Rational Design and Crystal Structure Determination of a 3-D Metal−Organic Jungle-Gym-like Open Framework

One ligand, two roles: novel pillar-layered metal-organic frameworks built with a 3D ligand and asymmetric inorganic nodes

Herein, we show two cases of pillar-layered MOFs which are built solely with one kind of three-dimensional (3D) ditopic ligand. The ligand in both structures functions not only as an intralayer linker in the layer but also as a "pillar" between adjacent layers. Such multi-functionality of the linker is accompanied by uncommon asymmetric 10-c metal hexamer or 7-c tetramer nodes, which have never been reported in previous 3D MOF structures.

Bodipy-Based Metal-Organic Frameworks Transformed in Solid States from 1D Chains to 2D Layer Structures as Efficient Visible Light Heterogeneous Photocatalysts for Forging C-B and C-C Bonds

Boron dipyrromethene (also known as bodipy), as a class of versatile and robust fluorophores and a structural analogue of porphyrins, has received a great deal of interests in the field of light-harvesting and energy-transfer processes. However, the fabrication of bodipy monomers into metal-organic frameworks (MOFs) and the exploitation of their potential still lags behind the porphyrin MOFs. In this work, two bodipy-based MOFs, BMOF 1D with 1D chain structure and BMOF 2D with 2D layer structure, were assembled by using dicarboxyl-functionalized bodipy ligands. BMOF 1D can also be converted to BMOF 2D by inserting additional ligands into BMOF 1D to cross-link the adjacent chains into the rhombic grid layer. During this process, spontaneous exfoliation occurred simultaneously and resulted in the formation of several hundred nanometer thickness BMOF 2D (nBMOF 2D), which can be further exfoliated into one-layer MOF nanosheets (BMON 2D) by using the ultrasonic liquid exfoliation method in a high yield. Featuring the distinct bodipy scaffolds in the porous frameworks, both BMOF 2D and BMON 2D displayed high reactivity and recyclability in the photocatalytic inverse hydroboration and cross-dehydrogenative coupling reactions to afford α-amino organoborons and α-amino amides in moderate to high yields. This work not only highlights the cascade utilization of ligand installation and ultrasonic liquid exfoliation methods to provide the single-layer MOF sheets in high yields but also advances the bodipy-based MOFs as a new type of heterogeneous photocatalysts in the forging of C-B and C-C bonds driven by visible light.

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.

Snapshots of Postsynthetic Modification in a Layered Metal-Organic Framework: Isometric Linker Exchange and Adaptive Linker Installation

Terminal ligand exchange and framework linker exchange have been frequently practiced as powerful tools to functionalize reticular structures such as metal-organic frameworks (MOFs). Herein, we report the postsynthetic modification (PSM) of a 6-connected layered MOF (hxl topology) to achieve a 12-connected fcu framework. In the PSM process, isometric linker exchange in the layers and linker installation between adjacent layers by the substitution of modulating ligands happen simultaneously. Snapshots of PSM at different time points reveal that the hxl domain is adaptively reorganized to create sites for new linker installation, and gradually the fcu domain dominates the crystal. Detailed kinetic analysis suggests that, although adaptive linker installation requires interlayer expansion of stackings in situ, it is kinetically faster than isometric linker exchange in the layers.

Linker Expansion and Its Impact on Switchability in Pillared-Layer MOFs

Linker elongation is an important method to systematically adjust porosity and pore size in isoreticular MOFs. In flexible structures, this approach opens the possibility for the systematic analysis of the building blocks and their contribution to the overall flexible behavior enabling tuning of the framework responsivity toward molecular stimuli. In this work, we report two new compounds isoreticular to the highly flexible pillared layer structure DUT-8(Ni) ([Ni₂(2,6-ndc)₂(dabco)]_n, 2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicylo[2.2.2]octane). Aromatic linker 2,6-ndc was substituted by longer carboxylic linkers, namely, 4,4'-biphenyldicarboxylate (4,4'-bpdc) and 4,4'-stilbenedicarboxylate (4,4'-sdc), while the dabco pillar was retained. The structural response of the new compounds toward the desolvation and adsorption of various fluids was studied using advanced in situ PXRD techniques, demonstrating distinct differences in the flexible behavior of three compounds and disclosing the impact of linker structure on the framework response. Theoretical calculations provide mechanistic insights and an energetic rationale for the pronounced differences in switchability observed. The energetics of linker bending and linker-linker dispersion interactions govern the phase transitions in investigated MOFs.

Construction of a zeolite A-type multivariate metal–organic framework for selective sensing of Fe3+ and Cr2O72−

A multivariant metal–organic framework (MTV-MOF) BUT-27 with a unique zeolite A structure was synthesized by post-synthetic linker exchange via single-crystal-to-single-crystal transformation.

Concentration-Dependent Seeding as a Strategy for Fabrication of Densely Packed Surface-Mounted Metal-Organic Frameworks (SURMOF) Layers

The layer‐by‐layer (LbL) method is a well‐established method for the growth of surface‐attached metal–organic frameworks (SURMOFs). Various experimental parameters, such as surface functionalization or temperature, have been identified as essential in the past. In this study, inspired by these recent insights regarding the LbL SURMOF growth mechanism, the impact of reactant solutions concentration on LbL growth of the Cu₂(F₄bdc)₂(dabco) SURMOF (F₄bdc²⁻=tetrafluorobenzene‐1,4‐dicarboxylate and dabco=1,4‐diazabicyclo‐[2.2.2]octane) in situ by using quartz‐crystal microbalance and ex situ with a combination of spectroscopic, diffraction and microscopy techniques was investigated. It was found that number, size, and morphology of MOF crystallites are strongly influenced by the reagent concentration. By adjusting the interplay of nucleation and growth, we were able to produce densely packed, yet thin films, which are highly desired for a variety of SURMOF applications.

Zeolite-Templated Carbon as a Stable, High Power Magnesium-Ion Cathode Material

One strategy to overcome the slow kinetics associated with electrochemical magnesium ion storage is to employ a permanently porous, capacitive cathode material together with magnesium metal as the anode. This strategy has begun to be employed, for example, using framework solids like Prussian blue analogues or porous carbons derived from metal-organic frameworks, but the cycling stability of an ordered, bottom-up synthesized, three-dimensional carbon framework toward magnesiation and demagnesiation in a shuttle-type battery remains unexplored. Zeolite-templated carbons (ZTCs) are a class of ordered porous carbonaceous framework materials with numerous superlative properties relevant to electrochemical energy storage. Herein, we report that ZTCs can serve as high-power cathode materials for magnesium-ion hybrid capacitors (MHCs), exhibiting high specific capacities (e.g., 113 mA h g^-1 after 100 cycles) with an average discharge voltage of 1.44 V and exceptional capacity retention (e.g., 76% after 200 cycles). ZTC-based MHCs meet or exceed the gravimetric energy densities of state-of-the-art batteries functioning on the Mg²⁺ shuttle, while simultaneously displaying far superior rate capabilities (e.g., 834 W kg^-1 at 600 mA g^-1).

Facile Orientational Control of M2L2P SURMOFs on ⟨100⟩ Silicon Substrates and Growth Mechanism Insights for Defective MOFs

Layer-by-layer growth of Cu₂(bdc)₂(dabco) surface-mounted metal-organic frameworks (SURMOFs) was investigated on silicon wafers treated with different surface anchoring molecules. Well-oriented growth along the [100] and [001] directions could be achieved with simple protocols: growth along the [100] direction was achieved by substrate pretreatment with 80 °C piranha, while growth along the [001] direction was enabled by only rinsing silicon with absolute ethanol. Growth along the [001] direction produced more homogeneous SURMOF films. Optimization to enhance [001]-preferred orientation growth revealed that small changes in the SURMOF growth sequence (the number of rinse steps and linker concentrations) have a noticeable impact on the final film quality and the number of misaligned crystals. This new straightforward protocol was used to successfully grow other layer pillar-type SURMOFs, including the growth of Cu₂(bdc)₂(bipy) with simultaneous suppression of framework interpenetration.

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.

Application of a Pillared-Layer Zn-Triazolate Metal-Organic Framework in the Dispersive Miniaturized Solid-Phase Extraction of Personal Care Products from Wastewater Samples

The pillared-layer Zn-triazolate metal-organic framework (CIM-81) was synthesized, characterized, and used for the first time as a sorbent in a dispersive micro-solid phase extraction method. The method involves the determination of a variety of personal care products in wastewaters, including four preservatives, four UV-filters, and one disinfectant, in combination with ultra-high performance liquid chromatography and UV detection. The CIM-81 MOF, constructed with an interesting mixed-ligand synthetic strategy, demonstrated a better extraction performance than other widely used MOFs in D-µSPE such as UiO-66, HKUST-1, and MIL-53(Al). The optimization of the method included a screening design followed by a Doehlert design. Optimum conditions required 10 mg of CIM-81 MOF in 10 mL of the aqueous sample at a pH of 5, 1 min of agitation by vortex and 3 min of centrifugation in the extraction step; and 1.2 mL of methanol and 4 min of vortex in the desorption step, followed by filtration, evaporation and reconstitution with 100 µL of the initial chromatographic mobile phase. The entire D-µSPE-UHPLC-UV method presented limits of detection down to 0.5 ng·mL-1; intra-day and inter-day precision values for the lowest concentration level (15 ng·mL-1)-as a relative standard deviation (in %)-lower than 8.7 and 13%, respectively; average relative recovery values of 115%; and enrichment factors ranging from ~3.6 to ~34. The reuse of the CIM-81 material was assessed not only in terms of maintaining the analytical performance but also in terms of its crystalline stability.

Reunderstanding the photoinduced charge transfer process of ammonium polyoxomolybdate

Protonation of 4-phenylpyridine can realize photoinduced electron transfer from the carboxyl group to 4-phenylpyridine and photochromism of ammonium polyoxomolybdate without proton transfer.

Structural diversity of zinc(ii) coordination polymers with octafluorobiphenyl-4,4′-dicarboxylate based on mononuclear, paddle wheel and cuboidal units

Certain modifications of the synthetic conditions lead to 6 novel coordination polymers, thus enriching the coordination chemistry of perfluorinated ligands.

A critical review of cathodes for rechargeable Mg batteries

Benefiting from a higher volumetric capacity (3833 mA h cm-3 for Mg vs. 2046 mA h cm-3 for Li) and dendrite-free Mg metal anode, reversible Mg batteries (RMBs) are a promising chemistry for applications beyond Li ion batteries. However, RMBs are still severely restricted by the absence of high performance cathodes for any practical application. In this review, we provide a critical and rigorous review of Mg battery cathode materials, mainly reported since 2013, focusing on the impact of structure and composition on magnesiation kinetics. We discuss cathode materials, including intercalation compounds, conversion materials (O2, S, organic compounds), water co-intercalation cathodes (V2O5, MnO2etc.), as well as hybrid systems using Mg metal anode. Among them, intercalation cathodes are further categorized by 3D (Chevrel phase, spinel structure etc.), 2D (layered structure), and 1D materials (polyanion: phosphate and silicate), according to the diffusion pathway of Mg2+ in the framework. Instead of discussing every published work in detail, this review selects the most representative works and highlights the merits and challenges of each class of cathodes. Advances in theoretical analysis are also reviewed and compared with experimental results. This critical review will provide comprehensive knowledge of Mg cathodes and guidelines for exploring new cathodes for rechargeable magnesium batteries.

Two-dimensional metal–organic framework nanosheets: synthesis and applications

Synthesis and applications of two-dimensional metal–organic framework nanosheets and their composites are summarized.

Box-like gel capsules from heterostructures based on a core–shell MOF as a template of crystal crosslinking

Cubic polymer capsules (PCs) were obtained using a crystal crosslinking (CC) method on core–shell MOF crystals, with a well-defined hollow cubic shape reflecting the heterostructure of the template.

High-Throughput Computational Screening of Multivariate Metal-Organic Frameworks (MTV-MOFs) for CO2 Capture

Multivariate metal-organic frameworks (MTV-MOFs) contain multiple linker types within a single structure. Arrangements of linkers containing different functional groups confer structural diversity and surface heterogeneity and result in a combinatorial explosion in the number of possible structures. In this work, we carried out high-throughput computational screening of a large number of computer-generated MTV-MOFs to assess their CO₂ capture properties using grand canonical Monte Carlo simulations. The results demonstrate that functionalization enhances CO₂ capture performance of MTV-MOFs when compared to their parent (unfunctionalized) counterparts, and the pore size plays a dominant role in determining the CO₂ adsorption capabilities of MTV-MOFs irrespective of the combinations of the three functional groups (-F, -NH₂, and -OCH₃) that we investigated. We also found that the functionalization of parent MOFs with small pores led to larger enhancements in CO₂ uptake and CO₂/N₂ selectivity than functionalization in larger-pore MOFs. Free energy contour maps are presented to visually compare the influence of linker functionalization between frameworks with large and small pores.

Post-synthetic transformation of a Zn(ii) polyhedral coordination network into a new supramolecular isomer of HKUST-1

A Zn-based porphyrin containing metal-organic material (porphMOM-1) was transformed into a novel Cu-based porphyrin-encapsulating metal-organic material (porph@HKUST-1-β) via a one-pot post-synthetic modification (PSM) process involving both metal ion exchange and linker installation of trimesic acid. HKUST-1-β is the first example of yao topology and is to our knowledge the first supramolecular isomer of the archetypal coordination network HKUST-1.

Coordination change, lability and hemilability in metal–organic frameworks

Deformation or cleavage/reformation of metal–ligand bonds in MOFs lies at the heart of chemical/thermal stability and dynamic/flexible behaviour, provides avenues for post-synthetic modification, and can enable novel or improved performance for a variety of applications.

Morphology Control of Metal-Organic Frameworks Based on Paddle-Wheel Units on Ion-Doped Polymer Substrate Using an Interfacial Growth Approach

A three-dimensional metal-organic framework (MOF) consisting of pillared square-grid nets based on paddle-wheel units was synthesized by interfacial self-assembly of the frameworks on a metal-ion-doped polymer substrate. Although this type of Cu-based MOF is typically synthesized by a two-step solvothermal method, the utilization of a metal-ion-doped polymer substrate as a metal source for the framework allowed for the one-pot growth of MOF crystals on the substrate. The morphology of the obtained MOF crystals could be controlled from tetragonal to elongated tetragonal with different aspect ratios by changing the concentrations of the dicarboxylate layer ligands and diamine pillar ligands. The present approach provides a new route for the design and synthesis of MOF crystals and thin films for future applications such as gas membranes, catalysts, and electronic devices.

Synthesis and stabilization of a hypothetical porous framework based on a classic flexible metal carboxylate cluster

A hypothetical porous network isomeric to MIL-88/MIL-101 has been realized by the introduction of terminal ligands and further stabilized by crosslinking.

The construction, structures, and functions of pillared layer metal–organic frameworks

Pillared layer metal–organic frameworks (PL-MOFs), belonging to one representative of porous materials, have witnessed major advances in the past few years.

Halogenated MOF-5 variants show new configuration, tunable band gaps and enhanced optical response in the visible and near infrared

We show that full halogenation of paradigm MOF-5 can tune the band gap and optical response between 1.0 and 4.2 eV leading to optical activity in the visible and infrared. Applications include photocatalysts, photoactive materials, and optoelectronics.

"Heterogeneity within order" in metal-organic frameworks

Metal-organic frameworks (MOFs) are constructed by linking inorganic units with organic linkers to make extended networks. Though more than 20 000 MOF structures have been reported most of these are ordered and largely composed of a limited number of different kinds building units, and very few have multiple different building units (heterogeneous). Although heterogeneity and multiplicity is a fundamental characteristic of biological systems, very few synthetic materials incorporate heterogeneity without losing crystalline order. Thus, the question arises: how do we introduce heterogeneity into MOFs without losing their ordered structure? This Review outlines strategies for varying the building units within both the backbone of the MOF and its pores to produce the heterogeneity that is sought after. The impact this heterogeneity imparts on the properties of a MOF is highlighted. We also provide an update on the MOF industry as part of this themed issue for the 150th anniversary of BASF.

Dynamic structural transformations of coordination supramolecular systems upon exogenous stimulation

This feature article comments on the dynamic structural transformations of coordination supramolecular systems, which can be triggered by exposure to various exogenous stimuli such as concentration, temperature, light and mechanical force, as well as their synergic effect.

A copper-based layered coordination polymer: synthesis, magnetic properties and electrochemical performance in supercapacitors

A layered copper-based coordination polymer ([Cu(hmt)(tfbdc)(H₂O)], Cu-LCP) has been synthesized, which shows ferromagnetic interactions between the adjacent copper(ii) ions and an excellent capacitive performance in supercapacitors.

An unprecedented high nuclearity catecholato-based Ti(IV)-architecture bearing labile pyridine ligands

We describe the synthesis and characterisation of a robust S4-symmetry titanium-based architecture bearing catecholato and pyridine ligands. This neutral complex formulated as [Ti10O12(cat)8(py)8] displays a tetrahedral inorganic core decorated by catecholato ligands with unusual coordination modes. In solution, the pyridine ligands are labile as shown in DOSY studies at variable temperature. The light absorption property in the visible domain (λmax = 411 nm, ε = 10 800) was also characterised for the complex.