Adsorption of aspartic acid on kaolinite

The interaction of aspartic acid with kaolinite was studied by potentiometric titrations and by adsorption measurements both at constant aspartate concentration (but varying pH) and at a constant pH of 5.5. The temperature was 25 degrees C, and the ionic medium 5 mM KNO3. Aspartic acid dissociation constants estimated from titrations agreed with those from the literature. The adsorption of aspartic acid to kaolinite was weak and varied only slightly with pH; 10-18% of 100 microM aspartic acid adsorbed to kaolinite at 100 m(2)L(-1) between pH 3 and 10. Data from the titrations and adsorption experiments were fitted closely by an extended constant-capacitance surface complexation model, in which monodentate outer-sphere complexes formed between deprotonated aspartic acid molecules and protonated sites on the variable-charge edges of the kaolinite crystals. There appeared to be no adsorption to the permanently charged crystal faces.     

Aromatic foldamers as scaffolds for metal second coordination sphere design

As metalloproteins exemplify, the chemical and physical properties of metal centers depend not only on their first but also on their second coordination sphere. Installing arrays of functional groups around the first coordination sphere of synthetic metal complexes is thus highly desirable, but it remains a challenging objective. Here we introduce a novel approach to produce tailored second coordination spheres. We used bioinspired artificial architectures based on aromatic oligoamide foldamers to construct a rigid, modular and well-defined environment around a metal complex. Specifically, aza-aromatic monomers having a tethered [2Fe–2S] cluster have been synthesized and incorporated in conical helical foldamer sequences. Exploiting the modularity and predictability of aromatic oligoamide structures allowed for the straightforward design of a conical architecture able to sequester the metal complex in a confined environment. Even though no direct metal complex–foldamer interactions were purposely designed in this first generation model, crystallography, NMR and IR spectroscopy concurred to show that the aromatic oligoamide backbone alters the structure and fluxional processes of the metal cluster.

Wrapping a [2Fe–2S] metal complex in an aromatic foldamer helix is introduced as a new approach to tailor a second coordination sphere.     

Structural and Spectroscopic Studies of Halocuprate(I) and Haloargentate(I) Complexes [M2XnX′4−n]2−

The complexes [Cu₂Br₄]^2?, [Cu₂I₄]^2?, [Cu₂I₂Br₂]^2?, [Cu₂I₃Cl]^2?, [Ag₂Cl₄]^2? have been characterized as their isomorphous bis(triphenylphosphoranylidene)ammonium ([Ph₃PNPPh₃]⁺ = PNP⁺) salts by single crystal structural determinations. All anions show the centrosymmetric doubly halogen‐bridged forms [XM(μ‐X)₂MX]^2? with three‐coordinate metal atoms that have been observed in [M₂X₄]^2? complexes with other large organic cations. In [Cu₂I₂Br₂]^2? the iodide ligands occupy the bridging positions and the bromide the terminal positions, while in [Cu₂I₃Cl]^2?, obtained in an attempt to prepare [Cu₂I₂Cl₂]^2?, two of the iodide ligands occupy the bridging positions with the third iodide and the chloride ligand occupying two statistically disordered terminal positions. In [Ag₂Cl₄]^2? the distortion from ideal trigonal coordination of the metal atom is greater than in the copper complexes, but less than in other previously reported [Ag₂Cl₄]^2? complexes with organic cations. The ν(MX) bands have been assigned in the far‐IR spectra, and confirm previous observations regarding the unexpectedly simple IR spectra of [Cu₂X₄]^2? complexes.     

Limitations of low resolution mass spectrometry in the electron capture negative ionization mode for the analysis of short- and medium-chain chlorinated paraffins

The analysis of complex mixtures of chlorinated paraffins (CPs) with short (SCCPs, C₁₀–C₁₃) and medium (MCCPs, C₁₄–C₁₇) chain lengths can be disturbed by mass overlap, if low resolution mass spectrometry (LRMS) in the electron capture negative ionization mode is employed. This is caused by CP congeners with the same nominal mass, but with five carbon atoms more and two chlorine atoms less; for example C₁₁H₁₇³⁷Cl³⁵Cl₆ (m/z 395.9) and C₁₆H₂₉³⁵Cl₅ (m/z 396.1). This can lead to an overestimation of congener group quantity and/or of total CP concentration. The magnitude of this interference was studied by evaluating the change after mixing a SCCP standard and a MCCP standard 1+1 (S+MCCP mixture) and comparing it to the single standards. A quantification of the less abundant C₁₆ and C₁₇ congeners present in the MCCP standard was not possible due to interference from the major C₁₁ and C₁₂ congeners in the SCCPs. Also, signals for SCCPs (C₁₀–C₁₂) with nine and ten chlorine atoms were mimicked by MCCPs (C₁₅–C₁₇) with seven and eight chlorine atoms (for instance C₁₀H₁₂Cl₁₀ by C₁₅H₂₄Cl₈). A similar observation was made for signals from C₁₅–C₁₇ CPs with four and five chlorine atoms resulting from SCCPs (C₁₀–C₁₂) with six and seven chlorine atoms (such as C₁₅H₂₈Cl₄ by C₁₀H₁₆Cl₆) in the S+MCCP mixture. It could be shown that the quantification of the most abundant congeners (C₁₁–C₁₄) is not affected by any interference. The determination of C₁₀ and C₁₅ congeners is partly disturbed, but this can be detected by investigating isotope ratios, retention time ranges and the shapes of the CP signals. Also, lower chlorinated compounds forming [M+Cl]^– as the most abundant ion instead of [M-Cl]^– are especially sensitive to systematic errors caused by superposition of ions of different composition and the same nominal mass.     

Structures and anion-binding properties of M4L6 tetrahedral cage complexes with large central cavities

Reaction of the bis-bidentate bridging ligand L(3), in which two bidentate chelating 3(2-pyridyl)pyrazole units are separated by a 3,3'-biphenyl spacer, with Co(II) salts affords tetranuclear cage complexes of composition [Co(4)(L(3))(6)]X(8)(X =[BF(4)](-), [ClO(4)](-), [PF(6)](-) or I(-)) in which four 6-coordinate Co(II) ions in an approximately tetrahedral array are connected by six bis-bidentate bridging ligands, one spanning each of the six edges of the Co(4) tetrahedron. In every case, X-ray crystallography reveals that the 'apical' Co(II) ion has a fac tris-chelate geometry, whereas the other three Co(II) ions have mer tris-chelate geometries, resulting in (non-crystallographic)C(3) symmetry for the cages; that this structure is retained in solution is confirmed by (1)H NMR spectroscopy of the paramagnetic cages. In every case one of the anions is located inside the central cavity of the cage, with the remaining seven outside. We found no clear evidence for an anion-based templating effect. The cage superstructure is sufficiently large to leave gaps in the centres of the faces through which the internal and external anions can exchange. Variable-temperature (19)F NMR spectroscopy was used to investigate the dynamic behaviour of the cages with X =[BF(4)](-) and [PF(6)](-) in MeCN solution: in both cases two separate signals, corresponding to external and internal anions, are clear at 233 K which have coalesced to a single signal at room temperature. Analysis of the linewidth of the minor signal (for the internal anion) at various temperatures below coalescence gave an activation energy for anion exchange of ca. 50 kJ mol(-1) in each case, a figure which suggests that anion exchange can occur via a conformational rearrangement of the cage superstructure in solution rather than opening of the cavity by cleavage of metal-ligand bonds.     

1,3-dialkyl- and 1,3-diaryl-3,4,5,6-tetrahydropyrimidin-2-ylidene rhodium(i) and palladium(II) complexes: synthesis, structure, and reactivity

The synthesis of novel 1,3-diaryl- and 1,3-dialkylpyrimidin-2-ylidene-based N-heterocyclic carbenes (NHCs) and their rhodium(i) and palladium(II) complexes is described. The rhodium compounds bromo(cod)[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene]rhodium (7), bromo(cod)(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)rhodium (8) (cod=eta(4)-1,5-cyclooctadiene, mesityl=2,4,6-trimethylphenyl), chloro(cod)(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)rhodium (9), and chloro(cod)[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene]rhodium (10) were prepared by reaction of [[Rh(cod)Cl](2)] with lithium tert-butoxide followed by addition of 1,3-dimesityl-3,4,5,6-tetrahydropyrimidinium bromide (3), 1,3-dimesityl-3,4,5,6-tetrahydropyrimidinium tetrafluoroborate (4), 1,3-di-2-propyl-3,4,5,6-tetrahydropyrimidinium bromide (6), and 1,3-di-2-propyl-3,4,5,6-tetrahydropyrimidinium tetrafluoroborate, respectively. Complex 7 crystallizes in the monoclinic space group P2(1)/n, and 8 in the monoclinic space group P2(1). Complexes 9 and 10 were used for the synthesis of the corresponding dicarbonyl complexes dicarbonylchloro(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)rhodium (11), and dicarbonylchloro[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene]rhodium (12). The wavenumbers nu(CO I)/nu(CO II) for 11 and 12 were used as a quantitative measure for the basicity of the NHC ligand. The values of 2062/1976 and 2063/1982 cm(-1), respectively, indicate that the new NHCs are among the most basic cyclic ligands reported so far. Compounds 3 and 6 were additionally converted to the corresponding cationic silver(i) bis-NHC complexes [Ag(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(2)]AgBr(2) (13) and [Ag[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene](2)]AgBr(2) (14), which were subsequently used in transmetalation reactions for the synthesis of the corresponding palladium(II) complexes Pd(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(2) (2+)(Ag(2)Br(2)Cl(4) (4-))(1/2) (15) and Pd[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene)(2)]Cl(2) (16). Complex 15 crystallizes in the monoclinic space group P2(1)/c, and 16 in the monoclinic space group C(2)/c. The catalytic activity of 15 and 16 in Heck-type reactions was studied in detail. Both compounds are highly active in the coupling of aliphatic and aromatic vinyl compounds with aryl bromides and chlorides with turnover numbers (TONs) up to 2000000. Stabilities of 15 and 16 under Heck-couplings conditions were correlated with their molecular structure. Finally, selected kinetic data for these couplings are presented.     

Wafer-scale periodic nanohole arrays templated from two-dimensional nonclose-packed colloidal crystals

This communication reports a simple yet versatile nonlithographic approach for fabricating wafer-scale periodic nanohole arrays from a large variety of functional materials, including metals, semiconductors, and dielectrics. Spin-coated two-dimensional (2D) nonclose-packed colloidal crystals are used as first-generation shadow masks during physical vapor deposition to produce isolated nanohole arrays. These regular nanoholes can then be used as second-generation etching masks to create submicrometer void arrays in the substrates underneath. Complex patterns with micrometer-scale resolution can be made by standard microfabrication techniques for potential device applications. These 2D-ordered nanohole arrays may find important technological applications ranging from subwavelength optics to interferometric biosensors.     

Stacking due to ionic transport number mismatch during sample sweeping on microchips

Sample stacking can occur in isoconductive buffer systems as a result of ion transport mismatches that cause changes in buffer conductivity during electrophoresis. Fluorescence imaging was used to examine this effect in the sweeping of hydrophobic dyes with sodium dodecyl sulfate (SDS) on microchips. Imaging revealed the occurrence of a stacking effect in a sodium borate buffer system in which the sample buffer and SDS-containing run buffer had the same initial conductivity. Injected sample plugs were first swept by SDS micelles and the swept band was then stacked at the trailing end of the sample zone. This effect is due to changes in conductivity at both the front and back interfaces of the injected sample plug and can be modeled by moving boundary equations. Maximum signal enhancements of 86-, 160- and 560-fold were obtained for Rhodamine 560, Rhodamine B and Rhodamine 6G, respectively, by the combination of sweeping and stacking within a 1 cm section of microchannel. Based on sample sweeping/stacking and manipulation of the electric field polarity, a method of trapping and concentrating analyte from multiple injections was also demonstrated.