journal menu
abstracts
Multi-component crystals composed of two different chiral molecules have the potential to form diastereomers. In the present study a selection of chiral amides and acids was employed to form multi-component crystals. Since growing these multi-component crystals from solution failed, solvent assisted grinding was used. The resulting diastereomeric pairs, which are present as polycrystalline powders showed distinctly different powder diffraction patterns. In order to elucidate the crystal structures direct space global optimization structure solution methods were successfully used in several cases. A potential application of these diastereomeric multi-component crystals is the determination of the absolute configuration of one of the two components based on the known absolute configuration of the other.[1] In addition, Raman spectroscopy and DSC were employed to determine thermodynamic properties. In subsequent grinding experiments racemic conglomerates and racemates formed the starting material. These experiments demonstrated in several cases a relationship between melting point differences and preferential formation of only one diastereomer.
abstracts
The first λ3,λ5-tetraphosphete contains a 4π-electron four-membered ring as the central structural unit of a dispirocyclic system and can thus be classified as an analogue to diphosphetes and cyclodiphosphazenes. According to its crystal structure the central P4 unit exhibits not only P–P bonds which are of equal length (P1–P2 2,139(1) Å, P1–P2A 2,142(1) Å), but also rhombic distortion (P1–P2–P1A 79,4(1)0, P2–P1–P2A 100,6(1)0).[1] Therefore its electronic structure cannot be described as 'Phosphacyclobutadiene' but either as a bis(ylide) or as a system with delocalized double bonds. After various quantum chemical calculations and an extensive examination of its reaction and coordination behavior failed to answer this question, we addressed the problem via a detailed analysis of its charge density distribution. The experimental charge density based on high resolution X-ray diffraction data collected at low temperature is determined by multipole least squares refinement using the program package XD2006.[2] In a first step, the static deformation density exhibits charge density which is located mainly outside of the P4 ring plane at the λ3-phosphorus atoms but simultaneously redistributed into the P–P bond area. In addition to that, a study of its topological properties and an inspection of the Laplacian of the electron density according to Bader's `Quantum Theory of Atoms in Molecules' (QTAIM)[3] further highlight the bonding features. They reveal polar Si–N, Si–C and P–N bonds with a decreasing amount of electrostatic contribution as well as four valence shell charge concentrations (thus sp3 hybridization) at each of the phosphorus atoms. Finally supported by theoretical calculations, the results illustrate the unique bonding situation in the P4 unit combining a high ylidic character with unusual not exclusively sigma-like P–P bonds.
