Adamantanes, nortricyclenes, and dendrimers with extended silicon backbones

By reaction of the hexabromoheptasilane MeSi(SiMe(2)SiMeBr(2))(3) (1 a) with H(2)O, H(2)S, NH(3), and H(2)NMe the heptasilaadamantanes MeSi(SiMe(2)SiMeO)(3) (4), MeSi(SiMe(2)SiMeS)(3) (5), MeSi(SiMe(2)SiMeNH)(3) (6 a), and MeSi(SiMe(2)SiMeNMe)(3) (6 b), respectively, were prepared in good to moderate yields. Molecular structures of 4, 5, 6 a, and 6 b were determined by X-ray crystallography. The symmetry of the cages is approximately C(3v), and the geometry around the nitrogen atoms is essentially planar. Ab initio SCF/HF calculations with the 6-31G* basis set confirm these results. Reduction of MeSi(SiMe(2)SitBuBr(2))(3) (1 b) with lithium naphthalenide afforded the heptasilanortricyclene MeSi(SiMe(2)SitBu)(3) (7). The (29)Si NMR spectrum of 7 consists of three signals with chemical shifts that agree closely with values predicted by ab initio calculations. (29)Si INADEQUATE spectra also strongly support the nortricyclene structure. Ab initio SCF/HF calculations were performed for the parent molecule Si(7)H(10), and the ring strain of the cage was estimated as 168.8 kJ mol(-1) by using the homodesmic reaction Si(7)H(10) + 3 Si(2)H(6)-->Si(13)H(28). Compound 1 a also served as the starting material for the preparation of first-generation dendrimer 2 a by reaction with six equivalents of Ph(2)MeSiLi. Subsequent protodearylation with HBr and reaction with (Me(2)PhSi)(2)SiMeK afforded second-generation dendrimer 3. All dendrimers were characterized by multinuclear NMR spectroscopy.     

Transport properties and point-contact spectra of RECu6 with RE=La,Ce and Pr

Measurements of the current-voltage characteristics on point-contacts, of the thermopower and the thermal conductivity between 1.3 and 300 K and of the electrical resistivity between 4.2 and 900 K of polycrystalline samples of ReCu₆ (RE=La, Ce, Pr, Gd) are presented. The anomalies of the heavy fermion system CeCu₆ are strong compared to the other systems, but the properties of LaCu₆ and PrCu₆ are also unusual, possibly caused by the complicated crystal structure and, for PrCu₆, by crystal field effects.     

Korrelation zwischen photoelektronen- und elektronen-spektren—IV: Der substituenteneinfluss auf das E1g(a)-π-orbital des benzols

By optimized PPP-SCF-CI-parameters and the conception of partial bond- and charge-localizing on the basis of a structural model of benzene the PE- and UV-spectroscopical dates of Fluor-, Chlor-, Brom-, Jodbenzene, Thiophenol, Toluene, Phenol and Aniline are interpreted within the error-limit of experimental values. By the influence of substituents on the π-e_1g(a)-orbital of benzene it is demonstrated, that the rigid use of Koopmans-Theorem cannot be presupposed, if n-orbitals are insufficient delocalized. Therefore the differences from Koopmans-Theorem are the greatest in Fluorbenzene and decrease with increasing donor-character. They are also significant in Aniline. The examination of this differences with the aid of the model-conception permits statements of two significant effects: (1) bond-length-changes and (2) orbital-decontraction or—contraction (changes in the effective nuclear charge number) caused by ionisation or excitation. Both effects are reverse. The second effect dominates in the examined compounds and causes changes in MO-energy on an average of - 0·333 [eV] (3·65%).     

Group 10 metal aminopyridinato complexes: synthesis, structure, and application as aryl-Cl activation and hydrosilane polymerization catalysts

(4-Methyl-pyridin-2-yl)(trimethylsilanyl)amine (ApSi-H) and tert-butyl(4-methyl-pyridin-2-yl)amine (AptBu-H) were synthesized via salt metathesis and aryl amination reactions, respectively. Lithiation of these two aminopyridines using n-BuLi and the reactions with [(dme)NiCl2] (dme = dimethoxyethane) or [(cod)PdCl2] (cod = cyclooctadiene) in THF at low temperature gave rise--after workup in hexane--to group 10 amido compounds, [(ApSi)4Ni2], [(AptBu)2Pd], [(AptBu-H)(AptBu)2Ni], [(AptBu)3(C2H5O)3Ni3OLi(thf)], and [(AptBu)2Ni(tBupy)2] (tBupy = 4-tert-butylpyridine). The aminopyridinato complexes were characterized by X-ray crystal structure analysis. The highly strained binding situation of the aminopyridinato ligands suggested that these compounds might be efficiently converted into catalytically active species. The applications of some of the synthesized complexes as Suzuki cross-coupling catalysts (activation of aryl chlorides) are described and [(ApSi)4Ni2] is a rare example of a "phosphine-free" catalyst system. A number of late transition metal complexes were found to successfully catalyze polymerization of MeH2SiSiH2Me toward soluble, linear poly(methylsilane). Remarkable activity was observed for [(ApSi)2Pd].     

Using a simple high-performance liquid chromatography separation and fraction collection methodology to achieve compound-specific isotopic analysis for dissolved organic compounds

A new application for the quantitative and isotopic analyses of dissolved inorganic and dissolved organic carbon compounds has been developed. Dissolved organic matter (DOM) in natural water samples can be separated on a high-performance liquid chromatography (HPLC) column and collected as fractions. Each discrete fraction can then be analyzed using the technique of St-Jean (Rapid Commun. Mass Spectrom. 2003; 17: 419-428) with a total inorganic carbon/total organic carbon (TIC/TOC) analyzer interfaced with a continuous-flow isotope ratio mass spectrometer. Experimental data using short-chain fatty acid standards (formic, acetic, and propionic acids) show that fraction recoveries of 100% are possible and that sample integrity is maintained. 13C-isotopic analyses of products prior to and subsequent to extraction and collection show no isotopic effects associated with the methodology, and errors are well within the accepted analytical uncertainty of the IRMS. Comparison of data from pure standards and organic-rich natural waters shows that quantitative analyses still need to be done with standards that more closely imitate the matrices of the samples, in order to acquire an appropriate calibration curve. Injections of organic-rich matrices on the HPLC column did not affect fraction recovery, nor did they create high background of partially retained organic compounds slowly released from the HPLC column, and hence 13C-isotopic results are relatively unaffected. The specific limitation on this methodology is the required use of carbon-free carrier solvents due to potential memory effects associated with the TIC/TOC analyzer. Further developments of this application could make routine compound-specific isotopic analyses (CSIA) for a wider range of organic materials possible.     

Synthesis of a hafnocene disilene complex

Reaction of the magnesium transmetalation product of a 1,2-dipotassiodisilane with hafnocene dichloride gives a disilene hafnocene complex. X-ray crystallography of the respective trimethylphosphane adduct provides structural proof for this assignment.     

Electronic and magnetic properties of the high-T c superconductor EuBa2Cu3O x

Electrical resistivity and magnetic susceptibility show that single phase EuBa₂Cu₃O _x becomes superconducting at 94 K. Magnetization and magnetoresistivity yield a lower critical field of H_c1500 G and a variation ofT _c with magnetic field of dT _c /dH1 K/T. The high temperature resistivity shows that at about 700 K due to oxygen loss the sample is driven into a semiconducting state, in which the superconductivity is destroyed. The normal state magnetic susceptibility measured in the superconducting and in the oxygen degased, semiconducting sample indicates that the sample is paramagnetic in the normal state. This paramagnetism is larger than the one expected for trivalent Eu and therefore has to be attributed in part to a magnetic moment on Cu. Eu is found to be stable trivalent between 300 and 4.2 K through Mößbauer-spectroscopy.