Polyhalides as scaffolds for supramolecular, ion-conducting crown ether stacks

"Crown ethers, such as dibenzo-18-crown-6 (DB18C6) are in principle perfect building blocks to be stacked on top of each other for one-dimensional (1D) channel formation. However, in the more than 1000 publications on crown ethers in the solid state, only one case was of channel formation described, but not as main focus of research.[1] We now present a way to systematically induce the stacking of DB18C6 with the help of polyhalides, which play the roles of scaffolds via halogen bonding.[2] These compounds can be considered as ""supramolecular straws"". Using for example potassium as couter ion for triiodide for example, we obtained a solid which contains three differently filled, parallel channels in the solid state, which are arranged between the polyhalide anions. Exchanging potassium with sodium by immersion of a single crystal into NaOH solution leads to a single-crystal-to-single-crystal transformation into a compound with two channel types. This transition from a system crystallizing initially in the P2-space group to yield a compound in Pccn is only possible under these very special conditions. We will further present how the ion transport through these channels can be quantified and which process is involved in ion exchange. The role of the polyhalide anions, which cannot be replaced by other linear anions, will be emphasized as well. "

Cr(II) complex: water reductant and starting compound for new Cr(III) compounds

The use of alkali aryloxide reagents in organo-metallic synthesis often depends on their solubility, a property derived from their structure. The regain of interest of alkali aryloxides also originates from the discovery of high-temperature superconducting compounds, which has generated a great interest in the formation of oxide materials and other ceramics. Thus, many alkoxides of yttrium and copper are common precursors for oxide materials. Moreover, the synthesis of heterobimetallic alkoxides has provided a facile route for obtaining soluble, volatile, and generally monomeric species. These heterobimetallic complexes can thus serve as valuable precursors for making metal oxides but it is not the only possible application for this type of compounds. These complexes can be used as starting compounds for syntheses of more complex structures. In this work, the salt elimination and ligand exchange reaction of chromium(II) chloride with lithium phenoxide yields a mixed metal lithium-chromium(II) phenoxide. Using this latter as intermediately formed starting material and combining the substitution reaction with an oxidation process, we have gained access to new polynuclear chromium(III) aryloxide complexes. While a 1D coordination polymer based on chromium(III) is obtained in a first reaction by serendipity, the controlled addition of water to the Cr(II) complex leads to three new discrete chromium(III) cluster compounds. The use of deuterated species allowed to confirm the oxidation based on the addition of water by detection of H2, HD and D2. During these investigations, we have also identified a THF-adduct of chromium(II) chloride, used in the literature as precursor in numerous syntheses, but with a hitherto unknown structure. Figure 1: Oxidation of a Cr(II) complex by water and formation of a new Cr(III) complex.

Polymorphism, what it is and how to identify it

Polymorphism is a very important phenomenon not only in basic research, but certainly in pharmaceutical industry and materials science. Polymorphs possess different properties, for instance the solubility or the mechanical resistance can differ dramatically from one polymorph to the other - properties which can be crucial for their application. Hence, it is important to be able to control the formation of polymorphs and to understand their formation. We here gave some insights into the basic knowledge of polymorph formation and their identification and characterization in order to give an overview on the current state of the art. In order to give interested peoples a tool in hand to test their compounds for polymorphism, we established a series of flow sheets to follow, depending on the class of compounds, hoping that they are useful for many scientists who are not so well acquainted with polymorphism. The presented schemes resume thus the identification steps for polymorphs. It should also help to use the term polymorph correctly in order to reduce the number of publications in which this term is not used in a correct way.