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

Enantiopure trigonal bipyramidal coordination cages templated by in situ self-organized D2h-symmetric anions

The control of a molecule's geometry, chirality, and physical properties has long been a challenging pursuit. Our study introduces a dependable method for assembling D3-symmetric trigonal bipyramidal coordination cages. Specifically, D2h-symmetric anions, like oxalate and chloranilic anions, self-organize around a metal ion to form chiral-at-metal anionic complexes, which template the formation of D3-symmetric trigonal bipyramidal coordination cages. The chirality of the trigonal bipyramid is determined by the point chirality of chiral amines used in forming the ligands. Additionally, these cages exhibit chiral selectivity for the included chiral-at-metal anionic template. Our method is broadly applicable to various ligand systems, enabling the construction of larger cages when larger D2h-symmetric anions, like chloranilic anions, are employed. Furthermore, we successfully produce enantiopure trigonal bipyramidal cages with anthracene-containing backbones using this approach, which would be otherwise infeasible. These cages exhibit circularly polarized luminescence, which is modulable through the reversible photo-oxygenation of the anthracenes.

Room-Temperature Superprotonic Conductivity beyond 10-1 S cm-1 in a Co(II) Coordination Polymer

An efficient design of crystalline solid-state proton conductors (SSPCs) is crucial for the progress of clean energy applications. Developing such materials to make them work at room temperature with a conductivity of ≥10-1 S cm-1 is of significant interest in terms of technical and commercial aspects. Utilizing the recently highlighted "coordinated-water-driven proton conduction" approach, herein, we have rationally synthesized two highly stable and scalable 1D Co(II) coordination polymers (CPs) as SSPCs, PCM-2 {[Co(bpy)(H2O)2(NO3)2]·H2O}n and PCM-3 {[Co2(bpy)2(SO4)2(H2O)6].4H2O}n, with distinct alignments in coordinated water and coordinated oxo-anions (nitrate and sulfate, respectively). The acidity of the metal-bound water molecules in PCM-2 is further enhanced through cooperative long-range continuous H bonds with coordinated Brønsted basic nitrates (proton acceptors), leading to ultrahigh superprotonic conductivities even at 25 °C (1.03 × 10-1 S cm-1 under 95% RH), and reached a maximum of 2.99 × 10-1 S cm-1 at 85 °C (95% RH). The conductivity at 25 °C is even higher than that of commercial Nafion 117 (6.74 × 10-2 S cm-1 at 100% RH). The absence of such an H-bonding interaction in PCM-3 (closed loops) resulted in a lesser conductivity of 5.87 × 10-5 S cm-1 (95% RH, 85 °C). PCM-2 represents the first example of SSPC exhibiting conductivity in the order 10-1 S cm-1 at ambient temperature (25 °C) with excellent recyclability.

In-situ surface enhanced Raman spectroscopy revealing the role of metal-organic frameworks on photocatalytic reaction selectivity on highly sensitive and durable Cu-CuBr substrate

Photocatalytic reactions using copper-based nanomaterials have emerged as a new paradigm in green technology. Selective photocatalysis is very important for improving energy utilization efficiency, and in order to directional improve catalytic selectivity, it is necessary to understand the mechanism of interfacial reactions at the molecular level. Therefore, a unique bifunctional Cu-CuBr substrate is first fabricated via an electrochemical method, which overcomes the instability of traditional copper-based materials and endows high surface-enhanced Raman spectroscopy (SERS) sensitivity and photocatalytic performance and can be stored stably for more than a year. Further modification of the surface with Metal-Organic Frameworks (MOFs) containing carboxyl functional groups can significantly tune the surface properties of the substrate. This increases the adsorption of cationic dyes to improve the SERS effect, and 10-10 M methylene blue can easily be detected with this substrate. Surprisingly, in-situ SERS monitoring of the interfacial photocatalytic dehalogenation reaction of aromatic halides through its intrinsic SERS effect reveal two competing selective reaction pathways, self-coupling and hydrogenation. Typically, the SERS spectra reveal that the latter's selectivity was greatly enhanced after MOFs modification, and the yield rate of the hydrogenated product increased from 27.6 % to 46.9 % (selectivity increased from 32.7 % to 51.5 %). This proves that the surface properties of catalysts, especially the affinity for reaction intermediates, can effectively regulate catalytic selectivity.

Transition from vehicle to Grotthuss proton transfer in a nanosized flask: cryogenic ion spectroscopy of protonated p-aminobenzoic acid solvated with D2O

Proton transfer (PT) is one of the most ubiquitous reactions in chemistry and life science. The unique nature of PT has been rationalized not by the transport of a solvated proton (vehicle mechanism) but by the Grotthuss mechanism in which a proton is transported to the nearest proton acceptor along a hydrogen-bonded network. However, clear experimental evidence of the Grotthuss mechanism has not been reported yet. Herein we show by infrared spectroscopy that a vehicle-type PT occurs in the penta- and hexahydrated clusters of protonated p-aminobenzoic acid, while Grotthuss-type PT is observed in heptahydrated clusters, indicating a change in the PT mechanism depending on the degree of hydration. These findings emphasize the importance of the usually ignored vehicle mechanism as well as the degree of hydration. It highlights the possibility of controlling the PT mechanism by the number of water molecules in chemical and biological environments.

Covalent Organic Frameworks: Reversible 3D Coalesce via Interlocked Skeleton-Pore Actions and Impacts on π Electronic Structures

Covalent organic frameworks (COFs) create extended two-dimensional (2D) skeletons and aligned one-dimensional (1D) channels, constituting a class of novel π architectures with predesignable structural ordering. A distinct feature is that stacks of π building units in skeletons shape the pore walls, onto which a diversity of different units can be assembled to form various pore interfaces, opening a great potential to trigger a strong structural correlation between the skeleton and the pore. However, such a possibility has not yet been explored. Herein, we report reversible three-dimensional (3D) coalescence and interlocked actions between the skeleton and pore in COFs by controlling hydrogen-bonding networks in the pores. Introducing carboxylic acid units to the pore walls develops COFs that can confine water molecular networks, which are locked by the surface carboxylic acid units on the pore walls via multipoint, multichain, and multidirectional hydrogen-bonding interactions. As a result, the skeleton undergoes an interlocked action with pores to shrink over the x-y plane and to stack closer along the z direction upon water uptake. Remarkably, this interlocked action between the skeleton and pore is reversibly driven by water adsorption and desorption and triggers profound effects on π electronic structures and functions, including band gap, light absorption, and emission.

A Pair of Multifunctional Cu(II)-Dy(III) Enantiomers with Zero-Field Single-Molecule Magnet Behaviors, Proton Conduction Properties and Magneto-Optical Faraday Effects

Multifunctional materials with a coexistence of proton conduction properties, single-molecule magnet (SMM) behaviors and magneto-optical Faraday effects have rarely been reported. Herein, a new pair of Cu(II)-Dy(III) enantiomers, [DyCu2(RR/SS-H2L)2(H2O)4(NO3)2]·(NO3)·(H2O) (R-1 and S-1) (H4L = [RR/SS] -N,N'-bis [3-hydroxysalicylidene] -1,2-cyclohexanediamine), has been designed and prepared using homochiral Schiff-base ligands. R-1 and S-1 contain linear Cu(II)-Dy(III)-Cu(II) trinuclear units and possess 1D stacking channels within their supramolecular networks. R-1 and S-1 display chiral optical activity and strong magneto-optical Faraday effects. Moreover, R-1 shows a zero-field SMM behavior. In addition, R-1 demonstrates humidity- and temperature-dependent proton conductivity with optimal values of 1.34 × 10-4 S·cm-1 under 50 °C and 98% relative humidity (RH), which is related to a 1D extended H-bonded chain constructed by water molecules, nitrate and phenol groups of the RR-H2L ligand.

High Proton Conductivity of Magnetically Ordered 2D Oxalate-Bridged [MnIICrIII] Coordination Polymers with Irregular Topology

Two heterometallic coordination polymers {[NH(CH₃)₂(C₂H₅)]₈[Mn₄Cl₄Cr₄(C₂O₄)₁₂]}_n (1) and {[NH(CH₃)-(C₂H₅)₂]₈[Mn₄Cl₄Cr₄(C₂O₄)₁₂]}_n (2) were obtained by slow evaporation of an aqueous solution containing the building block [A]₃[Cr(C₂O₄)₃] [A = (CH₃)₂(C₂H₅)NH⁺ or (CH₃)(C₂H₅)₂NH⁺] and MnCl₂·2H₂O. The isostructural compounds comprise irregular two-dimensional (2D) oxalate-bridged anionic layers [Mn₄Cl₄Cr₄(C₂O₄)₁₂]_n^8n- with a Shubnikov plane net fes topology designated as (4·8²), interleaved by the hydrogen-bonded templating cations (CH₃)₂(C₂H₅)NH⁺ (1) or (CH₃)(C₂H₅)₂NH⁺ (2). They exhibit remarkable humidity-sensing properties and very high proton conductivity at room temperature [1.60 × 10^-3 (Ω·cm)^-1 at 90% relative humidity (RH) of 1 and 9.6 × 10^-4 (Ω·cm)^-1 at 94% RH of 2]. The layered structure facilitates the uptake of water molecules, which contributes to the enhancement of proton conductivity at high RH. The better proton transport observed in 1 compared to that in 2 can be tentatively attributed to the higher hydrophilicity of the cations (CH₃)₂(C₂H₅)NH⁺, which is closely related to their affinity for water molecules. The original topology of the anionic networks in both compounds leads to the development of interesting magnetic phases upon cooling. The magnetically ordered ground state can be described as the coupling of ferromagnetic spin chains in which Mn²⁺ and Cr³⁺ ions are bridged by bis(bidentate) oxalate groups into antiferromagnetic planes through monodentate-bidentate oxalate bridges in the layers, which are triggered to long-range order below temperature 4.45 K via weaker interlayer interactions.

The proton conduction behavior of two 1D open-framework metal phosphates with similar crystal structures and different hydrogen bond networks

Two open-framework zinc phosphates [C₃N₂H₁₂][Zn(HPO₄)₂] (1) and [C₆N₄H₂₂]_0.5[Zn(HPO₄)₂] (2) were synthesized via hydrothermal reaction and characterized by powder X-ray diffraction, thermogravimetric analysis and scanning electron microscopy. Both compounds have a similar crystal structure and macroscopic morphology. However, the difference in equilibrium cations, in which the propylene diamine is for 1 and the triethylenetetramine is for 2, results in a significant distinction in the dense hydrogen grid. The diprotonated propylene diamine molecule in 1 is more favorable for forming a hydrogen-bond network in three dimensions than in 2, in which the twisted triethylenetetramine forms a hydrogen bond grid with the inorganic framework only in two dimensions owing to its large steric effect. This distinction further leads to a disparity in the proton conductivity of both compounds. The proton conductivity of 1 can reach 1.00 × 10^-3 S cm^-1 under ambient conditions (303 K and 75% RH) and then increase to 1.11 × 10^-2 S cm^-1 at 333 K and 99% RH, which is the highest value among the open-framework metal phosphate proton conductors operated in the same conduction. In contrast, the proton conductivity of 2 is four orders of magnitude smaller than 1 at 303 K and 75% RH and two orders smaller than 1 at 333 K and 99% RH.

f-block MOFs: A Pathway to Heterometallic Transuranics

A novel series of heterometallic f-block-frameworks including the first examples of transuranic heterometallic ²³⁸ U/²³⁹ Pu-metal-organic frameworks (MOFs) and a novel monometallic ²³⁹ Pu-analog are reported. In combination with theoretical calculations, we probed the kinetics and thermodynamics of heterometallic actinide(An)-MOF formation and reported the first value of a U-to-Th transmetallation rate. We concluded that formation of uranyl species could be a driving force for solid-state metathesis. Density of states near the Fermi edge, enthalpy of formation, band gap, proton affinity, and thermal/chemical stability were probed as a function of metal ratios. Furthermore, we achieved 97 % of the theoretical maximum capacity for An-integration. These studies shed light on fundamental aspects of actinide chemistry and also foreshadow avenues for the development of emerging classes of An-containing materials, including radioisotope thermoelectric generators or metalloradiopharmaceuticals.

Advances in Metal-Organic Frameworks MIL-101(Cr)

MIL-101(Cr) is one of the most well-studied chromium-based metal-organic frameworks, which consists of metal chromium ion and terephthalic acid ligand. It has an ultra-high specific surface area, large pore size, good thermal/chemical/water stability, and contains unsaturated Lewis acid sites in its structure. Due to the physicochemical properties and structural characteristics, MIL-101(Cr) has a wide range of applications in aqueous phase adsorption, gas storage and separation, and catalysis. In this review, the latest synthesis of MIL-101(Cr) and its research progress in adsorption and catalysis are reviewed.

Improvement of the Proton Conduction of Copper(II)-Mesoxalate Metal-Organic Frameworks by Strategic Selection of the Counterions

Three copper(II)/mesoxalate-based MOFs with formulas (H₃O)[Cu₉(Hmesox)₆(H₂O)₆Cl]·8H₂O (1), (NH₂Me₂)_0.4(H₃O)_0.6[Cu₉(Hmesox)₆(H₂O)₆Cl]·8H₂O (2), and (enH₂)_0.25(enH)_1.5[Cu₆(Hmesox)₃(mesox)(H₂O)₆Cl_0.5]Cl_0.5·5.25H₂O (3) were synthesized (H₄mesox = mesoxalic acid = 2,2-dihydroxypropanedioic acid, en = ethylenediamine). Essentially, all of the compounds display the same anionic network with a different arrangement of the cations, which have a remarkable effect on the proton conduction of the materials, ranging from 1.16 × 10^–4 S cm^–1 for 1 to 1.87 × 10^–3 S cm^–1 for 3 (at 80 °C and 95% RH). These compounds also display antiferromagnetic coupling among the copper(II) ions through both the carboxylate and alkoxido bridges. The values of the principal magnetic coupling constants were calculated by density functional theory (DFT), leading to congruent values that confirm the predominant antiferromagnetic nature of the interactions.

Three copper(II)-mesoxalate metal−organic frameworks were synthesized in the presence of three different cations: hydronium, dimethylammonium, and ethylenediammonium, which neutralize the charge of the anionic networks. Besides the crystallographic characterization and the investigation of the magnetic properties, the compounds show varying proton conductivities depending on the included cations. The proton conductivity increases 1 order of magnitude in the case of compound 3 (1.87 × 10⁻³ S cm⁻¹ at 80 °C and 95% RH), which contains ethylenediammonium cations.

Ratiometric and Colorimetric Optical Thermometers Using Emissive Dimeric and Trimeric {[Au(SCN)2 ]- }n Moieties Generated in d-f Heterometallic Assemblies

Gold complexes can generate excimers ([Au₂ ]→[Au₂ ]*) and exciplexes ([Au₃ ]→[Au₃ ]*) with light excitation. Four Gd^III and Y^III complexes were assembled with dimeric {[Au(SCN)₂ ]^- }₂ and trimeric {[Au(SCN)₂ ]^- }₃ bis(thiocyanato)gold(I) counterions. The vibrational signature associated with the Au⋅⋅⋅Au vibrational mode was probed with ultralow frequency (ULF) Raman spectroscopy as a function of temperature. Emission spectroscopy was used to explore photophysical properties. Two broad features in the high- and low-energy regions were associated with the fluorescence and phosphorescence of the gold entities, respectively. Temperature-dependent luminescence measurements showed that the emission color can be tuned from blue to green via cyan and white. Hence, these complexes can act as colorimetric thermometers. Additionally, a ratiometric thermal sensing ability was incorporated with high sensitivity up to 5 % K^-1 in the cryogenic temperature range.

Simultaneous Control of Pore-Space Partition and Charge Distribution in Multi-Modular Metal-Organic Frameworks

We report here a strategy for making anionic pacs type porous materials by combining pore space partition with charge reallocation. The method uses the first negatively charged pore partition ligand (2,5,8-tri-(4-pyridyl)-1,3,4,6,7,9-hexaazaphenalene, H-tph) that simultaneously enables pore partition and charge reallocation. Over two dozen anionic pacs materials have been made to demonstrate their excellent chemical stability and a high degree of tunability. Notably, Ni₃ -bdt-tph (bdt=1,4-benzeneditetrazolate) exhibits month-long water stability, while CoV-bdt-tph sets a new benchmark for C₂ H₂ storage capacity under ambient conditions for ionic MOFs. In addition to tunable in-framework modules, we show feasibility to tune the type and concentration of extra-framework counter cations and their influence on both stability and capability to separate industrial C₃ H₈ /C₃ H₆ and C₆ H₆ /C₆ H₁₂ mixtures.

Significantly lowered activation energy in proton conductor by Mg substitution in a layered Ni metal-organic framework

Designs and developments of proton conductors are highly important in chemistry and energy fields. In this study, a novel metal-organic framework H₂DAB-MgNi(ox)₃ was synthesized. X-ray powder diffraction, scanning transmission electron microscopy, and scanning transmission electron microscopy-energy-dispersive X-ray mapping measurements demonstrated that the H₂DAB-MgNi(ox)₃ had a solid-solution structure, with the homogeneous distribution of Mg and Ni elements. The proton conductivity of H₂DAB-MgNi(ox)₃ was enhanced from that of H₂DAB-Ni₂(ox)₃ at 95% relative humidity by Mg substitution.

Superprotonic Conductivity of a Functionalized Metal-Organic Framework at Ambient Conditions

Seeking fast proton transport pathways at ambient conditions is desirable but challenging. Here, we report a strategy to synthesize a composite material with a polyoxometalate (POM) and an ionic liquid (IL) confined in stable metal-organic framework (MOF) channels through electrostatic interaction. The obtained SO₃H-IL-PMo₁₂@MIL-101 possesses fast proton transfer, and its proton conductivity can reach 1.33 × 10^-2 S cm^-1 at ambient conditions (30 °C, 70% relative humidity (RH)), which is the highest value among the MOF-based proton conductors operated in an ambient environment. Therefore, it has the potential of becoming a room-temperature proton conductor without a humidifier. Importantly, the composite material is further fabricated into a composite membrane for proton-exchange membrane fuel cells (PEMFCs), which can deliver a power density of 0.93 mW cm^-2 at 30 °C and 98% RH. This result can lay a fundamental basis for the application of MOF-based proton conductors in the area of electrochemical energy conversion.

Sorption and Magnetic Properties of Oxalato-Based Trimetallic Open Framework Stabilized by Charge-Assisted Hydrogen Bonds

We report a new structure of {[Co(bpy)₂(ox)][{Cu₂(bpy)₂(ox)}Fe(ox)₃]}_n·8.5nH₂O NCU-1 presenting a rare ladder topology among oxalate-based coordination polymers with anionic chains composed of alternately arranged [Cu₂(bpy)₂(ox)]²⁺ and [Fe(ox)₃]³⁻ moieties. Along the a axis, they are separated by Co(III) units to give porous material with voids of 963.7 ų (16.9% of cell volume). The stability of this structure is assured by a network of stacking interactions and charge-assisted C-H…O hydrogen bonds formed between adjacent chains, adjacent cobalt(III) units, and alternately arranged cobalt(III) and chain motifs. The soaking experiment with acetonitrile and bromobenzene showed that water molecules (8.5 water molecules dispersed over 15 positions) are bonded tightly, despite partial occupancy. Water adsorption experiments are described by a D’arcy and Watt model being the sum of Langmuir and Dubinin–Serpinski isotherms. The amount of primary adsorption sites calculated from this model is equal 8.2 mol H₂O/mol, being very close to the value obtained from the XRD experiments and indicates that water was adsorbed mainly on the primary sites. The antiferromagnetic properties could be only approximately described with the simple Cu^II-ox-Cu^II dimer using H = −J·S₁·S₂, thus, considering non-trivial topology of the whole Cu-Fe chain, we developed our own general approach, based on the semiclassical model (SC) and molecular field (MF) model, to describe precisely the magnetic superexchange interactions in NCU-1. We established that Cu(II)-Cu(II) coupling dominates over multiple Cu(II)-Fe(III) interactions, with J_CuCu = −275(29) and J_CuFe = −3.8(1.6) cm⁻¹ and discussed the obtained values against the literature data.

Stable isomeric layered indium coordination polymers for high proton conduction

Stable isomeric layered indium coordination polymers with different coordinated anionic sites for high proton conduction.

Cationic complex directed thiostannate layers with excellent proton conduction and photocatalysis properties

Three isostructural thiostannates SnS-M (M = Fe, Mn and Zn) have been fabricated using metal-amine complex cations as structure-directing agents. These thiostannates are composed of typical two-dimensional lamellar [Sn3S7]n2n- anionic...

Porosity regulation of metal-organic frameworks for high proton conductivity by rational ligand design: mono- versus disulfonyl-4,4′-biphenyldicarboxylic acid

Porous crystalline metal-organic frameworks (MOFs) bearing sulfonic groups (–SO3H) are receiving increasing attention as solid-state proton-conductors because the –SO3H group can not only enhance the proton concentration but also form...

Encapsulating NH4Br in a metal organic framework: achieving remarkable proton conduction in a wide relative humidity range

Proton-conducting materials are key components for constructing high-energy-density electronic devices. In this work, by accumulating NH₄Br into the nanospace of the classical metal organic framework MIL-101-Cr, a proton conductivity as high as 1.53 × 10^-1 S cm^-1 was achieved at 363 K and 100% RH. The proton conduction of NH₄Br@MIL-101-Cr was also high even at lower relative humidity; for instance, it was ∼10^-2 S cm^-1 at 75% RH. The activation energy was calculated to be 0.11 eV for NH₄Br@MIL-101-Cr, indicative of tight H-bond networks and a low barrier to proton transfer, and confirming the occurrence of pure proton conduction as well.

Strain-Controlled Spin Transition in Heterostructured Metal-Organic Framework Thin Film

Metal-organic framework (MOF) thin films have recently attracted much attention as a new platform for surface/interface research, where unconventional structural and physical properties emerge. Among the many MOFs as candidates for fabrication of thin films, Hofmann-type MOFs {Fe(pz)[M(CN)₄]} [pz = pyrazine; M = Ni (Nipz), M = Pt (Ptpz)] are attractive, because they undergo spin transitions with concomitant structural changes. Here, we demonstrate the first example of a strain-controlled spin transition in heterostructured MOF thin films. The spin transition temperature of Ptpz can be controlled in the temperature range of 300-380 K by fabricating a nanometer-sized heterostructured thin film with a Nipz buffer layer, where the smaller lattice of Nipz causes epitaxial compressive strain to the Ptpz layer. The fabricated heterostructured thin film exhibited a remarkable increase in spin transition temperature with a dynamic structural transformation, confirmed by variable-temperature (VT) X-ray diffraction and VT Raman spectroscopy. By verifying interfacial strain in a heterostructured thin film, we can rationally control the characteristics of MOFs-not only spin transition but also various physical properties such as gas storage, catalysis, sensing, proton conductivity, and electrical properties, among others.

Humidity-Sensing Properties of an 1D Antiferromagnetic Oxalate-Bridged Coordination Polymer of Iron(III) and Its Temperature-Induced Structural Flexibility

A novel one-dimensional (1D) oxalate-bridged coordination polymer of iron(III), {[NH(CH₃)(C₂H₅)₂][FeCl₂(C₂O₄)]}_n (1), exhibits remarkable humidity-sensing properties and very high proton conductivity at room temperature (2.70 × 10⁻⁴ (Ω·cm)⁻¹ at 298 K under 93% relative humidity), in addition to the independent antiferromagnetic spin chains of iron(III) ions bridged by oxalate groups (J = −7.58(9) cm⁻¹). Moreover, the time-dependent measurements show that 1 could maintain a stable proton conductivity for at least 12 h. Charge transport and magnetic properties were investigated by impedance spectroscopy and magnetization measurements, respectively. Compound 1 consists of infinite anionic zig-zag chains [FeCl₂(C₂O₄)]_nⁿ⁻ and interposed diethylmethylammonium cations (C₂H₅)₂(CH₃)NH⁺, which act as hydrogen bond donors toward carbonyl oxygen atoms. Extraordinarily, the studied coordination polymer exhibits two reversible phase transitions: from the high-temperature phase HT to the mid-temperature phase MT at T ~213 K and from the mid-temperature phase MT to the low-temperature phase LT at T ~120 K, as revealed by in situ powder and single-crystal X-ray diffraction. All three polymorphs show large linear thermal expansion coefficients.

Quasielastic neutron scattering study on proton dynamics assisted by water and ammonia molecules confined in MIL-53

Dynamics of water and other small molecules confined in nanoporous materials is one of the current topics in condensed matter physics. One popular host material is a benzenedicarboxylate-bridging metal (III) complex abbreviated to MIL-53, whose chemical formula is M(OH)[C6H2(CO2)2R2] where M = Cr, Al, Fe and R = H, OH, NH2, COOH. These materials absorb not only water but also ammonia molecules. We have measured the quasi-elastic neutron scattering of MIL-53(Fe)-(COOH)2·2H2O and MIL-53(Fe)-(COOH)2·3NH3 which have full guest occupancy and exhibit the highest proton conductivity in the MIL-53 family. In a wide relaxation time region (τ = 10-12-10-8 s), two relaxations with Arrhenius temperature dependence were found in each sample. It is of interest that their activation energies are smaller than those of bulk H2O and NH3 liquids. The momentum transfer dependence of the relaxation time and the temperature dependence of the relaxation intensity suggest that the proton conduction is due to the Grotthuss mechanism with thermally excited H2O and NH3 molecules.

Single-crystal-to-single-crystal transformations among three Mn-MOFs containing different water molecules induced by reaction time: crystal structures and proton conductivities

Three Mn-MOFs {[Mn₃(μ₄-L)₂(H₂O)₇]·4H₂O}_n (1), {[Mn₃(μ₅-L)₂(H₂O)₆]·4H₂O}_n (2) and {[Mn₃(μ₇-L)₂(H₂O)₂]}_n (3) (H₃L = 5-(6-carboxypyridin-3-yl)isophthalic acid) were obtained under different reaction times and temperatures. Interestingly, induced by reaction time, compound 1 can lose one water molecule and SC-SC transform into compound 2. Similarly, compound 2 can also SC-SC transform into 3. Studies on two SC-SC transformation processes were carried out and the transformation mechanisms were deduced, which were verified by TG analyses. Different numbers of water molecules in the three compounds resulted in different coordination environments of the metal cation, coordination modes of the L^3- ligand, continuities of hydrogen bonds, dimensions of framework and porosities. The AC impendence spectra studies revealed that compounds 1-3 can enhance the proton conductivities of the Nafion composite membrane to about 47.77%, 36.88% and 21.28%, respectively. It is speculated that the highest proton conductivity of compound 1 may be due to its continuous hydrogen bond chain and highest water uptake, which were mainly decided by the number of water molecules.

Engineering of the XY Magnetic Layered System with Adeninium Cations: Monocrystalline Angle-Resolved Studies of Nonlinear Magnetic Susceptibility

An original example of modular crystal engineering involving molecular magnetic {Cu^II[W^V(CN)₈]}^- bilayers and adeninium cations (AdeH⁺) toward the new layered molecular magnet (AdeH){Cu^II[W^V(CN)₈]}·2H₂O (1) is presented. 1 crystallizes within the monoclinic C2 space group (a = 41.3174(12), b = 7.0727(3), c = 7.3180(2) Å, β = 93.119(3)°, and V = 2135 ų). The bilayer topology is based on a stereochemical matching between the square pyramidal shape of Cu^II moiety and the bicapped trigonal prismatic shape of [W^V(μ-CN)₅(CN)₃], and the separation between bilayers is significantly increased (by ∼50%; from ca. 9.5 to ca. 14.5 Å) compared to several former analogues in this family. This was achieved via a unique combination of (i) a 1D ribbonlike hydrogen bond system {AdeH⁺···H₂O···AdeH⁺···}_∞ exploiting planar water-assisted Hoogsteen···Sugar synthons with (ii) parallel 1D π-π stacks {AdeH⁺···AdeH⁺}_∞. In-plane 2D XY magnetism is characterized by a T_c close to 33 K, H_c,in-plane = 60 Oe, and H_{c,out-of-plane} = 750 Oe, high values of in-plane γ critical exponents (γ_b = 2.34(6) for H||b and γ_c = 2.16(5) for H||c), and a Berezinskii-Kosterlitz-Thouless (BKT) topological phase transition, deduced from crystal-orientation-dependent scaling analysis. The obtained values of in-plane ν critical exponents, ν_b = 0.48(5) for H||b and ν_c = 0.49(3) for H||c, confirm the BKT transition (ν_BKT = 0.5). Full-range angle-resolved monocrystalline magnetic measurements supported by dedicated calculations indicated the occurrence of nonlinear susceptibility performance within the easy plane in a magnetically ordered state. We refer the occurrence of this phenomenon to spontaneous resolution in the C2 space group, a tandem not observed in studies on previous analogues and rarely reported in the field of molecular materials. The above magneto-supramolecular strategy may provide a novel means for the design of 2D molecular magnetic networks and help to uncover the inherent phenomena.

Control of Proton-Conductive Behavior with Nanoenvironment within Metal-Organic Materials

Solid-state proton-conductive materials have been of great interest for several decades due to their promising application as electrolytes in fuel cells and electrochemical devices. Metal-organic materials (MOMs) have recently been intensively investigated as a new type of proton-conductive materials. The highly crystalline nature and structural designability of MOMs make them advantageous over conventional noncrystalline proton-conductive materials-the detailed investigation of the structure-property relationship is feasible on MOM-based proton conductors. This review aims to summarize and examine the fundamental principles and various design strategies on proton-conductive MOMs, and shed light on the nanoconfinement effects as well as the importance of hydrophobicity on specific occasions, which have been often disregarded. Besides, challenges and future prospects on this field are presented.

Insertion of single-ion magnets based on mononuclear Co(II) complexes into ferromagnetic oxalate-based networks

The 1 : 2 and 1 : 1 Co(ii) complexes of the L ligand (L = 6-(3,5-diamino-2,4,6-triazinyl)2,2'-bipyridine) with formulas [CoII(L)2](ClO4)2·0.5MeCN·Et2O (1) and [CoII(L)(CH3CN)2(H2O)](ClO4)2·MeCN (2) have been prepared. The structural and magnetic characterization of the two compounds shows that they contain octahedral high-spin Co(ii) and present a field-induced slow relaxation of the magnetization. 1 has been inserted into a bimetallic oxalate-based network leading to a novel achiral 3D compound of formula [CoII(L)2][MnIICrIII(ox)3]2·(solvate) (3) exhibiting ferromagnetic ordering below 4.6 K. EPR measurements suggest a weak magnetic coupling between the two sublattices.

Ion-conductive metal-organic frameworks

Metal-organic frameworks (MOFs) have emerged as a new class of ionic conductors because of their tuneable and highly ordered microporous structures. The ionic conduction of various ionic carriers, such as a proton (H⁺), hydroxide ion (OH^-), lithium ion (Li⁺), sodium ion (Na⁺), and magnesium ion (Mg²⁺), in the pores of MOFs has been widely investigated over the past decade. Reports reveal that the porous or channel structures of MOFs are fundamentally suitable as ion-conducting pathways. There are clear differences in the basic designs of ion-conductive MOFs, i.e., the introduction of ionic carriers and construction of efficient ion-conducting pathways, depending on the ionic carriers. We summarize the examples and fundamental design of highly ion-conductive MOFs with various types of ionic carriers.

Versatile oxalato-bridging modes: a novel three-dimensional framework structure of manganese(ii) oxalate complex [MnC2O4]·0.5H2O and the relationship with other manganese(ii) oxalates

An unknown manganese(ii) oxalate complex [MnC₂O₄]·0.5H₂O (1) was discovered with a novel three-dimensional structure, exhibiting versatile bridging modes of the oxalate ligand. The thermal and magnetic behaviors of this complex were studied and the relationship with four other manganese(ii) oxalates is also discussed.

Rational strategies for proton-conductive metal-organic frameworks

Since the transition of energy platforms, proton-conducting materials have played a significant role in broad applications for electrochemical devices. In particular, solid-state proton conductors (SSPCs) are emerging as the electrolyte in fuel cells (FC), a promising power generation technology, because of their high performance and safety for operating in a wide range of temperatures. In recent years, proton-conductive porous metal-organic frameworks (MOFs) exhibiting high proton-conducting properties (>10-2 S cm-1) have been extensively investigated due to their potential application in solid-state electrolytes. Their structural designability, crystallinity, and porosity are beneficial to fabricate a new type of proton conductor, providing a comprehensive conduction mechanism. For the proton-conductive MOFs, each component, such as the metal centres, organic linkers, and pore space, is manipulated by a judicious predesign strategy or post-synthetic modification to improve the mobile proton concentration with an efficient conducting pathway. In this review, we highlight rational design strategies for highly proton-conductive MOFs in terms of MOF components, with representative examples from recent years. Subsequently, we discuss the challenges and future directions for the design of proton-conductive MOFs.

Aliphatic amine mediated assembly of [M6(mna)6] (M = Cu/Ag) into extended two-dimensional structures: synthesis, structure and Lewis acid catalytic studies

New aliphatic amine directed two-dimensional cadmium coordination polymers were shown to exhibit Lewis-acid catalytic activity for the cyanation of imines.