Unusual and Highly Tunable Missing-Linker Defects in Zirconium Metal–Organic Framework UiO-66 and Their Important Effects on Gas Adsorption

Defining the Proton Topology of the Zr6-Based Metal-Organic Framework NU-1000

Metal-organic frameworks (MOFs) constructed from Zr6-based nodes have recently received considerable attention given their exceptional thermal, chemical, and mechanical stability. Because of this, the structural diversity of Zr6-based MOFs has expanded considerably and in turn given rise to difficulty in their precise characterization. In particular it has been difficult to assign where protons (needed for charge balance) reside on some Zr6-based nodes. Elucidating the precise proton topologies in Zr6-based MOFs will have wide ranging implications in defining their chemical reactivity, acid/base characteristics, conductivity, and chemical catalysis. Here we have used a combined quantum mechanical and experimental approach to elucidate the precise proton topology of the Zr6-based framework NU-1000. Our data indicate that a mixed node topology, [Zr6(μ3-O)4(μ3-OH)4(OH)4 (OH2)4](8+), is preferred and simultaneously rule out five alternative node topologies.

Crystallography of metal-organic frameworks

Metal-organic frameworks (MOFs) are one of the most intensely studied material types in recent times. Their networks, resulting from the formation of strong bonds between inorganic and organic building units, offer unparalled chemical diversity and pore environments of growing complexity. Therefore, advances in single-crystal X-ray diffraction equipment and techniques are required to characterize materials with increasingly larger surface areas, and more complex linkers. In addition, whilst structure solution from powder diffraction data is possible, the area is much less populated and we detail the current efforts going on here. We also review the growing number of reports on diffraction under non-ambient conditions, including the response of MOF structures to very high pressures. Such experiments are important due to the expected presence of stresses in proposed applications of MOFs - evidence suggesting rich and complex behaviour. Given the entwined and inseparable nature of their structure, properties and applications, it is essential that the field of structural elucidation is able to continue growing and advancing, so as not to provide a rate-limiting step on characterization of their properties and incorporation into devices and applications. This review has been prepared with this in mind.

A metal-organic framework containing unusual eight-connected Zr-oxo secondary building units and orthogonal carboxylic acids for ultra-sensitive metal detection

Two metal-organic frameworks (MOFs) with Zr-oxo secondary building units (SBUs) were prepared by using p,p'-terphenyldicarboxylate (TPDC) bridging ligands pre-functionalized with orthogonal succinic acid (MOF-1) and maleic acid groups (MOF-2). Single-crystal X-ray structure analysis of MOF-1 provides the first direct evidence for eight-connected SBUs in UiO-type MOFs. In contrast, MOF-2 contains twelve-connected SBUs as seen in the traditional UiO MOF topology. These structural assignments were confirmed by extended X-ray absorption fine structure (EXAFS) analysis. The highly porous MOF-1 is an excellent fluorescence sensor for metal ions with the detection limit of <0.5 ppb for Mn(2+) and three to four orders of magnitude greater sensitivity for metal ions than previously reported luminescent MOFs.

A combinatorial approach towards water-stable metal-organic frameworks for highly efficient carbon dioxide separation

A library of 20 UiO-66-derived metal-organic frameworks (MOFs) is synthesized in a combinatorial approach involving mixed ligand copolymerization and two post-synthetic modifications (PSMs) in tandem. Mixed ligand co-polymerization of benzene-1,4-dicarboxylic acid (BDC) and sodium 2-sulfoterephthalate (SS-BDC) with zirconium tetrachloride (ZrCl4 ) was used to prepare 5 groups of MOFs with the same UiO-66 topology but differing amounts of sulfate groups. These MOFs exhibit excellent water stabilities in a pH range of 1 to 12, together with high CO2 uptake capacities and selectivities.

Increasing the Stability of Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are a new category of advanced porous materials undergoing study by many researchers for their vast variety of both novel structures and potentially useful properties arising from them. Their high porosities, tunable structures, and convenient process of introducing both customizable functional groups and unsaturated metal centers have afforded excellent gas sorption and separation ability, catalytic activity, luminescent properties, and more. However, the robustness and reactivity of a given framework are largely dependent on its metal-ligand interactions, where the metal-containing clusters are often vulnerable to ligand substitution by water or other nucleophiles, meaning that the frameworks may collapse upon exposure even to moist air. Other frameworks may collapse upon thermal or vacuum treatment or simply over time. This instability limits the practical uses of many MOFs. In order to further enhance the stability of the framework, many different approaches, such as the utilization of high-valence metal ions or nitrogen-donor ligands, were recently investigated. This review details the efforts of both our research group and others to synthesize MOFs possessing drastically increased chemical and thermal stability, in addition to exemplary performance for catalysis, gas sorption, and separation.

Correlated defect nanoregions in a metal-organic framework

Throughout much of condensed matter science, correlated disorder is a key to material function. While structural and compositional defects are known to exist within a variety of metal-organic frameworks (MOFs), the prevailing understanding is that these defects are only ever included in a random manner. Here we show--using a combination of diffuse scattering, electron microscopy, anomalous X-ray scattering and pair distribution function measurements--that correlations between defects can in fact be introduced and controlled within a hafnium terephthalate MOF. The nanoscale defect structures that emerge are an analogue of correlated Schottky vacancies in rocksalt-structured transition metal monoxides and have implications for storage, transport, optical and mechanical responses. Our results suggest how the diffraction behaviour of some MOFs might be reinterpreted, and establish a strategy of exploiting correlated nanoscale disorder as a targetable and desirable motif in MOF design.

Rational design of metal–organic frameworks with anticipated porosities and functionalities

This highlight review will outline the recent advances on rational design of MOFs from both our and other groups based on their structure–property relationships, and provide a systematic overview of different methods for rational design of MOFs with desired porosities and functionalities.

A novel porous metal–organic framework from a new bis(acylhydrazone) ligand capable of reversibly adsorbing/desorbing water and small alcohol molecules

Weak coordination and π–π interactions link the metallomacrocycles into two interpenetrating 3D networks to form the material with hydrophilic pores.

Syntheses, crystal structures, and optical properties of five metal complexes constructed from a V-shaped thiophene-containing ligand and different dicarboxylate ligands

Five new metal complexes constructed from the V-shaped ligand 2,8-di(1H-imidazol-1-yl)dibenzothiophene (DIDP) and different aromatic dicarboxylic acids have been synthesized and structurally characterized in detail. All these complexes show optical semiconductive properties.

High valence 3p and transition metal based MOFs

This article focuses on high valence 3p and transition metal based metal organic frameworks.

Tuning pore size in a zirconium–tricarboxylate metal–organic framework

The water-stable frameworks, [Zr₆O₄(OH)₄(X)₆(btc)₂]·nH₂O, where X = formate, acetate, or propionate, exhibit tunable porosity by virtue of systematic modulation of the chain length of the monocarboxylate ligand X.

A molecular porous zirconium–organic material exhibiting highly selective CO2 adsorption, high thermal stability, reversible hydration, facile ligand exchange and exclusive dimerization of phenylacetylene

A multi-functional molecular porous zirconium-organic material, CAUMOF-12, has been discovered.

Water adsorption in UiO-66: the importance of defects

Density distributions of water in an ideal UiO-66 unit cell (left) and a unit cell missing linkers (right).

Band gap modulation of functionalized metal–organic frameworks

Metal–organic frameworks (MOFs) have been envisioned as alternatives to planar metallic catalysts for solar-to-fuel conversion.