Perylene Diimide as a Precise Graphene-like Superoxide Dismutase Mimetic

Here we show that the active portion of a graphitic nanoparticle can be mimicked by a perylene diimide (PDI) to explain the otherwise elusive biological and electrocatalytic activity of the nanoparticle construct. Development of molecular analogues that mimic the antioxidant properties of oxidized graphenes, in this case the poly(ethylene glycolated) hydrophilic carbon clusters (PEG–HCCs), will afford important insights into the highly efficient activity of PEG–HCCs and their graphitic analogues. PEGylated perylene diimides (PEGn–PDI) serve as well-defined molecular analogues of PEG–HCCs and oxidized graphenes in general, and their antioxidant and superoxide dismutase-like (SOD-like) properties were studied. PEGn–PDIs have two reversible reduction peaks, which are more positive than the oxidation peak of superoxide (O2•–). This is similar to the reduction peak of the HCCs. Thus, as with PEG–HCCs, PEGn–PDIs are also strong single-electron oxidants of O2•–. Furthermore, reduced PEGn–PDI, PEGn–PDI•–, in the presence of protons, was shown to reduce O2•– to H2O2 to complete the catalytic cycle in this SOD analogue. The kinetics of the conversion of O2•– to O2 and H2O2 by PEG8–PDI was measured using freeze-trap EPR experiments to provide a turnover number of 133 s–1; the similarity in kinetics further supports that PEG8–PDI is a true SOD mimetic. Finally, PDIs can be used as catalysts in the electrochemical oxygen reduction reaction in water, which proceeds by a two-electron process with the production of H2O2, mimicking graphene oxide nanoparticles that are otherwise difficult to study spectroscopically

Mechanism of Water Intrusion into Flexible ZIF-8: Liquid Is Not Vapor

Zeolitic Imidazolate Frameworks (ZIF) find application in storage and dissipation of mechanical energy. Their distinctive properties linked to their (sub)nanometer size and hydrophobicity allow for water intrusion only under high hydrostatic pressure. Here we focus on the popular ZIF-8 material investigating the intrusion mechanism in its nanoscale cages, which is the key to its rational exploitation in target applications. In this work, we used a joint experimental/theoretical approach combining in operando synchrotron experiments during high- pressure intrusion experiments, molecular dynamics simulations, and stochastic models to reveal that water intrusion into ZIF-8 occurs by a cascade filling of connected cages rather than a condensation process as previously assumed. The reported results allowed us to establish structure/function relations in this prototypical microporous material, representing an important step to devise design rules to synthesize porous media

Mechanistically-guided materials chemistry: synthesis of new ternary nitrides, CaZrN2_2 and CaHfN2_2

Recent computational studies have predicted many new ternary nitrides, revealing synthetic opportunities in this underexplored phase space. However, synthesizing new ternary nitrides is difficult, in part because intermediate and product phases often have high cohesive energies that inhibit diffusion. Here, we report the synthesis of two new phases, calcium zirconium nitride (CaZrN$_2$) and calcium hafnium nitride (CaHfN$_2$), by solid state metathesis reactions between Ca$_3$N$_2$ and $M$Cl$_4$ ($M$ = Zr, Hf). Although the reaction nominally proceeds to the target phases in a 1:1 ratio of the precursors via Ca$_3$N$_2$ + $M$Cl$_4$ $\rightarrow$ Ca$M$N$_2$ + 2 CaCl$_2$, reactions prepared this way result in Ca-poor materials (Ca$_xM_{2-x}$N$_2$, $x<1$). A small excess of Ca$_3$N$_2$ (ca. 20 mol\%) is needed to yield stoichiometric Ca$M$N$_2$, as confirmed by high-resolution synchrotron powder X-ray diffraction. In situ synchrotron X-ray diffraction studies reveal that nominally stoichiometric reactions produce Zr$^{3+}$ intermediates early in the reaction pathway, and the excess Ca$_3$N$_2$ is needed to reoxidize Zr$^{3+}$ intermediates back to the Zr$^{4+}$ oxidation state of CaZrN$_2$. Analysis of computationally-derived chemical potential diagrams rationalizes this synthetic approach and its contrast from the synthesis of MgZrN$_2$. These findings additionally highlight the utility of in situ diffraction studies and computational thermochemistry to provide mechanistic guidance for synthesis

Generation and Applications of Structure Envelopes for Metal-Organic Frameworks

Synthesis of polycrystalline, vs. single-crystalline porous materials, such as metal-organic frameworks (MOFs), is usually beneficial due to shorter synthetic time and higher yields. However, the structural characterization of these materials by X-ray powder diffraction can be complicated. Even more difficult, is to track structural changes of MOFs by in situ experiments. Hence, we designed several successful techniques for the structural investigation of porous MOFs. These methods utilize the Structure Envelope (SE) density maps. SEs are surfaces which describing the pore system with the framework. It was shown that these maps can be easily generated from the structure factors of a few (1 to 10) of the most intense low index reflections. Application of SE in Charge Flipping calculations shortens and simplifies structure determination of MOF materials. This method provides excellent MOF models which can be used as a good starting point for their refinement. However, the most interesting results have been found by using Difference Envelope Density (DED) analysis. DED plots are made by taking the difference between observed and calculated SE densities. This allows us to study guest related issues of MOFs such as, location of guest molecules in the pores, tracking activation of MOFs and gas loading, etc. We also have shown that, DED created from routine powder diffraction patterns might provide very important information about MOF structure itself. In fact DED can be used for study of interpenetration, substituents locations and effects conformational changes in the MOF ligands. Generation and analysis of SEs and DEDs are easy and straightforward. It provides the information needed to explain major deviations in structure-property relationship in MOFs. In our opinion, this method might become one of the important and routine techniques for MOFs structural analysis

Crystal Morphology As An Evidence Of Supramolecular Organization In Adducts Of 1,2-Bis(Chloromercurio)Tetrafluorobenzene With Organic Esters

Low melting organic esters, ethyl acetate (1), diethyl carbonate (2), methyl benzoate (3) and phenyl acetate (4), have been grown using the miniature zone melting in situ crystallization technique and their molecular and crystal structures have been determined. Co-crystallization of the esters 1-4 with Lewis acid 1,2-bis(chloromercurio)tetrafluorobenzene (II) resulted in three adducts with a ratio of organic to organometallic components of 1:1 (II·1, II·3, and II·4), and one complex with a corresponding ratio of 1:2 (2(II)·2). X-ray analysis revealed presence of supramolecular 1D chain associates in crystals of II·1 and II·3, which are similar to those found before in the crystal of individual II. The crystal structures of complexes 2(II)·2 and II·4 have demonstrated the presence of 2D layer supramolecular associates. Both 1D and 2D associates are stabilized mostly by strong Hg⋯Cl interactions between molecules of II, which in some cases are additionally strengthen via Hg⋯O, Hg⋯π, and π-π interactions between molecules of esters and II. Clear correlation between type of supramolecular associates and crystal morphology was found: complexes with 1D supramolecular associates grow in the form of needle-like crystals, while 2D associates promote formation of the plate-like crystals. Comparison of structural characteristics of esters 1-4 in crystals of individual compounds, complexes, and isolated state (quantum calculations and gas electron diffraction literature data) has demonstrated similarity of the molecular geometry but possibility of conformational variations. On the basis of correlation analysis of structural and IR spectroscopic data on all compounds under investigation and taking into account that sum of the van der Waals radii of the Hg and O is equal to 3.50 Å, it was shown that shorter Hg⋯O contacts should correspond to an additional interactions between these types of atoms. © 2011 American Chemical Society

Unusual Chemical Ratio, Z″ Values, and Polymorphism in Three New <i>N-</i>Methyl Aminopyridine–4-Nitrophenol Adducts

Cocrystallization of 4-nitrophenol (<b>I</b>) with <i>N</i>-methyl substituted aminopyridines, 4-<i>N</i>-methylaminopyridine <b>1</b>, 2-<i>N</i>-methylaminopyridine <b>2</b>, and 2-<i><i>N,N</i></i>-dimethylaminopyridine <b>3</b>, resulted in three novel adducts <b>1</b>·2­(<b>I</b>), <b>2</b>·3­(<b>I</b>), and <b>3</b>·3­(<b>I</b>), one of which, <b>2</b>·3­(<b>I</b>), was found in three polymorphic forms, <b>A</b>, <b>B</b>, and <b>C</b>. The single crystals were grown by slow evaporation from ethanol. The proton transfer from the phenoxy to the pyridine moieties was registered in all compounds. The adducts comprise pyridinium cations, 4-nitrophenolate anions, and varying in number neutral 4-nitrophenol molecules. Though the asymmetric hydrogen-bonded network involving the −N⁺H groups of pyridinium cations and the −C–O^– and −C–OH groups of 4-nitrophenol moieties is registered in the adducts, the delicate balance of noncovalent interactions that include CH···O hydrogen bonds and face-to-face stacking interactions between the extended antiparallel arrays of components controls the centrosymmetric packing. Although three polymorphs of <b>2</b>·3­(<b>I</b>) share several structural common features, they reveal significant differences in the conformation of the pyridinium cation, and the hydrogen-bonding patterns

Interconversion between Discrete and a Chain of Nanocages: Self-Assembly via a Solvent-Driven, Dimension-Augmentation Strategy

Using a ligand bearing a bulky hydrophobic group, a “shish kabob” of nanocages, has been assembled through either a one-fell-swoop or a step-by-step procedure by varying the dielectric constant of the assembly mixture. A hydrophobic solvent breaks down the chain to discrete nanocages, while a hydrophilic solvent reverses the procedure. Although the shish kabob of nanocages has exactly the same chemical composition and even the same Archimedean-solid structure as those of its discrete analogue, its gas-adsorption capacity is remarkably improved because assembly of a chain exposes the internal surface of an individual cage. This dimension-augmentation strategy may have general implications in the preparation of porous materials

Reversible Switching from Antiferro- to Ferromagnetic Behavior by Solvent-Mediated, Thermally-Induced Phase Transitions in a Trimorphic MOF-Based Magnetic Sponge System

Hydrothermal reactions of copper­(II) acetate, tetrazolate-5-carboxylate (tzc), and the neutral N-donor spacer ligand 1,3-di­(4-pyridyl)­propane (dpp) lead in a single reaction vial to the simultaneous formation of three different single-crystalline solvates [Cu­(tzc)­(dpp)]_<i>n</i>·0.5C₆H₁₄·0.5H₂O (<b>1</b>), [Cu­(tzc)­(dpp)]_<i>n</i>·4.5H₂O (<b>2</b>), and [Cu­(tzc)­(dpp)]_<i>n</i>·1.25C₆H₁₄ (<b>3</b>). All three structures were characterized by single crystal X-ray diffraction. None of these solvates can be prepared as phase-pure bulk materials, but reaction conditions similar to those used for single crystal synthesis yield a phase-pure polycrystalline bulk material of an additional forth solvate phase [Cu­(tzc)­(dpp)]_<i>n</i>·2H₂O (<b>4</b>). Investigations of its thermal properties by <i>in situ</i> temperature-dependent synchrotron-based powder diffraction experiments have shown interesting phase transitions upon heating in a helium stream. Initially, the precursor dihydrate <b>4</b> transforms to an anhydrous phase [Cu­(tzc)­(dpp)]_<i>n</i> (<b>6I</b>) <i>via</i> the intermediate monohydrate phase [Cu­(tzc)­(dpp)]_<i>n</i>·H₂O (<b>5</b>). Upon further heating, phase <b>6I</b> transforms to a new anhydrous polymorph <b>6II</b>, which transforms upon cooling to a further new phase <b>6III</b>. Thermogravimetric measurements performed in tandem with differential scanning calorimetry as well as infrared spectroscopic investigations are in agreement with these findings. The de/resolvation behavior is accompanied by a dramatic change in their magnetic properties: The dihydrate phase shows antiferromagnetic exchange interactions, whereas ferromagnetic properties are observed for the trimorphic anhydrate system. This magnetic sponge-like behavior can be reversibly cycled upon de/resolvation of the material

Comparison of zwitterionic N-alkylaminomethanesulfonic acids to related compounds in the Good buffer series

Several N-alkyl and N,N-dialkylaminomethanesulfonic acids were synthesized (as zwitterions and/or sodium salts) to be tested for utility as biological buffers at lower pH levels than existing Good buffer compounds (aminoalkanesulfonates with a minimum of two carbons between amine and sulfonic acid groups as originally described by Norman Good, and in common use as biological buffers). Our hypothesis was that a shorter carbon chain (one carbon) between the amino and sulfonic acid groups should lower the ammonium ion pK a values. The alkylaminomethanesulfonate compounds were synthesized in aqueous solution by reaction of primary or secondary amines with formaldehyde/sodium hydrogensulfite addition compound. The pK a values of the ammonium ions of this series of compounds (compared to existing Good buffers) was found to correlate well with the length of the carbon chain between the amino and sulfonate moeties, with a significant decrease in amine basicity in the aminomethanesulfonate compounds (pK a decrease of 2 units or more compared to existing Good buffers). An exception was found for the 2-hydroxypiperazine series which shows only a small pK a decrease, probably due to the site of protonation in this compound (as confirmed by X-ray crystal structure). X-ray crystallographic structures of two members of the series are reported. Several of these compounds have pK a values that would indicate potential utility for buffering at pH levels below the normal physiological range (pK a values in the range of 3 to 6 without aqueous solubility problems) – a range that is problematic for currently available Good buffers. Unfortunately, the alkylaminomethanesulfonates were found to degrade (with loss of their buffering ability) at pH levels below the pK a value and were unstable at elevated temperature (as when autoclaving) – thus limiting their utility

Reversible Switching from Antiferro- to Ferromagnetic Behavior by Solvent-Mediated, Thermally-Induced Phase Transitions in a Trimorphic MOF-Based Magnetic Sponge System

Hydrothermal reactions of copper­(II) acetate, tetrazolate-5-carboxylate (tzc), and the neutral N-donor spacer ligand 1,3-di­(4-pyridyl)­propane (dpp) lead in a single reaction vial to the simultaneous formation of three different single-crystalline solvates [Cu­(tzc)­(dpp)]_<i>n</i>·0.5C₆H₁₄·0.5H₂O (<b>1</b>), [Cu­(tzc)­(dpp)]_<i>n</i>·4.5H₂O (<b>2</b>), and [Cu­(tzc)­(dpp)]_<i>n</i>·1.25C₆H₁₄ (<b>3</b>). All three structures were characterized by single crystal X-ray diffraction. None of these solvates can be prepared as phase-pure bulk materials, but reaction conditions similar to those used for single crystal synthesis yield a phase-pure polycrystalline bulk material of an additional forth solvate phase [Cu­(tzc)­(dpp)]_<i>n</i>·2H₂O (<b>4</b>). Investigations of its thermal properties by <i>in situ</i> temperature-dependent synchrotron-based powder diffraction experiments have shown interesting phase transitions upon heating in a helium stream. Initially, the precursor dihydrate <b>4</b> transforms to an anhydrous phase [Cu­(tzc)­(dpp)]_<i>n</i> (<b>6I</b>) <i>via</i> the intermediate monohydrate phase [Cu­(tzc)­(dpp)]_<i>n</i>·H₂O (<b>5</b>). Upon further heating, phase <b>6I</b> transforms to a new anhydrous polymorph <b>6II</b>, which transforms upon cooling to a further new phase <b>6III</b>. Thermogravimetric measurements performed in tandem with differential scanning calorimetry as well as infrared spectroscopic investigations are in agreement with these findings. The de/resolvation behavior is accompanied by a dramatic change in their magnetic properties: The dihydrate phase shows antiferromagnetic exchange interactions, whereas ferromagnetic properties are observed for the trimorphic anhydrate system. This magnetic sponge-like behavior can be reversibly cycled upon de/resolvation of the material