A series of well-defined homopolymer-stabilized Pd colloids with varying metal particle size was used to study the Heck coupling reaction between aryl halides and olefins. Correlation of initial reaction rates and particle sizes determined by transmission electron microscopy demonstrates that the Heck reaction is a sensitive probe of metal surface structure. The presence of the colloid-stabilizing homopolymer affords a much more stable catalyst than that obtained using the colloid−precursor metal complexes alone. Extremely high total turnover numbers (TON, moles of substrate/moles of Pd; TON = 100 000) and turnover frequencies (TOF = TON/h; TOF > 80 000) are easily attainable with poly(vinylpyrrolidone)-stabilized colloidal palladium in the coupling of p-bromobenzaldehyde with butyl acrylate.

ExperimentalRefractive Indices: The ordinary and extraordinary refractive indices of drawn films were measured using an Abbe refractometer. The so-called trirefringence technique [31] was used to determine the three principle refractive indices of the drawn films.UV-vis: Polarized UV-vis measurements were performed on a Perkin Elmer Lambda 9 spectrophotometer in the wavelength range from 400 to 700 nm. Prior to the measurements, the drawn films were coated with a paraffin oil and sandwiched between two glass slides to eliminate the scattering of light at the film surface. The transmittances of the films were measured parallel (T 1 ) and perpendicular (T 2 ) to the drawing direction. The polarizing efficiency (PE) and single-piece transmittance (T sp ) were calculated from these measurements using Equations 1 and 2. PE = (T 2 ± T 1 )/(T 2 + T 1 )(1)Light Scattering in the Forward Direction: A collimated beam of light, originating from a halogen light source, illuminates the sample via an optical fiber. The measurements were performed using a green bandpass filter with a central wavelength of 561 nm and a full width at half maximum of 21 nm. Two linear dichroic polarizers are placed below and above the sample to control the polarization directions of the incident and transmitted light. The light passing through the system is collected by a light collector connected to a photomultiplier tube (PMT), which measures the light intensity in terms of absolute luminance (Cd/m 2 ). The light collector can rotate in a vertical plane over an angle y from 0 to 70. The sample with the polarizers is mounted on a sample holder that can rotate over an angle j from 0 to 360. Consequently, the spatial distribution of the light intensity in the forward direction can be evaluated in spherical coordinates. For a graphical representation of the three-dimensional light intensity distribution, the spherical coordinates y and j were converted into Cartesian coordinates x and y with x = sin y cos j and y = sin y sin j. Relative light intensities were obtained by dividing the absolute intensities by the measured intensity without a sample at x = 0 and y = 0 (i.e., the maximum of the incident light beam).Integral Light Intensity in the Forward and Backward Direction: The ratio of the light scattered in the forward and backward direction was measured with light from a laser (Spectra Physics, 2 mW helium neon laser, wavelength = 540 nm), which is polarized using a dichroic polarizer and subsequently split into two beams by a beam splitter. One beam is directed to the sample and an integrating sphere and the second, reference beam is transmitted directly to a photodiode. The measured intensity in the integrating sphere was divided by the reference intensity to correct for laser intensity fluctuations. The relative intensity of the light in the forward direction is measured by placing the sample in front of the integrating sphere. Subsequently, the same sample is placed at the rear of the integrating sphere and the relative intensity of light sc...

The low thermal stability of nanoparticles typically restricts their use in catalytic and other applications to low- to moderate-temperature conditions. We present a novel approach to the stabilization of nanosized noble metal particles by embedding them in a high-temperature stabilized hexa-aluminate matrix. The simple 'one-pot' approach is based on a microemulsion-templated sol-gel synthesis and yields mesoporous nanocomposite materials with pure textural porosity and excellent high-temperature stability up to about 1200 °C. To our knowledge, this is the first time that metal nanoparticles have been stabilized to such high temperatures. We furthermore find that the microemulsion templating allows a tailoring of the ceramic matrix without influencing the size of the embedded Pt particle. This opens up the possibility of a true multiscale engineering of nanocomposite materials. We see these novel materials therefore not only as very promising candidates for a broad range of high-temperature catalytic applications, but generally view this versatile synthesis route as a first step towards expanding the parameter range for nanoparticle applications.

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The metal‐vapor reactions of Co atoms with 1,3‐tBu2CpH (1a), 1,3‐tBu2CpH (1a), 1,2,4‐tBu3CpH (1b) and EtMe4CpH (1c) are described. With 1a the two mononuclear complexes [(η5‐tBu2Cp) (η4‐tBu2‐1,3‐cyclopentadiene)Co] (2a) and [(η5‐tBu2Cp)2Co] (3a) were isolated, together with the dinuclear cobalt cluster [{(η5‐tBu2Cp)Co}2H3] (4a) and trace amounts of the tetranuclear cluster [{(η5‐tBu2Cp)CoH}4] (5a). The molecular structures of 3a and 4a were determined by X‐ray diffraction. Reaction of 1b with Co atoms afforded a single product, the dinuclear cluster [{(η5‐tBu3Cp)Co}2H3] (4b), whose molecular structure was determined by single‐crystal X‐ray diffraction. Both, 4a and 4b exhibit extremely short Co‐Co distances [2.244(1) (4a) and 2.242(1) Å (4b)], as found for the Me5Cp analog [{(η5‐Me5Cp)Co}2H3] (4c). Reaction of an isomeric mixture of Me4EtCpH (1c) with Co atoms furnished the mononuclear sandwich [ (η5‐Me4EtCp)(η4‐Me4Et‐1,3‐ cyclopentadiene)Co] (2b), the trinuclear hydridocobalt cluster [{(η5‐Me4EtCp)Co}3H4] (6a) and the tetranuclear hydridocobalt cluster [{(η5‐Me4EtCp)Co}4H4] (5c). The molecular structure of 5c was determined by X‐ray crystallography and revealed a tetrahedral arrangement of the cobalt atoms. The electrochemical behavior of the dinuclear complexes 4a‐4c was studied by cyclic voltammetry. Reversible redox couples were found for all three compounds, with a correlation between the degree of alkyl substitution and their respective cathodic shifts. Compounds 4a and 4b react with CO to yield the mononuclear and dinuclear complexes [ (η5‐ CpR)Co(CO)2] (R1,3‐tBu2, 1,2,4‐tBu3) (7a and 7b) as well as [{(η5‐CpR)CO}2(CO)2] (8a and 8b). Reaction of the trinuclear hydridocobalt cluster 6a and its Me5Cp analog 6b with AgBF4 in the presence of PEt3 yielded the heteronuclear clusters [{η5‐Me4CPR)Co}3AgP(Et)3H4]+[BF4]− (REt, Me) (9a and 9b). 9a was structurally characterized by X‐ray crystallography.