Cationic ligand, immobilization

It was recently found that the modification of neutral phosphine ligands with cationic phenylguanidinium groups represents a very powerful tool with which to immobilize Rh-complexes in ionic liquids such as [BMIM][PFg] [76]. The guani-dinium-modified triphenylphosphine ligand was prepared from the corresponding iodide salt by anion-exchange with [NH4][PFg] in aqueous solution, as shown in Scheme 5.2-15. The iodide can be prepared as previously described by Stelzer et al. [73]. 

Lee et al. [103] synthesized a chiral Rh-complex with a bisphosphine-contain-ing cation as ligand (Fig. 41.8, 2) to improve the immobilization of the transition-metal complex within the ionic liquid. 

The first two methods have the advantage that no modification of the homogeneous catalyst is needed. Surface hydrogen-bonded catalysts are limited to cationic complexes, while physical entrapment is more widely applicable. However, both methods are very sensitive to the solvent properties of the reaction medium. The chemical methods of immobilization require modification of the ligand, and this may be quite laborious. In the case of irreversible catalyst deacti-... 

Much research has already been devoted in the past couple of years to (i) the immobilization of ATRP active metal catalysts on various supports to allow for catalyst separation and reycycling and (ii) ATRP experiments in pure water as the solvent of choice [62]. A strategy to combine these two demands with an amphiphilic block polymer has recently been presented. Two types of polymeric macroligands where the ligand was covalently linked to the amphiphilic poly(2-oxazo-line)s were prepared. In the case of ruthenium, the triphenylphosphine-functiona-lized poly(2-oxazoline)s described in section 6.2.3.2 were used, whereas in the case of copper as metal, 2,2 -bipyridine functionalized block copolymers were prepared via living cationic polymerization [63] of 2-methyl-2-oxazoline and a bipyridine-functionalized monomer as shown in Scheme 6.8. 

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