A Phosphate‐Based Silver–Bipyridine 1D Coordination Polymer with Crystallized Phosphoric Acid as Superprotonic Conductor

Variation in Catalytic Efficacies of a 2D pH-Stable MOF by Altering Activation Methods

Although it is well-known that the Lewis acidity of Metal-Organic Frameworks (MOFs) can effectively enhance their catalytic activity in organic transformations, access to these Lewis-acidic sites remains a key hurdle to widespread applications of Lewis-acidic catalysis by MOFs. Easy accessibility of strong Lewis acidic sites onto 2D MOFs by using proper activation methods can be a cornerstone in attaining desired catalytic performance. Herein, we report a new 2D chemically stable MOF, IITKGP-60, which displayed excellent framework robustness over a wide pH range (2-12). Benefiting from the abundant open metal sites (OMSs) and framework robustness, the catalytic activity of the developed material was explored in one-pot three-component Strecker reaction and Knoevenagel condensation reaction. Moreover, the developed catalyst is superior in catalyzing the reactions involving sterically hindered substrate (1-naphthaldehyde) with high turnover number. A comparative catalytic study was conducted using different activation methods (chloroform and methanol exchanged activated samples), highlighting the significant effect of activation methods on its catalytic performances. The sustainable synthetic pathway under solvent-free conditions for a broad scope of substrates using low catalyst loading and excellent recyclability made the developed pH-stable framework a promising heterogeneous catalyst.

pH-Stable Zn(II) Coordination Polymer as a Multiresponsive Turn-On and Turn-Off Fluorescent Sensor for Aqueous Medium Detection of Al(III) and Cr(VI) Oxo-Anions

Nowadays, coordination polymers (CPs) are promising candidates as sensory materials for their high sensitivity, improved selectivity, fast responsive nature, as well as good recyclability. However, poor chemical stability often makes their practical usage limited. Herein, employing a mixed ligand approach, we constructed a chemically robust CP, {[Zn2L2(DPA)2]·3H2O}n (IITKGP-70, IITKGP stands for the Indian Institute of Technology Kharagpur), which exhibited excellent framework robustness not only in water but also over a broad range of pH solutions (pH = 3-11). The developed framework displayed high selectivity and sensitivity for the detection of trivalent Al3+ ions and toxic hexavalent Cr(VI)-oxo anions in an aqueous medium. The developed framework exhibited an aqueous medium Al3+ turn-on phenomenon with a limit of detection (LOD) value of 1.29 μM, whereas a turn-off effect was observed for toxic oxo-anions (Cr2O72- and CrO42-) having LOD values of 0.27 and 0.71 μM, respectively. Both turn-on and turn-off mechanisms are speculated via spectroscopic methods coupled with several ex situ studies. Such a multiresponsive nature (both turn-on and turn-off) for aqueous medium detection of targeted cations and anions simultaneously in a single platform coupled with high robustness, ease of scalability, recyclability, and fast-responsive nature makes IITKGP-70 highly fascinating as a sensory material for real-world applications.

Synthesis, crystal structure and photophysical properties of bis-[2,6-di-fluoro-3-(pyridin-2-yl)pyridine-κN](tri-fluoro-methane-sulfonato-κO)silver(I)

In the title compound, [Ag(CF3SO3)(C10H6F2N2)2], the AgI centre adopts a highly distorted trigonal-planar coordination environment resulting from its coordination by one O atom of the tri-fluoro-methane-sulfonate anion and the pyridine N atoms of two crystallographically independent 2',6'-di-fluoro-2,3'-bi-pyridine ligands, which display very similar conformations to one another. Pairwise Ag⋯O-SO2CF3 - [Ag⋯O = 2.8314 (14) Å] inter-actions and inter-molecular C-H⋯O inter-actions between inversion-related units lead to the formation of an eight-membered cyclic dimer in which the silver atoms are separated by 6.2152 (3) Å. In the crystal, the dimers are linked through C-H⋯O hydrogen bonds, halogen⋯π and weak π-π stacking inter-actions, resulting in the formation of a three-dimensional supra-molecular network. The title compound exhibits a strong and broad emission band from 400 nm to 550 nm in solution and its photoluminescence quantum efficiency is estimated to be ca 0.2, indicating that the title compound could have applications as an emitting material in organic light-emitting diodes (OLEDs).

A new silver coordination polymer based on 4,6-diamino-2-pyrimidinethiol: synthesis, characterization and catalytic application in asymmetric Hantzsch synthesis of polyhydroquinolines

A highly efficient and stable heterogeneous coordination polymer (CP) was successfully prepared by hydrothermal combination of silver and 4,6-diamino-2-pyrimidinethiol. The prepared coordination polymer was characterized by FT-IR, XRD, TGA, SEM, EDX, X-ray mapping and Nitrogen adsorption–desorption analysis. The prepared Ag–CP exhibit excellent catalytic activity in multicomponent Hantzsch synthesis of polyhydroquinolines under mild reaction conditions in relatively short reaction times. The heterogeneity of the catalyst was confirmed by the hot filtration test; also, the catalyst was reused for at least four times under the optimized reaction conditions without any significant loss of its catalytic activity.

Structural and proton conductivity studies of fibrous π-Ti2O(PO4)2·2H2O: application in chitosan-based composite membranes

Although the fibrous polymorphic modification of titanium phosphate, π-Ti2O(PO4)2·2H2O (π-TiP) has been known for decades, its crystal structure has remained unsolved. Herewith, we report the crystal structure of π-TiP at room temperature, as determined from synchrotron radiation powder X-ray diffraction, and corroborated by 31P solid state NMR and accurate density functional theory calculations. In contrast to the previously reported ρ-TiP polymorph, the as-synthesized hydrated phase crystallizes in the monoclinic system (P21/c, a = 5.1121(2) Å, b = 14.4921(9) Å, c = 12.0450(11), β = 115.31(1)°, Z = 4), and is composed of corner-sharing titanium octahedra and phosphate units arranged in a pattern that is unique to the ρ-TiP polymorph. The unit cell was confirmed by electron diffraction, while the formation of planar packing imperfections and stacking faults along the [101] plane was revealed by HRTEM analysis. An in situ dehydration study of π-TiP, monitored by high-temperature powder X-ray diffraction, led to a new anhydrous monoclinic (P21/c, a = 5.1187(13) Å, b = 11.0600(21) Å, c = 14.4556(26), β = 107.65(2)°, Z = 4) phase that crystallizes at 500 °C. The latter resembles the packing fashion of the parental π-TiP, albeit titanium atoms are present in both distorted tetrahedral and octahedral coordination environments. Anhydrous π-TiP was found to partially rehydrate at room temperature, reversibly adopting the structure of the initial phase. The studies carried out under different conditions of leaching and impregnation with H3PO4 showed that π-TiP exhibits an extrinsic proton conductivity (1.3 × 10-3 S cm-1 at 90 °C and 95% RH) due to the presence of the protonated phosphate species bound on the particles surface, as revealed by 31P MAS-NMR spectroscopy data. The composite membranes of Chitosan (CS) matrices filled with H3PO4-impregnated π-TiP solid show an increment of proton conductivity up to 4.5 × 10-3 S cm-1, at 80 °C and 95% RH, which is 1.8-fold higher than those of the bare CS membranes.

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.

Immobilization of a Polar Sulfone Moiety onto the Pore Surface of a Humid-Stable MOF for Highly Efficient CO2 Separation under Dry and Wet Environments through Direct CO2-Sulfone Interactions

The stability of microporous metal-organic frameworks (MOFs) in moist environments must be taken into consideration for their practical implementations, which has been largely ignored thus far. Herein, we synthesized a new moisture-stable Zn-MOF, {[Zn₂(SDB)₂(L)₂]·2DMA}_n, IITKGP-12, by utilizing a bent organic linker 4,4'-sulfonyldibenzoic acid (H₂SDB) containing a polar sulfone group (-SO₂) and a N, N-donor spacer (L) with a Brunauer-Emmett-Teller surface area of 216 m² g^-1. This material displays greater CO₂ adsorption capacity over N₂ and CH₄ with high IAST selectivity, which is also validated by breakthrough experiments with longer breakthrough times for CO₂. Most importantly, the separation performance is largely unaffected in the presence of moisture of simulated flue gas stream. Temperature-programmed desorption (TPD) analysis shows the ease of the regeneration process, and the performance was verified for multiple cycles. In order to understand the structure-function relationship at the atomistic level, grand canonical Monte Carlo (GCMC) calculation was performed, indicating that the primary binding site for CO₂ is between the sulfone moieties in IITKGP-12. CO₂ is attracted to the bonded structure (V-shape) of the sulfone moieties in a perpendicular fashion, where C_CO2 is aligned with S, and the CO₂ axis bisects the SO₂ axis. Thus, the strategic approach to immobilize the polar sulfone moiety with a high number of inherent stronger M-N coordination and the absence of coordination unsaturation made this MOF potential toward practical CO₂ separation applications.

A "Thermodynamically Stable" 2D Nickel Metal-Organic Framework over a Wide pH Range with Scalable Preparation for Efficient C2 s over C1 Hydrocarbon Separations

The design and construction of "thermodynamically stable" metal-organic frameworks (MOFs) that can survive in liquid water, boiling water, and acidic/basic solutions over a wide pH range is highly desirable for many practical applications, especially adsorption-based gas separations with obvious scalable preparations. Herein, a new thermodynamically stable Ni MOF, {[Ni(L)(1,4-NDC)(H₂ O)₂ ]}_n (IITKGP-20; L=4,4'-azobispyridine; 1,4-NDC=1,4-naphthalene dicarboxylic acid; IITKGP stands for the Indian Institute of Technology Kharagpur), has been designed that displays moderate porosity with a BET surface area of 218 m²  g^-1 and micropores along the [10-1] direction. As an alternative to a cost-intensive, cryogenic, high-pressure distillation process for the separation of hydrocarbons, MOFs have recently shown promise for such separations. Thus, towards an application standpoint, this MOF exhibits a higher uptake of C₂ hydrocarbons over that of C₁ hydrocarbon under ambient conditions, with one of the highest selectivities based on the ideal adsorbed solution theory (IAST) method. A combination of two strategies (the presence of stronger metal-N coordination of the spacer and the hydrophobicity of the aromatic moiety of the organic ligand) possibly makes the framework highly robust, even stable in boiling water and over a wide range of pH 2-10, and represents the first example of a thermodynamically stable MOF displaying a 2D structural network. Moreover, this material is easily scalable by heating the reaction mixture at reflux overnight. Because such separations are performed in the presence of water vapor and acidic gases, there is a great need to explore thermodynamically stable MOFs that retain not only structural integrity, but also the porosity of the frameworks.