Ligand immobile complex

The procedures which allow for the immobilization of a metal complex on or in a solid are numerous and well described they result from more than thirty years of continuous and imaginative efforts. More difficult, and consequently less successful, work has been described regarding the syntheses of efficient catalysts from these precursors. An immobilized complex must show high activity and selectivity for the target reaction, must be easily recovered intact, and must be stable towards metal leaching under the reaction conditions. These two latter requirements are particularly important where asymmetric catalysis is the goal, because the metal and the ligands... 

Three approaches have been tested, as already described above for inorganic supports. The first attempts concern the direct reaction of transition metal carbonyls with unmodified organic polymers like poly-2-vinyl-pyridine.61 62 However, this kind of anchoring is restricted to only a few complexes. Various polymers have been functionalized with donor groups 63-72 ligand displacement reactions using these afforded the corresponding immobilized complexes. Finally, tests with modified complexes and unmodified polymers are scarce because of the low stability of these complexes under the conditions of reactions. 

The synthesis of aldehydes via hydroformylation of alkenes is an important industrial process used to produce in the region of 6 million tonnes a year of aldehydes. These compounds are used as intermediates in the manufacture of plasticizers, soaps, detergents and pharmaceutical products [7], While the majority of aldehydes prepared from alkene hydroformylation are done so in organic solvents, some research in 1975 showed that rhodium complexes with sulfonated phosphine ligands immobilized in water were able to hydroformylate propene with virtually complete retention of rhodium in the aqueous phase [8], Since catalyst loss is a major problem in the production of bulk chemicals of this nature, the process was scaled up, culminating in the Ruhrchemie-Rhone-Poulenc process for hydroformylation of propene, initially on a 120000 tonne per year scale [9], The development of this biphasic process represents one of the major transitions since the discovery of the hydroformylation reaction. The key transitions in this field include [10] ... 

The simple adsorption of ligand to surface, binding to a secondary ligand, or ligand immobilization through a coordination complex can be classified as this type of immobilization. This technique can be subdivided as follow ... 

Several diphosphine ligands have been applied and the corresponding complexes have been tested for the immobilization (Fig. 2.1.6.3). The activity of different free and immobilized complexes in the enantioselective hydrogenation of dimethyl itaconate and methyl a-acetamidoacrylate was investigated. In blank reactions over pure mesoporous materials no reaction took place. When rhodium supported on carriers was used as catalyst, no enantiomeric excess was observed. 

For the hydrogenation of methyl a-acetamidoacrylate, the best results were again obtained with the (S,S)-Me-Duphos ligand. The complex showed complete conversion with 94% or 93% ee in homogeneous and heterogeneous media, respectively. When methyl a-acetamidoacrylate was used as a probe molecule for enantioselective hydrogenations over RhDuphos immobilized on M41S-type materials, 99% ee and a TON of 2000 were obtained over RhDuphos/Al-MCM-41 (S/Rh = 2000 1), whereas RhDuphos/Al-MCM-48 resulted in 97% ee and a TON of 2900 (S/Rh = 3000 1). 

Ruthenium complexes with mixed bipyridyl ligands, immobilized inside a Nation film, may also be used as pH-sensitive sensor layers [90]. A completely different approach for a ratiometric imaging of pH sensor foils was developed for diagenetic studies of marine sediments, using the dual fluorescence excitation ratio of the pH-sensitive fluorophore 8-hydroxypyrene-l,3,6-trisulfonic acid (HPTS) [91]. Commonly used dual fluorophors with different absorption and emission maxima in the protonated and basic form for ratiometric measurements are the naphthofluorescein and seminaphthofluorescein derivates (SNARF and SNAFL) [92], It should be noted that ammonia or carbon dioxide can also be detected by some of these pH-sensitive materials [55,93]. 

The resulting noncovalently immobilized complexes have been used as ligand systems for both the Pd-catalyzed allylic amination reaction and the Rh-catalyzed hydroformylation. A glycine-urea functionalized PPh3 ligand, 4(S), was noncovalently attached to the immobilized dendritic support, and the application of this system in the Pd-catalyzed allylic amination attains similar yields and product distributions as the homogeneous analogue for the... 

Polynuclear PCMU can be synthesized by means of all the above described methods for the preparation of mononuclear PCMU, such as binding a few metals with polynucleating macromolecular ligands, immobilization of the ready-made and well characterized polynuclear complexes or clusters on polymers and assemblage of polymetallic sites on polymer supports. 

Polymer-immobilized complexes with heterocyclic ligands as catalysts 92UK257. 

Surface complexation — is complexation of metal ions by ligands immobilized on the electrode surface (-> electrode surface area). The ligands may be incorporated in the structure of a -> carbon paste electrode, covalently bound to the surface of a chemically modified electrode (-> surface-modified electrodes), or adsorbed (-> adsorption) on the electrode surface etc. Surface complexation is not confined to electrodes. It can occur on many surfaces, e.g., minerals, when in contact with metal ion solutions or solutions containing complexing ions (in the first case, the surface provides the ligand and the solution the metal ion, whereas in the second case, the surface provides the metal ion and the solution the ligand). Surface complexation can be an important step in the dissolution of solid phases [ii]. 

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