In Situ Single-crystal X-ray Diffraction Studies of Physisorption and Chemisorption of SO2 within a Metal-Organic Framework and Its Competitive Adsorption with Water

Living on an increasingly polluted planet, the removal of toxic pollutants such as sulfur dioxide (SO2) from the troposphere and power station flue gas is becoming more and more important. The CPO-27/MOF-74 family of metal-organic frameworks (MOFs) with their high densities of open metal sites is well suited for the selective adsorption of gases that, like SO2, bind well to metals and have been extensively researched both practically and through computer simulations. However, until now, focus has centered upon the binding of SO2 to the open metal sites in this MOF (called chemisorption, where the adsorbent-adsorbate interaction is through a chemical bond). The possibility of physisorption (where the adsorbent-adsorbate interaction is only through weak intermolecular forces) has not been identified experimentally. This work presents an in situ single-crystal X-ray diffraction (scXRD) study that identifies discrete adsorption sites within Ni-MOF-74/Ni-CPO-27, where SO2 is both chemisorbed and physisorbed while also probing competitive adsorption of SO2 of these sites when water is present. Further features of this site have been confirmed by variable SO2 pressure scXRD studies, DFT calculations, and IR studies.

Synthesis and Crystal Structure of the Europium(II) Hydride Oxide Iodide Eu5H2O2I4 Showing Blue-Green Luminescence

As the first europium(II) hydride oxide iodide, dark red single crystals of Eu5H2O2I4 could be synthesized from oxygen-contaminated mixtures of EuH2 and EuI2. Its orthorhombic crystal structure (a = 1636.97(9) pm, b = 1369.54(8) pm, c = 604.36(4) pm, Z = 4) was determined via single-crystal X-ray diffraction in the space group Cmcm. Anion-centred tetrahedra [HEu4]7+ and [OEu4]6+ serve as central building blocks interconnected via common edges to infinite ribbons parallel to the c axis. These ribbons consist of four trans-edge connected (Eu2+)4 tetrahedra as repetition unit, two H--centred ones in the inner part, and two O2--centred ones representing the outer sides. They are positively charged, according to ∞1{[Eu5H2O2]4+}, to become interconnected and charge-balanced by iodide anions. Upon excitation with UV light, the compound shows blue-green luminescence with the shortest Eu2+ emission wavelength ever observed for a hydride derivative, peaking at 463 nm. The magnetic susceptibility of Eu5H2O2I4 follows the Curie-Weiss law down to 100 K, and exhibits a ferromagnetic ordering transition at about 10 K.

The Role of the Bi3+ Lone Pair Effect in Bi(H3O)(SO4)2, Bi(HSO4)3, and Bi2(SO4)3

Three new members in the Bi₂O₃-SO₃-H₂O system are identified by single crystal X-ray diffraction and Rietveld refinement after a fundamental examination of this phase space. Bi(H₃O)(SO₄)₂ crystallizes in space group P2₁/c (no. 14, a = 1203.5(4), b = 682.9(2), c = 821.2(2) pm, β = 102.99(1)°, 861 independent reflections, 88 refined parameters, wR₂ = 0.14) homeotypic with Nd(H₃O)(SO₄)₂ featuring edge-sharing BiO₉ polyhedra. Bi(HSO₄)₃ crystallizes in a new structure type in space group P1 (no. 2, a = 492.04(7), b = 910.8(1), c = 1040.8(2) pm, α = 85.443(5)°, β = 86.897(5)°, γ = 74.542(4)°, 3227 independent reflections, 154 refined parameters, wR₂ = 0.05) comprising dimers of edge-sharing BiO₈ polyhedra. For Bi₂(SO₄)₃, a new modification crystallizing in space group P2₁/n (no. 14, a = 1308.03(7), b = 473.25(3), c = 1452.61(8) pm, β = 100.886(2)°, 3189 independent reflections, 155 refined parameters, wR₂ = 0.03) isotypic to Sb₂(SO₄)₃ with noncondensed BiO₇ polyhedra is presented. The role of the Bi³⁺ lone pair effect as elucidated by density functional theory (DFT) calculations is discussed for all three compounds with respect to their structural and optical properties. Additionally, the Bi³⁺ lone pair activity is compared to the recently reported borosulfates Bi(H₃O)[B(SO₄)₂]₄ and Bi₂[B₂(SO₄)₆]. Geometrical calculations based on structural data are correlated with electron localization function (ELF) calculations to establish the origin of the direction and strength of the lone pair stereoactivity of Bi³⁺ in oxidic compounds. Finally, the thermal properties of the three compounds are reported.

The tin sulfates Sn(SO4)2 and Sn2(SO4)3: crystal structures, optical and thermal properties

We report the crystal structures of two tin(IV) sulfate polymorphs Sn(SO4)2-I (P2₁/c (no. 14), a = 504.34(3), b = 1065.43(6), c = 1065.47(6) pm, β = 91.991(2)°, 4617 independent reflections, 104 refined parameters, wR₂ = 0.096) and Sn(SO4)2-II (P2₁/n (no. 14), a = 753.90(3), b = 802.39(3), c = 914.47(3) pm, β = 92.496(2)°, 3970 independent reflections, 101 refined parameters, wR₂ = 0.033). Moreover, the first heterovalent tin sulfate Sn2(SO4)3 is reported which adopts space group P1̄ (no. 2) (a = 483.78(9), b = 809.9(2), c = 1210.7(2) pm, α = 89.007(7)°, β = 86.381(7)°, γ = 73.344(7)°, 1602 independent reflections, 152 refined parameters, wR₂ = 0.059). Finally, SnSO4 - the only tin sulfate with known crystal structure - was revised and information complemented. The optical and thermal properties of all tin sulfates are investigated by FTIR, UV-vis, luminescence and ¹¹⁹Sn Mössbauer spectroscopy as well as thermogravimetry and compared.

Strong Lewis and Brønsted Acidic Sites in the Borosulfate Mg3 [H2 O→B(SO4 )3 ]2

Borosulfates provide fascinating structures and properties that go beyond a pure analogy to silicates. Mg₃[H₂O→B(SO₄)₃]₂ is the first borosulfate featuring a boron atom solely coordinated by three tetrahedra. Thus, the free Lewis acidic site forms a Lewis acid–base adduct with a water molecule. This is unprecedented for borosulfate chemistry and even for borates. Quantum chemical calculations on water exchange reactions with BF₃ and B(C₆F₅)₃ revealed a higher Lewis acidity for the borosulfate anion. Moreover, proton exchange reactions showed a higher Brønsted acidity than comparable silicates or phosphates. Additionally, Mg₃[H₂O→B(SO₄)₃]₂ was characterised by X‐ray diffraction, infrared spectroscopy, thermogravimetric analysis, and density functional theory (DFT) calculations.

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.

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).

Tb(HSO4)(SO4) - a green emitting hydrogensulfate sulfate with second harmonic generation response

Recently, sulfates have attracted attention as materials for non-linear optical applications. This compound class is extended by Tb(HSO₄)(SO₄), which is solvothermally synthesised from Tb₄O₇ and sulfuric acid. The compound crystallises in the non-centrosymmetric space group P2₁ (Z = 2, a = 665.03(5) pm, b = 659.41(5) pm, c = 680.24(5) pm, and β = 104.640(2)°) and is homeotypic with Ni₂In. The terbium ions adopt the indium sites and the sulfate and hydrogen sulfate anions are situated on the nickel sites. The compound shows green luminescence based on f-f-transitions and the positions of the f-d-excitation bands reveal a weak coordination behaviour of the sulfate anions. Tb(HSO₄)(SO₄) exhibits a second harmonic generation response comparable to KH₂PO₄ (KDP). Furthermore, the material is characterised by electrostatic calculations, infrared spectroscopy and thermal analysis.

Blue Excitement: The Lanthanide(III) Chloride Oxidomolybdates(VI) Ln3Cl3[MoO6] (Ln = La, Pr, and Nd) and Their Spectroscopic Properties

The lanthanide(III) chloride oxidomolybdates(VI) with the empirical formula Ln₃Cl₃[MoO₆] (Ln = La, Pr, and Nd) were synthesized by solid-state reactions utilizing the respective lanthanide trichloride, lanthanide sesquioxide (where available), and molybdenum trioxide together with lithium chloride as a fluxing agent. The title compounds crystallize in hexagonal space group P6₃/ m ( a = 942-926 pm, c = 542-533 pm, Z = 2). Besides tetracapped trigonal prismatically coordinated Ln³⁺ cations, noncondensed trigonal prismatic [MoO₆]^6- entities are found in the crystal structure. In addition to X-ray diffraction, the title compounds were also characterized by single-crystal Raman and infrared spectroscopy as well as measurements to determine their magnetic susceptibility and behavior at low temperatures. The most outstanding properties of the Ln₃Cl₃[MoO₆] representatives (Ln = La, Pr, and Nd), however, are of an optical nature, because their band gaps, determined by diffuse reflectance spectroscopy, show a significant shift toward lower energies compared to those of other rare-earth metal chloride molybdates with a different polyhedral arrangement. This culminates in La₃Cl₃[MoO₆]:Eu³⁺ exhibiting luminescence, which can be excited in the visible range of the electromagnetic spectrum by a blue light-emitting diode.

RE2[B2(SO4)6] (RE = Y, La–Nd, Sm, Eu, Tb–Lu): a silicate-analogous host structure with weak coordination behaviour

The very first thorough investigation of the rare earth borosulfates RE₂[B₂(SO₄)₆] with RE = Y, La–Nd, Sm, Eu and Tb–Lu focusses on synthetic aspects and especially optical properties.