K2Ca4[(UO2)(Si2O7)2]: A Uranyl Silicate with a One-Dimensional Chain Structure

Na3[Al2B6P4O22(OH)3](H2O)6 and Na3[Al2BP2O11](H2O)0.5: Two Remarkable Complex Aluminum Borophosphates

Two remarkable aluminum borophosphates (AlBPOs), namely, Na3[Al2B6P4O22(OH)3](H2O)6 (denoted as ABPO1) and Na3[Al2BP2O11](H2O)0.5 (denoted as ABPO2), have been designed and prepared by low-temperature flux syntheses. The exceptional open framework structure of ABPO1 is formed by a unique microanionic network [Al2B6P4O22(OH)3]n3-, which contains three types of 8-, 12-, and 16-membered ring (MR) tunnels. Interestingly, these tunnels are featured by a type of super-nanocage as large as ∼1.753 nm × 1.753 nm × 1.753 nm, which is the first example of AlBPOs containing extra-large cages. Importantly, it was found that Na+ can be partially exchanged by K+, Sr2+, Cd2+, and Ni2+, which means that it is a potential ionic exchanger for removing radionuclides and toxic cations. The structure of ABPO2 features a unique 2D anionic AlBPO layer composed of corner-sharing AlO6 octahedra and AlO4, BO4, and PO4 tetrahedra. To the best of our knowledge, this is the first example of both AlO6 octahedra and AlO4 tetrahedra being contained in the structure. 9-MRs can be observed along the b-axis. Herein, the syntheses and topological structures of ABPO1 and ABPO2 as well as elemental analysis, thermal stability, infrared spectroscopy, UV-vis diffuse reflectance, structural properties, and ionic exchange properties are also discussed.

Intrinsic Specific Activity Enhancement for Bifunctional Electrocatalytic Activity toward Oxygen and Hydrogen Evolution Reactions via Structural Modification of Nickel Organophosphonates

A comprehensive knowledge of the structure-activity relationship of the framework material is decisive to develop efficient multifunctional electrocatalysts. In this regard, two different metal organophosphonate compounds, [Ni(Hhedp)2]·4H2O (I) and [Ni3(H3hedp)2(C4H4N2)3]·6H2O (II) have been isolated through one-pot hydrothermal strategy by using H4hedp (1-hydroxyethane 1,1-diphosphonic acid) and N-donor auxiliary ligand (pyrazine; C4H4N2). The structures of synthesized materials have been established through single-crystal X-ray diffraction studies, which confirm that compound I formed a one-dimensional molecular chain structure, while compound II exhibited a three-dimensional extended structure. Further, the crystalline materials have participated as efficient electrocatalysts for the oxygen evolution and hydrogen evolution reactions (OER and HER) as compared to the state-of-the-art electrocatalyst RuO2. The electrocatalytic OER and HER performances show that compound II displayed better electrocatalytic performances toward OER (η10 = 305 mV) and HER (η10 = 230 mV) in alkaline (1 M KOH) and acidic (0.5 M H2SO4) media, respectively. Substantially, the specific activity has been assessed in order to measure the inherent electrocatalytic activity of the title electrocatalyst, which displays an enrichment of fourfold higher activity of compound II (0.64 mA/cm2) than compound I (0.16 mA/cm2) for the OER experiments. Remarkably, inclusion of an auxiliary pyrazine ligand into the metal organophosphonate structure (compound II) not only offers higher dimensionality along with significant enhancement of the overall bifunctional electrocatalytic performances but also improves the long-term stability, which is noteworthy for the family of hybrid framework materials.

Framework Uranyl Silicates: Crystal Chemistry and a New Route for the Synthesis

To date, uranyl silicates are mostly represented by minerals in nature. However, their synthetic counterparts can be used as ion exchange materials. A new approach for the synthesis of framework uranyl silicates is reported. The new compounds Rb₂[(UO₂)₂(Si₈O₁₉)](H₂O)_2.5 (1), (K,Rb)₂[(UO₂)(Si₁₀O₂₂)] (2), [Rb₃Cl][(UO₂)(Si₄O₁₀)] (3) and [Cs₃Cl][(UO₂)(Si₄O₁₀)] (4) were prepared at harsh conditions in "activated" silica tubes at 900 °C. The activation of silica was performed using 40% hydrofluoric acid and lead oxide. Crystal structures of new uranyl silicates were solved by direct methods and refined: 1 is orthorhombic, Cmce, a = 14.5795(2) Å, b = 14.2083(2) Å, c = 23.1412(4) Å, V = 4793.70(13) ų, R1 = 0.023; 2 is monoclinic, C2/m, a = 23.0027(8) Å, b = 8.0983(3) Å, c = 11.9736(4) Å, β = 90.372(3) °, V = 2230.43(14) ų, R1 = 0.034; 3 is orthorhombic, Imma, a = 15.2712(12) Å, b = 7.9647(8) Å, c = 12.4607(9) Å, V = 1515.6(2) ų, R1 = 0.035, 4 is orthorhombic, Imma, a = 15.4148(8) Å, b = 7.9229(4) Å, c = 13.0214(7) Å, V = 1590.30(14) ų, R1 = 0.020. Their framework crystal structures contain channels up to 11.62 × 10.54 Å filled by various alkali metals.

U(VI) Coordination Modes in Complex Uranium Silicates: Cs[(UO6)2(UO2)9(Si2O7)F] and Rb2[(PtO4)(UO2)5(Si2O7)]

Crystals of two new inorganic uranyl silicates, Cs[(UO6)2(UO2)9(Si2O7)F] (1) and Rb2[(PtO4)(UO2)5(Si2O7)] (2), were produced from melts in evacuated silica tubes. Their structures have been solved by direct methods: 1 is trigonal, P-31c, a = 10.2040(3), c = 17.1278(5) Å, V = 1544.45(10) Å3, R1 = 0.042; 2 is tetragonal, P4/mbm, a = 16.0400(24), c = 3.9231(6) Å, V = 1009.34(10) Å3, R1 = 0.045. 1 is the first example of cation–cation interactions between the uranyl polyhedra in uranyl silicates. Therein, UVI adopts three coordination modes, UO6 octahedra, UO6F, and UO7 pentagonal bipyramids, with the latter sharing common edges to form U2O12 dimers. Three dimers associate into six-membered rings via cation–cation interactions. The structure of 1 can be described as a complex uranyl fluoride silicate framework with channels filled by the U1 atoms and disordered Cs+ cations. 2 represents a new type of topology never observed before among the structures of uranyl compounds; it is also a first complex uranium platinum oxide. Therein, the UO6 tetragonal bipyramids share edges to form chains. Five such chains are stitched into a complex ribbon via the silicon polyhedra. The ribbons are connected into a framework by the PtO4 squares; rubidium atoms are located in the channels of the framework.

Role of aromatic vs. aliphatic amine for the variation of structural, electrical and catalytic behaviors in a series of silver phosphonate extended hybrid solids

Four inorganic-organic hybrid silver phosphonate compounds, [Ag(C10H8N2)(H4hedp)] (1), [Ag2(C10H8N2)(H3hedp)]·2H2O (2), [C4H12N2][Ag4(H2hedp)2] (3) and [C4H12N2][Ag10(H2hedp)4(H2O)2]·2H2O (4) (H5hedp = 1-hydroxyethane-1,1-diphosphonic acid), have been prepared by virtue of the variable amine-directed hydrothermal strategy. The subsequent roles of coordinated aromatic amine (4,4'-bipyridine) and coordination-free templated aliphatic amine (piperazine) are studied. The connectivity of the silver ions, diphosphonate units (hedp) and bipyridine moiety can give rise to the one-dimensional structure of 1 and two-dimensional layer structure of 2. In contrast, the silver ions and diphosphonate units are connected to form the tetrameric and pentameric silver cluster units in compound 3 and 4, respectively. Such clusters are rare examples of fundamental building units in the piperazine templated two-dimensional silver based layer structures. The room temperature dielectric studies show the extremely high dielectric permittivity of the amine templated compounds (3 and 4) compared to amine coordinated structures (1 and 2). The synthesized compounds also participate in various heterogenous catalytic reactions acting as active Lewis acid catalysts that are observed for the first time in the amine-templated metal organophosphonates. The observed band gaps and dielectric values suggest that compounds 3 and 4 are more promising candidates for electronic applications, while compounds 1 and 2 are comparatively better Lewis acid catalysts.

Colossal Dielectric Responses from the Wide Band Gap 2D-Semiconducting Amine Templated Hybrid Framework Materials

Two unprecedented organic amine templated silver organophosphonate hybrid solids have been synthesized hydrothermally by varying the molar ratio of the reactants. Both of the compounds consist of novel tetra- and penta-nuclear silver phosphonate basic building units. The dielectric constants are extremely large due to the charge separation of anionic metal phosphonate frameworks and cationic templated piperazine moieties in the compounds, as found for the first time in a hybrid organophosphate family. The conductivity and UV-visible absorption studies provide strong evidence about the semiconducting nature of the present compounds.

Crystal growth of novel 3D skeleton uranyl germanium complexes: influence of synthetic conditions on crystal structures

Five centrosymmetric uranyl germanate compounds, K₈BrF(UO₂)₃(Ge₂O₇)₂, Rb₆(UO₂)₃(Ge₂O₇)₂·0.5H₂O, Cs₆(UO₂)₂Ge₈O₂₁ and A⁺₂(UO₂)₃(GeO₄)₂ (A⁺ = Rb⁺, Cs⁺), were synthesized in this work. K₈BrF(UO₂)₃(Ge₂O₇)₂ and Rb₆(UO₂)₃(Ge₂O₇)₂·0.5H₂O were obtained under mixed KF-KBr flux and hydrothermal conditions, respectively. Both structures crystallized in the triclinic P1[combining macron] space group and have similar anionic frameworks featuring novel hexagon shaped 12-membered channels. The condensation of two different types of SBU [UGe₄] pentamers (A) and (A2) results in the formation of K₈BrF(UO₂)₃(Ge₂O₇)₂ and Rb₆(UO₂)₃(Ge₂O₇)₂·0.5H₂O frameworks. Cs₆(UO₂)₂Ge₈O₂₁ was obtained from a CsF-CsCl high temperature flux, and it also crystallized in the centrosymmetric triclinic P1[combining macron] space group. The structure of Cs₆(UO₂)₂Ge₈O₂₁ has a novel oxo-germanate layer composed of germanate tetrahedra and trigonal bipyramids. Two new SBU types, (4²·5²-A2) and (5⁴-A2) [UGe₄] pentamers, were found in the structure of Cs₆(UO₂)₂Ge₈O₂₁. A⁺₂(UO₂)₃(GeO₄)₂ (A⁺ = Rb⁺, Cs⁺) were synthesized by a high temperature/high pressure (HT/HP) technique, and both structures with oval-shaped 12-membered channels crystallized in the centrosymmetric orthorhombic Pnma space group. The extreme conditions led to the formation of [U₂Ge₂] tetramers (E), which consist of 7-coordinated U and 5-coordinated Ge. Different synthetic methods of uranyl germanate compounds resulted in a distinct coordination environment of the uranyl cations and a variety of U[double bond, length as m-dash]O and U-O bond lengths, further affecting the dimensionality and types of uranyl units and SBUs. The Raman and IR spectra of the five new phases were collected and analyzed.

Structural and Spectroscopic Investigation of Novel 2D and 3D Uranium Oxo-Silicates/Germanates and Some Statistical Aspects of Uranyl Coordination in Oxo-Salts

Synthesis, structural and spectroscopic characterization, and topological analysis of five novel uranyl-based silicates and germanates have been performed. The open-framework K₄(UO₂)₂Si₈O₂₀·4H₂O has been synthesized under hydrothermal conditions and is based upon [USi₆] heptamers interconnected via edge-sharing. Its structure is composed of sechser silicate layers with 4-, 8-, and 16-membered rings. The largest 16-membered rings have an average dimension of ∼8.93 × 9.42 Ų. β-K₂(UO₂)Si₄O₁₀ has been obtained by the high-temperature flux growth method. Its 3D framework contains a loop-branched sechser single layer with 4- and 8-membered rings and consists of the same [USi₆] heptamers as observed in K₄(UO₂)₂Si₈O₂₀·4H₂O. Na₆(UO₂)₃(Si₂O₇)₂ has also been synthesized from melted fluxes and represents a 2D layer structure composed by [USi₄] pentamers. Two iso-structural compounds A⁺(UO₂)(HGeO₄)·H₂O (A⁺ = Rb⁺, Cs⁺) were synthesized via the hydrothermal method, and their structures are of the α-uranophane type. The 2D layers consist of [U₂Ge₂] tetramer secondary building units (SBUs). The Raman spectra of all novel phases were collected, and bands were assigned according to the existing oxo-silicate rings and oxo-germanium units. Additionally, we performed a statistical investigation of the local coordination of uranyl ions in all known inorganic structures with different oxo-anions (TO_x, T = B³⁺, Si/Ge⁴⁺, P/As⁵⁺, S/Se/Te⁶⁺, Cr/Mo/W⁶⁺, P/As³⁺, and Se/Te⁴⁺). We found a direct correlation between the ionic potential of the central cations T in oxo-anions in their higher oxidation states and the coordination number of uranyl groups.

Formation of Open Framework Uranium Germanates: The Influence of Mixed Molten Flux and Charge Density Dependence in U-Silicate and U-Germanate Families

Seven novel open-framework uranyl germanates, K₂(UO₂)GeO₄, K₆(UO₂)₃Ge₈O₂₂, α-Cs₂(UO₂)Ge₂O₆, β-Cs₂(UO₂)Ge₂O₆, Cs₂(UO₂)GeO₄, and A(UO₂)₃(Ge₂O₇)₂ (A = [NaK₆Cl]⁶⁺, [Na₂Cs₆Cl₂]⁶⁺), were grown from different mixed molten fluxes. The three-dimensional (3D) structure of K₂(UO₂)GeO₄ with 8-ring channels can be built upon [UGe₄] pentamer secondary building units (SBUs). The 3D framework of K₆(UO₂)₃Ge₈O₂₂ with trapezoid (Ge₈O₂₂)^12- clusters consists of two types of [UGe₄] pentamers. The 3D framework of α-Cs₂(UO₂)Ge₂O₆ with 10-ring channels, crystallizing in the P2₁/ n space group, is constructed by [UGe₄] pentamers. The structure of β-Cs₂(UO₂)Ge₂O₆ contains achter (eight) single germanate chains and is composed of [UGe₆] heptamers and [UGe₄] pentamers. The structure of Cs₂(UO₂)GeO₄ with hexagonal 10-ring channels is composed of [U₃Ge₄] heptamers and twisting five-fold GeO₄ tetrahedra in four-membered Ge₄O₁₂ rings occur. 3D frameworks of NaK₆Cl(UO₂)₃(Ge₂O₇)₂ (space group Pnnm) and Na₂Cs₆Cl₂(UO₂)₃(Ge₂O₇)₂ ( P2₁/ c) can be constructed from the same SBUs [UGe₄] pentamers. Thermal stability of salt-inclusions was studied by TG and PXRD analysis. Analysis of charge density for the U-Si-O system indicates that the polymerization of silicate units reduces the cross-links of the 3D frameworks. The concept of SBUs combined with the cutting and gluing strategy was applied to understand and analyze the distinct 8-, 10-, 12-, and 14- membered channels for the uranyl germanate family. The charge density of all known 3D U-Si/Ge-O frameworks has been investigated, which shows strong correlations with chemical composition of corresponding phases. The increase of Si/O (Ge/O) ratios in silicate units results in the decrease of negative charge density. Moreover, the charge density increases with decreasing countercation size within the same Si/O ratio. The correlations can be used to predict inclusion phase formation within U-Si/Ge-O families. Raman spectra of the studied uranyl germanates were measured, and bands were assigned on the basis of structural features.

Novel amine templated three-dimensional zinc-organophosphonates with variable pore-openings

Amine templated three-dimensional zinc-organophosphates with different ring sizes have been obtained by varying the organic amine molecules. The figure shows the various net structures obtained during the study.