Chemistry - A European Journal

Aluminum 1,4‐benzenedicarboxylate Al(OH)[O₂CC₆H₄CO₂]⋅ [HO₂CC₆H₄CO₂H]_0.70 or MIL‐53 as (Al) has been hydrothermally synthesized by heating a mixture of aluminum nitrate, 1,4‐benzenedicarboxylic acid, and water, for three days at 220 °C. Its 3 D framework is built up of infinite trans chains of corner‐sharing AlO₄(OH)₂ octahedra. The chains are interconnected by the 1,4‐benzenedicarboxylate groups, creating 1 D rhombic‐shaped tunnels. Disordered 1,4‐benzenedicarboxylic acid molecules are trapped inside these tunnels. Their evacuation upon heating, between 275 and 420 °C, leads to a nanoporous open‐framework (MIL‐53 ht (Al) or Al(OH)[O₂CC₆H₄CO₂]) with empty pores of diameter 8.5 Å. This solid exhibits a Langmuir surface area of 1590(1) m² g⁻¹ together with a remarkable thermal stability, since it starts to decompose only at 500 °C. At room temperature, the solid reversibly absorbs water in its tunnels, causing a very large breathing effect and shrinkage of the pores. Analysis of the hydration process by solid‐state NMR (¹H, ¹³C, ²⁷Al) has clearly indicated that the trapped water molecules interact with the carboxylate groups through hydrogen bonds, but do not affect the hydroxyl species bridging the aluminum atoms. The hydrogen bonds between water and the oxygen atoms of the framework are responsible for the contraction of the rhombic channels. The structures of the three forms have been determined by means of powder X‐ray diffraction analysis. Crystal data for MIL‐53 as (Al) are as follows: orthorhombic system, Pnma (no. 62), a = 17.129(2), b = 6.628(1), c = 12.182(1) Å; for MIL‐53 ht (Al), orthorhombic system, Imma (no. 74), a = 6.608(1), b = 16.675(3), c = 12.813(2) Å; for MIL‐53 lt (Al), monoclinic system, Cc (no. 9), a = 19.513(2), b = 7.612(1), c = 6.576(1) Å, β = 104.24(1)°.

A self‐consistent system of additive covalent radii, R(AB)=r(A) + r(B), is set up for the entire periodic table, Groups 1–18, Z=1–118. The primary bond lengths, R, are taken from experimental or theoretical data corresponding to chosen group valencies. All r(E) values are obtained from the same fit. Both E–E, E–H, and E–CH₃ data are incorporated for most elements, E. Many E–E′ data inside the same group are included. For the late main groups, the system is close to that of Pauling. For other elements it is close to the methyl‐based one of Suresh and Koga [J. Phys. Chem. A 2001, 105, 5940] and its predecessors. For the diatomic alkalis MM′ and halides XX′, separate fits give a very high accuracy. These primary data are then absorbed with the rest. The most notable exclusion are the transition‐metal halides and chalcogenides which are regarded as partial multiple bonds. Other anomalies include H₂ and F₂. The standard deviation for the 410 included data points is 2.8 pm.

The concept of shape‐controlled synthesis is discussed by investigating the growth mechanisms for silver nanocubes, nanowires, and nanospheres produced through a polymer‐mediated polyol process. Experimental parameters, such as the concentration of AgNO₃ (the precursor to silver), the molar ratio between poly(vinylpyrrolidone) (PVP, the capping agent) and AgNO₃, and the strength of chemical interaction between PVP and various crystallographic planes of silver, were found to determine the crystallinity of seeds (e.g., single crystal versus decahedral multiply twinned particles). In turn, the crystallinity of a seed and the extent of the PVP coverage on the seed were both instrumental in controlling the morphology of final product. The ability to generate silver nanostructures with well‐defined morphologies provides a great opportunity to experimentally and systematically study the relationship between their properties and geometric shapes.

Carbon‐rich‐quick scheme: A carbon‐rich solid product made up of uniform micrometer‐sized spheres of tunable diameter has been synthesized by the hydrothermal carbonization of saccharides. These microspheres possess a core–shell chemical structure based on the different nature of the oxygen functionalities between the core and the outer layer (see figure).magnified image

A carbon‐rich solid product, here denoted as hydrochar, has been synthesized by the hydrothermal carbonization of three different saccharides (glucose, sucrose, and starch) at temperatures ranging from 170 to 240 °C. This material is made up of uniform spherical micrometer‐sized particles that have a diameter in the 0.4–6 μm range, which can be modulated by modifying the synthesis conditions (i.e., the concentration of the aqueous saccharide solution, the temperature of the hydrothermal treatment, the reaction time, and type of saccharide). The formation of the carbon‐rich solid through the hydrothermal carbonization of saccharides is the consequence of dehydration, condensation, or polymerization and aromatization reactions. The microspheres thus obtained possess, from a chemical point of view, a core–shell structure consisting of a highly aromatic nucleus (hydrophobic) and a hydrophilic shell containing a high concentration of reactive oxygen functional groups (i.e., hydroxyl/phenolic, carbonyl, or carboxylic).

Herein we report the synthesis of a crystalline graphitic carbon nitride, or g‐C₃N₄, obtained from the temperature‐induced condensation of dicyandiamide (NH₂C(NH)NHCN) by using a salt melt of lithium chloride and potassium chloride as the solvent. The proposed crystal structure of this g‐C₃N₄ species is based on sheets of hexagonally arranged s‐heptazine (C₆N₇) units that are held together by covalent bonds between C and N atoms which are stacked in a graphitic, staggered fashion, as corroborated by powder X‐ray diffractometry and high‐resolution transmission electron microscopy.

Upon light excitation MOF‐5 behaves as a semiconductor and undergoes charge separation (electrons and holes) decaying in the microsecond time scale. The actual conduction band energy value was estimated to be 0.2 V versus NHE with a band gap of 3.4 eV. Photoinduced electron transfer processes to viologen generates the corresponding viologen radical cation, while holes of MOF‐5 oxidizes N,N,N′,N′‐tetramethyl‐p‐phenylenediamine. One application investigated for MOF‐5 as a semiconductor has been the shape‐selective photocatalyzed degradation of phenol in aqueous solutions.

A systematic modulation of organic ligands connecting dinuclear paddle‐wheel motifs leads to a series of isomorphous metal‐organic porous materials that have a three‐dimensional connectivity and interconnected pores. Aromatic dicarboxylates such as 1,4‐benzenedicarboxylate (1,4‐bdc), tetramethylterephthalate (tmbdc), 1,4‐naphthalenedicarboxylate (1,4‐ndc), tetrafluoroterephthalate (tfbdc), or 2,6‐naphthalenedicarboxylate (2,6‐ndc) are linear linkers that form two‐dimensional layers, and diamine ligands, 4‐diazabicyclo[2.2.2]octane (dabco) or 4,4′‐dipyridyl (bpy), coordinate at both sides of Zn₂ paddle‐wheel units to bridge the layers vertically. The resulting open frameworks [Zn₂(1,4‐bdc)₂(dabco)] (1), [Zn₂(1,4‐bdc)(tmbdc)(dabco)] (2), [Zn₂(tmbdc)₂(dabco)] (3), [Zn₂(1,4‐ndc)₂(dabco)] (4), [Zn₂(tfbdc)₂(dabco)] (5), and [Zn₂(tmbdc)₂(bpy)] (8) possess varying size of pores and free apertures originating from the side groups of the 1,4‐bdc derivatives. [Zn₂(1,4‐bdc)₂(bpy)] (6) and [Zn₂(2,6‐ndc)₂(bpy)] (7) have two‐ and threefold interpenetrating structures, respectively. The non‐interpenetrating frameworks (1–5 and 8) possess surface areas in the range of 1450–2090 m²g⁻¹ and hydrogen sorption capacities of 1.7–2.1 wt % at 78 K and 1 atm. A detailed analysis of the sorption data in conjunction with structural similarities and differences concludes that porous materials with straight channels and large openings do not perform better than those with wavy channels and small openings in terms of hydrogen storage through physisorption.

Poly(aminoimino)heptazine, otherwise known as Liebig's melon, whose composition and structure has been subject to multitudinous speculations, was synthesized from melamine at 630 °C under the pressure of ammonia. Electron diffraction, solid‐state NMR spectroscopy, and theoretical calculations revealed that the nanocrystalline material exhibits domains well‐ordered in two dimensions, thereby allowing the structure solution in projection by electron diffraction. Melon ([C₆N₇(NH₂)(NH)]_n, plane group p2 gg, a=16.7, b=12.4 Å, γ=90°, Z=4), is composed of layers made up from infinite 1D chains of NH‐bridged melem (C₆N₇(NH₂)₃) monomers. The strands adopt a zigzag‐type geometry and are tightly linked by hydrogen bonds to give a 2D planar array. The inter‐layer distance was determined to be 3.2 Å from X‐ray powder diffraction. The presence of heptazine building blocks, as well as NH and NH₂ groups was confirmed by ¹³C and ¹⁵N solid‐state NMR spectroscopy using ¹⁵N‐labeled melon. The degree of condensation of the heptazine core was further substantiated by a ¹⁵N direct excitation measurement. Magnetization exchange observed between all ¹⁵N nuclei using a fp‐RFDR experiment, together with the CP‐MAS data and elemental analysis, suggests that the sample is mainly homogeneous in terms of its basic composition and molecular building blocks. Semiempirical, force field, and DFT/plane wave calculations under periodic boundary conditions corroborate the structure model obtained by electron diffraction. The overall planarity of the layers is confirmed and a good agreement is obtained between the experimental and calculated NMR chemical shift parameters. The polymeric character and thermal stability of melon might render this polymer a pre‐stage of g‐C₃N₄ and portend its use as a promising inert material for a variety of applications in materials and surface science.

Over the last decade the potential for N,N‐dialkyl(thio)urea derivatives to serve as active metal‐free organocatalysts for a wide range of synthetically useful reactions susceptible to the influence of general acid catalysis has begun to be realised. This article charts the development of these catalysts (with emphasis on the design principles involved), from early “proof‐of‐concept” materials to contemporary active chiral (bifunctional) promoters of highly selective asymmetric transformations.

A stereochemical study of polyhedral eight‐vertex structures is presented, based on continuous shape measures (CShM). Reference polyhedra, shape maps, and minimal‐distortion interconversion paths are presented for eight‐vertex polyhedral and polygonal structures within the CShM framework. The application of these stereochemical tools is analyzed for several families of experimental structures: 1) coordination polyhedra of molecular transition‐metal coordination compounds, classified by electron configuration and ligands; 2) edge‐bonded polyhedra, including cubane structures, realgar, and metal clusters; 3) octanuclear transition‐metal supramolecular architectures; and 4) coordination polyhedra in extended structures in inorganic solids. Structural classification is shown to be greatly facilitated by these tools, and the detection of less common structures, such as the gyrobifastigium, is straightforward.