M[B2(SO4)4] (M =Mn, Zn)—Syntheses and Crystal Structures of Two New Phyllosilicate Analogue Borosulfates

Borosulfates are a rapidly expanding class of silicate analogue materials, where the structural diversity is expected to be at least as large as known for silicates. However, borosulfates with cross-linking of the anionic network into two or even three dimensions are still very rare. Herein, we present two new representatives with phyllosilicate analogue topology. Through solvothermal reactions of ZnO and MnCl2·4H2O with boric acid in oleum (65% SO3), we obtained single-crystals of Mn[B2(SO4)4] (monoclinic, P21/n, Z = 2, a = 8.0435(4), b = 7.9174(4), c = 9.3082(4) Å, β = 110.94(1)◦, V = 553.63(5) Å3) and Zn[B2(SO4)4] (monoclinic, P21/n, Z = 2, a = 7.8338(4), b = 8.0967(4), c = 9.0399(4) Å, β = 111.26(1)◦, V = 534.36(5) Å3). The crystal structures reveal layer-like anionic networks with alternating viererand zwölfer-rings formed exclusively by corner-linked (SO4)and (BO4)-tetrahedra.


Introduction
The structural chemistry of silicates is dominated by anionic networks of corner-shared (SiO 4 )-tetrahedra and is still a fascinating field of research for scientists from different disciplines. Friedrich Liebau is one of the pioneers in silicate chemistry and introduced a classification in 1985 [1]. According to this classification, silicates can be assigned to different classes: Nesosilicates reveal exclusively isolated (SiO 4 )-tetrahedra. Isolated anionic moieties of corner-shared (SiO 4 )-tetrahedra are predominant in sorosilicates. The anionic substructures of cyclosilicates contain ring-like anionic moieties of corner-shared (SiO 4 )-tetrahedra with a Si:O ratio of 1:3. Inosilicates are formed by infinite anionic chains or bands. If the anionic network branches in two dimensions phyllosilicates occur, which reveal infinite anionic layers. Last but not least, silicates with 3D network like structures are called tectosilicates. The function of all previously introduced silicates is mostly related to their respective crystal structures, as well as the dimensionality of the anionic substructures [2][3][4][5].
Nowadays, several substitution variants of the aforementioned silicates are known. In these compounds, the central silicon atoms are partially or completely substituted by one or even two different atoms. Thus, the resulting anionic substructures differ by the respective atoms in the tetrahedral centers, which determine the total charge of the anionic network. Especially for alumosilicates, alumophosphates, and borophosphates, where aluminum, phosphorous, and boron atoms reside in the center of the tetrahedra, the correlation between syntheses, structure, and properties are well explored [6][7][8]. Probably, the most prominent examples are microporous alumosilicates, the so-called zeolites. Due to their open network structures, they are used as ion-conducting materials, absorbents, and catalysts [9,10].
A comparably new class of silicate analogue materials are borosulfates, with sulfur and boron occupying the position of silicon. According to Pauling's rules, networks of corner-shared tetrahedra are stable, if the respective central atoms are able to compensate the occurring charges, either by themselves or by extended crosslinking of the anionic network [11]. To branch networks with sulfur in the center of the tetrahedra into further dimensions, the high charge has to be compensated. Therefore, the combination of (SO 4 )-and (BO 4 )-tetrahedra [12][13][14][15][16][17][18][19][20][21][22][23][24][25]

Results and Discussion
Both new borosulfates were obtained in form of colorless crystals (see Figures Table 1 and X-ray crystallography for further data and details according the refinement). The crystal structures exhibit layer-like anionic subunits of vertex-shared (BO 4 )-and (SO 4 )-tetrahedra ( Figure 1) charge compensated by the respective divalent counter cations Mn 2+ and Zn 2+ . Each (BO 4 )-tetrahedron is coordinated by four different (SO 4 )-tetrahedra via common corners, whereas for each (SO 4 )-tetrahedron two oxygen atoms remain terminal. The Niggli formula for the anionic substructure of both borosulfates can be described as 2  (7), S1-O12 1.4125 (7), S1-O111 1.5263 (6), S1-O112 1.5229 (6) (7), S1-O12 1.4135(7), S1-O111 1.5247(6), S1-O112 1.5315 (6) (Figure 2, top). Each zwölfer ring is surrounded by six further zwölfer rings. Two of those six are opposite to another and are aligned along the crystallographic b-axis. Each one shares two common (BO 4 )-tetrahedra with the zwölfer ring. The common (BO 4 )-tetrahedra are linked via two separate (SO 4 )-tetrahedra, whereas each is part of one of the two zwölfer rings. As a result, a vierer ring is formed. The other four zwölfer rings share a common (BO 4 )(SO 4 )(BO 4 )-fragment and spread the network along the ac-diagonal, thereby creating a phyllosilicate analogue anionic layer. Each layer is terminated exclusively by (SO 4 )-tetrahedra (Figure 2, top). However, similarities to the borophosphates also occur. With a B:P ratio <1, compounds with condensed [B(PO 4 ) 4 ] 9− anions occur. In this form, highly charged P 5+ cations are located within the network as far away from each other as possible [29,30]. In the title compounds, the situation is similar. The highly charged sulfur atoms are in the center of the (SO 4 )-tetrahedra, coordinated to the low charged boron cations forming repetitive [B(SO 4 ) 4 ] blocks. Charge compensation of the borosulfates is achieved by Mn 2+ and Zn 2+ cations. They are packed in a stacked manner parallel to the crystallographic b-axis in the middle of the channels running through the anionic substructures. The cations are exclusively located within the center of the zwölfer rings. They are coordinated by four of their terminal oxygen atoms of (SO 4 )-tetrahedra and two oxygen atoms of terminal (SO 4 )-tetrahedra belonging to the anionic layers above and underneath (Figure 3). Besides    (133 mg (0.67 mmol), Fluka, Seelze, Germany) or ZnO (55 mg (0.68 mmol), Merck, Darmstadt, Germany) were loaded together with H 3 BO 3 (300 mg (4.85 mmol), Carl Roth, Karlsruhe, Germany), and 1 mL oleum (65% SO 3 , Merck, Darmstadt, Germany) into a thick-walled glass ampoule (l = 300 mm, ∅ = 16 mm, thickness of the tube wall = 1.8 mm), respectively. The ampoules were torch sealed under reduced pressure (0.01 mbar), and placed into a block-shaped resistance furnace and heated up to 523 K within 150 min. The temperature was maintained for 48 h and finally reduced to 298 K within 96 h. A large number of colorless block-shaped crystals were obtained for both borosulfates (Figures S1 and S2), and the yield was quantitative for ZnO and almost quantitative for MnCl 2 ·4H 2 O. The latter suffered from small impurities of (H 3 O)[B(SO 4 ) 2 ] (see X-ray crystallography). The crystals are very moisture sensitive and decompose immediately after exposure to air. Thus, the products were handled under strictly inert conditions. Caution: Oleum is a strong oxidizer, which needs careful handling. During and even after the reaction, the ampoules might be under remarkable pressure. It is mandatory to cool down the ampoules using liquid nitrogen prior to opening.

Synthesis of Mn[B 2 (SO 4 ) 4 ] and Zn[B 2 (SO 4 ) 4 ]: MnCl 2 ·4H 2 O
X-ray crystallography: The mother liquor was separated from the crystals via decantation. The acid-containing side of the ampoule was cooled with liquid nitrogen and several crystals were transferred into inert oil directly after opening. The remaining bulk material was transferred to a glovebox for further characterization. Under a polarization microscope, a suitable crystal was prepared, mounted onto a glass needle (∅ = 0.1 mm), and immediately placed into a stream of cold N 2 (measurement temperature see Table 1) inside the diffractometer (Bruker D8 Quest κ, Bruker, Karlsruhe, Germany). After unit cell determination, the reflection intensities were collected. Structure solutions and refinements were conducted using the SHELX program package [31].
X-ray powder diffraction was carried out with a Stoe StadiP powder diffractometer (Stoe & Cie GmbH, Darmstadt, Germany) in transmission geometry on a sample washed with CCl 4 and dried under reduced pressure before the measurement. To fill a capillary with a solid sample was impossible due to small amounts of adhesive solvent. Finally, a flat sample was irradiated with Ge(111)-monochromatized Mo-Kα 1 -radiation (λ = 0.7093 Å), which was detected using a Dectris Mythen 1K detector (Dectris AG, Baden-Daettwil, Germany). Due to the fast decomposition of the sample holder matrix, the measurement had to be conducted with a scan range from 2 to 41.9 • , with a step size of 2.1 • and step time of 11 s. Rietveld refinement was accomplished using TOPAS4.2 [32].  [33] using Direct Methods. Further atoms could be successfully located by difference Fourier techniques during refinement with SHELXL-2017/1 [34]. Multi-scan absorption correction was applied to the data using the program SADABS-2014/5 by Bruker [35]. Finally, the structure and model of Mn[B 2 (SO 4 ) 4 ] were refined to R 1 (all data) = 0.0271 and wR 2 (all data) = 0.0553 and for Zn[B 2 (SO 4 ) 4 ] to R 1 (all data) = 0.0186 and wR 2 (all data) = 0.0468. Selected bond lengths and angles are presented in the Tables S3, S4, S7 and S8. CSD

Conclusions
Reactions between oleum enriched with 65% SO 3 and boric acid in evacuated torch-sealed glass ampoules seem to be a powerful strategy to synthesize borosulfates with layer-like anionic substructures of divalent metals. As can be seen from this and our previous publications, not only the main group but also subgroup metals can be charge-compensating cations.  4 ] are the first representatives of layer-like borosulfates with the cations residing in the plane of the anionic network and the center of zwölfer rings. However, it is still unclear why cations with a comparable atomic radius like Zn 2+ and Mg 2+ act differently. This problem is currently under investigation using quantum chemical calculations.