cooling led either to a freezing of dynamic to static disorder or to low symmetric structures with enormously enlarged unit cells and multiple twinning that could not be resolved. The comparatively smooth ordering of (Bi₈)Tl[AlCl₄]₃ can thus be regarded a stroke of luck.