Small changes—Huge influences: NMR chemical shifts of Ni(II) complexes with polar substrates

Neutral Ni(II) complexes have been shown to be highly valuable as robust and versatile catalysts in olefin polymerization. But they show reduced reactivity when the polar monomers methyl acrylate and vinyl acetate are incorporated. To get further insight into this behavior, NMR chemical shift calculations were performed on the system [(N,O) Ni (H) (PMe3)] 1 (N,O = ‐N,O‐{2,6‐(3,5‐(F3C)2C6H3)2C6H3) NC(H)‐3,5‐I2‐2‐O‐C6H2}). The chemical shifts show reasonable agreement with experiment but are also extremely influenced by geometrical features of the complex as well as the inserted substrate. The first prominent feature, the low‐field shift of the C_carbonyl in the incorporated monomer, can only be reproduced when it is in close proximity to the Ni and in this way hinders the attack of a new monomer. Second, the almost 100 ppm difference in the chemical shift of the carbon of the two substrates directly bound to Ni can be reasoned by the different directionality of polarization as disclosed by natural bond orbital (NBO) analysis. © 2013 Wiley Periodicals, Inc.     

Synthesis and characterization of novel carbene complexes of phosphorus(V) fluorides with potential liquid-crystalline properties

A series of novel push–pushI and push–pullII carbene-stabilized complexes of phosphorus(V) fluorides bearing substituents with liquid-crystalline properties were synthesized by the oxidative addition of difluoroamines to phosphorus(III) halides. These octahedral complexes were characterized by NMR spectroscopy and X-ray analysis.     

On the interaction of two different types of ligands binding to the same molecule part I: basics and the transfer of the decoupled sites representation to systems with n and one binding sites

The decoupled sites representation (DSR) for one type of ligand allows to regard complex overall titration curves as sum of classical Henderson-Hasselbalch (HH) titration curves. In this work we transfer this theoretical approach to molecules with different types of interacting ligands (e.g. protons and electrons), prove the existence of decoupled systems for n ₁ and one binding sites for two different ligands, and point out some difficulties and limits of this transfer. A major difference to the DSR for one type of ligand is the loss of uniqueness of the decoupled system. However, all decoupled systems share a unique set of microstate probabilities and each decoupled system corresponds to a certain permutation of these microstate probabilities. Moreover, we show that the titration curve of a certain binding site in the original system can be regarded as linear combination of the titration curves of the individual sites of the decoupled system if the weights of the linear combination are substituted by functions in the activity of the second ligand. In the underlying model with only pairwise interaction, an important observation of our theoretical investigation is the following: Even though the binding sites of ligand L ₁ may not interact directly, they can show secondary interaction due to the interaction with the second type of ligand. This means, if the activity of the second ligand is fixed and we regard the 1-dimensional titration curve of an individual binding site for ligand L ₁ depending on its activity, we may observe a strong deviation from the classical HH shape in spite of non-interacting sites for ligand L ₁.     

Coexpression of CPR from Various Origins Enhances Biotransformation Activity of Human CYPs in S. pombe

Cytochrome P450 enzymes (CYPs or P450s) are the most important enzymes involved in the phase I metabolism of drugs (and other xenobiotics) in humans, and the corresponding drug metabolites are needed as reference substances for their structural confirmation and for pharmacological or toxicological characterization. We have previously shown that biotechnological synthesis of such metabolites is feasible by whole-cell biotransformation with human CYPs recombinantly expressed in the fission yeast Schizosaccharomyces pombe. It was the aim of this study to compare the activity of seven human microsomal CYPs (CYP2C9, CYP2D6, CYP3A4, CYP3A5, CYP3A7, CYP17, and CYP21) upon coexpression with NADPH-cytochrome P450 oxidoreductases (CPRs) from various origins, namely, human CPR (hCPR) and its homologues from fission yeast (ccr1) and the bishop’s weed Ammi majus (AmCPR), respectively. For this purpose, 28 recombinant strains were needed, with five of them having been constructed previously and 23 strains being newly constructed. Bioconversion experiments showed that coexpression of a CPR does not only influence the reaction rate but, in some cases, also exerts an influence on the metabolite pattern. For CYP3A enzymes, coexpression of hCPR yielded the best results, while for another two, hCPR was equally helpful as ccr1 (both CYP17 and CYP21) or AmCPR (CYP17 only), respectively. Interestingly, CYP2D6 displayed its highest activity when coexpressed with ccr1 and CYP2C9 with AmCPR. These results corroborate the view of CPR as a well-suited bio-brick in synthetic biology for the construction of artificial enzyme complexes.     

Hybrid polymer sol–gel material for UV-nanoimprint: microstructure and thermal densification

Hybrid polymer solutions suitable for UV-nanoimprint were synthesized by combination of an alkoxysilane binder mixture with silica nanoparticles. Hydrolysis and condensation reactions were monitored by NMR and viscosity measurements. Thereby long-term stable systems were produced as a prerequisite for industrial application. Dip-coating of glass substrates and subsequent UV-curing yielded thin films. Their thermal densification and microstructural evolution resulted in pure glassy porous coatings, which were in detail characterized by N₂-sorption measurements and ellipsometric porosimetry. Results emphasize the importance of the binder-particle interaction within these materials that are destined for the fabrication of microstructured surfaces by cost efficient and industrially feasible UV-based soft lithography. Structured glassy layers with high inorganic content show thermal stability up to >500 °C and have a high structure accuracy >85 %.     

Valence‐to‐Core X‐Ray Emission Spectroscopy of Iron–Carbonyl Complexes: Implications for the Examination of Catalytic Intermediates

Valence‐to‐core X‐ray emission spectroscopy (V2C XES) has been applied to a series of compounds relevant to both homogeneous catalysts and intermediates in heterogeneous reactions, namely [Fe(CO)₅], [Fe₂(CO)₉], [Fe₃(CO)₁₂], [Fe(CO)₃(cod)] (cod=cyclo‐octadienyl), [Fe₂Cp₂(CO)₄] (Cp=cyclo‐pentadienyl), [Fe₂Cp*₂(CO)₄] (Cp*=tetramethylcyclopentadienyl), and [FeCp(CO)₂(thf)][B(ArF)₄] (ArF=pentafluorophenyl). DFT calculations of the V2C XES spectra show very good agreement with experiment, which allows for an in depth analysis of the origins of the observed spectral signatures. It is demonstrated that the observed spectral features can be broken down into specific ligand and metal fragment contributions. The relative intensities of the observed features are further explained through a quantitative investigation of the metal 3p and 4p contributions to the spectra. The ability to use V2C XES to separate carbonyl, hydrocarbon, and solvent contributions is highlighted.     

Gels dried under supercritical and ambient conditions: a comparative study and their subsequent conversion to silica–carbon composite aerogels

In present work, we have prepared gels with various compositions of methyltrimethoxysilane—3-(2,3-epoxypropoxy) propyltrimethoxysilane (MTMS-GPTMS) using a two-step acid base sol–gel process. To make a comparative study between the two common drying routes, we prepared gels under supercritical and also under ambient conditions. The density of the supercritically dried hybrid aerogels lies between 0.18 and 0.31 gcm^?3, while the density of the ambient dried ones ranges between 0.35 and 0.42 gcm^?3. The surface area of MTMS-0.25 GPTMS aerogel dried under supercritical conditions, has been found to be 464 m² g^?1 with a pore volume and average pore diameter of 1.24 cm³ g^?1 and 11 nm respectively. The same composition dried under ambient conditions is found to have similar properties i.e. a BET surface area of 439 m² g^?1, pore volume of 1.22 cm³ g^?1 and average pore diameter of 11 nm. The aerogels were later pyrolyzed yielding silica/carbon composite aerogels. The pyrolized aerogels possessed a surface area as high as 207 m² g^?1 with a total pore volume of 0.98 cm³ g^?1. The pyrolysed aerogels were also calcined to yield carbon free materials.