Homogeneous Catalysis with Transition Metal Catalysts

Homogeneous Catalysis with Transition Metal Catalysts 

Most advances in industrial homogeneous catalysis are based on the development of organometallic catalysts. Thousands of organometallic complexes (i.e., compounds with metal-carbon bonds) have become known in the last few decades, and the rapid development of the organic chemistry of the transition metals has been driven by their potential applications as industrial catalysts [12]. 

The chemistry of organo transition metal catalysis is explained in terms of the reactivity of organic hgands boimd to the metal center. The d orbitals of the transition metals allow hgands such as H (hydride), CO, and alkenes to be boimd in such a way that they are activated towards further reactions. 

The most important reactions in catal5nic cycles are those involving ligands located in the coordination sphere of the same metal center. The molecular transformations generally require a loose coordination of the reactants to the central atom and facile release of the products from the coordination sphere. Both processes must proceed with an activation energy that is as low as possible, and thus extremely labile metal complexes are required. Such complexes have a vacant coordination site or at least one weakly bound hgand. 

Reasons for the binding power of transition metals are that they can exist in various oxidation states and that they can exhibit a range of coordination numbers. The coordination complexes can be classified by dividing the hgands into two groups ionic and neutral ligands [Til]. Ionic ligands include  

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Homogeneous Catalysis with Transition Metal Catalysts... 

I 2 Homogeneous Catalysis with Transition Metal Catalysts I—Fe°(CO)3 + HBF4 —... 

The hydroformylation of olefins is a type of CO insertion reaction that is one of the most important industrial applications of homogeneous catalysis with transition metal complexes (208,209). Conventional industrial processes (e.g., the Oxo process) typically use either cobalt- or rhodium-based catalysts and conduct the reaction in two-phase gas-liquid reactors. Efficient transfer of the reactants from the gas phase into the liquid phase is of primary importance to minimize inherent mass transfer limitations (208). Reactor design thus focuses on optimizing this mass transfer rate by maximizing the interfacial area between phases. An SCE process eliminates this transport restriction since the hydrogen... 

Homogeneous catalysis by transition metal clusters has been reviewed from the perspective of the specific transformations.Examples of very mixed-metal clusters catalyzing processes homogeneously are collected in Table IX. As is generally the case with homogeneous catalysis, the catalytic precursor is well defined, but the nature of the active catalyst is unclear. 

Homogeneous catalysis by transition metal complexes almost always involves processes in which product-catalyst separation and catalyst recycling are important issues. For years, researchers have worked to find effective ways to isolate metal-complex catalysts in phases separate from those containing the catalyst, usually by anchoring the metal complex to a solid surface. As summarized by Driessen-Holscher, it is now evident that the method that has met with most practical success in this direction involves the use of multiple liquid phases. For example, rhodium complexes with water-soluble sulfonated ligands are used to catalyze alkene hydroformyla-tion, and the aqueous-phase catalyst and the organic products are easily separated as insoluble liquid phases. 

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). 

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