Borosulfates Cs2B2S3O13, Rb4B2S4O17, andA3HB4S2O14(A= Rb, Cs) - Crystalline Approximants for Vitreous B2O3?

Cd[B2(SO4)4] and H2[B2(SO4)4] - a phyllosilicate-analogous borosulfate and its homeotypic heteropolyacid

Borosulfates consist of heteropolyanionic networks of corner-shared (SO₄)- and (BO₄)-tetrahedra charge compensated by metal or non-metal cations. The anionic substructures differ significantly, depending on the different branching of the silicate-analogous borosulfate building blocks. However, only one acid has been characterized by single crystal X-ray diffraction so far. Herein, we present H₂[B₂(SO₄)₄] as the first phyllosilicate analogue representative, together with the homeotypic representative Cd[B₂(SO₄)₄]. The latter can be considered the cadmium salt of the former. Their crystal structures and crystallographic relationship are elucidated. For H₂[B₂(SO₄)₄], the bonding situation is examined using Hirshfeld-surface analysis. Further, the optical and thermal properties of Cd[B₂(SO₄)₄] are investigated by FTIR and UV-Vis spectroscopy, thermogravimetry, as well as temperature-programmed powder X-ray diffraction.

Targeting Diverse Bridging Motifs within Actinide Borosulfates and Establishing an Unconventional Structural Hierarchy

The first actinide borosulfates, (UO₂)[B(SO₄)₂(SO₃OH)] (TSUBOS-1) and (UO₂)₂[B₂O(SO₄)₃(SO₃OH)₂] (TSUBOB-1), were synthesized solvothermally in oleum using UO₃. The classical borosulfate crystal structure of TSUBOS-1 is partially consistent with an established conventional hierarchy. Uranyl pentagonal bipyramids limit the anionic network linkages and isolate sulfate tetrahedra within the anionic network. Therefore, the classical one-dimensional chain established in the hierarchy does not fully describe the structure. The structure of TSUBOB-1 is the first actinide borosulfate that contains an unconventional borate-to-borate bridging mode (denoted B-O-B) and a zero-dimensional oxoanionic unit consisting of one sulfate tetrahedron that shares vertices with two B-O-B bridged borate tetrahedra that each share a vertex with two sulfate tetrahedra. As this structure departs from the existing structural hierarchy, a modified approach for understanding the unconventional borosulfate substructure and dimensionality is proposed, together with a new graphical notation. In the course of our synthesis experiments, a novel uranyl disulfate compound (UO₂)₂[(S₂O₇)(SO₃OH)₂] (TSUDS) was isolated and characterized. The structure of TSUDS is a framework consisting of uranyl pentagonal bipyramids and sulfate tetrahedra. Each uranyl pentagonal bipyramid is surrounded by five sulfate tetrahedra, two of which share a vertex creating a disulfate with a S-O-S bridging mode. The uranyl bipyramids are linked to one another via the singular sulfate or disulfate groups.

Structural Characterization and ORR Activity of a Copper Complex Borate and an Unexpected [Ni(atta)(SO4)0.5]+ Borate-Sulfate

Two metal templated borates, [Ni(atta)(SO₄)_0.5]·[B₅O₆(OH)₄] (1) and [Cu(1-MI)₄]·[B₅O₆(OH)₄]₂·1-MI₂ (2), have been synthesized. The structures were determined by single-crystal X-ray diffraction and further characterized by Fourier transform infrared (FTIR), elemental analysis, and powder X-ray diffraction (PXRD). The structure of 1 consists of [B₅O₆(OH)₄]^- clusters and [Ni(atta)(SO₄)_0.5]⁺ complexes, which shows a very rare Ni-O-S bond. 1 and 2 exhibit a hydrogen-bonded network formed by [B₅O₆(OH)₄]^- clusters. Oxygen reduction reaction (ORR) activities of 1 and 2 were explored.

Polycations Stabilised by Borosulfates: [Au 3 Cl 4 ][B(S 2 O 7 ) 2 ] and the One‐Dimensional Metal [Au 2 Cl 4 ][B(S 2 O 7 ) 2 ](SO 3 )

(SO₄)‐rich silicate analogue borosulfates are able to stabilise cationic cluster‐like and chain‐like aggregates. Single crystals of [Au₃Cl₄][B(S₂O₇)₂] and [Au₂Cl₄][B(S₂O₇)₂](SO₃) were obtained by solvothermal reaction with SO₃, and the electronic properties were investigated by means of density functional theory–based calculations. [Au₃Cl₄][B(S₂O₇)₂] exhibits a cluster‐like cation, and the cationic gold‐chloride strands in [Au₂Cl₄][B(S₂O₇)₂](SO₃) are found to resemble one‐dimensional metallic wires. This is confirmed by polarisation microscopy.

Triple-Vertex Linkage of (BO4 )-Tetrahedra in a Borosulfate: Synthesis, Crystal Structure, and Quantum-Chemical Investigation of Sr[B3 O(SO4 )4 (SO4 H)]

Borosulfates are classified as silicate analogue materials. The number of crystallographically characterized compounds is still limited, whereas the structural diversity is already impressive. The anionic substructures of borosulfates exhibit vertex-connected (BO₄ )- and (SO₄ )-tetrahedra, whereas bridging between two (SO₄ )- or even between two (BO₄ )-tetrahedra is scarce. The herein presented compound Sr[B₃ O(SO₄ )₄ (SO₄ H)] is the first borosulfate with a triple-vertex linkage of three (BO₄ ) tetrahedra via one common oxygen atom. DFT calculations complement the experimental studies. Bader charges (calculated for all atoms) as well as charge-density calculations give hint to the electron distribution within the anionic substructure and density-of-states calculations support the interpretation of the bonding situation.

Prediction of Novel van der Waals Boron Oxides with Superior Deep-Ultraviolet Nonlinear Optical Performance

Deep-ultraviolet nonlinear optical (DUV NLO) materials are attracting increasing attention because of their structural diversity and complexity. Using the two-dimensional (2D) crystal structure prediction method combined with the first-principles calculations, here we propose layered 18-membered-ring (18MR) boron oxide B₂ O₃ polymorphs as high-performance NLO materials. 18MR-B₂ O₃ with the AA and AB stackings are potential DUV NLO materials. The superior performing 18MR-B₂ O₃ ^AB has an unprecedentedly high second harmonic generation coefficient of 1.63 pm V^-1 , the largest among the DUV NLO materials, three times larger than that of the advanced DUV NLO material KBe₂ BO₃ F₂ and comparable to that of β-BaB₂ O₄ . Its unusually large birefringence of 0.196 at 400 nm guarantees the phase-matching wavelength λ_PM to reach this material's extreme absorption edge of ≈154 nm.

Synthesis-Controlled Polymorphism and Optical Properties of Phyllosilicate-Analogous Borosulfates M[B2 (SO4 )4 ] (M=Mg, Co)

Increased synthetic control in borosulfate chemistry leads to the access of various new compounds. Herein, the polymorphism of phyllosilicate-analogous borosulfates is unraveled by adjusting the oleum (65 % SO3 ) content. The new polymorphs β-Mg[B2 (SO4 )4 ] and α-Co[B2 (SO4 )4 ] both consist of similar layers of alternating borate and sulfate tetrahedra, but differ in the position of octahedrally coordinated cations. The α-modification comprises cations between the layers, whereas in the β-modification cations are embedded within the layers. With this new synthetic approach, phase-pure compounds of the respective polymorphs α-Mg[B2 (SO4 )4 ] and β-Co[B2 (SO4 )4 ] were also achieved. Tanabe-Sugano analysis of the Co2+ polymorphs reveal weak ligand field splitting and give insights into the coordination behavior of the two-dimensional borosulfate anions for the first time. DFT calculations confirmed previous in silico experiments and enabled an assignment of the polymorphs by comparing the total electronic energies. The compounds are characterized by single-crystal XRD, PXRD, FTIR, and UV/Vis/NIR spectroscopy, thermogravimetric analysis (TGA), and density functional theory (DFT) calculations.

Ni[B2 (SO4 )4 ] and Co[B2 (SO4 )4 ]: Unveiling Systematic Trends in Phyllosilicate Analogue Borosulfates

Borosulfates are compounds analogous to silicates, with heteropolyanionic subunits of vertex-linked (SO₄ )- and (BO₄ )-tetrahedra. In contrast to the immense structural diversity of silicates, the number of borosulfates is yet very limited and the extent of their properties is still unknown. This is particularly true for representatives with phyllosilicate and tectosilicate analogue anionic substructures. Herein, we present Ni[B₂ (SO₄ )₄ ] and Co[B₂ (SO₄ )₄ ], two new borosulfates with phyllosilicate analogue topology. While the anionic subunits of both structures are homeotypic, the positions of the charge compensating cations differ significantly: Ni^II is located between the borosulfate layers, while Co^II -in contrast-is embedded within the layer. Detailed analysis of these two structures based on single-crystal X-ray diffraction, magnetochemical investigations, X-ray photoelectron spectroscopy, and quantum chemical calculations, unveiled the reasons for this finding. By in silico comparison with other divalent borosulfates, we uncovered systematic trends for phyllosilicate analogues leading to the prediction of new species.

From S-O-S to B-O-S to B-O-B Bridges: Ba[B(S2O7)2]2 as a Model System for the Structural Diversity in Borosulfate Chemistry

Various different possible connection patterns of sulfate and borate tetrahedra enable a vast structural diversity in borosulfates, a rather new class of silicate-analogous compounds. Here we unravel a direct relationship from S-O-S to B-O-S to B-O-B bridges for the first time in borosulfate chemistry. Solvothermal synthesis in pure oleum (65% SO₃) yielded the first alkaline earth metal borosulfate comprising S-O-S bridges: Ba[B(S₂O₇)₂]₂ (I2/a, Z = 4, a = 1160.77(9) pm, b = 891.44(7) pm, c = 2130.26(19) pm, β = 104.0341(17)°) contains molecular [B(S₂O₇)₂]^- anions of a central boron atom and two chelating disulfate groups. By using equal amounts of sulfuric acid and oleum solely B-O-S bridges were obtained in Ba[B₂(SO₄)₄] (Pnna, Z = 4, a = 1279.08(18) pm, b = 1280.0(2) pm, c = 731.70(11) pm) featuring one-dimensional _∞¹[B(SO₄)_4/2]^- chains. The thermal analysis on Ba[B(S₂O₇)₂]₂ revealed the conversion from S-O-S bridges to B-O-S bridges in Ba[B₂(SO₄)₄] and to B-O-B bridges in Ba[B₂O(SO₄)₃] by a successive release of SO₃. Thus, BaO-B₂O₃-SO₃ is the first quaternary system for borosulfates uniting all three possible connection patterns enabling us to understand the fascinating but systematic chemistry in such systems. Both new compounds were also characterized by means of X-ray powder diffraction, electrostatic calculations, and infrared spectroscopy assisted by density functional theory (DFT).

Borosulfates-Synthesis and Structural Chemistry of Silicate Analogue Compounds

Borosulfates are oxoanionic compounds consisting of condensed sulfur- and boron-centered tetrahedra. Hitherto, they were mostly achieved from solvothermal syntheses in SO3 -enriched sulfuric acid, or from reactions with the superacid H[B(HSO4 )4 ]. The crystal structures are very similar to those of the corresponding class of silicates and their substitution variants, especially regarding the typical structural motif of corner-sharing tetrahedra. However, the borosulfates are supposed to be even more versatile, because (BO3 ) units might also be part of the anionic network. The following article deals with detailed reports on the different synthesis strategies, the crystal chemistry of borosulfates in comparison to silicates, and their hitherto identified properties.

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 vierer- and zwölfer-rings formed exclusively by corner-linked (SO4)- and (BO4)-tetrahedra.

RE(SO4)[B(OH)4](H2O), RE(SO4)[B(OH)4](H2O)2, and RE(SO4)[B(OH)4](H2O)·H2O: Rare-Earth Borate-Sulfates Featuring Three Types of Layered Structures

Using hydrothermal reactions, three series of rare-earth borate-sulfates, namely, RE(SO₄)[B(OH)₄](H₂O) (RE = La (1), Sm (2), Eu (3)), RE(SO₄)[B(OH)₄](H₂O)₂ (RE = Pr (4), Nd (5), Sm (6), Eu (7), Gd (8)), and RE(SO₄)[B(OH)₄](H₂O)·H₂O (RE = Tb (9), Dy (10), Ho (11), Er (12), Tm (13), Yb (14), Lu (15), Y (16)), have been synthesized, which represent the first rare-earth borate-sulfate mixed-anion compounds. All these compounds possess the same fundamental building anionic units of SO₄ and B(OH)₄ tetrahedra; however, they exhibit three different types of two-dimensional (2D) layered structures composed of 1D RE-B-O and RE-S-O chains. The rare-earth borate chains are similar in all compounds, while the rare-earth sulfate chains differ in each type of compound due to the various coordination modes of sulfate groups. On the basis of the measured UV-vis diffuse reflectance spectra, the optical band gaps of compounds 2, 3, 6, and 7 are estimated to be 4.66, 4.53, 4.62, and 4.50 eV, respectively. Luminescence studies show that compounds 2, 3, 6, and 7 exhibit strong emission in the orange or red regions. Furthermore, thermal analysis and magnetic susceptibility measurements for these four representative compounds have also been performed.

Synthesis, Crystal Structure, and Properties of Bi3 TeBO9 or Bi3 (TeO6 )(BO3 ): A Non-Centrosymmetric Borate-Tellurate(VI) of Bismuth

Pale-yellow single crystals of the new borate tellurate(VI) Bi₃ TeBO₉ were obtained by reaction of stoichiometric amounts of Bi₂ O₃ , B₂ O₃ , and Te(OH)₆ at 780 °C. The non-centrosymmetric crystal structure (P6₃ , Z=2, a=8.7454(16), c=5.8911(11) Å, 738 refl., 43 param, R1=0.037, wR2=0.093) contains isolated trigonal-planar BO₃ units and nearly undistorted TeO₆ octahedra. The Bi³⁺ cations are located in between in octahedral voids. The BiO₆ octahedra are significantly distorted to a [3+3] pattern (2.25/2.50 Å) due to the ns² configuration. According to the structural features, the formula can be written as Bi₃ (TeO₆ )(BO₃ ). Alternatively, the structure can also be described as hcp of oxygen with Te^VI and Bi^III in octahedral voids and B^III in trigonal- planar voids. The vibrational spectra show the typical features of BO₃ and TeO₆ units with a significant ¹⁰ B/¹¹ B isotopic splitting of the IR-active B-O valence mode (1248 and 1282 cm^-1 ). The UV/Vis spectrum shows an optical band edge with an onset around 480 nm (2.6 eV). MAS-NMR spectra of ¹¹ B show an anisotropic signal with a quadrupole coupling constant of C_Q =2.55 MHz. and a very small deviation from rotational symmetry (η=0.2). The isotropic chemical shift is 20.1 ppm. The second harmonic generation (SHG) test was positive with an activity comparable to potassium dihydrogen phosphate (KDP). Bi₃ TeBO₉ decomposes in air at 825 °C to Bi₂ TeO₅ .