Probing the Antiaromaticity and Coordination Chemistry of Bowl-Shaped Zinc(II) Norcorrole.

Zinc norcorrole was prepared as its pyridine complex (ZnNc·pyridine) by metalation of freebase norcorrole. The ZnNc·pyridine complex is distinctly bowl-shaped, as demonstrated by both X-ray crystallography and nuclear magnetic resonance (NMR) spectroscopy. NMR spectroscopy showed characteristic ring current deshielding effects, with different magnitudes on either face of the bowl-shaped complex. Exchanging the pyridine ligand with the bidentate ligand DABCO results in the formation of a stable (ZnNc)2·DABCO sandwich complex, which was also characterized by crystallography and NMR spectroscopy. The NMR resonances of the axial ligands in all of the complexes demonstrate that the paratropic ring current in zinc norcorrole is approximately 40 nA/T, which is comparable in magnitude to the diatropic ring current in porphyrin. Analysis of the ligand-exchange processes on addition of DABCO to ZnNc·pyridine showed that ZnNc coordinates to axial nitrogen-containing ligands with approximately 1000-fold higher binding constants than analogous zinc porphyrins.

Hydrogen-bonded frameworks containing aliphatic 3D linkers show high-capacity water vapour sorption.

Hydrogen-bonded frameworks were prepared from a tetra-amidinium component and three-dimensional cubane and bicyclopentane dicarboxylate linkers. Despite the incorporation of aliphatic components, the frameworks demonstrate strong and reversible uptake of water vapour, with one of the frameworks showing water uptake at very low relative humidity.

Robust Carborane-Based Metal-Organic Frameworks for Hexane Separation.

Hexane isomers play a vital role as feedstocks and fuel additives in the petrochemical industry. However, their similar physical and chemical properties lead to significant challenges in the separation process. Traditional thermal separation techniques are energy-intensive and lead to significant carbon footprint penalties. As such, there is a growing demand for the development of less energy-intensive nonthermal separation methods. Adsorption-based separation methods, such as using solid sorbents or membranes, have emerged as promising alternatives to distillation. Here, we report the successful synthesis of two novel metal-organic frameworks (MOFs), NU-2004 and NU-2005, by incorporating a carborane-based three-dimensional (3D) linker and using aluminum and vanadium nodes, respectively. These MOFs exhibit exceptional thermal stability and structural rigidity compared to other MIL-53 analogues, which is further corroborated using synchrotron studies. Furthermore, the inclusion of the quasi-spherical 3D linker in NU-2004 demonstrates significant advancements in the separation of hexane isomers compared to other MIL MOFs containing two-dimensional (2D) and aliphatic 3D linkers.

Expanding Linker Dimensionality in Metal-organic Frameworks for sub-Ångstrom Pore Control for Separation Applications.

Metal-organic frameworks (MOFs) are a class of porous materials with high surface areas, which are acquiring rapid attention on an exponential basis. A significant characteristic of MOFs is their ability to act as adsorbents to selectively separate component mixtures of similar size, thereby addressing the technological need for an alternative approach to conventional distillation methods. Recently, MOFs comprising a 3-Dimensional (3D) linker have shown outstanding capabilities for difficult separations compared to the parent 2-Dimensional (2D) analogue. 3D-linkers with a polycyclic core are underrepresented in the MOF database due to the widespread preferred use of 2D-linkers and the misconceived high-cost of 3D linkers. We summarize the recent research of 3D-linker MOFs and highlight their beneficial employment for selective gas and hydrocarbon adsorption and separation. Furthermore, we outline forecasts in this area to create a platform for widespread adoption of 3D-linkers in MOF synthesis.

Engineering Metal-Organic Frameworks for Selective Separation of Hexane Isomers Using 3-Dimensional Linkers.

Metal-organic frameworks (MOFs) are highly tunable materials with potential for use as porous media in non-thermal adsorption or membrane-based separations. However, many separations target molecules with sub-angstrom differences in size, requiring precise control over the pore size. Herein, we demonstrate that this precise control can be achieved by installing a three-dimensional linker in an MOF with one-dimensional channels. Specifically, we synthesized single crystals and bulk powder of NU-2002, an isostructural framework to MIL-53 with bicyclo[1.1.1]pentane-1,3-dicarboxylic acid as the organic linker component. Using variable-temperature X-ray diffraction studies, we show that increasing linker dimensionality limits structural breathing relative to MIL-53. Furthermore, single-component adsorption isotherms demonstrate the efficacy of this material for separating hexane isomers based on the different sizes and shapes of these isomers.

Halide-selective, proton-coupled anion transport by phenylthiosemicarbazones.

Phenylthiosemicarbazones (PTSCs) are proton-coupled anion transporters with pH-switchable behaviour known to be regulated by an imine protonation equilibrium. Previously, chloride/nitrate exchange by PTSCs was found to be inactive at pH 7.2 due to locking of the thiourea anion binding site by an intramolecular hydrogen bond, and switched ON upon imine protonation at pH 4.5. The rate-determining process of the pH switch, however, was not examined. We here develop a new series of PTSCs and demonstrate their conformational behaviour by X-ray crystallographic analysis and pH-switchable anion transport properties by liposomal assays. We report the surprising finding that the protonated PTSCs are extremely selective for halides over oxyanions in membrane transport. Owing to the high chloride over nitrate selectivity, the pH-dependent chloride/nitrate exchange of PTSCs originates from the rate-limiting nitrate transport process being inhibited at neutral pH.

In Situ Investigation of Multicomponent MOF Crystallization during Rapid Continuous Flow Synthesis.

Access to the potential applications of metal-organic frameworks (MOFs) depends on rapid fabrication. While there have been advances in the large-scale production of single-component MOFs, rapid synthesis of multicomponent MOFs presents greater challenges. Multicomponent systems subjected to rapid synthesis conditions have the opportunity to form separate kinetic phases that are each built up using just one linker. We sought to investigate whether continuous flow chemistry could be adapted to the rapid formation of multicomponent MOFs, exploring the UMCM-1 and MUF-77 series. Surprisingly, phase pure, highly crystalline multicomponent materials emerge under these conditions. To explore this, in situ WAXS was undertaken to gain an understanding of the formation mechanisms at play during flow synthesis. Key differences were found between the ternary UMCM-1 and the quaternary MUF-7, and key details about how the MOFs form were then uncovered. Counterintuitively, despite consisting of just two ligands UMCM-1 proceeds via MOF-5, whereas MUF-7 consists of three ligands but is generated directly from the reaction mixture. By taking advantage of the scalable high-quality materials produced, C6 separations were achieved in breakthrough settings.

Acridinone-based anion transporters.

The arrangement of hydrogen bond donors around a central lipophilic scaffold has proven to be a successful strategy in the development of potent chloride transporters. In this work, we revisit an acridinone 1,9-bis(thio)urea motif which had previously shown promise as an anion sensor and expand the series of compounds by appending a variety of electron-withdrawing groups to the peripheral phenyl moieties. High levels of activity were achieved by the most effective compounds in the series, which facilitated strictly electroneutral transport.

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.

25 Years of Reticular Chemistry.

At its core, reticular chemistry has translated the precision and expertise of organic and inorganic synthesis to the solid state. While initial excitement over metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) was undoubtedly fueled by their unprecedented porosity and surface areas, the most profound scientific innovation of the field has been the elaboration of design strategies for the synthesis of extended crystalline solids through strong directional bonds. In this contribution we highlight the different classes of reticular materials that have been developed, how these frameworks can be functionalized, and how complexity can be introduced into their backbones. Finally, we show how the structural control over these materials is being extended from the molecular scale to their crystal morphology and shape on the nanoscale, all the way to their shaping on the bulk scale.

Tetrapodal Anion Transporters.

Synthetic anion transporters that facilitate chloride transport are promising candidates for channelopathy treatments. However, most anion transporters exhibit an undesired side effect of facilitating proton transport via interacting with fatty acids present in the membrane. To address the limitation, we here report the use of a new tetrapodal scaffold to maximize the selective interaction with spherical chloride over binding the carboxylate headgroup of fatty acids. One of the new transporters demonstrated a high selectivity for chloride uniport over fatty acid-induced proton transport while being >10 times more active in chloride uniport than strapped calixpyrroles that were previously the only class of compounds known to possess similar selectivity properties.

Mixed donor, phenanthroline photoactive MOFs with favourable CO2 selectivity.

Mixed donor phenanthroline-carboxylate linkers were combined with MnII or ZnII to form photoactive MOFs with large pore apertures. The MOFs display high CO2 adsorption capacities, which consequently causes structural framework flexibility, and align with favorable metrics for selective CO2 capture. The photophysical properties of the MOFs were investigated, with the MnII MOF giving rise to short triplet LMCT lifetimes.

Enhancing Multicomponent Metal-Organic Frameworks for Low Pressure Liquid Organic Hydrogen Carrier Separations.

The resurgence of interest in the hydrogen economy could hinge on the distribution of hydrogen in a safe and efficient manner. Whilst great progress has been made with cryogenic hydrogen storage or liquefied ammonia, liquid organic hydrogen carriers (LOHCs) remain attractive due to their lack of need for cryogenic temperatures or high pressures, most commonly a cycle between methylcyclohexane and toluene. Oxidation of methylcyclohexane to release hydrogen will be more efficient if the equilibrium limitations can be removed by separating the mixture. This report describes a family of six ternary and quaternary multicomponent metal-organic frameworks (MOFs) that contain the three-dimensional cubane-1,4-dicarboxylate (cdc) ligand. Of these MOFs, the most promising is a quaternary MOF (CUB-30), comprising cdc, 4,4'-biphenyldicarboxylate (bpdc) and tritopic truxene linkers. Contrary to conventional wisdom that adsorptive interactions with larger, hydrocarbon guests are dominated by π-π interactions, here we report that contoured aliphatic pore environments can exhibit high selectivity and capacity for LOHC separations at low pressures. This is the first time, to the best of our knowledge, where selective adsorption for cyclohexane over benzene is witnessed, underlining the unique adsorptive behavior afforded by the unconventional cubane moiety.

Acoustomicrofluidic assembly of oriented and simultaneously activated metal-organic frameworks.

The high surface area and porosity, and limitless compound and network combinations between the metal ions and organic ligands making up metal-organic frameworks (MOFs) offer tremendous opportunities for their use in many applications. While numerous methods have been proposed for the synthesis of MOF powders, it is often difficult to obtain oriented crystals with these techniques. Further, the need for additional post-synthesis steps to activate the crystals and release them from the substrate presents a considerable production challenge. Here, we report an acoustically-driven microcentrifugation platform that facilitates fast convective solutal transport, allowing the synthesis of MOF crystals in as short as five minutes. The crystals are not only oriented due to long-range out-of-plane superlattice ordering aided by molecular dipole polarization under the acoustoelectric coupling, but also simultaneously activated during the synthesis process.

Light-harvesting, 3rd generation RuII/CoII MOF with a large, tubular channel aperture.

A photoactive, hetero-metallic CoII/RuII-based metal-organic framework (MOF) with a large channel aperture, ca. 21 Å, is reported. The photophysical properties of the MOF are derived from the RuII nodes giving rise to emission centred at ca. 620 nm and relatively long triplet 3MLCT lifetimes. In addition to the optical attributes, the 1H-imidazo [4,5-f][1,10]-phenanthroline ligand imparts structural functionality to the MOF which is composed of alternating CoII- and RuII-based nodes of Δ and Λ helicity. The framework maintains its integrity upon activation and shows gas sorption behaviour that is characteristic of mesoporous materials promoting high CO2 sorption capacities and selectivities over N2.

Assembly, disassembly and reassembly: a "top-down" synthetic strategy towards hybrid, mixed-metal {Mo10Co6} POM clusters.

Polyoxometalates (POMs) are commonly prepared using a "bottom-up" synthetic procedure. The alternative "top-down" approach of disassembling a pre-formed POM unit to generate new synthetic intermediates is promising, but relatively comparatively underused. In this paper, a rationale for the top-down method is provided, demonstrating that this approach can generate compounds that are fundamentally inaccessible from simple bottom-up assembly. We demonstrate this principle through the synthesis of a series of 10, new, mixed-metal, hybrid compounds with the general formula [TBA]2[MoVI10CoII6O30(RpPO3)6(RcCOO)2(L)x(H2O)6] (TBA = tetrabutylammonium, Rp = phosphonate moiety, Rc = carboxylate moiety, L = pyridyl ligand, and x = 2-4), including a one-dimensional polyoxometalate-based coordination polymer. We propose that these structures are generated from {MoxO3x-1} fragments that cannot be accessed from bottom-up assembly alone. The POM clusters are stabilised by three distinct classes of organic ligand - organophosphonate, carboxylate and pyridyl ligands - which can each be substituted independantly, thus providing a controlled route to ligand functionalisation.

CUB-5: A Contoured Aliphatic Pore Environment in a Cubic Framework with Potential for Benzene Separation Applications.

One prominent aspect of metal organic frameworks (MOFs) is the ability to tune the size, shape, and chemical characteristics of their pores. MOF-5, with its open cubic connectivity of Zn4O clusters joined by two-dimensional, terephthalate linkers, is the archetypal example: both functionalized and elongated linkers produce isoreticular frameworks that define pores with new shapes and chemical environments. The recent scalable synthesis of cubane-1,4-dicarboxylic acid (1,4-H2cdc) allows the first opportunity to explore its application in leading reticular architectures. Herein we describe the use of 1,4-H2cdc to construct [Zn4O(1,4-cdc)3], referred to as CUB-5. Isoreticular with MOF-5, CUB-5 adopts a cubic architecture but features aliphatic, rather than aromatic, pore surfaces. Methine units point directly into the pores, delivering new and unconventional adsorption locations. Our results show that CUB-5 is capable of selectively adsorbing high amounts of benzene at low partial pressures, promising for future investigations into the industrial separation of benzene from gasoline using aliphatic MOF materials. These results present an effective design strategy for the generation of new MOF materials with aliphatic pore environments and properties previously unattainable in conventional frameworks.

Cadmium tris(dithiocarbamate) ionic liquids as single source, solvent-free cadmium sulfide precursors.

The first cadmium-based ionic liquids (ILs) have been developed, along with a family of cadmium dialkyldithiocarbamate salts, (cation)[Cd(R2dtc)3], in the pursuit of single source molecular precursors that thermolyse to form cadmium sulfide. Pyrrolidinium cadmium dialkyldithiocarbamate salts, (C4C1py)[Cd(R2dtc)3], salts were established to be ILs through thorough thermal and structural investigation.