Dirk Leusser

A comparison of a microfocus X-ray source and a conventional sealed tube for crystal structure determination

Experiments are described in which a direct comparison was made between a conventional 2 kW water-cooled sealed-tube X-ray source and a 30 W air-cooled microfocus source with focusing multilayer optics, using the same goniometer, detector, radiation (Mo Kα), crystals and software. The beam characteristics of the two sources were analyzed and the quality of the resulting data sets compared. The Incoatec Microfocus Source (IµS) gave a narrow approximately Gaussian-shaped primary beam profile, whereas the Bruker AXS sealed-tube source, equipped with a graphite monochromator and a monocapillary collimator, had a broader beam with an approximate intensity plateau. Both sources were mounted on the same Bruker D8 goniometer with a SMART APEX II CCD detector and Bruker Kryoflex low-temperature device. Switching between sources simply required changing the software zero setting of the 2θ circle and could be performed in a few minutes, so it was possible to use the same crystal for both sources without changing its temperature or orientation. A representative cross section of compounds (organic, organometallic and salt) with and without heavy atoms was investigated. For each compound, two data sets, one from a small and one from a large crystal, were collected using each source. In another experiment, the data quality was compared for crystals of the same compound that had been chosen so that they had dimensions similar to the width of the beam. The data were processed and the structures refined using standard Bruker and SHELX software. The experiments show that the IµS gives superior data for small crystals whereas the diffracted intensities were comparable for the large crystals. Appropriate scaling is particularly important for the IµS data.

Structure/Reactivity Studies on an α-Lithiated Benzylsilane: Chemical Interpretation of Experimental Charge Density

Modern organic synthesis (e.g., of natural products) is virtually impossible without employment of enantiomerically enriched compounds. In many cases, alkyllithium compounds are key intermediates for the generation of these stereogenic substances. In recent years, the lithiated carbon atom in silicon-substituted benzyllithium compounds has become a focus of interest because it is possible to maintain its stereogenic information. Starting from a highly enantiomerically enriched benzylsilane, (R,S)-2 x quinuclidine could be obtained, and the absolute configuration at the metalated carbon atom was determined by X-ray diffraction analysis. In solution, a quartet was found in the (13)C NMR spectrum for the metalated carbon atom because of coupling between carbon and lithium, indicating a fixed lithium carbon contact at room temperature. After reaction of (R,S)-2 x quinuclidine with trimethylchlorostannane, the trapped product (S,S)-4 was obtained with a dr > or = 98:2 with inversion of the configuration at the metalated carbon. Multipole refinement against high-resolution diffraction data and subsequent topological analysis of the benchmark system (R,S)-2 x quinuclidine provide insight in the electronic situation and thus the observed stereochemical course of the transformations. Surprisingly, the negative charge generated at the carbanion hardly couples into the phenyl ring. The neighboring silicon atom counterbalances this charge by a pronounced positive charge. Therefore, the alpha-effect of the silicon atom is caused not just by a polarization of the electron density but also by an electrostatic bond reinforcement. Furthermore, the experimentally determined electrostatic potential unequivocally explains the observed back side attack of an electrophile under inversion of the stereogenic center with high diastereomeric ratios.

Highly soluble acenes as semiconductors for thin film transistors

Acene derivatives 3, 5, 7 and 9 bearing two trimethoxyphenylethynyl substituents have been synthesized. The optical properties of these acenes have been investigated by UV/Vis and fluorescence spectroscopy and their eletrochemical properties have been elucidated by cyclic voltammetry (CV). X-Ray analysis of the anthracene and tetracene derivatives 3 and 5 has revealed a 1-D π-stacking in crystals of 3 and 5. In contrast to their parent hydrocarbons, the present disubstituted acenes are highly soluble in different solvents. Spin-coating has afforded amorphous thin films that exhibit field effect mobility. Best performance has been achieved for the OTFTs prepared from the pentacene derivative 7 with a field effect mobility of 1.9 × 10−5 cm2 V−1 s−1 and an on/off ratio of 103.

Carbanion or Amide? First Charge Density Study of Parent 2-Picolyllithium†

The negative charge originating from deprotonation of the methyl group is distributed over the 2-picolyl ring. Bonding properties derived from the electron density distribution support the enamide character of picolyllithium (PicLi; the picture shows the deformation density of [2-PicLi x PicH](2)), but electrophilic attack occurs at the deprotonated C atom. This reactivity is rationalized by the electrostatic potential, which guides electrophiles towards the nucleophilic C atom.

Di(2-pyridyl)-Amides and -Phosphides: Syntheses, Reactivity, Structures, Raman-Experiments and Calculations

Abstract The 2-pyridyl containing compounds (2-Py)2NH 1, (2-Py)2PH 2, Me2Al(2-Py)2N 3, Me2Al(2-Py)2P 4, Et2Al(2-Py)2N 5, Et2Al(2-Py)2P 6 and Et2Al(2-Py)2NAlEt37 have been synthesized and analyzed by solid state structure determination, FT-Raman spectroscopy and theoretical calculations in order to elucidate the charge density distribution. All di(2-pyridyl) amides and -phosphides coordinate the R2Al+ fragment via both ring nitrogen atoms, but the Lewis basicity of the central two-coordinated nitrogen atom in 5 is high enough to coordinate a second equivalent AlEt3 to form the Lewis acid base adduct Et2Al(2-Py)2NAlEt37. Several density functionals (BLYP, B3LYP, BPW91) have been examined in relation to various basis sets (6-31G, 6-31+G, 6-31G(d), 6-31+G(d)). This computational tool facilitates the unambiguous assignment of the Raman ring vibration frequencies. The shift to higher wavenumbers proceeding from the parent di(2-pyridyl)amine 1 and di(2-pyridyl)-phosphane 2 to the metal complexes 3 and 4 indicates partial double bond localization in the ring positions 3 and 5. This effect is more pronounced in the di(2-pyridyl)amide complexes than in the phosphide. Due to the higher electronegativity of the central nitrogen atom in 3, 5 and 7 compared to the bridging two-coordinated phosphorus atom in 4 and 6 the di(2-pyridyl)amide is the harder Lewis base. In the phosphides nearly all charge density couples into the rings leaving the central phosphorus atom only attractive for soft metals.

An experimental charge density study of two isomers of hexasilabenzene.

The similarities and differences between carbon and its heavier congener silicon generate challenging synthetic targets, especially when multiply bonded systems are concerned. The first stable compound with a Si=Si bond goes back to West et al. in 1981, and conjugated systems with Si= Si double bonds were pioneered in 1997 by Weidenbruch et al. Whether silicon analogues of benzene can show aromatic character is still a point of constant debate. The aromatic nature of silabenzenes has been predicted theoretically, but the synthesis of a stable silabenzene was not accomplished until Tokitoh et al. reported the sterically encumbered 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl-substituted monosila derivative. At the same time, Ando et al. independently reported the synthesis of 1,4-disila Dewar benzene. Only two years later, Sekiguchi et al. accomplished the synthesis of 1,2-disilabenzene by reacting RSi SiR (R = Si(CH(SiMe3)2iPr) with PhC CH in a formal [2+2+2] cycloaddition reaction. Recently, in a cooperative effort with the Roesky group, we reported the synthesis of a 1,4-disilabenzene by reacting [{PhC(NtBu)2}Si]2 with diphenyl alkyne. [8] With regards to homonuclear systems, the Scheschkewitz group recently made groundbreaking progress with the isolation of ring and cage isomers of hexasilabenzene (Scheme 1), which prompted the present experimental charge-density study. The dark-green-colored six-membered ring system 1 rearranges upon heating or UV irradiation to the red silicon cage compound 2 with a bridged propellane structure. An analogous transformation for fully saturated silicon compounds under irradiative conditions has been described by Kira and co-workers. We propose a transition of 1 to 2 by the reaction pathway in Scheme 1 (bottom). The transformation proceeds by the breaking of the Si1 Si3 and Si2 Si4 bonds in 1, followed by a twist of the four-membered silicon ring and the formation of the new Si1 Si2 and Si3 Si4 bonds (see Scheme 1 and video in the Supporting Information). In the following, we analyze the bonding situation in the ring (1) and cage (2) isomer of hexasilabenzene (TipSi)6 on the basis of experimental charge-density investigations (Figure 1). High-resolution X-ray data with (sinq/l)max =

Chemical interpretation of molecular electron density distributions

In this study, the two small molecules HS(CH)(CH2), 1, and F(CH)4F, 2, are presented, which yield different chemical interpretations when one and the same density is interpreted either by means of Natural Bond Orbital and subsequent Natural Resonance Theory application or by the Quantum Theory of Atoms In Molecules. The first exhibits a SC bond in the orbital based approach, whereas the density based Quantum Theory of Atoms In Molecules detects no corresponding bond. In F(CH)4F a F···F bond is detected in the density based approach, whereas in the orbital based approach no corresponding bond is found. Geometrical reasons for the presence of unexpected and the absence of expected bond critical points are discussed.

A Unifying Bonding Concept for Metal Hydrosilane Complexes

Experimental and theoretical charge density studies and molecular orbital analyses suggest that the complexes [Cp2Ti(PMe3)SiH2Ph2] (1) and [Cp2Ti(PMe3)SiHCl3] (2) display virtually the same electronic structures. No evidence for a significant interligand hypervalent interaction could be identified for 2. A bonding concept for transition-metal hydrosilane complexes aims to identify the true key parameters for a selective activation of the individual M-Si and Si-H bonds.

Experimental charge density distribution of non-coordinating sp3 carbanions in [Mg{(pz*)3C}2]

In this communication we present the experimental charge density distribution in [Mg{(pz*)(3)C}(2)] (1), (pz* = 3,5-dimethylpyrazolyl), containing two non-coordinating sp(3) carbanionic lone-pairs.

Solid-State Photolysis of Anthracene-Linked Ammonium Salts: The Search for Topochemical Anthracene Photodimerizations

Abstract The reaction of 9-N,N-dimethylaminomethylanthracene 1 with aromatic carboxylic acids gave crystalline salts (2a, 2b, 3a and 3b), which were irradiated in the solid state. Whereas salt 3a was selectively transformed to the dimer 4a, the other salts were photoinert. The results were rationalized on the basis of X-ray structure analysis. In the presence of oxygen, solid-state irrradiation of anthracene salt 3b leads to selective photooxygenation. The selectivity of both solid-state photoreactions was not observed in solution.