Christopher J. Sumby

Post-synthetic structural processing in a metal-organic framework material as a mechanism for exceptional CO2/N2 selectivity

Here we report the synthesis and ceramic-like processing of a new metal-organic framework (MOF) material, [Cu(bcppm)H2O], that shows exceptionally selective separation for CO2 over N2 (ideal adsorbed solution theory, S(ads) = 590). [Cu(bcppm)H2O]·xS was synthesized in 82% yield by reaction of Cu(NO3)2·2.5H2O with the link bis(4-(4-carboxyphenyl)-1H-pyrazolyl)methane (H2bcppm) and shown to have a two-dimensional 4(4)-connected structure with an eclipsed arrangement of the layers. Activation of [Cu(bcppm)H2O] generates a pore-constricted version of the material through concomitant trellis-type pore narrowing (b-axis expansion and c-axis contraction) and a 2D-to-3D transformation (a-axis contraction) to give the adsorbing form, [Cu(bcppm)H2O]-ac. The pore contraction process and 2D-to-3D transformation were probed by single-crystal and powder X-ray diffraction experiments. The 3D network and shorter hydrogen-bonding contacts do not allow [Cu(bcppm)H2O]-ac to expand under gas loading across the pressure ranges examined or following re-solvation. This exceptional separation performance is associated with a moderate adsorption enthalpy and therefore an expected low energy cost for regeneration.

Capturing snapshots of post-synthetic metallation chemistry in metal-organic frameworks.

Post-synthetic metallation is employed strategically to imbue metal–organic frameworks (MOFs) with enhanced performance characteristics. However, obtaining precise structural information for metal-centred reactions that take place within the pores of these materials has remained an elusive goal, because of issues with high symmetry in certain MOFs, lower initial crystallinity for some chemically robust MOFs, and the reduction in crystallinity that can result from carrying out post-synthetic reactions on parent crystals. Here, we report a new three-dimensional MOF possessing pore cavities that are lined with vacant di-pyrazole groups poised for post-synthetic metallation. These metallations occur quantitatively without appreciable loss of crystallinity, thereby enabling examination of the products by single-crystal X-ray diffraction. To illustrate the potential of this platform to garner fundamental insight into metal-catalysed reactions in porous solids we use single-crystal X-ray diffraction studies to structurally elucidate the reaction products of consecutive oxidative addition and methyl migration steps that occur within the pores of the Rh-metallated MOF, 1·[Rh(CO)2][Rh(CO)2Cl2]. Obtaining precise structural information for metal-centred reactions that take place within the pores of metal–organic frameworks continues to be an elusive goal. Now, a flexible framework has been synthesized that enables the direct elucidation of the products of post-synthetic metallation reactions and subsequent chemical transformations by single-crystal X-ray crystallography. Camera image:

Kinetically controlled porosity in a robust organic cage material

Microporous materials are of significant interest owing to their central role in gas storage, separation processes, and catalysis. Recently, microporous molecular solids composed of discrete, shape-persistent organic cages have received growing attention because they possess unique properties that set them apart from conventional, extended network materials, such as zeolites, metal–organic frameworks, and covalent organic frameworks. For example, molecular solids are readily solution-processable, provide facile access to multicomponent materials by mix-and-match synthesis, and, by virtue of their noncovalent intermolecular packing, can exhibit advanced properties, such as adsorbatetriggered on/off porosity switching. Unlike extended networks, where solvent-accessible voids are linked through rigid covalent framework solids composed of discrete organic cages predominantly aggregate by relatively weak dispersion forces. Predicting the crystal structures of such weakly aggregating materials is a long-standing challenge in solid-state chemistry, and is, in this field, inherently coupled to estimating the ultimate porosity of a molecular solid from its building units, as different polymorphs can afford solids with dramatically different surface areas. Accordingly, relatively few examples of porous organic solids have been reported. Nevertheless, recent work from the laboratories of Cooper and Mastalerz have demonstrated that the porosity of such materials can be modified through crystal engineering strategies and synthetic processing. 10] Herein we describe the synthesis and characterization of a novel, permanently porous, shape-persistent cage molecule (C1) that is constructed entirely from thermodynamically robust carbon–carbon bonds and has the molecular formula C112H62O2 (Scheme 1). Furthermore, we demonstrate kinetically controlled access to two crystalline polymorphs C1a and C1b that possess dramatically different N2 porosities: polymorph C1a, which is nonporous to N2, and polymorph C1b, which affords a BET surface area of 1153 m g .

Synthesis of a zinc(II) imidazolium dicarboxylate ligand metal-organic framework (MOF): a potential precursor to MOF-tethered N-heterocyclic carbene compounds

Two new isomeric dinitrile ligands containing imidazolium salt cores have been synthesized from cyanoanilines in good yield. These have been converted to the corresponding dicarboxylic acids using hydrobromic acid, or the dicarboxylic acids were synthesized directly from the analogous cyanoaniline starting material in a two-step, one-pot reaction. The crystal structures of three of the four compounds are reported. Two reaction pathways with metals are possible for the dicarboxylic acids, initially giving rise to metal carboxylate based metal-organic frameworks (MOFs) as described in this work or N-heterocyclic carbene (NHC) metal complexes en route to the synthesis of MOFs containing tethered NHC complexes. The reaction of 1,3-bis(4-carboxyphenyl)imidazolium bromide with zinc nitrate in dimethylformamide (DMF) gave a one-dimensional (1D) MOF containing the intact imidazolium salt. The potentially porous structure, formed from close packing of the 1D necklace-type polymers, contains channels occupied by disordered DMF solvate molecules. A formate anion also bridges the zinc secondary building units in the 1D polymer, which has an undulating structure resulting from the U-shaped conformation of the dicarboxylate imidazolium ligand.

Mixed-Matrix Membranes

Research into extended porous materials such as metal-organic frameworks (MOFs) and porous organic frameworks (POFs), as well as the analogous metal-organic polyhedra (MOPs) and porous organic cages (POCs), has blossomed over the last decade. Given their chemical and structural variability and notable porosity, MOFs have been proposed as adsorbents for industrial gas separations and also as promising filler components for high-performance mixed-matrix membranes (MMMs). Research in this area has focused on enhancing the chemical compatibility of the MOF and polymer phases by judiciously functionalizing the organic linkers of the MOF, modifying the MOF surface chemistry, and, more recently, exploring how particle size, morphology, and distribution enhance separation performance. Other filler materials, including POFs, MOPs, and POCs, are also being explored as additives for MMMs and have shown remarkable anti-aging performance and excellent chemical compatibility with commercially available polymers. This Review briefly outlines the state-of-the-art in MOF-MMM fabrication, and the more recent use of POFs and molecular additives.

Emerging applications of metal–organic frameworks

Metal–organic frameworks are a unique class of materials well known for their crystallinity and ultra-high porosity. Since their first report over fifteen years ago, research in this area has sought to actively exploit these properties, especially in gas adsorption. In this article we canvass some emerging topics in the field of MOF research that show promise for new applications in areas such as biotechnology, catalysis, and microelectronics.

Network structures of cyclotriveratrylene and its derivatives

The host molecule cyclotriveratrylene can be incorporated into network structures by acting as a hydrogen bond acceptor or as a chelating ligand for Group 1 metals. New CTV-based ligands with pyridyl functional groups have been synthesised for use as multifunctional ligands in coordination networks. Resultant hydrogen bonded network structures and coordination networks show a range of chain, 2-D and 3-D structures with hexagonal 63 nets predominating. Recent results are discussed including highly complex 3-D networks where the molecular hosts pack in a tetrameric back-to-back fashion.

Anion-directed self-assembly of metallosupramolecular coordination polymers of the radialene ligand hexa(2-pyridyl)[3]radialene

Hexa(2-pyridyl)[3]radialene was prepared by reaction of di-2-pyridylmethane with tetrachlorocyclopropene. Reaction of this ligand with silver nitrate and silver hexafluorophosphate gave two different one-dimensional coordination polymers each with a 2:1 metal–ligand composition. X-ray crystal structures revealed that the structure and topology of the coordination polymer is strongly influenced by the anion. In the reaction with silver nitrate the ligand self-assembled to give a helical one-dimensional coordination polymer, but when silver hexafluorophosphate was used, a coordination polymer, composed of Ag3L2 prismate cages linked by linear-bridging silver atoms, was obtained.

AIMs: a new strategy to control physical aging and gas transport in mixed-matrix membranes

The effect of controlling interactions between the components in a mixed-matrix membrane at the molecular level has been explored. A systematic series of soluble metal–organic polyhedra (MOPs) of varying external organic chain length were prepared and applied within polymer membranes to produce anti-aging intercalated membranes (AIMs). Use of a soluble porous additive allowed for intimate mixing between the polymer and the porous additive, eliminating the formation of non-selective gas transport voids at the interface, typically found in traditional mixed-matrix membranes. Moreover, the molecular interaction thus created provided a valuable tool for tailoring the physical aging rates of the membranes. Aging was slowed by a factor of three with the optimal tBu-MOP additive, and viscosity measurements revealed they held the strongest MOP–polymer interaction, confirming the utility of the AIMs approach. MOP loading was therefore able to be optimized for the maximum anti-aging effect by monitoring the relative change in viscosity. Absolute gas permeability scaled with the MOP external organic chain length, revealing solubility-driven diffusion.

Building blocks for cyclotriveratrylene-based coordination networks

The incorporation of three-fold symmetric organic host molecules into coordination polymers should allow for the construction of new and interesting network structures, capable of multiple inclusion behaviour. A range of new multi-dentate bridging ligands/molecular hosts have been prepared by appending nitrogen-containing heterocycles to either cyclotricatechylene, or cyclotriguaiacylene cores. These compounds were obtained in a single-step reaction from readily available precursors, with moderate to good yields, and characterised by a combination of NMR spectroscopy, mass spectrometry and elemental analysis. Two of the new compounds were characterised by X-ray crystallography, revealing different modes of self inclusion behaviour, which indicate the potential importance of [small pi]-donor stabilisation by CTV derivatives in host-guest chemistry.