Polymer-supported ligand-metal complexes

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

The complexation of neutral molecules by ligands covalently bound to polymer supports requires the study of a set of parameters different from those involving the complexation reactions of metal ions. While the latter emphasizes ion exchange mechanisms, the former relies on reactions such as acid/base interactions for selectivity. The role of the polymer support in molecular complexation reactions has been the focus of this part of our research studies are being undertaken to determine whether the polymer acts only as an inert matrix on which to bond appropriate ligands, or whether it can also influence the ligand-molecule interaction. 

N. E. Leadbeater, M. Marco, Preparation of polymer-supported ligands and metal complexes for use in catalysis, Ghent. Rev. 102 (2002) 3217. 

The desire to convert benzene directly to phenol with 30% hydrogen peroxide was mentioned in Chap. 4. A polymer-supported salicylimine vanadyl complex (1 mol%) was used to catalyze this reaction. Phenol was obtained in 100% yield at 30% conversion.217 There was no leaching of the metal. The catalyst was recycled ten times after which it started to break up. Oxidation of ligands is often a problem with oxidation catalysts. Inorganic supports not subject to such oxidation need to be tried to extend the life of such catalytic agents. 

Several approaches to immobilize transition metal catalysts on polymer supports have been reported in the literature. The most representative ones are (1) immobilization of ligands as well as ligand-metal complexes by covalent and/or coordination bonds (2) adsorption of catalysts on the supports (3) formation of ionic pairs between, for example, the surface of the support bearing anionic functional groups and cationic metal species and (4) entrapment of catalysts... 

Few other asymmetrie reactions have been performed using insoluble or soluble polymer-supported ligands. The first example is a Mukaiyama-aldol condensation between silyl ketene acetal and different aldehydes using polymeric Box analog of 99 as chiral ligands and Cu(OTf)2 as metal soiu ce in water (Scheme 147) [216]. When using benzaldehyde as substrate, yields were very low (12-34%) and ee were moderate (40-62%) whatever the polymer-supported Box. The same level of enantioseleetivity was observed with other aldehydes while the yield was better with all the ligand/Cu complexes used. 

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. 

IV. 10. N.E. Leadbeater, M. Marco - Preparation of Polymer-Supported Ligands and Metal Complexes for Use in Catalysis, Chem. Rev. 102,3217,2002. 

The most frequently used organic supports are polystyrene and styrene-divinylbenzene copolymer beads with functional groups such as diphenylpho.sphine covalently bonded. The polymer-anchored catalyst complex can then be obtained, for example, by displacement of a ligand already co-ordinated to a soluble metal complex (Cornils and Herrmann, 1996) ... 

In addition to imprinted acid-base catalysts [49-55], attempts to imprint metal complexes have been reported and constitute the current state of the art [46, 47]. In most cases of metal-complex imprinting, ligands of the complexes are used as template molecules, which aims to create a cavity near the metal site. Molecular imprinting of metal complexes exhibits several notable features (i) attachment of metal complex on robust supports (ii) surrounding of the metal complex by polymer matrix and (iii) production of a shape selective cavity on the metal site. Metal complexes thus imprinted have been appHed to molecular recognition [56, 57], reactive complex stabilization [58, 59], Hgand exchange reaction [60] and catalysis [61-70]. 

---