Crystal structures of the tetrachloridoaluminates(III) of rubidium(I), silver(I), and lead(II)

2.1 Preparative results and role of the ionic liquid

The dissolution of AgCl, RbCl, or PbCl₂ in the Lewis-acidic ionic liquid [BMIm]Cl·4AlCl₃ (BMIm=1-n-butyl-3-methylimidazolium) at 180–200°C and subsequent cooling of the reaction mixture to room temperature resulted in the precipitation of colorless, air sensitive, plank-shaped crystals of Ag[AlCl₄], Rb[AlCl₄] or Pb[AlCl₄]₂. Crystals of the silver and rubidium compound were not obtained at all or only in insufficient quality for single-crystal measurements by reactions in pristine AlCl₃, whereas proper crystallization of the lead salt was also successful in an AlCl₃ melt. The role of the IL can be classified as that of a solvent, supporting the crystal growth, as well as of a reagent, providing the [AlCl₄]⁻ ions for all three compounds.

2.2 The crystal structure of Ag[AlCl₄]

X-ray diffraction analysis on a single crystal of silver(I)-tetrachloridoaluminate(III) revealed a monoclinic structure in the space group P2₁/c (no. 14) with four formula units per unit cell and lattice parameters a=711.8(1), b=661.1(1), c=1343.4(2) pm and β=92.26(1)° at T=296(1) K. Atomic parameters and interatomic distances are listed in Tables S1 and S2 of the Supporting Information available online. In the structure of Ag[AlCl₄], each silver atom is surrounded by two η²- and two η¹-coordinating [AlCl₄]⁻ tetrahedra, forming a distorted octahedral coordination sphere of chloride ions (Fig. 2). The coordination number of six is unusually high for silver(I) ions, as it is only found in about 4% of its coordination compounds, according to the Inorganic Crystal Structure Database (ICSD) [38].

Fig. 2:

(a) Crystal structure of Ag[AlCl₄]. (b) Distorted octahedral coordination of the silver ion formed by chloride ions of four [AlCl₄]⁻ tetrahedra. Ellipsoids comprise 80% of the probability densities of the atoms at T=296 K.

The position of the silver ions in the structure of Ag[AlCl₄] could not be determined reliably with the use of a second-order displacement tensor. It was necessary to introduce a second atomic position instead, to compensate for the irregular shape of the electron density, which can be attributed to an increased oscillation of the atom in the soft anionic structure of [AlCl₄]⁻ tetrahedra. The Ag–Cl distances range from 262.3(3) to 305.2(3) pm, which deviates significantly from the 277.4(1) pm observed in AgCl [39]. When compared to the benzene stabilized C₆H₆-Ag[AlCl₄] however, the interatomic distances match almost perfectly, with a range of 259(2)–304(3) pm observed in the organometallic compound [40]. The [AlCl₄] ⁻ tetrahedra are slightly distorted, with Al–Cl distances of 212.3(2)–215.1(2) pm. This distortion is a common phenomenon for compounds containing tetrahedral anions and has also been observed in other tetrachloridoaluminates, e.g. Li[AlCl₄], Na[AlCl₄], or K[AlCl₄] [1], [17], [23]. Therefore, it will not be mentioned further in the following. When considering only silver(I) and aluminum(III) cations, the structure of Ag[AlCl₄] can be regarded as a distorted derivate of the β-tin structure type (Fig. S2a). In Ag[AlCl₄], two of these networks are interwoven and inverted against each other (Fig. S2b). Alternatively, the structure can be described as a distorted hexagonal close packing of chloride ions whose hexagonal layers are parallel to the (101) plane. Silver(I) ions occupy 1/4 of the octahedral voids and aluminum atoms fill 1/8 of the tetrahedral voids, respectively (Fig. S2c).

It might be assumed that Cu[AlCl₄] has a crystal structure similar to that of Ag[AlCl₄], however, the lighter homolog forms a tetragonal structure – space group P4̅2c – that shows a slightly distorted cubic close packing of chloride ions with copper ions occupying 1/4 of the tetrahedral voids (Fig. 3a). Unexpectedly, there is a high degree of similarity between the crystal structures of Ag[AlCl₄] and Cu[AlCl₄]₂, i.e. the copper(II) salt (Fig. 3b). Both structures crystallize in the same space group type and both coinage metal ions are surrounded by six chloride ions, originating from four [AlCl₄]⁻ units, in a distorted octahedral shape. The distortion in Cu[AlCl₄]₂ was attributed to the Jahn-Teller effect, resulting in an elongation of the axial Cu–Cl distances. Therefore, the η²-coordinating [AlCl₄]⁻ ions occupy the equatorial coordination plane while the axial ligands only bind by a vertex to the copper(II) ion. Contrary to that, there is no such ordered “functionality” of [AlCl₄]⁻ ions in Ag[AlCl₄]; no distinction between equatorial and axial ligands can be made based on the coordination mode of the [AlCl₄]⁻ ligands (Fig. 3c).

Fig. 3:

Crystal structures of: (a) Cu[AlCl₄], (b) Cu[AlCl₄]₂ and (c) Ag[AlCl₄]. Ellipsoids comprise 99.99% of the probability density of atoms at T=296 K (a) or 80% at 296 K (b, c).

2.3 The crystal structure of Rb[AlCl₄]

Mairesse et al. analyzed the crystal structure of Rb[AlCl₄] based on powder X-ray diffraction data in 1979 [7] and suspected it to belong to the baryte structure type [41]. However, they were unable to obtain single crystals for a full structural analysis, which has now been successful through the use of ILs.

Single-crystal X-ray diffraction experiments revealed the orthorhombic crystal structure of rubidium(I)-tetrachloridoaluminate in the space group Pnma (no. 62) with four formula units per unit cell and lattice parameters a=1114.8(2), b=708.9(1) and c=926.3(1) pm at 296(1) K. Atomic parameters and interatomic distances are listed in Tables S3 and S4 of the Supporting Information. The crystal structure of Rb[AlCl₄] is composed of rubidium(I) cations and tetrahedral [AlCl₄]⁻ anions (Fig. 4a). In fact, the assumption made by Mairesse et al. is correct: Rb[AlCl₄] is isostructural to BaSO₄ (Fig. 4b). Therefore, its relation to other tetrachloridoaluminates of monovalent cations, such as Cs[AlCl₄] [7] or Tl[AlCl₄], [4] which also crystallize in the baryte structure type, is apparent.

Fig. 4:

Isotypic crystal structures of: (a) Rb[AlCl₄] and (b) BaSO₄ [41]. Ellipsoids comprise 70% of the probability density of atoms at 296 K. For BaSO₄, no anisotropic displacement parameters were given.

Each rubidium atom is surrounded by 12 chloride ions in an irregular polyhedral sphere that can be described as a pentagon below and a capped hexagon above the central atom (Fig. 5). Both polygons are heavily distorted, as already reported for the baryte structure [42]. Corresponding to the high coordination number (c.n.=12), the Rb–Cl distances in this coordination sphere range from 336.7(2) pm to 433.5(2) pm (average 374(32) pm), showing a significant increase compared to the 329.1(2) pm in RbCl (c.n.=6) [43] and to that of other tetrachloridometalates, such as Rb₂[ZnCl₄] (c.n.=7; 8) or Rb₂[CdCl₄] (c.n.=9), in which the distances range from 321.8(1) to 386.1(1) pm [44], [45]. However, all interatomic distances in Rb[AlCl₄] exceeding this previously observed range, are secondary bonds to chloride ions that belong to an already coordinating [AlCl₄]⁻ ion, rendering it a bidentate chelating ligand.

Fig. 5:

The irregular coordination environment around the [RbCl₁₂]¹¹⁻ core unit built from seven [AlCl₄]⁻ ions in Rb[AlCl₄]. Ellipsoids comprise 70% of the probability density of atoms at T=296 K.

2.4 The crystal structure of Pb[AlCl₄]₂

In 2012, Müller successfully crystallized Pb[AlCl₄]₂ from a pristine AlCl₃ melt [34]. Single-crystal diffraction performed on these crystals revealed an orthorhombic structure that he categorized to be isotypic to α-Sr[GaCl₄]₂. X-ray analysis of single crystals that precipitated from IL confirmed the structure model by Müller. Thus, the ionic liquid has no influence on the crystal structure and simply acts as a solvent and reactant. The crystal structure of lead(II)-bis(tetrachloridoaluminate) is orthorhombic with the space group Pbca (no. 61), eight formula units per unit cell and lattice parameters a=1220.2(1), b=1034.4(1) and c=2022.0(2) pm at 296(1) K. Atomic parameters and interatomic distances are listed in Tables S5 and S6 of the Supporting Information. In the crystal structure each Pb²⁺ ion is surrounded by nine chloride ions belonging to one η¹- and four η²-coordinating [AlCl₄]⁻ tetrahedra. The coordination of the lead cation can be described as a significantly distorted variant of either a tricapped trigonal prism or a capped square-antiprism (Fig. 6b). The Pb–Cl distances in Pb[AlCl₄]₂ range from 286.1(1) to 345.3(1) pm, which is very similar to the distance range in PbCl₂ (286(4)–364(4) pm) [46]. Moreover, the two coordination polyhedra of the lead cations are almost identical, with somewhat reduced distortion of the tricapped trigonal prism in PbCl₂ (Fig. 6c).

Fig. 6:

(a) Crystal structure of Pb[AlCl₄]₂. (b) and (c) Coordination of nine chloride ions in a distorted tricapped prism around a lead(II) cation in the crystal structure of 3 and PbCl₂, respectively. Ellipsoids comprise 90% of the probability density of atoms at 296 K. No anisotropic displacement parameters were given for PbCl₂ [46].

To the best of our knowledge, the only other compound containing lead(II) ions coordinated to complex chloridometalate anions is the isostructural Pb[GaCl₄]₂, which was also mentioned by Müller [34]. All Pb–Cl distances in the crystal structure of the gallium compound are up to 10 pm shorter than in the aluminum analog, while still giving rise to the same coordination environment. The Ga–Cl distances are about 4–5 pm longer than the Al–Cl distances, and the unit cell volume differs by less than 2%. Therefore, the differences between the aluminate and the gallate can be interpreted as a shift of the chloride ions, caused by the lower electronegativity of aluminum (Pauling values: Pb 1.9, Ga 1.8, Al 1.6). As mentioned before, Pb[AlCl₄]₂ is isotypic to α-Sr[GaCl₄]₂, [34] which also means that it is a derivative of the cuprite structure type. The aluminum atoms, representing the center of the anionic tetrahedra, replace the copper(I) ions, whereas the lead(II) ions take the positions of the oxide anions. In accordance with the lower symmetry, the packing is slightly distorted, but the two interwoven cristobalite networks can still be recognized within the structure of Pb[AlCl₄]₂ (Fig. S3). It might be somewhat surprising that Pb[AlCl₄]₂ is isotypic to α-Sr[GaCl₄]₂ rather than to RT-Sr[AlCl₄]₂ [15], [25]. Both strontium compounds crystallize in the orthorhombic space group type Pbca. However, the aluminate shows a coordination environment for the strontium(II) ions that comprises four η²-coordinating [AlCl₄]⁻ tetrahedra with the next nearest chloride ligand lying over 450 pm away from the central atom, making a nine-fold coordination, as described for the lead compound, improbable. Furthermore, the room-temperature modification of Sr[AlCl₄]₂ is derived from a tetragonal high-temperature structure – space group I4₁/acd – that exists above 180°C [25]. As a result, the unit cell of RT-Sr[AlCl₄]₂ shows only minimal deviation from tetragonal metrics with ∆(a–b)=15 pm compared to the ∆(a–b)=186 pm observed for the unit cell of Pb[AlCl₄]₂, which makes finding a direct structural relation between those two compounds difficult. It might be argued that the lone pair of lead(II) plays a role in that.

4.1 Synthesis

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 syntheses took place in the ionic liquid [BMIm]Cl·4AlCl₃, which acted as solvent and reactant.

For the synthesis of Ag[AlCl₄], the ampoule was loaded with 97.4 mg AgCl (0.68 mmol, 99.998%, Alfa Aesar), 151.6 mg [BMIm]Cl (0.87 mmol, 98%, Sigma Aldrich, dried under vacuum at 100°C), and 454.1 mg AlCl₃ (3.4 mmol, sublimed three times). The evacuated and sealed ampoule was heated at 180°C for 60 h.

Rb[AlCl₄] was obtained from a mixture of 82.0 mg RbCl (0.68 mmol, 99%, abcr), 150.6 mg [BMIm]Cl (0.86 mmol, 98%, Sigma Aldrich, dried under vacuum at 100°C), and 450.9 mg AlCl₃ (3.38 mmol, sublimed three times). The evacuated and sealed ampoule was heated at 180°C for 60 h.

The synthesis of Pb[AlCl₄]₂ was realized by mixing 186.6 mg PbCl₂ (0.67 mmol, 98%, Sigma Aldrich), 150.0 mg [BMIm]Cl (0.86 mmol, 98%, Sigma Aldrich, dried under vacuum at 100°C), and 450.0 mg AlCl₃ (3.38 mmol, sublimed three times). The evacuated and sealed ampoule was heated at 200°C for 24 h.

The mixture was cooled to room temperature at ΔT/t=−6 K h⁻¹. All title compounds were obtained as colorless, plank shaped, air sensitive crystals alongside with recrystallized hexagonal AlCl₃ platelets. The IL was removed by washing the products with dry dichloromethane under inert gas conditions three times. The product of the Ag[AlCl₄] synthesis could not be treated under these conditions as the crystals decayed upon contact with dichloromethane. All products were obtained in an estimated yield of 50–60%, being contaminated only by a recrystallized of AlCl₃ present in excess. Respective powder diffractograms can be found in Figs. S4–S6 of the Supporting Information.

4.2 EDX analysis

Energy dispersive X-ray (EDX) spectroscopy was employed to check the chemical composition of the crystals, using a SU8020 (Hitachi) SEM equipped with a Silicon Drift Detector (SDD) X-Max^N (Oxford). However, several problems impeded the interpretation of the measured data. Since the samples could not be polished due to their high sensitivity to moisture, the surface of the crystals was uneven and the crystals themselves tilted. Furthermore, the compound partially decomposes in the high-energetic electron beam (U_a=25 kV) that is necessary to activate the metal atoms for this measurement. EDX analysis was therefore mainly used as a qualitative analysis to confirm the suspected composition. In that regard, we were able to confirm the examined crystals to be ternary compounds containing aluminum, chlorine and the respective second metal. Impurities in the form of oxygen were also detected, which can be attributed to the carbon pad used for preparation of the single crystals under inert atmosphere. These pads are known to trap large amounts of oxygen, which we were unable to remove under dynamic vacuum even after long treatment times.

4.3 X-ray crystal structure determination

Single-crystal X-ray diffraction was measured on a four-circle Kappa APEX II CCD diffractometer (Bruker) with a graphite(002) monochromator and a CCD detector at T=296(1) K. MoKα radiation (λ=71.073 pm) was used. After integration [47], a multi-scan absorption correction was applied by using Sadabs [48] within the Bruker Apex3 software suite [49]. The initial structure solution was performed with Shelxt [50] and further refinement processed in Shelxl against F_o² [51], [52].

Ag[AlCl₄]: monoclinic; space group P2₁/c (no. 14); T=296(1) K; a=711.8(1), b=661.1(1), c=1343.4(2) pm, β=92.26(1)°, V=631.8(1)×10⁶ pm³; Z=4; ρ_calcd.=2.91 g cm⁻³; μ(MoKα)=4.9 mm⁻¹; 2θ_max=51.8°, −5≤h≤8, −8≤k≤8, −16≤l≤16; 7517 measured, 1227 unique reflections, R_int=0.047, R_σ=0.030; 65 parameters, R₁ [878 F_o>4σ(F_o)]=0.040, wR₂ (all F_o²)=0.083, GooF=1.11, min./max. residual electron density: −0.45/0.53 e×10⁻⁶ pm⁻³. For atomic parameters see Tables S1 and S2 of the Supporting Information.

Rb[AlCl₄]: orthorhombic; space group Pnma (no. 62); T=296(1) K; a=1114.8(1), b=708.9(1), c=926.3(1), V=732.0(2)×10⁶ pm³; Z=4; ρ_calcd.=2.31 g cm⁻³; μ(MoKα)=8.2 mm⁻¹; 2θ_max=53.4°, −14≤h≤14, −8≤k≤5, −11≤l≤11; 8344 measured, 846 unique reflections, R_int=0.049, R_σ=0.024; 34 parameters, R₁ [601 F_o>4σ(F_o)]=0.039, wR₂ (all F_o²)=0.098, GooF=1.01, min./max. residual electron density: −0.48/0.90 e×10⁻⁶ pm⁻³. For atomic parameters see Tables S3 and S4 of the Supporting Information.

Pb[AlCl₄]₂: orthorhombic; space group Pbca (no. 61); T=296(1) K; a=1220.2(1), b=1034.4(1), c=2022.0(2) pm; V=2551.9(3)×10⁶ pm³; Z=8; ρ_calcd.=2.84 g cm⁻³; μ(MoKα)=15.0 mm⁻¹; 2θ_max=70.2°, −19≤h≤18, −15≤k≤16, −31≤l≤32; 38501 measured, 5638 unique reflections, R_int=0.043, R_σ=0.029; 101 parameters, R₁ [3952 F_o>4σ(F_o)]=0.028, wR₂ (all F_o²)=0.038, GooF=1.165, min./max. residual electron density: −1.59/1.72 e×10⁻⁶ pm⁻³. For atomic parameters see Tables S5 and S6 of the Supporting Information.

Further details of the crystal structure determination are available from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (Germany), E-mail: crysdata@fiz-karlsruhe.de, on quoting the deposition numbers CSD-1958743 (Ag[AlCl₄]), CSD-1958744 (Rb[AlCl₄]), and CSD-1958745 (Pb[AlCl₄]₂).