Campbell J. Coghlan

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:

Zirconium-based metal-organic framework for removal of perrhenate from water

The efficient removal of pertechnetate (TcO4(-)) anions from liquid waste or melter off-gas solution for an alternative treatment is one of the promising options to manage (99)Tc in legacy nuclear waste. Safe immobilization of (99)Tc is of major importance because of its long half-life (t1/2 = 2.13 × 10(5) yrs) and environmental mobility. Different types of inorganic and solid-state ion-exchange materials have been shown to absorb TcO4(-) anions from water. However, both high capacity and selectivity have yet to be achieved in a single material. Herein, we show that a protonated version of an ultrastable zirconium-based metal-organic framework can adsorb perrhenate (ReO4(-)) anions, a nonradioactive surrogate for TcO4(-), from water even in the presence of other common anions. Synchrotron-based powder X-ray diffraction and molecular simulations were used to identify the position of the adsorbed ReO4(-) (surrogate for TcO4(-)) molecule within the framework.

Hetero-bimetallic metal–organic polyhedra

Porous metal-organic polyhedra (MOPs), constructed from heterometallic Pd(II)-M(II) (M = Cu, Ni, Zn) paddlewheel nodes and 5-tert-butyl-1,3-benzenedicarboxylate organic links, were prepared in which the Pd(II) ions preferentially line the inner surface of the cage molecules. Careful activation produces co-ordinatively unsaturated 3d transition metal sites on the external MOP surfaces giving rise to H2 adsorption enthalpies in excess of -12 kJ mol(-1).

Studies with an immobilized metal affinity chromatography cassette system involving binuclear triazacyclononane-derived ligands: Automation of batch adsorption measurements with tagged recombinant proteins

This study describes the determination of the adsorption isotherms and binding kinetics of tagged recombinant proteins using a recently developed IMAC cassette system and employing automated robotic liquid handling procedures for IMAC resin screening. These results confirm that these new IMAC resins, generated from a variety of different metal-charged binuclear 1,4,7-triaza-cyclononane (tacn) ligands, interact with recombinant proteins containing a novel N-terminal metal binding tag, NT1A, with static binding capacities similar to those obtained with conventional hexa-His tagged proteins, but with significantly increased association constants. In addition, higher kinetic binding rates were observed with these new IMAC systems, an attribute that can be positively exploited to increase process productivity. The results from this investigation demonstrate that enhancements in binding capacities and affinities were achieved with these new IMAC resins and chosen NT1A tagged protein. Further, differences in the binding performances of the bis(tacn) xylenyl-bridged ligands were consistent with the distance between the metal binding centres of the two tacn moieties, the flexibility of the ligand and the potential contribution from the aromatic ring of the xylenyl group to undergo π/π stacking interactions with the tagged proteins.

Particle size effects in the kinetic trapping of a structurally-locked form of a flexible MOF

The application of metal–organic frameworks (MOFs) for gas storage, molecular separations and catalysis necessitates careful consideration of the particle size and structuralisation (e.g. pelletisation, surface-anchoring) of a material. Recently, particle size has been shown to dramatically alter the physical and structural properties of certain MOFs, but overall there is limited information on how the particle size affects the properties of flexible MOFs. Here we demonstrate that the particle size of a flexible MOF, specifically the as-synthesised form of [Cu(bcppm)H2O]·S (H2bcppm = bis(4-(4-carboxyphenyl)-1H-pyrazolyl)methane, S = solvent) (1), correlates with the rate of structural reorganisation from a “kinetically-trapped”, activated 3D form of this MOF to an “open” 2D form of the structure. We also outline two methods for synthetically reducing the particle size of 1 at room temperature, using 0.1 M NaOH (for two reaction times: 0.5 and 16 h) and with the sodium salt of the ligand Na2bcppm, producing crystals of 85 ± 15, 280 ± 14 and 402 ± 41 nm, respectively.

Mapping-Out Catalytic Processes in a Metal–Organic Framework with Single-Crystal X-ray Crystallography

Single-crystal X-ray crystallography is employed to characterize the reaction species of a full catalytic carbonylation cycle within a MnII -based metal-organic framework (MOF) material. The structural insights explain why the Rh metalated MOF is catalytically competent toward the carbonylation of MeBr but only affords stoichiometric turn-over in the case of MeI. This work highlights the capability of MOFs to act as platform materials for studying single-site catalysis in heterogeneous systems.

A Unique 3D Nitrogen-Doped Carbon Composite as High-Performance Oxygen Reduction Catalyst

The synthesis and properties of an oxygen reduction catalyst based on a unique 3-dimensional (3D) nitrogen doped (N-doped) carbon composite are described. The composite material is synthesised via a two-step hydrothermal and pyrolysis method using bio-source low-cost materials of galactose and melamine. Firstly, the use of iron salts and galactose to hydrothermally produceiron oxide (Fe2O3) magnetic nanoparticle clusters embedded carbon spheres. Secondly, magnetic nanoparticles diffused out of the carbon sphere when pyrolysed in the presence of melamine as nitrogen precursor. Interestingly, many of these nanoparticles, as catalyst-grown carbon nanotubes (CNTs), resulted in the formation of N-doped CNTs and N-doped carbon spheres under the decomposition of carbon and a nitrogen environment. The composite material consists of integrated N-doped carbon microspheres and CNTs show high ORR activity through a predominantly four-electron pathway.

Hydrogen adsorption in azolium and metalated N-heterocyclic carbene containing MOFs

Azolium and metalated N-heterocyclic carbene (NHC) functionalised metal–organic frameworks (MOFs) have been investigated as adsorbents and heterogenous catalysts. Here we describe the structures of two sets of 1D polymeric structures, {[M2(μ2-HCOO)(HL)2](NO3)·xDMF}n (M = Zn, x = 1, 1; Cu, x = 1.75, 2) and {[M3(HL)4(H2O)4](NO3)2·xDMF}n (M = Co, x = 2.75, 3; Mg, x = 0, 4; Mn, x = 1.5, 5), prepared from reactions of 1,3-bis(3-carboxyphenyl)-1H-imidazol-3-ium bromide (H3LBr) with M(NO3)2·xH2O (M = Zn, Cu, Co and Mg) and MnCl2·4H2O. These pack as porous and non-porous 3D materials, respectively. In the case of the known Zn(II) material 1, which we reveal to be porous, we also report metalation of the NHC precursor concomitant with framework synthesis to give {[Zn2(μ2-HCOO)(HL)1.6(L–Cu–Br)0.4](NO3)0.6·0.75DMF}n (1-Cu). Compounds 1, 2, and 1-Cu show H2 adsorption enthalpies of −9.9, −9.1, and −9.7 kJ mol−1 respectively, and allow the effect of micropores decorated with charged imidazolium moieties, or NHC–CuBr entities, on H2 adsorption to be probed.

Utilising hinged ligands in MOF synthesis: a covalent linking strategy for forming 3D MOFs

Here we show that connecting two equivalents of a bis-pyrazolymethane ‘hinged’ link by a carbon–carbon bond characteristically ‘extends’ the 2D layered metal–organic frameworks (MOFs) typically formed with such compounds into 3D MOF materials. 1,1,2,2-Tetrakis[4-(4-carboxyphenyl)-1H-pyrazol-1-yl]ethane (L) was prepared in three steps and upon reaction with late transition metals, namely copper(II), cadmium(II) and zinc(II), gave 3D MOFs [Cu2(L)(H2O)2] 1.4DMF and [M2L]·xDMF (M = Zn, x = 1; M = Cd, x = 1.75). The 3D MOFs display gating behaviour in their adsorption isotherms, consistent with 3rd generation flexible structures. Furthermore, the 3D MOFs showed appreciable affinity for CO2 at 293 K however, due to the larger pore sizes, molecular sieving of CO2/N2 was not observed. Reaction of L with cobalt(II) gave a 3D hydrogen-bonded network incorporating 1D coordination polymer chains that is topologically equivalent to the Zn and Cd MOFs. The strategy outlined here demonstrates a novel route for designing more chemically and thermally robust 3D MOFs from 2D layered materials.

Isolation and Structure of a Hydrogen-bonded 2,2′:6′,2″-Terpyridin-4′-one Acetic Acid Adduct

Herein, we report the crystal structure of a key intermediate in the synthesis of 4′-substituted-terpyridines. Our findings confirm that the terpyridin-4′-one intermediate as generated from the condensation reaction of the corresponding triketone precursor with ammonium acetate is isolated as a hydrogen-bonded adduct with acetic acid, and not, as previously reported, as the acetate salt of a protonated pyridine nitrogen. This finding provides a rationale for the behaviour and structure of substituted terpyridin-4′-ones and pyridones in both the solid state and in solution.