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.

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.

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.

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.

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 Mn^II or Zn^II to form photoactive MOFs with large pore apertures.

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.

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.

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.

Hydrogen Bonding of O-Ethylxanthate Compounds and Neutron Structural Determination of C–H···S Interactions

A range of ethylxanthate (EtXn) salts, containing either protic or aprotic cations (guanidinium (1), methylammonium (2), dimethylammonium (3), trimethylammonium (4), tetramethylammonium (5), tetraethylammonium (6), and tetrapropylammonium (7)), have been synthesised and structurally characterised. The cations in these compounds differ in their degree of hydrogen-bonding ability, i.e. the number of donor groups, with significant structural consequences. Compounds 1–4 contain cations that are able to form N–H···S hydrogen bonds, with six, three, two, and one donor groups in 1–4 respectively. The number of donor atoms affects greatly the dimensionality of the hydrogen-bonding networks in the solid state. The structure of 1 has a 3-D hydrogen-bonding network, 2 and 3 form 2-D sheets and 1-D chains respectively, whereas the lone NH donor group in 4 has strong hydrogen bonding only within a discrete cation–anion pair. The tetraalkylammonium salts 5–7 have no strong hydrogen bonding, with only C–H···S and C–H···O interactions possible. To determine unambiguously the presence of such interactions, single-crystal Laue neutron diffraction data were obtained for compound 5, providing a fully anisotropic model, which can be used to rationalise potential close interactions in the other structures. The neutron structure of 5 confirms the existence of C–H···S hydrogen bonds, with the H···S distance falling well within the sum of the van der Waals radii of the atoms. The close-packing in 5–7 is mediated solely through these weak interactions, with the size of the cations influencing the structures.