Highly Selective Sorption and Separation of CO₂ from a Gas Mixture of CO₂ and CH₄ at Room Temperature by a Zeolitic Organic–Inorganic Ionic Crystal and Investigation of the Interaction with CO₂

Mixed gas cosorption and gas chromatographic investigations demonstrate that a zeolitic organic–inorganic ionic crystal K₂[Cr₃O­(OOCH)₆(4-etpy)₃]₂[α-SiW₁₂O₄₀]·2H₂O [<b>1</b>·2H₂O] (etpy = ethylpyridine) with a pore diameter of 3.5 Å possesses high separation ability of carbon dioxide (kinetic diameter 3.3 Å) over methane (3.76 Å) at room temperature in the presence of water vapor. Monte Carlo simulation combined with density functional theory calculation suggests that carbon dioxide molecules diffuse into the one-dimensional channels and interact initially with the potassium ions and then with the oxygen atoms of silicododecatungstates, which are confirmed with carbon dioxide sorption enthalpy and <i>in situ</i> IR spectroscopy

Synthesis and Reversible Transformation of Cu_<i>n</i>‑Bridged (<i>n</i> = 1, 2, or 4) Silicodecatungstate Dimers

Three copper-bridged sandwich-type silicodecatungstate dimers, TBA₈[Cu­(γ-SiW₁₀O₃₄)₂(CH₃CONH)₂]·4H₂O (<b>Cu-1</b>, TBA = tetra-<i>n-</i>butylammonium), TBA₈H₄[Cu₂(γ-SiW₁₀O₃₆)₂H₂O]·11H₂O·CH₃COCH₃ (<b>Cu-2</b>), and TBA₈H₂[Cu₄(γ-SiW₁₀O₃₆)₂(CH₃COO)₂]·5H₂O (<b>Cu-4</b>) have been selectively synthesized by reactions of divacant lacunary TBA₄[H₄(γ-SiW₁₀O₃₆)] (SiW10) with copper acetate in organic media. The copper cation(s) in <b>Cu-1</b>, <b>Cu-2</b>, and <b>Cu-4</b> possess square-planar four-coordinate (<b>Cu-1</b>), square-pyramidal five-coordinate (<b>Cu-2</b>), and octahedral six-coordinate (<b>Cu-4</b>) geometries, respectively. These compounds can reversibly be transformed simply by controlling the copper/SiW10 molar ratios in solutions

Concerted Functions of Anions and Cations in a Molecular Ionic Crystal with Stable Three-Dimensional Micropores

The molecular ionic crystal [Cr₃O­(OOCCHCH₂)₆(H₂O)₃]₃[α-PW₁₂O₄₀]·15H₂O [<b>Ia</b>] with stable three-dimensional micropores and a minimum aperture of 3.3 Å was synthesized with a phosphododecatungstate [α-PW₁₂O₄₀]^3– (polyoxometalate, POM) and a macrocation with acrylate ligands [Cr₃O­(OOCCHCH₂)₆­(H₂O)₃]⁺. The porous structure of <b>Ia</b> was basically constructed by an arrangement of macrocations forming a six-membered ring: vinyl groups (CHCH₂) of adjacent macrocations were aligned parallel to each other, suggesting a weak dispersion force between them. A guest-free phase [Cr₃O­(OOCCHCH₂)₆­(H₂O)₃]₃­[α-PW₁₂O₄₀] [<b>Ib</b>] was formed by the treatment of <b>Ia</b> in vacuo at room temperature without any structure change. Compound <b>Ib</b> showed shape-selective sorption of CO₂ and C₂H₂ (molecular size = 3.3 Å) over N₂ (3.6 Å) and methane (3.7 Å), and the sorption enthalpy of C₂H₂ was larger than that of CO₂. The high affinity toward C₂H₂ was further confirmed as follows: the Monte Carlo simulations of the optimized geometries of C₂H₂ in <b>Ib</b> showed that both hydrogen atoms were in the vicinity of the surface oxygen atoms of POMs. The gas sorption profiles showed a much faster diffusion for C₂H₂. All these results suggest that the anion and cation mainly play the guest-binding and structure-directing roles, respectively, (i.e., concerted functions) in an ionic crystal with stable three-dimensional micropores

Amphiprotic Properties of a Bis(μ-hydroxo)divanadium(IV)-Substituted γ‑Keggin-Type Silicodecatungstate Containing Two Different Kinds of Hydroxyl Moieties

A bis­(μ-hydroxo)­divanadium­(IV)-substituted γ-Keggin-type silicodecatungstate, (TBA)₄[γ-SiV^IV₂W₁₀O₃₆(μ-OH)₄] (<b>1</b>), possesses two different kinds of hydroxyl groups and can work as an amphiprotic species to accept and donate proton(s). Dehydrative condensation reactions of <b>1</b> with methanol and formic acid proceed on more basic hydroxyl groups between two vanadium atoms without the deprotonation of more acidic hydroxides between two tungsten atoms to form (TBA)₄[γ-SiV^IV₂W₁₀O₃₆(μ-OH)₃(μ-OR)] (<b>2·R</b>, R = Me, Et, Pr; <b>3</b>, R = C­(O)­H), showing Brønsted base properties of the hydroxyl groups between two vanadium atoms. On the other hand, the hydroxyl groups between tungsten atoms exhibit Brønsted acid properties and react with pyridine (Py) and TBAOH to form (TBA)₄X­[γ-SiV^IV₂W₁₀O₃₇(μ-OH)₃] (<b>PyH·4</b>, X = PyH; <b>TBA·4</b>, X = TBA). DFT calculations for [γ-SiV^IV₂W₁₀O₃₆(μ-OH)₄]^4– in water also support both the acidic and basic nature of hydroxyl groups in <b>1</b>

Porous Ionic Crystals Modified by Post-Synthesis of K₂[Cr₃O(OOCH)₆(etpy)₃]₂[α-SiW₁₂O₄₀]·8H₂O through Single-Crystal-to-Single-Crystal Transformation

Post-synthesis modification of a porous ionic crystal proceeded via two steps (acid treatment followed by ion-exchange) in an aqueous solution and a single-crystal-to-single-crystal manner. Compound K₂[Cr₃O­(OOCH)₆(etpy)₃]₂[α-SiW₁₂O₄₀]·8H₂O (etpy = 4-ethylpyridine) [<b>1a</b>] is a porous ionic crystal with one-dimensional channels, which can accommodate guests such as water, alcohols, and halocarbons. Crystals of <b>1a</b> were immersed in an aqueous HCl solution (acid treatment), and the etpy ligand which was exposed to the one-dimensional channel was removed and exchanged with water. The formula of the resulting compound was (etpyH⁺)₂[Cr₃O­(OOCH)₆(etpy)₂(H₂O)]₂[α-SiW₁₂O₄₀]·6H₂O [<b>2a</b>], and K⁺ ions, which are potential guest binding sites, were simultaneously removed by this treatment. Reincorporation of K⁺ ions was attempted by immersion of <b>2a</b> into an aqueous CH₃COOK solution (ion-exchange), and K₂[Cr₃O­(OOCH)₆(etpy)_2.5(H₂O)_0.5]₂[α-SiW₁₂O₄₀]·8H₂O [<b>3a</b>] was formed. Increase in sorption capacity by the two-step post-synthesis modification was confirmed by sorption isotherms and Monte Carlo-based simulations using water as a probe molecule. The role of K⁺ ions as water binding sites was confirmed by water sorption isotherms of alkali metal ion-exchanged compounds

Synthesis and Reversible Transformation of Cu_<i>n</i>‑Bridged (<i>n</i> = 1, 2, or 4) Silicodecatungstate Dimers

Three copper-bridged sandwich-type silicodecatungstate dimers, TBA₈[Cu­(γ-SiW₁₀O₃₄)₂(CH₃CONH)₂]·4H₂O (<b>Cu-1</b>, TBA = tetra-<i>n-</i>butylammonium), TBA₈H₄[Cu₂(γ-SiW₁₀O₃₆)₂H₂O]·11H₂O·CH₃COCH₃ (<b>Cu-2</b>), and TBA₈H₂[Cu₄(γ-SiW₁₀O₃₆)₂(CH₃COO)₂]·5H₂O (<b>Cu-4</b>) have been selectively synthesized by reactions of divacant lacunary TBA₄[H₄(γ-SiW₁₀O₃₆)] (SiW10) with copper acetate in organic media. The copper cation(s) in <b>Cu-1</b>, <b>Cu-2</b>, and <b>Cu-4</b> possess square-planar four-coordinate (<b>Cu-1</b>), square-pyramidal five-coordinate (<b>Cu-2</b>), and octahedral six-coordinate (<b>Cu-4</b>) geometries, respectively. These compounds can reversibly be transformed simply by controlling the copper/SiW10 molar ratios in solutions

Copper-Catalyzed Oxidative Cross-Coupling of <i>H</i>‑Phosphonates and Amides to <i>N</i>‑Acylphosphoramidates

A simple combination of copper(II) acetate (Cu(OAc)₂) and an appropriate base could promote oxidative cross-coupling of <i>H</i>-phosphonates and amides using air as a terminal oxidant. The substrate scope was broad with respect to both dialkyl <i>H</i>-phosphonates and nitrogen nucleophiles (including oxazolidinone, lactam, pyrrolidinone, urea, indole, and sulfonamide derivatives), giving the corresponding P–N coupling products in moderate to high yields

Redox-Induced Reversible Uptake–Release of Cations in Porous Ionic Crystals Based on Polyoxometalate: Cooperative Migration of Electrons with Alkali Metal Ions

Redox-active porous ionic crystals based on polyoxometalates (POM) were synthesized. By treating the crystal with an aqueous solution of ascorbic acid (reducing reagent) and KCl, one-electron reduction of POM proceeded followed by simultaneous uptake of K⁺. Interestingly, the reduction did not proceed without KCl, and the molecular size of ascorbic acid was too large to enter the porous crystal lattice. The time courses of reduction and K⁺ uptake were monitored by UV–vis spectroscopy and atomic absorption spectrometry (AAS), respectively. Both profiles could be reproduced by the linear driving force (LDF) model with similar rate constants. The reduced crystal could be oxidized with aqueous chlorine solution followed by the release of K⁺, and the redox cycles were reversible. The water sorption properties of the crystals could be controlled by the types of alkali metal ions incorporated. The Cs⁺ uptake and the simultaneous reduction of the crystal proceeded much faster than in the case of K⁺, which is in line with the trends in the Gibbs energies of hydration of alkali metal ions. Complete selectivity to Cs⁺ was observed in the uptake of ions from an aqueous binary mixture of Cs⁺ and Na⁺. All these results suggest the cooperative migration of electrons with alkali metal ions and the redox induced ion-exchange in porous ionic crystals based on POM

Synthesis and Characterization of Molecular Hexagons and Rhomboids and Subsequent Encapsulation of Keggin-Type Polyoxometalates by Molecular Hexagons

Structural control among hexagonal (trimer), rhomboidal (dimer), and infinite-chain supramolecular complexes with three different supporting ligands of ethylenediamine (en), <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethylethylenediamine (en*), and 1,2-bis­(diphenyl)­phosphinoethane (dppe) [(en)­Pd­(L)]₃(OTf)₆ <b>1t·OTf</b>, [(en*)­Pd­(L)]₂(PF₆)₄ <b>2d·PF</b>_<b>6</b>, and [(dppe)­Pd­(L)­(OTf)₂]_∞ <b>3·OTf</b> (OTf = trifluoromethane sulfonate; L = 1,3-bis­(4-pyridylethynyl)­benzene) in the solid and solution states was investigated. The encapsulation of a large Keggin-type polyoxometalate [α-PW₁₂O₄₀]^3– by these complexes was also examined. As the steric bulkiness of the supporting ligands increased in the order of en < en* < dppe, the hexagonal, rhomboidal, and infinite-chain structures were obtained, as confirmed by X-ray crystallography. In solution, equilibrium between the molecular hexagon (<b>1t·OTf</b>/<b>2t·PF</b>_<b>6</b>) and the molecular rhomboid (<b>1d·OTf</b>/<b>2d·PF</b>_<b>6</b>) was observed in the en/en* ligand systems, whereas <b>3·OTf</b> with the dppe ligand did not exhibit equilibrium and instead existed as a single species. These phenomena were established by cold-spray ionization mass spectroscopy (CSI-MS) and ¹H diffusion ordered NMR spectroscopy (DOSY). The addition of the highly negatively charged Keggin-type phosphododecatungstate [α-PW₁₂O₄₀]^3– to a solution of <b>2t/2d·PF</b>_<b>6</b> resulted in the encapsulation of the tungstate species in the cavity of the molecular hexagon to form {[(en*)­Pd­(L)]₃[⊃α-PW₁₂O₄₀]}­(PF₆)₃ <b>2t·</b>[α-PW₁₂O₄₀]^3–, as confirmed by a combination of ¹H and ³¹P DOSY and CSI-MS spectral data

Synthesis, Structure Characterization, and Reversible Transformation of a Cobalt Salt of a Dilacunary γ‑Keggin Silicotungstate and Sandwich-Type Di- and Tetracobalt-Containing Silicotungstate Dimers

A cobalt salt of a γ-Keggin dilacunary silicotungstate, {CoL₅}₂[γ-SiW₁₀O₃₄L₂] [<b>Co-SiW10</b>; L = <i>N</i>,<i>N</i>-dimethylformamide (DMF) or H₂O], could be synthesized by the cation-exchange reaction of TBA₄[γ-H₄SiW₁₀O₃₆] (TBA = tetra-<i>n</i>-butylammonium) with 2 equiv of Co­(NO₃)₂ with respect to TBA₄[γ-H₄SiW₁₀O₃₆] in a mixed solvent of DMF and acetone (97% yield). Each <b>Co-SiW10</b> was linked by water molecules via a hydrogen-bonding network. Besides <b>Co-SiW10</b>, various kinds of isostructural <b>M-SiW10</b> could be synthesized via the same procedure as that for <b>Co-SiW10</b> (M = Mn²⁺, Fe²⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Cd²⁺). By the reaction of <b>Co-SiW10</b> with 1 equiv of TBA₆[γ-H₂SiW₁₀O₃₆] in acetone, a silicotungstate dimer pillared by two cobalt cations with a significantly slipped dimer configuration, TBA₆[Co₂(γ-H₃SiW₁₀O₃₆)₂]·3H₂O (<b>Co2</b>), could be synthesized. By the reaction of <b>Co-SiW10</b> with 3 equiv of TBAOH in acetone, a tetracobalt-containing sandwich-type silicotungstate, TBA₆[{Co­(H₂O)}₂(μ₃-OH)₂{Co­(H₂O)₂}₂(γ-H₂SiW₁₀O₃₆)₂]·5H₂O (<b>Co4</b>), could be synthesized. Compound <b>Co4</b> possessed the tetracobalt–oxygen core, [{Co­(H₂O)}₂(μ₃-OH)₂{Co­(H₂O)₂}₂]⁶⁺, identical with those of previously reported Weakley-type sandwich polyoxometalates, [Co₄(H₂O)₂(XM₉O₃₄)₂]^<i>n</i>− (X = P⁵⁺, Si⁴⁺, Ge⁴⁺, As⁵⁺ or V⁵⁺; M = Mo⁶⁺ or W⁶⁺). The reversible transformation between these three compounds (<b>Co-SiW10</b> ⇆ <b>Co2</b>, <b>Co-SiW10</b> ⇆ <b>Co4</b>, and <b>Co2</b> ⇆ <b>Co4</b>) took place by the addition and/or subtraction of required components in appropriate solvents, affording the desired products in high yields (71–93% yields)