Showing papers on "Palladium published in 1995"

Palladium(0)-Catalyzed Cross-Coupling Reaction of Alkoxydiboron with Haloarenes: A Direct Procedure for Arylboronic Esters

Abstract: G e o r g T h i e m e V e r l a g K G , R ü d i g e r s t r a ß e 1 4 , 7 0 4 6 9 S t u t t g a r t , G e r m a n y 823 T . I S H I Y A M A , M . M U R A T A , N . M I Y A U R A * ( H O K K A I D O U N I V E R S I T Y , S A P P O R O , J A P A N ) Palladium(0)-Catalyzed Cross-Coupling Reaction of Alkoxydiboron with Haloarenes: A Direct Procedure for Arylboronic Esters J. Org. Chem. 1995, 60, 7508–7510.

Palladium reagents and catalysts : innovations in organic synthesis

Abstract: The Basic Chemistry of Organopalladium Compounds. Classification of the Reactions Involving Pd(II) Compounds and Pd(0) Complexes in This Book. Oxidative Reactions with Pd(II) Compounds. Catalytic Reactions with Pd(0) and Pd(II). Various Reactions Catalyzed by Pd(II) and Pd(0). Index.

Visualization of Surfactants on Nanostructured Palladium Clusters by a Combination of STM and High-Resolution TEM.

Abstract: Scanning tunneling microscopy (STM) and high-resolution transmission electron microscopy (TEM) have been used to determine the dimensions of a series of palladium clusters stabilized by tetraalkylammonium salts. Electrochemically prepared colloids were used in which the average diameter of the inner metal core was varied between 2 and 4 nanometers, and the size of the ammonium ions was adjusted in the series + N( n -C 4 H 9 ) 4 < + N( n -C 8 H 17 ) 4 < + N( n -C 18 H 37 ) 4 . The difference between the mean diameter determined by STM and that measured by TEM allows the determination of the thickness of the protective surfactant layer. On the basis of these studies, a model of the geometric properties of ammonium-stabilized palladium clusters has been proposed. Suggestions for the mechanism of the STM imaging process are also made.

Steam reforming of methanol over Pd/ZnO: Effect of the formation of PdZn alloys upon the reaction

Abstract: The catalytic performance of Pd/ZnO for steam reforming of methanol (CH 3 OH + H 2 O → CO 2 + 3H 2 ) was greatly improved by previously reducing the catalysts at higher temperatures. The original catalytic functions of metallic palladium were greatly modified as a result of the formation of PdZn alloys. Over the catalysts containing alloys, formaldehyde species formed in the reaction were suggested to be effectively attacked by water, being transformed into carbon dioxide and hydrogen. By contrast, the formaldehyde species decomposed selectively to carbon monoxide and hydrogen over catalysts containing metallic palladium.

Guest-Induced Organization of a Three-Dimensional Palladium(II) Cagelike Complex. A Prototype for "Induced-Fit" Molecular Recognition.

Abstract: Instead of the lock-and-key model, current understanding of molecular recognition in a biological system is based on the \"induced-fit'' mechanism, in which specific substrates induce the organization of the recognition site of a receptor. Although models for induced-fit have been provided by flexible artificial hosts which have restricted conformations only if they recognize a specific guest,' there are few examples of induced-fit models in which a guest induces the organization of a host i t ~ e l f . ~ . ~ Reported here is the successful demonstration of such a phenomenon in a coordination approach to a three-dimensional cagelike Pd(II) complex 1?-6 Thus, the complex I assembles in high yields only in the presence of specific guest molecules (Scheme 1). A tridentate ligand 1,3,5-tris(4-pyridy1methyl)benzene' (2,6 mM) was treated with (en)Pd(N03)2* (3, 9 mM) and sodium 4-methoxyphenylacetate (4*Na, 15 mM) at ambient temperature in water. The NMR spectrum, obtained by a control experiment carried out in DzO, showed the assembly of a single component in a high yield ('90%). Characterization of this component as the cagelike complex 1 mainly follows from its electrospray ionization mass spectroscopy (ESI-MS)9%'0 and NMR.\" In

Methane combustion over palladium catalysts: The effect of carbon dioxide and water on activity

Abstract: The influence of water and carbon dioxide on the activity of a series of palladium catalysts for the combustion of methane has been studied. It has been found that both compounds have an inhibitory effect but the extent of this depends on the experimental conditions. When both are present in the feedstream, the effect of carbon dioxide is negligible, the total decrease in activity only being equal to that of water alone. It is concluded that both water and carbon dioxide poison the same active sites on the palladium oxide with water able to displace carbon dioxide to form an inactive surface hydroxide. The magnitude of the effect due to both additives decreases with increasing reaction temperature and conversion. This is due both to the increased amount of water producted in the reaction reducing the effect of added water or carbon dioxide, and to the decreasing stability of the surface hydroxide at higher temperatures. In relation to practical applications, it is concluded that carbon dioxide will have no effect on catalytic activity under normal working conditions but water present in the feedstream may reduce the activity of the catalysts very significantly at lower temperatures. However, above about 450°C the effect of water is also expected to be very small.

Investigation of the active state of supported palladium catalysts in the combustion of methane

Abstract: Possible relationships between the structure and activity of supported palladium catalysts for the oxidation of methane have been explored. Samples have been exposed to a wide range of oxidation and reduction pretreatments and the activity for methane oxidation has been determined as a function of time-on-stream, both for pre-oxidised and for pre-reduced samples. Parallel experiments on the rate and extent of oxidation at 300°C have been performed. The results show that the rate of oxidation of pre-reduced palladium samples is very sensitive to the previous thermal history and samples retain a ‘memory’ of their treatment. In all cases a monolayer of adsorbed oxygen is taken up very rapidly but further oxidation is slower, although the samples approach the bulk oxide state within a few minutes. There are no obvious correlations between the catalytic activity for methane oxidation and the previous treatment of the samples, suggesting a sensitive dependence of activity on the morphology of the samples. It is clear, however, that metallic palladium is not active for methane oxidation. Similarly, chemisorbed oxygen is not as active as oxidised palladium. Moreover, it is found that there is no loss of activity when the palladium is fully oxidised, suggesting that previously reported changes in activity may reflect long-term morphological effects rather than simple over-oxidation of palladium particles.

Size dependence of the lattice parameter of small palladium particles

Abstract: Small palladium particles with mean diameters ranging from 1.4 to 5 nm have been prepared within a plasma polymer matrix from a vinyltrimethylsilane monomer. Electron diffraction has shown a decrease of the Pd lattice parameter with decreasing size of palladium cluster. These experimental data are used to deduce the value of the surface stress coefficient f=6.0\ifmmode\pm\else\textpm\fi{}0.9 N/m. A contraction of the lattice constant of small palladium particles with decreasing of cluster size, to our best knowledge, has not been reported in the literature up to now. The results suggest that the dilatation of the Pd lattice constant, often reported in the literature, can be explained in terms of incorporation of impurities like carbon, hydrogen, and oxygen, or pseudomorphism in the case of crystalline supports.

Luminescent Rhenium/Palladium Square Complex Exhibiting Excited State Intramolecular Electron Transfer Reactivity and Molecular Anion Sensing Characteristics

Abstract: Rigid macrocycles based on cis bridging ligation of transition metals represent an unusual,'-4 and largely new,5 class of compounds having tremendous promise in host-guest, inclusion, and molecular recognition chemistry. Most of the available systems are (1) derived from pt(I1) or Pd(I1) triflate species, (2) tetranuclear with respect to metal content, and therefore, (3) arranged in square or box-like geometries, with metal atoms defining the comers. We report a simple, but potentially very useful, variant and extension of this theme: incorporation of visible-light-addressable, luminescent metal-ligand components within a square assembly. Induction of photoluminescent characteristics is particularly attractive in the context of eventual molecular sensing applications because it suggests an altemative to 'H NMR spectroscopy for detection of guest inclusion. Induction of luminescence also opens up the possibility of electronic excited state reactivity and possible manipulation of reactivity by encapsulated guests. The light-emitting square complex, 1; was prepared by combining the chromophore fac-Re(CO)F1(4,4'-bpy)2 (2)6 (105 mg, 0.170 "01) (4,4'-bpy = 4,4'-bipyridine) with Pd(dppp)(triflate)zId (139 mg, 0.170 m o l ) (dppp = 1,3-(diphenylphosphino)propane) in 50 mL of dry CH2C12. After 12 h of stirring, the pale yellow product was collected as a powder on a glass frit, washed with methylene chloride, and dried under vacuum. The yield was 83%. Elemental analysis data are consistent with the formation of either 1 or a larger assembly with 1:l Re:Pd stoichiometry.' FAB+ mass spectral measurements (m-NBA) yielded intense peaks at m/z = 2871.7 (2871.2 expected for l*H+), 2721.0 (1 triflate) and 2564.9 (1 triflate 4,4'bpy), thereby establishing the tetranuclear metal content of the compound. No peaks for compounds of higher mass (e.g., hexanuclear or octanuclear complexes) were found. FT-IR and 'H NMR measurements (supporting information) yielded additional data supportive of molecular square formation.* Figure 1 shows that the lowest energy transition in the electronic absorption spectrum of 2 is shifted substantially red by formation of the square. The shift is similar to that observed upon ligand protonation in 26 and is consistent with assignment

New synthetic applications of water-soluble acetate pd/tppts catalyst generated in situ. evidence for a true pd(0) species intermediate

Abstract: Studies on the sp-sp intermolecular coupling reactions with the palladium water-soluble catalyst prepared in situ from palladium(II) acetate and sulfonated triphenylphosphine P(C6H4-m-SO3Na)(3) (TPPTS) in a homogeneous acetonitrile-water system, without Cu(I) promotor, afford diynes with moderate yields (45-65%). Under the same conditions, the sp(2)-sp coupling of 2-iodophenols or 2-iodoanilines and terminal alkynes followed by intramolecular cyclization gives indolic and furanic products in good yields (60-99%). Under these aqueous conditions, an efficient and short synthesis of eutypine illustrated the synthetic potentiality of the coupling. Furthermore, through a series of kinetic and P-31 NMR experiments, we have demonstrated that a mixture of Pd(OAc)(2) and TPPTS affords spontaneously a palladium(0) complex, through formation of bivalent complex Pd(OAc)(2)-(TPPTS)(2). A detailed mechanism of the reaction has been investigated thoroughly and the pertinent rate constants measured. The resulting palladium(0) complex reacts with phenyl iodide via an oxidative addition. This complex is considerably less reactive than the corresponding complex generated from PPh(3), probably due to steric effects.

Development of an active Ga2O3 supported palladium catalyst for the synthesis of methanol from carbon dioxide and hydrogen

Abstract: A significant effect of the support on the catalytic activity of palladium-based catalysts for methanol synthesis from carbon dioxide and hydrogen was observed, where Pd/Ga 2 O 3 was more active than Cu/ZnO by a factor of 2 in yield and 20 in turnover frequency

Facile Electrophilic Iodination of Icosahedral Carboranes. Synthesis of Carborane Derivatives with Boron-Carbon Bonds via the Palladium-Catalyzed Reaction of Diiodocarboranes with Grignard Reagents

Abstract: Electrophilic diiodination reactions of icosahedral closo1,2-C2B IOHI 2 and closo1,7-C2B IOHI 2 using 2 molar equiv of iodine monochloride in the presence of catalytic amounts of aluminum chloride yielded the corresponding clos0-9,12-12-1,2-C2B10H10 and closo-9,10-I2-1,7-C2B10H10 complexes in excellent yields. Palladium-mediated cross-coupling reactions of these diiodocarboranes with a variety of alkyl, aralkyl, and aryl Grignard reagents were reinvestigated, and it was demonstrated that the addition of copper(1) iodide as a cocatalyst is crucial to the success of this reaction. A reaction mechanism involving the initial formation of binary organocopper species (RCu; R = alkyl, aralkyl, aryl) followed by the reaction of (a-carborany1)palladium iodides (LzPd(Cb1)I and L2Pd(CbR)I; L = PPh3, Cb = c~oso-~ ,~ -C~BIOHIO, closo-1 7-C2BloHlo) via a four-centered transition state is proposed. The molecular structures of closo-9,10-12-1,7-C2B10H10, 1, and C~O~O-~,~~-(C~H~)~-~,~-C~BIOHIO, 10, have been determined. Crystallographic data are as follows: for 1, monoclinic, space group P21lc, a = 13.2719-