Wilfried Hoffbauer

VALENCE ISOMERIZATION IN THE SOLID STATE : FROM 1,3-DIPHOSPHACYCLOBUTANE-2,4-DIYL TO 1,2-DIHYDRO-1,2-DIPHOSPHETE

The butadiene-like phosphanylcarbene 2 is, according to ab initio calculations, the intermediate in the conversion of 1 into 3 in the solid state [Eq. (a)]; it is only 1.3 kJ mol-1 higher in energy than 1. For this conversion, only minor changes in the endocyclic bonds are required throughout the entire reaction. R2 N=2,2,6,6-Me4 C5 H6 N.

Structure analyses of Ba-silicate glasses

Abstract The structures of four different Ba-silicate glasses were analysed applying a broad spectrum of diffraction and spectroscopic methods, atomic force microscopy and computer simulations. This collaborative study offers new insights into the influence of the network modifier BaO. 3 at.% carbon were incorporated into two of the glasses with the intention to obtain additional information about the effect of partial substitution of carbon for oxygen on the considered structures. A structure model for Ba-silicate glasses with compositions close to BaSi 2 O 5 is discussed that differs with respect to other models proposed in previous studies. We propose that [SiO 4 ] tetrahedra do not exclusively form six-membered rings, but also considerable numbers of smaller rings arranged in (slightly) folded layers which alternate with barium layers and/or barium chains. Additionally, the occurrence of tetrahedral chains, possibly isolated rings and of regions in which the [SiO 4 ] tetrahedra form a silica-like network is assumed.

A comprehensive NMR study of cubic and hexagonal boron nitride

A variety of techniques and measurements on all NMR accessible nuclei allow one to obtain a complete and precise set of chemical shift and quadrupole coupling parameters for both boron and nitrogen in cubic and hexagonal boron nitride. For hexagonal boron nitride, 11B to 15N cross polarization under magic angle sample spinning conditions is demonstrated at natural isotope abundance. The presented approach for NMR characterization of the crystalline boron nitrides should also be applicable to structurally related composite materials, nanotubes, and amorphous ceramics.

Valenzisomerisierung im Festkörper: vom 1,3-Diphosphacyclobutan-2,4-diyl zum 1,2-Dihydro-1,2-diphosphet

Das butadienanaloge Phosphanylcarben 2 ist Ab-initio-Rechnungen zufolge das Intermediat der Umlagerung von 1 zu 3 im Feststoff [Gl. (a)]. Seine Energieliegt nur 1.3 kJ mol−1 hoher als die von 1, und es sind bei dieser Umlagerung wahrend der gesamten Reaktion lediglich kleine Anderungen bezuglich der endocyclischen Bindungen erforderlich. R2N = 2,2,6,6-Me4C5H6N.

Metastable Se6 as a ligand for Ag+: from isolated molecular to polymeric 1D and 2D structures

Attempts to prepare the hitherto unknown Se(6)(2+) cation by the reaction of elemental selenium and Ag[A] ([A](-) = [Sb(OTeF(5))(6)](-), [Al(OC(CF(3))(3))(4)](-)) in SO(2) led to the formation of [(OSO)Ag(Se(6))Ag(OSO)][Sb(OTeF(5))(6)](2)1 and [(OSO)(2)Ag(Se(6))Ag(OSO)(2)][Al(OC(CF(3))(3))(4)](2)2a. 1 could only be prepared by using bromine as co-oxidant, however, bulk 2b (2a with loss of SO(2)) was accessible from Ag[Al(OC(CF(3))(3))(4)] and grey Se in SO(2) (chem. analysis). The reactions of Ag[MF(6)] (M = As, Sb) and elemental selenium led to crystals of 1/∞{[Ag(Se(6))](∞)[Ag(2)(SbF(6))(3)](∞)} 3 and {1/∞[Ag(Se(6))Ag](∞)}[AsF(6)](2)4. Pure bulk 4 was best prepared by the reaction of Se(4)[AsF(6)](2), silver metal and elemental selenium. Attempts to prepare bulk 1 and 3 were unsuccessful. 1-4 were characterized by single-crystal X-ray structure determinations, 2b and 4 additionally by chemical analysis and 4 also by X-ray powder diffraction, FT-Raman and FT-IR spectroscopy. Application of the PRESTO III sequence allowed for the first time (109)Ag MAS NMR investigations of 4 as well as AgF, AgF(2), AgMF(6) and {1/∞[Ag(I(2))](∞)}[MF(6)] (M = As, Sb). Compounds 1 and 2a/b, with the very large counter ions, contain isolated [Ag(Se(6))Ag](2+) heterocubane units consisting of a Se(6) molecule bicapped by two silver cations (local D(3d) sym). 3 and 4, with the smaller anions, contain close packed stacked arrays of Se(6) rings with Ag(+) residing in octahedral holes. Each Ag(+) ion coordinates to three selenium atoms of each adjacent Se(6) ring. 4 contains [Ag(Se(6))(+)](∞) stacks additionally linked by Ag(2)(+) into a two dimensional network. 3 features a remarkable 3-dimensional [Ag(2)(SbF(6))(3)](-) anion held together by strong Sb-FAg contacts between the component Ag(+) and [SbF(6)](-) ions. The hexagonal channels formed by the [Ag(2)(SbF(6))(3)](-) anions are filled by stacks of [Ag(Se(6))(+)](∞) cations. Overall 1-4 are new members of the rare class of metal complexes of neutral main group elemental clusters, in which the main group element is positively polarized due to coordination to a metal ion. Notably, 1 to 4 include the commonly metastable Se(6) molecule as a ligand. The structure, bonding and thermodynamics of 1 to 4 were investigated with the help of quantum chemical calculations (PBE0/TZVPP and (RI-)MP2/TZVPP, in part including COSMO solvation) and Born-Fajans-Haber-cycle calculations. From an analysis of all the available data it appears that the formation of the usually metastable Se(6) molecule from grey selenium is thermodynamically driven by the coordination to the Ag(+) ions.

Incorporation of chromium carbenes in a silica matrix by sol-gel processing: application to aminolysis of alkoxycarbene complexes

Chromium carbene complexes have been immobilized in a silica matrix by sol-gel processing based on the polycondensation of (trialkoxy)silylethylphosphane ligands and tetraalkoxysilanes. The microstructure of the material obtained depends on the gelation conditions. In situ gelation of alkoxy- or aminocarbene complexes with tetramethoxysilane (TMOS) affords mesoporous materials with a homogeneous distribution of the metal complex. The metal carbene moiety is accessible for small substrates as demonstrated for the aminolysis of the incorporated methoxy(phenyl)carbene complex which slows down with increasing bulk of the amine.

Constitution and crystallization behaviour of ultrathin physical vapor deposited (PVD) Al2O3/SiO2 laminates

Abstract Ultrathin Al 2 O 3 /SiO 2 multilayers were produced by physical vapor deposition (PVD) using a double source jumping beam PVD-coater. Al 2 O 3 /SiO 2 multilayer formation is controlled by the electron beam jumping frequency yielding double-layer thicknesses of about 2, 5, 9, and 30 nm. The as-deposited Al 2 O 3 /SiO 2 laminates are non-crystalline and display periodical contrast modulations in TEM cross-sections as long as the nominal thickness of the Al 2 O 3 /SiO 2 double-layer is >5 nm. EDX line scans and 29 Si–MAS–NMR spectroscopy provide evidence of nanosized pure SiO 2 and pure Al 2 O 3 layers. XRD analyses show that films consisting of 30 nm thick Al 2 O 3 and SiO 2 layers at 1000°C form transition alumina only. Transition alumina plus minor amounts of mullite appear at 1000°C in alumino silicate coatings with intermediate Al 2 O 3 and SiO 2 layer thickness (5 and 9 nm), while only mullite occurs in samples with 2 nm thick compositional modulations. The crystallization of PVD-produced alumino silicate films with double layer thicknesses >5 nm behaves similar to diphasic (type II) mullite precursors, while Al 2 O 3 /SiO 2 double-layers 2 nm thick behave like single phase mullite precursors (type I). The latter is surprising because of the diphasic character of the double layers. Obviously, two conditions are required for mullite formation at 1000°C: Interdiffusion-produced chemical homogeneization between Al 2 O 3 and SiO 2 layers, and formation of homogeneization zones large enough for mullite nucleation (about 2 to 5 nm in size).

LAXS and MAS NMR studies on the temperature-induced changes of non-crystalline single phase type mullite precursors

Abstract The structural short-range-order of heat-treated non-crystalline precursors with stoichiometric mullite composition (72 wt% Al2O3, 28 wt% SiO2) has been analyzed by large angle X-ray spectroscopy (LAXS), and by 27 Al nuclear magnetic resonance (NMR) spectroscopy under magic angle spinning (MAS) conditions. The precursors were prepared from tetraethoxysilane (TEOS) and aluminium-sec-butoxide (Al(OBus)3) by a special slow hydrolysis method. 27 Al MAS NMR analyses reveal that the heat-treated mullite precursors have three different Al sites, which, according to literature data are assigned to tetrahedral (fourfold coordinated) [4] Al , pentahedral (fivefold coordinated) [5] Al , and octahedral (sixfold coordinated) [6] Al . The fraction of [5] Al increases with the calcination temperature, whereas the [4] Al and [6] Al percentage decrease. The results of X-ray radial distribution function (RDF) calculations are consistent with the 27 Al NMR data. The RDF indicates a gradually increasing degree of polymerization of TO4 (T=Al and Si), AlO5, and AlO6 polyhedra with the calcination temperature.

Dynamic properties of P4O6S and P4O7: 31P spin–echo and 31P MAS–NMR investigations

The rotational dynamics of P4O6S and P4O7 in the solid state were studied by means of 31P NMR spectra of spinning and static powder samples in the temperature range of 153-295 K and 295-388 K, respectively. All spectra were simulated to confirm the type of the motion and to extract the time scales as a function of the temperature. Good agreement between experimental and theoretical data was obtained on the basis of a three-site jump model. For P4O6S, the activation energy and the pre-exponential factor derived from the lineshape simulations amount to 51(2) kJ/mol and 6(3) x 10(15) s(-1). For P4O7, the spectral analysis yields an activation energy of 67(1) kJ/mol and a pre-exponential factor of 6(2) x 10(14) s(-1). The dynamic behavior was checked independently by lineshape analyses under both MAS and static conditions. Activation energies are consistent within the errors for the lineshape analyses. Additionally, we have analyzed spin-lattice relaxation measurements, which show the correct trends for the activation energies.

B2Si2O6 - ein amorphes anorganisches Netzwerk mit Si-Si-Bindungen

B2Si2O6 has been synthesized via sol-gel proces- sing, reacting Si2Cl6 with boric acid in abs. dioxane. The re- sulting amorphous product is the first borosilicate containing Si-Si bonds. B2Si2O6 has been characterized by IR, NMR, DTA, XANES, and TEM. The spectroscopic results indicate that BSiO3 is an individual phase and not a mixture of Si2O3 and B2O3. A random network model of B2Si2O6 can be de- rived, where structural units of Si2O6 are connected via com- mon vertices to trigonal planar coordinated boron atoms.