Multidirectional Solvent-Induced Structural Transformation in Designing a Series of Polycatenated Cobalt(II) Coordination Polymers: Impact on Carbon Dioxide and Hydrogen Uptake

Coordination polymers with external stimuli-responsive structural transformation acquired paramount importance in the advanced material research field due to their eye-catching application to deal with the existing challenging issue, and Co(II) metal complex with d⁷ electronic configuration is a renowned candidate for kinetic accountability and has the potentiality of structural transformation. Bearing these factors in mind, here, a Co(II) congener of a previously reported high hydrogen-adsorbing Cu(II)-based coordination polymer (CP), {[Cu(4-bpe)(2-ntp)]}_n [where 2-ntp^2- = 2-nitroterephthalate and 4-bpe = 1,2-bis-(4-pyridyl)ethane], has been synthesized to study the metal change impact on hydrogen adsorption and solvent-induced structural transformation with their impact on hydrogen uptake. This modified framework has a 2D + 2D → 3D inclined polycatenated framework as comparable to our previously published Cu(II) framework. Here, on the variation of different solvents, the labile Co(II)-containing framework exhibits a structural change through single-crystal to single-crystal (SC-SC) structural transformation and results in three new framework structures. All four frameworks are structurally characterized by elemental analysis, IR, PXRD, TGA, and single-crystal X-ray diffraction. The desolvated parent framework with exposed metal centers exhibits excellent results of H₂ adsorption of 1.3 wt % (145 cc/g) at 77 K and pressure of 1 bar with structural sustainability and CO₂ uptake of 130 cc/g at 195 K and 1 bar. For the other three solvent-mediated structural derivatives, H₂ and CO₂ adsorption have been studied, and the results are correlated with their structure.

The Complexity of Comparative Adsorption of C6 Hydrocarbons (Benzene, Cyclohexane, n-Hexane) at Metal-Organic Frameworks

The relatively stable MOFs Alfum, MIL-160, DUT-4, DUT-5, MIL-53-TDC, MIL-53, UiO-66, UiO-66-NH₂, UiO-66(F)₄, UiO-67, DUT-67, NH₂-MIL-125, MIL-125, MIL-101(Cr), ZIF-8, ZIF-11 and ZIF-7 were studied for their C₆ sorption properties. An understanding of the uptake of the larger C₆ molecules cannot simply be achieved with surface area and pore volume (from N₂ sorption) but involves the complex micropore structure of the MOF. The maximum adsorption capacity at p p₀^-1 = 0.9 was shown by DUT-4 for benzene, MIL-101(Cr) for cyclohexane and DUT-5 for n-hexane. In the low-pressure range from p p₀^-1 = 0.1 down to 0.05 the highest benzene uptake is given by DUT-5, DUT-67/UiO-67 and MIL-101(Cr), for cyclohexane and n-hexane by DUT-5, UiO-67 and MIL-101(Cr). The highest uptake capacity at p p₀^-1 = 0.02 was seen with MIL-53 for benzene, MIL-125 for cyclohexane and DUT-5 for n-hexane. DUT-5 and MIL-101(Cr) are the MOFs with the widest pore window openings/cross sections but the low-pressure uptake seems to be controlled by a complex combination of ligand and pore-size effect. IAST selectivities between the three binary mixtures show a finely tuned and difficult to predict interplay of pore window size with (critical) adsorptive size and possibly a role of electrostatics through functional groups such as NH₂.

Building Unit Extractor for Metal-Organic Frameworks

Metal-organic frameworks (MOFs) have relevance in extensive applications such as gas adsorption, separation, and energy storage. The tunability demonstrated by MOFs has encouraged research on MOF database generation via distinct methodologies. One of the crucial stages of these procedures is pre-processing, which often includes extraction of the building units (BUs). The process of BU extraction is intricate, and it is further amplified with the presence of solvent molecules/ions in the structure. This work presents MOF BU developer (mBUD), a platform to deconstruct the BUs, such as metal nodes, organic linkers, and functional groups of the MOF structure. The deconstruction algorithm has been assessed on the MOF structures of the CoRE MOF 2019 database. A total of 2,580 BUs have been extracted and provided as a database. This platform has been utilized to create a ready-to-use database of unique BUs deconstructed from the CoRE MOF database. We have also provided the web version of mBUD that can be easily used to extract BUs. These BUs can be employed to develop hypothetical MOF structures. It is envisaged that the BU database built with the deconstruction platform will aid the design of novel application-specific MOFs.

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.

Recent progresses in luminescent metal-organic frameworks (LMOFs) as sensors for the detection of anions and cations in aqueous solution

The discharge of excessive metal ions and anions into water bodies leads to the serious pollution of water and environment, which in turn has a certain impact on industry, agriculture, and human life. Because of the unique advantages of luminescent metal-organic frameworks (LMOFs), they have been successfully explored as various fluorescent probes to quickly and effectively detect these pollutants. This perspective not only introduces the design strategy and classification of LMOFs, especially the construction methods of water-stable LMOFs, but also reports the latest progresses in some LMOFs between 2016 and 2020 as well as expounds the mechanisms of LMOFs for detecting anions and cations. Moreover, the luminescence properties of LMOFs are related to the selection of metal ions, the structure of organic ligands, the pore size, and the interaction of guest molecules. Finally, the further development of LMOFs is summarized and prospected in this field.

Crystal structures and the ferroelectric properties of homochiral metal-organic frameworks constructed from a single chiral ligand

MOFs have proven to be promising candidates for designing ferroelectric materials. Herein, two new homochiral MOFs, Co-MOF-1 and Co-MOF-2, have been synthesized using the chiral ligand, HL (HL = phenyl-((pyridin-4-ylmethyl)-amino)-acetic acid), and Co(NO₃)₂·6H₂O. Co-MOF-1 was obtained via a two-step synthetic route involving a hydrogel to Zn-MOF conversion and a dissolution-recrystallization process. Co-MOF-2 was directly synthesized by a coordination reaction between chiral ligand, HL, and Co(NO₃)₂·6H₂O under hydrothermal conditions. We investigate the correlation between the ferroelectric properties of the samples and their crystal structures. The ferroelectric properties of Co-MOF-1 and Co-MOF-2 are drastically different. Indeed, Co-MOF-2 shows an obvious hysteretic behavior, while a clear electric hysteresis loop was not observed for Co-MOF-1. These significant disparities may be attributed to the different molecular dipole moments in Co-MOF-1 and Co-MOF-2. The different octahedral coordination units in the molecular structures of the Co-MOFs may alter the dipole moments of the molecules, resulting in the absence of a hysteresis loop for Co-MOF-1.

From lamellar net to bilayered-lamella and to porous pillared-bilayer: reversible crystal-to-crystal transformation, CO2 adsorption, and fluorescence detection of Fe3+, Al3+, Cr3+, MnO4−, and Cr2O72− in water

Layered coordination polymers of different network topologies have showed reversible structural transformations and fluorescence detection toward Fe³⁺, Al³⁺, Cr³⁺, MnO₄^–, and Cr₂O₇^2– in water.

Vapour-driven crystal-to-crystal transformation showing an interlocking switch of the coordination polymer chains between 1D and 3D

A rare crystal-to-crystal transformation occurs between 1D coordination polymer chains and 3D mechanically interlocked structures via reversible opening and closing of the homogeneous chains without any change in the coordination mode and composition.