Metal complex homogeneous catalysis

Keywords H-P addition Unsaturated carbon linkage Transition metal complex Homogeneous catalysis... 

Our understanding of organic reactions catalyzed by soluble metal complexes ( homogeneous catalysis ) is based on the properties and stoichiometric reactions of organometallic complexes, defined as molecules containing metal-carbon bonds. Significant aspects are summarized below, but for details the reader is recommended to one of the excellent texts cited at the end of this Appendix. 

Dr. Creutz s research interests include kinetics and mechanisms of ground and excited-state reactions of transition metal complexes, homogeneous catalysis in water, and charge transfer processes in nanoscale clusters. 

Catalytic functions of metal ions and their complexes The homogeneous catalytic isomerisation of olefins by transition metal catalysts Homogeneous catalysis... 

To be historically correct, there were earlier examples of metal-mediated homogeneous catalysis. For example, the Hg -catalyzed hydration of acetylene to acetaldehyde became an industrial process in 1912. There is an intermediate /r-acetylene complex to activate the substrate. The lead chamber process to make sulfuric acid (NO catalysis) is even older but does not involve metals or metal complexes as catalysts [134]. 

In order for a substrate to be activated by a transition metal in homogeneous catalysis, prior interaction of the substrate with the transition metal catalyst is required. When the complex is coordinafively saturated with an 18 electron configuration [1,7], coordinafive unsaturation is usually created by dissociation of all or part of a bound ligand from the complex thus allowing the resulting coordinafively unsaturated transition metal fragment to enter the catalytic cycle (Scheme 1.4). 

To explore the catalytic chemistry of metal surfaces, and in particular of small metal particles, we shall have to seek the help of adjacent areas of science. These will include the study under UHV conditions of chemisorbed hydrocarbons, concerning which much is now known homogeneous catalysis by metal complexes, and catalysis by complexes adsorbed on surfaces (to a more limited extent) organometallic chemistry in general and of course theoretical chemistry. 

Unfortunately, the aforementioned problems have not been adequately discussed in literature, although there is considerable interest in the preparation of structurally homogeneous macromolecular metal complexes for catalysis, photochemistry, biological applications, etc. This chapter reports on the specificity of the processes for formation of macromolecular complexes through both routes polymerlike transformations and copolymerization of metal-containing monomers. In addition, the predictive potential of reactions and the ways of preventing some of them will be discussed. 

The plenary lectures at the 14th I.C.C.C. Meeting at Toronto have been published, as have the papers presented at the Bressanone Conference. Several other useful reviews of reactions involving metal ions have been published. The oxidation-reduction of the cobalt centre in vitamin Big has been discussed and recent developments in the bioinorganic chemistry of this complex have also been described. -Electron-transfer catalysts involving metalloenzyme systems have also been reviewed by Williams. The role of transition metals in homogeneous catalysis has been described and homo-lytic oxidation and reduction reactions of organic compounds by metallic ions have been reviewed. ... 

Khan, M. M. T. 1974, Homogeneous Catalysis by Metal Complexes, Vol. II, Activation of Alkenes and Alkynes, Academic Press New York - London... 

G. W. ParshaH, Homogeneous Catalysis The applications and Chemistry of Catalysis by Soluble Transition Metal Complexes,Johm. Wiley Sons, Inc., New York, 1980, 240 pp. An excellent treatment of catalysis by coordination compounds. 

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

Many transition metal complexes dissolve readily in ionic liquids, which enables their use as solvents for transition metal catalysis. Sufficient solubility for a wide range of catalyst complexes is an obvious, but not trivial, prerequisite for a versatile solvent for homogenous catalysis. Some of the other approaches to the replacement of traditional volatile organic solvents by greener alternatives in transition metal catalysis, namely the use of supercritical CO2 or perfluorinated solvents, very often suffer from low catalyst solubility. This limitation is usually overcome by use of special ligand systems, which have to be synthesized prior to the catalytic reaction. 

Since no special ligand design is usually required to dissolve transition metal complexes in ionic liquids, the application of ionic ligands can be an extremely useful tool with which to immobilize the catalyst in the ionic medium. In applications in which the ionic catalyst layer is intensively extracted with a non-miscible solvent (i.e., under the conditions of biphasic catalysis or during product recovery by extraction) it is important to ensure that the amount of catalyst washed from the ionic liquid is extremely low. Full immobilization of the (often quite expensive) transition metal catalyst, combined with the possibility of recycling it, is usually a crucial criterion for the large-scale use of homogeneous catalysis (for more details see Section 5.3.5). 

In comparison to heterogeneous catalyzed reactions, homogeneous catalysis offers several important advantages. The catalyst complex is usually well defined and can be rationally optimized by ligand modification. Every metal center can be active in the reaction. The reaction conditions are usually much milder (T usually < 200 °C), and selectivities are often much higher than with heterogeneous catalysts. 

These advantages notwithstanding, the proportion of homogeneous catalyzed reactions in industrial chemistry is still quite low. The main reason for this is the difficulty in separating the homogeneously dissolved catalyst from the products and by-products after the reaction. Since the transition metal complexes used in homogeneous catalysis are usually quite expensive, complete catalyst recovery is crucial in a commercial situation. 

In comparison with traditional biphasic catalysis using water, fluorous phases, or polar organic solvents, transition metal catalysis in ionic liquids represents a new and advanced way to combine the specific advantages of homogeneous and heterogeneous catalysis. In many applications, the use of a defined transition metal complex immobilized on a ionic liquid support has already shown its unique potential. Many more successful examples - mainly in fine chemical synthesis - can be expected in the future as our loiowledge of ionic liquids and their interactions with transition metal complexes increases. 

The past fifteen years have seen evidence of great interest in homogeneous catalysis, particularly by transition metal complexes in solution predictions were made that many heterogeneous processes would be replaced by more efficient homogeneous ones. There are two motives in these changes—first, intellectual curiosity and the belief that we can define the active center with... 

A discussion of the different types of solute-solute and solute-solvent interactions acting in homogeneous catalysis by transition metal complexes. E. Cesarotti, R. Ugo and L. Kapan, Coord. Chem. Rev., 1982,43, 275-298 (47). 

For a general account of transition-metal-NHC complexes in homogeneous catalysis, see Yong BS, Nolan SP (2003) Chemtracts 16 205... 

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