Multicomponent Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) are constructed from metal ions or clusters and organic linkers. Typical MOFs are rather simple, comprising just one type of joint and linker. An additional degree of structural complexity can be introduced by using multiple different components that are assembled into the same framework In the early days of MOF chemistry, conventional wisdom held that attempting to prepare frameworks starting from such a broad set of components would lead to multiple different phases. However, this review highlights how this view was mistaken and frameworks comprising multiple different components can be deliberately designed and synthesized. When coupled to structural order and periodicity, the presence of multiple components leads to exceptional functional properties that can be understood at the atomic level.

Trisequential Postsynthetic Modification of a Tagged IRMOF-9 Framework.

Tailoring the pore environments of metal-organic frameworks (MOFs) is key to improving their performance and expanding their applicability. Postsynthetic methods, wherein an already synthesized MOF undergoes further chemical reactions, present many advantages for such tailoring and lead to much interesting new chemistry. However, this method has seldom been pushed farther than two reaction steps on the organic component. Here we report a three-step sequence starting from an alkenyl group on the biphenyl backbone of an IRMOF-9 analogue. The alkene is converted to an oxirane group and subsequently to a 1,2-azidoalcohol. The ultimate product is a framework functionalized with an aziridine ring. The reaction efficiency of each step is high, which suppresses the formation of undesired functional groups and the buildup of unintended multivariate frameworks. The synthesis of each framework was attempted via a direct synthetic method employing the appropriately functionalized biphenyldicarboxylate ligand. In general, this met with failure, which demonstrates the power and utility of postsynthetic methods for preparing new materials.

Pressure promoted low-temperature melting of metal-organic frameworks.

Metal-organic frameworks (MOFs) are microporous materials with huge potential for chemical processes. Structural collapse at high pressure, and transitions to liquid states at high temperature, have recently been observed in the zeolitic imidazolate framework (ZIF) family of MOFs. Here, we show that simultaneous high-pressure and high-temperature conditions result in complex behaviour in ZIF-62 and ZIF-4, with distinct high- and low-density amorphous phases occurring over different regions of the pressure-temperature phase diagram. In situ powder X-ray diffraction, Raman spectroscopy and optical microscopy reveal that the stability of the liquid MOF state expands substantially towards lower temperatures at intermediate, industrially achievable pressures and first-principles molecular dynamics show that softening of the framework coordination with pressure makes melting thermodynamically easier. Furthermore, the MOF glass formed by melt quenching the high-temperature liquid possesses permanent, accessible porosity. Our results thus imply a route to the synthesis of functional MOF glasses at low temperatures, avoiding decomposition on heating at ambient pressure.

Thermal Elimination of Ethylene from Cyclobutyl Groups Characterized by X-ray Crystallography in a Metal-Organic Framework Matrix.

Methods to synthesize and characterize aromatic molecules with vinyl substituents are sought after yet limited in the literature. Here, we introduce cyclobutyl groups into a metal-organic framework (MOF) matrix that are poised to produce ethylene upon heating. The expulsion of ethylene produces vinyl groups on an aromatic core, which are isolated by the crystalline matrix of the framework. This enables full characterization of the thermolysis by single-crystal X-ray diffraction. Further, we modify the vinyl groups by a bromine addition reaction. Importantly, the two transformations happen in a single-crystal-to-single-crystal manner without changing the overall network structure of the parent framework. New insights into the structural and synthetic chemistry of this important class of compound are generated. Installing reactive vinyl tags in materials by the high temperature thermolysis of cyclobutyl groups is a powerful strategy for altering their physicochemical characteristics.

Overcoming Fundamental Limitations in Adsorbent Design: Alkene Adsorption by Non-porous Copper(I) Complexes.

Purifying alkenes from alkanes requires cryogenic distillation. This consumes energy equivalent to countries of ca. 5 million people. Replacing distillation with adsorption processes would significantly increase energy efficiency. Trade-offs between kinetics, selectivity, capacity, and heat of adsorption have prevented production of an optimal adsorbent. We report adsorbents that overcome these trade-offs. [Cu-Br]3 and [Cu-H]3 are air-stable trinuclear complexes that undergo reversible solid-state inter-molecular rearrangements to produce dinuclear [Cu-Br⋅(alkene)]2 and [Cu-H⋅(alkene)]2 . The reversible solid-state rearrangement, confirmed in situ using powder X-ray diffraction, allows adsorbent design trade-offs to be overcome, coupling low heat of adsorption (-10 to -17 kJ mol-1alkene ), high alkene:alkane selectivity (47; 29), and uptake capacity (>2.5 molalkene mol-1Cu3 ). Most remarkably, [Cu-H]3 displays fast uptake and regenerates capacity within 10 minutes.

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.

Catalysts Confined in Programmed Framework Pores Enable New Transformations and Tune Reaction Efficiency and Selectivity.

Controlling chemical reactions in porous heterogeneous catalysts is a tremendous challenge because of the difficulty in producing uniform active sites that can be tuned with precision. However, analogous to enzymes, when a catalytic pocket provides complementary close contacts and favorable intermolecular interactions with the reaction participants, the reaction efficiency and selectivity may be tuned. Here, we report an isoreticular family of catalysts based on the multicomponent metal-organic framework MUF-77. The microenvironment around the site of catalysis was successfully programmed by introducing functional groups (modulators) to the organic linkers at sites remote from the catalytic unit. The framework catalysts produced in this way exhibit several unique features, including the simultaneous enhancement of both reactivity and stereochemical selectivity in aldol reactions, the ability to catalyze Henry reactions that cannot be accomplished by homogeneous analogs, and discrimination between different reaction pathways (Henry versus aldol) that compete for a common substrate.

A Robust Ethane-Trapping Metal-Organic Framework with a High Capacity for Ethylene Purification.

The separation of ethane from ethylene is of prime importance in the purification of chemical feedstocks for industrial manufacturing. However, differentiating these compounds is notoriously difficult due to their similar physicochemical properties. High-performance porous adsorbents provide a solution. Conventional adsorbents trap ethylene in preference to ethane, but this incurs multiple steps in separation processes. Alternatively, high-purity ethylene can be obtained in a single step if the adsorbent preferentially adsorbs ethane over ethylene. We herein report a metal-organic framework, MUF-15 (MUF, Massey University Framework), constructed from inexpensive precursors that sequesters ethane from ethane/ethylene mixtures. The productivity of this material is exceptional: 1 kg of MOF produces 14 L of polymer-grade ethylene gas in a single adsorption step starting from an equimolar ethane/ethylene mixture. Computational simulations illustrate the underlying mechanism of guest adsorption. The separation performance was assessed by measuring multicomponent breakthrough curves, which illustrate that the separation performance is maintained over a wide range of feed compositions and operating pressures. MUF-15 is robust, maintains its performance in the presence of acetylene, and is easily regenerated by purging with inert gas or by placing under reduced pressure.

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.

Harnessing Bottom-Up Self-Assembly To Position Five Distinct Components in an Ordered Porous Framework.

Positioning a diverse set of building blocks in a well-defined array enables cooperativity amongst them and the systematic programming of functional properties. The extension of this concept to porous metal-organic frameworks (MOFs) is challenging since the installation of multiple components in a well-ordered framework requires careful design of the lattice topology, judicious selection of building blocks, and precise control of the crystallization parameters. Herein, we report how we met these challenges to prepare the first quinary MOF structure, FDM-8, by bottom-up self-assembly from two metals, ZnII and CuI , and three distinct carboxylate- and pyrazolate-based linkers. With a surface area of 3643 m2 g-1 , FDM-8 contains hierarchical pores and shows outstanding methane-storage capacity at high pressure. Furthermore, functional groups introduced on the linkers became compartmentalized in predetermined arrays in the pores of the FDM-8 framework.

The First Observation of Hidden Hysteresis in an Iron(III) Spin-Crossover Complex.

Molecular magnetic switches are expected to form the functional components of future nanodevices. Herein we combine detailed (photo-) crystallography and magnetic studies to reveal the unusual switching properties of an iron(III) complex, between low (LS) and high (HS) spin states. On cooling, it exhibits a partial thermal conversion associated with a reconstructive phase transition from a [HS-HS] to a [LS-HS] phase with a hysteresis of 25 K. Photoexcitation at low temperature allows access to a [LS-LS] phase, never observed at thermal equilibrium. As well as reporting the first iron(III) spin crossover complex to exhibit reverse-LIESST (light-induced excited spin state trapping), we also reveal a hidden hysteresis of 30 K between the hidden [LS-LS] and [HS-LS] phases. Moreover, we demonstrate that FeIII spin-crossover (SCO) complexes can be just as effective as FeII systems, and with the advantage of being air-stable, they are ideally suited for use in molecular electronics.

Systematic Tuning of the Luminescence Output of Multicomponent Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) exhibit a broad range of luminescence characteristics due to the vast array of metal ions and organic linkers available as building blocks. Systematic control over the emissive output of MOFs is highly sought after. Methods for tuning emission profiles are emerging based largely on luminescent metal ions and the encapsulation of emissive guests. Herein, we show how the functionalization of the organic linkers of a series of multicomponent MUF-77 (MUF = Massey University Framework) materials can methodically tune their spectral output. This was quantified by chromaticity diagrams. White-light emission was obtained by combining the photophysical characteristics of the three distinct organic fluorophores present in these materials. Our results also show that both (i) energy transfer interactions between the organic components and (ii) noncovalent interactions with guests can also be harnessed to tune the emission. These results establish multicomponent metal-organic frameworks as fluorescent materials with unique spectral characteristics.

Modulating the Performance of an Asymmetric Organocatalyst by Tuning Its Spatial Environment in a Metal-Organic Framework.

Systematically tuning the spatial environment around the active sites of synthetic catalysts is a difficult challenge. Here, we show how this can be accomplished in the pores of multicomponent metal-organic frameworks. This relies on embedding a catalytic unit in a pore of the MUF-77 framework and then tuning its environment by introducing different functional groups to the surrounding linkers. This approach benefits from the structural regularity of MUF-77, which places each component in a precise location to circumvent disorder. Prolinyl groups, which are catalytically competent toward asymmetric aldol reactions, were selected as the catalytic unit. Since every prolinyl group is positioned in an identical environment, correlations between the pore architecture and the activity of these single-site catalysts can be elucidated. Systematic engineering of the pore structure, which is achieved by installing modulator groups on the framework linkers, impacts on the reaction rate and the enantiomeric excess of the aldol products. Furthermore, the spatial environment around the proline catalyst can override its innate stereochemical preference to dictate the preferred enantiomer of the reaction product. These results offer a new way to design three-dimensional active site environments for synthetic catalysts.

An Isoreticular Series of Zinc(II) Metal-Organic Frameworks Derived from Terpyridylcarboxylate Ligands.

An isoreticular family of seven microporous metal-organic frameworks of the general form [ZnL] have been synthesized, where L is a 4'-substituted 2,2':6',2″-terpyridine-4,4″-dicarboxylate ligand. Each framework adopts an interpenetrated zeolitic gismondine (gis-c) topology and possesses one-dimensional square channels with ca. 9.0 Šapertures running down the crystallographic c axis. Gas adsorption measurements with N2, H2, CH4, and CO2 confirm their permanent porosity. The ligand functional groups, which include phenyl, 2-tolyl, 4-chlorophenyl, 4-nitrophenyl, 2-thienyl, 3-thienyl, and 4-pyridyl, line the channel walls and tune the gas adsorption properties of these materials.

Catalytically Active Bimetallic Nanoparticles Supported on Porous Carbon Capsules Derived From Metal-Organic Framework Composites.

We report a new methodology for producing monometallic or bimetallic nanoparticles confined within hollow nitrogen-doped porous carbon capsules. The capsules are derived from metal-organic framework (MOF) crystals that are coated with a shell of a secondary material comprising either a metal-tannic acid coordination polymer or a resorcinol-formaldehyde polymer. Platinum nanoparticles are optionally sandwiched between the MOF core and the shell. Pyrolysis of the MOF-shell composites produces hollow capsules of porous nitrogen-doped carbon that bear either monometallic (Pt, Co, and Ni) or alloyed (PtCo and PtNi) metal nanoparticles. The Co and Ni components of the bimetallic nanoparticles are derived from the shell surrounding the MOF crystals. The hollow capsules prevent sintering and detachment of the nanoparticles, and their porous walls allow for efficient mass transport. Alloyed PtCo nanoparticles embedded in the capsule walls are highly active, selective, and recyclable catalysts for the hydrogenation of nitroarenes to anilines.

Systematic ligand modulation enhances the moisture stability and gas sorption characteristics of quaternary metal-organic frameworks.

Complex metal-organic frameworks (MOFs) that maintain high structural order promise sophisticated and tunable properties. Here, we build on our strategy of using combinations of structurally distinct ligands to generate a new isoreticular series of ordered quaternary Zn4O-carboxylate MOFs. Rational design of the framework components steers the system toward multicomponent MOFs and away from competing phases during synthesis. Systematic ligand modulation led to the identification of a set of frameworks with unusually high stability toward water vapor. These frameworks lose no porosity after 100 days' exposure to ambient air or 20 adsorption-desorption cycles up to 70% relative humidity. Across this series of frameworks, a counterintuitive relationship between the length of pendant alkyl groups and framework stability toward water vapor emerges. This phenomenon was probed via a series of gas and vapor adsorption experiments together with Grand Canonical Monte Carlo (GCMC) simulations, and could be rationalized on the basis of the propensity of the frameworks to adsorb water vapor and the proximity of the adsorbed water molecules to the water-sensitive metal clusters. Systematic variation of the pore volume and topography also tunes the CO2 and CH4 gas adsorption behavior. Certain of these materials display increases in their adsorption capacities of 237% (CO2) and 172% (CH4) compared to the parent framework.

Metal-organic framework nanocrystals as sacrificial templates for hollow and exceptionally porous titania and composite materials.

We report a strategy that employs metal-organic framework (MOF) crystals in two roles for the fabrication of hollow nanomaterials. In the first role the MOF crystals provide a template on which a shell of material can be deposited. Etching of the MOF produces a hollow structure with a predetermined size and morphology. In combination with this strategy, the MOF crystals, including guest molecules in their pores, can provide the components of a secondary material that is deposited inside the initially formed shell. We used this approach to develop a straightforward and reproducible method for constructing well-defined, nonspherical hollow and exceptionally porous titania and titania-based composite nanomaterials. Uniform hollow nanostructures of amorphous titania, which assume the cubic or polyhedral shape of the original template, are delivered using nano- and microsized ZIF-8 and ZIF-67 crystal templates. These materials exhibit outstanding textural properties including hierarchical pore structures and BET surface areas of up to 800 m(2)/g. As a proof of principle, we further demonstrate that metal nanoparticles such as Pt nanoparticles, can be encapsulated into the TiO2 shell during the digestion process and used for subsequent heterogeneous catalysis. In addition, we show that the core components of the ZIF nanocrystals, along with their adsorbed guests, can be used as precursors for the formation of secondary materials, following their thermal decomposition, to produce hollow and porous metal sulfide/titania or metal oxide/titania composite nanostructures.

Functionalisation of MUF-15 enhances CO2/CH4 selectivity in mixed-matrix membranes.

MUF-15 (MUF = Massey University Framework) is a metal-organic framework with pores that can be tuned by ligand functionalisation. Crystallites of MUF-15 and derivatives were blended with the organic polymer 6FDA-DAM to produce mixed-matrix membranes (MMMs). At a loading of 30 wt%, membranes with MUF-15-F, MUF-15 with an appended fluoro group, exhibited a CO2 permeability of 1300 Barrer and CO2/CH4 selectivity of 37.1. These values surpass membranes with the parent MUF-15 and exceed the Robeson upper bound.

Programmed pore architectures in modular quaternary metal-organic frameworks.

To generate metal-organic frameworks (MOFs) that are complex and modular yet well ordered, we present a strategy employing a family of three topologically distinct linkers that codes for the assembly of a highly porous quaternary MOF. By introducing substituted analogues of the ligands, a set of eight isoreticular frameworks is delivered, with the MOF structure systematically varied while the topology is maintained. To combat randomness and disorder, the substitution patterns of the ligands are designed to be compatible with their crystallographic site symmetries. MOFs produced in this way feature "programmed pores"--multiple functional groups compartmentalized in a predetermined array within a periodic lattice--and are capable of complex functional behavior. In these examples unconventional CO2 sorption trends, including capacity enhancements close to 100%, emerge from synergistic effects. Future PP-MOFs may be capable of enzyme-like heterogeneous catalysis and ultraselective adsorption.

µ-1,6-Dioxo-1,6-di-phenyl-hexane-3,4-diolato-bis-[(2,2'-bi-pyridine)-chlorido-copper(II)] dihydrate.

The reaction of CuCl2 with 1,6-diphenyl-1,3,5,6-hexa-netetrone and 2,2'-bi-pyridine (bipy) in ethanol gave crystals of the corresponding bimetallic complex, [Cu2(C18H12O4)Cl2(C10H8N2)2]·2H2O. The mol-ecule is centrosymmetric with each CuII ion coordinated to two oxygen atoms from the tetronediate, two nitro-gen atoms from a bipy ligand and one coordinated chloride ion. A water mol-ecule of crystallization forms hydrogen bonds to the chloride ions, linking the mol-ecules into a chain parallel to the bc-face diagonal.