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).     

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.     

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.     

P.E.HSQC: a simple experiment for simultaneous and sign-sensitive measurement of (1JCH+DCH) and (2JHH+DHH) couplings

The angular information content of residual dipolar couplings between nuclei of fixed distance makes the accurate and sign-sensitive measurement of (¹J_CH + D_CH) and (²J_HH + D_HH) couplings highly desirable. Experiments published so far are typically highly specialized for the effective measurement of a subset of couplings. The P.E.HSQC presented here, is an E.COSY based experiment which allows the simultaneous measurement of all heteronuclear and homonuclear couplings within CH, CH₂, and CH₃ groups in a single spectrum with the necessary precision and sign information. The simplicity of the approach and the absence of artefacts like phase distortions due to antiphase evolution make it ideally suited for coupling determination of organic molecules at natural abundance.     

Solid phase extraction and diffusive gradients in thin films techniques for determination of total and labile concentrations of Cd(II), Cu(II), Ni(II) and Pb(II) in Black Sea water

Total dissolved and labile concentrations of Cd(II), Cu(II), Ni(II) and Pb(II) were determined at six locations of the Bourgas Gulf of the Bulgarian Black Sea coast. Solid phase extraction procedure based on monodisperse, submicrometer silica spheres modified with 3-aminopropyltrimethoxysilane followed by the electrothermal atomic absorption spectrometry (ETAAS) was developed and applied to quantify the total dissolved metal concentrations in sea water. Quantitative sorption of Cd, Cu, Ni and Pb was achieved in the pH range 7.5–8, for 30?min, adsorbed elements were easily eluted with 2?mL 2?mol?L^?1 HNO₃. Since the optimal pH for quantitative sorption coincides with typical pH of Black Sea water (7.9–8.2), on-site pre-concentration of the analytes without any additional treatment was possible. Detection limits achieved for total dissolved metal quantification were: Cd 0.002?µg?L^?1, Cu 0.005?µg?L^?1, Ni 0.03?µg?L^?1, Pb 0.02?µg?L^?1 and relative standard deviations varied from 5–13% for all studied elements (for typical Cd, Cu, Ni and Pb concentrations in Black Sea water). Open pore diffusive gradients in thin films (DGT) technique was employed for in-situ sampling and pre-concentration of the sea water and in combination with ETAAS was used to determine the proportion of dynamic (mobile and kinetically labile) species of Cd(II), Cu(II), Ni(II) and Pb(II) in the sea water. Obtained results showed strong complexation for Cu and Pb with sea water dissolved organic matter. The ratios between DGT-labile and total dissolved concentrations found for Cu(II) and Pb(II) were in the range 0.2–0.4. For Cd and Ni, these ratios varied from 0.6 to 0.8, suggesting higher degree of free and kinetically labile species of these metals in sea water.     

Electron-structure calculations and bond order analysis using density functional theory of cationic dinuclear arene ruthenium complexes

The structure and nature of the metal-metal bonding interaction in the cationic complexes [(eta6-C6Me6)2Ru2(mu2-H)3]+ (1), [(eta6-C6Me6)2Ru2(mu2-H)2(mu2-1,4-SC6H4Br)]+ (2), [(eta6-C6Me6)2Ru2(mu2-H)(mu2-1,4-SC6H4Br)2]+ (3), and [(eta6-C6Me6)2Ru2(mu2-1,4-SC6H4Br)3]+ (4) have been studied at the density functional theory (DFT) level using molecular orbital (MO) theory, bond order (BO) analysis, bond decomposition energy (BDE), electron localization function (ELF), and Laplacian of the density methods. The results show that there is no direct bond between the two ruthenium atoms in 1-4, the MO interaction within the diruthenium backbone being stabilized by the bridging ligands. For complex 1, the ELF clearly shows that the bond within the diruthenium backbone is through the three bridging hydride ligands, which act as a sort of glue by forming three-center two-electron bonds characterized by (Ru, H, Ru) basins with 1.8 e mostly located in the H atomic basin.     

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.