Gold catalysis: in situ EXAFS study of homogeneous oxidative esterification.

Gold complexes were prepared and investigated as catalysts for the oxidative esterification of aldehydes. Stabilisation by pyridine ligands gave good conversions and the in situ extended X-ray absorption fine structure (EXAFS) study of the reactions indicated that the reaction mixtures contained only mononuclear gold species. Thus, this is the first proof for a homogeneous gold-catalysed oxidation reaction; the presence of nanoparticles could be excluded experimentally.

Spectroscopic set-up for simultaneous UV-Vis/(Q)EXAFS in situ and in operando studies of homogeneous reactions under laboratory conditions.

A novel experimental set-up for in operando studies of homogeneous catalyzed reactions under laboratory conditions has been developed and tested. It combines time-resolved X-ray absorption spectroscopy with UV/Vis spectroscopy. The reaction solution is stirred in a vessel and pumped in a circle by a peristaltic free gear-wheel through a measurement cell. The X-ray and UV/Vis beams probe the same sample volume of the cell orthogonally. Reactants can be added to the reaction mixture in the course of the measurements and a defined gas atmosphere can be adjusted up to a pressure of 10 bar. The in situ reduction of cerium(IV) ammonium nitrate to cerium(III) by isopropanol is studied as a test reaction with quick-XANES and UV/Vis measurements with a time resolution of 60 s and 1 s, respectively.

Nanostructured copper oxide on silica-zirconia mixed oxides by chemical implantation.

N,N-Dialkylcarbamato complexes of copper(II), [Cu(O(2)CNR(2))(2)] (R = All = allyl, C(3)H(5); iPr, CH(CH(3))(2)) were prepared with the aim of functionalizing silica and nanostructured silica-zirconia matrices. The mixed matrices for the grafting reactions were prepared by copolymerizing MAPTMS (methacryloxypropyltrimethoxysilane), the precursor for the silica matrix, with the zirconium tetranuclear derivative [Zr(4)O(2)(OMc)(12)] (OMc = methacrylate), the precursor for the zirconia nanoparticles. Suspension of the silica and silica-zirconia matrices in a solution of the copper dialkylcarbamate led to the functionalization of the respective substrates. The composition, microstructure, morphology, and physicochemical nature of the copper species grafted on the matrices were investigated by FTIR, X-ray photoelectron spectroscopy (XPS), EPR, X-ray absorption spectroscopy (XAS), XRD, TEM, and dinitrogen adsorption. The effect of selected experimental parameters (the nature of the copper precursor and of the matrix, grafting time, thermal treatment) on the grafting reaction was investigated. The Cu/Si ratio is increased by increasing the grafting time and the ZrO(2)-SiO(2) matrix is more reactive to attack by the carbamato complexes than either prepared or commercial SiO(2). After functionalization of the matrix, thermal treatment yielded nanostructured copper(II) oxide clusters, average diameter 12-15 nm, uniformly supported on the silica and on the silica-zirconia matrices.

Halide ligand-more than just sigma-donors? A structural and spectroscopic study of homologous organonickel complexes.

The isoleptic organonickel complexes [(bpy)Ni(Mes)X] (bpy ) 2,2'-bipyridine; Mes ) 2,4,6-trimethylphenyl; X ) F,Cl, Br, or I, and for comparison X ) OMe and SCN) have been investigated by multiple spectroscopic means.Their structures have been determined in part by single-crystal X-ray diffraction, the full series by extended X-ray absorption fine structure. The long-wavelength charge transfer absorptions (mainly metal-to-ligand charge transfer)obtain contributions from the mesityl coligand but are almost invariable upon variation of X. UV-vis spectroscopy allowed investigation of the solvolysis reaction [(bpy)Ni(Mes)X] + Solv a [(bpy)Ni(Mes)(Solv)]+ + X-, which occurs very fast for X ) I (k ) 0.176(4) M(-1) s(-1)) or Br but very slow for X ) Cl (k ) 5.18(5) x 10(-5) M(-1) cm(-1))or F. Quantum chemical (density functional theory) calculations on the geometry, electronic states, and electronic transitions (time-dependent density functional theory) are very helpful for detailed insight into the role the X coligands play in these complexes. The combination of methods reveals rather strong, highly covalent Ni-X bonds for all halide coligands but marginal pi-donation.

Microemulsion approach to neodymium, europium, and ytterbium oxide/hydroxide colloids--effects of precursors and preparation parameters on particle size and crystallinity.

Colloids based on lanthanides or their oxides have a great potential in the areas of optical and magnetic materials. In this study the confined space of reverse micellar systems formed by water in cyclohexane was used to precipitate particles based on neodymium, europium, and ytterbium. The morphology and structural properties of the prepared colloids were determined by transmission electron microscopy, IR spectroscopy, and X-ray diffraction and absorption measurements. The size of the obtained systems as determined by dynamic light scattering ranged from a few nanometers to several hundreds of nanometers in diameter, depending on the reaction conditions. The employed surfactant was found to have a major influence on the particle size and morphology. In contrast to the ionic surfactants sodium dodecyl sulfate and cetyltrimethylammonium bromide, the nonionic surfactant Triton X-100 generally delivered very small and unagglomerated particles. The precursor counterion had a similar effect, depending on its ability to coordinate to the particle surface, and prevented particle growth and agglomeration. The influence of further parameters such as the pH of the aqueous starting solutions, the mixing methodology, and the preparation temperature was also investigated. Applying increased temperatures the particles exhibited a higher crystallinity, and at the same time the particle size was drastically increased.

Platinum species in the pores of NaX, NaY and NaA zeolites studied using EPR, XAS and FTIR spectroscopies.

The results of X-band EPR, X-ray absorption and Fourier transform infrared spectroscopy on Pt(NH(3))(4)(2+) exchanged NaX, NaY and NaA zeolites reveal after oxygen calcination at 573 K that diamagnetic Pt(2+) is not the only product. Calcination provides Pt(3+) cations, but depending on the heating rate, the decomposition of amino groups during calcination also produces hydrogen that reduces Pt(3+) to Pt(2+) and Pt(+). NaX (Si/Al = 1.23) has a more negative framework charge than NaY (Si/Al = 2.31), so Pt(3+) can be stabilized only in NaX, whereas lower oxidation states of Pt such as Pt(+) can be stabilized in both, NaX and NaY, and neither of the paramagnetic Pt cations are stabilized in NaUSY (Si/Al = 3). The autoreduction process allows controlling the number of Pt(3+) and Pt(+) in the NaX zeolite by changing the calcination heating rate: a heating rate of 1.25 K min(-1) gives only Pt(+), but 0.5 K min(-1) gives a Pt(3+)/Pt(+) ratio close to 1. The structure of the support is also important for the synthesis of Pt species. While isolated paramagnetic Pt ions were stabilized in faujasite zeolites (NaX and NaY), a paramagnetic Pt dimer was obtained in a Linde type A zeolite (LTA, Si/Al = 1) by applying the same preparation methods. The fraction of paramagnetic Pt species which were characterized by X-band EPR spectroscopy amounts to 2-18% of the total Pt in the zeolites, the remaining Pt must be diamagnetic.

The amplitude reduction factor and the cumulant expansion method: crucial factors in the structural analysis of alkoxide precursors in solution.

The transition-metal alkoxide yttrium 2-methoxyethoxide Y(OEtOMe)(3) in solution is studied as a model system of the large class of alkoxide precursors used in the sol-gel process by means of EXAFS spectroscopy. The discussion is focused on the amplitude reduction factor S (2)(0) and the cumulant expansion method. If asymmetry is present in the radial distribution function, the determination of the correct structural model can only be achieved by balancing multiple Gaussian shell fits against only one shell fit with a third cumulant C3. A method to identify the best model, based on statistical parameters of the EXAFS fit, is proposed and checked with two well-known reference compounds, Y(5)O(O(i)Pr)(13) and Y(acac)(3).3H(2)O, and applied to the structurally unknown solution of Y(OEtOMe)(3) in 2-methoxyethanol. The two references are also used to discuss the transferability of S(2)(0) values, determined from reference compounds to unknown samples. A model-free procedure to identify the correct amplitude reduction factor S(2)(0) by making use of fits with different k-weighting schemes is critically investigated. This procedure, which does not require any crystallographic data, is used for the case of Y(OEtOMe)(3) in solution, where significant differences of the amplitude reducing factor of both the oxygen and yttrium shell in comparison to the reference Y(5)O(O(i)Pr)(13) were found. With such a detailed analysis of EXAFS data, a reliable characterization of Y(OEtOMe)3 in 2-methoxyethanol by means of EXAFS spectroscopy is possible. The decameric structure unit found in solid Y(OEtOMe)(3) is not preserved, but rather, a pentameric framework similar to that in Y5O(O(i)Pr)(13) is formed.

Structural investigations on the hydrolysis and condensation behavior of pure and chemically modified alkoxides. 2. Germanium alkoxides.

Structural investigations on the hydrolysis and condensation behavior of germanium alkoxides were for the first time performed by means of X-ray absorption fine structure and Raman spectroscopy. The studies reveal that germanium alkoxides are monomeric in nature and undergo very fast hydrolysis and condensation reactions upon water addition. However, the chelation of germanium alkoxides by acetylacetone does not take place even 48 h after mixing, and any change in hydrolysis and condensation behavior is not observed after acetylacetone addition. When mixed with prehydrolyzed silicon alkoxide, the structures of germanium alkoxides are not modified. Both Si and Ge precursors are insensitive to the presence of each other in the reaction solution even after 48 h of aging. The addition of water to this mixture catalyzes the hydrolysis and condensation reactions very fast and leads to the formation of Ge-O-Ge (and consequently Si-O-Si) homocondensation products.

Structural investigations on the hydrolysis and condensation behavior of pure and chemically modified alkoxides. 1. Transition metal (Hf and Ta) alkoxides.

Structural investigations on the hydrolysis and condensation behavior of hafnium and tantalum alkoxides were for the first time performed by means of X-ray absorption fine structure and Raman spectroscopy. The studies reveal that both of the alkoxides are dimeric in nature and instantaneously undergo hydrolysis and condensation reactions upon the addition of water. The results indicate that the chemical reaction of the alkoxides with acetylacetone occurs immediately with an increase from 6- to 8-fold coordination around the metal. As a consequence of the coordination by a bidentate ligand, hydrolysis and condensation reactions are hindered in solutions of the chemically modified Hf(OnBu)4 and Ta(OEt)5. Furthermore, the investigations demonstrate that the structure of metal alkoxides is not altered after mixing with prehydrolyzed silicon tetraethoxide, and even after 48 h, both of the species remain as separate entities in the mixture. The addition of water to this mixture starts the hydrolysis and condensation reactions instantaneously and leads to the formation of a M-O-M homocondensation product due to the different reactivity of the two alkoxides.

Mechanistic insights into stereoselective catalysis-the effects of counterions in a CuII-bissulfoximine-catalyzed Diels-Alder reaction.

The initial steps of an enantioselective Diels-Alder reaction catalyzed by a CuII-bissulfoximine complex were followed by EXAFS (EXAFS=extended X-ray absorption fine structure), EPR (EPR=electron paramagnetic resonance) spectroscopy (CW-EPR, FID-detected EPR, pulse ENDOR, HYSCORE; CW=continuous wave; ENDOR=electron nuclear double resonance; HYSCORE=hyperfine sublevel correlation; FID=free induction decay), and UV-visible spectroscopy. The complexes formed between the parent CuX2 (X=Cl-, Br-, TfO-, SbF6-) salts, the chiral bissulfoximine ligand (S,S)-1, and N-(1-oxoprop-2-en-1-yl)oxazolidin-2-one (2) as the substrate in CH2Cl2 were investigated in frozen and fluid solution. In all cases, penta- or hexacoordinated CuII centers were established. The complexes with counterions indicating high stereoselectivity (TfO- and SbF6-) reveal one unique species in which substrate 2 binds to pseudoequatorial positions (via O atoms), shifting the counterions to axial locations. On the other hand, those lacking stereoselectivity (X=Cl- and Br-) form two species in which the parent halogen anions remain at equatorial positions preventing the formation of geometries compatible with those found for X=TfO- and SbF6-.

Structure and magnetization of small monodisperse platinum clusters.

Magnetization measurements of well-characterized monodisperse Pt clusters consisting of 13+/-2 atoms in a zeolite confirm the predicted extraordinary magnetic polarization with up to 8 unpaired electrons on a cluster, corresponding to a magnetic moment of 0.65(5) microB per atom. The effect is partly quenched by hydrogen chemisorption. The study provides insight into the electronic structure of the cluster and is fundamental for an understanding of how magnetism develops in small clusters.

Investigations into the metal species of the homogeneous iron(III) catalyzed Michael addition reactions.

An investigation into the species formed in the first step of the solvent free homogeneous Michael reaction of alpha,beta-unsaturated ketones with 2-oxocyclopentanecarboxylate (1) is presented. This reaction is catalyzed by FeCl(3).6H(2)O (2) and Fe(ClO(4))(3).9H(2)O (3). EXAFS, XANES, Raman and UV-Vis studies were carried out to explain the experimentally found higher catalytic activity of Fe(ClO(4))(3).9H(2)O (3) compared to FeCl(3).6H(2)O (2). A very intense pre-edge peak is found for a 1.6 mol% solution of FeCl(3).6H(2)O (2) in 1, suggesting a tetrachloroferrate(III) compound to be present in this solution. This is proved by UV-Vis and Raman spectroscopy. The counterion of this anionic complex is an octahedral [Fe(III)(1-H)(2)(H2O2)](+) complex with two deprotonated 2-oxocyclopentanecarboxylate (1) as the chelating ligand, (1-H)(-), as suggested by the examination of the XANES region, the obtained coordination numbers from the EXAFS analysis and by UV-Vis and Raman spectroscopies. In summary, the anion-cation species [Fe(III)Cl(4)](-)[Fe(III)(-H)(2)(H2O2)](+) is formed with FeCl(3).6H(2)O (2), whereas in the case of Fe(ClO(4))(3).9H(2)O (3) XAFS, Raman and UV-Vis investigations suggest the presence of a complex of the form [Fe(III)(1-H)(2)(H2O2)](+)[ClO(4)](-). The obtained results are discussed to explain the reduced catalytic activity of FeCl(3).6H(2)O (2) in comparison to Fe(ClO(4))(3).9H(2)O (3).

Structure of dense hydrogen fluoride gas from neutron diffraction and molecular dynamics simulations.

The gas phase of hydrogen fluoride has been investigated by neutron diffraction experiments at three different particle densities. All investigated states are within the liquid-gas coexistence region of hydrogen fluoride. From the obtained diffraction data we deduced information about the local structure of the gas phase, which consists of small agglomerates. This has been expected as liquid hydrogen fluoride forms the strongest hydrogen bonds known. Molecular dynamics simulations with a modified potential have been carried out for all experimentally investigated states. The results confirmed that the size of the formed agglomerates in the gas phase is growing with increasing density of the gas phase.

Synthesis, characterization, EXAFS investigation and antibacterial activities of new ruthenium(III) complexes containing tetradentate Schiff base.

A series of new hexa-coordinated ruthenium(III) complexes of the type [Ru(X)(2-atmp-ba)(EPh3)] (where H2-2-atmp-ba=N,N'-bis(2-aminothiophenol)benzoylacetone; X=Cl or Br; E=P or As) have been prepared by reacting [RuX3(EPh3)3] (where X=Cl or Br; E=P or As) with tetradentate Schiff base ligand (H2-2-atmp-ba) in 1:1 molar ratio. The complexes have been characterized by elemental analyses, Infra red, electronic, electron paramagnetic resonance spectroscopy and cyclic voltammetry. In order to confirm the coordination and structure of the complexes extended X-ray absorption fine structure spectroscopy (EXAFS) studies have been carried out. Based on the above data, an octahedral structure has been confirmed for the complexes. The new complexes were also screened for their antibacterial properties.

Spectroscopic investigations of bis(sulfoximine) copper(II) complexes and their relevance in asymmetric catalysis.

The structure of Cu(II) complex 3 formed within the course of a stereoselective Diels-Alder reaction was investigated by EXAFS, CW-EPR at X- and W-band, HYSCORE, pulsed ENDOR, and UV-vis spectroscopy. The experimental techniques indicate that the chiral bis(sulfoximine) ligand (S,S)-1 and the dienophile form a tetragonally distorted complex in CH(2)Cl(2). The ligand binds to the Cu(II) center via the imine nitrogens, whereas the dienophile interacts via the carbonyl oxygen atoms. The additional sites of the first coordination sphere are occupied by counterions and, presumably, solvent molecules. At the axial position, a triflate anion binds via an oxygen atom.

Structural investigation of high-valent manganese-salen complexes by UV/Vis, Raman, XANES, and EXAFS spectroscopy.

XANES and EXAFS spectroscopic studies at the Mn-K- and Br-K-edge of reaction products of (S,S)-(+)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) chloride ([(salen)Mn(III)Cl], 1) and (S,S)-(+)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) bromide ([(salen)Mn(III)Br], 2) with 4-phenylpyridine N-oxide (4-PPNO) and 3-chloroperoxybenzoic acid (MCPBA) are reported. The reaction of the Mn(III) complexes with two equivalents of 4-PPNO leads to a hexacoordinated compound, in which the manganese atom is octahedrally coordinated by four oxygen/nitrogen atoms of the salen ligand at an average distance of approximately 1.90 A and two additional, axially bonded oxygen atoms of the 4-PPNO at 2.25 A. The oxidation state of this complex was determined as approximately +IV by a comparative study of Mn(III) and Mn(V) reference compounds. The green intermediate obtained in reactions of MCPBA and solutions of 1 or 2 in acetonitrile was investigated with XANES, EXAFS, UV/Vis, and Raman spectroscopy, and an increase of the coordination number of the manganese atoms from 4 to 5 and the complete abstraction of the halide was observed. A formal oxidation state of IV was deduced from the relative position of the pre-edge 1s-->3d feature of the X-ray absorption spectrum of the complex. The broad UV/Vis band of this complex in acetonitrile with lambda(max)=648 nm was consistent with a radical cation structure, in which a MCPBA molecule was bound to the Mn(IV) central atom. An oxomanganese(V) or a dimeric manganese(IV) species was not detected.