Giant anisotropic thermal expansion of copper-cyanido flat layers with flexible copper nodes.

Flat layers are usually considered as structurally rigid motifs in two-dimensional (2D) materials. In this work, we demonstrate that a flat honeycomb-layer composed of distorted tri-coordinate copper ions bridged with cyanidos in (tetraethylammonium)Cu2(CN)3 exhibits high in-plane flexibility. This resulted in an extremely large anisotropic 2D-thermal expansion.

Metal-organic framework/Nb4C3Tx MXene composites for ultrasensitive detection of dopamine.

An easy, in situ growth approach led to the formation of several composites of metal-organic framewoks and Nb4C3Tx MXenes mixed intimately at the submicron scale. The high affinity of MXene surface for dopamine, enhanced by a nanostructuration induced by MOFs, resulted in superior sensing performances. The system exhibited good linearity over the 1-100 nM range, with an excellent limit of detection of 0.2 nM.

A novel style of 2D Hofmann-type coordination polymer incorporated trigonal prismatic coordination geometry with bidentate co-ligands.

A novel 2D Hofmann-type framework was prepared with a bidentate co-ligand, 5,5'-dimethyl-2,2'-bipyridyl (dmbpy), which forces the curvature of the layer. X-ray diffraction analysis demonstrated that the coordination polymers, MnII(dmbpy)[MVN(CN)4] (MV = Mn (1) and Cr (2)), formed a considerably corrugated 2D cyanide-bridged network with a quasi C4v symmetric building unit, [CrVN(CN)4]2-, and trigonal prismatic coordination geometry around MnII. Compound 2 demonstrated a metamagnetic-like ordering at 14.4 K, caused by the intra- and inter-layer antiferromagnetic interactions between CrV (S = 1/2) and MnII (S = 5/2), and a weak ferromagnetic behaviour at 2 K reflecting the single-ion anisotropy of CrV and structural anisotropy.

Strongly Enhanced Polarization in a Ferroelectric Crystal by Conduction-Proton Flow.

Ion conductors comprising noncentrosymmetric frameworks have emerged as new functional materials. However, strongly correlated polarity functionality and ion transport have not been achieved. Herein, we report a ferroelectric proton conductor, K2MnN(CN)4·H2O (1·H2O), exhibiting the strong correlation between its polar skeleton and conductive ions that generate anomalous ferroelectricity via the proton-bias phenomenon. The application of an electric field of ±1 kV/cm (0.1 Hz) on 1·H2O at 298 K produced the ferroelectricity (polarization = 1.5 × 104 µC/cm2), which was enhanced by the ferroelectric-skeleton-trapped conductive protons. Furthermore, the strong polarity-proton transport coupling of 1·H2O induced a proton-rectification-like directional ion-conductive behavior that could be adjusted by the magnitude and direction of DC electric fields. Moreover, 1·H2O exhibited reversible polarity switching between the polar 1·H2O and its dehydrated form, 1, with a centrosymmetric structure comprising an order-disorder-type transition of the nitrido-bridged chains.

Janus-Type Mixed-Valent Copper-Cyanido Honeycomb Layers.

The synthesis of Janus-type layers, which possess front and back sides that consist of different structures, remains a major challenge in the field of two-dimensional materials. In this study, two Janus-type layered coordination polymers, namely, CuII(NEtH2)(NMe2H·H2O)CuI(CN)3 (1) and CuII(NMe2H)(NMe2H·H2O)CuI(CN)3 (2), were synthesized via a simple one-pot procedure using copper(II) nitrate and sodium cyanido in mixed solutions of dimethylamine and ethylamine. Uniquely, 1 and 2 were composed of cyanido-bridged neutral layers and exhibited a CuICuII mixed-valent state. Meanwhile, using a solution of pure dimethylamine for the synthesis yielded the monovalent three-dimensional framework (NMe2H2)[CuI2(CN)3] (3). Results indicated that the simultaneous use of two mixed amines gave rise to the controlled reduction of CuII ions during the reaction. In addition, each face of the layers was coordinated by different amines on the axial positions of the CuII sites, resulting in anisotropic Janus layers. Furthermore, the thermal expansion behavior of 2 was investigated, demonstrating that the neutral [CuICuII(CN)3] layer was relatively rigid compared with the analogous anionic [CuI2(CN)3]- layer.

Structural-transformation-induced Drastic Luminescence Changes in an Organic-Inorganic Hybrid [ReN(CN)4 ]2- Salt Triggered by Chemical Stimuli.

We synthesized a (1-propylpyridinium)2 [ReN(CN)4 ]-type organic-inorganic hybrid exhibiting water-vapor-induced drastic structural changes of the [ReN(CN)4 ]2- assemblies. Specifically, upon exposure to water vapor, dehydrated nitrido-bridged chains were converted to hydrated cyanido-bridged tetranuclear clusters via rearrangements of large molecular building units in the crystals. These switchable assembly forms display substantially different photo-physical properties, although in both cases the emission is caused by a metal-centered d-d transition. The nitrido-bridged chain exhibited a near-infrared (749 nm) emission, which blue-shifted as the temperature increased, while a visible (561 nm) emission and its red shift was demonstrated by the cyanido-bridged cluster.

Efficient water-based purification of metal-organic polyhedra by centrifugal ultrafiltration.

An efficient water-based purification strategy for metal-organic polyhedra (MOPs) using commercially available centrifugal ultrafiltration membranes was developed. Having a diameter above 3 nm, MOPs were almost fully retained by the filters, while free ligands and other impurities were washed away. MOP retention also enabled efficient counter-ion exchange. This method paves the way for the application of MOPs with biological systems.

Water-Soluble Ionic Metal-Organic Polyhedra as a Versatile Platform for Enzyme Bio-immobilization.

Metal-organic polyhedra (MOPs) can act as elementary structural units for the design of modular porous materials; however, their association with biological systems remains greatly restricted by their typically low stabilities and solubilities in water. Herein, we describe the preparation of novel MOPs bearing either anionic or cationic groups and exhibiting a high affinity for proteins. Simple mixing of the protein bovine serum albumin (BSA) and ionic MOP aqueous solutions resulted in the spontaneous formation of MOP-protein assemblies, in a colloidal state or as solid precipitates depending on the initial mixing ratio. The versatility of the method was further illustrated using two enzymes, catalase and cytochrome c, with different sizes and isoelectric points (pI's) below and above 7. This mode of assembly led to the high retention of catalytic activity and enabled recyclability. Furthermore, the co-immobilization of cytochrome c with highly charged MOPs resulted in a substantial 44-fold increase of its catalytic activity.

The structure and modified properties of a self-dimerised Cu(II) inclusion complex in γ-cyclodextrins.

Inclusion structures incorporating more than one guest molecule are elusive because confinement alters their molecular properties. We report the solid-state characterization of an inclusion complex comprising two γ-cyclodextrins and two [Cu(2-pyridinemethanolate)(2-pyridinemethanol)]PF6 units. Quantum calculation reveals that interfragment charge transfer occurs. The confined Cu fragment and the unincluded "linear chain [Cu(2-pyridinemethanolate)(2-pyridinemethanol)]PF6" exhibit different properties.

Negative Thermal Expansion of Undulating Coordination Layers through Interlayer Interaction.

The negative thermal expansion (NTE) of solid-state materials is of significance in various fields, but a very rare phenomenon. In this study, we carried out a meta-analysis for the anisotropic thermal expansion behavior of fifteen two-dimensional coordination polymers [M(salen)]2[M'(CN)4(solvent)] (M = Mn, Fe; M' = MnN, ReN, Pt, Pt(I2)x; x = 0.18, 0.45, 0.85, 1.0; solvent = H2O, MeOH, MeCN) with a newly synthesized [Fe(salen)]2[MnN(CN)4(MeCN)]. Consequently, we successfully demonstrate the unusual NTE of the undulating coordination layers by an expansion deformation of the layers via strong interlayer interaction within the layer stacking. Notably, the layer volume of [Mn(salen)]2[ReN(CN)4] with its powder form decreases with a large NTE coefficient, αlayer-volume = -27 × 10-6 K-1 (100-500 K). This is a significantly large value despite the increase in layer thickness along the layer contraction based on the anisotropic transformation of undulating layers. Conversely, the analysis demonstrates that the chemical modification of the layers to enhance intralayer interaction rather than interlayer interaction switches a direction of the layer anisotropy, yielding positive thermal expansion materials with the coefficient of the layer volume reaching +92 × 10-6 K-1.

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%.

The ligand field in low-crystallinity metal-organic frameworks investigated by soft X-ray core-level absorption spectroscopy.

The ligand field (LF) of transition metal ions is a crucial factor in realizing the mechanism of novel physical and chemical properties. However, the low-crystallinity state, including the amorphous state, precludes the clarification of the electronic structural relationship of transition metal ions using crystallographic techniques, ultraviolet and infrared optical methods, and magnetometry. Here, we demonstrate that soft X-ray 2p → 3d core-level absorption spectroscopy (L2,3-edge XAS) systematically revealed the local 3d electronic states, including in the LF, of nitrogen-coordinated transition-metal ions for low-crystallinity cyanide-bridged metal-organic frameworks (MOFs) M[Ni(CN)4] (MNi; M = Mn, Fe, Co, Ni) and Ni[Pd(CN)4] (NiPd). In NiNi and NiPd, N-coordinated Ni ions with square-planar symmetry exhibit strong orbital hybridization and ligand-to-metal charge transfer effects. In MnNi, FeNi, and CoNi, the correlation between the crystalline electric field splitting in the LF and the transition metal-nitrogen bonding length is revealed using the multiplet LF theory. Regardless of the different local symmetries, our results indicate that L2,3-edge XAS is a powerful tool for gaining element-specific knowledge about the transition-metal ion characterizing the functionality of low-crystallinity MOFs and will be the foundation for an attractive platform, such as adsorption/desorption materials.

Flexibility Control of Two-Dimensional Coordination Polymers by Crystal Morphology: Water Adsorption and Thermal Expansion.

Layer flexibility in two-dimensional coordination polymers (2D-CPs) contributes to several functional materials as it results in anisotropic structural response to external stimuli. Chemical modification is a common technique for modifying layer structures. This study demonstrates that crystal morphology of a cyanide-bridged 2D-CP of type [Mn(salen)]2 [ReN(CN)4 ] (1) consisting of flexible undulating layers significantly impacts the layer configuration and assembly. Nanoplates of 1 showed an in-plane contraction of layers with a longer interlayer distance compared to the micrometer-sized rod-type particles. These effects by crystal morphology on the structure of the 2D-CP impacted the structural flexibility, resulting in dual-functional changes: the enhancement of the sensitivity of structural transformation to water adsorption and modification of anisotropic thermal expansion of 1. Moreover, the nanoplates incorporated new adsorption sites within the layers, resulting in the uptake of an additional water molecule compared to the micrometer-sized rods.

Downsizing of Nanocrystals While Retaining Bistable Spin Crossover Properties in Three-Dimensional Hofmann-Type {Fe(pz)[Pt(CN)4]}-Iodine Adducts.

Mastering nanostructuration of functional materials into electronic devices is presently an essential task in materials science. This is particularly relevant for spin crossover (SCO) compounds, whose properties are extremely sensitive to size reduction. Indeed, the search for materials displaying strong cooperative hysteretic SCO properties operative at the nanoscale close near room temperature is extremely challenging. In this context, we describe here the synthesis and characterization of 20-30 nm surfactant-free nanocrystals of the FeII Hofmann-type polymer {FeII(pz)[PtII,IVIx(CN)4]} (pz = pyrazine), which affords the first example of a robust three-dimensional coordination polymer, substantially keeping operational thermally induced SCO bistability at such a scale.

Guest-selective and reversible magnetic phase switching in a pseudo-pillared-layer porous magnet.

A novel porous magnet consisting of cationic two-dimensional (2-D) layers extended by FeIII-CN-NiII linkages and pseudo-pillar dianions was synthesized. The size-selective guest adsorption behaviour of water and methanol molecules originates from the narrow bottle-neck-type pores in the flexible pseudo-pillared-layer structure, which results in the switching of the magnetic phases from antiferromagnetic to ferromagnetic, involving significant changes in the interlayer distance.

Crystalline assembly of metal-organic polyhedra driven by ionic interactions with polyoxometalates.

Charge-driven self-assembly of cationic zirconium-based metal-organic polyhedra (MOPs) with polyoxometalates (POMs) leads to a series of porous crystalline salts, prepared by simple mixing of soluble precursors. The reactivity of immobilized POMs was greatly increased, as demonstrated by their fast reduction by hydrazine vapors, without loss of structural integrity.

Guest-Tunable Excited States in a Cyanide-Bridged Luminescent Coordination Polymer.

The excited-state energy was tuned successfully by guest molecules in a cyanide-bridged luminescent coordination polymer (CP). Methanol or ethanol vapor reversibly and significantly changed the luminescent color of the CP between green and yellow (Δλem = 32 nm). These vapors did not significantly affect the environment around the luminophore in the ground state of the CP, whereas they modulated the excited states for the resulting bathochromic shift. The time-resolved photoluminescent spectra of the CP systems showed that solvent adsorption enhanced the energetic relaxation in the excited states. Furthermore, time-resolved infrared spectroscopy indicated that cyanide bridging in the CP became more flexible in the excited states than that in the ground state, highlighting the sensitivity of the excited states to external stimuli, such as the guest vapor. Overall, guest-tunable excited states will allow the more straightforward design of sensing materials by characterizing the transient excited states.

Metal Complex Lipids for Fluid-Fluid Phase Separation in Coassembled Phospholipid Membranes.

We demonstrate a fluid-fluid phase separation in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) membranes using a metal complex lipid of type [Mn(L1)] (1; HL1=1-(2-hydroxybenzamide)-2-(2-hydroxy-3-formyl-5-hexadecyloxybenzylideneamino)ethane). Small amount of 1 produces two separated domains in DMPC, whose phase transition temperatures of lipids (Tc ) are both lower than that of the pristine DMPC. Variable temperature fluorescent microscopy for giant-unilamellar vesicles of DMPC/1 hybrids demonstrates that visible phase separations remain in fluid phases up to 37 °C, which is clearly over the Tc of DMPC. This provides a new dimension for the application of metal complex lipids toward controlling lipid distributions in fluid membranes.

Host-Guest Interaction Modulation in Porous Coordination Polymers for Inverse Selective CO2 /C2 H2 Separation.

Controlling gas sorption by simple pore modification is important in molecular recognition and industrial separation processes. In particular, it is challenging to realize the inverse selectivity, which reduces the adsorption of a high-affinity gas and increases the adsorption of a low-affinity gas. Herein, an "opposite action" strategy is demonstrated for boosting CO2 /C2 H2 selectivity in porous coordination polymers (PCPs). A precise steric design of channel pores using an amino group as an additional interacting site enabled the synergetic increase in CO2 adsorption while suppressing the C2 H2 adsorption. Based on this strategy, two new ultramicroporous PCP physisorbents that are isostructural were synthesised. They exhibited the highest CO2 uptake and CO2 /C2 H2 volume uptake ratio at 298 K. Origin of this specific selectivity was verified by detailed density functional theory calculations. The breakthrough separation performances with remarkable stability and recyclability of both the PCPs render them relevant materials for C2 H2 purification from CO2 /C2 H2 mixtures.

pH-Dependent ion permeability control of a modified amphotericin B channel through metal complexation.

Amphotericin B incorporating 2,2'-bipyridine (bpy-AmB) forms a membrane channel exhibiting pH-dependent Ca2+ ion permeability with a selective response to Cu2+ ions. The coordination structure at bpy sites depends on the pH and metal ions can control the association state of bpy-AmB in the membrane.