Nature of the chemical bond between metal atoms and oxide surfaces: new evidences from spin density studies of K atoms on alkaline earth oxides

We have studied the interaction of K atoms with the surface of polycrystalline alkaline-earth metal oxides (MgO, CaO, SrO) by means of CW- and Pulsed-EPR, UV-Vis-NIR spectroscopies and DFT cluster model calculations. The K adsorption site is proposed to be an anionic reverse corner formed at the intersection of two steps, where K binds by more than 1 eV, resulting in thermally stable species up to about 400 K. The bonding has small covalent and large polarization contributions, and the K atom remains neutral, with one unpaired electron in the valence shell. The interaction results in strong modifications of the K electronic wave function which are directly reflected by the hyperfine coupling constant, (K)a(iso). This is found to be a very efficient "probe" to measure the degree of metal-oxide interaction which directly depends on the substrate basicity. These results provide an original and general model of the early stages of the metal-support interaction in the case of ionic oxides.     

δ‐Methyl Branching in the Side Chain Makes the Difference: Access to Room‐Temperature Discotics

Although discotic liquid crystals are attractive functional materials, their use in electronic devices is often restricted by high melting and clearing points. Among the promising candidates for applications are [15]crown‐5 ether‐based liquid crystals with peripheral n‐alkoxy side chains, which, however, still have melting points above room temperature. To overcome this problem, a series of o‐terphenyl and triphenylene [15]crown‐5 ether derivatives was prepared in which δ‐methyl‐branched alkoxy side chains of varying lengths substitute the peripheral linear alkoxy chains. The mesomorphic properties of the novel crown ethers were studied by differential scanning calorimetry, polarizing optical microscopy, and X‐ray diffraction. δ‐Methyl branching indeed lowers melting points resulting in room‐temperature hexagonal columnar mesophases. The mesophase widths, which ranged from 87 to 30 K for o‐terphenyls, significantly increased to 106–147 K for the triphenylenes depending on the chain lengths, revealing the beneficial effect of a flat mesogen, due to improved π–π interactions.     

On-Line Coupling of Separation Techniques to NMR

The hyphenation of chromatographic separation techniques with NMR spectroscopy is one of the most powerful and time-saving methods for the separation and structural elucidation of unknown compounds and molecular compositions of mixtures. Most of the routinely used NMR flow-cells have detection volumes between 40–180 μL for conventional separations with analytical columns, and the newest designs employ detection volumes in the order of 200 nL for capillary separations. The low flow rates used in capillary chromatography permit the use of deuterated solvents. Unequivocal structural assignment of unknown chromatographic peaks is possible by two-dimensional stopped-flow capillary HPLC-NMR experiments.     

Stereoselective hydroxylation of an achiral cyclopentanecarboxylic acid derivative using engineered P450s BM-3

Substrate engineered, achiral carboxylic acid derivative was biohydroxylated with various mutants of cytochrome P450 BM-3 to give two out of the four possible diastereoisomers in high de and ee. The BM-3 mutants exhibit up to 9200 total turnovers for hydroxylation of the engineered substrate, which without the protecting group is not transformed by this enzyme.     

Fabrication of Metal‐Block‐Copolymer Composite Films by a Palladium‐Catalyzed Electroless Nickel‐Plating Process

Summary: Homogeneous films of PI‐b‐PDMAEMA are prepared on top of silicon (100) substrates. The free film surface shows microdomains of PDMAEMA within a PI matrix. These microdomains act as templates for the highly site‐selective synthesis of metal nanoparticles via palladium‐catalyzed electroless nickel plating. The particle formation is studied by atomic force microscopy in tapping mode and implications for a redox reaction and a nanoparticle growth mechanism on the surface of nanopatterned films are discussed.

Chemical structure of the PI‐b‐PDMAEMA copolymer and AFM phase image of a PI‐b‐PDMAEMA film on Si (100) substrate.     

Laccase-catalyzed carbon-carbon bond formation: oxidative dimerization of salicylic esters by air in aqueous solution

The laccase catalyzed oxidative dimerization of salicylic esters, a rare example of a laccase-catalyzed carbon-carbon bond formation, was studied. This reaction allows the use of air as stoichiometric oxidant and proceeds in aqueous solution. The preparative scope and the mechanism of the method, which provides a new and convenient access to functionalized biaryls under mild conditions, were investigated.     

Novel rearrangements and formation of 2,5-dihydro-1,2,5-oxoniastannaboratolesvia methanolysis of zwitterionic η2-(alkyn-1-yl-borate)alkenyltin compounds

Organo-substituted 2,5-dihydro-1,2,5-oxoniastannaboratoles3 were prepared by methanolysis of zwitterionic ²-(alkyn-l-ylborate)alkenyltin compounds1. Analogously, the bis[²-(alkyn-l-yl-borate)alkenyltin derivative2 reacts with an excess of methanol to give a dimeric MeO-bridged 2,5-dihydro-1,2,5-oxoniastannaboratole (10a)₂. Various intermediates could be identified by NMR spectroscopy, and the molecular structure of(10a) ₂ was determined by X-ray analysis. The structures of several products of the methanolysis indicate that protolysis of an Sn-C=bond occurs first, followed by an MeO/alkenyl exchange reaction.Dedicated to the 100th Anniversary of A. N. Nesmeyanov's birth.Laboratorium für Anorganische Chemie, Universität Bayreuth, D-95440 Bayreuth, Germany Institut für Anorganische Chemie, Universität-GH Essen, Universitätsstr. 5–7, D-45117 Essen, Germany. Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1185–1196, September, 1999.