Guest-Specific Function of a Flexible Undulating Channel in a 7,7,8,8-Tetracyano-p-quinodimethane Dimer-Based Porous Coordination Polymer

Advancing Pore-Space-Partitioned Metal-Organic Frameworks with Isoreticular Cluster Concept

Trigonal planar M3(O/OH) trimers are among the most important clusters in inorganic chemistry and are the foundational features of multiple high-impact MOF platforms. Here we introduce a concept called isoreticular cluster series and demonstrate that M3(O/OH), as the first member of a supertrimer series, can be combined with a higher hierarchical member (double-deck trimer here) to advance isoreticular chemistry. We report here an isoreticular series of pore-space-partitioned MOFs called M3M6 pacs made from co-assembly between M3 single-deck trimer and M3x2 double-deck trimer. Important factors were identified on this multi-modular MOF platform to guide optimization of each module, which enables the phase selection of M3M6 pacs by overcoming the formation of previously-always-observed same-cluster phases. The new pacs materials exhibit high surface area and high uptake capacity for CO2 and small hydrocarbons, as well as selective adsorption properties relevant to separation of industrially important mixtures such as C2H2/CO2 and C2H2/C2H4. Furthermore, new M3M6 pacs materials show electrocatalytic properties with high activity.

Selective sorption of oxygen and nitrous oxide by an electron donor-incorporated flexible coordination network

Incorporating strong electron donor functionality into flexible coordination networks is intriguing for sorption applications due to a built-in mechanism for electron-withdrawing guests. Here we report a 2D flexible porous coordination network, [Ni₂(4,4'-bipyridine)(VTTF)₂]n(1) (where H₂VTTF = 2,2'-[1,2-bis(4-benzoic acid)-1,2ethanediylidene]bis-1,3-benzodithiole), which exhibits large structural deformation from the as-synthesized or open phase (1α) into the closed phase (1β) after guest removal, as demonstrated by X-ray and electron diffraction. Interestingly, upon exposure to electron-withdrawing species, 1β reversibly undergoes guest accommodation transitions; 1α⊃O₂ (90 K) and 1α⊃N₂O (185 K). Moreover, the 1β phase showed exclusive O₂ sorption over other gases (N₂, Ar, and CO) at 120 K. The phase transformations between the 1α and 1β phases under these gases were carefully investigated by in-situ X-ray diffraction, in-situ spectroscopic studies, and DFT calculations, validating that the unusual sorption was attributed to the combination of flexible frameworks and VTTF (electron-donor) that induces strong interactions with electron-withdrawing species.

Developing Water-Stable Pore-Partitioned Metal-Organic Frameworks with Multi-Level Symmetry for High-Performance Sorption Applications

A new perspective is proposed in the design of pore-space-partitioned MOFs that is focused on ligand symmetry properties sub-divided here into three hierarchical levels: 1) overall ligand, 2) ligand substructure such as backbone or core, and 3) the substituent groups. Different combinations of the above symmetry properties exist. Given the close correlation between nature of chemical moiety and its symmetry, such a unique perspective into ligand symmetry and sub-symmetry in MOF design translates into the influences on MOF properties. Five new MOFs have been prepared that exhibit excellent hydrothermal stability and high-performance adsorption properties with potential applications such as C₃ H₆ /C₂ H₄ and C₂ H₂ /CO₂ selective adsorption. The combination of high stability with high benzene/cyclohexane selectivity of ≈13.7 is also of particular interest.

Porous functional metal–organic frameworks (MOFs) constructed from different N-heterocyclic carboxylic ligands for gas adsorption/separation

This review mainly summarizes the recent progress of MOFs composed of N-heterocyclic carboxylate ligands in gas sorption/separation. This work may help to understand the relationship between the structures of MOFs and gas sorption/separation.

Enhanced Adsorption Selectivity of Carbon Dioxide and Ethane on Porous Metal-Organic Framework Functionalized by a Sulfur-Rich Heterocycle

Porous metal-organic framework [Zn₂(ttdc)₂(bpy)] (1) based on thieno [3,2-b]thiophenedicarboxylate (ttdc) was synthesized and characterized. The structure contains intersected zig-zag channels with an average aperture of 4 × 6 Å and a 49% (v/v) guest-accessible pore volume. Gas adsorption studies confirmed the microporous nature of 1 with a specific surface area (BET model) of 952 m²·g^-1 and a pore volume of 0.37 cm³·g^-1. Extensive CO₂, N₂, O₂, CO, CH₄, C₂H₂, C₂H₄ and C₂H₆ gas adsorption experiments at 273 K and 298 K were carried out, which revealed the great adsorption selectivity of C₂H₆ over CH₄ (IAST selectivity factor 14.8 at 298 K). The sulfur-rich ligands and double framework interpenetration in 1 result in a dense decoration of the inner surface by thiophene heterocyclic moieties, which are known to be effective secondary adsorption sites for carbon dioxide. As a result, remarkable CO₂ adsorption selectivities were obtained for CO₂/CH₄ (11.7) and CO₂/N₂ (27.2 for CO₂:N₂ = 1:1, 56.4 for CO₂:N₂ = 15:85 gas mixtures). The computational DFT calculations revealed the decisive role of the sulfur-containing heterocycle moieties in the adsorption of CO₂ and C₂H₆. High CO₂ adsorption selectivity values and a relatively low isosteric heat of CO₂ adsorption (31.4 kJ·mol^-1) make the porous material 1 a promising candidate for practical separation of biogas as well as for CO₂ sequestration from flue gas or natural gas.

Hierarchical Metal-Organic Aerogel as a Highly Selective and Sustainable CO2 Adsorbent

Typical amorphous aerogels pose great potential for CO₂ adsorbents with high surface areas and facile diffusion, but they lack well-defined porosity and specific selectivity, inhibiting utilization of their full functionality. To assign well-defined porous structures to aerogels, a hierarchical metal-organic aerogel (HMOA) is designed, which consists of well-defined micropores (d ∼ 1 nm) by coordinative integration with chromium(III) and organic ligands. Due to its hierarchical structure with intrinsically flexible coordination, the HMOA has excellent porous features of a high surface area and a reusable surface with appropriate binding energy for CO₂ adsorption. The HMOA features high CO₂ adsorption capacity, high CO₂/N₂ IAST selectivity, and vacuum-induced surface regenerability (100% through 20 cycles). Further, the HMOA could be prepared via simple ambient drying methods while retaining the microporous network. This unique surface-tension-resistant micropore formation and flexible coordination systems of HMOA make it a potential candidate for a CO₂ adsorbent with industrial scalability and reproducibility.

On the Role of Flexibility for Adsorptive Separation

Chemical separations, mostly based on heat-driven techniques such as distillation, account for a large portion of the world's energy consumption. In principle, differential adsorption is a more energy-efficient separation method, but conventional adsorbent materials are still not effective for many industry-relevant mixtures. Porous coordination polymers (PCPs), or metal-organic frameworks (MOFs), are attractive for their well-defined, designable, modifiable, and flexible structures connecting to various potential applications. While the importance of the structural flexibility of MOFs in adsorption-based functions has been demonstrated, the understanding of this special feature is still in its infancy and mostly stays at the periodic structural transformation at the equilibrium state and the special shapes of single-component adsorption isotherms. There are many confusions about the categorization and roles of various types of flexibility. This Account discusses the role of flexibility of MOFs for adsorptive separation, mainly from the thermodynamic and kinetic points of view.As the classic type of framework flexibility, guest-driven structural transformations and the corresponding adsorption isotherms can be thermodynamically described by the energies of the host-guest system. The highly guest-specific pore-opening action showing contrasting single-component adsorption isotherms is regarded as a strategy for achieving molecular sieving without the need for aperture size control, but its effect and role for mixture separation are still controversial. Quantitative mixture adsorption/separation experiments showed that the common periodic (cooperative) pore-opening action leads to coadsorption of molecules smaller than the opened aperture, while the aperiodic (noncooperative) one can achieve inversed molecular sieving under a thermodynamic mechanism.The energy barrier and structure in the nonequilibrium state are also important for flexibility and adsorption/separation. With suitable energy barriers between metastable structures, new types of framework flexibility such as aperture gating can be realized. While kinetically controlled gating flexibility is usually ignored because of the difficulty of characterization or considered as disadvantageous for separation because of the variable aperture size, it plays a critical role in most kinetic separation systems, including adsorbents conventionally regarded as rigid. With the concept of gating flexibility, the meanings of aperture and guest sizes for judging molecular sieving need to be reconsidered. Gating flexibility depends on not only the host itself but also the guest, the host-guest interaction, and the external environment such as temperature, which can be rationally tuned to achieve special adsorption/separation behaviors such as inversed temperature dependence, molecular sieving, and even inversed thermodynamic selectivity. The comprehensive understanding of the thermodynamic and kinetic bases of flexibility will give a new horizon for next-generation separation materials beyond MOFs and adsorbents.

Open metal site (OMS)-inspired investigation of adsorption and catalytic functions in a porous metal-organic framework (MOF)

In this article, we report the adsorption and catalytic study of the three-dimensional (3D) metal-organic framework (MOF) {Mn₂(1,4-bdc)₂(DMF)₂} (1) (1,4-bdcH₂, 1,4-benzene dicarboxylic acid; DMF, N,N-dimethylformamide) together with the synthesis and structure of two new Mn(II)-MOFs {Mn₃(Br-bdc)₃(DMF)₄} (2) and {Mn₃(NO₂-bdc)₃(DMF)₄} (3) (Br-bdcH₂, 2-bromo-1,4-benzene dicarboxylic acid; NO₂-bdcH₂, 2-nitro-1,4-benzene dicarboxylic acid) under solvothermal conditions. Compounds 2 and 3 have two-dimensional (2D) extended structures and feature trimeric {Mn₃(CO₂)₆} units that serve as secondary building units for the frameworks. The desolvated compound of 1, denoted as 1', having potential Mn(II) open metal sites (OMSs) lined in a one-dimensional (1D) Mn-chain interconnected by carboxylate groups, exhibits guest-selective adsorption of solvent vapours wherein the compound shows a stepwise profile with H₂O vapour, while a gated isotherm was recorded with MeOH. After realizing the favourable interaction of 1' with polar solvent molecules, we have used Mn(II) OMSs in 1' for efficient cyanosilylation reactions of aromatic aldehydes. We have recorded 100% conversion for eight aromatic aldehydes, while several other aldehydes showed appreciable conversion. Notably, the recorded conversions in the case of many substrates are higher than those for many other reported MOF catalysts.

Highly Selective Separation of Benzene and Cyclohexane in a Spatially Confined Carborane Metallacage

Separation of light hydrocarbons (C₁-C₉) represents one of the "seven chemical separations to change the world". Boron clusters can potentially play an important role in chemical separation, due to their unique three-dimensional structures and their ability to promote a potentially rich array of weak noncovalent interactions. Herein, we report the rational design of metallacages with carborane functionality and cooperative dihydrogen binding sites for the highly selective capture of cyclohexane molecules. The metallacage 1, bearing the ligand 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPT), can produce cyclohexane with a purity of 98.5% in a single adsorption-desorption cycle from an equimolar mixture of benzene and cyclohexane. In addition, cyclohexene molecules can be also encapsulated inside the metallacage 1. This selective encapsulation was attributed to spatial confinement effects, C-H···π interactions, and particularly dihydrogen-bond interactions. This work suggests exciting future applications of carborane cages in supramolecular chemistry for the selective adsorption and separation of alkane molecules and may open up a new research direction in host-guest chemistry.

Phenanthrene[2]arene: synthesis and application as nonporous adaptive crystals in the separation of benzene from cyclohexane

Phenanthrene[2]arene 1 with excellent adsorption capacity for benzene was synthesized in high yield. Activated crystals of 1 have been successfully used to separate PhH from equimolar mixture of PhH and Cy and the purity of the PhH can reach 98.4%.

Harnessing Shape Complementarity for Upgraded Cyclohexane Purification through Adaptive Bottlenecked Pores in an Imidazole-Containing MOF

Shape complementarity is a biological craft for precisely binding substrates at protein-protein interfaces. An analogy to such a function can be drawn conceptually for crystalline porous solids; yet the manifested entities are rare in reticular chemistry. The bottleneck-shaped pores carved out of a metal-organic framework, Zn(MIBA)₂ (aka. MAF-stu-13), can perfectly accommodate benzene molecules. Remarkably, its framework adapts to the optimal guest binding-the enhanced host-guest interactions in the neck in turn minimize the guest-guest repulsion in the pore to the extent it turns into attraction-as demonstrated by the combined X-ray structural and DFT computational studies. This adaptive material can be used for liquid-phase production of ultrahigh-purity (≥99 %) cyclohexane, achieving a balance between uptake capacity and separation selectivity and surpassing the performances of other porous and nonporous crystals reported recently (e.g. product purity 99.4 % vs. 97.5 % to date).

Spiers Memorial Lecture: Coordination networks that switch between nonporous and porous structures: an emerging class of soft porous crystals

Coordination networks (CNs) are a class of (usually) crystalline solids typically comprised of metal ions or cluster nodes linked into 2 or 3 dimensions by organic and/or inorganic linker ligands. Whereas CNs tend to exhibit rigid structures and permanent porosity as exemplified by most metal-organic frameworks, MOFs, there exists a small but growing class of CNs that can undergo extreme, reversible structural transformation(s) when exposed to gases, vapours or liquids. These "soft" or "stimuli-responsive" CNs were introduced two decades ago and are attracting increasing attention thanks to two features: the amenability of CNs to design from first principles, thereby enabling crystal engineering of families of related CNs; and the potential utility of soft CNs for adsorptive storage and separation. A small but growing subset of soft CNs exhibit reversible phase transformations between nonporous (closed) and porous (open) structures. These "switching CNs" are distinguished by stepped sorption isotherms coincident with phase transformation and, perhaps counterintuitively, they can exhibit benchmark properties with respect to working capacity (storage) and selectivity (separation). This review addresses fundamental and applied aspects of switching CNs through surveying their sorption properties, analysing the structural transformations that enable switching, discussing structure-function relationships and presenting design principles for crystal engineering of the next generation of switching CNs.

Reversing Benzene/Cyclohexane Selectivity through Varying Supramolecular Interactions Using Aliphatic, Isoreticular MOFs

Effective solid-state adsorbent materials, such as metal organic frameworks (MOFs), rely upon tailored void spaces for selective adsorption of one component from a mixture. This is particularly crucial when separating challenging mixtures such as benzene (Bz) and cyclohexane (Cy) requiring a highly expensive and energy intensive process. Employing bulky "3D-linkers" to construct MOFs leads to materials with unique, contoured pore shapes which consequently allow for significant control over guest adsorption preferences. Investigation into these selectivity preferences is key to identifying suitable materials for industrial separations and is an area currently underexplored. Here, we provide an in-depth investigation exploring the selectivity path between planar and 3D-linkers and their preference to adsorb either Cy or Bz. To validate this principle, the adsorption selectivity of Cy and Bz in 3DL-MOF-1 ([Zn₄O(pdc)₃] (pdc = bicylo[1.1.1]pentane-1,3-dicarboxylate), CUB-5, and MOF-5 was explored. MOF-5 exhibits a selective preference for Cy adsorption at low pressures, contrary to popular belief, while CUB-5 and 3DL-MOF-1 are Bz selective. DFT-D3 calculations and breakthrough simulations explore this behavior and highlight CUB-5 and MOF-5 as strong candidates for future separation materials.

Metal-Organic Frameworks for Highly Selective Separation of Xylene Isomers and Single-Crystal X-ray Study of Aromatic Guest-Host Inclusion Compounds

Separation of hydrocarbon molecules, such as benzene/cyclohexane and o-xylene/m-xylene/p-xylene, is relevant due to their widespread application as chemical feedstock but challenging because of their similar boiling points and close molecular sizes. Physisorption separation could offer an energy-efficient solution to this problem, but the design and synthesis of sorbents that exhibit high selectivity for one of the hydrocarbons remain a largely unmet challenge. Herein, we report a new heterometallic MOF with a unique tortuous shape of channels decorated with aromatic sorption sites [Li₂Zn₂(bpy)(ndc)₃] (NIIC-30(Ph), bpy = 4,4'-bipyridine, ndc^2- = naphthalene-1,4-dicarboxylate) and study of its benzene/cyclohexane and xylene vapor and liquid separation. For an equimolar benzene/cyclohexane mixture, it is possible to achieve a 10-fold excess of benzene in the adsorbed phase. In the case of xylenes, microporous framework NIIC-30(Ph) demonstrates outstanding selective sorption properties and becomes a new benchmark for m-/o-xylene separation. In addition, NIIC-30(Ph) is stable enough to carry out at least three separation cycles of benzene/cyclohexane mixtures or ternary o-xylene/m-xylene/p-xylene mixtures both in the liquid and in the vapor phase. Insights into the performance of NIIC-30(Ph) are gained from X-ray structural studies of each aromatic guest inclusion compound.

Flexible amorphous metal-organic frameworks with π Lewis acidic pore surface for selective adsorptive separations

Selective separation of light hydrocarbons (LHs) and adsorption of volatile organic compounds (VOCs) remain expensive and complex scientific challenges in the petrochemical industry. Shape-selective adsorbent materials can cost-effectively face these demands. Two new porous, dynamic and amorphous metal-organic frameworks (MOFs), NEU-3 [= Zn(PMDA)(Py)2] and NEU-4 [= Fe(PMDA)(Py)2] are disclosed. These MOFs along with NEU-1c [= Zn(BPDI)(Py)2] and NEU-2 [= Fe(BPDI)(Py)2] display an electron-deficient pore surface due to predesigned π-electron-deficient ligands. They are unique smart guest-responsive materials owing to their π Lewis acidic pore surface and presumably their framework flexibility. A variety of effective adsorptions and adsorptive separations is achieved by using beds of NEU-1c, NEU-2, NEU-3 and NEU-4. Promising for further investigations into the petrochemical industry, NEU-4 shows ultrahigh benzene adsorption, recognition capability, selectivity for benzene over its analogues, and high stability and regenerability.

Selective adsorptive separation of cyclohexane over benzene using thienothiophene cages

The selective separation of benzene (Bz) and cyclohexane (Cy) is one of the most challenging chemical separations in the petrochemical and oil industries. In this work, we report an environmentally friendly and energy saving approach to separate Cy over Bz using thienothiophene cages (ThT-cages) with adaptive porosity. Interestingly, cyclohexane was readily captured selectively from an equimolar benzene/cyclohexane mixture with a purity of 94%. This high selectivity arises from the C–H⋯S, C–H⋯π and C–H⋯N interactions between Cy and the thienothiophene ligand. Reversible transformation between the nonporous guest-free structure and the host–guest assembly, endows this system with excellent recyclability with minimal energy requirements.

Selective adsorptive separation of cyclohexane was realized from an equimolar benzene and cyclohexane mixture via crystalline thienothiophene cages with a selectivity of 94%.

Adsorptive Separation of Benzene, Cyclohexene, and Cyclohexane by Amorphous Nonporous Amide Naphthotube Solids

Benzene hydrogenation is an important industrial process. The reaction is incomplete, resulting in a mixture of benzene, cyclohexane, and/or cyclohexene that have to be separated before any further reactions. The currently used extractive and azeotropic distillations are operationally complex and energy intensive. Adsorptive separation provides an alternative energy-efficient method. However, the separation of the ternary mixture by adsorptive separation has not yet been reported. In the present research, we report two macrocyclic hosts with hydrogen-bonding sites in their cavities that are able to separate the ternary mixture of benzene, cyclohexene, and cyclohexane. N-H⋅⋅⋅π interactions were found to play a key role in the selective separation. In addition, fast adsorption, high loading ratios, and easy recycling are achieved with the present system, which is promising for practical applications.

Metal-organic tube or layered assembly: reversible sheet-to-tube transformation and adaptive recognition

Rational preparation of an adaptive cavity-like enzyme is a great challenge for chemists. Herein, a new self-assembly strategy for the rational preparation of metal-organic tubes with nano-channels has been developed; both 1D metal-organic tube and corresponding 2D layered assemblies can be selectively synthesized driven by different templates; reversible sheet-to-tube transformation can be realized and the key intermediate has been identified. Furthermore, the newly formed nano-channel displays excellent polarity-selectivity for encapsulation of guest molecules, and can be further expanded or contracted through guest-driven adaptive deformation; even induced by very similar guest molecules, the adaptive deformations can also be obviously distinguished. Finally, the key chemicals benzene/hexane with a similar size can also be effectively separated by such nano-channels in the liquid phase. Our work not only provides a new synthetic strategy for the rational synthesis of metal-organic tubes with a reversible sheet-to-tube transformation character, but also gives a potential method for the construction of adaptive host-like enzymes and an in-depth understanding of the nature of adaptive host and guest molecules.

Exceptionally effective benzene/cyclohexane separation using a nitro-decorated metal-organic framework

Using a series of isoreticular MOFs with void connections of varying diameters, the selective separation of benzene and cyclohexane in both liquid and vapor phases is shown. The driving force of the highly efficient adsorption process is the formation of weak interactions between the adsorbed benzene molecules and the host framework.

Tuning the Molecular and Cationic Affinity in a Series of Multifunctional Metal-Organic Frameworks Based on Dodecanuclear Zn(II) Carboxylate Wheels

A series of new zinc(II)-thiophene-2,5-dicarboxylate (tdc) MOFs based on novel dodecanuclear wheel-shaped building blocks has been synthesized in almost quantative yields. Single-crystal X-ray diffraction analyses reveal 3D porous frameworks with a complex composition [Zn₁₂(tdc)₆(glycolate)₆(dabco)₃] where glycolate is a deprotonated polyatomic alcohol (ethylene glycol, EgO₂, 1; 1,2-propanediol, PrO₂, 2; 1,2-butanediol, BuO₂, 3; 1,2-pentanediol, PeO₂, 4; glycerol, GlO₂, 5) and dabco is 1,4-diazo[2.2.2.]bicyclooctane. All compounds 1-5 are isostructural except for pendant groups of the diols decorating the surface of channels. The adsorption of small gases (N₂, CO₂, CH₄, C₂H₂, C₂H₄, C₂H₆) and larger hydrocarbons (benzene, cyclohexane) both in liquid and vapor phases was thoroughly investigated for all compounds. The zero-coverage adsorption enthalpies, Henry constants, and selectivity factors by various models are calculated and discussed. The versatile adsorption functionality of the title series results from the variable nature of the diol and could be tailored for a specific adsorbate system. For example, 1 shows excellent selectivity of benzene over cyclohexane (20:1 for vapors, 92:1 for liquid phase), while 4 demonstrates unprecedented adsorption preference of cyclohexane over benzene (selectivity up to 5:1). The compound 5 demonstrates great adsorption selectivity for CO₂/N₂ (up to 75.1), CO₂/CH₄ (up to 7.7), C₂H₂/CH₄ (up to 14.2), and C₂H₄/CH₄ (up to 9.4). Also, due to polar nature of the pores, 5 features size-selective sorption of alkaline metal cations in order Li⁺ > Na⁺ > K⁺ > Cs⁺ as well as a notable luminescent response for cesium(I) ions and urea.

Concomitant Use of Tetrathiafulvalene and 7,7,8,8-Tetracyanoquinodimethane within the Skeletons of Metal-Organic Frameworks: Structures, Magnetism, and Electrochemistry

In search of multifunctional metal-organic frameworks (MOFs), redox-active donors and acceptors, namely, tetrathiafulvalene (TTF) and 7,7,8,8-tetracyanoquinodimethane (TCNQ), were concomitantly used as skeletal components with diamagnetic metal nodes (Cd and Zn) to construct unique framework materials. Six isostructural frameworks were synthesized by diffusion of metal salts, TTF(py)₄, and either paramagnetic Li(TCNQ) or diamagnetic H₂TCNQ. They were characterized by single-crystal X-ray diffraction and FT-IR and UV-vis-NIR spectroscopy, and their physical properties were studied, including two postsynthetic modifications involving crystal-to-crystal transformations following a solid-solution reaction with I₂. The highly colored crystals of two isostructural Zn and Cd frameworks contain undulating Cd-TTF(py)₄ layers entwined with TCNQ in a chicken-wire net as part of the skeleton of the MOF as well as TCNQ intercalated within the channels, while nitrate anions are occluded within the cavities formed by the pyridine moieties. Reaction with I₂ replaces each intercalated TCNQ^•- within the channels with I₃^-. The optical properties and the electron paramagnetic resonance (EPR) spectra indicate the presence of only radical TCNQ^•- in the parent compounds, while the magnetic susceptibilities enabled an estimation of the amount of TCNQ^•- ( S = 1/2) leading to almost paramagnetic behavior. Solid-state electrochemistry provides evidence of several one-electron redox states corresponding to the electroactive cores.

Isolated π-Interaction Sites in Mesoporous MOF Backbone for Repetitive and Reversible Dynamics in Water

We report the introduction of π-interaction sites into a series of chemically robust metal-organic frameworks (MOFs), MOF-526, -527, and -528, with progressively increased pore size, 1.9-3.7 nm, and the inclusion and release of large organic molecules in water. The mesopores in these MOFs lead to fast adsorption kinetics, whereas the π-interaction between isolated porphyrin units in the MOF backbone and polycyclic structure of the organic guests provides excellent reversibility. Specifically, seven large organic dyes were quantitatively captured by the porphyrin units of these MOFs in a 2:1 molar ratio, exhibiting unprecedented kinetics for MOFs [e.g., 4.54 × 10⁵ L/mol for rhodamine B] at an extremely low concentration (10 ppm) in water. Rotational-echo double-resonance NMR experiments revealed that the distance between the guest molecules and porphyrin units in MOFs was in the range from 3.24 to 3.37 Å, confirming the specific π-interaction. Repetitive and reversible dynamics was achieved in these MOFs for 10 complete inclusion-release cycles without any decay in performance, which is ideally suited for the removal and recycle of large polycyclic organic molecules from water. The performance of MOF-526 rivals that of state-of-the-art carbon and polymers.

Rational synthesis and dimensionality tuning of MOFs from preorganized heterometallic molecular complexes

Rational synthesis of heterometallic MOFs was carried out by the judicious choice of pivalate complexes and a tricarboxylate linker defining their dimensionality.

Detection of adsorbates on emissive MOF surfaces with X-ray photoelectron spectroscopy

The luminescence of azobenzene chromophore struts in a metal organic framework is quenched by nitroaromatic guests. X-ray photoelectron spectroscopic methods verify the emission changes are due to the surface adsorption of the guest molecules rather than encapsulation.

Guest-Responsive Metal-Organic Frameworks as Scaffolds for Separation and Sensing Applications

Metal-organic frameworks (MOFs) have evolved to be next-generation utility materials because of their serviceability in a wide variety of applications. Built from organic ligands with multiple binding sites in conjunction with metal ions/clusters, these materials have found profound advantages over their other congeners in the domain of porous materials. The plethora of applications that these materials encompass has motivated material chemists to develop such novel materials, and the catalogue of MOFs is thus ever-escalating. One key feature that MOFs possess is their responsiveness toward incoming guest molecules, resulting in changes in their physical and chemical properties. Such uniqueness generally arises owing to the influenceable ligands and/or metal units that govern the formation of these ordered architectures. The suitable host-guest interactions play an important role in determining the specific responses of these materials and thus find important applications in sensing, catalysis, separation, conduction, etc. In this Account, we focus on the two most relevant applications based on the host-guest interactions that are carried out in our lab, viz., separation and sensing of small molecules. Separation of liquid-phase aromatic hydrocarbons by less energy-intensive adsorption processes has gained attention recently. Because of their tailored structures and functionalized pore surfaces, MOFs have become vital candidates in molecular separation. Prefunctionalization of MOFs by astute choice of ligands and/or metal centers results in targeted separation processes in which the molecular sieving effect plays a crucial role. In this view, separation of C₆ and C₈ liquid aromatic hydrocarbons, which are essential feedstock in various chemical industries, is one area of research that requires significant attention because of the gruesome separation techniques adopted in such industries. Also, from the environmental perspective, separation of oil/water mixtures demands significant attention because of the hazards of marine oil spillage. We have achieved successful separation of such by careful impregnation of hydrophobic moieties inside the nanochannels of MOFs, resulting in unprecedented efficiency in oil/water separation. Also, recognition of small molecules using optical methods (fluorescence, UV, etc.) has been extended to achieve sensing of various neutral species and anions that are important from environmental point of view. Incorporation of secondary functional groups has been utilized to sense nitroaromatic compounds (NACs) and other small molecules such as H₂S, NO, and aromatic phenols. We have also utilized the postfunctionalization strategy via ion exchange to fabricate MOFs for sensing of environmentally toxic and perilous anionic species such as CN^- and oxoanions. Our current endeavors to explore the applicability of MOFs in these two significant areas have widened the scope of research, and attempts to fabricate MOFs for real-time applications are underway.

Doping of metal-organic frameworks towards resistive sensing

Coordination polymerization leads to various metal-organic frameworks (MOFs) with unique physical properties and chemical functionalities. One of the challenges towards their applications as porous materials is to make MOFs optimally conductive to be used as electronic components. Here, it is demonstrated that Co-MOF-74, a honeycomb nano-framework with one-dimensionally arranged cobalt atoms, advances its physical properties by accommodating tetracyanochinodimethan (TCNQ), an acceptor molecule. Strong intermolecular charge transfer reduces the optical band gap down to 1.5 eV of divalent TCNQ and enhances the electrical conduction, which allows the MOF to be utilized for resistive gas- and photo-sensing. The results provide insight into the electronic interactions in doped MOFs and pave the way for their electronic applications.

Micropore Formation of [Zn2(Oxac) (Taz)2]·(H2O)2.5 via CO2 Adsorption

As-synthesized [Zn₂(Oxac) (Taz)₂]·(H₂O)_2.5, referred to as ZOTW_2.5, was prepared from aqueous methanol solutions of Zn₅(CO₃)₂(OH)₆ and two kinds of ligands of 1,2,4-triazole (Taz) and oxalic acid (Oxac) at 453 K for 12 h. The crystal structure was determined by the Rietveld method. As-synthesized ZOTW_2.5 was pretreated at 383 K and 1 mPa for t_pt h, ZOTW_x(t_pth). ZOTW_x(≥3h) showed a type I adsorption isotherm for N₂ at 77 K having a saturation amount (V_s) of 180 mg/g, but that pretreated shortly showed only 1/10 in V_s. CO₂ was adsorbed at 303 K in sigmoid on nonporous ZOTW_x(≤2h) and in Langmuir-type on ZOTW_x(≥3h) to reach the adsorption amount of 120 mg/g at 700 Torr. N₂ adsorption on ZOTW_x(≤2h)deCO₂, degassed after CO₂ adsorption on ZOTW_x(≤2h), was promoted 5-fold from 180 mg/g on ZOTW_x(t_pth) and ZOTW_x(≥3h)deCO₂ up to ca. 1000 mg/g. The interaction of CO₂ and H₂O molecules in micropores may lead to a new route for micropore formation.

Metal–organic frameworks with Lewis acidity: synthesis, characterization, and catalytic applications

In this highlight, we review the recent development in the design and synthesis of metal–organic frameworks with Lewis acidity, the characterization techniques of Lewis acid sites, and their applications in heterogeneous catalysis.

Harnessing Lewis acidic open metal sites of metal–organic frameworks: the foremost route to achieve highly selective benzene sorption over cyclohexane

Based on the tactical utilization of the Lewis acidic open metal sites (OMS) functionality; for an OMS-rich, microporous, water-stable series of metal–organic frameworks (M-MOF-74), vapor sorption based efficient selectivity for benzene over cyclohexane has been realized, crucial from industrial separation frontier.

Structural, Electronic, and Magnetic Characterization of a Dinuclear Zinc Complex Containing TCNQ(-) and a μ-[TCNQ-TCNQ](2-) Ligand

A dinuclear zinc complex containing both a σ-dimerized 7,7,8,8-tetracyanoquinodimethane (TCNQ) ligand ([TCNQ-TCNQ](2-)) and TCNQ(-) was synthesized for the first time. This is the first instance of a single molecular complex with a bridging [TCNQ-TCNQ](2-) ligand. Each zinc center is coordinated with two 2,2'-bipyrimidines and one TCNQ(-), and the remaining coordination site is occupied by a [TCNQ-TCNQ](2-) ligand, which bridges the two zinc centers. The complex facilitates π-stacking of TCNQ(-) ligands when crystallized, which gives rise to a near-IR charge-transfer transition and strong antiferromagnetic coupling.