Tuning the Spin‐Crossover Behaviour of a Hydrogen‐Accepting Porous Coordination Polymer by Hydrogen‐Donating Guests

A Hoffman‐like coordination polymer with appreciable porosity and uncoordinated pyridyl groups, namely, [Fe(2,5‐bpp){Au(CN)₂}₂] ? x Solv (2,5‐bpp=2,5‐bis(pyrid‐4‐yl)pyridine; Solv=solvent), was synthesised and characterised. A series of fascinating spin‐crossover behaviours with abrupt, stepwise and hysteretic features were obtained by exchange with a range of protic solvents (ethanol, n‐propanol, isopropyl alcohol, sec‐butanol and isobutanol). Guest–host hydrogen‐bonding interactions involving the H‐accepting site of the framework are primarily responsible for the pronounced cooperativity of these spin‐crossover behaviours. Meanwhile, the tunable critical temperatures over a range of about 130 K are presumably attributable to a certain degree of competition between internal pressure and local electronic influences of solvents.     

Recognition of 1,3‐Butadiene by a Porous Coordination Polymer

The separation of 1,3‐butadiene from C₄ hydrocarbon mixtures is imperative for the production of synthetic rubbers, and there is a need for a more economical separation method, such as a pressure swing adsorption process. With regard to adsorbents that enable C₄ gas separation, [Zn(NO₂ip)(dpe)]_n (SD‐65; NO₂ip=5‐nitroisophthalate, dpe=1,2‐di(4‐pyridyl)ethylene) is a promising porous material because of its structural flexibility and restricted voids, which provide unique guest‐responsive accommodation. The 1,3‐butadiene‐selective sorption profile of SD‐65 was elucidated by adsorption isotherms, in situ PXRD, and SSNMR studies and was further investigated by multigas separation and adsorption–desorption‐cycle experiments for its application to separation technology.     

Control of Interpenetration and Gas‐Sorption Properties of Metal–Organic Frameworks by a Simple Change in Ligand Design

In metal–organic framework (MOF) chemistry, interpenetration greatly affects the gas‐sorption properties. However, there is a lack of a systematic study on how to control the interpenetration and whether the interpenetration enhances gas uptake capacities or not. Herein, we report an example of interpenetration that is simply controlled by the presence of a carbon–carbon double or single bond in identical organic building blocks, and provide a comparison of gas‐sorption properties for these similar frameworks, which differ only in their degree of interpenetration. Noninterpenetrated ( SNU‐70 ) and doubly interpenetrated ( SNU‐71 ) cubic nets were prepared by a solvothermal reaction of [Zn(NO₃)₂] ? 6 H₂O in N,N‐diethylformamide (DEF) with 4‐(2‐carboxyvinyl)benzoic acid and 4‐(2‐carboxyethyl)benzoic acid, respectively. They have almost‐identical structures, but the noninterpenetrated framework has a much bigger pore size (ca. 9.0×9.0 Å) than the interpenetrated framework (ca. 2.5×2.5 Å). Activation of the MOFs by using supercritical CO₂ gave SNU‐70′ and SNU‐71′ . The simulation of the PXRD pattern of SNU‐71′ indicates the rearrangement of the interpenetrated networks on guest removal, which increases pore size. SNU‐70′ has a Brunauer–Emmett–Teller (BET) surface area of 5290 m² g^?1, which is the highest value reported to date for a MOF with a cubic‐net structure, whereas SNU‐71′ has a BET surface area of 1770 m² g^?1. In general, noninterpenetrated SNU‐70′ exhibits much higher gas‐adsorption capacities than interpenetrated SNU‐71′ at high pressures, regardless of the temperature. However, at P<1 atm, the gas‐adsorption capacities for N₂ at 77 K and CO₂ at 195 K are higher for noninterpenetrated SNU‐70′ than for interpenetrated SNU‐71′ , but the capacities for H₂ and CH₄ are the opposite; SNU‐71′ has higher uptake capacities than SNU‐70′ due to the higher isosteric heat of gas adsorption that results from the smaller pores. In particular, SNU‐70′ has exceptionally high H₂ and CO₂ uptake capacities. By using a post‐synthetic method, the C?C double bond in SNU‐70 was quantitatively brominated at room temperature, and the MOF still showed very high porosity (BET surface area of 2285 m² g^?1).     

Switchable Guest Molecular Dynamics in a Perovskite‐Like Coordination Polymer toward Sensitive Thermoresponsive Dielectric Materials

A new perovskite‐like coordination polymer [(CH₃)₂NH₂][Cd(N₃)₃] is reported which undergoes a reversible ferroelastic phase transition. This transition is due to varied modes of motion of the [(CH₃)₂NH₂]⁺ guest accompanied by a synergistic deformation of the [Cd(N₃)₃]^? framework. The unusual two‐staged switchable dielectric relaxation reveals the molecular dynamics of the polar cation guest, which are well controlled by the variable confined space of the host framework. As the material switches from the ferroelastic phase to the paraelastic phase, a remarkable increase of the rotational energy barrier is detected. As a result, upon heating at low temperature, this compound shows a notable change from a low to a high dielectric state in the ferroelastic phase. This thermoresponsive host–guest system may serve as a model compound for the development of sensitive thermoresponsive dielectric materials and may be key to understanding and modulating molecular/ionic dynamics of guest molecules in confined space.     

Targeted Manipulation of Metal–Organic Frameworks To Direct Sorption Properties

Metal–organic frameworks are promising materials for manifold applications. This Minireview highlights approaches for the fine‐tuning of specific sorption properties (e.g. capacity, selectivity, and breathing behavior) of this interesting class of materials. Central aspects covered are the control over the crystal morphology, the targeted tuning of sorption properties by judicious choice of metal centers and linkers, and the preparation of host–guest systems. We want to introduce the reader to these topics on the basis of the manipulation of a handful of outstanding prototypical metal–organic frameworks.     

Unprecedented Sulfone‐Functionalized Metal–Organic Frameworks and Gas‐Sorption Properties

Gas storage : A new, sulfone‐functionalized dicarboxylate‐based ligand (see figure) is capable of directing the formation of novel metal–organic frameworks with unprecedented organic and inorganic secondary building units. A high CO₂ uptake with remarkable selectivity over CH₄, N₂, and H₂ was observed at near‐ambient temperature.

     

Encapsulation of an Ionic Metalloporphyrin into a Zeolite Imidazolate Framework in situ for CO2 Chemical Transformation via Host–Guest Synergistic Catalysis

It is a great challenge to rationally integrate multiple reactive sites into a composite material with confined nanospace, which can be applied as a nanoreactor to facilitate targeting catalytic reaction. In this work, an ionic metalloporphyrin has been encapsulated in situ into ZIF‐8 for a solvent‐free synthesis of cyclic carbonates from CO₂ and epoxides without any co‐catalyst under 1 atm CO₂.     

Thermally Stable Porous Hydrogen‐Bonded Coordination Networks Displaying Dual Properties of Robustness and Dynamics upon Guest Uptake

Two series of microporous lanthanide coordination networks of the general formula, {[Ln(ntb)Cl₃] ? x H₂O}_n (series 1 : monoclinic C2/c, Ln=Sm and Tb; series 2 : hexagonal P3₁/c, Ln=Sm and Eu; ntb=tris(benzimidazol‐2‐ylmethyl)amine, x=0–4) have been synthesized and characterized by IR, elemental analyses, thermal gravimetry, and single‐crystal and powder X‐ray diffraction methods. In both series, the monomeric [Ln(ntb)Cl₃] coordination units are consolidated by N? H???Cl or C? H???Cl hydrogen bonds to sustain three‐dimensional (3D) networks. However, the different modes of hydrogen bonding in the two series lead to crystallization of the same [Ln(ntb)Cl₃] monomers in different forms (monoclinic vs. hexagonal), consequently giving rise to distinct porous structures. The resulting hydrogen‐bonded coordination networks display high thermal stability and robustness in water removal/inclusion processes, which was confirmed by temperature‐dependent single‐crystal‐to‐single‐crystal transformation measurements. Adsorption studies with H₂, CO₂, and MeOH have been carried out, and reveal distinct differences in adsorption behavior between the two forms. In the case of MeOH uptake, the monoclinic network shows a normal type I isotherm, whereas the hexagonal network displays dynamic porous properties.     

Guest‐Responsive Function of a Dynamic Metal–Organic Framework with a π Lewis Acidic Pore Surface

A 3D dynamic coordination framework with an electron‐deficient pore surface has been synthesized by using Zn^II (having a variable coordination number) and a predesigned flexible π‐electron‐deficient core‐based ligand, exhibiting chemical separations based on pore surface functionalization (π Lewis acidic pore surfaces and open metal sites) and framework flexibility, giving rise to a unique smart guest‐responsive material.     

Metal–Organic Spheres as Functional Systems for Guest Encapsulation

Music of the spheres : Infinite coordination polymerization of Zn²⁺ ions and a multitopic ligand produces metal–organic micro‐ and nanospheres that can be used as functional matrices. The spheres can encapsulate combinations of active substances, such as organic dyes, magnetic nanoparticles, or luminescent quantum dots (see image), which results in spheres that are luminescent in the blue, green, and red regions of the spectrum.

     

Programmable Polymer‐Based Supramolecular Temperature Sensor with a Memory Function

A new class of polymeric thermometers with a memory function is reported that is based on the supramolecular host–guest interactions of poly(N‐isopropylacrylamide) (PNIPAM) with side‐chain naphthalene guest moieties and the tetracationic macrocycle cyclobis(paraquat‐p‐phenylene) (CBPQT⁴⁺) as the host. This supramolecular thermometer exhibits a memory function for the thermal history of the solution, which arises from the large hysteresis of the thermoresponsive LCST phase transition (LCST=lower critical solution temperature). This hysteresis is based on the formation of a metastable soluble state that consists of the PNIPAM–CBPQT⁴⁺ host–guest complex. When heated above the transition temperature, the polymer collapses, and the host–guest interactions are disrupted, making the polymer more hydrophobic and less soluble in water. Aside from providing fundamental insights into the kinetic control of supramolecular assemblies, the developed thermometer with a memory function might find use in applications spanning the physical and biological sciences.     

Reversible Guest Binding in a Non‐Porous FeII Coordination Polymer Host Toggles Spin Crossover

Formation of either a dimetallic compound or a 1 D coordination polymer of adiponitrile adducts of [Fe(bpte)]²⁺ (bpte=[1,2‐bis(pyridin‐2‐ylmethyl)thio]ethane) can be controlled by the choice of counteranion. The iron(II) atoms of the bis(adiponitrile)‐bridged dimeric complex [Fe₂(bpte)₂(μ₂‐(NC(CH₂)₄CN)₂](SbF₆)₄ ( 2 ) are low spin at room temperature, as are those in the polymeric adiponitrile‐linked acetone solvate polymer {[Fe(bpte)(μ₂‐NC(CH₂)₄CN)](BPh₄)₂ ? Me₂CO} ( 3? Me₂CO). On heating 3? Me₂CO to 80 °C, the acetone is abruptly removed with an accompanying purple to dull lavender colour change corresponding to a conversion to a high‐spin compound. Cooling reveals that the desolvate 3 shows hysteretic and abrupt spin crossover (SCO) S=0?S=2 behaviour centred at 205 K. Non‐porous 3 can reversibly absorb one equivalent of acetone per iron centre to regenerate the same crystalline phase of 3? Me₂CO concurrently reinstating a low‐spin state.     

Cover Picture: Metal–Organic Spheres as Functional Systems for Guest Encapsulation (Angew. Chem. Int. Ed. 13/2009)

The infinite coordination polymerization…? of metal ions and multitopic organic ligands is explored to fabricate metal–organic micro‐ and nanospheres that can be used as functional matrices. In their Communication on page 2325 ff., D. Maspoch and co‐workers show how this simple process affords spheres that encapsulate active substances, such as magnetic nanoparticles, organic dyes, and quantum dots, to result in multifunctional spheres. Marianne Verdoux is thanked for the cover graphic design.

     

Tuning Single‐Molecule Dynamics in Functionalized Mesoporous Silica

Diffusion of single molecules of a substituted terrylene diimide dye in functionalized mesoporous silica films was monitored by single‐molecule fluorescence microscopy. By varying the chemical nature and density of the functional groups, the diffusion dynamics of the dye molecules can be controlled precisely. The picture shows a sketch of a dye molecule in a pore, diffusion data for different phenyl functionalization densities, and the trajectory of one molecule in a cyanopropyl‐functionalized film.