Oxygen Evolution Reaction Driven by Charge Transfer from a Cr Complex to Co-Containing Polyoxometalate in a Porous Ionic Crystal

Considerable efforts have been devoted to developing oxygen evolution reaction (OER) catalysts based on transition metal oxides. Polyoxometalates (POMs) can be regarded as model compounds of transition metal oxides, and cobalt-containing POMs (Co-POMs) have received significant interest as candidates. Nanocomposites based on Co-POMs have been reported to show high OER activities due to synergistic effects among the components; however, the role of each component is unclear due to its complex structure. Herein, we utilize porous ionic crystals (PICs) based on Co-POMs, which enable a composition–structure–function relationship to be established to understand the origin of the synergistic catalysis. Specifically, a Keggin-type POM [α-CoW12O40]6– and a Cr complex [Cr3O­(OOCCH2CN)6(H2O)3]+ are implemented as PIC building blocks for the OER under nonbasic conditions. The potentially OER-active but highly soluble [α-CoW12O40]6– was successfully anchored in the crystalline PIC matrix via Coulomb interactions and hydrogen bonding induced by polar cyano groups of the Cr complex. The PIC exhibits efficient and sustained OER catalytic activity, while each building block is inactive. The Tafel slope of the linear sweep voltammetry curve and the relatively large kinetic isotope effect value suggest that elementary steps closely related to the OER rate involve single-electron and proton transfer reactions. Electrochemical and spectroscopic studies clearly show that the synergistic catalysis originates from the charge transfer from the Cr complex to [α-CoW12O40]6–; the increased electron density of [α-CoW12O40]6– may increase its basicity and accelerate proton abstraction as well as enhance electron transfer to stabilize the reaction intermediates adsorbed on [α-CoW12O40]6–

Fast Conduction of Organic Cations in Metal Sulfate Frameworks

We demonstrated a new method of synthesizing crystalline organic cation conductors. The conductivities of various organic cations involved in a one-dimensional zinc sulfate framework were studied. The optimized structure (EMIm)₂[Zn­(SO₄)₂] exhibited an ionic conductivity of 3.8 × 10^–3 S cm^–1 at 210 °C, which is comparable to that of highly conductive organic ionic plastic crystals. The high ionic conductivity is attributable to the defect structures of the organic cations in the inorganic frameworks. The pulsed-field gradient solid-state NMR technique revealed that the self-diffusion coefficient of organic cations in the zinc sulfate at 80 °C is comparable to that of the popular ionic liquid EMIm-BF₄ at 30 °C, which indicates that liquid-like fast transporting of organic cation is achieved in the robust crystal structure

Fast Conduction of Organic Cations in Metal Sulfate Frameworks

We demonstrated a new method of synthesizing crystalline organic cation conductors. The conductivities of various organic cations involved in a one-dimensional zinc sulfate framework were studied. The optimized structure (EMIm)₂[Zn­(SO₄)₂] exhibited an ionic conductivity of 3.8 × 10^–3 S cm^–1 at 210 °C, which is comparable to that of highly conductive organic ionic plastic crystals. The high ionic conductivity is attributable to the defect structures of the organic cations in the inorganic frameworks. The pulsed-field gradient solid-state NMR technique revealed that the self-diffusion coefficient of organic cations in the zinc sulfate at 80 °C is comparable to that of the popular ionic liquid EMIm-BF₄ at 30 °C, which indicates that liquid-like fast transporting of organic cation is achieved in the robust crystal structure

Control of Molecular Rotor Rotational Frequencies in Porous Coordination Polymers Using a Solid-Solution Approach

Rational design to control the dynamics of molecular rotors in crystalline solids is of interest because it offers advanced materials with precisely tuned functionality. Herein, we describe the control of the rotational frequency of rotors in flexible porous coordination polymers (PCPs) using a solid-solution approach. Solid-solutions of the flexible PCPs [{Zn­(5-nitroisophthalate)_<i>x</i>(5-methoxyisophthalate)_1–<i>x</i>(deuterated 4,4′-bipyridyl)}­(DMF·MeOH)]_<i>n</i> allow continuous modulation of cell volume by changing the solid-solution ratio <i>x</i>. Variation of the isostructures provides continuous changes in the local environment around the molecular rotors (pyridyl rings of the 4,4′-bipyridyl group), leading to the control of the rotational frequency without the need to vary the temperature

Control of Molecular Rotor Rotational Frequencies in Porous Coordination Polymers Using a Solid-Solution Approach

Rational design to control the dynamics of molecular rotors in crystalline solids is of interest because it offers advanced materials with precisely tuned functionality. Herein, we describe the control of the rotational frequency of rotors in flexible porous coordination polymers (PCPs) using a solid-solution approach. Solid-solutions of the flexible PCPs [{Zn­(5-nitroisophthalate)_<i>x</i>(5-methoxyisophthalate)_1–<i>x</i>(deuterated 4,4′-bipyridyl)}­(DMF·MeOH)]_<i>n</i> allow continuous modulation of cell volume by changing the solid-solution ratio <i>x</i>. Variation of the isostructures provides continuous changes in the local environment around the molecular rotors (pyridyl rings of the 4,4′-bipyridyl group), leading to the control of the rotational frequency without the need to vary the temperature

Control of Molecular Rotor Rotational Frequencies in Porous Coordination Polymers Using a Solid-Solution Approach

Rational design to control the dynamics of molecular rotors in crystalline solids is of interest because it offers advanced materials with precisely tuned functionality. Herein, we describe the control of the rotational frequency of rotors in flexible porous coordination polymers (PCPs) using a solid-solution approach. Solid-solutions of the flexible PCPs [{Zn­(5-nitroisophthalate)_<i>x</i>(5-methoxyisophthalate)_1–<i>x</i>(deuterated 4,4′-bipyridyl)}­(DMF·MeOH)]_<i>n</i> allow continuous modulation of cell volume by changing the solid-solution ratio <i>x</i>. Variation of the isostructures provides continuous changes in the local environment around the molecular rotors (pyridyl rings of the 4,4′-bipyridyl group), leading to the control of the rotational frequency without the need to vary the temperature

Dynamics of Linkers in Metal–Organic Framework Glasses

Metal–organic framework (MOF) glasses have emerged as a new class of organic–inorganic hybrid glass materials. Considerable efforts have been devoted to unraveling the macroscopic dynamics of MOF glasses by studying their rheological behavior; however, their microscopic dynamics remain unclear. In this work, we studied the effect of vitrification on linker dynamics in ZIF-62 by solid-state 2H nuclear magnetic resonance (NMR) spectroscopy. 2H NMR relaxation analysis provided a detailed picture of the mobility of the ZIF-62 linkers, including local restricted librations and a large-amplitude twist; these details were verified by molecular dynamics. A comparison of ZIF-62 crystals and glasses revealed that vitrification does not drastically affect the fast individual flipping motions with large-amplitude twists, whereas it facilitates slow cooperative large-amplitude twist motions with a decrease in the activation barrier. These observations support the findings of previous studies, indicating that glassy ZIF-62 retains permanent porosity and that short-range disorder exists in the alignment of ligands because of distortion of the coordination angle

Lanthanide-Based Porous Coordination Polymers: Syntheses, Slow Relaxation of Magnetization, and Magnetocaloric Effect

Two lanthanide-containing structurally analogous porous coordination polymers (PCPs) have been isolated with the general molecular formula [Ln₂(L₁)₂(H₂O)₄(ox)]_<i>n</i>.4<i>n</i>H₂O (where L₁ = fumarate, ox = oxalate; Ln = Dy (<b>1</b>), Gd (<b>2</b>)). Thermogravimetric analysis (TGA) and TG-MS measurements performed on <b>1</b> and <b>2</b> suggest that not only the solvated water molecules in the crystal lattice but also the four coordinated water molecules on the respective lanthanides in <b>1</b> and <b>2</b> are removed upon activation. Due to the removal of the waters, <b>1</b> and <b>2</b> lost their crystallinity and became amorphous, as confirmed by powder X-ray diffraction (PXRD). We propose the molecular formula [Ln₂(L₁)₂(ox)]_<i>n</i> for the amorphous phase of <b>1</b> and <b>2</b> (where Ln = Dy (<b>1′</b>), Gd (<b>2′</b>)) on the basis of XANES, EXAFS, and other experimental investigations. Magnetization relaxation dynamics probed on <b>1</b> and <b>1′</b> reveal two different relaxation processes with effective energy barriers of 53.5 and 7.0 cm^–1 for <b>1</b> and 45.1 and 6.4 cm^–1 for <b>1′</b>, which have been rationalized by detailed ab initio calculations. For the isotropic lanthanide complexes <b>2</b> and <b>2′</b>, magnetocaloric effect (MCE) efficiency was estimated through detailed magnetization measurements. We have estimated −Δ<i>S</i>_<i>m</i> values of 52.48 and 41.62 J kg^1– K^–1 for <b>2′</b> and <b>2</b>, respectively, which are one of the largest values reported for an extended structure. In addition, a 26% increase in −Δ<i>S</i>_m value in <b>2′</b> in comparison to <b>2</b> is achieved by simply removing the passively contributing (for MCE) solvated water molecule in the lattice and coordinated water molecules

Direct Synthesis of Hierarchically Porous Metal–Organic Frameworks with High Stability and Strong Brønsted Acidity: The Decisive Role of Hafnium in Efficient and Selective Fructose Dehydration

The direct synthesis of metal–organic frameworks (MOFs) with strong Brønsted acidity is challenging because the functional groups exhibiting Brønsted acidity (e.g., sulfonic acid groups) often jeopardize the framework integrity. Herein, we report the direct synthesis of two hierarchically porous MOFs named NUS-6 composed of either zirconium (Zr) or hafnium (Hf) clusters with high stability and strong Brønsted acidity. Via the modulated hydrothermal (MHT) synthesis, these two MOFs can be easily synthesized at a low temperature (80 °C) with high throughput. They exhibit BET surface areas of 550 and 530 m2 g–1 for Zr and Hf one, respectively, and a unique hierarchically porous structure of coexisting micropores (∼0.5, ∼0.7, and ∼1.4 nm) and mesopores (∼4.0 nm) with dangling sulfonic acid groups. Structural analysis reveals that the hierarchical porosity of NUS-6 is a result of missing linkers and clusters of the parental UiO-66 framework. These unique features make NUS-6 highly efficient and selective solid acid catalysts for dehydration of fructose to 5-hydroxymethylfurfural (HMF), in which NUS-6­(Hf) demonstrates a superior performance versus that of NUS-6­(Zr) because of the stronger Brønsted acidity contributed from Hf-μ3-OH groups as well as smaller pore sizes suitable for the restriction of unwanted side reactions. Our results have demonstrated for the first time the unique attributes of Hf-MOFs featured by superior stability and Brønsted acidity that can be applied as heterogeneous catalysts in biobased chemical synthesis