Polymer supported transition metal complexes

Sherrington, D. C. Polymer-supported transition metal complex alkene epoxidation catalysts. Spec. Publ. - R. Soc. Chem. 1998, 216, 220-228. 

Polystyrene-based resins have been used widely as supports for metal complex catalysts and other reactive species. These polymers, however, have a drawback in their limited thermo-oxidative stability [1,2]. The scope for application is therefore restricted, particularly in polymer-supported transition metal complex oxidation catalysts [3]. Consequently there is a need for the development of polymer supports with a much higher intrinsic thermo-oxidative stability. Polybenzimidazoles and polyimides are likely candidates in this respect. 

C. U. Pittman, Jr., Catalysis by Polymer-Supported Transition Metal Complexes, Chapter 5 in Polymer-Supported Reactions in Organic Synthesis, P. Hodge, D. C. Sherrington, Eds., pp. 249-291, Wiley, New York, 1980. 

Figure 1. Preparation of polymer-supported transition metal complexes by ligand exchange.

A wide variety of polymers have been used to prepare polymer supported transition metal complexes for a variety of applications, as shown in Table I. 

In an effort to overcome the necessity for high-dilution techniques during palladium-mediated macro( clization reactions, the use of polymer-supported transition-metal complexes offers an attractive solution. Reaction concentrations of up to 0.5 mol dm can now be used. ... 

Ionic liquids have already been demonstrated to be effective membrane materials for gas separation when supported within a porous polymer support. However, supported ionic liquid membranes offer another versatile approach by which to perform two-phase catalysis. This technology combines some of the advantages of the ionic liquid as a catalyst solvent with the ruggedness of the ionic liquid-polymer gels. Transition metal complexes based on palladium or rhodium have been incorporated into gas-permeable polymer gels composed of [BMIM][PFg] and poly(vinyli-dene fluoride)-hexafluoropropylene copolymer and have been used to investigate the hydrogenation of propene [21]. 

The literature of supported transition metal complexes has been thoroughly reviewed. In this article, the chemistry of supported complexes is covered in general terms by class of solid support these include metal oxides, clay minerals, zeolites, polymers, and ion-exchange resins. 

Possible alternatives to cross-linked polymer supports are soluble and colloidal polymers. They would require large scale ultrafiltration for industrial use. Although ultrafiltration is not yet economical for desalination of seawater, it might be for a separation of a more expensive product. One example is the catalytic partial hydrogenation of soybean oil (361 with soluble polymer-bound transition metal complexes. Solid inorganic supports such as silica gel and alumina are usually not subject to these physical attrition and filtration problems. 

A viable process for manufacturing polyolefin-clay nanocomposifes by in situ polymerization requires adequate catalytic activity, desirable polymer microstructure, and physical properties including processibility, a high level of clay exfoliation fhaf remains stable under processing conditions and, preferably, inexpensive catalysf components. The work described in the previous two sections focused on achieving in situ polymerization with clay-supported transition metal complexes, and there was less emphasis on optimization of polymer properties and/or clay dispersion. Since 2000, many more comprehensive studies have been undertaken that attempt to characterize and optimize the entire system, from the supported catalyst to the nanocomposite material. The remainder of this chapter covers work published in the past decade on clay-polyolefin nanocomposites of ethylene and propylene homopolymers, as well as their copolymers, made by in situ polymerization. The emphasis is on the catalyst compositions and catalyst-clay interactions that determine the success of one-step methods to synthesize polyolefins with enhanced physical properties. 

Some of the earlier attempts to make supported transition metal complexes were directed toward physically trapping the complexes in the pores of molecular sieves (Rony, 1969 Meir and Uytterhoeven, 1973), adsorbing them on carbon, silica or alumina (Robinson et al., 1969 Acres et al., 1966), and supporting them on proteins (Wilson and Whitesides, 1978). We shall exclude these from our discussion and restrict ourselves only to those cases where the metal complex is chemically bound to an organic polymer. 

Functionalized polymers incorporating neutral, metal binding ligands such as phosphine were prepared as early as 1959 (Rabinowitz and Marcus, 1961 Issleib and Tzschach, 1959). After Merriheld introduced the concept of solid-phase synthesis, the basic idea of using polymer-immobilized transition metal complexes as catalysts burgeoned, and many more polymeric supports containing neutral donor ligands have been prepared. 

The Fischer-Tropsch activity of resin 5 and the unique reaction conditions have important consequences. The use of a reaction solvent raises the possibility of controlling heat removal in this appreciably exothermic process. The apparent homogeneous nature of the catalytic species suggests that other soluble Fischer-Tropsch catalysts may be forthcoming. Finally, CpCo-(00)2 possesses catalytic activity not found in soluble CpCo-(00)2 this demonstrates that attachment to a polymer support not only may induce changes in catalytic activity of a transition metal complex, but also might give rise to completely new activity (51,52,53). 

Support-bound transition metal complexes have mainly been prepared as insoluble catalysts. Table 4.1 lists representative examples of such polymer-bound complexes. Polystyrene-bound molybdenum carbonyl complexes have been prepared for the study of ligand substitution reactions and oxidative eliminations [51], Moreover, well-defined molybdenum, rhodium, and iridium phosphine complexes have been prepared on copolymers of PEG and silica [52]. Several reviews have covered the preparation and application of support-bound reagents, including transition metal complexes [53-59]. Examples of the preparation and uses of organomercury and organo-zinc compounds are discussed in Section 4.1. 

Schuchardt U, Dos Santos EN, Santos Dias F (1989) Butadiene oligomerization and telomerization catalyzed by transition metal complexes supported on organic polymers. J Mol Catal 55 340-352... 

Another application of hyperbranched polymers as supports for catalysts is their use as backbones for the covalent attachment of organometallic fragments. NCN-pincer complexes (NCN-pincer = 2,6-bis[(dimethylamino)-methyl] phenyl anion) are attractive building blocks for catalytic reactions [20,21], Covalent introduction of the transition-metal complexes can also be of interest for visualization and imaging of dendritic polymers by transmission electron microscopy (TEM). 

Many heterogeneous catalysts have been reported in the past to be prepared by anchoring or grafting, processes whereby stable, covalent bonds are formed between an homogeneous transition metal complex and an inert polymer or inorganic support [1-4] The aim was to combine the potential versatility and selectivity of homogeneous catalysts with the practical advantages of a solid material [5]... 

In conclusion, we have shown that attachment of transition metal complexes to polymer supported triphenylphosphine leads to air stable, versatile immobilised catalysts that are as active as their homogeneous analogues and have the advantage that they can be re-used numerous times. Work is currently underway to exploit the activity of other polymer-supported organometallic complexes in metal-mediated organic synthesis. 

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