Synthesis,spectral and thermal studies on pyrazolatebridged palladium(II) coordination polymers

Synthesis, spectroscopic characterization and thermal behavior of pyrazolate-bridged palladium complexes [Pd(μ-Pz)₂]_n (1), [Pd(μ-mPz)₂]_n (2), [Pd(μ-dmPz)₂]_n (3), [Pd(μ-IPz)₂]_n (4) {pyrazolate (Pz^–), 4-methylpyrazolate (mPz^–), 3,5-dimethylpyrazolate (dmPz^–), 4-iodopyrazolate (IPz^–)} have been described in this work. The exobidentate coordination mode of pyrazolato ligands in 1–4 was inferred on basis of IR spectroscopic evidences. TG investigations indicated that the introduction of substituents at the 4 position in the pyrazolyl moiety into coordination polymers do not affect significantly their thermal stability, whereas at the 3 and 5 position reduced the stability of the main chain. Metal palladium was the final product of the thermal decompositions, which was identified by X-ray powder diffraction.     

A three‐dimensional supramolecular framework based on the interlinkage of an interpenetrating diamondoid coordination network

In the development of coordination‐driven crystalline materials, O‐ and N‐atom donors from carboxylate and pyridyl‐based ligands are widely used classes of multidentate bridging ligands composed of several terminal coordinating groups linked by either rigid or flexible spacers. The rigidity of the ligands can play a vital role in the determination of the structures formed. A new Cd^II supramolecular compound, namely poly[μ‐adipato‐κ²O ¹:O⁴‐μ‐adipato‐κ⁴O ¹,O ^1′:O ⁴,O^4′‐diaquabis[μ‐1,4‐bis(pyridin‐4‐yl)‐1,3‐butadiene‐κ²N :N ′]dicadmium(II)], [Cd₂(C₆H₈O₄)₂(C₁₄H₁₂N₂)₂(H₂O)₂]_n , (I), has been synthesized by the self‐assembly of Cd(NO₃)₂·4H₂O, adipic acid (hexane‐1,6‐dioic acid; H₂adp) and the dipyridyl ligand 1,4‐bis(pyridin‐4‐yl)buta‐1,3‐diene (1,4‐bpbd) under hydrothermal conditions. Single‐crystal X‐ray diffraction analysis reveals that each Cd^II centre is located in a distorted octahedral coordination environment, coordinated by one water O atom, three carboxylate O atoms from two different adp²⁻ ligands and two N atoms from two different 1,4‐bpbd ligands. The Cd(H₂O) units are interconnected by the μ₂,κ²‐adp²⁻, μ₂,κ⁴‐adp²⁻ and 1,4‐bpbd ligands, which lie across centres of inversion, to give a 6⁶‐ dia network. Large cavities within a single diamondoid network permit the mutual threefold interpenetration of crystallographically equivalent frameworks. Hydrogen‐bonding interactions between the coordinated water molecules and adp²⁻ carboxylate O atoms anchor the interpenetrating networks into a unique three‐dimensional supramolecular structure. Topologically, taking the coordinated water molecules and Cd^II centres as nodes, the whole architecture can be simplified as a binodal (3,7)‐connected supramolecular framework. The identity of (I) was further characterized by IR spectroscopy, elemental analysis, thermogravimetric analysis and powder X‐ray diffraction. The solid‐state photoluminescence properties of (I) were also investigated.     

Synthesis and application of palladium stearates as precursors for the preparation of palladium nanoparticles

This report successfully demonstrates the synthesis and application of palladium stearates. It was found that the branching of the carboxylate anion of metal precursors could influence the size and shape of palladium nanoparticles (PdNPs). Worm‐like nanowires were formed when using the branched isomer palladium isostearate (PdISt₂), while triangular nanoparticles were produced in a majority when using the normal form: palladium stearate (PdSt₂). Furthermore, when applying CO₂ to the system, both types of PdNPs transformed into more spherical shapes with smaller sizes. The formation of carbamates from the amine stabilizer with CO₂ could prevent the further growth and aggregation of PdNPs. The PdNPs were tested as catalysts for the hydrogenation of styrene, and higher catalytic activities were achieved with PdNPs that were prepared with the assistance of CO₂. Copyright © 2012 John Wiley & Sons, Ltd.     

Electrochemical milling and faceting: size reduction and catalytic activation of palladium nanoparticles

Improved performance through milling: A method for enhancing the catalytic activity of supported metal nanoparticles is reported. This method enhances the activity for the ethanol electro-oxidation of a supported palladium catalyst. The much higher catalytic performance is ascribed to the increased electrochemically active surface area as well as the generation of high-index facets at the milled nanoparticle surface.     

High-throughput kinetic study of hydrogenation over palladium nanoparticles: combination of reaction and analysis

The hydrogenation of 1-acetylcyclohexene, cyclohex-2-enone, nitrobenzene, and trans-methylpent-3-enoate catalyzed by highly active palladium nanoparticles was studied by high-throughput on-column reaction gas chromatography. In these experiments, catalysis and separation of educts and products is integrated by the use of a catalytically active gas chromatographic stationary phase, which allows reaction rate measurements to be efficiently performed by employing reactant libraries. Palladium nanoparticles embedded in a stabilizing polysiloxane matrix serve as catalyst and selective chromatographic stationary phase for these multiphase reactions (gas-liquid-solid) and are coated in fused-silica capillaries (inner diameter 250 microm) as a thin film of thickness 250 nm. The palladium nanoparticles were prepared by reduction of palladium acetate with hydridomethylsiloxane-dimethylsiloxane copolymer and self-catalyzed hydrosilylation with methylvinylsiloxane-dimethylsiloxane copolymer to obtain a stabilizing matrix. Diphenylsiloxane-dimethylsiloxane copolymer (GE SE 52) was added to improve film stability over a wide range of compositions. Herein, we show by systematic TEM investigations that the size and morphology (crystalline or amorphous) of the nanoparticles strongly depends on the ratio of the stabilizing polysiloxanes, the conditions to immobilize the stationary phase on the surface of the fused-silica capillary, and the loading of the palladium precursor. Furthermore, hydrogenations were performed with these catalytically active stationary phases between 60 and 100 degrees C at various contact times to determine the temperature-dependent reaction rate constants and to obtain activation parameters and diffusion coefficients.     

Stereoselective hydrosilylation of enals and enones catalysed by palladium nanoparticles

A highly versatile and efficient hydrosilylation method by palladium nanoparticle catalysis allows the direct and chemoselective synthesis of 1) enolsilanes of high isomeric purity, 2) saturated aldehydes or ketones, or 3) the corresponding saturated acetals from α,β-unsaturated aldehydes or ketones. The choice of the product is determined by simply switching the solvent from THF to mixtures of THF/water or THF/alcohol.     

Nano-electrochemical detection of hydrogen or protons using palladium nanoparticles: distinguishing surface and bulk hydrogen.

The benefits of using nanoparticle-modified electrodes are exemplified through the electrochemical detection of protons and/or hydrogen. It is shown that a palladium-nanoparticle-modified boron-doped diamond allows voltammetric information relating to the relative roles played by the surface and the bulk metal to be obtained for the proton-hydrogen system at palladium surfaces which is not accessible using palladium macroelectrodes or microelectrodes.     

On the ratio of processes of adsorbed oxygen layer formation and palladium surface layer dissolution at linear anodic potential sweep

The surface layer dissolution on certain palladium deposits (Pd/Pt) and smooth polycrystalline Pd in sulfuric acid solutions is quantitatively studied at anodic linear potential sweep using atomic absorption analysis. The strong effect of the conditions of Pd/Pt electrolytic generation on the processes of palladium dissolution and oxygen adsorption is shown. The scanning electron microscopy method is used to trace the relationship between the synthesis conditions and the palladium deposit morphology. It is shown that palladium dissolution in the course of the anodic potential sweep is the most active for deposits with the highest specific surfaces. At the oxygen adsorption potentials, the charge value consumed in Pd dissolution decreases as the potential sweep rate increases.     

A closer look: magnetic behavior of a three-dimensional cyanometalate coordination polymer dominated by a trace amount of nanoparticle impurity

The structure and properties of the K{Ni[Au(CN)(2)](3)} coordination polymer, prepared as a powder at room temperature and recrystallized hydrothermally, are reported. The structure of K{Ni[Au(CN)(2)](3)} contains triply-interpenetrated Prussian Blue type pseudo-cubic arrays assembled from the alternation of octahedral Ni(II) centers and [Au(CN)(2)](-) bridging units. SQUID magnetometry studies have shown that K{Ni[Au(CN)(2)](3)} displays typical paramagnetic behavior for isolated Ni(II) centers down to 1.8 K. However, the magnetic behavior of the samples prepared under hydrothermal conditions suggests a superparamagnetic signature superimposed onto a paramagnetic background. After investigating the samples by transmission electron microscopy, it was determined that, in addition to K{Ni[Au(CN)(2)](3)}, the high-temperature (125, 135, and 165 degrees C) aqueous reaction of Ni(NO(3))(2)6 H(2)O with KAu(CN)(2) also led to the formation of nanoparticles of NiO and Au as minor side products, and that these dominated the magnetic behavior. Nanoparticles of various sizes and shapes were observed, depending on the reaction conditions. Samples containing nanoparticles were found to be superparamagnetic, exhibiting blocking temperatures of approximately 17-22 K, consistent with the behavior expected for NiO nanoparticles. These results illustrate the extreme care that must be taken when examining the physical properties of apparently analytically pure materials prepared by hydrothermal methods.     

Redox reactions in palladium catalysis: on the accelerating and/or inhibiting effects of copper and silver salt additives in cross-coupling chemistry involving electron-rich phosphine ligands

Challenging a catalytic cycle: Pd(0) catalysts are readily oxidized by Cu and Ag salts to give dinuclear Pd(I) complexes and Cu(I) or Ag(I) cubanes (see scheme). The reactivities of the resulting Pd(I) dimers are consistent with several observations of additive effects in cross-coupling chemistry. The results indicate the possibility for alternative catalytic cycles involving dinuclear Pd(I) complexes over the currently accepted synergistic cycles involving Pd(0)/Pd(II) intermediates and Cu or Ag.     

Formation and thermal stability of BMI-based interpenetrating polymers for gas separation membranes

Semi-interpenetrating polymer networks (semi-IPNs) were prepared by sol–gel technique through in situ polymerization of bismaleimide (BMI) in thermoplastic polyetherimide (PEI) as well as in polysulfone (PSF). This synthesis route allows arresting thermoset/thermoplastic phase separation at an early stage by solidifying the semi-IPNs through membrane phase inversion. The phase separation could be observed visually in the casting solution or by optical microscope on the surface of the produced membranes. These semi-IPNs with a density lower than their thermoplastic base polymer allowed easier water penetration during membrane phase inversion. This led to improved membrane morphology that was confirmed by scanning electron microscopy. Membranes fabricated from these semi-IPN materials had thinner skin layers and longer straight fingers perpendicular to membrane surface. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) showed that these semi-IPNs membranes have improved glass transition temperatures but a lower thermal stability. However, at ambient conditions, these membranes with their improved structure and morphology showed superior gas separation characteristics compared to base polymers. For example, the permeance was increased by 12–15 times without a significant decrease in the selectivity of oxygen over nitrogen in air separation experiments.     

Rationally designed pincer-type heck catalysts bearing aminophosphine substituents: Pd IV intermediates and palladium nanoparticles

The aminophosphine-based pincer complexes [C6H3-2,6-(XP(piperidinyl)2)2Pd(Cl)] (X=NH 1; X=O 2) are readily prepared from cheap starting materials by sequential addition of 1,1',1'-phosphinetriyltripiperidine and 1,3-diaminobenzene or resorcinol to solutions of [Pd(cod)(Cl)2] (cod=cyclooctadiene) in toluene under N2 in "one pot". Compounds 1 and 2 proved to be excellent Heck catalysts and allow the quantitative coupling of several electronically deactivated and sterically hindered aryl bromides with various olefins as coupling partners at 140 degrees C within very short reaction times and low catalyst loadings. Increased reaction temperatures also enable the efficient coupling of olefins with electronically deactivated and sterically hindered aryl chlorides in the presence of only 0.01 mol % of catalyst. The mechanistic studies performed rule out that homogeneous Pd 0 complexes are the catalytically active forms of 1 and 2. On the other hand, the involvement of palladium nanoparticles in the catalytic cycle received strong experimental support. Even though pincer-type Pd IV intermediates derived from 1 (and 2) are not involved in the catalytic cycle of the Heck reaction, their general existence as reactive intermediates (for example, in other reactions) cannot be excluded. On the contrary, they were shown to be thermally accessible. Compounds 1 and 2 show a smooth halide exchange with bromobenzene to yield their bromo derivatives in DMF at 100 degrees C. Experimental observations revealed that the halide exchange most probably proceeded via pincer-type Pd IV intermediates. DFT calculations support this hypothesis and indicated that aminophosphine-based pincer-type Pd IV intermediates are generally to be considered as reactive intermediates in reactions with aryl halides performed at elevated temperatures.     

Effect of the structure of bridging ligands on the structure and adsorption properties of 3D-coordinated copper(II) and cobalt(II) formate polymers

Increasing length of the bipyridyl bridging ligands in a series of similar complexes facilitates the formation of microporous coordination polymers. The structure of one of these compounds was found by the Rietveld method. These compounds were found to differ in their adsorption caparcities relative to methanol, nitrogen, and hydrogen. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 1, pp. 39–43, January–February, 2006.