Proton Conductivity Control by Ion Substitution in a Highly Proton-Conductive Metal–Organic Framework

Proton conductivity through two-dimensional (2-D) hydrogen-bonding networks within a layered metal–organic framework (MOF) (NH₄)₂(H₂adp)­[Zn₂(ox)₃]·3H₂O (H₂adp = adipic acid; ox = oxalate) has been successfully controlled by cation substitution. We synthesized a cation-substituted MOF, K₂(H₂adp)­[Zn₂(ox)₃]·3H₂O, where the ammonium ions in a well-defined hydrogen-bonding network are substituted with non-hydrogen-bonding potassium ions, without any apparent change in the crystal structure. We successfully controlled the proton conductivity by cleavage of the hydrogen bonds in a proton-conducting pathway, showing that the 2-D hydrogen-bonding networks in the MOF truly contribute to the high proton conductivity. This is the first example of the control of proton conductivity by ion substitution in a well-defined hydrogen-bonding network within a MOF

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

Control of Crystalline Proton-Conducting Pathways by Water-Induced Transformations of Hydrogen-Bonding Networks in a Metal–Organic Framework

Structure-defined metal–organic frameworks (MOFs) are of interest because rational design and construction allow us to develop good proton conductors or possibly control the proton conductivity in solids. We prepared a highly proton-conductive MOF (NH₄)₂(adp)­[Zn₂(ox)₃]·<i>n</i>H₂O (abbreviated to <b>1·</b><i><b>n</b></i><b>H</b>_<b>2</b><b>O</b>, adp: adipic acid, ox: oxalate, <i>n</i> = 0, 2, 3) having definite crystal structures and showing reversible structural transformations among the anhydrate (<b>1</b>), dihydrate (<b>1·2H</b>_<b>2</b><b>O</b>), and trihydrate (<b>1·3H</b>_<b>2</b><b>O</b>) phases. The crystal structures of all of these phases were determined by X-ray crystallography. Hydrogen-bonding networks consisting of ammonium ions, water molecules, and carboxylic acid groups of the adipic acids were formed inside the two-dimensional interlayer space in hydrated <b>1·2H</b>_<b>2</b><b>O</b> and <b>1·3H</b>_<b>2</b><b>O</b>. The crystal system of <b>1</b> or <b>1·2H</b>_<b>2</b><b>O</b> (<i>P</i>2₁/<i>c</i>, No. 14) was changed into that of <b>1·3H</b>_<b>2</b><b>O</b> (<i>P</i>1̅, No. 2), depending on water content because of rearrangement of guests and acidic molecules. Water molecules play a key role in proton conduction as conducting media and serve as triggers to change the proton conductivity through reforming hydrogen-bonding networks by water adsorption/desorption processes. Proton conductivity was consecutively controlled in the range from ∼10^–12 S cm^–1 (<b>1</b>) to ∼10^–2 S cm^–1 (<b>1·3H</b>_<b>2</b><b>O</b>) by the humidity. The relationships among the structures of conducting pathways, adsorption behavior, and proton conductivity were investigated. To the best of our knowledge, this is the first example of the control of a crystalline proton-conducting pathway by guest adsorption/desorption to control proton conductivity using MOFs

Control of Crystalline Proton-Conducting Pathways by Water-Induced Transformations of Hydrogen-Bonding Networks in a Metal–Organic Framework

Structure-defined metal–organic frameworks (MOFs) are of interest because rational design and construction allow us to develop good proton conductors or possibly control the proton conductivity in solids. We prepared a highly proton-conductive MOF (NH₄)₂(adp)­[Zn₂(ox)₃]·<i>n</i>H₂O (abbreviated to <b>1·</b><i><b>n</b></i><b>H</b>_<b>2</b><b>O</b>, adp: adipic acid, ox: oxalate, <i>n</i> = 0, 2, 3) having definite crystal structures and showing reversible structural transformations among the anhydrate (<b>1</b>), dihydrate (<b>1·2H</b>_<b>2</b><b>O</b>), and trihydrate (<b>1·3H</b>_<b>2</b><b>O</b>) phases. The crystal structures of all of these phases were determined by X-ray crystallography. Hydrogen-bonding networks consisting of ammonium ions, water molecules, and carboxylic acid groups of the adipic acids were formed inside the two-dimensional interlayer space in hydrated <b>1·2H</b>_<b>2</b><b>O</b> and <b>1·3H</b>_<b>2</b><b>O</b>. The crystal system of <b>1</b> or <b>1·2H</b>_<b>2</b><b>O</b> (<i>P</i>2₁/<i>c</i>, No. 14) was changed into that of <b>1·3H</b>_<b>2</b><b>O</b> (<i>P</i>1̅, No. 2), depending on water content because of rearrangement of guests and acidic molecules. Water molecules play a key role in proton conduction as conducting media and serve as triggers to change the proton conductivity through reforming hydrogen-bonding networks by water adsorption/desorption processes. Proton conductivity was consecutively controlled in the range from ∼10^–12 S cm^–1 (<b>1</b>) to ∼10^–2 S cm^–1 (<b>1·3H</b>_<b>2</b><b>O</b>) by the humidity. The relationships among the structures of conducting pathways, adsorption behavior, and proton conductivity were investigated. To the best of our knowledge, this is the first example of the control of a crystalline proton-conducting pathway by guest adsorption/desorption to control proton conductivity using MOFs

Control of Crystalline Proton-Conducting Pathways by Water-Induced Transformations of Hydrogen-Bonding Networks in a Metal–Organic Framework

Structure-defined metal–organic frameworks (MOFs) are of interest because rational design and construction allow us to develop good proton conductors or possibly control the proton conductivity in solids. We prepared a highly proton-conductive MOF (NH₄)₂(adp)­[Zn₂(ox)₃]·<i>n</i>H₂O (abbreviated to <b>1·</b><i><b>n</b></i><b>H</b>_<b>2</b><b>O</b>, adp: adipic acid, ox: oxalate, <i>n</i> = 0, 2, 3) having definite crystal structures and showing reversible structural transformations among the anhydrate (<b>1</b>), dihydrate (<b>1·2H</b>_<b>2</b><b>O</b>), and trihydrate (<b>1·3H</b>_<b>2</b><b>O</b>) phases. The crystal structures of all of these phases were determined by X-ray crystallography. Hydrogen-bonding networks consisting of ammonium ions, water molecules, and carboxylic acid groups of the adipic acids were formed inside the two-dimensional interlayer space in hydrated <b>1·2H</b>_<b>2</b><b>O</b> and <b>1·3H</b>_<b>2</b><b>O</b>. The crystal system of <b>1</b> or <b>1·2H</b>_<b>2</b><b>O</b> (<i>P</i>2₁/<i>c</i>, No. 14) was changed into that of <b>1·3H</b>_<b>2</b><b>O</b> (<i>P</i>1̅, No. 2), depending on water content because of rearrangement of guests and acidic molecules. Water molecules play a key role in proton conduction as conducting media and serve as triggers to change the proton conductivity through reforming hydrogen-bonding networks by water adsorption/desorption processes. Proton conductivity was consecutively controlled in the range from ∼10^–12 S cm^–1 (<b>1</b>) to ∼10^–2 S cm^–1 (<b>1·3H</b>_<b>2</b><b>O</b>) by the humidity. The relationships among the structures of conducting pathways, adsorption behavior, and proton conductivity were investigated. To the best of our knowledge, this is the first example of the control of a crystalline proton-conducting pathway by guest adsorption/desorption to control proton conductivity using MOFs

Design and Synthesis of Hydroxide Ion–Conductive Metal–Organic Frameworks Based on Salt Inclusion

We demonstrate a metal–organic framework (MOF) design for the inclusion of hydroxide ions. Salt inclusion method was applied to an alkaline-stable ZIF-8 (ZIF = zeolitic imidazolate framework) to introduce alkylammonium hydroxides as ionic carriers. We found that tetrabutylammonium salts are immobilized inside the pores by a hydrophobic interaction between the alkyl groups of the salt and the framework, which significantly increases the hydrophilicity of ZIF-8. Furthermore, ZIF-8 including the salt exhibited a capacity for OH^– ion exchange, implying that freely exchangeable OH^– ions are present in the MOF. ZIF-8 containing OH^– ions showed an ionic conductivity of 2.3 × 10^–8 S cm^–1 at 25 °C, which is 4 orders of magnitude higher than that of the blank ZIF-8. This is the first example of an MOF-based hydroxide ion conductor

Design and Synthesis of Hydroxide Ion–Conductive Metal–Organic Frameworks Based on Salt Inclusion

We demonstrate a metal–organic framework (MOF) design for the inclusion of hydroxide ions. Salt inclusion method was applied to an alkaline-stable ZIF-8 (ZIF = zeolitic imidazolate framework) to introduce alkylammonium hydroxides as ionic carriers. We found that tetrabutylammonium salts are immobilized inside the pores by a hydrophobic interaction between the alkyl groups of the salt and the framework, which significantly increases the hydrophilicity of ZIF-8. Furthermore, ZIF-8 including the salt exhibited a capacity for OH^– ion exchange, implying that freely exchangeable OH^– ions are present in the MOF. ZIF-8 containing OH^– ions showed an ionic conductivity of 2.3 × 10^–8 S cm^–1 at 25 °C, which is 4 orders of magnitude higher than that of the blank ZIF-8. This is the first example of an MOF-based hydroxide ion conductor

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