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- 1,3,4-Oxadiazole (1)
- 1,3,4-oxadiazole (1)
- C. High pressure (1)
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In the title compound, C13H14BN3O, the aziridine ring is an almost equilateral triangle, the CC distance being slightly shorter than the CN distances, probably because of the dative BN bond. The five-membered ring, composed of two C atoms and N, B and O atoms, is fused with the aziridine ring to form a six-membered ring with a chair conformation.
Crystal structures of four different di-aryl-1,3,4-oxadiazole compounds (aryl = 2-pyridyl-, 3-pyridyl-, 2-aminophenyl-, 3-aminophenyl-) are determined. Crystallization of di(2-pyridyl)-1,3,4-oxadiazole yielded monoclinic and triclinic polymorphs. The structures are characterized by the occurrence of ππ interactions. Additionally, in case of the aminophenyl compounds intra- as well as intermolecular hydrogen bonds are found that influence the packing motif as well. Since these molecules are often used as ligands in metalorganic complexes similarities and differences of the molecular conformation between the molecules in the pure crystals and that of the ligands in the complexes are discussed.
Three new compounds have been synthesized based on the molecular motif 2-[2,6-bis(trifluoromethyl)phenyl]-5-phenyl-1,3,4-oxadiazole, with subsequent CF3-substitution in the ortho-positions of the phenylene ring. The crystal structures of the compounds have been determined by single crystal X-ray diffraction. All compounds have a monoclinic structure. The solid state structure of the compounds is influenced by the electronic properties of the fluorine atoms, leading to the occurrence of CH...F, and CF...ϖ interactions, partly replacing ϖϖ interactions usually observed in the crystal structures of 2,5-diphenyl-1,3,4-oxadiazole derivatives. Other significant interactions than those involving fluorine appear only in rare cases. The strong impact of the fluorine atoms on the intra- and intermolecular interactions, and the molecular conformation lead to novel inputs for the understanding of molecular recognition, supramolecular assembly, and crystal packing of fluorine containing compounds.
High-pressure studies on molecular crystals - relations between structure and high-pressure behavior
(2008)
This paper summarizes attempts to understand structureproperty relationships for a large class of aromatic diphenyl-1,3,4-oxadiazole molecules. Starting from the investigation of the crystal structure several common packing motifs as well as characteristic differences are derived. Many different molecules show a rather planar conformation in the solid state. A stronger intermolecular twist is only observed for compounds with substituents occupying the ortho-positions of the phenyl rings. Most crystal structures are characterized by the formation of stacks leading to intense ππ acceptordonor interactions between oxadiazole and phenyl rings. High-pressure investigations result in a soft compression behavior typical for organic molecular crystals. The bulk behavior may be described by the Murnaghan equation of state with similar coefficients (bulk modulus and its pressure derivative) for nearly all investigated compounds but also for related substances. The compression shows a strong anisotropy resulting from the specific features and packing motifs of the crystal structure. This is clearly indicated by a corresponding strain analysis. Additionally to the crystal structure the Raman spectrum was also investigated under increasing pressure. The different pressure behavior of external and internal modes reflects the difference between intra- and intermolecular interactions.
The supermolecular mechanism of the template-mediated oxidative polymerization of pyrrole is investigated in detail. It is shown for the first time that 2,5-bis(pyrrol-2-yl)pyrrolidyl complexes with aromatic sulfonic acids such as %#946;-naphtalenesulfonic acid or para-toluenesulfonic acid are formed in the early polymerization step. The crystalline acidbase complexes are isolated and their crystal structures are analyzed. A 2,5-bis(pyrrol-2-yl)pyrrolidyl cation is common to both complexes and forms an inner salt with the sulfonic acid anion. As concluded from NMR investigations the synthesis using sulfonic acids results in a high stereo selectivity with respect to the trans conformation of the pyrrolidyl ring compared with the classical synthesis routine of this compound using hydrochloric acid. The needle-like crystals of the complexes act as hard templates during the next step of the polymerization and result in tubular morphologies of the polypyrrole.
Structure determination of two asymmetrically substituted oxadiazoles from powder diffraction data
(2008)
The crystal structures of the 1,3,4 oxadiazole compounds N,N-dimethyl-N-[4-(1,3,4-oxadiazol-2-yl)phenyl]amine (1) and 2-methyl-5-phenyl-1,3,4-oxadiazole (2) have been determined. In case of 1 no adequate crystals were available; therefore the structure was solved at room temperature from X-ray powder diffraction data using the method of simulated annealing. This solution is compared to a second one obtained by applying the molecular replacement method. Subsequent Rietveld refinements combined with the so called two stage method based on the data collected to 1.6 Å resolution yielded an Rwp value of 7.27% for 1. Compound 1 crystallizes in the orthorhombic space group P212121 with lattice parameters of a = 7.599(4) Å, b = 6.004(2) Å, c = 21.736(3) Å. The crystal structure of 2 was solved by means of single crystal structure analysis (monoclinic space group P21/c, a = 8.010(3) Å, b = 10.783(4) Å, c = 19.234(7) Å, β = 90.794(9)°).
Diphenyl-1,3,4-oxadiazole (DPO) crystallization experiments from solutions clearly reveal the polymorphism of the substance. Besides the formerly known centrosymmetric monoclinic structure with space group P21/c (DPO I) a new monoclinic structure with the non-centrosymmetric space group Cc is found (DPO II): a=2.4134(4) nm, b=2.4099(3) nm, c=1.2879(2) nm,?=110.048(3)°, and V=7.0363(17) nm3. The asymmetric unit contains six independent molecules in a complex packing motif. A re-determination of the crystal structure of DPO I at room temperature gives lattice parameters a=0.51885(6) nm, b=1.8078(2) nm, c=1.21435(14) nm, ?=93.193(3)°, and V=1.1373(2) nm3. X-ray measurements at 363 K show a significant increase of the unit cell volume by 1.6%. Differences between both structures concerning morphology and characteristic Raman bands are outlined in detail. DSC investigations show an irreversible transition from DPO I to DPO II at 97 °C. DPO II does not show any transition in the temperature range up to the melting point at 141 °C. The non-centrosymmetric DPO II structure shows triboluminescence.