Solution State-Like Reactivity of a Flexible Crystalline Werner-Type Metal Complex.

Flexible crystalline solids exhibit unique properties in response to external stimuli like heat and light. However, challenges exist in developing crystalline solids that have similar degrees of flexibility as in solution. Herein, we report the preparation of the new flexible crystalline metal complex [Cd(CF3SO3)2(4-spy)4] (4-spy=4-styrylpyridine), which contains photoreactive 4-spy ligand. Unlike traditional solids, this metal complex displays solution state-like [2+2] photocycloaddition reactivity. Specifically, UV irradiation of the crystalline material leads to formation of the same diverse array of dimers and cis isomer that are generated by photoreaction in the solution state. In addition, the photoresponsive flexibility of the solid leads to a photosalient effect and photo-induced formation of pores. The origin of the solution state-like photoreactivity of the solid is related to properties of the Cd(II) cation and fluorinated CF3SO3 anion, and the multi-orientational arrangement of the 4-spy ligands.

Entropically driven melting of Cu-based 1D coordination polymers.

We investigated the melting behavior of four CPs with one-dimensional structures from a thermodynamic point-of-view. The difference in melting points depending on the crystal structures is observed. The interactions within the crystals were analyzed using DFT calculations. These analyses suggest that entropic terms dominate the melting points.

Selective Gas Sensing under a Mixed Gas Flow with a One-Dimensional Copper Coordination Polymer.

Structural changes of the coordination polymer associated with gas adsorption (gate opening-type adsorption) can be linked to bulk physical properties such as magnetism, electrical conductivity, and dielectric properties. To enable real-space sensing applications, it is imperative to have a system where the selective adsorption of mixed gases can be correlated with physical properties. In this report, we demonstrate that a crystalline sample of one-dimensional (1D) coordination polymer exhibits selective CO2 adsorption while simultaneously displaying dielectric switching behavior in a mixed N2/CO2 gas environment. In the crystal of {[Cu2(2-TPA)4(pz)]·CH3CN}n (1·CH3CN), where 2-TPA and pz are 2-thiophencarboxylate and pyrazine, respectively, paddle wheel-type units of [Cu2(2-TPA)4] are bridged by pz, forming a 1D chain structure. One of the two crystallographically independent 2-TPA units was interacted with the pz moiety of the adjacent 1D chain by π···π interactions, forming a two-dimensional (2D) layer parallel to the ab plane. Activated 1 shows selective CO2 adsorption by a gate opening-type adsorption mechanism, indicating that the CO2 adsorption process is accompanied by a structural change. The change in the real part of dielectric permittivity (ε') under the mixed N2/CO2 gas flow is a result of the selective CO2 adsorption, which was supported by the enthalpy changes (ΔH) associated with CO2 adsorption in two methods: CO2 adsorption isotherms and temperature-dependent measurements of ε' under a mixed N2/CO2 gas flow. The calculated ΔH values were found to be in good agreement across both methods. The CO2 ratio in the mixed N2/CO2 gas flow increased, and the switching ratio of ε' (Δε') also increased. Notably, Δε' exhibited a marked increase beyond the pressure required for gate opening adsorption.

Composite with a Glassy Nonporous Coordination Polymer Enhances Gas Adsorption Selectivity.

A glass-crystal composite (g-NCP/PCP), comprising a glassy nonporous coordination polymer (g-NCP) and a crystalline porous coordination polymer (PCP)/metal-organic framework, was synthesized by using a melt-quenched method. Compared to that of the PCP itself, g-NCP/PCP has an enhanced gas adsorption selectivity. The results should stimulate further studies of the chemistry of g-NCP/PCP glass-crystal composites.

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.

Mechanical Force Induced Formation of Extrinsic Micropores in Coordination Polymers.

Mechanical force can be employed not only to efficiently synthesize new materials under environmentally friendly conditions but also to change the macroscopic and microscopic properties of materials. Although coordination polymers (CPs) are attractive functional materials because they possess high structural designability and diversity, mechanical force-induced structural and functional changes of CPs are challenging issues. In this study, two one-dimensional CPs, one a densely packed nonporous CP [Cu2(bza)4(pyr)] (1) and the other a porous CP [Cu2(1-nap)4(pyr)] (2) (bza = benzoate, 1-nap = 1-naphthoate and pyr = pyrimidine), were subjected to ball-milling to assess the effect of mechanical force on their porosities. Ball mill treatments were found to induce an amorphization and cause a 30 fold enhancement of the CO2 adsorption amount at 195 K and P/P0 ∼ 1 for 1 and a slightly decreased CO2 adsorption amount for 2. The results of thorough characterization studies suggest that the formation of extrinsic micropores in addition to extrinsic mesopores/macropores between particles takes place by ball milling.

One-Step Preparation of Fe/N/C Single-Atom Catalysts Containing Fe-N4 Sites from an Iron Complex Precursor with 5,6,7,8-Tetraphenyl-1,12-Diazatriphenylene Ligands.

Fe/N/C single-atom catalysts containing Fe-Nx sites prepared by pyrolysis are promising cathode materials for fuel cells and metal-air batteries due to their high oxygen reduction reaction (ORR) activities. We have developed iron complexes containing N2- or N3-chelating coordination structures with preorganized aromatic rings in a 1,12-diazatriphenylene framework tethering bromo substituents as precursors to precisely construct Fe-N4 sites in an Fe/N/C catalyst. One-step pyrolysis of the iron complex with carbon black forms atomically dispersed Fe-N4 sites without iron aggregates. X-ray absorption spectroscopy (XAS) and electrochemical measurements revealed that the iron complex with N3-coordination is more effectively converted to Fe-N4 sites catalyzing ORR with a TOF value of 0.21 e site-1 s-1 at 0.8 V vs. RHE. This indicates that the formation of Fe-N4 sites is controlled by precise tuning of the chemical structure of the iron complex precursor.

Dipole fluctuation and structural phase transition in hydrogen-bonding molecular assemblies of mononuclear CuII complexes with polar fluorobenzoate ligands.

A series of mononuclear CuII complexes, [CuII(4-FBA)2(py)2(H2O)] (1), [CuII(3-FBA)2(py)2(H2O)] (2), and [CuII(3,4-F2BA)2(py)2(H2O)] (3), where 4-FBA = 4-fluorobenzoate, 3-FBA = 3-fluorobenzoate, 3,4-F2BA = 3,4-difluorobenzoate, and py = pyridine, respectively, was synthesized and the complexes crystallographically identified. All the CuII complex crystals share a one-dimensional O-H⋯O hydrogen-bonding chain substructure, although the mutual alignment of fluorinated benzoate (FxBA) ligands exhibits subtle differences among the various compounds, i.e., FxBA ligands align in an antiparallel fashion in crystals 1 and 3, while 3-FBA ligands in crystal 2 are interdigitated with a tilt along the a axis. Reversible phase transitions were found upon heating at 170.7, 171.3, and 267.5 K for crystals 1, 2, and 3, respectively; all crystals showed approximately 3% expansion and shrinkage of the intermolecular O-H⋯O hydrogen bond distances associated with the thermally activated orientational fluctuations of the FxBA ligands in crystals 1 and 3. The increase in dielectric constant with increasing temperature, at 240 K, activated molecular fluctuation in the 3,4-F2BA ligands in crystal 3. Heat capacity measurements indicated that both the expansion and shrinkage of hydrogen bonds, and the molecular fluctuation in 3,4-F2BA ligands, contributed to phase transition, and the latter caused dipole fluctuation, resulting in a dielectric anomaly in crystal 3.

Reversible hydrogen adsorption at room temperature using a molybdenum-dihydrogen complex in the solid state.

Reversible H2 storage under mild conditions is one of the most important targets in the field of materials chemistry. Dihydrogen complexes are attractive materials for this target because they possess moderate adsorption enthalpy as well as adsorption without cleavage of the H-H bond. In spite of these advantages, H2 adsorption studies of dihydrogen complexes in the solid state are scarce. We herein present H2 adsorption properties of the 16-electron precursor complex ([Mo(PCy3)2(CO)3]) in the solid state synthesized by two procedures. One is the direct synthesis under an Ar atmosphere (1), and the other is removal of the N2-adduct under vacuum (2). 2 showed ideal Langmuir type reversible ad/desorption of H2 above room temperature, whereas 1 showed irreversible adsorption. The adsorption enthalpy of 2 was larger than that in THF solution. Using DFT calculation, this difference was explained by the absence of the agostic interaction in the solid state.

Alkyl decorated metal-organic frameworks for selective trapping of ethane from ethylene above ambient pressures.

The trapping of paraffins is beneficial compared to selective olefin adsorption for adsorptive olefin purification from a process engineering point of view. Here we demonstrate the use of a series of Zn2(X-bdc)2(dabco) (where X-bdc2- is bdc2- = 1,4-benzenedicarboxylate with substituting groups X, DM-bdc2- = 2,5-dimethyl-1,4-benzenedicarboxylate or TM-bdc2- = 2,3,5,6-tetramethyl-1,4-benzenedicarboxylate and dabco = diazabicyclo[2.2.2.]octane) metal-organic frameworks (MOFs) for the adsorptive removal of ethane from ethylene streams. The best performing material from this series is Zn2(TM-bdc)2(dabco) (DMOF-TM), which shows a high ethane uptake of 5.31 mmol g-1 at 110 kPa, with a good IAST selectivity of 1.88 towards ethane over ethylene. Through breakthrough measurements a high productivity of 13.1 L kg-1 per breakthrough is revealed with good reproducibility over five consecutive cycles. Molecular simulations show that the methyl groups of DMOF-TM are forming a van der Waals trap with the methylene groups from dabco, snuggly fitting the ethane. Further, rarely used high pressure coadsorption measurements, in pressure regimes that most scientific studies on hydrocarbon separation on MOFs ignore, reveal an increase in ethane capacity and selectivity for binary mixtures with increased pressures. The coadsorption measurements reveal good selectivity of 1.96 at 1000 kPa, which is verified also through IAST calculations up to 3000 kPa. This study overall showcases the opportunities that pore engineering by alkyl group incorporation and pressure increase offer to improve hydrocarbon separation in reticular materials.

A Temporarily Pore-Openable Porous Coordination Polymer for Guest Adsorption/Desorption.

Flexible porous coordination polymers (PCPs)/metal-organic frameworks are unique materials that have potential applications as components of highly efficient separation, sensor, and actuator systems. In general, the structures of flexible PCPs drastically change upon guest loading. In this investigation, we uncovered the rare one-dimensional PCP [Cu2(bza)4(2-apyr)] (1; bza = benzoate and 2-apyr = 2-aminopyrimidine), which exhibits a unique type of flexibility involving temporary pore opening. Single-crystal X-ray diffraction analysis revealed that desolvated 1 and ethyl acetate (AcOEt)-loaded (1·AcOEt) and CO2-loaded (1·CO2) 1 have isolated pores. In the case of 1, the pore structure prevents guest penetration. In addition, the isolated pore structures of 1·AcOEt and 1·CO2 block guest release. However, 1 participates in reversible adsorption/desorption of AcOEt and CO2 because pore opening occurs temporarily. The CO2 adsorption/desorption isotherms of 1 are type I and dissimilar to those observed in traditional flexible PCPs with adsorption/desorption hysteresis. The lesser conventional flexibility displayed by 1 could offer new insight into the design of flexible PCPs.

Gas sorption of nano-porous supramolecules formed by multi-hydrogen bonded coordination capsules.

[{ReI(CO)3(Hbim)}3(tpta)]2 (1, Hbim- = 2,2'-biimidazolate monoanion, tpta = 2,4,6-tripyridyl-1,3,5-triazine) was prepared as a nano-space supramolecule by using a new group of H-bonded coordination capsules. The hamburger bun-shaped half unit [{ReI(CO)3(Hbim)}3(tpta)] contains six intermolecular H-bonds of Hbim- ligands with complementary dual NHN types, and three [ReI(CO)3(Hbim)] are coordinated by bridging tridentate tpta. Interestingly, mechanical grinding easily would convert single crystals of 1 to an amorphous state with minor crystallinity while maintaining the nano-space pores. The ground sample can reversibly uptake and release small molecules such as CO2 and (CH2Cl)2.

A hydrogen-bonded organic framework based on redox-active tri(dithiolylidene)cyclohexanetrione.

Redox-active hexakis(4-carboxyphenyl) tri(dithiolylidene)cyclohexanetrione (CPDC) was synthesized. The CPDC-based porous framework, constructed via anomalistic helical hydrogen-bonding, exhibites permanent porosity and photoconductivity.

Influence of carbohydrate polymer shaping on organic dye adsorption by a metal-organic framework in water.

A number of studies have been conducted to develop new metal-organic frameworks (MOFs) as adsorbents for the removal of contaminants from polluted water. However, few reports exist describing detailed and thorough examinations of the effects of shaping on the adsorption properties of MOFs. In this study, a thorough analysis and comparison was conducted of the Orange II and Rhodamine B dye adsorption properties of unshaped MIL-100(Fe) (MIL) particles and alginate polymer-shaped MIL beads (MIL-alg). The adsorption affinities of Orange II and Rhodamine B for unshaped MIL were observed to be higher than those for shaped MIL-alg because partial coating of the surface of MIL particles by alginate polymer weakens adsorption forces. Kinetic analysis using a two-compartment model indicates that the contribution of the slow step in the mechanistic pathway for adsorption is more pronounced in MIL-alg compared to MIL because slow dye diffusion takes place in the alginate polymer. We believe that these fundamental findings will have a beneficial impact on approaches to design shaped MOFs that display improved dye removal performance.

Structural Phase Transitions of a Molecular Metal Oxide.

The structural phase of a metal oxide changes with temperature and pressure. During phase transitions, component ions move in multidimensional metal-oxygen networks. Such macroscopic structural events are robust to changes in particle size, even at scales of around 10 nm, and size effects limiting these transitions are particularly important in, for example, high-density memory applications of ferroelectrics. In this study, we examined structural transitions of the molecular metal oxide [Na@(SO3 )2 (n-BuPO3 )4 MoV 4 MoVI 14 O49 ]5- (Molecule 1) at approximately 2 nm by using single-crystal X-ray diffraction analysis. The Na+ encapsulated in the discrete metal-oxide anion exhibited a reversible order-disorder transition with distortion of the Mo-O molecular framework induced by temperature. Similar order-disorder transitions were also triggered by chemical pressure induced by removing crystalline solvent molecules in the single-crystal state or by substituting the countercation to change the molecular packing.

Responsive Four-Coordinate Iron(II) Nodes in FePd(CN)4.

Metal node design is crucial for obtaining structurally diverse coordination polymers (CPs) and metal-organic frameworks with desirable properties; however, FeII ions are exclusively six-coordinated. Herein, we present a cyanide-bridged three-dimensional (3D) CP, FePd(CN)4 , bearing four-coordinate FeII ions, which is synthesized by thermal treatment of a two-dimensional (2D) six-coordinate FeII CP, Fe(H2 O)2 Pd(CN)4 ⋅4 H2 O, to remove water molecules. Atomic-resolution transmission electron microscopy and powder X-ray and neutron diffraction measurements revealed that the FePd(CN)4 structure is composed of a two-fold interpenetrated PtS topology network, where the FeII center demonstrates an intermediate geometry between tetrahedral and square-planar coordination. This four-coordinate FeII center with the distorted geometry can act as a thermo-responsive flexible node in the PtS network.

Controlling the gate-sorption properties of solid solutions of Werner complexes by varying component ratios.

Methods relying on the use of solid solutions can be used to produce solid materials having finely controlled physical properties. In the current investigation, we utilized this protocol to prepare solid solutions derived from two different Werner complexes in order to assess the effects of component ratios on acetone vapor adsorption properties. For this purpose, microcrystalline solid solutions with the basic elemental composition α-[Cu(PF6)2(py)4]x[Cu(CF3SO3)2(py)4]1-x (x = 0.75 and 0.5) (α-PAC-2-PF6/CF3SO3(x = 0.75 and 0.5), py = pyridine) were prepared by hexane induced precipitation of acetone solutions of [Cu(PF6)2(py)4] (PAC-2-PF6) and [Cu(CF3SO3)2(py)4] (PAC-2-CF3SO3). The results of acetone sorption isotherm measurements show that gate opening and closing pressures of the solid solutions are dependent on the composition ratios of PAC-2-CF3SO3 and PAC-2-PF6. Specifically, an increase in CF3SO3- anion content induces weakening of the interactions with acetone as a consequence of expansion of lattice constants and also strengthening host-host interactions. These effects cause an increase in gate opening and closing pressures.

Coordinated Water as New Binding Sites for the Separation of Light Hydrocarbons in Metal-Organic Frameworks with Open Metal Sites.

Metal-organic frameworks with open metal sites are promising materials for gas separations. Particularly, the M2(dobdc) (dobdc4- = 2,5-dioxidobenzenedicarboxylate, M2+ = Co2+, Mn2+, Fe2+, ...) framework has been the Drosophila of this research field and has delivered groundbreaking results in terms of sorption selectivity. However, many studies focus on perfect two-component mixtures and use theoretical models, e.g., the ideal adsorbed solution theory, to calculate selectivities. Within this work, we shed light on the comparability of these selectivities with values obtained from propane/propene multicomponent measurements on the prototypical Co2(dobdc) framework, and we study the impact of impurities like water on the selectivity. Despite the expected capacity loss, the presence of water does not necessarily lead to a decreased selectivity. Density functional theory calculations of the binding energies prove that the water molecules adsorbed to the metal centers introduce new binding sites for the adsorbates.

Understanding the interactions between the bis(trifluoromethylsulfonyl)imide anion and absorbed CO2 using X-ray diffraction analysis of a soft crystal surrogate.

The selective carbon dioxide (CO2) absorption properties of ionic liquids (ILs) are highly pertinent to the development of methods to capture CO2. Although it has been reported that fluorinated components give ILs enhanced CO2 solubilities, it has been challenging to gain a deep understanding of the interactions occurring between ILs and CO2. In this investigation, we have utilized the soft crystalline material [Cu(NTf2)2(bpp)2] (NTf2‒ = bis(trifluoromethylsulfonyl)imide, bpp = 1,3-bis-(4-pyridyl)propane) as a surrogate for single-crystal X-ray diffraction analysis to visualize interactions occurring between CO2 and NTf2‒, the fluorinated IL component that is responsible for high CO2 solubility. Analysis of the structure of a CO2-loaded crystal reveals that CO2 interacts with both fluorine and oxygen atoms of NTf2‒ anions in a trans rather than cis conformation about the S-N bond. Theoretical analysis of the structure of the CO2-loaded crystal indicates that dispersion and electrostatic interactions exist between CO2 and the framework. The overall results provide important insight into understanding and improving the CO2 absorption properties of ILs.

A Zn(ii) metal-organic framework with dinuclear [Zn2(N-oxide)2] secondary building units.

We report the synthesis, structural characterisation, and adsorption properties of a three-dimensional metal-organic framework [Zn(pydcao)(DMF)] (H2-pydcao = 3,5-pyridinedicarboxylic acid N-oxide) that has an unprecedented [Zn2(N-oxide)2] secondary building unit.