Investigation of the stereodynamics of tris‐(α‐diimine)–transition metal complexes by enantioselective dynamic MEKC

Enantiomerization of octahedral tris(α‐diimine)–transition metal complexes was investigated by enantioselective dynamic MEKC. Varying both the transition metal ion (Fe²⁺, Fe³⁺, and Ni²⁺) and the bidentate diimine ligand (1,10‐phenanthroline and 2,2′‐bipyridyl), the enantiomer separations were performed either in a 100 mM sodium tetraborate buffer (pH 9.3) or in a 100 mM sodium tetraborate/sodium dihydrogenphosphate buffer (pH 8.0) both containing sodium cholate as chiral surfactant. The unified equation of dynamic chromatography was employed to determine apparent reaction rate constants from the electropherograms showing distinct plateau formation. Apparent activation parameters ΔH^? and ΔS^? were calculated from temperature‐dependent measurements between 10.0 and 35.0°C in 2.5 K steps. It was found that the nature of the central metal ion and the ligand strongly influence the enantiomerization barrier. Surprisingly, complexes containing the 2,2′‐bipyridyl ligand show highly negative activation entropies between ?103 and ?116 J (K mol)^?1 while the activation entropy of tris(1,10‐phenanthroline) complexes is positive indicating a different mechanism of interconversion. Furthermore, it was found that the Ni²⁺ complexes are stereostable under the conditions investigated here making them a lucent target as enantioselective catalysts.     

On the Suitability of Different Online Gait Generation Methods for Pre-Defined Footsteps

Detailed dynamical modeling is the basis for simulation and model based control. In this contribution the Projection Equation is used for the modeling of a biped walking machine, resulting in the equations of motion which are needed for gait generation and verification of its stability. For biped robots one main issue is the generation of stable trajectories for the center of mass (CoM). Several different approaches based on the Zero Moment Point (ZMP) scheme have been presented in the past. Due to the complex dynamic structure of bipedal robots most of the considered algorithms use a linear inverted pendulum as a simplified model. This results in a decoupling of the ZMP equations in lateral and forward direction, but limits the trajectories to a constant height of the CoM. An extension of the well known LQR theory by future reference values has been proposed. This model based approach seems to perform quite well, but does not allow the consideration of constraints on the position of the ZMP. This limitation is removed by the use of Model Predictive Control (MPC) with inequality constraints. By extending this approach to a time invariant one the restriction to a constant height of the CoM is no longer necessary. Both methods as well as the time invariant approach for variable CoM heights have been evaluated in simulations and will be experimentally verified on a real robot soon. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Transmission of 3 keV Ne7+ ions through nanocapillaries etched in polymer foils: evidence for capillary guiding

We report unprecedented transmission experiments of 3 keV Ne7+ ions through capillaries of 100 nm diameter and 10 microm length produced by etching ion tracks in a polymer foil. We studied foils tilted up to +/-20 degrees for which the incident ions are forced to interact with the capillary surface. Surprisingly, the majority of Ne7+ ions were found to survive the surface scattering events in their initial charge state. The angular distributions of the transmitted particles indicate propagation of the Ne7+ ions along the capillary axis. This capillary guiding of the Ne7+ ion provides evidence that the inner walls of the capillaries become charged and electron capture from the surface is suppressed in a self-organizing process.     

Untersuchungen an oxidischen Katalysatoren. XIX. Über die thermische Alterung eines Al2O3/Cr2O3/K2O-Tränkkatalysators für die Dehydrocyclisierung und seine Regenerierung

Studies on Oxide Catalysts. XIX. On the Thermal Aging of an Al₂O₃/Cr₂O₃/K₂O Dehydrocyclization Catalyst Made by Impregnation and its Regeneration An Al₂O₃/Cr₂O₃/K₂O catalyst was aged at different temperatures in hydrogen flow. These samples and the regenerated ones (prepared by oxidation and subsequent reduction under mild conditions of the aged samples) were investigated by means of reflectance spectroscopy, EPR, oxygen chemisorption, determination of oxidizability and microcatalytic measurements of the activity in the dehydrocyclization of n-hexane. The reversible and irreversible processes occurring during the thermal aging are discussed. The importance of the oxidizability of the chromia for the regenerability of the catalysts is explained.     

Zur Konkurrenz von Stickstoff- und Schwefel-Donoratomen in Zinkkomplexen: [Zn(bims)2][SiF6] · 5MeOH und [ZnCl(paps)]2(Zn2Cl6) (bims = Bis(2-benzimidazolylmethyl)sulfid; paps = o,o′-(N,N′-Dipicolinyliden)diazadiphenyldisulfid)

Competition between Nitrogen and Sulfur Donor Atoms in Zinc(II) Complexes: [Zn(bims)₂][SiF₆] · 5MeOH and [ZnCl(paps)]₂(Zn₂Cl₆) (bims = bis(2-benzimidazolylmethyl)sulfide; paps = o,o′-(N,N′-dipicolinylidene)diazadiphenyl-disulfide) [Zn(bims)₂](SiF₆) · 5 MeOH ( 1 ) was synthesized by reaction of Zn(SiF₆) with the ligand bis(2-benzimidazolylmethyl)sulfide (bims). Zinc is tetrahedrally coordinated by four benzimidazole nitrogen atoms of the two ligand molecules, the sulfur atom does not coordinate. From reaction of ZnCl₂ with o,o′-(N,N′-dipicolinylidene)diazadiphenyldisulfide (paps) [ZnCl(paps)]₂(Zn₂Cl₆) ( 2 ) was obtained. Here, an octahedral coordination by four nitrogen atoms, one sulfur atom of the ligand and one chloride ion was found. Both compounds were characterized by infrared and ¹H-NMR-spectroscopy as well as by single crystal X-ray structure analysis. Space groups and structural data: 1: P1 , a = 9.904(2), b = 10.951(3), c = 19.356(2) Å, α = 91.08(2), β = 91.11(2), γ = 95.74(2)°, R1 = 0.0631; 2: P1 , a = 9.051(3), b = 11.110(3), c = 15.170(4) Å, α = 92.99(2), β = 105.93(2), γ = 107.79(2)°, R1 = 0.0585.     

Untersuchungen an Oxidischen Katalysatoren. XXX [1] Charakterisierung der Me2+ -Kationenlokalisierung in CaNaY-, MgNaY- und CaMgNaY-Zeolithen

Studies on Oxide Catalysts. XXX. Characterization of the Me²⁺ Localization in Zeolites CaNaY, MgNaY, and CaMgNaY Data for exchange of the cations Ca²⁺, Mg²⁺, as well as Ca²⁺ and Mg²⁺ into the zeolite NaY have been determined. From this it was concluded on the localization of Me²⁺ cations in hydrated samples. The adsorption of ammonia in the temperature region of 420 to 670 K, the determination of the heats of immersion in water and nitromethane and the Me²⁺ ? CO-interaction indicated by IR spectroscopy yielded informations on the localization of cations in dehydrated samples. The results show that cations Ca²⁺ more selectively than cations Mg²⁺ occupy positions in the small cages. In the case of the competitive cation exchange this fact causes a site directed distribution of cations in zeolite cages.     

Tripodal Ligands: Design of Distorted Coordination Polyhedra in Biomimetic Metal Complexes. Crystal structures of [Zn(SCN)(ntb)](SCN) · iPrpOH and [Fe(acac)(ntb)](ClO4)2 · 2CH2Cl2 · iPrpOH,ntb = N-tris(2-benzimidazolylmethyl)amine

The tripodal ligand N-tris(2-benzimidazolylmethyl)-amine (ntb) was used for the preparation of zinc(II) and iron(III) complexes, [Zn(SCN)(ntb)](SCN) · ⁱPrpOH ( 1 ) and [Fe(acac)(ntb)](ClO₄)₂ · 2CH₂Cl₂ · ⁱPrpOH ( 2 ). 1 has a highly distorted trigonal-bipyramidal ZnN₅ coordination geometry. The donor atoms are nitrogens of one amine, three benzimidazoles and one SCN^?. A striking feature of the complex is the length of the Zn? N_amine bond of 2.539(6)Å. The octahedral N₄O₂ coordination sphere of the iron in 2 is less distorted than that of the zinc in 1 . The metal is surrounded by an amine and three benzimidazole nitrogens of the ligand and two oxygens of the bidentate acetylacetonate co-ligand. The Fe? O bond lengths differ by about 0.1 Å. As for the unusual long Zn? N bond in 1 this is a result of a trans effect. 1 crystallizes in the space group P1 with: a = 9.530(1)Å, b = 13.402(1)Å, c = 13.578(2)Å, α = 98.83(1), β = 95.19(1), γ = 101.21(1)°, Z = 2; 2 is also triclinic, space group P1 , with: a = 9.875(6)Å, b = 12.929(10)Å, c = 18.635(15)Å, α = 94.95(8)°, β = 101.01(6)°, γ = 111.09(4)°, Z = 2.     

Determining the Diastereoselectivity of the Formation of Dipeptidonucleotides by NMR Spectroscopy

Proteins are composed of l -amino acids, but nucleic acids and most oligosaccharides contain d -sugars as building blocks. It is interesting to ask whether this is a coincidence or a consequence of the functional interplay of these biomolecules. One reaction that provides an opportunity to study this interplay is the formation of phosphoramidate-linked peptido RNA from amino acids and ribonucleotides in aqueous condensation buffer. Here we report how the diastereoselectivity of the first peptide coupling of the peptido RNA pathway can be determined in situ by NMR spectroscopy. When a racemic mixture of an amino acid ester was allowed to react with an 5′-aminoacidyl nucleotide, diastereomeric ratios of up to 72 : 28 of the resulting dipeptido nucleotides were found by integration of ³¹P- or ¹H-NMR peaks. The highest diastereomeric excess was found for the homochiral coupling product d -Ser-d -Trp, phosphoramidate-linked to adenosine 5′-monophosphate with its d -ribose ring. When control reactions with an N-acetyl amino acid and valine methyl ester were run in organic solvent, the diastereoselectivity was found to be lower, with diastereomeric ratios≤62 : 38. The results from the exploratory study thus indicate that the ribonucleotide residue not only facilitates the coupling of lipophilic amino acids in aqueous medium but also the formation of a homochiral dipeptide. The methodology described here may be used to search for other stereoselective reactions that shed light on the origin of homochirality.