Michaela Flock

Group 4 metallocene complexes of disilenes, digermenes, and a silagermene.

Reactions of 1,2-dipotassiotetrakis(trimethylsilyl)disilane with group 4 metallocene dichlorides lead to the formation of the respective metallocene 1,1,2,2-tetrakis(trimethylsilyl)disilene complexes. While the disilene titanocene complex could be structurally characterized, the zirconocene and hafnocene compounds, which are believed to possess some degree of bis-[bis(trimethylsilyl)silylene] character, can only be isolated in substance as the respective trimethylphosphane adducts. Analogous metallocene 1,1,2,2-tetrakis(trimethylsilyl)digermene complexes and a tetrakis(trimethylsilyl)silagermene complex were prepared. Instead of metallocene 1,1,2,2-tetrakis(trimethylsilyl)distannene complexes, four-membered rings composed of a metallocene and three bis(trimethylsilyl)stannylene units were obtained.

The Role of 2,6-Diaminopyridine Ligands in the Isolation of an Unprecedented, Low-Valent Tin Complex

Stabilization of the central atom in an oxidation state of zero through coordination of neutral ligands is a common bonding motif in transition-metal chemistry. However, the stabilization of main-group elements in an oxidation state of zero by neutral ligands is rare. Herein, we report that the transamination reaction of the DAMPY ligand system (DAMPY=2,6-[ArNH-CH2 ]2 (NC5 H3 ) (Ar=C6 H3 -2,6-iPr2 )) with Sn[N(SiMe3 )2 ]2 produces the DIMPYSn complex (DIMPY=(2,6-[ArNCH]2 (NC5 H3 )) with the Sn atom in a formal oxidation state of zero. This is the first example of a tin compound stabilized in a formal oxidation state of zero by only one donor molecule. Furthermore, three related low-valent Sn(II) complexes, including a [DIMPYSn(II) Cl](+) [SnCl3 ](-) ion pair, a bisstannylene DAMPY{Sn(II) [N(SiMe3 )2 ]2 }2 , and the enamine complex MeDIMPYSn(II) , were isolated. Experimental results and the conclusions drawn are also supported by theoretical studies at the density functional level of theory and (119) Sn Mössbauer spectroscopy.

Intramolecular interactions in β-alanine, 3-aminopropanal and 3-aminopropanol

Abstract The intramolecular interactions in the local minima on the potential energy surfaces of β-alanine, 3-aminopropanal and 3-aminopropanol are discussed on the basis of electron density maps. The resulting characterization is in agreement with criteria using bond lengths and vibration frequencies. The CO group is found to be the most important structural feature in these compounds.

Formation of Formal Disilene Fluoride Adducts

Reactions of trisilylated silylfluorides with potassium tert-butoxide were found to give disilylated fluorosilylenoids. The latter undergo a self-condensation reaction, leading to the formation of beta-fluorodisilanyl anions, which may also be considered as fluoride adducts of disilenes. The stereochemical outcome of this self-condensation depends on the bulkiness of the silyl substituents. Thus, mixtures of diastereomers are obtained in some cases. Reaction of a disilene adduct with magnesium bromide triggers metal fluoride elimination and formation of the respective tetrasilylated disilene. Attempts to exchange one silyl group of the starting material for a methyl, phenyl, or tert-butyl group led to a different course of reaction for the methyl and phenyl cases. The tert-butyl-substituted example gave the expected disilene adduct, but it was not the main product.

Silylanions: inversion barriers and NMR chemical shifts.

Alpha-substituent effects on inversion barriers and NMR chemical shifts have been studied on a set of silyl anions, [X(3-n)Y(n)Si](-) (X, Y=H, CH(3), and SiH(3)). The MP2/6-31+G* optimized structures show a pattern of increasing inversion barriers with augmenting numbers of methyl substituents. The highest barrier of 48.5 kcalmol(-1) is obtained for the (CH(3))(3)Si(-) ion. The silyl group displays the opposite effect by decreasing the inversion barrier to a minimum of 16.3 kcalmol(-1) in (SiH(3))(3)Si(-). The influence of counterions on these barriers is probed by addition of a lithium or potassium cation. In most cases, a decrease of the energy barriers with respect to the bare anions is observed. The (29)Si NMR chemical shifts calculated at the IGLO-DFT and GIAO-MP2 level of theory are also analyzed in view of the substituents and counterions.

Ab initio SCF investigation of 3-aminopropanol and 3-aminopropanal

Abstract Results of ab initio SCF (4-31G) studies of the potential energy surfaces of 3-aminopropanol and 3-aminopropanal are reported. Geometry data of all local minima are given and various intramolecular interactions are deduced from these data. All reaction paths in the potential energy surface of 3-aminopropanal and selected reaction paths in the potential energy surface of 3-aminopropanol are discussed.

Stable Silenolates and Brook-Type Silenes with Exocyclic Structures.

The first silenolates with exocyclic structures [(Me3Si)2Si(Si2Me4)2SiC(R)O]−K+ (2a: R = 1-adamantyl; 2b: mesityl; 2c: o-tolyl) were synthesized by the reaction of the corresponding acylcyclohexasilanes 1a–c with KOtBu. NMR spectroscopy and single-crystal X-ray diffraction analysis suggest that the aryl-substituted silenolates 2b,c exhibit increased character of functionalized silenes as compared to the alkyl-substituted derivative 2a due to the different coordination of the K+ counterion to the SiC(R)O moiety. 2b,c, thus, reacted with ClSiiPr3 to give the exocyclic silenes (Me3Si)2Si(Si2Me4)2Si=C(OSiiPr3)R (3b: R = Mes; 3c: o-Tol), while 2a afforded the Si-silylated acylcyclohexasilane 1d. The thermally remarkably stable compound 3b, which is the first isolated silene with the sp2 silicon atom incorporated into a cyclopolysilane framework, could be fully characterized structurally and spectroscopically.

Structure based descriptors for the estimation of colloidal interactions and protein aggregation propensities.

The control of protein aggregation is an important requirement in the development of bio-pharmaceutical formulations. Here a simple protein model is proposed that was used in molecular dynamics simulations to obtain a quantitative assessment of the relative contributions of proteins’ net-charges, dipole-moments, and the size of hydrophobic or charged surface patches to their colloidal interactions. The results demonstrate that the strength of these interactions correlate with net-charge and dipole moment. Variation of both these descriptors within ranges typical for globular proteins have a comparable effect. By comparison no clear trends can be observed upon varying the size of hydrophobic or charged patches while keeping the other parameters constant. The results are discussed in the context of experimental literature data on protein aggregation. They provide a clear guide line for the development of improved algorithms for the prediction of aggregation propensities.

Molecular Structures of the Digermanes Me2XGeGeXMe2 (X = Me, Cl, and H) : An Ab Initio Study Combined with Experimental Investigation by Raman Spectroscopy and Gas Electron Diffraction

Ab initio calculations were carried out to investigate the potential-energy surface for internal rotation of the methylated digermanes hexamethyldigermane Me(3)GeGeMe(3) ( 1), dichlorotetramethyldigermane Me(2)ClGeGeClMe(2) ( 2), and tetramethyldigermane Me(2)HGeGeHMe(2) ( 3). Different basis sets [6-31+G(d), SDD, aug-cc-pVTZ] were employed at the DFT and MP2 levels of theory to optimize structures and to calculate energies and vibrational frequencies. For 1, one minimum representing a staggered structure was located on the potential-energy surface. For 2 and 3, antiperiplanar conformations with C 2 symmetry were found to be the global minima. Additionally, synclinal minima were located for 2 and 3 when certain basis sets were employed. Determination of structural parameters in the gas phase by gas electron diffraction confirmed the computed predictions for all three compounds. For 2 and 3, the ratios of antiperiplanar to synclinal conformer were detected to be 90:10 (328 K) and 72:28 (293 K), respectively, by gas electron diffraction. The experimentally determined GeGe bond lengths in 1, 2, and 3 in the gas phase are 241.4(1), 242.7(2) (averaged for antiperiplanar and synclinal), and 241.7(1) pm (equal for antiperiplanar and synclinal). Only averaged structures were observed, using Raman spectroscopy, for 2 and 3 because the wavenumber differences are small between conformers and there is only a small contribution from the second conformer in each case. For 2, the crystal structure was also determined by X-ray diffraction. An anticlinal structure (with Cl atoms eclipsing the C atoms) was found with a GeGe bond length of 242.1 pm.

Ab initio SCF investigation of γ-hydroxybutyric acid

Abstract The potential energy surface of γ-hydroxybutyric acid was investigated by means of ab initio 4-31G SCF calculations. Sixty-six symmetry unique local minima are contained in this potential energy surface. Intramolecular interactions and all reactions, which involve the intramolecular hydrogen bond CO−O−H … O−H are discussed. Combination with the results for γ-aminobutyric acid yields a comparison of hydrogen bonds involving the groups −OH and −NH 2 .