Strategies for the anchoring of metal complexes, clusters, and colloids inside nanoporous alumina membranes

Two complementary strategies are presented for the anchoring of molecular palladium complexes, of cobalt or platinum clusters or of gold colloids inside the nanopores of alumina membranes. The first consists in the one step condensation of an alkoxysilyl functional group carried by the metal complex with the hydroxy groups covering the surface of the membrane pores. Thus, using the short-bite alkoxysilyl-functionalized diphosphane ligands (Ph2P)2N(CH2)3Si(OMe)3 (1) and (Ph2P)2N(CH2)4SiMe2(OMe)] (2) derived from (Ph2P)2NH (dppa) (dppa bis(diphenylphosphanyl)amine), the palladium complexes [Pd(dmba)(kappa2-P,P-(Ph2P)2N(CH2)3Si(OMe)3)] Cl (3) and [Pd(dmba)[kappa2-P,P-(Ph2P)2N(CH2)4SiMe2(OMe)]]Cl (4) (dmba-H = dimethylbenzylamine). respectively, were tethered to the pore walls. After controlled thermal treatment. confined and highly dispersed palladium nanoparticles were formed and characterized by transmission electron microscopy (TEM). This method could not be applied to the cobalt cluster [Co4(CO)8(mu-dppa)[mu-P,P-(Ph2P)2N(CH2)4SiMe2(OMe)]] (7) owing to its too limited solubility. However, its anchoring was achieved by using the second method which consisted of first derivatizing the pore walls with 1 or 2. The covalent attachment of the diphosphane ligands provides a molecular anchor that allows subsequent reaction with the cluster [Co4(CO)10(mu-dppa)] 6 to generate anchored 7 and this step was monitored by UV/Vis spectroscopy. In addition, the presence of carbonyl ligands in the cluster provides for the first time a very sensitive spectroscopic probe in the IR region which confirms both cluster incorporation and the retaining of its molecular nature inside the membrane. The presence of the bridging dppa ligand in 6 provides additional stabilization and accounts for the selectivity of the procedure. Using this method, platinum clusters (diameter ca. 2 nm) and gold colloids (diameter ca. 13 nm) were immobilized after passing their solution through the functionalized membrane pores. The resulting membranes were characterized by TEM which demonstrated the efficiency of the complexation and showed the high dispersion of the metal loading. The successful application of these methods has demonstrated that nanoporous alumina membranes are not only unique supports to incorporate metal complexes, clusters, or colloids but can also be regarded as functional matrices or microreactors, thus opening new fields for applications.     

Metal Nanoparticles on Stainless Steel Surfaces as Novel Heterogeneous Catalysts

The goal of this work was the development of a novel type of heterogeneous catalyst, consisting of bare metal nanoparticles on stainless steel foils, which can be shaped to any kind of architecture and, if necessary, heated electrically. Solutions of pre-prepared, ligand protected and monodispersed gold, palladium, platinum and rhodium nanoparticles were sprayed onto stainless steel foils, followed by the careful removal of the ligand molecules by an oxygen plasma treatment. Due to this, bare particles become irreversibly fixed on the steel support. It could be shown that the original particle sizes do not change during the plasma treatment. Foils, densely coated with the nanoparticles, were used for gas phase catalyses in a self-made reactor at room temperature or at 60 °C. Hydrogenation of 1,3-butadiene at 15 nm Pd and 2 nm Pt, CO oxidation at 16 nm, 8 nm and 1.4 nm gold and NO reduction with NH₃ at 2 nm Rh particles were performed, indicating that the novel catalysts might in principle be applicable in technical processes if the experimental conditions like form and temperature would be optimized. Dedicated to Professor Dieter Fenske on the occassion of his 65th birthday.     

NMR in metal cluster compounds compared to glasses

The field and temperature dependence of the³¹P nuclear spin lattice relaxation rate in the metal cluster compound Ru₅₅(P(t-Bu)₃)₁₂Cl₂₀ follows a power law: 1/T ₁ ∝T ⁿ B ^?m , withn=1.5±0.1 at 3.25 T andn=1.3±0.1 at 6.45 T;m ? 1.4. Such dependences have so far only been observed in inorganic glasses and been attributed to two level systems. The correspondence suggests that the relaxation rate is due to interaction of theP-nuclear moment with electronic spins of stochastically moving charge carriers, which are thought to be responsible for the electrical conductivity through hopping between neigboring cluster molecules.     

Electrochemistry of capillary systems with narrow pores. IV. Dialysis potentials

The dialysis potentials of different collodion membranes with graded pore sizes and electrochemical activities have been measured in dilute aqueous KCl solutions as functions of concentration. It is possible to predict the value of the diffusion potential within a few millivolts on the basis of electrical conductivity data obtained with the same membranes. In general, the measured values are lower than those calculated. It is assumed that this difference is caused by the membranes having a distribution of pore sizes.     

Über das Alkaloid Lonicerin aus Callichilia barteri. 137. Mitteilung über Alkaloide

The alkaloid lonicerine was isolated from Callichilia barteri (Apocynaceae). By chemical and spectroscopic evidence it could be shown to be 16-epi-aspidodasycarpine ( 1 ).     

A fluorimetric assay for cortisol

A simple, rapid and sensitive fluorimetric assay for the quantitative determination of cortisol is reported. The assay is based on the formation of a fluorescent dye when cortisol is incubated with a mixture of sulfuric acid and acetic acid. The fluorescence spectrum recorded for the resulting dye shows a maximum extinction at 475 nm and a maximum emission at 525 nm. The solvent 2-methyl-4-pentanone was used for extraction and was found to act as a fluorescence amplifier. A limit of detection of 2.7 μM was achieved, making it possible to forego solvent evaporation. The assay suffers minor interference from 11-deoxycortisol which exhibits low fluorescence at λ _ex: 460 nm; λ _em: 505 nm. Typical standard deviations were below 4%. We validated the assay using a biotransformation with recombinant Schizosaccharomyces pombe which regioselectively hydroxylates 11-deoxycortisol to cortisol. The method described herein is suitable for preliminary screening of microorganisms capable of steroid hydroxylation.     

Thermisches Verhalten von Cyclopropa[c]chromenen

The Cyclopropa[c]chromenes 14 , trans-and cis- 15 , trans-and cis- 16 and 17 rearrange on heating > 200° in N, N-diethylaniline to give 2-alkyl-2H-chromenes 7, 8, 21, 22. The rate determining step of this rearrangement is the ‘homoelectrocyclic’ ring opening of the cyclopro-pa[c]chromenes to give ω-allyl-quinomethanes of type 4. These intermediates show fast [1,5s] and [1,7a] H-shifts, followed by electrocyclic ring closure. Deuterium labelling experiments are in agreement with this mechanism. The remarkable dependence of the rates of rearrangement with respect to the stereochemistry of the cyclopropa[c]chromenes (cf. table 2) suggests that in the first step only one of the two possible disrotatory modes of ring opening is involved.     

Forward and backward pericyclic photochemical reactions have intermediates in common, yet cyclobutenes break the rules.

Photochemical pericyclic reactions are believed to proceed via a so-called pericyclic minimum on the lowest excited potential surface (S(1)), which is common to both the forward and backward reactions. Such a common intermediate has never been directly detected. The photointerconversion of 1,3-butadiene and cyclobutene is the prevailing prototype for such reactions, yet only diene ring closure proceeds with the stereospecificity that the Woodward-Hoffmann rules predict. This contrast seems to exclude a common intermediate. Using ultrafast spectroscopy, we show that the excited states of two cyclobutene/diene isomeric pairs are linked by not one, but by two common minima, p* and ct*. Starting from the diene side (cyclohepta-1,3-diene and cycloocta-1,3-diene), electrocyclic ring closure passes via the pericyclic minimum p*, whereas ct* is mainly responsible for cis-trans isomerization. Starting from the corresponding cyclobutenes (bicyclo[3.2.0]heptene-6 and bicyclo[4.2.0]octene-7), the forbidden isomer is formed from ct*. The path branches at the first (S(2)/S(1)) conical intersection towards p* and ct*. The fact that the energetically unfavorable ct* path can compete is ascribed to a dynamic effect: the momentum in C=C twist direction, acquired--such as in other olefins--in the Franck-Condon region of the cyclobutenes.     

Alstonia scholaris: Struktur des Indolalkaloides Narelin

Alstonia scholaris: The structure of the indole alkaloid nareline Besides the known akuammidine, picralinal, picrinine and pseudoakuammigine a new indole alkaloid called nareline (M=352) was isolated from Alstonia scholaris R. BR. , which belongs to the plant family of Apocynaceae. Its structure 2 was deduced by single crystal X-ray diffraction. 2 represents the absolute configuration. The spectroscopic data of 2 and its derivatives (Scheme 1) as well as their chemical behavior support this structure. In biogenetic sense nareline is related to the bases akuammiline ( 4 ) and picraline ( 5 ) (Scheme 2). In contrast to those the C-atom 5 is exocyclic and represents an aldehyde group which forms together with the oxygen atom of the N (4)-hydroxylamine group a cyclic half acetale. - By oxidation (CrO₃/CH₃COOH) of 2 the oxindol derivative 19 (oxonareline) is formed which contains a cyclic acetal as a partial structure element (Scheme 4).     

Critical parameters and optimization of a magnesium-selective liquid membrane electrode for application to human blood serum

Summary The selectivity of a new magnesium ionophore (ETH 7025) induced in membranes of different compositions is experimentally studied in view of the ion activities in human serum. The required selectivity coefficient against calcium for the application of an ion-selective magnesium electrode to human serum is calculated for the worst case. Other critical parameters for the application of a liquid PVC-based ion-selective membrane to undiluted human serum discussed are: the sensor lifetime which is related to the lipophilicity of the carrier as well as the ruggedness of the membrane against interactions with components of the relatively lipophilic sample.