The organometallic chemistry of nitrogenases

Nitrogenases mediate the reduction of many substrates other than dinitrogen and a summary is given. Several chemical systems that mimic aspects of nitrogenase reactivity, including transition metal complexes of alkynes and olefins, are outlined. Their protonation has been studied and their relevance to reduction of alkynes and olefins by nitrogenase is assessed. Cyclopropene is reduced by molybdenum nitrogenase to propene and cyclopropane. The reactions of cyclopropene with different transition metal complexes are discussed and a study of interactions of cyclopropenes with models for the active site of the nitrogenase enzyme are described. These models include transition metal hydrides, such as [FeH(H₂)(dmpe)₂][BPh₄] and [MoH₄(dppe)₂] reducing 3,3-dimethylcyclopropene and cyclopropene. Products observed upon protonation and deuteration of several platinum-cyclopropene complexes are presented and a mechanism for their formation is proposed.     

Homogeneous and heterogeneous catalysis: bridging the gap through surface organometallic chemistry

Surface organometallic chemistry is an area of heterogeneous catalysis which has recently emerged as a result of a comparative analysis of homogeneous and heterogeneous catalysis. The chemical industry has often favored heterogeneous catalysis, but the development of better catalysts has been hindered by the presence of numerous kinds of active sites and also by the low concentration of active sites. These factors have precluded a rational improvement of these systems, hence the empirical nature of heterogeneous catalysis. Catalysis is primarily a molecular phenomenon, and it must involve well-defined surface organometallic intermediates and/or transition states. Thus, one must be able to construct a well-defined active site, test its catalytic performance, and assess a structure-activity relationship, which will be used, in turn-as in homogeneous catalysis-to design better catalysts.By the transfer of the concepts and tools of molecular organometallic chemistry to surfaces, surface organometallic chemistry can generate well-defined surface species by understanding the reaction of organometallic complexes with the support, which can be considered as a rigid ligand. This new approach to heterogeneous catalysis can bring molecular insight to the design of new catalysts and even allow the discovery of new reactions (Ziegler-Natta depolymerization and alkane metathesis). After more than a century of existence, heterogeneous catalysis can still be improved and will play a crucial role in solving current problems. It offers an answer to economical and environmental problems faced by industry in the production of molecules (agrochemicals, petrochemicals, pharmaceuticals, polymers, basic chemicals).     

Shape control and associated magnetic properties of spinel cobalt ferrite nanocrystals

By combining nonhydrolytic reaction with seed-mediated growth, high-quality and monodisperse spinel cobalt ferrite, CoFe(2)O(4), nanocrystals can be synthesized with a highly controllable shape of nearly spherical or almost perfectly cubic. The shape of the nanocrystals can also be reversibly interchanged between spherical and cubic morphology through controlling nanocrystal growth rate. Furthermore, the magnetic studies show that the blocking temperature, saturation, and remanent magnetization of nanocrystals are solely determined by the size regardless the spherical or cubic shape. However, the shape of the nanocrystals is a dominating factor for the coercivity of nanocrystals due to the effect of surface anisotropy. Such magnetic nanocrystals with distinct shapes possess tremendous potentials in fundamental understanding of magnetism and in technological applications of magnetic nanocrystals for high-density information storage.     

Pyridine elaboration through organometallic intermediates: regiochemical control and completeness

Pyridines carrying heterosubstituents (such as carboxy, amido, amino, alkoxy or trifluoromethyl groups or solely individual halogen atoms) can be readily and site selectively metalated. Subsequent reaction with a suitable electrophile opens rational access to a wealth of new building blocks for the synthesis of biologically active compounds. This approach relies on organometallic methods, which are both efficacious and extremely flexible as far as the substitution site and the product structure are concerned (86 references).     

Syntheses of organometallic monomers containing cobalt and manganese

Direct electrophilic substitution of the cyclopentadienyl ring of cymantrene has been used to prepare five cymantrenyl-monomers with unsaturated sidechains. Direct substitution of cymantrene with acryloyl- and methacryloyl chloride under Friedel-Crafts conditions has led to acryloyl- and methacryloylcymantrene. The complexes have been characterized using ¹H and ¹³C NMR, IR and elemental analysis. Both complexes polymerize under treatment with AIBN at 60–70°C. The chelated complex [n⁵-C₅H₄(CH₂CH=CH₂)]-Co[(NH)₂C₆H₄], was prepared from the treatment of [n⁵-C₅H₄(CH₂CH=CH₂)]Co(CO)₂ with I₂ followed by (NH)₂C₆H₄ and base. The complex was characterized with ¹H NMR and IR but failed to undergo free-radical polymerization. The strategies used to prepare organometallic polymers are discussed.     

金属有机化学中的绿色化学反应进展

着重综述了金属有机化学领域中所涉及的与绿色化学概念有关的一些化学反应,如水相反应、超临界流体及离子液体中的反应、固态反应、原子经济性及过渡金属催化的有机反应。     

Origins of thermodynamically stable superhydrophobicity of boron nitride nanotubes coatings

Superhydrophobic surfaces are attractive as self-cleaning protective coatings in harsh environments with extreme temperatures and pH levels. Hexagonal phase boron nitride (h-BN) films are promising protective coatings due to their extraordinary chemical and thermal stability. However, their high surface energy makes them hydrophilic and thus not applicable as water repelling coatings. Our recent discovery on the superhydrophobicity of boron nitride nanotubes (BNNTs) is thus contradicting with the fact that BN materials would not be hydrophobic. To resolve this contradiction, we have investigated BNNT coatings by time-dependent contact angle measurement, thermogravimetry, IR spectroscopy, and electron microscopy. We found that the wettability of BNNTs is determined by the packing density, orientation, length of nanotubes, and the environmental condition. The origins of superhydrophobicity of these BNNT coatings are identified as (1) surface morphology and (2) hydrocarbon adsorbates on BNNTs. Hydrocarbon molecules adsorb spontaneously on the curved surfaces of nanotubes more intensively than on flat surfaces of BN films. This means the surface energy of BNNTs was enhanced by their large curvatures and thus increased the affinity of BNNTs to adsorb airborne molecules, which in turn would reduce the surface energy of BNNTs and make them hydrophobic. Our study revealed that both high-temperature and UV-ozone treatments can remove these adsorbates and lead to restitution of hydrophilic BN surface. However, nanotubes have a unique capability in building a hydrophobic layer of adsorbates after a few hours of exposure to ambient air.     

稀土金属有机化学的最新进展

综述了我们近年来在稀土金属有机化合物的合成,结构以及它们在有机合成和极性单体聚合中的应用的新进展。     

Coordination and organometallic chemistry of cyclophosphazenes and polyphosphazenes

This review describes the chemistry of cyclophosphazene and polyphosphazene ligand systems and their transition and organometallic complexes. The structures of the ligands and the complexes are discussed.     

Reactivity of the metal-silicon bond in organometallic chemistry

The reactivity of molecules containing a metal-silicon bond is currently attracting considerable attention owing to its relevance to a number of important stoichiometric and catalytic transformations. The focus of this review concerns those stoichiometric reactions involving the metal-silicon bond which result in isolable Si-containing metal complex(es). Catalytic reactions are therefore only considered when strong evidence for M---Si intermediate complexes is provided. Reactions in which the silicon atom leaves the metal complex are not examined. For convenience, the reactions will be classified into insertion and migration reactions, although this terminology does not necessarily have a mechanistic implication and is somewhat arbitrary since many reaction products could belong to one or the other section.     

Synthesis of organometallic PNA oligomers by click chemistry

The facile side-specific insertion, on the solid phase, of one or two ferrocene moieties into peptide nucleic acid (PNA) oligomers by click chemistry is presented.     

Application of computer modeling to surface organometallic chemistry

Computer modeling studies of several surface organometallic complexes have been performed. These studies demonstrate the utility of the modeling technique as well as providing insight into the interaction between the surface organometallic species and the support surface. Models for hydroxylated oxide supports for silica, γ-alumina and magnesia are described. The surface structures for MORh(η³-C₃H₅)₂ (MAl, Si), Mg/[HFeOs₃(CO)₁₃]⁻, and (μ-H) (μ-OMM) Os₃(CO)₁₀ (MAl,Si) are modeled to determine the preferred arrangement of the organometallic species on the hydroxylated support. In the modeling procedures van der Waals interaction energies and non-bonded contacts as a function of orientation of the cluster with respect to the surface are considered. Relaxation of the cluster and Coulombic interaction energies are also considered where appropriate.     

Use of organometallic chemistry for hydrotreatment catalyst development

This paper describes our use of organometallic chemistry to develop enhanced hydrotreatment catalysts. The approach involves (1) identifying the most active catalytic metals, (2) choosing precursors that will be easily activated into these materials under mild conditions, and (3) then increasing the surface area to provide a highly active catalyst. We describe our efforts in studying hydrodenitrogenation (HDN) reactions, including homogeneous reaction chemistry of the C? N bond, development of enhanced HDN catalysts for coal liquids, and some applications of organometallic chemistry towards coal liquefaction.     

Formation of organometallic polymer nanorods using a nanoporous alumina template and the conversion to magnetic ceramic nanorods

Polyferrocenylsilane nanorods were prepared using a porous anodic aluminium oxide template followed by chemical etching; pyrolysis was used to obtain magnetic iron oxide-containing ceramic nanorods.     

Some organometallic chemistry of ruthenium(II)

Stoichiometric and catalytic reaction of Ru(II) phosphine complexes with alkynes, olefins, and enynes are described. The hydride complex RuCl(CO)H(PPh₃)₃ (1) reacts with the double bond of a cis-enyne whereas it reacts with triple bonds of trans-enynes. Metathesis of vinyl silanes with olefins are catalyzed by 1 where β-Si elimination is the key step. Dimerizations of ^tBu- and Me₃Si-substituted acetylanes into the corresponding butatrienes are catalyzed by Ru(II) active species as studied by isolation of the intermediates. A model reaction for the crucial step of the catalytic cycle, formation of a Ru vinylidene complex from acetylene, has been fully simulated by ab initio-MO calculations.