Spektroskopische Untersuchung von Tetramminzink-und Tetrammincadmium-tetrachloroplatinat(II)

Zusammenfassung Die Festkörperelektronenspektren sowie die Infrarotspektren (im Bereich 4000–100 cm^–1) der Verbindungen [Zn(NH₃)₄]-PtCl₄ und [Cd(NH₃)₄]PtCl₄ werden gemessen und kurz besprochen.

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Spectroscopic investigation of tetramminzinc and tetrammincadmium tetrachloroplatinates(II)

The reflectance-electronic and IR-spectra of solid [Zn(NH₃)₄]PtCl₄ and [Cd(NH₃)₄]PtCl₄ are measured and briefly discussed.

     

Exceedingly Fast Oxygen Atom Transfer to Olefins via a Catalytically Competent Nonheme Iron Species

The reaction of [Fe(CF₃SO₃)₂(PyNMe₃)] with excess peracetic acid at ?40 °C leads to the accumulation of a metastable compound that exists as a pair of electromeric species, [Fe^III(OOAc)(PyNMe₃)]²⁺ and [Fe^V(O)(OAc)(PyNMe₃)]²⁺, in fast equilibrium. Stopped‐flow UV/Vis analysis confirmed that oxygen atom transfer (OAT) from these electromeric species to olefinic substrates is exceedingly fast, forming epoxides with stereoretention. The impact of the electronic and steric properties of the substrate on the reaction rate could be elucidated, and the relative reactivities determined for the catalytic oxidations could be reproduced by kinetic studies. The observed fast reaction rates and high selectivities demonstrate that this metastable compound is a truly competent OAT intermediate of relevance for nonheme iron catalyzed epoxidations.     

Organoplatinum-Bridged Cyclotribenzylene Dimers

Cyclotribenzylenes (CTBs) combining carbonitrile (−CN) and alkyne (−C₂H) substituents were synthesized as racemic mixtures and resolved by HPLC on chiral stationary phases. Two of these compounds were used to prepare platinum-bridged CTB dimers, in which Pt^II is bound to the CTBs via Pt−alkynyl bonds in cis configuration. The organometallic complexes were examined by mass spectrometry and NMR spectroscopy, which indicated that they were obtained as mixtures of diastereoisomers (a meso or syn form and a pair of chiral or anti forms) when racemic CTBs were used. Enantiomerically pure complexes were prepared from resolved CTBs, which allowed us to distinguish the NMR signals of the chiral and meso forms in the diastereoisomeric mixtures. In certain conditions, the platinum complexes played the role of a pincer π-alkynyl ligand for Cu(I) coming from the copper iodide used as a synthetic auxiliary. The Cu⁺ cations could be easily removed by treatment with NaCN, affording the mononuclear bis-cyclotribenzylene complexes. These compounds failed to lead to metallo-cryptophanes by coordination of two [M(dppp)]²⁺ complex subunits (M=Pd, Pt; dppp=1,3-bis(diphenylphosphino)propane), each to two carbonitrile substituents belonging to different CTBs, pointing to the superiority of the one pot self-assembly processes for the preparation of metallo-cryptophanes.     

Pt/[Fe]ZSM-5 modified by Na and Cs cations: an active and selective catalyst for dehydrogenation of n-alkanes to n-alkenes

Pt clusters within [Fe]ZSM-5 channels provide active and stable sites for the selective catalytic dehydrogenation of n-alkanes to n-alkenes. Cs and Na cations titrate acid sites and inhibit skeletal isomerization and cracking side reactions.     

Cyanide-bridged tetradecanuclear RuII3MII11 clusters (MII=ZnII and CuII) based on the high connectivity building block [Ru3(HAT)(CN)12]6-: structural and photophysical properties

Reaction of the anionic cyanometallate chromophore [{Ru(CN)4}3(micro3-HAT)]6- with [MII(tren)]2+ complexes (M=ZnII, CuII) provides discrete tetradecanuclear clusters of formula [{MII(tren)(micro-CN)}11{Ru3(HAT)(CN)}]16+; the weak luminescence of the Ru3 chromophore is substantially enhanced in the presence of ZnII ions, whereas it is completely quenched when CuII centers are present.     

Surface characterization of platinum electrodes

The quantitative analysis of the different surface sites on platinum samples is attempted from pure voltammetric data. This analysis requires independent knowledge of the fraction of two-dimensional (111) and (100) domains. Specific site-probe reactions are employed to achieve this goal. Irreversibly-adsorbed bismuth and tellurium have been revealed to be sensitive to the presence of (111) terrace domains of different width whereas almost all sites involved in (100) ordered domains have been characterized through germanium adatoms. The experimental protocol follows that used with well-defined single-crystal electrodes and, therefore, requires careful control of the surface cleanliness. Platinum basal planes and their vicinal stepped surfaces have been employed to obtain calibration plots between the charge density measured under the adatom redox peak, specific for the type of surface site, and the corresponding terrace size. The evaluation of the (100) bidimensional domains can also be achieved using the voltammetric profiles, once the fraction of (111) ordered domains present in the polyoriented platinum has been determined and their featureless contribution has been subtracted from the whole voltammetric response. Using that curve, it is possible to perform a deconvolution of the adsorption states of the polycrystalline sample different from those related to (111) domains. The fraction of (100)-related states in the deconvoluted voltammogram can then be compared to that expected from the independent estimation coming from the charge involved in the redox process undergone by the irreversibly-adsorbed germanium and thus check the result of the deconvolution. The information about the surface-site distribution can also be applied to analyze the voltammetric profile of nanocrystalline platinum electrodes.     

Electrochemically-driven conformational shift in mono- and di-copper constrained macrotricyclic cyclen receptors

An electrochemical study of mono- and di-copper constrained cyclen macrotricycles is presented. Electrochemical data in DMF solution indicate that the reduction of dinuclear complexes occurs in two steps in the -0.4 to -0.8 V vs.AgCl/Ag potential range yielding CuII CuI and CuI CuI species further reduced to Cu metal at highly negative potentials. Mononuclear complexes are reduced in two steps to CuI and Cu metal. Electrochemical data suggest that reduction of both mononuclear and dinuclear complexes approach a square scheme involving electrochemically-driven conformational shifts for metal ions. The presence of endo- and exo-forms of the complexes are revealed by changes in the electrochemical response of the complexes in the presence of tetraethylammonium chloride, 1-azabicyclo[2.2.2]octane and diazabicyclo[2.2.2]octane competing ligands.     

Ion-specific and reversible wetting of imidazolium-based minigels

Cross-linked imidazolium-based [poly(ViEtIm +Br -)] microparticles were synthesized, and their wetting properties were studied by optical microscopy, after addition of aqueous solutions of sodium halides. Particle wetting showed ion specificity due to counterion binding, described by Desnoyer's model. The interaction between anions and the microparticles allowed exchanging halogenides between them in a reversible way. A salt-independent characteristic wetting time was found as well as a decreasing power law with salt concentration, for the network diffusion coefficient. It modified the polymer network elasticity as ion concentration increased, making the network softer.     

Structural and textural evolution during folding of layers of layered double hydroxides

Layers of a layered double hydroxide, containing aluminum 4-fold coordinated, were partially folded in order to obtain a fibrous hydrotalcite-like compound. The hydrotalcite layers, in the presence of an anionic surfactant (sodium dodecyl sulfate) after hydrothermal treatment for 2 weeks, acquire a mesoporous-like arrangement. The transformation was monitored by techniques sensitive to structural and textural properties. Results suggest that brucite-like layers can be joined throughout unsaturated coordinated aluminum, that is, tetrahedral aluminum which links through hydrogen bonds to form aluminum octahedrally coordinated. The fractal dimension parameter was very sensitive to evolution from layered to fibrous hydrotalcites.     

Study of the interaction in clusters formed by phenol and CH3X (X=CN,F,Cl) molecules

The characteristics of the interaction between phenol and acetonitrile, methyl fluoride and methyl chloride were studied. The most stable structures for clusters containing one or two CH3X molecules and one phenol moiety were located by means of ab initio and density functional theory calculations. Phenol-acetonitrile dimer presents two almost equally stable structures; one of them is a typical linearly hydrogen bonded minimum, whereas in the other one, a C-H...pi contact is established accompanied by a distorted O-H...N hydrogen bond. Although the latter minimum presents the larger interaction energy, deformation effects favor the formation of the linear hydrogen bonded one. In complexes with methyl fluoride and methyl chloride, this arrangement is the most stable structure and no linear hydrogen bonded structures were located. Our best estimates for the interaction energies amount to -27.8, -21.6, and -19.7 kJ/mol for clusters of phenol with acetonitrile, methyl fluoride, and methyl chloride, respectively. The main contribution to the stabilization of these clusters is of electrostatic nature, although in structures where a C-H...pi contact is present, the dispersion contribution is also significant. In clusters formed by phenol and two CH3X units, the most stable arrangement corresponds to a head to tail disposal with O-H...X, C-H...X, and C-H...pi contacts forming a cycle. Only for this type of arrangement, three body effects are non-negligible even though they constitute a minor effect. The results also indicate that interactions with methyl fluoride and methyl chloride are of similar intensity, although weaker than with acetonitrile. Significant frequency shifts are predicted for the O-H stretching, which increase when increasing the number of CH3X molecules.