E-Selective Manganese-Catalyzed Semihydrogenation of Alkynes with H2 Directly Employed or In Situ-Generated.

Selective semihydrogenation of alkynes with the Mn(I) alkyl catalyst fac-[Mn(dippe)(CO)3(CH2CH2CH3)] (dippe = 1,2-bis(di-iso-propylphosphino)ethane) as a precatalyst is described. The required hydrogen gas is either directly employed or in situ-generated upon alcoholysis of KBH4 with methanol. A series of aryl-aryl, aryl-alkyl, alkyl-alkyl, and terminal alkynes was readily hydrogenated to yield E-alkenes in good to excellent isolated yields. The reaction proceeds at 60 °C for directly employed hydrogen or at 60-90 °C with in situ-generated hydrogen and catalyst loadings of 0.5-2 mol %. The implemented protocol tolerates a variety of electron-donating and electron-withdrawing functional groups, including halides, phenols, nitriles, unprotected amines, and heterocycles. The reaction can be upscaled to the gram scale. Mechanistic investigations, including deuterium-labeling studies and density functional theory (DFT) calculations, were undertaken to provide a reasonable reaction mechanism, showing that initially formed Z-isomer undergoes fast isomerization to afford the thermodynamically more stable E-isomer.

Toward the Recovery of Platinum Group Metals from a Spent Automotive Catalyst with Supported Ionic Liquid Phases.

We present a novel approach for the separation and recovery of Pt and Pd leached from a spent automotive catalyst relying on conventional and polymerized supported ionic liquid phases (SILPs and polySILPs, respectively). A variety of parameters with possible effects on the separation behavior, namely, acidity and concentration of the platinum group metal (PGM) containing solution, as well as different SILP and polySILP loadings, were evaluated for the separation of PGMs in the presence of high concentrations of Al, Fe, Zn, and Ce. The polySILP material demonstrated the ability to separate the PGMs from major accompanying interferences in a single separation step, while problems arising from ionic liquid leaching in the case of SILPs could be avoided. Moreover, the use of supported ionic liquid phases allowed the drastic reduction of the amount of required ionic liquid compared to conventional liquid-liquid separation, while avoiding problems arising from emulsion formation. Subsequent stripping experiments lead to further purification of the PGMs and finally desorption from the solid material into a pure solution. Eventually, the concept of chemisorbed polySILPs provides a new and convenient approach for the recycling of platinum group metals.

Influence of the Ionic Liquid on the Activity of a Supported Ionic Liquid Phase FeII Pincer Catalyst for the Hydrogenation of Aldehydes.

The catalytic hydrogenation of different aldehydes to the corresponding alcohols was investigated using an FeII hydride pincer complex as catalyst in the supported ionic liquid phase (SILP) reaction mode. Two different ionic liquids of the type [X4441][NTf2] with X=N or P were applied with mesoporous silica gel as support, which was coated first with a chemisorbed monolayer of the corresponding modified IL to remove acidic surface OH-groups and to prevent IL leaching. Quantitative conversion with turn-over frequencies in the order of 1000 h- 1 were obtained for various aromatic and heteroaromatic aldehydes and highly selective aldehyde reduction was observed also for substrates containing reducible C=C bonds. Aldehydes with longer aliphatic chains or cycloalkyl substituents, however, showed no conversion here, in contrast to a previous study with an imidazolium-based ionic liquid. These differences were ascribed primarily to differences in substrate/ionic liquid interactions. Whereas [N4441][NTf2] and [P4441][NTf2] gave essentially identical results for different substrates in single-batch reactions, prolonged use of the catalyst in repeated reaction cycles lead to a quick drop-off in catalyst activity in [P4441][NTf2], but a continuous, quantitative conversion in [N4441][NTf2].

Calcium l-tartrate complex formation in neutral and in hyperalkaline aqueous solutions.

The complex formation reaction between the l-tartrate (Tar2-) and calcium ions taking place in neutral and in hyperalkaline (pH > 13) aqueous solutions has been investigated. It was demonstrated that upon NaOH addition the solubility of the CaTar(s) precipitate significantly increases. Conductometric and freezing point depression measurements further confirmed that in this process water soluble species are formed as a result of a reaction between the CaTar(s) and the hydroxide ion (or, conversely, between Ca(OH)2(s) and the Tar2- ion). 13C NMR spectroscopic measurements yielded the value of pK3 = 15.4 ± 0.2 for the proton dissociation of one of the alcoholic OH groups of Tar2- (at 25.0 °C and 4 M Na(Cl) ionic strength). Upon addition of calcium ions to an alkaline Tar2- solution, the 1H NMR signal gradually broadened and the 13C-satellite peaks split to two components, which also indicate complexation. From H2/Pt potentiometric titrations performed with solutions in the 13.6 ≤ pH ≤ 14.4 range, it was observed, that this complex formation is accompanied by a hydroxide ion consuming process. The titration curves can be best described via assuming the formation of the CaTarH-1-(aq) (lg ß11-1 = -11.2 ± 0.1) and CaTarH-22-(aq) (lg ß11-2 = -25.3 ± 0.1) complexes. In hyperalkaline solutions, these two species account for more than 90-99% of the calcium ions present and the contribution of the other reasonable and well-established calcium-containing solution species is rather small. The possible structures of the above complexes have been modeled via ab initio calculations. The stoichiometries are consistent both with species containing coordinated alcoholate group(s) and with mixed Ca(ii)-hydroxo-tartrato complexes. From the data available at present, both types of structures can be considered as chemically reasonable.

Synthesis of high-quality, well-characterized CaAlFe-layered triple hydroxide with the combination of dry-milling and ultrasonic irradiation in aqueous solution at elevated temperature.

The combination of mechanochemical and ultrasonic treatment was applied to synthesize CaAlFe-layered triple hydroxides with carbonate or chloride anions in the interlamellar space. The optimal parameters of the preparation were explored by altering the initial ratio of the metal ions and the temperature of ultrasonic irradiation. The resulting triple hydroxides were characterized by X-ray diffractometry, infrared and X-ray absorption spectroscopies, thermogravimetric analysis and scanning electron microscopy. The products were close-to-phase-pure CaAlFe-layered triple hydroxides. Elevation of the temperature transformed the CaAlFe-Cl(-)-layered triple hydroxide to rare oxyhalides (Ca2FeO3Cl and Ca12Al14O32Cl2).

Aggregation of negatively charged colloidal particles in the presence of multivalent cations.

The aggregation and charging behavior of sulfate and carboxyl latex particles in the presence of different multivalent salts was studied. Time-resolved light scattering and electrophoresis are the main experimental techniques used. In particular, the influence of the type of counterion is investigated. The main conclusion is that the valence of the counterion is highly relevant in determining the aggregation behavior, whereas its chemical nature is rather unimportant. Multivalent ions of higher valence destabilize the suspensions more effectively, in particular, by shifting the critical coagulation concentration (CCC) to lower values. This behavior reflects the classical Schulze-Hardy rule. Comparison with literature data reveals that the presently investigated systems behave similarly to the ones described earlier, but the observed dependence on valence is weaker than in some other systems. Moreover, we observe a slowdown of the aggregation at high electrolyte concentrations. This slowdown can be explained by the greater viscosity of the electrolyte solutions under these conditions.

Using low-frequency IR spectra for the unambiguous identification of metal ion-ligand coordination sites in purpose-built complexes.

One of the aims of our long-term research is the identification of metal ion-ligand coordination sites in bioinspired metal ion-C- or N-protected amino acid (histidine, tyrosine, cysteine or cystine) complexes immobilised on the surface of chloropropylated silica gel or Merrifield resin. In an attempt to reach this goal, structurally related, but much simpler complexes have been prepared and their metal ion-ligand vibrations were determined from their low-frequency IR spectra. The central ions were Mn(II), Co(II), Ni(II) or Cu(II) and the ligands (imidazole, isopropylamine, monosodium malonate) were chosen to possess only one-type of potential donor group. The low-frequency IR spectra were taken of the complexes for each ion-ligand combination and the typical metal ion-functional group vibration bands were selected and identified. The usefulness of the obtained assignments is demonstrated on exemplary immobilised metal ion-protected amino acid complexes.

Predicting aggregation rates of colloidal particles from direct force measurements.

Direct force measurements between negatively charged colloidal latex particles of a diameter of 1 µm were carried out in aqueous solutions of various inorganic monovalent and multivalent cations with the multiparticle colloidal probe technique based on the atomic force microscope (AFM). The observed force profiles were rationalized within the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). In the presence of monovalent and divalent cations, this theory was capable to describe the force profiles correctly down to distances of a few nm. At shorter distances, however, a strong non-DLVO attraction was identified. For more highly charged cations, an additional and more long-ranged non-DLVO attractive force is observed, and it was interpreted by surface charge heterogeneities. On the basis of these force profiles, the aggregation rates, which were independently measured by light scattering, can be predicted relatively well. The main conclusion of this study is that, in the present system, direct force measurements do capture the principal interactions driving aggregation in colloidal suspensions.

Multinuclear complex formation in aqueous solutions of Ca(II) and heptagluconate ions.

The equilibria and structure of complexes formed between the Ca(2+) ion and the heptagluconate (Hglu(-)) ion in both neutral and alkaline solutions have been studied. In alkaline solutions an uncharged, multinuclear complex is formed with the composition of Ca3Hglu2(OH)4 (or [Ca3Hglu2H(-4)](0)) with an unexpectedly high stability constant (lg ß(32-4) = 14.09). The formation of the trinuclear complex was deduced from potentiometry and confirmed by freezing-point depression measurements and conductometry as well. The binding sites of Hglu(-) were determined from NMR measurements. Besides the carboxylate group, the O atoms on the second and third carbon atoms proved to be the most probable sites for Ca(2+) binding.

Histidine-rich branched peptides as Cu(II) and Zn(II) chelators with potential therapeutic application in Alzheimer's disease.

Two histidine-rich branched peptides with one lysine as a branching unit have been designed and synthesized by solid-phase peptide synthesis. Their complex formation with Cu(II) and Zn(II) as well as their ability to attenuate the metal-ion induced amyloid aggregation has been characterized. Both peptides can keep Cu(II) and Zn(II) in complexed forms at pH 7.4 and can bind two equivalents of metal ions in solutions with excess metal. The stoichiometry, stability and structure of the complexes formed have been determined by pH potentiometry, UV-Vis spectrophotometry, circular dichroism, EPR and NMR spectroscopy and ESI-MS. Both mono- and bimetallic species have been detected over the whole pH range studied. The basic binding mode is either a tridentate {N(amino), N(amide), N(im)} or a histamine-type of coordination which is complemented by the binding of far imidazole or amino groups leading to macrochelate formation. The peptides were able to prevent Cu(II)-induced Aß(1-40) aggregation but could not effectively compete for Zn(II) in vitro. Our results suggest that branched peptides containing potential metal-binding sites may be suitable metal chelators for reducing the risk of amyloid plaque formation in Alzheimer's disease.

Superoxide dismutase mimicking Cu(II)-mixed amino acid complexes covalently grafted onto silica gel--an FT-IR study.

Our recent work concerning the synthesis, characterisation and testing of bioinspired electron transfer catalysts is described in this contribution. The catalysts were various Cu(II) complexes having mixed C- or N-protected amino acids (L-histidine and L-tyrosine) as ligands covalently grafted onto surface-modified silica gel. The resulting materials were structurally characterised by FT-IR spectroscopy, and their superoxide dismutase activities were tested. The covalently anchored Cu(II) complexes displayed appreciable activities in the test reaction; thus, they may be considered as promising candidates as durable electron transfer catalysts approaching the efficiency of the enzyme mimicked.