Zwitterionic Spirocyclic λ5Si‐Silicates with Two Bidentate Acetohydroximato(2–) or Benzohydroximato(2–) Ligands: Synthesis,Structure, and Dynamic Stereochemistry

The syntheses of the zwitterionic spirocyclic λ⁵Si‐silicates 7–14 are described. The chiral zwitterions contain a pentacoordinate (formally negatively charged) silicon atom and a tetracoordinate (formally positively charged) nitrogen atom, the ate and onium center being connected by an alkylene group. The zwitterions each contain two identical bidentate diolato(2–) ligands that formally derive from acetohydroximic acid or benzohydroximic acid. The stereochemistry and dynamic behavior of these compounds were investigated by experimental and theoretical methods. For this purpose, the zwitterionic λ⁵Si‐silicates 7–14 were studied by solution (¹H, ¹³C, ²⁹Si) and solid‐state (¹³C, ¹⁵N, and ²⁹Si CP/MAS) NMR experiments. In addition, compounds 7 , 8 , 10 , 11 , and 13 were structurally characterized by single‐crystal X‐ray diffraction. The dynamic behavior (intramolecular enantiomerization) of 7 and 13 in solution was studied by VT ¹H NMR experiments. These experimental studies were completed by ab initio investigations of the related anionic model species 15 . The chiral compounds 7–14 exist as (λ)‐ and (δ)‐enantiomers in the solid state and in solution. The trigonal‐bipyramidal structure of the respective Si‐coordination polyhedra, with the two carbon‐linked oxygen atoms in the axial sites, is the energetically most favorable one. The (λ)‐ and (δ)‐enantiomers of 7–14 are configurationally stable in solution on the NMR time scale ([D₆]DMSO, room temperature). They undergo an intramolecular (λ)/(δ)‐enantiomerization (twist‐type mechanism), with an activation free enthalpy of δG^{ = 72–73 kJ mol^–1 (experimentally established for 7 and 13 ; calculated energy barrier for the model species 15 : 66.0 kJ mol^–1).     

Spirocyclic zwitterionic lambda5Si-silicates with two bidentate ligands derived from alpha-amino acids or alpha-hydroxycarboxylic acids: synthesis, structure, and stereodynamics

A series of zwitterionic lambda(5)Si-silicates with a (2,2,6,6-tetramethylpiperidinio)methyl group and two identical bidentate ligands derived from glycine, (S)-alanine, (S)-phenylalanine, glycolic acid, (S)-lactic acid, (S)-3-phenyllactic acid, or (S)-mandelic acid were synthesized and structurally characterized (solution and solid-state NMR spectroscopy; single-crystal X-ray diffraction). The chiral lambda(5)Si-silicates with ligands derived from optically active alpha-amino acids or alpha-hydroxycarboxylic acids were isolated as enantiomerically and diastereomerically pure compounds that undergo a (Lambda)/(Delta)-epimerization in solution.     

Q.E.COSY: determining sign and size of small deuterium residual quadrupolar couplings using an extended E.COSY principle

Residual quadrupolar couplings contain important structural information comparable with residual dipolar couplings. However, the measurement of sign and size of especially small residual quadrupolar couplings is difficult. Here, we present an extension of the E.COSY principle to spin systems consisting of a Spin 1 coupled to a spin ½ nucleus, which allows the determination of the sign of the quadrupolar coupling of the Spin 1 nucleus relative to the heteronuclear coupling between the spins. The so‐called Q.E.COSY approach is demonstrated with its sign‐sensitivity using variable angle NMR, stretched gels and liquid crystalline phases applied to various CD and CD₃ groups. Especially the sign‐sensitive measurement of residual quadrupolar couplings that remain unresolved in conventional deuterium 1D spectra is shown. Copyright © 2016 John Wiley & Sons, Ltd.     

Raffinose Capped Silver Nanoparticles: A New Localized Surface Plasmon Resonance Based Sensor for Selective Quantification of Cr(VI) in Waste Waters

In this study, a new method for selective determination of Cr(VI) in water samples at pH 4 is presented using raffinose capped silver nanoparticles (Ag/Raff NPs) as an optical sensor. The method is based on the variation of LSPR absorption band intensity as a result of electrostatic interaction between the negatively charged Ag/Raff NPs and positive Cr(III) ions, in-situ produced by chemical reduction of Cr(VI) with ascorbic acid, combined with the fast kinetics of Cr(III) coordination to the –OH groups of the capping agent on the nanoparticle surface, further causing the nanoparticle aggregation. The calibration curve for Cr(VI) is linear in the range 2.5–7.5 μmol L⁻¹, the limit of quantification achieved is 1.9 μmol L⁻¹, and values of relative standard deviation vary from 3 to 5% for concentration level 1.9–7.5 μmol L⁻¹. The interference studies performed in the presence of various metal ions show very good selectivity of Ag/Raff NPs toward Cr(VI) species. The added–found method is used to confirm the accuracy and precision of developed analytical approach.     

Synthesis and structural characterization of neutral higher-coordinate silicon(IV) complexes with tridentate dianionic chelate ligands

The neutral pentacoordinate silicon(IV) complexes 8 and 9 with an SiO2N3 skeleton and the neutral hexacoordinate silicon(IV) complex 10.1/2 CH3CN with an SiO4N2 skeleton were synthesized, starting from tetra(cyanato-N)silane or tetra(thiocyanato-N)silane. Compounds 8 and 9 contain one tridentate dianionic ligand derived from 4-[(2-hydroxyphenyl)amino]pent-3-en-2-one and two monodentate singly charged cyanato-N or thiocyanato-N ligands bound to the silicon(IV) coordination center, whereas the silicon(IV) center of 10 is coordinated by two of these tridentate dianionic ligands. All compounds were characterized by single-crystal X-ray diffraction and solid-state and solution NMR spectroscopy. To get more information about the stereochemistry of the compounds studied, the experimental investigations were complemented by computational studies.     

Spektrophotometrische Untersuchung der Eisen(III)-Komplexe mit o-Methylbenzamidoxim

The complex formation of Fe³⁺ with o-methyl benzamide oxime was studied spectrophotometrically in methanol solution. The stepwise process gives complexes 1∶1, 1∶2 and 1∶3. The formation constants are lgK ₁ = lg β₁ = 1,88 ± 0,12, lgK ₂ = 3,53 ± 0,2, lgK ₃ = 4,96 ± 0,2, lg β₂ = 1,65 ± 0,32 and lg β₃ = 1,43 ± 0,4, whereK ₃ = β₁ · β₂ · β₃. All measurements were carried out at 25°C and an ionic strength μ=1.