Series of Cadmium-Organic Frameworks Based on Mixed Flexible and Rigid Ligands: Single-Crystal-to-Single-Crystal Transformations, Sorption, and Luminescence Properties

Here, we present a series of Cd(II) coordination polymers containing two types of ligands: sterically rigid terephthalate derivatives (bdc-NO22- and bdc-Br2-) and flexible bis(2-methylimidazolyl)propane (bmip). The combination of two types of ligands is used to obtain and characterize compounds by single crystal and powder X-ray diffraction, FT-IR, elemental analysis, and TGA. Guest exchange results in structural transformations. 2-fold interpenetrated 1·DMF and 2·DMF rapidly undergo to 4-fold interpenetrated 1·Et2O, 1·EtOH, and 1·H2O, or 2·Et2O, respectively. Also, changes in the coordinating numbers and length of the N,N'-donor bmip ligand were observed according to single crystal X-ray analysis. Activated guest-free compounds [Cd(bdc-NO2)(bmip)] (1) and [Cd(bdc-Br)(bmip)] (2) are shown to be porous with a BET surface area of 103 and 283 m2·g-1, respectively. Moreover, both compounds demonstrate gate-opening behavior of ethylene adsorption isotherms at low pressures (<1 bar) and highly selective adsorption of benzene over cyclohexane or lower alcohols. Also, both compounds demonstrate a strong dependence of the maximum of the photoluminescence emission on an excitation wavelength. As a result, the photoluminescence color changes from white to red and from blue to red through green and yellow for compounds 1 and 2, respectively, with excitation wavelength changing from 360 to 540 nm.

Recent advances in fluorescence-based chemosensing of organoarsenic feed additives using luminescence MOFs, COFs, HOFs, and QDs

Organoarsenics are low-toxicity compounds that are used widely as feed additives to promote livestock growth, enhance meat pigmentation, and fight against intestinal parasites. The organoarsenic compounds are commonly found in poultry waste and the degradation of organoarsenic produces the toxic carcinogen inorganic arsenic such as As(V) and As(III), which results in severe arsenic pollution of soil and groundwater. As a consequence, there exists a high necessity to develop suitable sensing methods for the trace detection and quantification of organoarsenic feed additives in wastewater. Among various detection methods, in particular, fluorescence-based sensing has become a popular and efficient method used extensively for sensing water contaminants and environmental contaminants. In the recent past, a wide variety of fluorescence chemosensors have been designed and employed for the efficient sensing and quantification of the concentration of organoarsenic feed additives in different environmental samples. This review article systematically highlights various fluorescence chemosensors reported to date for fluorescence-based sensing of organoarsenic feed additives. The fluorescence sensors discussed in this review are classified and grouped according to their structures and functions, and in each section, we provide a detailed report on the structure, photophysics, and fluorescence sensing properties of different chemosensors. Lastly, the future perspectives on the design and development of practically useful sensor systems for selective and discriminative sensing of organoarsenic compounds have been stated.

MOFs in the time domain

Many of the proposed applications of metal-organic framework (MOF) materials may fail to materialize if the community does not fully address the difficult fundamental work needed to map out the 'time gap' in the literature - that is, the lack of investigation into the time-dependent behaviours of MOFs as opposed to equilibrium or steady-state properties. Although there are a range of excellent investigations into MOF dynamics and time-dependent phenomena, these works represent only a tiny fraction of the vast number of MOF studies. This Review provides an overview of current research into the temporal evolution of MOF structures and properties by analysing the time-resolved experimental techniques that can be used to monitor such behaviours. We focus on innovative techniques, while also discussing older methods often used in other chemical systems. Four areas are examined: MOF formation, guest motion, electron motion and framework motion. In each area, we highlight the disparity between the relatively small amount of (published) research on key time-dependent phenomena and the enormous scope for acquiring the wider and deeper understanding that is essential for the future of the field.

The electrochemical reduction of a flexible Mn(II) salen-based metal-organic framework

A new metal-organic framework (MOF) containing a Mn(II) salen complex (BET surface area = 967 ± 6 m² g^-1) undergoes a reversible crystalline-to-amorphous transformation. Experimental studies and computational calculations show that the MOF is stable to a one-electron reduction at more anodic potentials than the corresponding discrete complex.

Zinc(ii) and cadmium(ii) amorphous metal-organic frameworks (aMOFs): study of activation process and high-pressure adsorption of greenhouse gases

Two novel amorphous metal-organic frameworks (aMOFs) with chemical composition {[Zn2(MTA)]·4H2O·3DMF} n (UPJS-13) and {[Cd2(MTA)]·5H2O·4DMF} n (UPJS-14) built from Zn(ii) and Cd(ii) ions and extended tetrahedral tetraazo-tetracarboxylic acid (H4MTA) as a linker were prepared and characterised. Nitrogen adsorption measurements were performed on as-synthesized (AS), ethanol exchanged (EX) and freeze-dried (FD) materials at different activation temperatures of 60, 80, 100, 120, 150 and 200 °C to obtain the best textural properties. The largest surface areas of 830 m2 g-1 for UPJS-13 (FD) and 1057 m2 g-1 for UPJS-14 (FD) were calculated from the nitrogen adsorption isotherms for freeze-dried materials activated at mild activation temperature (80 °C). Subsequently, the prepared compounds were tested as adsorbents of greenhouse gases, carbon dioxide and methane, measured at high pressures. The maximal adsorption capacities were 30.01 wt% CO2 and 4.84 wt% CH4 for UPJS-13 (FD) and 24.56 wt% CO2 and 6.38 wt% CH4 for UPJS-14 (FD) at 20 bar and 30 °C.

What Lies beneath a Metal-Organic Framework Crystal Structure? New Design Principles from Unexpected Behaviors

The rational design principles established for metal-organic frameworks (MOFs) allow clear structure-property relationships, fueling expansive growth for energy storage and conversion, catalysis, and beyond. However, these design principles are based on the assumption of compositional and structural rigidity, as measured crystallographically. Such idealization of MOF structures overlooks subtle chemical aspects that can lead to departures from structure-based chemical intuition. In this Perspective, we identify unexpected behavior of MOFs through literature examples. Based on this analysis, we conclude that departures from ideality are not uncommon. Whereas linker topology and metal coordination geometry are useful starting points for understanding MOF properties, we anticipate that deviations from the idealized crystal representation will be necessary to explain important and unexpected behaviors. Although this realization reinforces the notion that MOFs are highly complex materials, it should also stimulate a broader reexamination of the literature to identify corollaries to existing design rules and reveal new structure-property relationships.

Crystal transformation in Mn(II) metal-organic frameworks based on a one-dimensional chain precursor

The solvothermal reaction of Mn(ii) salts and 5-((4'-(tetrazol-5''-yl)benzyl)oxy)isophthalic acid (H3L) affords an Mn(ii) based coordination polymer Mn(H2L)2(H2O)2 (1), which possesses a one-dimensional (1D) chain structure. Using 1 as the precursor, three Mn(ii) metal-organic frameworks, Mn3L2(2,2'-bpy)2·5H2O (2), Mn3L2(H2O)4 (3), and Mn4L2(HL)(H2O)5·0.5H2O (4), with three-dimensional (3D) networks can be obtained by different strategies of crystal transformation. Upon introduction of 2,2'-bipyridine (2,2'-bpy) as the ligand and 2,2'-biquinoline-4,4'-dicarboxylic acid as the structural-directing agent, 1 undergoes irreversible crystal transformation into 2 and 3, respectively, and 1 can be transformed into 4 by increasing the reaction temperature. Interestingly, the irreversible structural transformation of 3 into 2 can be carried out by adding a 2,2'-bpy ligand. Notably, after the removal of coordinated water molecules, 1 and 3 exhibit good catalytic performance for the cyanosilylation reaction even at 0 °C.

Dynamic porous coordination polymers built-up from flexible 4,4'-dithiodibenzoate and rigid N-based ligands

Herein we report the design, synthesis, structural characterisation and functional testing of a series of Cu(ii) coordination polymers containing flexible 4,4'-dithiodibenzoate ligand (4,4'-DTBA), with or without auxiliary N-donor ligands. Reaction of Cu(ii) with 4,4'-DTBA yielded a 1D coordination polymer (1) based on Cu(ii) paddlewheel units connected by 4,4'-DTBA, to form cyclic loop chains with intramolecular voids that exhibit reversible structural transformations upon subsequent solvent exchange in methanol to afford a new, crystalline, permanently-porous structure (1'). However, when the same reaction was run with pyridine, it formed a porous 2D coordination polymer (2). We have attributed the difference in dimensionality seen in the two products to the coordination of pyridine on the axial site of the Cu(ii) paddle-wheel, which forces flexible 4,4'-DTBA to adopt a different conformation. Reactions in the presence of 4,4'-bipyridine (4,4'-bpy) afforded two new, flexible, 2D coordination polymers (3 & 4). Lower concentrations of 4,4'-bpy afforded a structure (3) built from 1D chains analogous to those in 1 and connected through 4,4'-bpy linkers coordinated to the axial positions. Interestingly, 3 showed reversible structural transformations triggered by either solvent exchange or thermal treatment, each of which yielded a new crystalline and permanently porous phase (3'). Finally, use of higher concentrations of 4,4'-bpy led to a coordination polymer (4) based on a distorted CuO₃N₂ trigonal bipyramid, rather than on the Cu(ii) paddlewheel. The connection of these motifs by 4,4'-DTBA resulted in a zig-zag 1D chain connected through 4,4'-bpy ligands to form a porous 2D network. Interestingly, 4 also underwent reversible thermal transformation to yield a microporous coordination polymer (4').

Sequential Solid-State Transformations Involving Consecutive Rearrangements of Secondary Building Units in a Metal-Organic Framework (MOF)

Solid-state transformations in metal-organic frameworks (MOFs) are important and have led to the creation of new MOF structures. Solid-state transformations from interpenetrated to non-interpenetrated networks involving rearrangement of secondary building units (SBUs) in a single-crystal-to-single-crystal (SCSC) fashion have not been explored to date. Herein, we report the sequential, thermally stimulated solid-state transformations in a barium-organic framework (UPC-600). The two-fold interpenetrated framework of UPC-600 is converted at 373 K into UPC-601, a non-interpenetrated framework. This proceeds in a SCSC fashion and involves the rearrangement of two proximate rod-shaped SBUs in different nets to generate a new rod-shaped SBU. At 473 K, a continuous solid-state transformation involving a second rearrangement occurred, UPC-601 converted into UPC-602 by the rearrangement of the 1D rod-shaped SBU to a 2D layer SBU. This is the first example of such a thermally driven stepwise transformation involving simultaneous cleavage and regeneration of multiple bonds.

Interlinker Hydrogen Bonds Govern CO2 Adsorption in a Series of Flexible 2D Diacylhydrazone/Isophthalate-Based MOFs: Influence of Metal Center, Linker Substituent, and Activation Temperature

Four new layered flexible metal–organic frameworks (MOFs) containing a diacylhydrazone moiety, namely, guest-filled [Zn₂(iso)₂(tdih)₂]_n (1), [Zn₂(NH₂iso)₂(tdih)₂]_n (2), [Cd₂(iso)₂(tdih)₂]_n (3) and [Cd₂(NH₂iso)₂(tdih)₂]_n (4) were synthesized using terephthalaldehyde di-isonicotinoylhydrazone (tdih) as a linear ditopic linker as well as isophtalate (iso) or 5-aminoisophthalate (NH₂iso) as angular colinkers. The MOFs with hexacoordinated cadmium centers feature two-dimensional pore systems as compared to the MOFs with pentacoordinated zinc centers showing either zero-dimensional or mixed zero-/one-dimensional voids, as evidenced by single-crystal X-ray diffraction. In contrast to the frameworks based on isophtalates which do not show any significant gas uptakes, introduction of amino-substituted linker enables CO₂ adsorption. Gently activated aminoisophthalate-based frameworks, that is, guest-exchanged in methanol and heated to 100 °C, show reversible gated CO₂ adsorptions at 195 K, whereas the increase of activation temperature to 150 °C or more leads to one-step isotherms and lower adsorption capacities. X-ray diffraction and IR spectroscopy reveal significant structural differences in interlayer hydrogen bonding upon activation of materials at higher temperatures. The work emphasizes the role of hydrogen bonds in crystal engineering of layered materials and the importance of activation conditions in such systems.

Interplay between a metal center and functionalization of isophthalate linker leads to remarkable diversity of structures and properties in the series of layered flexible metal−organic frameworks. Intriguing adsorption properties include stepwise gated CO₂ adsorptions and strong dependence on activation conditions. The role of hydrogen bonds in crystal engineering of layered materials is underscored by activation−structure−adsorption correlations.

Reversible Interlayer Sliding and Conductivity Changes in Adaptive Tetrathiafulvalene-Based Covalent Organic Frameworks

Ordered interlayer stacking is intrinsic in two-dimensional covalent organic frameworks (2D COFs) and has strong implications on COF's optoelectronic properties. Reversible interlayer sliding, corresponding to shearing of 2D layers along their basal plane, is an appealing dynamic control of both structures and properties, yet it remains unexplored in the 2D COF field. Herein, we demonstrate that the reversible interlayer sliding can be realized in an imine-linked tetrathiafulvalene (TTF)-based COF TTF-DMTA. The solvent treatment induces crystalline phase changes between the proposed staircase-like sql net structure and a slightly slipped eclipsed sql net structure. The solvation-induced crystallinity changes correlate well with reversible spectroscopic and electrical conductivity changes as demonstrated in oriented COF thin films. In contrast, no reversible switching is observed in a related TTF-TA COF, which differs from TTF-DMTA in terms of the absence of methoxy groups on the phenylene linkers. This work represents the first 2D COF example of which eclipsed and staircase-like aggregated states are interchangeably accessed via interlayer sliding, an uncharted structural feature that may enable applications such as chemiresistive sensors.

Temperature-induced structural transformations accompanied by changes in magnetic properties of two copper coordination polymers

Two ferromagnetic copper compounds have been synthesized under different temperature, which represented the rare example of structural transformations resulting from the coordination modes of organic ligands supported by magnetic results.

Metal–organic framework based antibiotic release and antimicrobial response: an overview

MOF and MOF based heterostructures for antibacterial applications.

An N,N'-diethylformamide solvent-induced conversion cascade within a metal-organic framework single crystal

Crystals of a two-dimensional (2D) metal-organic framework (MOF) [Cd3(BTB)2(DEF)4]·2(DEF)0.5 (1; BTB = benzene-1,3,5-tribenzolate; DEF = N,N'-diethylformamide) immersed in a solution of trans-1,2-bis(4-pyridyl)ethylene (BPEE) yields an interpenetrated three-dimensional (3D) MOF of [Cd3(BTB)2(BPEE)(H2O)2]·(BPEE)·xSol (2). Crystals of MOF 2, in turn, undergo a cascade conversion when immersed in DEF, yielding [Cd3(BTB)2(BPEE)1.8(DEF)0.9(H2O)0.8]·xSol (3a) over 100 seconds and [Cd3(BTB)2(BPEE)2(DEF)2]·xSol (4) after one hour, before finally shuttling back to MOF 1 after six hours.

Thermal Reactivity in Metal Organic Materials (MOMs): From Single-Crystal-to-Single-Crystal Reactions and Beyond

Thermal treatment is important in the solid-state chemistry of metal organic materials (MOMs) because it can create unexpected new structures with unique properties and applications that otherwise in the solution state are very difficult or impossible to achieve. Additionally, high-temperature solid-state reactivity provide insights to better understand chemical processes taking place in the solid-state. This review article describes relevant thermally induced solid-state reactions in metal organic materials, which include metal organic frameworks (MOFs)/coordination polymers (CPs), and second coordination sphere adducts (SSCs). High temperature solid-state reactivity can occur in a single-crystal-to-single crystal manner (SCSC) usually for cases where there is small atomic motion, allowing full structural characterization by single crystal X-ray diffraction (SC-XRD) analysis. However, for the cases in which the structural transformations are severe, often the crystallinity of the metal-organic material is damaged, and this happens in a crystal-to-polycrystalline manner. For such cases, in the absence of suitable single crystals, structural characterization has to be carried out using ab initio powder X-ray diffraction analysis or pair distribution function (PDF) analysis when the product is amorphous. In this article, relevant thermally induced SCSC reactions and crystal-to-polycrystalline reactions in MOMs that involve significant structural transformations as a result of the molecular/atomic motion are described. Thermal reactivity focusing on cleavage and formation of coordination and covalent bonds, crystalline-to-amorphous-to-crystalline transformations, host-guest behavior and dehydrochlorination reactions in MOFs and SSCs will be discussed.

2D Oligosilyl Metal-Organic Frameworks as Multi-state Switchable Materials

We report the synthesis of a set of 2D metal-organic frameworks (MOFs) constructed with organosilicon-based linkers. These oligosilyl MOFs feature linear Si_n Me_2n (C₆ H₄ CO₂ H)₂ ligands (lin-Si_n , n=2, 4) connected by Cu paddlewheels. The stacking arrangement of the 2D sheets is dictated by van der Waals interactions and is tunable by solvent exchange, leading to reversible structural transformations between many crystalline and amorphous phases.

Polarity-Induced Excited-State Intramolecular Proton Transfer (ESIPT) in a Pair of Supramolecular Isomeric Multifunctional Dynamic Metal-Organic Frameworks

A pair of supramolecular isomers of Cd^II -based MOF have been synthesized by utilizing a flexible N,N'-donor linker and a dicarboxylate with ESIPT (excited-state intramolecular proton transfer) fluorophore by varying the reaction media. One of the MOFs has a 3D four-fold interpenetrating framework with guest solvent in the structure that undergoes a solvent-dependent crystalline-to-crystalline structural transformation, which has been extensively studied by powder XRD and IR spectroscopy. The other MOF is structurally rigid in nature and has a two-fold interpenetrating structure without any guest molecules. Both the compounds show moderate CO₂ adsorption and one of them, the MOF with the four-fold interpenetrating structure, also shows moderately high H₂ adsorption. Furthermore, both the compounds show interesting luminescence behavior. In the solid state, the two compounds show single-peak spectra, whereas upon suspension of these compounds in polar solvents, the maxima split into two peaks with a large Stokes shift. On the other hand, in nonpolar solvents, only one emission maximum is observed. This solvatochromic dual-emission phenomenon is due to ESIPT, which has been extensively studied.

Complex Phase Behaviour and Structural Transformations of Metal-Organic Frameworks with Mixed Rigid and Flexible Bridging Ligands

Two new heteroleptic metal-organic framework materials show strong adsorption of H₂ and ethanol. [Co₂ (L1)(bdc)₂ ], where L1=N¹ ,N⁴ -bis(4-pyridinylmethyl)-2,5-dimethylbenzene-1,4-diamine and bdc is benzene-1,4-dicarboxylate, has a twofold interpenetrating pillared layer structure with pcu topology. It has a stepped, hysteretic EtOH adsorption that can be related to complicated phase and structural transformation behaviour that occurs on de-solvation and re-solvation, including major conformational changes to the geometry of the flexible L1 ligand. [Co₂ (L1)(bpdc)₂ ], where bpdc=biphenyl-4,4'-dicarboxylate, has a unique six-connected self-catenating framework structure. Solvation changes occur without significant structural change and a partially-hydrolysed material binds its own decomposition products as guests.

Solvent switching smart metal–organic framework as a catalyst of reduction and condensation

The organization of a Zn-based metal–organic framework (MOF) as the first solvent switching catalyst has been achieved via in situ ligand incorporation.

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.

Multivariant synthesis, crystal structures and properties of four nickel coordination polymers based on flexible ligands

Four new Ni(ii) metal–organic frameworks with diverse structures have been synthesized under different conditions. The structural transformation from compound 4 to 3 has also been investigated.