Activation‐Dependent Breathing in a Flexible Metal–Organic Framework and the Effects of Repeated Sorption/Desorption Cycling

A non‐interpenetrated metal–organic framework with a paddle‐wheel secondary building unit has been activated by direct thermal evacuation, guest exchange with a volatile solvent, and supercritical CO₂ drying. Conventional thermal activation yields a mixture of crystalline phases and some amorphous content. Exchange with a volatile solvent prior to vacuum activation produces a pure breathing phase with high sorption capacity, selectivity for CO₂ over N₂ and CH₄, and substantial hysteresis. Supercritical drying can be used to access a guest‐free open phase. Pressure‐resolved differential scanning calorimetry was used to confirm and investigate a systematic loss of sorption capacity by the breathing phase as a function of successive cycles of sorption and desorption. A corresponding loss of sample integrity was not detectable by powder X‐ray diffraction analysis. This may be an important factor to consider in cases where flexible MOFs are earmarked for industrial applications.     

Activation-Dependent Breathing in a Flexible Metal–Organic Framework and the Effects of Repeated Sorption/Desorption Cycling

A non-interpenetrated metal–organic framework with a paddle-wheel secondary building unit has been activated by direct thermal evacuation, guest exchange with a volatile solvent, and supercritical CO₂ drying. Conventional thermal activation yields a mixture of crystalline phases and some amorphous content. Exchange with a volatile solvent prior to vacuum activation produces a pure breathing phase with high sorption capacity, selectivity for CO₂ over N₂ and CH₄, and substantial hysteresis. Supercritical drying can be used to access a guest-free open phase. Pressure-resolved differential scanning calorimetry was used to confirm and investigate a systematic loss of sorption capacity by the breathing phase as a function of successive cycles of sorption and desorption. A corresponding loss of sample integrity was not detectable by powder X-ray diffraction analysis. This may be an important factor to consider in cases where flexible MOFs are earmarked for industrial applications.     

Crossover Sorption of C2H2/CO2 and C2H6/C2H4 in Soft Porous Coordination Networks

Porous sorbents are materials that are used for various applications, including storage and separation. Typically, the uptake of a single gas by a sorbent decreases with temperature, but the relative affinity for two similar gases does not change. However, in this study, we report a rare example of “crossover sorption,” in which the uptake capacity and apparent affinity for two similar gases reverse at different temperatures. We synthesized two soft porous coordination polymers (PCPs), [Zn₂(L1)(L2)₂]_n (PCP-1) and [Zn₂(L1)(L3)₂]_n (PCP-2) (L1= 1,4-bis(4-pyridyl)benzene, L2=5-methyl-1,3-di(4-carboxyphenyl)benzene, and L3=5-methoxy-1,3-di(4-carboxyphenyl)benzene). These PCPs exhibits structural changes upon gas sorption and show the crossover sorption for both C₂H₂/CO₂ and C₂H₆/C₂H₄, in which the apparent affinity reverse with temperature. We used in situ gas-loading single-crystal X-ray diffraction (SCXRD) analysis to reveal the guest inclusion structures of PCP-1 for C₂H₂, CO₂, C₂H₆, and C₂H₄ gases at various temperatures. Interestingly, we observed three-step single-crystal to single-crystal (sc-sc) transformations with the different loading phases under these gases, providing insight into guest binding positions, nature of host–guest or guest-guest interactions, and their phase transformations upon exposure to these gases. Combining with theoretical investigation, we have fully elucidated the crossover sorption in the flexible coordination networks, which involves a reversal of apparent affinity and uptake of similar gases at different temperatures. We discovered that this behaviour can be explained by the delicate balance between guest binding and host–guest and guest-guest interactions.     

An Americium‐Containing Metal–Organic Framework: A Platform for Studying Transplutonium Elements

The synthesis, structure, and spectroscopic characterization of the first transplutonium metal–organic framework (MOF) is described. The preparation and structure of Am‐GWMOF‐6, [Am₂(C₆H₈O₄)₃(H₂O)₂][(C₁₀H₈N₂)], is analogous to that of the isostructural trivalent lanthanide‐only containing material GWMOF‐6. The presented MOF architecture is used as a platform to probe Am³⁺ coordination chemistry and guest‐enhanced luminescent emission, whereas the framework itself provides a means to monitor the effects of self‐irradiation upon crystallinity over time. Presented here is a discussion of these properties and the opportunities that MOFs provide in the structural and spectroscopic study of actinides.     

Windmill Co4{Co4(μ4‐O)} with 16 Divergent Branches Forming a Family of Metal–Organic Frameworks: Organic Metrics Control Topology,Gas Sorption,and Magnetism

A series of highly connected metal–organic frameworks (MOFs), [Co₈(O)(OH)₄(H₂O)₄(ina)₈](NO₃)₂ ? 2 C₂H₅OH ? 4 H₂O ( 1 ), [Co₈(O)(OH)₄(H₂O)₄(pba)₈](NO₃)₂ ? 8 C₂H₅OH ? 28 H₂O ( 2 ), and [Co₈(O)(OH)₄(H₂O)₄(pbba)₈](NO₃)₂ ? guest ( 3 ), in which ina=isonicotinate, pba=4‐pyridylbenzoate, and pbba=4‐(pyridine‐4‐yl)phenylbenzoate, is reported. These MOFs contain a new secondary building unit (SBU), with a square Co₄(μ₄‐O) central unit having the rare μ₄‐O^2? motif, which is decorated by the other four peripheral cobalt atoms through μ₃‐OH in a windmill‐like shape. This SBU holds 16 divergent connecting organic ligands, pyridyl‐carboxylates, to form three different frameworks. The high porosity of desolvated 2 is shown by the efficient gas absorption of N₂, CO₂, CH₄, and H₂. In addition, 1 and 2 exhibit unusual canted antiferromagnetic behavior with spin‐glass‐like relaxation, with blocking temperatures that are fairly high, 20 K ( 1 ) and 10 K ( 2 ), for cobalt materials. The relationship between the metal clusters and linkers has been studied, in which the size and rotational degrees of freedom of the ligands are found to control the topology, gas sorption, and magnetic properties.     

Crystal structures of the clathrate inclusion compounds formed by the host bis(isothiocyanato) tetrakis (4-methylpyridine) iron (II) with benzene,m-xylene andp-xylene as guests

Single crystal X-ray structures of three 1 : 1 (guest: host) compounds in which the Fe(NCS)₂ (4-methylpyridine)₄ complex is the host component and benzene,m-xylene andp-xylene are the respective guest components, are reported. The crystals of the inclusion compounds are tetragonalI4₁/a, with:a = 17.08(1),c = 23.66(3) Å (benzene clathrate);a = 17.17(1),c = 24.02(2) Å (m-xylene clathrate) anda = 17.12(1),c = 23.93(3) Å (p-xylene clathrate);Z = 8. The host complex molecule has the octahedral type of coordination of the N₆ donor system. The isothiocyanate ligands are intrans positions and related by twofold axial symmetry. The symmetry axis runs diagonally between the neighbouring, equatorial 4-methylpyridine (MePy) ligands. The two symmetrically independent McPy ligands form angles ofca. 45 and 55° with the equatorial Fe-N₄ plane. The absorption sites for guest molecules lie on centres of symmetry. Non-centrosymmetricm-xylene molecules occupy these sites randomly distributed over two orientations related by symmetry. The structures belong to the class oforganic zeolites since the cavities occupied by the guest molecules are interconnected to form a three-dimensional network.     

Engineered Bifunctional Luminescent Pillared-Layer Frameworks for Adsorption of CO2 and Sensitive Detection of Nitrobenzene in Aqueous Media

Through a dual-ligand synthetic approach, five isoreticular primitive cubic (pcu)-type pillared-layer metal–organic frameworks (MOFs), [Zn₂(dicarboxylate)₂(NI-bpy-44)] ⋅ x DMF ⋅ y H₂O, in which dicarboxylate=1,4-bdc ( 1 ), Br-1,4-bdc ( 2 ), NH₂-1,4-bdc ( 3 ), 2,6-ndc ( 4 ), and bpdc ( 5 ), have been engineered. MOFs 1 – 5 feature twofold degrees of interpenetration and have open pores of 27.0, 33.6, 36.8, 52.5, and 62.1 %, respectively. Nitrogen adsorption isotherms of activated MOFs 1′ – 5′ at 77 K all displayed type I adsorption behavior, suggesting their microporous nature. Although 1′ and 3′ – 5′ exhibited type I adsorption isotherms of CO₂ at 195 K, MOF 2′ showed a two-step gate-opening sorption isotherm of CO₂. Furthermore, MOF 3′ also had a significant influence of amine functions on CO₂ uptake at high temperature due to the CO₂–framework interactions. MOFs 1 – 5 revealed solvent-dependent fluorescence properties; their strong blue-light emissions in aqueous suspensions were efficiently quenched by trace amounts of nitrobenzene (NB), with limits of detection of 4.54, 5.73, 1.88, 2.30, and 2.26 μm , respectively, and Stern–Volmer quenching constants (K_sv) of 2.93×10³, 1.79×10³, 3.78×10³, 4.04×10³, and 3.21×10³ m ⁻¹, respectively. Of particular note, the NB-included framework, NB@ 3 , provided direct evidence of the binding sites, which showed strong host–guest π–π and hydrogen-bonding interactions beneficial for donor–acceptor electron transfer and resulting in fluorescence quenching.     

Modular,Homochiral, Porous Coordination Polymers: Rational Design,Enantioselective Guest Exchange Sorption and Ab Initio Calculations of Host–Guest Interactions

Two new, homochiral, porous metal–organic coordination polymers [Zn₂(ndc){(R)‐man}(dmf)]?3DMF and [Zn₂(bpdc){(R)‐man}(dmf)]?2DMF (ndc=2,6‐naphthalenedicarboxylate; bpdc=4,4′‐biphenyldicarboxylate; man=mandelate; dmf=N,N′‐dimethylformamide) have been synthesized by heating Zn^II nitrate, H₂ndc or H₂bpdc and chiral (R)‐mandelic acid (H₂man) in DMF. The colorless crystals were obtained and their structures were established by single‐crystal X‐ray diffraction. These isoreticular structures share the same topological features as the previously reported zinc(II) terephthalate lactate [Zn₂(bdc){(S)‐lac}(dmf)]?DMF framework, but have larger pores and opposite absolute configuration of the chiral centers. The enhanced pores size results in differing stereoselective sorption properties: the new metal–organic frameworks effectively and stereoselectively (ee up to 62 %) accommodate bulkier guest molecules (alkyl aryl sulfoxides) than the parent [Zn₂(bdc){(S)‐lac}(dmf)]?DMF, while the latter demonstrates decent enantioselectivity toward precursor of chiral anticancer drug sulforaphane, CH₃SO(CH₂)₄OH. The new homochiral porous metal–organic coordination polymers are capable of catalyzing a highly selective oxidation of bulkier sulfides (2‐NaphSMe (2‐C₁₀H₇SMe) and PhSCH₂Ph) that could not be achieved by the smaller‐pore [Zn₂(bdc){(S)‐lac}(dmf)]?DMF. The sorption of different guest molecules (both R and S isomers) into the chiral pores of [Zn₂(bdc){(S)‐lac}(dmf)]?DMF was modeled by using ab initio calculations that provided a qualitative explanation for the observed sorption enantioselectivity. The high stereo‐preference is accounted for by the presence of coordinated inner‐pore DMF molecule that forms a weak C? H???O bond between the DMF methyl group and the (S)‐PhSOCH₃ sulfinyl group.     

Encapsulation of Silver Nanoparticles in an Amine‐Functionalized Porphyrin Metal–Organic Framework and Its Use as a Heterogeneous Catalyst for CO2 Fixation under Atmospheric Pressure

A new porphyrin‐based compound, [Zn₃(C₄₀H₂₄N₈)(C₂₀H₈N₂O₄)₂(DEF)₂](DEF)₃ ( 1 ; DEF=N,N‐diethylformamide), has been synthesized by employing 5,10,15,20‐tetrakis(4‐pyridyl)porphyrin, 1,2‐diamino‐3,6‐bis(4‐carboxyphenyl)benzene, and Zn²⁺ salt at 100 °C under solvothermal conditions. The structure, as determined by single‐crystal XRD studies, is three‐dimensional with threefold interpenetration. The usefulness of free −NH₂ groups in the ligand was exploited for anchoring silver nanoparticles through a simple solution‐based route. The silver‐loaded sample, Ag@ 1 , was characterized by powder XRD, energy‐dispersive X‐ray spectroscopy, high‐resolution TEM, SEM, X‐ray photoelectron spectroscopy, and inductively coupled plasma MS analysis, which clearly indicated that silver nanoparticles with a size of 3.83 nm were uniformly distributed within the metal–organic framework (MOF). The Ag@ 1 sample was evaluated for possible catalytic activity for the carboxylation of a terminal alkyne by employing CO₂ under atmospheric pressure; this gave excellent results. The Ag@ 1 catalyst was found to be robust, active, and recyclable. The present studies suggest that porphyrin MOFs not only exhibit interesting structures, but also show good heterogeneous catalytic activity towards the fixation of CO₂.     

Solvent‐Dependent Synthesis from Layer to Microporous Pillared‐Layer Framework for Selective Sorption of Gas Light Hydrocarbons

Three metal‐organic frameworks, [Zn₃(bdc)₃(DMA)₂] ( 1 ) (H₂bdc = terephthalic acid; DMA = N,N‐dimethylacetamide), [Zn₃(bdc)₃(DMA)(DMPU)] ( 2 ) (DMPU = 1,3‐dimethylpropyleneurea), and [Zn₃(bdc)₄(Hdma)₂] ( 3 ) [Hdma = protonated dimethylamine (Me₂NH₂)] were synthesized by using different solvents. The solvents determine the framework architectures range from layer to microporous pillared‐layer framework. The microporous framework has 8‐connected bcg topology and shows selective sorption ability for gas light hydrocarbons.     

Fluorescent Cadmium Bipillared-Layer Open Frameworks: Synthesis,Structures, Sensing of Nitro Compounds,and Capture of Volatile Iodine

Fluorescent Cd metal–organic frameworks (MOFs), [Cd₂(dicarboxylate)₂(NI-bpy-44)₂] (dicarboxylate=benzene-1,4-dicarboxylate (1,4-bdc, 1 ), 2-bromobenzene-1,4-dicarboxylate (Br-1,4-bdc, 2 ), 2-nitrobenzene-1,4-dicarboxylate (NO₂-1,4-bdc, 3 ), biphenyl-4,4′-dicarboxylate (bpdc, 4 ); NI-bpy-44=N-(pyridin-4-yl)-4-(pyridin-4-yl)-1,8-naphthalimide)), featuring non- and twofold interpenetrating pcu -type bipillared-layer open structures with sufficient free voids of 58.4, 51.4, 51.5, and 41.4 %, respectively, have been hydro(solvo)thermally synthesized. MOFs 1 – 4 emitted solid-state blue or cyan fluorescence emissions at 447±7 nm, which mainly arose from NI-bpy-44 and are dependent on the incorporated solvents. After immersing the crystalline samples in different solvents, that is, H₂O and DMSO ( 1 and 2 ) as well as nitrobenzene and phenol ( 1 – 4 ), they exhibited a remarkable fluorescence quenching effect, whereas o-xylene and p-xylene ( 4 ) caused significant fluorescence enhancement. The sensing ability of MOFs 1 – 4 toward nitro compounds carried out in the vapor phase showed that nitrobenzene and 2-nitrophenol displayed detectable fluorescence quenching with 1 , 2 , and 4 whereas 4-nitrotoluene was an effective fluorescence quencher for 1 and 2 ; this is most likely attributed to their electron-deficient properties and higher vapor pressures. Moreover, MOFs 1 – 4 are highly reusable for quick capture of volatile iodine, as supported by clear crystal color change and also by immense fluorescence quenching responses owing to the donor–acceptor interaction. Low-pressure CO₂ adsorption isotherms indicate that activated materials 1′ – 4′ are inefficient at taking up CO₂.     

A highly porous metal-organic framework: structural transformations of a guest-free MOF depending on activation method and temperature

A doubly interpenetrating porous metal–organic framework ( SNU‐77 ) has been synthesized from the solvothermal reaction of the extended carboxylic acid tris(4′‐carboxybiphenyl)amine (H₃TCBPA) and Zn(NO₃)₂ ? 6H₂O in N,N‐dimethylacetamide (DMA). SNU‐77 undergoes single‐crystal‐to‐single‐crystal transformations during various activation processes, such as room‐temperature evacuation, supercritical CO₂ drying, and high temperature evacuation, to afford SNU‐77R , SNU‐77S , and SNU‐77H , respectively. These guest‐free MOFs exhibited different fine structures with different window shapes and different effective window sizes at room temperature. Variable‐temperature synchrotron single‐crystal X‐ray analyses reveal that the guest‐free structure is also affected by changes in temperature. Despite the different fine structures, SNU‐77R , SNU‐77S , and SNU‐77H show similar gas sorption properties due to the nonbreathing nature of the framework and an additional structural change upon cooling to cryogenic gas sorption temperature. SNU‐77H exhibits a large surface area (BET, 3670 m² g^?1), a large pore volume (1.52 cm³ g^?1), and exceptionally high uptake capacities for N₂, H₂, O₂, CO₂, and CH₄ gases.     

A host–guest complex of di­aqua­bis­[1‐hydroxy‐2(1H)‐pyridine­thionato‐O,S]­magnesium(II) and 2,2′‐di­thio­bis­(pyridine N‐oxide)

The title compound, [Mg(C₅H₄NOS)₂(H₂O)₂]·C₁₀H₈N₂O₂S₂, is a two‐component host–guest material. The 2,2′‐di­thio­bis(pyridine N‐oxide) molecule has crystallographic twofold symmetry. The metal complex lies on an inversion centre and associates via C—H?S interactions into chains which thread the 2,2′‐di­thio­bis­(pyridine N‐oxide) lattice in perpendicular directions. Hydro­gen bonds exist between the water mol­ecules of the di­aqua­magnesium units and the N—O groups of the host lattice.     

Design and Development of Zeolite-Based Catalytic Processes for Aromatics Production

Processes for the production of xylenes, which occur in an integrated aromatic complex, are discussed. A brief overview of the work carried out at Indian Petrochemicals Corporation Limited for the development of zeolite-based catalytic processes for the production of aromatics is presented. This includes xylene isomerization, transalkylation and disproportionation of C₇ and C₉ aromatics for maximization of xylenes, selective disproportionation of toluene and selective alkylation of mono-alkylaromatics to p-dialkylbenzene. Achievements in the commercialization of zeolite-based catalysts and processes for isomerization of m-xylene to p- and o-xylene along with dealkylation of ethylbenzene, and for selective ethylation of ethylbenzene to produce p-diethylbenzene are highlighted.     

Formation of a one‐dimensional water chain containing two linking patterns of planar water tetramers

The self‐assembly of the title dinuclear complex, namely (μ‐p‐phenylenediamine‐N,N,N′,N′‐tetraacetato)bis[aqua(1,10‐phenanthroline)nickel(II)] dodecahydrate, [Ni₂(C₁₄H₁₂N₂O₈)(C₁₂H₈N₂)₂(H₂O)₂]·12H₂O, through intricate noncovalent interactions results in a two‐dimensional sheet‐like structure. The dimer lies about an inversion centre at the centre of the p‐phenylenediamine ring. Uncoordinated water molecules form one‐dimensional chains in which cyclic water tetramers act as two types of building blocks. The water molecules play a significant role in the stabilization of the three‐dimensional supramolecular framework. Intramolecular `aryl–metal chelate ring' π–π interactions are also observed.     

Poly[[aquabis(μ2‐isonicotinato‐κ3N:O,O′)cadmium(II)] 1,4‐di‐3‐pyridyl‐2,3‐diaza‐1,3‐butadiene hemisolvate]

The title compound, {[Cd(C₆H₄NO₂)₂(H₂O)]·0.5C₁₂H₁₀N₄}_n, presents an intricate three‐dimensional network with cavities traversing it in three orthogonal directions, where the (disordered) guest molecules lodge. The compound is a member of a series of coordination polymers presenting the same main host framework but with guests of variable size and geometry, to which the flexible skeleton seems to adapt. The disorder in the structure is explained in terms of an apparently well defined specificity in the position/orientation of the guest molecules, as determined by the main framework.     

Guest-mediated phase transitions in a flexible pillared-layered metal–organic framework under high-pressure

The guest-dependent flexibility of the pillared-layered metal–organic framework (MOF), Zn₂bdc₂dabco·X(guest), where guest = EtOH, DMF or benzene, has been examined by high-pressure single crystal X-ray diffraction. A pressure-induced structural phase transition is found for the EtOH- and DMF-included frameworks during compression in a hydrostatic medium of the guest species, which is dependent upon the nature and quantity of the guest in the channels. The EtOH-included material undergoes a phase transition from P4/mmm to C2/m at 0.69 GPa, which is accompanied by a change in the pore shape from square to rhombus via super-filling of the pores. The DMF-included material undergoes a guest-mediated phase transition from I4/mcm to P4/mmm at 0.33 GPa via disordering of the DMF guest. In contrast, the benzene-included framework features a structure with rhombus-shaped channels at ambient pressure and shows direct compression under hydrostatic pressure. These results demonstrate the large influence of guest molecules on the high-pressure phase behavior of flexible MOFs. Guest-mediated framework flexibility is useful for engineering MOFs with bespoke pore shapes and compressibility.

The guest-dependent flexibility of the pillared-layered metal–organic framework (MOF), Zn₂bdc₂dabco·X(guest), where guest = EtOH, DMF or benzene, has been examined by high-pressure single crystal X-ray diffraction.     

1H NMR refinement of the structure of the guest sublattice and molecular dynamics in the ultrathin channels of [Zn2(C8H4O4)2(C6H12N2)]·n(H3C)2NCHO

Localization and molecular mobility of the ligands ([C₈H₄O₄]²⁺ and [C₆H₁₂N₂]⁰) of the host lattice and (CH₃)₂NCHO dimethyl formamide guest molecules in the inclusion compound [Zn₂(C₈H₄O₄)₂(C₆H₁₂N₂)]·n(H₃C)₂NCHO were studied on the basis of ¹H NMR data. At room temperature, the longest axes of the dimethyl formamide guest molecules are ordered in parallel to the C ₄ symmetry axes, and the symmetry planes of these molecules are disordered, while preserving the tetragonal crystal system of the inclusion compound. At lower temperatures, a phase transition takes place in view of the ordering in the guest sublattice. Original Russian Text Copyright ? 2009 by A. V. Sabylinskii, S. P. Gabuda, S. G. Kozlova, D. N. Dybtsev, and V. P. Fedin __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 50, No. 3, pp. 443–450, May–June, 2009.     

Infrared Spectroscopic Study on the Hofmann-dadn-type and the Td-dahxn-type clathrates

Infrared spectra of M(1,10-diaminodecane)Ni(CN)₄ · 1,5 G (M=Co, Ni or Cd; G=o-xylene, m-xylene, p-xylene) and Cd(l,6-diaminohexane)M(CN)₄ · C₆H₆ (M=Cd or Hg) clathrates are reported. The 1,10-diaminodecane and 1,6-diaminohexane molecules in the host permit the inclusion of bulky guest molecules. The spectral data of clathrates were compared with those of the corresponding host. The spectral features suggest that these compounds are similar in structure to other Hofmann-type and Hofmann-T_d type clathrates, respectively.     

Formation of a Pillar[5]arene‐Based Two‐Dimensional Poly‐Pseudo‐Rotaxane: Threading and Crosslinking by the Same Guest Molecules

A one‐pot reaction of the A1/A2‐thiopyridyl pillar[5]arene L with silver(I) trifluoroacetate in the presence of the linear dinitrile guest C8 , [CN(CH₂)_nCN, n=8], afforded the first example of a two‐dimensional (2D) poly‐pseudo‐rotaxane {[(μ₄‐Ag)₂( C8 @ L )₂(μ ‐C8 )](CF₃CO₂)₂}_n. Surprisingly, in this structure the C8 guest not only threads into the pillar[5]arene unit but also crosslinks the 1D coordinative polymeric arrays. The formation of the 2D poly‐pseudo‐rotaxane is driven by an adaptive rearrangement of the components that minimizes the steric clashes not only between the threaded guests but also between the threaded and crosslinked guests where crosslinking occurs. A pathway for the formation of the 2D poly‐pseudo‐rotaxane is proposed.