Structure supported metal complexes

In the last three decades, we have designed and successfully prepared various supported metal complexes on oxide surfaces that exhibit unique catalytic activities and selectivities that are different from those of their homogeneous analogues [3,4,9, 12-15]. With the aid of several sophisticated spectroscopic techniques, the structures and roles of catalytically active species on surfaces have been characterized and identified [3, 4,9,12-25]. Chemical interactions between metal complexes and oxide surfaces can provide new reactivity of metal species by the construction of a spatially controlled reaction environment and the formation of unsaturated active metal species, leading to high catalytic activity, selectivity and durability [21-25]. 

Catalysts were some of the first nanostructured materials applied in industry, and many of the most important catalysts used today are nanomaterials. These are usually dispersed on the surfaces of supports (carriers), which are often nearly inert platforms for the catalytically active structures. These structures include metal complexes as well as clusters, particles, or layers of metal, metal oxide, or metal sulfide. The solid supports usually incorporate nanopores and a large number of catalytic nanoparticles per unit volume on a high-area internal surface (typically hundreds of square meters per cubic centimeter). A benefit of the high dispersion of a catalyst is that it is used effectively, because a large part of it is at a surface and accessible to reactants. There are other potential benefits of high dispersion as well— nanostructured catalysts have properties different from those of the bulk material, possibly including unique catalytic activities and selectivities. 

In the following paragraphs, methods of preparation and characterization of structurally simple supported metal complexes are summarized, and examples are presented that illustrate characterization data and support general conclusions about structure, bonding, reactivity, and catalysis. 

The methods of structure determination of supported nanoclusters are essentially the same as those mentioned previously for supported metal complexes. EXAFS spectroscopy plays a more dominant role for the metal clusters than for the complexes because it provides good evidence of metal-metal bonds. Combined with density functional theory, EXAFS spectroscopy has provided much of the structural foundation for investigation of supported metal clusters. EXAFS spectroscopy provides accurate determinations of metal-metal distances ( 1-2%), but it gives only average structural information and relatively imprecise values of coordination numbers. EXAFS spectroscopy provides structure data that are most precise when the clusters are extremely small (containing about six or fewer atoms) and nearly uniform (Alexeev and Gates, 2000). 

The theoretical parameters characterizing Ir4 in zeolite NaX (Fig. 3) indicate Ir-O distances of about 2.2 A, in good agreement with EXAFS data (Ferrari et al., 1999) and approximately equal to the metal-oxygen bond distances found experimentally and theoretically for supported metal complexes, as discussed above. When the structure of Fig. 3 is rotated 60°, the theory indicates an Ir-O distance of about 2.7 A, in agreement with the longer distances observed by EXAFS spectroscopy (but this agreement may be fortuitous). 

The literature was reviewed to describe the newest efforts to synthesize and characterize supported polynuclear metal complexes as adsorbents and catalysts. This review includes our attempts to model the equilibrium structures and properties of the metal complexes, using simple quantum mechanics, as a means to understand better the interactions between the surface and the metal complexes. Special attention is directed towards the characterization of the supported metal complexes before and after ligand removal. We compare these modeling results with observations in the literature so as to understand better the fundamental processes that govern the interactions between the metal complexes and the surfaces. With this enhanced understanding of these governing factors, it should be easier to prepare oxide solids decorated with metal complexes having the desired physico-chemical properties. 

Chemists have prepared metal complexes containing metal atoms/ions as a means to understand better the structure, chemical bonding, and properties of metals and metal ions. One of the first efforts to affix these metal complexes to a surface as a means to create a supported catalyst was reported by Ballard followed by reports collected by Yermakov, et al. and Basset et al. We distinguish here between metal complexes that contain zero-valent metals and those that show metal cations and we limit this review to complexes containing metal ions as others have published extensive reviews of zero-valent, metal clusters and their chemistryIn our previous three reviews on the chemistry of supported, polynuclear metal complexes, we described efforts to synthesize and characterize oxide-supported, metal complexes as adsorbents, catalysts and precursors to supported metal oxides. In one application of this technology, efforts were... 

Two crystal structures of metal complexes of a,a-trehalose 133 are reported. Two Cd (tren) residues are chelated by the Glcp-02,03 and the Glcp-02, 03 diolato moieties, respectively, in the dinuclear complex 135 (O Fig. 29). As in free a,a-trehalose in the crystalline state, direct intramolecular hydrogen bonds are not found due to conformational restraints, but two sequences of the type 02 ---H-0 ---H-06 with a linking water molecule H2O" are observed (the reversed direction is found in free a,a-trehalose) [153]. Only one of the two Glcp-02,03-chelation sites of a, a -trehalose is chosen in a mononuclear complex with Ni-Me3tren, the iVA, iV -trimethyl analog of Ni-tren, in which no support by an intramolecular... 

In the case of most sol-gel materials, there is (by definition) no hope of producing crystalline samples, and the presence of the sihcate component will invariably interfere with efforts to obtain accurate analytical data. Despite these limitations, many techniques famihar to the coordination chemist have been successfully applied to the study of immobilized metal complexes, and new techniques are emerging that together provide—albeit at a lower resolution than is possible with X-ray crystallography—detailed information about the environment, homogeneity, and dynamics of TM complexes immobilized in silica materials. Many of the techniques used in the characterization of supported reagents of all types are discussed in detail in the book by Clark et al. (215). Techniques such as EXAFS, which are independent of the physical state of the sample, are widely applied and provide detailed structural information (57, 97, 216). The UV/vis and luminescence spectroscopies can often be used without any additional consideration, particularly when optically transparent gel samples are under smdy. Similarly, vibrational spectroscopies have been used extensively for the characterization of sihca-supported metal complexes for many years. 

Gel-immobilized catalytic systems (GCS) represent swelled polymer composites in which active sites of the particular metal complex are inunobilized. Graft copolymers of ethylene-propylene rubber (EPRu) and ligands of 4-vinylpyridine, acrylic acid, vinylpyrrolidone, organophosphorus compounds etc. act as a polymeric supports (polymeric phases) [140]. The structure of metal complex sites immobilized in a polymer gel is presented by the following scheme ... 

Supported metal complexes and clusters with well-defined structures offer the advantages of catalysts that are selective and structures that can be understood in depth. Such catalysts can be synthesized precisely with organometallic precursors, as illustrated in this review. Synthetic methods are illustrated with examples, including silica-supported chromium and titanium complexes for alkene polymerization rhodium carbonyls bonded predominantly at crystallographically specific sites in a zeolite and metal clusters, including Ir4, Rhg, OsjC, and bimetallics. 

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