Stoichiometric and catalytic reactivity of organometallic fragments supported on inorganic oxides

The reaction of some organometallic complexes with the surfaces of inorganic oxides leads to the formation of surface organometallic complexes, chemically bound to the surface yet retaining many features of their molecular structure. These surface organometallic complexes can therefore be considered to belong to both the molecular and solid states. In cases where such complexes have been structurally characterised, their reactivity can be interpreted with molecular concepts. In this review article, the stoichiometric and catalytic reactivity of some relatively well-defined surface organometallic fragments is surveyed. Many elementary steps which have precedent in molecular organometallic chemistry and homogeneous catalysis have now been demonstrated with surface organometallic fragments, including reversible ligand binding, oxidative addition, reductive elimination, protonation, heterolytic metal—carbon bond cleavage, electrophilic CH bond activation and insertion into metal—carbon bonds. In some cases, the supported organometallic complexes are highly effective low temperature catalysts, a phenomenon which is not always observed with molecular analogues nor with conventionally prepared heterogeneous catalysts. Applications of surface organometallic chemistry to catalytic alkane hydrogenolysis, olefin isomerisation and hydrogenation, the Fischer—Tropsch synthesis and the water—gas shift reaction are discussed. Proposed mechanisms for several representative catalytic cycles are presented.     

Polymer-bound metal complexes as catalysts: Synthesis, characterization, reactivity and catalytic activity in E-H bond activation

An account of recent research of our groups is presented, focusing on the preparation of several polystyrene anchored complexes and their use as catalysts in functionalization of alkenes. The procedures for covalently binding the ligands or ligand precursors are outlined, as well as for coordination of the metal ions. Instrumental techniques used for the characterization of the polymer-bound complexes have been highlighted. The heterogenized complexes are used as catalysts for the functionalization of alkenes, namely for oxidative aminations, hydroaminations and several types of oxidations, and results obtained using the catalysts prepared are presented. The main advantages of the use of the prepared supported catalyst are specified, namely their recyclability and their higher activity, also confirmed in many cases by comparison with their homogeneous counterparts. An account of the success in the use of EPR to characterize the V^IVO- and Cu^II-systems is also given.     

负载氧化钒催化剂的多波长拉曼光谱研究:结构识别和定量

探究负载金属氧化物的结构是确立催化剂结构和催化性能之间相互关系的首要条件. 在众多表征技术中,多波长拉曼光谱结合了共振拉曼和由不同波长激发的非共振拉曼,不仅在识别负载金属氧化物团簇的结构,而且在定量方面已经成为强有力的工具. 本文以两个负载氧化钒体系(VOx/SiO2,VOx/CeO2)为例,阐述了如何利用该技术研究活性氧化物团簇的多相结构,并理解氧化物团簇和载体之间复杂的相互作用. 由多波长拉曼光谱得到的定性和定量信息能为设计更有效的负载金属氧化物催化剂提供基本的依据.     

Preparation, Characterization and Catalysis of Intrazeolite Molybdenum, Cobalt and Cobalt–Molybdenum Sulfide Clusters: A Molecular Approach to Hydrodesulfurization Catalysts

Better understandings of the nature of Co–Mo sulfide catalysts are of great importance to a rational design of highly active hydrodesulfurization catalysts on a molecular level. Synthesis of uniform binary sulfide clusters well-defined in structure and thermally stabilized on a support is desirable for such purposes. In the present study, successful preparations, using metal carbonyls as precursors, of Mo, Co and Co–Mo sulfide clusters encaged in zeolite are reported. The structure, location and catalytic properties of the clusters are described on the basis of XPS, XAFS, XRD, XRF, IR, HREM and adsorptions of benzene and NO. Implications for the generation of catalytic synergy between Co and Mo sulfides are presented in brief. It is suggested that the host–guest interactions between zeolite framework oxygens and precursor molecules and product clusters are crucial to the size and structure of the intrazeolite clusters.     

Ligand-Free Metal Clusters

Recent advances in the synthesis and spectroscopic characterization of ligand-free metal clusters immobilized in cryogenic rare gas matrices have contributed greatly to the understanding of electronic, geometric, dynamic, and chemical bonding properties of a wide range of uni- and bimetallic clusters as a function of nuclearity and metal type. The knowledge accumulated on molecular metal aggregates devoid of ligands and isolated on various supports will form an important data base for gauging the reliability of quantum chemical calculations on metal clusters, as well as for comprehending certain aspects of chemisorption on, and catalysis by, supported metal clusters. It can be envisaged that information on ligand-free metal clusters entrapped in a wide range of matrix environments in conjunction with the data for these same metal clusters in the gas phase and in molecular beams will probably contribute towards understanding metal-support interactions and to the designed synthesis of a new breed of high-technology heterogeneous catalysts in the not too distant future.     

Transition metal clusters and supported species with metal-carbon bonds from first-principles quantum chemistry

We discuss the impact of density functional electronic structure calculations for understanding the organometallic chemistry of transition metal (TM) surface complexes and clusters. Examples will cover three types of systems, mainly of interest in the context of heterogeneous catalysis: (i) supported carbonyl complexes of rhenium on MgO and of rhodium in zeolites, (ii) TM clusters with CO ligands and adsorbates, and (iii) metal clusters exhibiting chemical bonds with atomic carbon. The first group of case studies promotes the concept that surface groups of oxide supports are bonded to TM complexes in the same way as common (poly-dentate) ligands are bonded in coordination compounds. The second group of examples demonstrates various “ligand effects” of TM clusters. Finally, we illustrate how carbido centers stabilize TM clusters and modify the propensity for adsorption at the surface of such clusters.     

Advances in late transition metal catalysts for olefin polymerization/oligomerizarion

In this review article, we have consolidated our recent studies on late transition metal catalysts (mainly Fe, Co) for olefin polymerization/oligomerization. A series of bisiminopyridyl Co(II) and Fe(II) complexes were synthesized. These catalysts when activated with MAO in aromatic or aliphatic hydrocarbon solvents, oligomerize or polymerize ethylene to α-olefins or high molecular weight polymers with exceptionally high activities and selectivities. The electronic and steric effects of allyloxy and benzyloxy substituted bisiminopyridyl Fe(II) and Co(II) complexes were also investigated. The influence of catalyst structure and temperature on the polymerization activity, thermal properties and molecular weight were discussed. The effects of heterogenization of these catalysts on silica and modified SBA-15 were analyzed. The polymerization of polar monomers such as vinyl ethers and methyl methacrylate was tested and no specific trends in activity and polymer molecular weight with changes in steric bulkiness around the metal center were observed with the same catalyst system.     

Advanced chemical design with supported metal complexes for selective catalysis

This review covers several recent topics of novel catalyst design with supported metal complexes on oxide surfaces for selective catalysis such as chiral self-dimerization to create asymmetric oxidative coupling catalysis, surface functionalization with achiral reagents to promote asymmetric catalysis, and molecular imprinting to design shape-selective catalysis. The new concepts and designs find wide applications to a variety of selective catalysts.     

High nuclearity metal clusters: Miniature bulk of unusual structures and properties?

Recent developments in cluster synthesis have produced many high nuclearity metal clusters of discretesize andshape approaching that of small particles. Some of these clusters have metal arrangements resemblingfragments of metallic lattices and thus may be considered as aminiature bulk. Some are related to the quasicrystalline phase. Yet others have little or no structural features in common with that of the bulk. These metal clusters of definitivesize andshape provide an opportunity for the study of the evolution of band structure fromatomic tomolecular to thebulk. The focus of this review is on the unusual structures and properties of well-defined high nuclearity metal clusters and their possible relations or variant to the bulk state. Specifically, interesting electronic, optical, and magnetic properties of metal clusters in the quantum-size regime are described. Structural systematics of high nuclearity metal clusters, ranging from thecluster-of-clusters to thelayer-by-layer growth sequence, are discussed. It is hoped that further studies of the structures and properties of large metal cluster compounds of discretesize andshape will shed light on how, when, and why metallic or other bulk behavior begins and ends.     

Linear Copper Complex Arrays as Versatile Molecular Strings: Syntheses,Structures, Luminescence,and Magnetism

The defined linear arrangement of metal atoms in discrete coordination complexes or polymers is still one of the most intriguing challenges in synthetic chemistry. These chain arrangements are of fundamental importance, because of their potential applications as molecular wires and single molecule magnets (SMM) in microelectronic devices on a molecular scale. Oligonuclear Group 11 metal complexes with suitable bridging ligands, specifically those that are based on copper as the first choice of a cheap precursor coinage metal, are of particular interest in this regard. This is due to the superior luminescence properties of these linear clusters that often show d¹⁰⋅⋅⋅d¹⁰ interactions in their molecular structures. The combination of Cu^I with heavier coinage metal ions results in tunable emissive arrays that are also stimuli-responsive. Thus, both linear multinuclear Cu^I and linear heteropolymetallic Cu^I/Ag^I as well as Cu^I/Au^I clusters are excellent candidates for applications in molecular/organic light-emitting devices (OLEDs). Alternatively, paramagnetic multinuclear cupric arrays are prominent as potential molecular wires with enhanced magnetic properties through multiple coupled d⁹ centers. This Review covers the whole range of linear multinuclear assemblies of cuprous and cupric ions in complexes and coordination polymers, their syntheses, photophysical behavior, and magnetic properties. Moreover, recent advances in the rapidly progressing field of hetero-Cu^I/Ag^I and Cu^I/Au^I molecular strings are also discussed.     

Polyamide-bound metal complex catalysts: Synthesis,characterization and catalytic properties

A series of polyamides having different numbers of methylene groups in their repeating units have been synthesized by interfacial polycondensation of terephthaloyl chloride with piperazine and aliphatic diamines H₂N(CH₂)_nNH₂ (n = 2, 6, 10). These materials, which have high thermal stability, were used for immobilization of rhodium and platinum complexes. Chloroplatinic acid and the compounds PtCl₂(CH₃CN)₂ and [RhCl(CO)₂]₂ were used as precursors of the supported catalysts. Low molecular weight analogues of the polyamides were prepared for a study of the coordination mode between the metal ion and the polymer by IR spectroscopy. The results suggest that the carbonyl oxygen of the polyamide is the site of coordination to both rhodium and platinum. The bound catalysts exhibited high activity in hydrosilylation of hexene-1. The activities of the rhodium complexes were found to be dependent on the structure of the polyamide support, decreasing with increasing distance between the amide groups, and closely paralled the changes in the degree of crystallinity of the polymers. Repeated use of the polymers bearing rhodium complexes showed that the bond between the metal and polyamide is fairly stable.     

Vanadium oxide surface studies

The vanadium oxides can exist in a range of single and mixed valencies with a large variety of structures. The large diversity of physical and chemical properties that they can thus possess make them technologically important and a rich ground for basic research. Here we assess the present status of the microscopic understanding of the physico-chemical properties of vanadium oxide surfaces. The discussion is restricted to atomically well-defined systems as probed by surface techniques. Following a brief review of the properties of the bulk oxides the electronic and geometric structure of their clean single crystal surfaces and adsorption studies, probing their chemical reactivity, are considered. The review then focuses on the growth and the surface properties of vanadium oxide thin films. This is partitioned into films grown on oxide substrates and those on metal substrates. The interest in the former derives from their importance as supported metal oxide catalysts and the need to understand the two-dimensional overlayer of the so-called “monolayer” catalyst. On the single crystal metal substrates thin oxide layers with high structural order and interesting properties can be prepared. Particular attention is given to ultrathin vanadium oxide layers, so-called nano-layers, where novel phases, stabilised by the substrate, form.     

Supported molecular iridium catalysts: resolving effects of metal nuclearity and supports as ligands

The performance of a supported catalyst is influenced by the size and structure of the metal species, the ligands bonded to the metal, and the support. Resolution of these effects has been lacking because of the lack of investigations of catalysts with uniform and systematically varied catalytic sites. We now demonstrate that the performance for ethene hydrogenation of isostructural iridium species on supports with contrasting properties as ligands (electron-donating MgO and electron-withdrawing HY zeolite) can be elucidated on the basis of molecular concepts. Spectra of the working catalysts show that the catalytic reaction rate is determined by the dissociation of H(2) when the iridium, either as mono- or tetra-nuclear species, is supported on MgO and is not when the support is the zeolite. The neighboring iridium sites in clusters are crucial for activation of both H(2) and C(2)H(4) when the support is MgO but not when it is the zeolite, because the electron-withdrawing properties of the zeolite support enable even single site-isolated Ir atoms to bond to both C(2)H(4) and H(2) and facilitate the catalysis.     

Metal-cluster compounds: Model systems for nanosized metal particles

Meta-cluster compounds can be exploited advantageously to study the evolution, with increasing size of the molecules of the physical properties of metal clusters from molecular to bulk-metal behavior. The metal-cluster molecules are well-defined, stoichiometric, chemical compounds. The molecules consist of a metal core of a variable number of atoms, surrounded by a shell of ligand atoms or molecules. Depending on the compound, the type of metal atom may be varied, whereas the core size can be changed from a few up to several thousands of atoms. Accordingly, these materials provide excellent model systems for monodisperse metal particles, embedded in a dielectric matrix, and can be investigated by the well-known experimental techniques of solid-state physics. © 1998 John Wiley & Sons, Ltd.     

Application of integrated computer simulation approach to solid surfaces and interfaces

The atomistic understanding of the structure, reactivity, and electronic properties of solid surfaces and interfaces are essential for the design of novel catalysts and electronics/photonics devices which have high-performance and unexplored properties. Computational chemistry is expected not only to rationalize the experimental results but also to predict new features. We have applied integrated computer simulation methods including quantum chemistry, periodic density functional theory, molecular dynamics, embedded atom method, and atomic force microscopy simulation to various topics related to solid surfaces and interfaces. In the present paper, we reviewed our recent activities on supported metal catalysts, metal clusters, atomic force microscopy simulation, high-temperature superconductors, tribology, Si semiconductor and V₂O₅ catalysts. Our activities also involve the generation of a lot of new computer simulation codes. We emphasize that the integrated computer simulation system provides not only methods for scientific studies but also a key technology for industrial innovations in research and development.     

New supramolecular transition metal catalysts

The covalent connection of a catalytically active transition metal center with a water-soluble receptor (host molecule) generates a new type of supramolecular catalyst in which the features of molecular recognition, phase transfer catalysis and transition metal catalysis are combined in a single system. The first examples of this principle make use of commercially available β-cyclodextrin (β-CD) as the receptor and rhodium complexes of diphosphanes as the catalytically active center, these being covalently connected to one another via a spacer. In competitive hydrogenation of certain olefins, unusual degrees of substrate selectivity based on molecular recognition are observed, not possible by conventional transition metal catalysts. The two-phase (water/organic) hydrogenation of nitro-aromatics also is a smooth process catalyzed by these supramolecular complexes. They also constitute an unusually active catalyst system for the selective hydroformylation of higher olefins such as 1-octene in a two-phase system. Dendrimers having diphosphane moieties on the surface provide ligands for transition metals, the corresponding metal complexes (e.g., Pd) functioning as efficient catalysts which can be recycled due to their nanoscopic properties.     

Catalysis by metal nanoparticles embedded on metal-organic frameworks

The present review describes the use of metal-organic frameworks (MOFs) as porous matrices to embed metal nanoparticles (MNPs) and occasionally metal oxide clusters, which are subsequently used as heterogeneous catalysts. The review is organized according to the embedded metal including Pd, Au, Ru, Cu, Pt, Ni and Ag. Emphasis is also given in the various methodologies reported for the formation of the NPs and the characterization techniques. The reactions described with this type of solid catalysts include condensation, hydrogenations, carbon-carbon coupling, alcohol oxidations and methanol synthesis among others. Remaining issues in this field have also been indicated.     

新一代高活性后过渡金属烯烃聚合催化剂

介绍了近几年发展起来的新一代后期过渡金属(Fe,Co,Ni,Pd)烯烃聚合催化剂,对催化剂的结构、性能及催化烯烃聚合进行了阐述。     

Functional Polyoxometalate Thin Films via Electrostatic Layer-by-Layer Self-Assembly

Polyoxometalates (POMs) comprise a structurally diverse class of inorganic transition metal oxygen clusters which—owing to their unique electronic properties—hold promise for a host of technological applications such as electrochromic windows, sensors, or heterogeneous catalysts, prototypic examples of which will be briefly exemplified. The integration of POMs into functional architectures and devices, however, necessitates the development of general methods that allow positioning these clusters in well-defined supramolecular architectures, thin films, or mesophases. This short review highlights recent advances in the preparation of composite multilayers fabricated by electrostatic layer-by-layer self-assembly (ELSA) of POMs and a variety of water-soluble cationic species, including transition metal complexes, cationic surfactants, polycations and bipolar pyridine.     

Platinum Group Metal Clusters: From Gas‐Phase Structures and Reactivities towards Model Catalysts

Transition‐metal clusters have long been proposed as model systems to study heterogeneous catalysts. In this Concept article we show how advanced spectroscopic techniques can be used to determine the structures of gas‐phase transition‐metal clusters and their complexes with small molecules. Combined with computational studies, this can help to develop an understanding of the reactivity of these catalytic models.