Transition Metal Complexes as Homogeneous Catalysts

In particular, if we have a complex that normally has n ligands when the oxidation state of the central metal is z, but prefers (n-l-2) ligands when the oxidation state is increased to z -V 2), we have the prerequisites for facile oxidative addition of a polyatomic molecule such as H2 to form two new ligands (here, hydrido ligands, H ) by breaking a covalent bond within the molecule and taking two electrons from the metal atom M (reaction 18.9). The reverse process is called reductive elimination. 

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One of the potential advantages of transition metal complexes as homogeneous catalysts is the possibility of variation of the ligands and the correlation of catalytic activity with some function of this variable. Few catalytic systems have been examined in the necessary detail, however, and the hydro-formylation reaction is no exception. Much of the work in this area has therefore taken a more empirical approach to catalyst modification. The result has been the production of an extensive patent literature of little scientific value other than to suggest areas in need of further study. 

The hydroformylation of olefins is the most important industrial process using transition metal complexes as homogeneous catalysts. Rhodium complexes are the most efficient catalysts for this reaction in terms of both activity and selectivity [3]. 

The polymerization of olefins and di-olefins is one of the most important targets in polymer science. This review article describes recent progress in this field and deals with organo-transition metal complexes as polymerization catalysts. Recent developments in organometallic chemistry have prompted us to find a precise description of the mechanism of propagation, chain transfer, and termination steps in the homogeneously metal-assisted polymerization of olefins and diolefins. Thus, this development provides an idea for designing any catalyst systems that are of interest in industry. 

In the case of transition metal complexes, their activity as homogeneous catalysts is well documented, but here the purpose is immobilization in a solid that should made possible the easy recovery of the complex from the reaction mixture and its reuse or operation under continuous flow. Immobilization has the benefit of avoiding complex dimerization and aggregation that is one of the common deactivation pathways for metallic complexes as homogeneous catalysts in many types of reactions, including oxidations and reductions. 

Two significant communications indicate the considerable potential of transition metal complexes as multifunctional homogeneous catalysts in the silane field (5, 53). Here the same catalyst activates silanes toward different substrates and it is probable that all proceed via a common metal hydrido intermediate. Both Co2(CO)8 and (Ph3P)3CoHX [X = H2, N2, or (H)Si(OEt)j] catalyze 0-silylation and hydrosilylation the hydrogen on Si may be replaced by R O, R COO, R CONH, or R3SiO [e.g., Eqs. (117)-(120)], and excellent yields of silylated product result. Phenolic groups do... 

Oxidation of alcohols to carbonyl compounds is an important reaction. Stoichiometric oxidants such as chromates, permanganates and MO4 (M = Ru, Os) are the commonly used reagents [19a,59,60]. However, they are going out of favour increasingly because they create heavy metal wastes . In view of this, development of environmentally friendly heterogeneous catalysts for alcohol oxidation is very important. In the use of catalytic amounts of transition metal salts or complexes as homogeneous catalysts for the oxidation of alcohols [61-64], separation of the catalyst from the reaction mixture and its subsequent recovery in active form is cumbersome. Heterogeneous catalysts for this kind of reaction are therefore necessary [65]. Clearly, encapsulation and/or immobilization of known... 

The interaction of saturated C—H and C—C bonds with heterogeneous metal catalysts forms the basis of widely applied reactions such as isomerization, cracking, and re-forming of alkanes. In recent years, much attention has been devoted to the selective activation of C—H bonds by transition metal complexes in homogeneous solution under mild conditions.601 604 In principle, an alkane can undergo oxidative addition to a noble metal complex according to... 

Homogeneous catalysis with defined soluble transition metal complexes as catalysts has become one of the most effective means of transforming simple olefins into more valuable materials. The technically important hydroformylation of olefins to aldehydes or alcohols the Wacker process the dimerization of propylene to linear hexenes the oligomerization of ethylene to linear a-olefins are only a few examples. A feature common to all these processes is the insertion of a substrate olefin molecule, which is coordinatively bonded to the transition metal center M, into a metal-carbon or metal-hydrogen bond present at the same center ... 

One attraction of the homogeneous ZN-catalyst is that it may contain a single well-defined transition metal complex as the catalytic species. This was the motivation of our earlier investigation of the Cp2TiCl2/AlR2Cl catalyst. However, the heterogeneity in the microstructures of anl-PP and the radiolabeling determination of c distrlbution showed that there are two or more catalytic species in the ansa-metallocene/MAO systems. 

Maraval V, Laurent R, Caminade AM, Majoral JP (2000) Phosphorus-containing dendrimers and their transition metal complexes as efficient recoverable multicenter homogeneous catalysts in organic synthesis. Organometallics 19 4025 029... 

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. 

Apart from the activation of a biphasic reaction by extraction of catalyst poisons as described above, an ionic liquid solvent can activate homogeneously dissolved transition metal complexes by chemical interaction. 

The most common catalyst used to date is chloroplatinic acid (also known, after its discoverer, as Speier s catalyst) it is now clear that, contrary to earlier views (23), hydrosilylation is a homogeneous process (25, 208). A major problem is that of reproducibility, and efforts are being made to utilize soluble transition metal complexes. Information about such systems has been used in the interpretation of some related catalytic heterogeneous reactions (232). 

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