Tadpole-Like Polar Molecule Encapsulated in a Two-in-One Supramolecular Cage: Molecular Motion, Phase Transition and Ferroelectricity.

Molecule-inclusive closed cage compounds present a unique platform for molecular motion in an isolated environment. This study showcases the incorporation of a tadpole-like polar molecule (1-propyl-1H-imidazole, PIm) into a supramolecular cage formed by duad semicage p-tert-butylcalix[4]arene. The ferroelectric phase transition as well as the cage-confined motion of encapsulated PIm was studied in detail. The unusual quadrastable state of the PIm in the paraelectric phase allows for the modulation of dipolar polarization over a broad temperature/frequency range. This compound represents the first example of a clathrate molecular ferroelectric featuring a molecule-inclusive supramolecular cage, and it also contributes to the understanding of cage-confined molecular dynamics.

Metal-Organic Framework Supported Low-Nuclearity Cluster Catalysts for Highly Selective Carbon Dioxide Electroreduction to Ethanol.

It is still a great challenge to achieve high selectivity of ethanol in CO2 electroreduction reactions (CO2RR) because of the similar reduction potentials and lower energy barrier of possible other C2+ products. Here, we report a MOF-based supported low-nuclearity cluster catalysts (LNCCs), synthesized by electrochemical reduction of three-dimensional (3D) microporous Cu-based MOF, that achieves a single-product Faradaic efficiency (FE) of 82.5% at -1.0 V (versus the reversible hydrogen electrode) corresponding to the effective current density is 8.66 mA cm-2. By investigating the relationship between the species of reduction products and the types of catalytic sites, it is confirmed that the multi-site synergism of Cu LNCCs can increase the C-C coupling effect, and thus achieve high FE of CO2-to-ethanol. In addition, density functional theory (DFT) calculation and operando attenuated total reflectance surface-enhanced infrared absorption spectroscopy further confirmed the reaction path and mechanism of CO2-to-EtOH.

Unlocking single molecule magnetism: a supramolecular strategy for isolating neutral MnIII salen-type dimer in crystalline environments.

In [Mn(5-MeOsalen)(Cl)]2(dibenzo[24]crown-8), dibenzo[24]crown-8 formed a supramolecule via multi-point interactions with the [Mn(5-MeOsalen)(Cl)] dimer. The dimer was magnetically isolated with ST = 4 and weak interdimer magnetic interactions. The crystal exhibited single-molecule magnet behaviour with an anisotropic barrier of 26(1) K, which is the highest among the Mn-salen series reported to date.

A Mixed Protonic-Electronic Conductor Base on the Host-Guest Architecture of 2D Metal-Organic Layers and Inorganic Layers.

The key to designing and fabricating highly efficient mixed protonic-electronic conductors materials (MPECs) is to integrate the mixed conductive active sites into a single structure, to break through the shortcomings of traditional physical blending. Herein, based on the host-guest interaction, an MPEC is consisted of 2D metal-organic layers and hydrogen-bonded inorganic layers by the assembly methods of layered intercalation. Noticeably, the 2D intercalated materials (≈1.3 nm) exhibit the proton conductivity and electron conductivity, which are 2.02 × 10-5 and 3.84 × 10-4 S cm-1 at 100 °C and 99% relative humidity, much higher than these of pure 2D metal-organic layers (>>1.0 × 10-10 and 2.01×10-8 S cm-1 ), respectively. Furthermore, combining accurate structural information and theoretical calculations reveals that the inserted hydrogen-bonded inorganic layers provide the proton source and a networks of hydrogen-bonds leading to efficient proton transport, meanwhile reducing the bandgap of hybrid architecture and increasing the band electron delocalization of the metal-organic layer to greatly elevate the electron transport of intrinsic 2D metal-organic frameworks.

A Proton Conductive Porous Framework of an 18-Crown-6-Ether Derivative Networked by Rigid Hydrogen Bonding Modules.

A rigid hydrogen-bonded organic framework (HOF) was constructed from a C3 -symmetric hexatopic carboxylic acid with a hydrophilic 18-crown-6-ether (18C6) component. Despite the flexible macrocyclic structure with many conformations, the derivative with three 4,4'-dicarboxy-o-terphenyl moieties in the periphery yielded a rigid layered porous framework through directional intermolecular hydrogen bonding. Interestingly, the HOF possesses 1D channels with bottleneck composed of 18C6 rings. The HOF shows proton conductivity (1.12×10-7  S cm-1 ) through Grotthuss mechanism (Ea =0.27 eV) under 98 %RH. The present unique water channel structure provides an inspiration to create molecular porous materials.

Fluoride-bridged dinuclear dysprosium complex showing single-molecule magnetic behavior: supramolecular approach to isolate magnetic molecules.

Using Na-encapsulated benzo[18]crown-6 (Na)(B18C6) as a counter cation, we successfully magnetically isolated a fluoride-bridging Dy dinuclear complex {[(PW11O39)Dy(H2O)2]2F} (Dy2POM) with lacunary Keggin ligands. (Na)(B18C6) formed two types of tetramers through C-H⋯O, π⋯π and C-H⋯π interactions, and each tetramer aligned in one dimension along the c-axis to form two types of channels. One channel was partially penetrated by a supramolecular cation from the ±a-axis direction, dividing the channel in the form of a "bamboo node". Dy2POM was spatially divided by this "bamboo node," which magnetically isolated one portion from the other. The temperature dependence of the magnetic susceptibility indicated a weak ferromagnetic interaction between the Dy ions bridged by fluoride. Dy2POM exhibited the magnetic relaxation characteristics of a single-molecule magnet, including the dependence of AC magnetic susceptibility on temperature and frequency. Magnetic relaxation can be described by the combination of thermally active Orbach and temperature-independent quantum tunneling processes. The application of a static magnetic field effectively suppressed the relaxation due to quantum tunneling.

Large breathing effect in ZIF-65(Zn) with expansion and contraction of the SOD cage.

The flexibility and guest-responsive behavior of some metal-organic frameworks (MOFs) indicate their potential in the fields of sensors and molecular recognition. As a subfamily of MOFs, the flexible zeolitic imidazolate frameworks (ZIFs) typically feature a small displacive transition due to the rigid zeolite topology. Herein, an atypical reversible displacive transition (6.4 Å) is observed for the sodalite (SOD) cage in flexible ZIF-65(Zn), which represents an unusually large breathing effect compared to other ZIFs. ZIF-65(Zn) exhibits a stepwise II → III → I expansion between an unusual ellipsoidal SOD cage (8.6 Å × 15.9 Å for II) and a spherical SOD cage (15.0 Å for I). The breathing behavior of ZIF-65(Zn) varies depending on the nature of the guest molecules (polarity and shape). Computational simulations are employed to rationalize the differences in the breathing behavior depending on the structure of the ZIF-65(Zn) cage and the nature of the guest-associated host-guest and guest-guest interactions.

Solvent dependence of crystal structure and dielectric relaxation in ferromagnetic [MnCr(oxalate)3]- salt.

[MnCr(oxalate)3]- possesses a two-dimensional ferromagnetic network that is an ideal system for the construction of multifunctional molecular materials based on ferromagnetism. This is because additional functions, such as ferroelectricity, can be hybridised by incorporating functional cations between the layers. However, the majority of [MnCr(oxalate)3]- networks readily incorporate solvent molecules upon crystallisation, and it is sometimes difficult to measure the crystal physical properties because of the collapse associated with desolvation. Upon desolvation, the polar crystal (CBA+)([18]crown-6)[MnCr(oxalate)3]-(CH3OH) (1·CH3OH) (CBA+ = 4-carboxybutan-1-aminium) underwent a crystal-to-crystal transformation to form (CBA+)([18]crown-6)[MnCr(oxalate)3]-, 1. Furthermore, this change was accompanied by hydrogen bond reorganisation in the (CBA+)([18]crown-6) supramolecular assembly. Both crystals exhibited ferromagnetic ordering at approximately 5 K. In crystal 1, a "merry-go-round" motion of [18]crown-6 was observed, with an activation energy of 41.41 kJ mol-1, which resulted in dielectric relaxation. This crystal-to-crystal structural transformation provides a strategy for designing multifunctional hybrid materials, in which an additional function arises from molecular motion.

Silver(I)-Based Molecular Perovskite Energetic Compounds with Exceptional Thermal Stability and Energetic Performance.

In recent years, molecular perovskite energetic materials have attracted more attention because of their simple synthesis processes, high thermal stabilities, excellent performances, and great significance as a design platform for energetic materials. To explore the possibility of the application of molecular perovskite energetic materials in heat-resistant explosives, four silver(I)-based molecular perovskite energetic compounds, (H2A)[Ag(ClO4)3], where H2A = piperazine-1,4-diium (H2pz2+) for PAP-5, 1-methyl-piperazine-1,4-diium (H2mpz2+) for PAP-M5, homopiperazine-1,4-diium (H2hpz2+) for PAP-H5, and 1,4-diazabicyclo[2.2.2]octane-1,4-diium (H2dabco2+) for DAP-5, were synthesized by a one-pot self-assembly strategy and structurally characterized. The single-crystal structures indicated that PAP-5, PAP-M5, and DAP-5 possess cubic perovskite structures while PAP-H5 possesses a hexagonal perovskite structure. Differential thermal analyses showed that their onset decomposition temperatures are >308.3 °C. For PAP-5 and DAP-5, they have not only exceptional calculated detonation parameters (D values of 8.961 and 8.534 km s-1 and P values of 42.4 and 37.9 GPa, respectively) but also the proper mechanical sensitivity (impact sensitivities of ≤10 J for PAP-5 and 3 J for DAP-5 and friction sensitivities of ≤5N for both PAP-5 and DAP-5) and thus are of interest as potential heat-resistant primary explosive components.

Room-temperature ferroelectric and ferroelastic orders coexisting in a new tetrafluoroborate-based perovskite.

The coexistence of multiferroic orders has attracted increasing attention for its potential applications in multiple-state memory, switches, and computing, but it is still challenging to design single-phase crystalline materials hosting multiferroic orders at above room temperature. By utilizing versatile ABX3-type perovskites as a structural model, we judiciously introduced a polar organic cation with easily changeable conformations into a tetrafluoroborate-based perovskite system, and successfully obtained an unprecedented molecular perovskite, (homopiperazine-1,4-diium)[K(BF4)3], hosting both ferroelectricity and ferroelasticity at above room temperature. By using the combined techniques of variable-temperature single-crystal X-ray structural analyses, differential scanning calorimetry, and dielectric, second harmonic generation, and piezoresponse force microscopy measurements, we demonstrated the domain structures for ferroelectric and ferroelastic orders, and furthermore disclosed how the delicate interplay between stepwise changed dynamics of organic cations and cooperative deformation of the inorganic framework induces ferroelectric and ferroelastic phase transitions at 311 K and 455 K, respectively. This instance, together with the underlying mechanism of ferroic transitions, provides important clues for designing advanced multiferroic materials based on organic-inorganic hybrid crystals.

Nitroprusside as a promising building block to assemble an organic-inorganic hybrid for thermo-responsive switching materials.

We present a new hybrid compound, namely (Me2NH2)[KFe(CN)5(NO)], possessing a unique nitroprusside-based inorganic host framework in 4-connected sra topology encapsulating organic guest cations. The flexible host-guest hydrogen bonds and synchronously deformed inorganic framework give rise to thermal-responsive switching behaviours on both thermal expansion and nonlinear optical properties during the phase transition at around room temperature.

Enhancing switchable dielectric property for crystalline supramolecular rotor compounds by adding polar components.

Two new compounds were obtained by assembling the [(2-methoxy-5-nitro-anilinium)(18-crown-6)]+ cation with non-polar PF6- and polar SO3CF3- anions, respectively. Benefiting from its polar anion, the SO3CF3- compound reveals a more significant dielectric switching behaviour during phase transition, demonstrating an effective strategy to enhance the dielectric property by adding polar components.

Intermediate-sized molecular sieving of styrene from larger and smaller analogues.

Molecular sieving can lead to ultrahigh selectivity and low regeneration energy because it completely excludes all larger molecules via a size restriction mechanism. However, it allows adsorption of all molecules smaller than the pore aperture and so separations of complicated mixtures can be hindered. Here, we report an intermediate-sized molecular sieving (iSMS) effect in a metal-organic framework (MAF-41) designed with restricted flexibility, which also exhibits superhydrophobicity and ultrahigh thermal/chemical stabilities. Single-component isotherms and computational simulations show adsorption of styrene but complete exclusion of the larger analogue ethylbenzene (because it exceeds the maximal aperture size) and smaller toluene/benzene molecules that have insufficient adsorption energy to open the cavity. Mixture adsorption experiments show a high styrene selectivity of 1,250 for an ethylbenzene/styrene mixture and 3,300 for an ethylbenzene/styrene/toluene/benzene mixture (orders of magnitude higher than previous reports). This produces styrene with a purity of 99.9%+ in a single adsorption-desorption cycle. Controlling/restricting flexibility is the key for iSMS and can be a promising strategy for discovering other exceptional properties.

Unprecedented water-controlled rotator-stator conversion of supramolecular rotors in crystals.

The rotational dynamics, dielectric response and phase transitions of a unique crystalline supramolecular rotor are controlled by the existence/absence of guest water molecules causing prominent effects on the supramolecular interactions. Such an unprecedented rotationally bistable rotor can promote the understanding of precise control of molecular rotors in crystals.

Partially Fluorinated Cu(I) Triazolate Frameworks with High Hydrophobicity, Porosity, and Luminescence Sensitivity.

Solvothermal reactions of 3-methyl-5-trifluoromethyl-1,2,4-triazole (Hfmtz) with Cu(CH3COO)2 at 120 °C in the presence of Cl- generate two partially fluorinated coordination polymers: i.e., [Cu4Cl(fmtz)3] (1 or MAF-51) and [Cu7Cl(fmtz)6] (2 or MAF-52). Single-crystal X-ray diffraction revealed 1 to have a three-dimensional (3D) nonporous structure with pcu topology consisting of 6-connected Cu4(µ4-Cl) clusters and 2 to possess a highly porous (void ratio 48%) 3D bnn network consisting of 5-connected Cu5(µ5-Cl) clusters. Benefiting from the hydrophobic pendant groups, complete coordination of the ligand N atoms, and strong M-N coordination bonds, 1 and 2 possess high water stability (exposed to water for at least 1 year) and hydrophobicity (water contact angles of 141° and 148°, respectively). The N2 sorption isotherm of activated 2 gave Langmuir/BET surface areas of 1023/848 m2 g-1 and a pore volume of 0.365 cm3 g-1. Moreover, 2 can adsorb large amounts of benzene and methanol but barely adsorb water. Both 1 and 2 show phosphorescence of Cu(I) complexes, but only that of porous 2 is sensitive to O2, showing a linear Stern-Volmer response below 1 mbar with an ultrahigh Ksv value of 5234 bar-1 and ultralow limit of detection of 1.9 ppm.

Selective Aerobic Oxidation of a Metal-Organic Framework Boosts Thermodynamic and Kinetic Propylene/Propane Selectivity.

Efficient adsorptive separation of propylene/propane (C3 H6 /C3 H8 ) is highly desired and challenging. Known strategies focus on either the thermodynamic or the kinetic mechanism. Here, we report an interesting reactivity of a metal-organic framework that improves thermodynamic and kinetic adsorption selectivity simultaneously. When the metal-organic framework is heated under oxygen flow, half of the soft methylene bridges of the organic ligands are selectively oxidized to form the more polar and rigid carbonyl bridges. Mixture breakthrough experiments showed drastic increase of C3 H6 /C3 H8 selectivity from 1.5 to 15. For comparison, the C3 H6 /C3 H8 selectivities of the best-performing metal-organic frameworks Co-MOF-74 and KAUST-7 were experimentally determined to be 6.5 and 12, respectively. Gas adsorption isotherms/kinetics, single-crystal X-ray diffraction, and computational simulations revealed that the oxidation gives additional guest recognition sites, which improve thermodynamic selectivity, and reduces the framework flexibility, which generate kinetic selectivity.

Supercooling Behavior and Dipole-Glass-like Relaxation in a Three-Dimensional Water Framework.

The dynamic behaviors of a new type of three-dimensional (3D) water framework symbiotic with 1D stacking organic guests, including an order-disorder transition of hydrogen atoms, a supercooling phenomenon during phase transition, and a dipole-glass-like relaxation behavior due to locally trapped water molecules, are presented. This extremely scarce 3D water framework, together with the rich dynamic behaviors it exhibits, provides new clues to design new ice-like models for promoting the fundamental understanding of the dynamic behavior of water in diverse solid-states.

Non-3d Metal Modulation of a Cobalt Imidazolate Framework for Excellent Electrocatalytic Oxygen Evolution in Neutral Media.

Cobalt imidazolate frameworks are classical electrocatalysts for the oxygen evolution reaction (OER) but suffer from the relatively low activity. Here, a non-3d metal modulation strategy is presented for enhancing the OER activity of cobalt imidazolate frameworks. Two isomorphous frameworks [Co4 (MO4 )(eim)6 ] (M=Mo or W, Heim=2-ethylimidazole) having Co(eim)3 (MO4 ) units and high water stabilities were designed and synthesized. In different neutral media, the Mo-modulated framework coated on a glassy carbon electrode shows the best OER performances (1 mA cm-2 at an overpotential of 210 mV in CO2 -saturated 0.5 m KHCO3 electrolyte and 2/10/22 mA cm-2 at overpotential of 388/490/570 mV in phosphate buffer solution) among non-precious metal catalysts and even outperforms RuO2 . Spectroscopic measurements and computational simulations revealed that the non-3d metals modulate the electronic structure of Co for optimum reactant/product adsorption and tailor the energy of rate-determining step to a more moderate value.

Spin-reorientation-induced magnetodielectric coupling effects in two layered perovskite magnets.

Spin-reorientation-induced magnetodielectric coupling effects were discovered in two layered perovskite magnets, [C6H5CH2CH2NH3]2[MCl4] (M = Mn2+ and Cu2+), via isothermal magnetodielectric measurements on single-crystal samples. Specifically, peak-like dielectric anomalies and spin-flop transitions appeared simultaneously at around ±34 kOe for the canted antiferromagnet (M = Mn2+) at below 44.3 K, while a low-field (1 kOe) controlled magnetodielectric effect was observed in the "soft" ferromagnet (M = Cu2+) at below 9.5 K. These isothermal magnetodielectric effects are highly reproducible and synchronous with the field-induced magnetization at different temperatures, well confirming the essential role of spin reorientation on inducing magnetodielectric coupling effects. These findings strongly imply that the layered perovskite magnets are new promising organic-inorganic hybrid systems to host magnetodielectric coupling effects.

Electrochemical Exfoliation of Pillared-Layer Metal-Organic Framework to Boost the Oxygen Evolution Reaction.

Two-dimensional (2D) materials and ultrathin nanosheets are advantageous for elevating the catalysis performance and elucidating the catalysis mechanism of heterogeneous catalysts, but they are mostly restricted to inorganic or organic materials based on covalent bonds. We report an electrochemical/chemical exfoliation strategy for synthesizing metal-organic 2D materials based on coordination bonds. A catechol functionalized ligand is used as the redox active pillar to construct a pillared-layer framework. When the 3D pillared-layer MOF serves as an electrocatalyst for water oxidation (pH 13), the pillar ligands can be oxidized in situ and removed. The remaining ultrathin (2 nm) nanosheets of the metal-organic layers are an efficient catalyst with overpotentials as low as 211 mV at 10 mA cm-2 and a turnover frequency as high as 30 s-1 at an overpotential of 300 mV.