Solution pH: A Selectivity Switch in Aqueous Organometallic Catalysis—Hydrogenation of Unsaturated Aldehydes Catalyzed by Sulfonatophenylphosphane–Ru Complexes

The equilibribrium distribution of water-soluble ruthenium hydrides [HRuCl(tppms)₃] and [H₂Ru(tppms)₄] (tppms = (3-sulfonatophenyl)diphenylphosphane) in the reaction with H₂ is governed by the pH value. As a consequence, the selectivity in the hydrogenation of cinnamaldehyde for reaction at C=C or C=O can be completely inverted by changing the pH value (see drawing below).     

Nickel: An Element with Wide Application in Industrial Homogeneous Catalysis

The efficiency and future development of the chemical industry are closely linked to catalysis. It has been estimated, for example, that 60 to 70% of all industrial chemicals have involved the use of a catalyst at some point during their manufacture. In the past two decades the share of the market credited to homogeneous transition metal catalysis increasead to 10–15%. Besides cobalt, which is used mainly in hydroformylation reactions, nickel is the most frequently used metal. Many carbon–carbon bond formation reactions can be carried out with high selectivity if catalyzed by organonickel complexes. Such reactions include, inter alia, carbonylation reactions, cyclic and linear oligomerization and polymerization reactions of monoenes and dienes, and hydrocyanation reactions. It was Reppe and Wilke who pioneered and shaped the field of homogeneous nickel catalysis. Great impetus was also given to the development of organonickel chemistry by Wilke and his students. Research in this area has contributed immensely towards an understanding of the reactions involved in catalysis.—This review is primarily concerned with nickel-catalyzed reactions which are of interest both preparatively and industrially; some mechanistic aspects are also dealt with.     

Nitrogen Donors in Organometallic Chemistry and Homogeneous Catalysis

Homogeneous catalysis has been responsible for many major recent developments in synthetic organic chemistry. The combined use of organometallic and coordination chemistry has produced a number of new and powerful synthetic methods for important classes of compounds in general and for optically active substances in particular. For this purpose, complexes with optically active ligands have been used, most of them coordinating through phosphorus. More recent developments have highlighted the use of “nitrogen-donors”, particularly as they are easily obtained from the “chiral pool”. However, the remarkable achievements in this area have been based on an empirical approach. This article attempts to bridge the gap between the synthetic and the coordination chemist. The first section discusses the rates of formation and dissociation of complexes with nitrogen donors, their relationship to the rates of product formation, and presents the factors which induce homolytic cleavage of M? C bonds. It also provides a summary of the main types of organometallic complexes formed by metal centers coordinated to nitrogen donors and their reactivity patterns. The second section highlights the most significant, homogeneously catalyzed reactions involving complexes with nitrogen ligands. Foremost among them are the asymmetric aspects of hydrogenation (particularly those involving boranes as reducing agents), hydrosilylation, cyclopropanations, Diels-Alder reactions, aldol condensations, alkylation of aldehydes, conjugate addition reactions, Grignard cross-coupling reactions, allylic alkylations, oxidation reactions, olefin epoxidations, and di-hydroxylation of olefins.     

Water-Soluble Ligands,Metal Complexes,and Catalysts: Synergism of Homogeneous and Heterogeneous Catalysis

Rapid developments in the field of catalysis are leading to an increased demand for tailor-made catalysts. Water-soluble complex catalysts, which are being intensively investigated at the present time, combine the advantages of homogeneous and heterogeneous catalysis: simple and complete separation of the product from the catalyst, high activity, and high selectivity. From the large number of available water-soluble ligands, the appropriate catalysts can be developed for many reactions. The industrial applications in the fields of hydrogenation and hydroformylation have already indicated the wide scope of this type of catalyst. In addition, the annual production of 300 000 tons of butyraldehyde through application of water-soluble rhodium complexes at Hoechst AG in Oberhausen, Germany, has demonstrated the industrial importance of the concept of complex-catalyzed reactions in aqueous two-phase systems. The efficient operation of catalytic processes increasingly requires the loss-free recycling of the noble metal catalyst, and this can be simply and economically realized in two-phase systems. Special applications in biochemical problems open up developments in the field of water-soluble transition metal complexes that far transcend the familiar kinds of homogeneous catalysis. In the near future, the investigation and application of metal complex catalysts that are compatible with the physiological, cheap, and environmentally friendly solvent, water, is likely to become a high priority in catalysis research.     

Otto Roelen,Pioneer in Industrial Homogeneous Catalysis

Otto Roelen discovered the oxo synthesis (hydroformylation) in 1938, and despite all the problems created by the war years he was able to explore successfully the fundamental aspects related to its application up to the point of building the first plant. At the same time he laid the groundwork for industrial utilization of homogeneous organometallic catalysts. Almost simultaneously, his contemporary, Walter Hieber, was investigating the basic chemistry of the same catalysts, but with no knowledge whatsoever of their potential application. Hydroformylation today constitutes one of the most important industrial examples of a homogeneous catalytic process. For corrigendum see DOI: 10.1002/anie.199423481     

MOF Encapsulated AuPt Bimetallic Nanoparticles for Improved Plasmonic-induced Photothermal Catalysis of CO2 Hydrogenation

Exploring new catalytic strategies for achieving efficient CO₂ hydrogenation under mild conditions is of great significance for environmental remediation. Herein, a composite photocatalyst Zr-based MOF encapsulated plasmonic AuPt alloy nanoparticles (AuPt@UiO-66-NH₂) was successfully constructed for the efficient photothermal catalysis of CO₂ hydrogenation. Under light irradiation at 150 °C, AuPt@UiO-66-NH₂ achieved a CO production rate of 1451 μmol g_metal⁻¹ h⁻¹ with 91 % selectivity, which far exceeded those obtained by Au@Pt@UiO-66-NH₂ with Pt shell on Au (599 μmol g_metal⁻¹ h⁻¹) and Au@UiO-66-NH₂ (218 μmol g_metal⁻¹ h⁻¹). The outstanding performances of AuPt@UiO-66-NH₂ were attributed to the synergetic effect originating from the plasmonic metal Au, doped active metal Pt, and encapsulation structure of UiO-66-NH₂ shell. This work provides a new way for photothermal catalysis of CO₂ and a reference for the design of high-performance plasmonic catalysts.     

Biosensor Based on Self‐Assembling Glucose Oxidase and Dendrimer‐Encapsulated Pt Nanoparticles on Carbon Nanotubes for Glucose Detection

A novel amperometric glucose biosensor based on layer‐by‐layer (LbL) electrostatic adsorption of glucose oxidase (GOx) and dendrimer‐encapsulated Pt nanoparticles (Pt‐DENs) on multiwalled carbon nanotubes (CNTs) was described. Anionic GOx was immobilized on the negatively charged CNTs surface by alternatively assembling a cationic Pt‐DENs layer and an anionic GOx layer. Transmission electron microscopy images and ζ‐potentials proved the formation of layer‐by‐layer nanostructures on carboxyl‐functionalized CNTs. LbL technique provided a favorable microenvironment to keep the bioactivity of GOx and prevent enzyme molecule leakage. The excellent electrocatalytic activity of CNTs and Pt‐DENs toward H₂O₂ and special three‐dimensional structure of the enzyme electrode resulted in good characteristics such as a low detection limit of 2.5 μM, a wide linear range of 5 μM–0.65 mM, a short response time (within 5 s), and high sensitivity (30.64 μA mM^?1 cm^?2) and stability (80% remains after 30 days).     

Efficient Hydrogenation of Benzaldehydes Over Mesopolymer‐Entrapped Pt Nanoparticles in Water

Trapped! Mesopolymer‐entrapped Pt nanoparticles serve as an efficient catalyst for the hydrogenation of benzaldehydes in water at ambient temperature. In comparison with the commercial Pt/alumina catalyst, the Pt/FDU‐14 catalyst shows superior performance in the hydrogenation reaction. The catalyst afforded high turnover frequencies and was reusable more than nine times without loss of activity or selectivity.

     

Synthesis and Characterization of Dendrimer‐Encapsulated Bimetallic Core‐Shell PdPt Nanoparticles

Using a successive method, PAMAM dendrimer‐encapsulated bimetallic PdPt nanoparticles have been successfully prepared with core‐shell structures (Pd@Pt DENs). Evidenced by UV‐vis spectra, high resolution transmission electron microscopy, and X‐ray energy dispersive spectroscopy (EDS), the obtained Pd@Pt DENs are monodispersed and located inside the cavity of dendrimers, and they show a different structure from monometallic Pt or Pd and alloy PdPt DENs. The core‐shell structure of Pd@Pt DENs is further confirmed by infrared measurements with carbon monoxide (IR‐CO) probe. In order to prepare Pd@Pt DENs, a required Pd/Pt ratio of 1:2 is determined for the Pt shell to cover the Pd core completely. Finally, a mechanism for the formation of Pd@Pt DENs is proposed.     

Properties of Dendrimer‐Encapsulated Pt Nanoparticles Doped Polypyrrole Composite Films and Their Electrocatalytic Activity for Glucose Oxidation

A type of novel electroanalytical sensing nanobiocomposite material was prepared by electropolymerization of pyrrole containing poly(amidoamine) dendrimers‐encapsulated platinum nanoparticles (Pt‐PAMAM), and glucose oxidase (GOx). The Pt nanoparticles encapsulated in PAMAM are nearly monodisperse with an average diameter of 3 nm, and they provide electrical conductivity. Polypyrrole acts as a polymer backbone to give stable and homogeneous cast thin films, and it also defines the electrical conductivity. Both Polypyrrole and PAMAM can provide a favorable microenvironment to keep the bioactivity of enzymes such as glucose oxidase. The homogeneity of GOx/Pt‐PAMAM‐PPy nanobiocomposite films was characterized by atomic force microscopy (AFM). Amperometric biosensors fabricated with these materials were characterized electrochemically using cyclic voltammetry (CV), electrochemical impedance spectra (EIS) and amperometric measurements in the presence of hydrogen peroxide or glucose. All those show the resultant biosensor sensitivity was strongly enhanced within the nanobiocomposite film. The optimized glucose biosensor displayed a sensitivity of 164 μA mM^?1 cm^?1, a linear range of 0.2 to 600 μM, a detection limit of 10 nM, and a response time of <3 s.     

Trimethylphosphane Complexes of Nickel,Cobalt, and Iron—Model Compounds for Homogeneous Catalysis

One of the greatest achievements of organometallic chemistry in the last ten years has been the experimental proof that transition metal-to-carbon bonds are thermodynamically about as stable as those between main group elements and carbon. The present contribution demonstrates how simply constituted alkylnickel, -cobalt, and -iron complexes are obtained by means of a kinetic stabilization using suitable neutral ligands and what information these model compounds can provide with respect to the course of processes in homogeneous catalyses.     

Intermediate Product–Catalyst Complexes in the Homogeneous Hydrogenation of Styrene Derivatives with Parahydrogen and Cationic RhI Catalysts

The catalyst-bound intermediate 1 is detected in the homogeneous hydrogenation of styrene derivatives in the presence of cationic Rh^I catalysts 2 . The detachment from the catalyst of the ethylbenzene 3 formed turns out to be slow and possibly rate-determining in some cases.     

包钯纳米颗粒的中空介孔硅铝酸盐纳米球的合成及其在多步催化反应和尺寸选择催化氢化中的应用

最近, 具有中空核壳结构的纳米材料在催化领域中有着深入的研究和广泛的应用. 在本文中, 我们使用一种简单易行的方法合成了一种包裹钯纳米颗粒的中空介孔硅铝酸盐纳米球(简写为Pd@HMAN). 首先, 通过一种先原位合成钯纳米粒子再对其进行二氧化硅包裹的方法, 在Brij56-环己烷-水的反相胶束中合成了具有核壳结构的包裹钯纳米颗粒的二氧化硅纳米球(简写为Pd@SiO₂). 然后, 使用CTAB, Na₂CO₃和NaAlO₂试剂, 通过简单的碱性条件下刻蚀Pd@SiO₂的过程, 我们成功得到了具有多孔性能的中空核壳型Pd@HMAN纳米催化剂. 由于硅铝酸盐外壳具有酸催化作用, 并且内核钯纳米颗粒又是一种高活性的催化媒介, 因此, 这种复合的多功能纳米催化剂能够很好的应用于多步催化反应中. 此外, 通过一个简单的热处理的方法, 能够缩小硅铝酸盐外壳上的孔道, 我们发现这种孔道调节后的Pd@HMAN纳米催化剂在尺寸选择性氢化反应中有很好的应用前景.     

Enantioselective Hydrogenation of Olefins with Iridium–Phosphanodihydrooxazole Catalysts

High turnover numbers and up to 98% ee were obtained in the catalytic hydrogenation of unfunctionalized aryl-substituted olefins with iridium–phosphanyldihydrooxazole complexes 1 (see reaction scheme). The anion of the complex—for example, hexafluorophosphate or tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BARF⁻)—has a remarkable effect on the reactivity and longevity of the catalyst.     

Knitting N‐doped Hierarchical Porous Polymers to Stabilize Ultra‐small Pd Nanoparticles for Solvent‐Free Catalysis

Hierarchical porous polymers with more than one pore size distributions can effectively support noble metal catalysts and circumvent the limitation of the diffusion of the reactants, and thus exhibit both excellent catalytic activity and superior diffusive properties. Herein, we report a simple one‐step Friedel–Crafts reaction to knit a series of benzene heterocycle monomers, such as indane, indoline, indole and tetrahydronaphthalene to obtain hierarchical porous polymers with high surface areas and/or abundant N sites. These polymers can be directly used to immobilize Pd ions, and stabilize Pd nanoparticles during the thermal reduction process to obtain Pd/polymer catalysts. In particular, indoline‐ and indole‐based polymers with high N content up to 7 wt % exhibit outstanding ability to stabilize uniform ultrasmall Pd nanoparticles. The obtained Pd‐polymers exhibit excellent catalytic activity in the solvent‐free oxidation of benzyl alcohol with O₂.     

Enantioselective Homogeneous Hydrogenation of Monosubstituted Pyridines and Furans

Summary.  The first case of an enantioselective hydrogenation of monosubstituted pyridines and furans with homogeneous rhodium diphosphine catalysts with low but significant enantioselectivities and catalyst activities is reported. Best enantioselectivities (ees of 24–27%) were obtained for the hydrogenation of 2- and 3-pyridine carboxylic acid ethyl ester and 2-furan carboxylic acid with catalysts prepared in situ from [Rh(nbd)₂]BF₄ and the chiral ligands diop, binap, or ferrocenyl diphosphines of the josiphos type. Turnover numbers (ton) were in the order of 10–20, turnover frequencies (tof) usually 1–2 h⁻¹. Diphosphines giving 6- or 7-ring chelates led to higher ees than 1,2-diphosphines; otherwise, no clear correlation between ligand properties and catalytic performance was found. In some experiments black precipitates were observed at the end of the reaction, indicating the decomposition of the homogeneous catalysts for certain ligand/metal/substrate combinations. Received April 5, 2000. Accepted (revised) May 2, 2000