Superstructure of a Substituted Zeolitic Imidazolate Metal–Organic Framework Determined by Combining Proton Solid‐State NMR Spectroscopy and DFT Calculations

We report the supercell crystal structure of a ZIF‐8 analog substituted imidazolate metal–organic framework (SIM‐1) obtained by combining solid‐state nuclear magnetic resonance and powder X‐ray diffraction experiments with density functional theory calculations.     

Selective Host–Guest Interaction between Metal Ions and Metal–Organic Frameworks Using Dynamic Nuclear Polarization Enhanced Solid‐State NMR Spectroscopy

The host–guest interaction between metal ions (Pt²⁺ and Cu²⁺) and a zirconium metal–organic framework (UiO‐66‐NH₂) was explored using dynamic nuclear polarization‐enhanced ¹⁵N{¹H} CPMAS NMR spectroscopy supported by X‐ray absorption spectroscopy and density functional calculations. The combined experimental results conclude that each Pt²⁺ coordinates with two NH₂ groups from the MOF and two Cl^? from the metal precursor, whereas Cu²⁺ do not form chemical bonds with the NH₂ groups of the MOF framework. Density functional calculations reveal that Pt²⁺ prefers a square‐planar structure with the four ligands and resides in the octahedral cage of the MOF in either cis or trans configurations.     

Postsynthetic Improvement of the Physical Properties in a Metal–Organic Framework through a Single Crystal to Single Crystal Transmetallation

A single crystal to single crystal transmetallation process takes place in the three‐dimensional (3D) metal–organic framework (MOF) of formula Mg^II₂{Mg^II₄[Cu^II₂(Me₃mpba)₂]₃}?45 H₂O ( 1 ; Me₃mpba^4?=N,N′‐2,4,6‐trimethyl‐1,3‐phenylenebis(oxamate)). After complete replacement of the Mg^II ions within the coordination network and those hosted in the channels by either Co^II or Ni^II ions, 1 is transmetallated to yield two novel MOFs of formulae Co₂^II{Co^II₄[Cu^II₂(Me₃mpba)₂]₃}?56 H₂O ( 2 ) and Ni₂^II{Ni^II₄[Cu^II₂(Me₃mpba)₂]₃}? 54 H₂O ( 3 ). This unique postsynthetic metal substitution affords materials with higher structural stability leading to enhanced gas sorption and magnetic properties.     

A Series of Lanthanide Metal–Organic Frameworks with Interesting Adjustable Photoluminescence Constructed by Helical Chains

Based on the isonicotinic acid (HIN=pyridine‐4‐carboxylic acid), seven lanthanide metal–organic frameworks (MOFs) with the formula [Ln(IN)₂L] (Ln=Eu ( 1 ), Tb ( 2 ), Er ( 3 ), Dy ( 4 ), Ho ( 5 ), Gd ( 6 ), La ( 7 ), L=OCH₂CH₂OH) have been synthesized by mixing Ln₂O₃ with HIN under solvothermal conditions, and characterized by single‐crystal X‐ray diffraction, powder X‐ray diffraction, infrared spectroscopy, and fluorescence spectroscopy. Crystal structural analysis shows that compounds 1–6 are isostructural, crystallize in a chiral space group P2₁2₁2₁, whereas compound 7 crystallizes in space group C2/c. Nevertheless, they all consist of new intertwined chains. Simultaneously, on the basis of the above‐mentioned compounds, we have realized a rational design strategy to form the doped Ln MOFs [(Eu_xTb_1?x)(IN)₂L] (x=0.35 ( 8 ), x=0.19 ( 9 ), x=0.06 ( 10 )) by utilizing Tb^III as the second “rare‐earth metal”. Interestingly, the photoluminescence of [(Eu_xTb_1?x)(IN)₂L] are not only adjustable by the ratios of Eu/Tb, but also temperature or excitation wavelength.     

Pressure‐Induced Bond Rearrangement and Reversible Phase Transformation in a Metal–Organic Framework

Pressure‐induced phase transformations (PIPTs) occur in a wide range of materials. In general, the bonding characteristics, before and after the PIPT, remain invariant in most materials, and the bond rearrangement is usually irreversible due to the strain induced under pressure. A reversible PIPT associated with a substantial bond rearrangement has been found in a metal–organic framework material, namely [tmenH₂][Er(HCOO)₄]₂ (tmenH₂²⁺=N,N,N′,N′‐tetramethylethylenediammonium). The transition is first‐order and is accompanied by a unit cell volume change of about 10 %. High‐pressure single‐crystal X‐ray diffraction studies reveal the complex bond rearrangement through the transition. The reversible nature of the transition is confirmed by means of independent nanoindentation measurements on single crystals.     

The Construction of Open GdIII Metal–Organic Frameworks Based on Methanetriacetic Acid: New Objects with an Old Ligand

The preparation, X‐ray crystallography and magnetic investigation of the first examples of methanetriacetate (mta)‐containing lanthanide(III) complexes of formulae [Gd(mta)(H₂O)₃]_n ? 4 n H₂O ( 1 ) [Gd(mta)(H₂O)₃]_n ? 2 n H₂O ( 2 ) and [Gd₂(mta)₂(H₂O)₂]_n ? 2 n H₂O ( 3 ) are described herein. This tripodal ligand promotes the formation of 6³ networks; thus 1 consists of a honeycomb structure, whereas in 2 two of these layers are condensed to form a rare five‐connected two‐dimensional (4⁸6²) network. Compound 3 can be seen as an aggregation of 6³ layers leading to a three‐dimensional (6,6)‐connected binodal (4¹²6³)(4⁹6⁶)‐ nia net, in which the gadolinium(III) ions and the mta ligands act as octahedral and as trigonal prismatic nodes, respectively. The magnetic properties of 1 – 3 were investigated in the temperature range 1.9–300 K. A close fit to the Curie law ( 1 ) and weak either antiferro‐ [J=?0.0063(1) cm^?1 ( 2 )] or ferromagnetic [J=+0.0264(6) cm^?1 ( 3 )] interactions between the Gd^III ions are observed; the different exchange pathways involved [extended tris‐bidentate mta ( 1 ) and μ‐O(1);κ²O(1),O(2) ( 2 and 3 ) plus single syn–syn carboxylate‐mta ( 3 )] accounting for these magnetic features. The nature and magnitude of the magnetic interactions, between the Gd^III ions in 1 – 3 , agree with the small amount of data existing in the literature for these kind of bridges.     

A Versatile AlIII‐Based Metal–Organic Framework with High Physicochemical Stability

A unique Al^III‐based metal–organic framework (467‐MOF) with two types of square channels has been designed and synthesized by using a flexible tricarboxylate ligand under solvothermal conditions. 467‐MOF exhibits superior thermal and chemical stability and, moreover, shows high CO₂ sorption selectivity over H₂, with a selectivity, based on the ideal adsorbed solution theory (IAST) of approximately 45 at 273 or 293 K. Furthermore, its solvent‐dependent photoluminescence makes it an applicable sensor in the detection of nitrobenzene explosives through fluorescence quenching.     

Switch‐On Fluorescence of a Perylene‐Dye‐Functionalized Metal–Organic Framework through Postsynthetic Modification

A perylene dye was introduced directly as a linker into a metal–organic framework (MOF) during synthesis. Depending on the dye concentration in the MOF synthesis mixture, different fluorescent materials were generated. The successful incorporation of the dye was proven by using ¹³C and ²⁷Al MAS NMR spectroscopy, by solution NMR spectroscopy after digestion of the MOF sample, and by synthesizing a reference dye without connecting groups, which could coordinate on the metal–oxo cluster inside the MOF. Fluorescence quenching effects of the MOF linker, 2‐aminoterephthalate, were observed and overcome by postsynthetic modification with acetic anhydride. We show here for the first time that amino groups, which can be used as anchoring points for covalent attachment of other molecules, are responsible for fluorescence quenching. Thus, a very promising strategy to implement switchable fluorescence into MOFs is shown here.     

Control of Channel Size for Selective Guest Inclusion with Inlaid Anionic Building Blocks in a Porous Cationic Metal–Organic Host Framework

Step by step : By attaching anionic building blocks of variable bulk to a cationic metal–organic framework, stepwise channel‐size adjustment of the resulting porous three‐dimensional host framework is achieved (see picture). This method is a new and viable approach for materials with predesigned nanopores for application in molecular recognition and selective guest inclusion.

     

Characterization of Zn‐Containing Metal–Organic Frameworks by Solid‐State 67Zn NMR Spectroscopy and Computational Modeling

Metal–organic frameworks (MOFs) are an extremely important class of porous materials with many applications. The metal centers in many important MOFs are zinc cations. However, their Zn environments have not been characterized directly by ⁶⁷Zn solid‐state NMR (SSNMR) spectroscopy. This is because ⁶⁷Zn (I=5/2) is unreceptive with many unfavorable NMR characteristics, leading to very low sensitivity. In this work, we report, for the first time, a ⁶⁷Zn natural abundance SSNMR spectroscopic study of several representative zeolitic imidazolate frameworks (ZIFs) and MOFs at an ultrahigh magnetic field of 21.1 T. Our work demonstrates that ⁶⁷Zn magic‐angle spinning (MAS) NMR spectra are highly sensitive to the local Zn environment and can differentiate non‐equivalent Zn sites. The ⁶⁷Zn NMR parameters can be predicted by theoretical calculations. Through the study of MOF‐5 desolvation, we show that with the aid of computational modeling, ⁶⁷Zn NMR spectroscopy can provide valuable structural information on the MOF systems with structures that are not well described. Using ZIF‐8 as an example, we further demonstrate that ⁶⁷Zn NMR spectroscopy is highly sensitive to the guest molecules present inside the cavities. Our work also shows that a combination of ⁶⁷Zn NMR data and molecular dynamics simulation can reveal detailed information on the distribution and the dynamics of the guest species. The present work establishes ⁶⁷Zn SSNMR spectroscopy as a new tool complementary to X‐ray diffraction for solving outstanding structural problems and for determining the structures of many new MOFs yet to come.     

The Metal–Organic Framework MIL‐53(Al) Constructed from Multiple Metal Sources: Alumina,Aluminum Hydroxide,and Boehmite

Three aluminum compounds, namely alumina, aluminum hydroxide, and boehmite, are probed as the metal sources for the hydrothermal synthesis of a typical metal–organic framework MIL‐53(Al). The process exhibits enhanced synthetic efficiency without the generation of strongly acidic byproducts. The time‐course monitoring of conversion from different aluminum sources into MIL‐53(Al) is achieved by multiple characterization that reveals a similar but differentiated crystallinity, porosity, and morphology relative to typical MIL‐53(Al) prepared from water‐soluble aluminum salts. Moreover, the prepared MIL‐53(Al) constructed with the three insoluble aluminum sources exhibit an improved thermal stability of up to nearly 600 °C and enhanced yields. Alumina and boehmite are more preferable than aluminum hydroxide in terms of product porosity, yield, and reaction time. The adsorption performances of a typical environmental endocrine disruptor, dimethyl phthalate, on the prepared MIL‐53(Al) samples are also investigated. The improved structural stability of MIL‐53(Al) prepared from these alternative aluminum sources enables double‐enhanced adsorption performance (up to 206 mg g^?1) relative to the conventionally obtained MIL‐53(Al).     

Molecular Level Characterization of the Structure and Interactions in Peptide‐Functionalized Metal–Organic Frameworks

We use density functional theory, newly parameterized molecular dynamics simulations, and last generation ¹⁵N dynamic nuclear polarization surface enhanced solid‐state NMR spectroscopy (DNP SENS) to understand graft–host interactions and effects imposed by the metal–organic framework (MOF) host on peptide conformations in a peptide‐functionalized MOF. Focusing on two grafts typified by MIL‐68‐proline ( ‐Pro ) and MIL‐68‐glycine‐proline ( ‐Gly‐Pro ), we identified the most likely peptide conformations adopted in the functionalized hybrid frameworks. We found that hydrogen bond interactions between the graft and the surface hydroxyl groups of the MOF are essential in determining the peptides conformation(s). DNP SENS methodology shows unprecedented signal enhancements when applied to these peptide‐functionalized MOFs. The calculated chemical shifts of selected MIL‐68‐NH‐ Pro and MIL‐68‐NH‐ Gly‐Pro conformations are in a good agreement with the experimentally obtained ¹⁵N NMR signals. The study shows that the conformations of peptides when grafted in a MOF host are unlikely to be freely distributed, and conformational selection is directed by strong host–guest interactions.