Absorption of CO₂ and CS₂ into the Hofmann-Type Porous Coordination Polymer: Electrostatic versus Dispersion Interactions

Absorption of CO₂ and CS₂ molecules into the Hofmann-type three-dimensional porous coordination polymer (PCP) {Fe­(Pz)­[Pt­(CN)₄]}_<i>n</i> (Pz = pyrazine) was theoretically explored with the ONIOM­(MP2.5 or SCS-MP2:DFT) method, where the M06-2X functional was employed in the DFT calculations. The binding energies of CS₂ and CO₂ were evaluated to be −17.3 and −5.2 kcal mol^–1, respectively, at the ONIOM­(MP2.5:M06-2X) level and −16.9 and −4.4 kcal mol^–1 at the ONIOM­(SCS-MP2:M06-2X) level. It is concluded that CS₂ is strongly absorbed in this PCP but CO₂ is only weakly absorbed. The absorption positions of these two molecules are completely different: CO₂ is located between two Pt atoms, whereas one S atom of CS₂ is located between two Pz ligands and the other S atom is between two Pt atoms. The optimized position of CS₂ agrees with the experimentally reported X-ray structure. To elucidate the reasons for these differences, we performed an energy decomposition analysis and found that (i) both the large binding energy and the absorption position of CS₂ arise from a large dispersion interaction between CS₂ and the PCP, (ii) the absorption position of CO₂ is mainly determined by the electrostatic interaction between CO₂ and the Pt moiety, and (iii) the small binding energy of CO₂ comes from the weak dispersion interaction between CO₂ and the PCP. Important molecular properties relating to the dispersion and electrostatic interactions, which are useful for understanding and predicting gas absorption into PCPs, are discussed in detail

Proton-Conductive Magnetic Metal–Organic Frameworks, {NR₃(CH₂COOH)}[M_a^IIM_b^III(ox)₃]: Effect of Carboxyl Residue upon Proton Conduction

Proton-conductive magnetic metal–organic frameworks (MOFs), {NR₃(CH₂COOH)}­[M_a^IIM_b^III(ox)₃] (abbreviated as <b>R–M</b>_<b>a</b><b>M</b>_<b>b</b>: R = ethyl (Et), <i>n</i>-butyl (Bu); M_aM_b = MnCr, FeCr, FeFe) have been studied. The following six MOFs were prepared: <b>Et–MnCr</b>·2H₂O, <b>Et–FeCr</b>·2H₂O, <b>Et–FeFe</b>·2H₂O, <b>Bu–MnCr</b>, <b>Bu–FeCr</b>, and <b>Bu–FeFe</b>. The structure of <b>Bu–MnCr</b> was determined by X-ray crystallography. Crystal data: trigonal, <i>R</i>3<i>c</i> (#161), <i>a</i> = 9.3928(13) Å, <i>c</i> = 51.0080(13) Å, <i>Z</i> = 6. The crystal consists of oxalate-bridged bimetallic layers interleaved by {NBu₃(CH₂COOH)}⁺ ions. <b>Et–MnCr</b>·2H₂O and <b>Bu–MnCr</b> (R–MnCr MOFs) show a ferromagnetic ordering with <i>T</i>_C of 5.5–5.9 K, and <b>Et–FeCr</b>·2H₂O and <b>Bu–FeCr</b> (R–FeCr MOFs) also show a ferromagnetic ordering with <i>T</i>_C of 11.0–11.5 K. <b>Et–FeFe</b>·2H₂O and <b>Bu–FeFe</b> (R–FeFe MOFs) belong to the class II of mixed-valence compounds and show the magnetism characteristic of Néel N-type ferrimagnets. The Et-MOFs (<b>Et–MnCr</b>·2H₂O, <b>Et–FeCr</b>·2H₂O and <b>Et–FeFe</b>·2H₂O) show high proton conduction, whereas the Bu–MOFs (<b>Bu–MnCr</b>, <b>Bu–FeCr</b>, and <b>Bu–FeFe</b>) show moderate proton conduction. Together with water adsorption isotherm studies, the significance of the carboxyl residues as proton carriers is revealed. The R–MnCr MOFs and the R–FeCr MOFs are rare examples of coexistent ferromagnetism and proton conduction, and the R–FeFe MOFs are the first examples of coexistent Néel N-type ferrimagnetism and proton conduction

Promotion of Low-Humidity Proton Conduction by Controlling Hydrophilicity in Layered Metal–Organic Frameworks

We controlled the hydrophilicity of metal–organic frameworks (MOFs) to achieve high proton conductivity and high adsorption of water under low humidity conditions, by employing novel class of MOFs, {NR₃(CH₂COOH)}­[MCr­(ox)₃]·<i>n</i>H₂O (abbreviated as <b>R-MCr</b>, where R = Me (methyl), Et (ethyl), or Bu (<i>n</i>-butyl), and M = Mn or Fe): <b>Me-FeCr</b>, <b>Et-MnCr</b>, <b>Bu-MnCr</b>, and <b>Bu-FeCr</b>. The cationic components have a carboxyl group that functions as the proton carrier. The hydrophilicity of the cationic ions was tuned by the NR₃ residue to decrease with increasing bulkiness of the residue: {NMe₃(CH₂COOH)}⁺ > {NEt₃(CH₂COOH)}⁺ > {NBu₃(CH₂COOH)}⁺. The proton conduction of the MOFs increased with increasing hydrophilicity of the cationic ions. The most hydrophilic sample, <b>Me-FeCr</b>, adsorbed a large number of water molecules and showed a high proton conductivity of ∼10^–4 S cm^–1, even at a low humidity of 65% relative humidity (RH), at ambient temperature. Notably, this is the highest conductivity among the previously reported proton-conducting MOFs that operate under low RH conditions

Promotion of Low-Humidity Proton Conduction by Controlling Hydrophilicity in Layered Metal–Organic Frameworks

We controlled the hydrophilicity of metal–organic frameworks (MOFs) to achieve high proton conductivity and high adsorption of water under low humidity conditions, by employing novel class of MOFs, {NR₃(CH₂COOH)}­[MCr­(ox)₃]·<i>n</i>H₂O (abbreviated as <b>R-MCr</b>, where R = Me (methyl), Et (ethyl), or Bu (<i>n</i>-butyl), and M = Mn or Fe): <b>Me-FeCr</b>, <b>Et-MnCr</b>, <b>Bu-MnCr</b>, and <b>Bu-FeCr</b>. The cationic components have a carboxyl group that functions as the proton carrier. The hydrophilicity of the cationic ions was tuned by the NR₃ residue to decrease with increasing bulkiness of the residue: {NMe₃(CH₂COOH)}⁺ > {NEt₃(CH₂COOH)}⁺ > {NBu₃(CH₂COOH)}⁺. The proton conduction of the MOFs increased with increasing hydrophilicity of the cationic ions. The most hydrophilic sample, <b>Me-FeCr</b>, adsorbed a large number of water molecules and showed a high proton conductivity of ∼10^–4 S cm^–1, even at a low humidity of 65% relative humidity (RH), at ambient temperature. Notably, this is the highest conductivity among the previously reported proton-conducting MOFs that operate under low RH conditions

Proton Conduction Study on Water Confined in Channel or Layer Networks of La^IIIM^III(ox)₃·10H₂O (M = Cr, Co, Ru, La)

Proton conduction of the La^IIIM^III compounds, LaM­(ox)₃·10H₂O (abbreviated to <b>LaM</b>; M = Cr, Co, Ru, La; ox^2– = oxalate) is studied in view of their networks. <b>LaCr</b> and <b>LaCo</b> have a ladder structure, and the ladders are woven to form a channel network. <b>LaRu</b> and <b>LaLa</b> have a honeycomb sheet structure, and the sheets are combined to form a layer network. The occurrence of these structures is explained by the rigidness versus flexibility of [M­(ox)₃]^3– in the framework with large La^III. The channel networks of <b>LaCr</b> and <b>LaCo</b> show a remarkably high proton conductivity, in the range from 1 × 10^–6 to 1 × 10^–5 S cm^–1 over 40–95% relative humidity (RH) at 298 K, whereas the layer networks of <b>LaCr</b> and <b>LaCo</b> show a lower proton conductivity, ∼3 × 10^–8 S cm^–1 (40–95% RH, 298 K). Activation energy measurements demonstrate that the channels filled with water molecules serve as efficient pathways for proton transport. <b>LaCo</b> was gradually converted to La^IIICo^II(ox)_2.5·4H₂O, which had no channel structure and exhibited a low proton conductivity of less than 1 × 10^–10 S cm^–1. The conduction–network correlation of LaCo­(ox)_2.5·4H₂O is reported

Direct Synthesis of Prussian Blue Nanoparticles in Liposomes Incorporating Natural Ion Channels for Cs⁺ Adsorption and Particle Size Control

Coordination polymer (CP) nanoparticles (NPs) formed by a self-assembly of organic ligands and metal ions are one of the attractive materials for molecular capture and deliver/release in aqueous media. Control of particle size and prevention of aggregation among CP NPs are important factors for improving their adsorption capability in water. We demonstrate here the potential of a liposome incorporating an antibiotic ion channel as a vessel for synthesizing Prussian blue (PB) NPs, being a typical CP. In the formation of PB NPs within liposomes, the influx rate of Fe²⁺ ions into liposome encapsulated [Fe­(CN)₆]^3– through channels was fundamental for the change of NPs’ sizes. The optimized PB NP–liposome composite showed higher adsorption capacity of Cs⁺ ions than that of aggregated PB NPs that are prepared without liposome in aqueous media

Vapor-Induced Conversion of a Centrosymmetric Organic–Inorganic Hybrid Crystal into a Proton-Conducting Second-Harmonic-Generation-Active Material

Chemical responsivity in materials is essential to build systems with switchable functionalities. However, polarity-switchable materials are still rare because inducing a symmetry breaking of the crystal structure by adsorbing chemical species is difficult. In this study, we demonstrate that a molecular organic–inorganic hybrid crystal of (NEt4)2[MnN(CN)4] (1) undergoes polarity switching induced by water vapor and transforms into a rare example of proton-conducting second-harmonic-generation-active material. Centrosymmetric 1 transforms into noncentrosymmetric polar 1·3H2O and 1·MeOH by accommodating water and methanol molecules, respectively. However, only water vapor causes a spontaneous single-crystal-to-single-crystal transition. Moreover, 1·3H2O shows proton conduction with 2.3 × 10–6 S/cm at 298 K and a relative humidity of 80%

Reversible Chemisorption of Sulfur Dioxide in a Spin Crossover Porous Coordination Polymer

The chemisorption of sulfur dioxide (SO₂) on the Hofmann-like spin crossover porous coordination polymer (SCO-PCP) {Fe­(pz)­[Pt­(CN)₄]} has been investigated at room temperature. Thermal analysis and adsorption–desorption isotherms showed that ca. 1 mol of SO₂ per mol of {Fe­(pz)­[Pt­(CN)₄]} was retained in the pores. Nevertheless, the SO₂ was loosely attached to the walls of the host network and completely released in 24 h at 298 K. Single crystals of {Fe­(pz)­[Pt­(CN)₄]}·<i>n</i>SO₂ (<i>n</i> ≈ 0.25) were grown in water solutions saturated with SO₂, and its crystal structure was analyzed at 120 K. The SO₂ molecule is coordinated to the Pt^II ion through the sulfur atom ion, Pt–S = 2.585(4) Å. This coordination slightly stabilizes the low-spin state of the Fe^II ions shifting the critical temperatures of the spin transition by 8–12 K. DFT calculations have been performed to rationalize these observations