Ionothermal Synthesis of Sulfidobismuth spiro‐Dicubane Cations

Abstract Bi2S3 was dissolved in the presence of NaCl in the ionic liquid [BMIm]Cl ⋅ 4AlCl3 (BMIm=1‐n‐butyl‐3‐methylimidazolium) through annealing the mixture at 180 °C. Upon cooling to room temperature, orange, air‐sensitive crystals of Na(Bi7S8)[S(AlCl3)3]2[AlCl4]2 (1) precipitated. X‐ray diffraction on single‐crystals of 1 revealed a triclinic crystal structure that contains (Bi7S8)5+ spiro‐dicubanes, [S(AlCl3)3]2− tetrahedra triples, isolated [AlCl4]− tetrahedra, and sodium cations.


Synthesis Results and Substitution Attempts
A mixture of NaCl and Bi 2 S 3 in the molar ratio of 2 : 1 was dissolved in the ionic liquid [BMIm]Cl · 4AlCl 3 [18] and solidified AlCl 3 (Figure S1, Supporting Information). Energy dispersive X-ray spectroscopy (EDX) confirmed the composition, particularly the presence of sodium in 1.
Analogous syntheses using the chlorides of other alkali metals (M = K, Rb, Cs) yielded crystals of similar color, shape and composition, according to EDX measurements. X-ray diffraction experiments on these crystals showed a high degree of diffuse scattering on the reflection rows parallel the b* axis, as well as low Bragg intensities. We suppose stacking faults to cause these effects, which also affect the crystal structure of 1 but to a lesser extent. Although a full crystal structure analysis was impossible, at least approximate lattice parameters for the compounds with M = K, Cs could be determined (

Crystal Structure of Na(Bi 7 S 8 )[S(AlCl 3 ) 3 ] 2 [AlCl 4 ] 2
Sodium(I)-spiro [8,8] (2), [18] and Na + cations in the inter-layer space (d � 260 pm). While the silver compound 2 has a high layer symmetry that was reduced to P1 À through stacking, in the sodium compound 1, the molecular units themselves are distorted and the deviations from the layer group p � 3m1 (no. 72) [40] are larger than in 2. This goes hand in hand with an ordered distribution of the   monovalent cations in 1 and seems to be associated with the size and the polarizability of the M + cation. Among the discussed monovalent cations, Na + is the smallest and hardest (Shannon radii for sixfold coordination in pm: Na 102, Ag 115, K 138, Rb 152, Cs 167). [41] The unit cell of the sodium compound 1 is twice as large as that of the silver compound 2. Conventions for the setting of unit cells make b the stacking vector for 1, while it is c for 2. The thickness of the layer packages in both compounds differ only by few percent. The bases of the unit cells that are parallel to the layers are also related: While in 2, the a and b vectors define a rhombic grid, following the trigonal layer symmetry, the vectors a and c of 1 describe a rectangle, which corresponds to an orthohexagonal setting (a -c ffi ffi ffi 3 p = 0.9 pm), yet without centering. The reflections h + l = 2n + 1, which violate the reflection conditions for the B centering and define the larger unit cell for 1, are much weaker than those with even index sums. In the crystal structure of 1 (Figure 1), the different y coordinates of the Na + cations in the same layer interspace are the most obvious deviations from the B centering. If the disordered Ag + distribution in 2 is neglected, the structure of 1 can be seen as crystallizing in a klassengleiche sub-group of index 2 of the space group of compound 2. [42] A representation of the structure of 2 in the "orthohexagonal" setting can be found in the Supporting Information ( Figure S2). The positions of the spiro-atoms in 1 and 2 (Table S3) deviate only by Δx = 0.014, Δy = 0.002, and Δz = À 0.020, supporting the close relation between the two structures.
The two isolated [AlCl 4 ] À tetrahedra show significant differences. The tetrahedron around Al7 is almost regular with bond lengths ranging from 213.0(5) pm to 213.9(5) pm, while the Al8 polyhedron shows four slightly deviating distances between 212.0(5) pm and 215.6(5) pm. All AlÀ Cl distances are in accordance with those observed in Na[AlCl 4 ] [43] and their variation can be correlated with secondary Cl···Bi or Cl···Na bonds.
The anionic group [S(AlCl 3 ) 3 ] 2À consists of three [AlSCl 3 ] 2À tetrahedra that share their sulfur vertex ( Figure 2). All polyhedra point in the same direction. The pseudo-symmetry is 3 m (C 3v ), while the crystallographic symmetry is only 1 (C 1 ). Despite the tilting of the tetrahedra, the SÀ Al bond lengths are rather uniform [226.5(5)-228.2 (5) 4 ]. [17] Most likely a gallium analogue exists in (Bi 3 GaS 5 )[Ga 3 Cl 10 ] 2 [GaCl 4 ] 2 ·S 8 , [39] assuming some erroneous assignment of S and Cl, which have very similar scattering factors. Moreover, the selenium compound (Bi 4 Se 4 )[Se(GaCl 3 ) 3 ][GaCl 4 ] exists, which corroborates the atom assignment. [28] All investigated crystals of 1 suffered from stacking disorder that caused streaks of diffuse scattering on the reflection rows parallel to b*. The stacking disorder, or twinning in the case of large domains, is a consequence of the trigonal pseudosymmetry of the layer packages, which permits a rotation of the stacking vector by � 120°. Because of the translational pseudosymmetry, represented by the above-mentioned groupsubgroup relationship, antiphase boundaries can be expected in addition. Although the selected crystal appeared to be relatively unaffected by stacking faults, its real structure manifested itself as unusually high residual electron densities, especially in the immediate vicinity of the bismuth atoms.

Conclusions
Lewis-acidic ionic liquids that contain an excess of AlCl 3 proved their ability to activate crystalline Bi 2 S 3 at moderate temperatures.
Although the IL is a non-oxidizing solvent, it can replace hot nitric acid, which is commonly used for this purpose. Together with NaCl and the anionic part of the IL, the dissolved Bi 2 S 3 forms the complex structured salt Na(Bi 7 S 8 )[S(AlCl 3 ) 3 ] 2 [AlCl 4 ] 4 . Its layered structure includes cationic spiro-dicubanes (Bi 7 S 8 ) 5 + and anionic tetrahedra triples [S(AlCl 3 ) 3 ] 2À . Analogous syntheses using the heavier alkali metal ions M + (M = K, Rb, Cs) yielded compounds of the same kind, whose crystal structures, however, could not be determined because of extensive stacking disorder. The method is not limited to Bi 2 S 3 and can also be used to activate other (sulfidic) minerals. Similar ionometallurgical approaches [44] have the potential to substitute conventional processes, such as roasting of sulfidic ores, which are associated with high temperatures and the formation of gases with high environmental impact.

Experimental Section
Synthesis: All compounds were handled in an argon-filled glove box (MBraun; p(O 2 )/p 0 < 1 ppm, p(H 2 O)/p 0 < 1 ppm). The reactions were carried out in silica ampules with a length of 120 mm and a diameter of 14 mm. Na(Bi 7  . The evacuated and sealed ampule was heated at 180°C for 6 d and subsequently tilted and cooled to room temperature at ΔT/ t = À 6 K/h À 1 . The IL was decanted from the precipitated colorless AlCl 3 and the deep red crystals of 1, which were obtained in sizes of 0.03 to 1 mm. The crystals of 1 were identified visually, according to their color and shape, and separated mechanically from other crystalline species and most of the IL. No further treatment was applied to these crystals, as the small amounts of residual IL on the crystal surface did not impede the following investigations. The excess sodium cations were not detected in any precipitate and are assumed to remain dissolved in the IL. EDX Spectroscopy: EDX measurements were conducted using a SU8020 (Hitachi) SEM equipped with a Silicon Drift Detector (SDD) X-Max N (Oxford) to check the chemical composition of the crystals. However, several problems impeded the interpretation of the measured data. The [AlCl 4 ] À ions partially decompose in the highenergetic electron beam (U a = 25 kV) that is necessary to activate bismuth for this measurement. [45] Considering these factors, we were able to support the composition of Na(Bi 7  Powder X-ray Diffraction: Data collection was performed at 296(2) K with an X'Pert Pro MPD diffractometer (PANalytical) equipped with a Ge(220) hybrid-monochromator using Cu-K α1 radiation (λ = 154.056 pm). Due to their sensitivity to moisture, the samples were contained in a glass capillary (Hilgenberg) with an outer diameter of 0.3 mm. A Le Bail fit was conducted for the lattice parameters of Na(Bi 7 S 8 )[S(AlCl 3 ) 3 ] 2 [AlCl 4 ] 2 based on the values gathered in the SCXRD measurements at 100(2) K with TOPAS. [46] 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 CCDdetector at T = 100(2) K. Mo-K α radiation (λ = 71.073 pm) was used. The datasets were corrected for background, polarization and Lorentz factor using the APEX3 software suite. [47] After integration, [47] a numerical absorption correction based on an optimized crystal description was applied. [48] The initial structure solution was performed with JANA2006 [49] and further refinement processed in SHELXL against F o 2 . [50][51][52] The unit cell setting was chosen so that (a) all angles are smaller than 90°and (b) one face of the cell is parallel to the layer packages in the structure, which also simplifies the group-subgroup relationship to the silver compound. Na(Bi 7 S 8 )[S(AlCl 3 ) 3 ] 2 [AlCl 4 ] 2 : triclinic; P1 À (no. 2); T = 100(2) K; a = 1846.4(1) pm, b = 1905.1(2) pm, c = 1065.5(2) pm, α = 84.07(1)°, β = 89.55(1)°, γ = 49.68(1)°; V = 2883.9(4) × 10 6 pm 3 ; Z = 2; 1 calcd. = 3.450 g cm À 3 ; μ(Mo-K α ) = 23.4 mm À 1 ; 2θ max = 54.0°, À 23 � h � 23, À 24 � k � 24, À 13 � l � 13; 61896 measured, 12231 unique reflections, R int = 0.087, R σ = 0.098; 469 parameters, R 1 [7989 F o > 4σ(F o )] = 0.046, wR 2 (all F o 2 ) = 0.071, GooF = 1.095, min./max. residual electron density: À 3.87/5.67 e × 10 À 6 pm À 3 . For atomic parameters see Table S2 of the Supporting Information.