Classification and synthesis of nickel pincer complexes

Over the past decades, the pincer ligands have attracted an increasing interest due to the unique properties of the coordination compounds they form. These monoanionic tridentate ligands are of great importance in organometallic and coordination chemistry. Their complexes with transition metals are used as homogeneous catalysts for various processes and also as functional materials with specified properties. The metal complexes formed by the pincer ligands provide an efficient alternative to the existing catalysts based on noble metals and, hence, the use of these complexes is a promising task of the modern chemical science. Therefore, nickel as the most accessible and inexpensive analog of palladium and platinum is of great practical interest. In this review, we consider the diversity of nickel complexes with pincer ligands, as well as the existing methods for their preparation and practical application.     

A new coupling reaction for the synthesis of ruthenium half-sandwich complexes with sterically demanding cyclopentadienyl ligands

The reaction of RuCl(3)(solv.)(n) with tert-butylacetylene in methanol or ethanol leads to the formation of chloro-bridged half-sandwich complexes with sterically demanding cyclopentadienyl ligands, which are of high interest as starting materials for the synthesis of novel Ru catalysts.     

Oxide- and zeolite-supported molecular metal complexes and clusters: physical characterization and determination of structure, bonding, and metal oxidation state

This article is a review of the physical characterization of well-defined site-isolated molecular metal complexes and metal clusters supported on metal oxides and zeolites. These surface species are of interest primarily as catalysts; as a consequence of their relatively uniform structures, they can be characterized much more precisely than traditional supported catalysts. The properties discussed in this review include metal nuclearity, oxidation state, and ligand environment, as well as metal-support interactions. These properties are determined by complementary techniques, including transmission electron microscopy; X-ray absorption, infrared, Raman, and NMR spectroscopies; and density functional theory. The strengths and limitations of these techniques are assessed in the context of results characterizing samples that have been investigated thoroughly and with multiple techniques. The depth of understanding of well-defined metal complexes and metal clusters on supports is approaching that attainable for molecular analogues in solution. The results provide a foundation for understanding the more complex materials that are typical of industrial catalysts.     

Adsorption of NO and CO over transition-metal-incorporated mesoporous catalytic materials

Novel high-surface-area mesoporous catalysts of type Al-MCM-41 containing transition metals such as iron, nickel, cobalt, zinc, copper, and cobalt were prepared and characterized using techniques such as BET, FTIR, ICP-MS, XPS, and XRD. The XPS measurements indicated that the transition-metal particles are located in the bulk or pore channels of the Al-MCM-41 materials. A detailed in situ FTIR investigation undertaken on the adsorption and disproportionation of NO and CO over the transition-metal-Al-MCM-41 mesoporous catalysts indicated the formation of various NO/CO species or complexes with active metal sites. The structure and dynamics of the metal activated complex and reactive species formed during the CO/NO reaction together with advantages of these catalysts and the influence of reaction temperature and pressure have been studied. NO adsorption at room temperature leads to the formation of adsorbed N(2)O, NO(2), MNO(2), MNO, and [M(NO)(2)] complexes. CO adsorption at room temperature leads to the formation of physisorbed carbon dioxide and cationic Lewis acid carbonyl moieties as well as transition-metal carbonyl complexes. The copper mesoporous catalysts prepared by different procedures (ion exchanged and as-synthesized) were compared for their interactions with CO and NO probe molecules.     

Electrophilic Activation of Silicon–Hydrogen Bonds in Catalytic Hydrosilations

Hydrosilation reactions represent an important class of chemical transformations and there has been considerable recent interest in expanding the scope of these reactions by developing new catalysts. A major theme to emerge from these investigations is the development of catalysts with electrophilic character that transfer electrophilicity to silicon by Si‐H activation. This type of mechanism has been proposed for catalysts ranging from Group 4 transition metals to Group 15 main group species. Additionally, other electrophilic silicon species, such as silylene complexes and η³‐H₂SiRR′ complexes, have been identified as intermediates in hydrosilation reactions. In this Review, different types of catalysts are compared to highlight the range of hydrosilation mechanisms that feature electrophilic silicon centers. The importance of these catalysts to the development of new hydrosilation reactions is also discussed.     

Encapsulation of tetraazamacrocyclic complexes of cobalt(II), copper(II) and oxovanadium(IV) in zeolite-Y and their use as catalysts for the oxidation of styrene

Encapsulation of tetraazamacrocyclic complexes of Co(II), Cu(II) and V(IV) into zeolite-Y has been accomplished, and the resulting materials were used as heterogeneous catalysts for aerobic oxidation of styrene. The materials were prepared by a ship-in-a-bottle method, in which the transition metal cations were first ion-exchanged into zeolite-Y and then reacted with ethylenediamine, followed by acetylacetone. The pure tetraazamacrocyclic complexes were characterized by FTIR, solid UV–Vis and elemental analysis. The structural integrity throughout the immobilization procedure, the successful immobilization of the macrocyclic complexes, and the loadings of metal ions and macrocyclic ligands were determined by characterization techniques such as FTIR, diffuse reflection UV–Vis, inductively coupled plasma atomic emission spectroscopy, scanning electron microscopy, TG/DTA and powder X-ray diffraction. Compared with their homogeneous analogues, the catalytic properties of the encapsulated macrocyclic complexes in the oxidation of styrene with air were investigated. The immobilized complexes proved to be active catalysts and could be reused without significant loss in activity.     

Platinum Group Organometallics Based on “Pincer” Complexes: Sensors,Switches, and Catalysts

Since the first reports in the late 1970s on transition metal complexes containing pincer‐type ligands—named after the particular coordination mode of these ligands—these systems have attracted increasing interest owing to the unusual properties of the metal centers imparted by the pincer ligand. Typically, such a ligand comprises an anionic aryl ring which is ortho,ortho‐disubstituted with heteroatom substituents, for example, CH₂NR₂, CH₂PR₂ or CH₂SR, which generally coordinate to the metal center, and therefore support the M−C σ bond. This commonly results in a terdentate and meridional coordination mode consisting of two metallacycles which share the M−C bond. Detailed studies of the formation and the properties of a large variety of pincers containing platinum group metal complexes have provided direct access to both a fundamental understanding of a variety of reactions in organometallic chemistry and to a range of new applications of these complexes. The discovery of alkane dehydrogenation catalysts, the mechanistic elucidation of fundamental transformations (for example, C−C bond activation), the construction of the first metallodendrimers for sustainable homogeneous catalysis, and the engineering of crystalline switches for materials processing represent only a few of the many highlights which have emanated from these numerous investigations. This review discusses the synthetic methodologies that are currently available for the preparation of platinum group metal complexes containing pincer ligands and especially emphasizes different applications that have been realized in materials science such as the development and engineering of sensors, switches, and catalysts.     

Molybdenum and tungsten imido alkylidene complexes as efficient olefin-metathesis catalysts

Catalytic olefin metathesis has quickly emerged as one of the most often-used transformations in modern chemical synthesis. One class of catalysts that has led the way to this significant development are the high-oxidation-state alkylidene complexes of molybdenum. In this review key observations that resulted in the discovery and development of molybdenum- and tungsten-based metathesis catalysts are outlined. An account of the utility of molybdenum catalysts in the synthesis of biologically significant molecules is provided as well. Another focus of the review is the use of chiral molybdenum complexes for enantioselective synthesis. These highly efficient catalysts provide unique access to materials of exceptional enantiomeric purity and often without generating solvent waste.     

含金属笼型倍半硅氧烷的研究进展

倍半硅氧烷作为催化剂载体硅胶表面结构与性能研究的模型,可以通过表征其表面反应性质来直观认识硅胶负载型催化剂的作用机制。过去几十年来,倍半硅氧烷的研究呈现飞跃式的发展态势,开发出许多新化合物和新合成方法,并在一些催化过程中得到应用。将倍半硅氧烷作为金属化合物的配体,极大地丰富了元素化学的内容。本文重点介绍了合成含金属笼型倍半硅氧烷的相关进展,同时介绍了含金属笼型倍半硅氧烷在聚合物材料应用中的研究。     

Ethylene polymerization promoted by nickel complexes

The nickel complexes are of special relevance to catalysis for ethylene oligomerization and polymerization. Beyond the famous commercial SHOP process for ethylene oligomerization, among recent progress of nickel catalysts, various nickel complexes containing different ligands such as the bidentate and tridentate ligands are of interest. In contrast to the importance of hetereogeneous catalysis, the homogeneous catalyst is a small share for polyolefins, while the well-defined complexes affect the microstructure of the resultant polyolefin. The nickel catalysts often perform ethylene activations for inner olefins and the branched polyethylene with broad or bimodal molecular weight distribution. The catalytic behavior will be affected by adaptation of ligands coordinating around the nickel center. In addition, the auxiliary ligand Ph₃P can improve the catalytic activity by one order of magnitude, and its active center can be confirmed through isolating and characterizing the reliable intermediate. The text was submitted by the authors in English.     

Selective Reduction of Aryl Substituted Chalcones with Zn/Ammonium/Ethanol/Water

The selective reduction of carbon-carbon double bonds of α, β-unsaturated carbonyl compounds has attracted great interest in organic synthesis. Of many reductive systems described in the literature, transitional metal such as Pd, Rh, Pt, Ni, Cu, Ir and their complexes are usually utilized as catalysts, hydrides of Sn, Se, Te, B and sodium dithionite have been developed to catalyze selective reduction of α, β-unsaturated carbonyl compounds.     

Recent progress in homogeneous supported asymmetric catalysis: example of the BINAP and the BOX ligands

Industrial developments of asymmetric catalysis are slowed down by cost and toxicity of the soluble transition metal complexes which are used as catalysts. This problem could be solved by using homogeneous supported catalysis among other technologies. The article analyzes recent results obtained in the fields of catalysis using transition metal complexes grafted on organic or inorganic materials. Two types of ligand are considered: BINAP and BOX. In these two cases several approaches are still under investigations and some of them appear to give a higher performance, such as the polymer with ligand in the main chain and with rigid spacer in case of the BINAP and the ligand grafted on silica particles in the case of BOX.     

Preparation of Porous Polymers Based on the Building Blocks of Cyclophosphazene and Cage-like Silsesquioxane and Their Use as Basic Catalysts for Knoevenagel Reactions

Phosphazene-containing porous materials are of a great interest due to their unique properties, caused by the synergetic presence of nitrogen and phosphorus atoms, and have found applications as adsorbents, basic catalysts, etc. On the other hand, cage-like silsesquioxanes are ideal building blocks for the preparation of covalently-linked porous materials. Here two new phosphazene-functionalized organosilsesquioxane cage-based porous polymers were synthesized successively by a Friedel-Crafts reaction of hexapyrrolylcyclotriphosphazene with octavinylsilsesquioxane in the presence of AlCl₃ and CF₃SO₃H as catalysts. The nature of acid catalysts barely influenced the character of pores due to the interaction of catalysts with basic nitrogen atoms of phosphazene units. The obtained polymers exhibited high efficiency as metal-free catalysts for the Knoevenagel reaction. This work opens new perspectives in the use of porous polymers based on cage-like organosiloxane compounds as basic catalysts for various reactions.     

Synthesis and Application of Novel Chiral Poly-nuclear-Mn(Ⅲ) Catalysts on Asymmetric Epoxidation of Alkenes

The interest of the coordination chemistry of manganese has been driver by the important roles of metalloenzymes and highly valuable catalysts in olefin expoxidation.1 Jacobsen Salen-Mn complexes with a simple structure have been commercially utilized to catalyze asymmetric epoxidation of unfunctionalized cis-alkenes2, but the catalytic enantioselectivity for trans-alkenes, unfortunately, are lower and this kinds of complexes are unstable and difficult to be recovered for reuse.3 In order to improve the catalytic activity and recyclability, many new catalysts including the supported catalysts,heterogeneous catalysts and else modified catalysts have been studied, however their comprehensive effects are unsatisfying.4Recently, some studies in interrelated realm showed that the catalytic performance of bi- or poly-nuclear complexes was superior to that of monomer.5 Meanwhile, our previous studies also showed that properly increasing the molecular weight of catalysts as well as the extent of conjugation of active center would not only result in high activity or reactivity but also its stability and recyclablity, aiding product isolation and catalyst recovery.6For these reasons we designed and synthesized the chiral poly-Mn(Ⅲ) complexes in which active sites were conjugated in certain distance side by side though central nucleus of 4, 6-dihydroxy- isophthalaldehyde (see the scheme). These novel Mn(Ⅲ) complexes have been investigated for the first time as catalysts (lmol%)for the asymmetric epoxidation of alkenes by using pure urea-H2O2 as oxidant and NH4OAc as additive in CH2Cl2/MeOH, showing high activity and good enantioselectivity. All reactions were finished in 1.5h. Rather surprisingly, a marginal increase in ee was observed when the concentration of the substrate was increased from 0.01 to 0.5M. The poly-nuclear complex formation enhanced the catalyst's reactivity and stability. It, unlike mononuclear, could be easily recovered and reused several cycles with a decline of yield but a keep of ee's.A low concentration requirement of catalyst, high ee and chemical activity, mild reaction conditions and catalytic recycles render the kinds of complexes attractive.     

Single-Site Carbon-Supported Metal-Oxo Complexes in Heterogeneous Catalysis: Structure,Reactivity, and Mechanism

When early transition metal complexes are molecularly grafted onto catalyst supports, well-defined, surface-bound species are created, which are highly active and selective single-site heterogeneous catalysts (SSHCs) for diverse chemical transformations. In this minireview, we analyze and summarize a less conventional type of SSHC in which molybdenum dioxo species are grafted onto unusual carbon-unsaturated scaffolds, such as activated carbon, reduced graphene oxide, and carbon nanohorns. The choice of earth-abundant, low-toxicity, versatile metal constituents, and various carbon supports illustrates “catalyst by design” principles and yields insights into new catalytic systems of both academic and technological interest. Here, we summarize experimental and computational investigations of the bonding, electronic structure, reaction scope, and mechanistic pathways of these unusual catalysts.     

稀土催化材料的应用及研究进展

稀土元素具有未充满电子的4f轨道和镧系收缩等特征,当用作催化剂的活性组分或载体时常常表现出独特的催化性能. 稀土催化材料的研究和发展为La和Ce等高丰度轻稀土元素的高质、高效利用提供了有效的途径. 目前稀土催化材料在石油化工、化石燃料的催化燃烧、机动车尾气净化、工业废气治理和固体氧化物燃料电池等领域发挥着重要的作用. 本文综述了稀土催化材料的应用以及理论研究进展,重点讨论了稀土元素对所涉及催化剂的结构、活性和稳定性等的影响.     

Selective oxidation of hydrocarbons catalyzed by iron-containing heterogeneous catalysts

Recent studies on iron-based heterogeneous catalysts for selective oxidation of hydrocarbons are reviewed with emphasis on the partial oxidation of methane and the epoxidation of alkenes. High dispersion of iron sites is essentially important for the selective oxidations. The effective catalysts include immobilized or encapsulated iron complexes, iron-doped metal oxides such as Fe³⁺-doped silica, iron-containing microporous and mesoporous materials, and iron-containing compounds with isolated iron sites typified by iron phosphate. The structure-reactivity relationships and the factors affecting the catalytic performances are discussed with the aim to uncover the requirements of the active iron sites in target-selective oxidation.     

Selective oxidation of hydrocarbons catalyzed by iron-containing heterogeneous catalysts

Recent studies on iron-based heterogeneous catalysts for selective oxidation of hydrocarbons are reviewed with emphasis on the partial oxidation of methane and the epoxidation of alkenes. High dispersion of iron sites is essentially important for the selective oxidations. The effective catalysts include immobilized or encapsulated iron complexes, iron-doped metal oxides such as Fe³⁺-doped silica, iron-containing microporous and mesoporous materials, and iron-containing compounds with isolated iron sites typified by iron phosphate. The structure-reactivity relationships and the factors affecting the catalytic performances are discussed with the aim to uncover the requirements of the active iron sites in target-selective oxidation.     

Monomeric cyclopentadiene molybdenum oxides and their carbonyl precursors as epoxidation catalysts

Monomeric cyclopentadiene molybdenum oxides were among the first synthesized high oxidation state organometallics. However, their syntheses and applications have long been overshadowed by their rhenium(VII) congeners, in particular by (pentamethylcyclopentadienyl)trioxorhenium(VII) and methyltrioxorhenium(VII). Only during the last decade a renaissance of the cyclopentadienyl molybdenum oxides is starting. Both with respect to synthetic access ways and to applications as oxidation catalysts in homogeneous and heterogeneous phase the interest in cyclopentadienyl molybdenum oxides and their tricarbonyl precursors is increasing. This results from the usually time-easy and straightforward synthetic procedures leading to these compounds and their easy heterogenizability, both on mesoporous materials and in ionic liquids. The catalytic performance, especially in olefin epoxidation with tert-butylhydroperoxide being the oxidant is very good and the derivatization to reach taylor made catalysts seems to be much easier than in the case of the organorhenium oxides. Another advantage in comparison to the latter complexes is the possible presence of two organic functionalities, which can be independently modified, e.g. for heterogenization of the complexes and for the enhancing of their catalytic activity at the same time.     

A New Zirconocene Complex and Its Catalytic Properties for Olefin Polymerization

As metallocene olefin polymerization catalysts,unbridged metallocenecomplexeshave re-ceived much attention because they are relatively easy to be synthesized and can produce notonly polyethylene butalso oligo-or polypropylenes with differentstereoregularities and prop-erties depending on their structures[1 ,2 ] .It has been known that a rather small structuralchange for a metallocene compound may cause a remarkable effect on its catalytic properties,such as catalytic activity,polymer molecule w…