Crystal Engineering of a New Hexafluorogermanate Pillared Hybrid Ultramicroporous Material Delivers Enhanced Acetylene Selectivity.

Hybrid ultramicroporous materials (HUMs), metal-organic platforms that incorporate inorganic pillars, are a promising class of porous solids. A key area of interest for such materials is gas separation, where HUMs have already established benchmark performances. Thanks to their ready compositional modularity, we report the design and synthesis of a new HUM, GEFSIX-21-Cu, incorporating the ligand pypz (4-(3,5-dimethyl-1H-pyrazol-4-yl)pyridine, 21) and GeF62- pillaring anions. GEFSIX-21-Cu delivers on two fronts: first, it displays an exceptionally high C2H2 adsorption capacity (≥5 mmol g-1) which is paired with low uptake of CO2 (<2 mmol g-1), and, second, a low enthalpy of adsorption for C2H2 (ca. 32 kJ mol-1). This combination is rarely seen in the C2H2 selective physisorbents reported thus far, and not observed in related isostructural HUMs featuring pypz and other pillaring anions. Dynamic column breakthrough experiments for 1:1 and 2:1 C2H2/CO2 mixtures revealed GEFSIX-21-Cu to selectively separate C2H2 from CO2, yielding ≥99.99% CO2 effluent purities. Temperature-programmed desorption experiments revealed full sorbent regeneration in <35 min at 60 °C, reinforcing HUMs as potentially technologically relevant materials for strategic gas separations.

Heteroleptic Tripalladium(II) Cages.

There is a concerted attempt to develop self-assembled metallo-cages of greater structural complexity, and heteroleptic PdII cages are emerging as prime candidates in these efforts. Most of these are dinuclear: few examples of higher nuclearity have been reported. We demonstrate here a robust method for the formation of tripalladium(II) cages from the 2 : 3 : 3 combination of a tritopic ligand, PdII , and a selection of ditopic ligands of the correct size and geometry.

Self-Assembly Synthesis of a [2]Catenane CoII Single-Molecule Magnet.

We describe herein the self-assembly synthesis of an octanuclear CoII [2]catenane {[Co4 (H2 L)6 ]2 16+ } formed by the mechanical interlocking of two {[Co4 (H2 L)6 ]8+ } rectangles of unprecedented topology. Subtle manipulation of the synthetic conditions allows the isolation of a mixed-valence [Co2 III /Co2 II ]10+ non-catenated rectangle. The CoII centers in the [2]catenane exhibit slow relaxation of their magnetic moment, i. e. single-molecule magnet properties, dominated by quantum tunneling and Raman relaxation processes. This work shows that metallo-supramolecular chemistry can precisely control the organization of single-molecule magnets in topologically complex arrangements.

Breaking the trade-off between selectivity and adsorption capacity for gas separation.

The trade-off between selectivity and adsorption capacity with porous materials is a major roadblock to reducing the energy footprint of gas separation technologies. To address this matter, we report herein a systematic crystal engineering study of C2H2 removal from CO2 in a family of hybrid ultramicroporous materials (HUMs). The HUMs are composed of the same organic linker ligand, 4-(3,5-dimethyl-1H-pyrazol-4-yl)pyridine, pypz, three inorganic pillar ligands, and two metal cations, thereby affording six isostructural pcu topology HUMs. All six HUMs exhibited strong binding sites for C2H2 and weaker affinity for CO2. The tuning of pore size and chemistry enabled by crystal engineering resulted in benchmark C2H2/CO2 separation performance. Fixed-bed dynamic column breakthrough experiments for an equimolar (v/v = 1:1) C2H2/CO2 binary gas mixture revealed that one sorbent, SIFSIX-21-Ni, was the first C2H2 selective sorbent that combines exceptional separation selectivity (27.7) with high adsorption capacity (4 mmol·g-1).

Functionalized Iron-Nitrogen-Carbon Electrocatalyst Provides a Reversible Electron Transfer Platform for Efficient Uranium Extraction from Seawater.

Uranium extraction from seawater provides an opportunity for sustainable fuel supply to nuclear power plants. Herein, an adsorption-electrocatalysis strategy is demonstrated for efficient uranium extraction from seawater using a functionalized iron-nitrogen-carbon (Fe-Nx -C-R) catalyst, comprising N-doped carbon capsules supporting FeNx single-atom sites and surface chelating amidoxime groups (R). The amidoxime groups bring hydrophilicity to the adsorbent and offer surface-specific binding sites for UO2 2+ capture. The site-isolated FeNx centres reduce adsorbed UO2 2+ to UO2 + . Subsequently, through electrochemical reduction of the FeNx sites, unstable U(V) ions are reoxidized to U(VI) in the presence of Na+ resulting in the generation of solid Na2 O(UO3 ·H2 O)x , which can easily be collected. Fe-Nx -C-R reduced the uranium concentration in seawater from ≈3.5 ppb to below 0.5 ppb with a calculated capacity of ≈1.2 mg g-1 within 24 h. To the best of the knowledge, the developed system is the first to use the adsorption of uranyl ions and electrodeposition of solid Na2 O(UO3 .H2 O)x for the extraction of uranium from seawater. The important discoveries guide technology development for the efficient extraction of uranium from seawater.

Thermal decomposition of hybrid ultramicroporous materials (HUMs).

Hybrid Ultramicroporous Materials (HUMs) are porous coordination materials with exemplary gas sorption and separation characteristics, but relatively poor thermal stability when compared to other porous coordination polymers or metal-organic frameworks (MOFs). The origin of this poor thermal stability has not yet been experimentally verified. Therefore, we investigate the thermal decomposition mechanisms of representative HUMs with the general formulae [M(SiF6)(L)2] or [M(SiF6)(L)(H2O)2], where M = Ni(ii), Cu(ii) or Zn(ii) and L = pyrazine or 4,4'-bipyridine. We find that two decomposition mechanisms dominate: (i) the fragmentation of the XF62- pillar into gaseous XF4 and fluoride, and (ii) direct sublimation of the N-donor ligand. The former process dictates the overall thermal stability of the material. We also demonstrate that HF is a possible decomposition product from certain hydrated HUM materials.

Synthetic Approaches to Metallo-Supramolecular CoII Polygons and Potential Use for H2O Oxidation.

Metal-directed self-assembly has been applied to prepare supramolecular coordination polygons which adopt tetrahedral (1) or trigonal disklike topologies (2). In the solid state, 2 assembles into a stable halide-metal-organic material (Hal-MOM-2), which catalyzes H2O oxidation under photo- and electrocatalytic conditions, operating with a maximum TON = 78 and TOF = 1.26 s-1. DFT calculations attribute the activity to a CoIII-oxyl species. This study provides the first account of how CoII imine based supramolecules can be employed as H2O oxidation catalysts.

Modulation of Crystal Packing via the Tuning of Peripheral Functionality for a Family of Dinuclear Mesocates.

A family of four novel pyrazinyl-hydrazone based ligands have been synthesized with differing functionality at the 5-position of the central aromatic ring. Previous work has shown such ligands to form dinuclear triple mesocates which pack to form hexagonal channels capable of gas sorption. The effect of the peripheral functionality of the ligand on the crystal packing was investigated by synthesizing complexes 1 to 4 which feature amino, bromo, iodo and methoxy substituents respectively. Complexes 1 to 3 crystallized in the same hexagonal space group P63 /m and featured 1D channels. However, on closer inspection while the packing of 1 is mediated by hydrogen bonding interactions, the packing of complexes 2 and 3 are not, due to a subtlety different π-π stacking interaction enforced by the halogen substituent. The more bulky nature of the methoxy substituent of 4 results in the complex crystallizing in the triclinic space group P-1, featuring an entirely different crystal packing.

Cyclic Aliphatic Hydrocarbons as Linkers in Metal-Organic Frameworks: New Frontiers for Ligand Design.

In this Minireview we outline the development of cyclic aliphatic moieties as ligands in metal-organic frameworks (MOFs), with a focus on the relationship between ligand design and synthesis and the properties of the subsequent materials. Aliphatic ligands have received considerably less attention than aromatic analogues in MOF chemistry but offer advantages in their unique combinations of geometric and electronic properties which are unattainable from conventional ligands. Here, we focus on rigid and semi-rigid backbone moieties derived from monocyclic and fused polycyclic aliphatic backbones, including cyclohexane and adamantane, cubane and bicyclo[2.2.2]octane, and discuss the synthetic chemistry of these species along with their potential importance as the next generation of building blocks for microporous materials.

Using Complementary Ligand Denticity to Direct Metallosupramolecular Structure about Metal Ions with Square-Planar Geometry.

An understanding of methods to control the structure of self-assembled architectures is central to the efforts of chemists interested in self-assembly synthesis. Metal ions with square-planar geometry are a favorite of metallosupramolecular chemists, as there are only a limited number of possible ligand arrangements around the metal ion. Two such arrangements, firstly four monodentate donor sites (1,1,1,1), and secondly two monodentate donors and one bidentate donor (1,1,2) exemplify the symmetry interaction and ligand directed approaches, respectively. Symmetry interaction approaches using two bidentate sites (2,2) or a monodentate and tridentate site (1,3) have not received the same level of attention. In these arrangements, two complementary sites combine at the metal ion(s). This Minireview seeks to detail strategies employed to direct structure in systems with these arrangements, and is illustrated with key exemplars from the field.

Highly efficient electrocatalytic hydrogen evolution promoted by O-Mo-C interfaces of ultrafine ß-Mo2C nanostructures.

Optimizing interfacial contacts and thus electron transfer phenomena in heterogeneous electrocatalysts is an effective approach for enhancing electrocatalytic performance. Herein, we successfully synthesized ultrafine ß-Mo2C nanoparticles confined within hollow capsules of nitrogen-doped porous carbon (ß-Mo2C@NPCC) and found that the surface layer of molybdenum atoms was further oxidized to a single Mo-O surface layer, thus producing intimate O-Mo-C interfaces. An arsenal of complementary technologies, including XPS, atomic-resolution HAADF-STEM, and XAS analysis clearly reveals the existence of O-Mo-C interfaces for these surface-engineered ultrafine nanostructures. The ß-Mo2C@NPCC electrocatalyst exhibited excellent electrocatalytic activity for the hydrogen evolution reaction (HER) in water. Theoretical studies indicate that the highly accessible ultrathin O-Mo-C interfaces serving as the active sites are crucial to the HER performance and underpinned the outstanding electrocatalytic performance of ß-Mo2C@NPCC. This proof-of-concept study opens a new avenue for the fabrication of highly efficient catalysts for HER and other applications, whilst further demonstrating the importance of exposed interfaces and interfacial contacts in efficient electrocatalysis.

Trace CO2 capture by an ultramicroporous physisorbent with low water affinity.

CO2 accumulation in confined spaces represents an increasing environmental and health problem. Trace CO2 capture remains an unmet challenge because human health risks can occur at 1000 parts per million (ppm), a level that challenges current generations of chemisorbents (high energy footprint and slow kinetics) and physisorbents (poor selectivity for CO2, especially versus water vapor, and/or poor hydrolytic stability). Here, dynamic breakthrough gas experiments conducted upon the ultramicroporous material SIFSIX-18-Ni-ß reveal trace (1000 to 10,000 ppm) CO2 removal from humid air. We attribute the performance of SIFSIX-18-Ni-ß to two factors that are usually mutually exclusive: a new type of strong CO2 binding site and hydrophobicity similar to ZIF-8. SIFSIX-18-Ni-ß also offers fast sorption kinetics to enable selective capture of CO2 over both N2 (S CN) and H2O (S CW), making it prototypal for a previously unknown class of physisorbents that exhibit effective trace CO2 capture under both dry and humid conditions.

A symmetry interaction approach to [M2L2]4+ metallocycles and their self-catenation.

Dinuclear [M2L2]n+ (M = Pd2+ or Pt2+) metallo-rectangles have a rich history, particularly in molecular recognition, including self-catenation events. These structures are mostly generated via a ligand-directed approach with cis-capped M2+ corners. We report here a symmetry interaction approach using 3 : 1 complementary denticity, giving a new route to [M2L2]4+ rectangles and their self-catenation.

Self-assembly and Cycling of a Three-state Pdx Ly Metallosupramolecular System.

The use of stimuli to induce reversible structural transformations in metallosupramolecular systems is of keen interest to chemists seeking to mimic the way that Nature effects conformational changes in biological machinery. While a wide array of stimuli have been deployed towards this end, stoichiometric changes have only been explored in a handful of examples. Furthermore, switching has generally been between only two distinct states. Here we use a simple 2-(1-(pyridine-4-methyl)-1H-1,2,3-triazol-4-yl)pyridine "click" ligand in combination with PdII in various stoichiometries and concentrations to quantitatively access and cycle between three distinct species: a [PdL2 ]2+ monomer, a [Pd2 L2 ]4+ dimer, and a [Pd9 L12 ]18+ cage.

Self-assembly of M4L4 tetrahedral cages incorporating pendant P[double bond, length as m-dash]S and P[double bond, length as m-dash]Se functionalised ligands.

Herein, the synthesis of metal-organic tetrahedral cages featuring flexible thio- and selenophosphate-based ligands is described. The cages were prepared by sub-component self-assembly of A[double bond, length as m-dash]P(OC6H4NH2-4)3 (A = S, Se) or S[double bond, length as m-dash]P(SC6H4NH2-4)3, 2-pyridinecarboxaldehyde, and either Fe[BF4]2 or Co[BF4]2. Preliminary host-guest studies into the ability of the pendant P[double bond, length as m-dash]S and P[double bond, length as m-dash]Se groups to interact with suitable substrates will be discussed.

Tunable Synthesis of Hollow Metal-Nitrogen-Carbon Capsules for Efficient Oxygen Reduction Catalysis in Proton Exchange Membrane Fuel Cells.

Atomically dispersed metal catalysts anchored on nitrogen-doped (N-doped) carbons demand attention due to their superior catalytic activity relative to that of metal nanoparticle catalysts in energy storage and conversion processes. Herein, we introduce a simple and versatile strategy for the synthesis of hollow N-doped carbon capsules that contain one or more atomically dispersed metals (denoted as H-M-Nx-C and H-Mmix-Nx-C, respectively, where M = Fe, Co, or Ni). This method utilizes the pyrolysis of nanostructured core-shell precursors produced by coating a zeolitic imidazolate framework core with a metal-tannic acid (M-TA) coordination polymer shell (containing up to three different metal cations). Pyrolysis of these core-shell precursors affords hollow N-doped carbon capsules containing monometal sites (e.g., Fe-Nx, CoNx, or Ni-Nx) or multimetal sites (Fe/Co-Nx, Fe/Ni-Nx, Co/Ni-Nx, or Fe/Co/Ni-Nx). This inventory allowed exploration of the relationship between catalyst composition and electrochemical activity for the oxygen reduction reaction (ORR) in acidic solution. H-Fe-Nx-C, H-Co-Nx-C, H-FeCo-Nx-C, H-FeNi-Nx-C, and H-FeCoNi-Nx-C were particularly efficient ORR catalysts in acidic solution. Furthermore, the H-Fe-Nx-C catalyst exhibited outstanding initial performance when applied as a cathode material in a proton exchange membrane fuel cell. The synthetic methodology introduced here thus provides a convenient route for developing next-generation catalysts based on earth-abundant components.

Solvent Dependent Spin-Crossover and Photomagnetic Properties in an Imidazolylimine FeII Complex.

The synthesis and physico-chemical characterization of an FeII complex [Fe(L1)3 ](ClO4 )2 ⋅CH3 CN⋅0.5H2 O, 1, incorporating a bidentate imidazolylimine-based ligand are reported. Complex 1 crystallises as the mer-isomer and the crystal lattice is replete with hydrogen bonding interactions between ClO4 - anions, solvent molecules and imidazole N-H groups. Variable-temperature structural, magnetic, photomagnetic and optical reflectivity techniques have been deployed to fully characterise the spin-crossover (SCO) behaviour in 1 along with its desolvated phase, 1⋅desolv. Variable-temperature (1.8-300 K) magnetic-susceptibility measurements reveal a broad two-step full SCO for 1 (T1/2 =158 and 184 K) and photomagnetic experiments at 10 K under white-light irradiation revealed complete photo-induced SCO. 1⋅desolv displays considerably different magnetic behaviour with sharp single-step SCO accompanied by a thermal hysteresis (T1/2↑ =105 K, T1/2↓ =95 K) in addition to full photo-induced SCO at lower temperatures.

Reversible Transformation between a [PdL2 ]2+ "Figure-of-Eight" Complex and a [Pd2 L2 ]4+ Dimer: Switching On and Off Self-Recognition.

Structural changes to metallosupramolecular assemblies resulting in the release or uptake of guests are currently well established, whereas transformations turning on and off specific self-recognition are far less developed. We report a novel ligand (2,6-bis(1-(3-pyridin-4-yl)phenyl-1H-1,2,3-triazol-4-yl)pyridine) possessing a tridentate central metal-binding site flanked by two pendant pyridyl arms. In a 2:1 ratio with PdII metal ions, a spiro-type [PdL2 ]2+ "Figure-of-eight" complex forms with the central tridentate binding pocket unoccupied. The introduction of an additional one equivalent of PdII metal ion results in the conversion to a dimeric [Pd2 L2 ]4+ molecule with the tridentate pocket occupied. There is site-specific self-recognition between dimers in solution with strong NOE peaks between adjacent molecules. The self-recognition between dimers can be turned off in two ways: firstly, adding another equivalent of PdII metal ion brings about binding to the previously uncoordinated pyridyl arms that are key to the self-recognition event, and; secondly, addition of sufficient ligand to return the stoichiometry to 2:1 regenerates the [PdL2 ]2+ complex. Hence, the self-recognition event can be turned on or off through simple variation of L:PdII stoichiometry.

A supramolecular porous material comprising Fe(ii) mesocates.

The dinuclear mesocate [Fe2L3](BF4)4, 1, is a supramolecular building block for a microporous material. Structural analysis reveals that extensive noncovalent interactions in the solid state generate a 3D framework with microporous channels. These channels are permanently accessible to incoming guest molecules and adsorption isotherms demonstrate that the material has a high selectivity for CO2 over N2.