Organometallic complexes, immobilization

The method of catalyst immobilization is one for the reasons for the success of the SAPC approach. Rather than covalently linking an organometallic complex to a support—which usually leads to loss of catalytic efficiency and leaching of the metal—it is the catalyst-philic phase that is immobilized. 

For this complex, molecular chemistry does not adequately model the surface reactivity and the latter is strongly influenced by the presence of surface hydroxyl groups [22]. The organometallic fragments immobilized on silica have been reacted with trimethylphosphine to afford different silica-supported phosphine complexes of rhodium. The course of the reaction depends strongly on the hydroxyl content of the silica surface [23] (Scheme 7.2). 

In periphery-functionalized dendritic catalysts, the functional groups at the surface determine the solubility and miscibility and thus the precipitation properties. Many dendrimers functionalized with organometallic complexes do not dissolve in apolar solvents, and the presence of multiple metal centers at the periphery facilitates precipitation upon addition of this type of solvent. It is emphasized that the use of dendrimer-immobilized catalysts with the goal of recovery through precipitation is worthwhile only if the tendency to precipitation of the dendritic system exceeds that of its non-dendritic equivalent. 

A special case is the test of immobilization of rhodium-diphosphine complexes on all-silica materials. After the first immobilization step in dichloromethane, the solid material had a yellow color and a Rh content of 0.07 mmol g was found. After the extraction with methanol, the entire amount of organometallic complex was washed out and the final material had again the original white color. No rhodium was detected in ICP-AES analysis of this sample. However, in the case of aluminum-containing materials the orange color obtained after the immobilization of the rhodium complexes in dichloromethane is clearly maintained even after extraction in methanol. 

Different immobilization methods were applied for Jacobsen s catalyst. The entrapment of the organometallic complex in the supercages of the dealuminated zeolite was achieved without noticeable loss of activity and selectivity. The immobilized catalysts were reusable and did not leach. For the oxidation of (-)-a-pinene the system used only O2 at RT instead of sodium hypochloride at 0 °C. There was a disadvantage in the use of pivalic aldehyde for oxygen transformation via the corresponding peracid. This results in the formation of pivalic acid, which has to be separated from the reaction mixture. 

The second example demonstrated immobilization via ship in a bottle , ionic, metal center, and covalent bonding approaches of the metal-salen complexes. Zeolites X and Y were highly dealuminated by a succession of different dealumi-nation methods, generating mesopores completely surrounded by micropores. This method made it possible to form cavities suitable to accommodate bulky metal complexes. The catalytic activity of transition metal complexes entrapped in these new materials (e.g, Mn-S, V-S, Co-S, Co-Sl) was investigated in stereoselective epoxidation of (-)-a-pinene using 02/pivalic aldehyde as the oxidant. The results obtained with the entrapped organometallic complex were comparable with those of the homogeneous complex. 

The organic and organometallic complexes of transition metals are especially important in catalysis and photovoltaics, on the basis of their redox and electron-mediating properties. Whilst most complex compounds can be studied in (organic) solution-phase experiments, their solid-state electrochemistry (often in an aqueous electrolyte solution environment) is in general also easily accessible by attaching microcrystalline samples to the surface of electrodes. Quite often, the voltammetric characteristics of a complex in the solid state will differ remarkably from its characteristics monitored in solution. Consequently, chemical, physical or mechanistic data are each accessible via the voltammetry of immobilized microparticles. 

A new immobilization method designed specifically to convert liquid-phase reactants has been developed [7, 8], The catalytic materials consist of a thin film that resides on a high-surface-area support, such as controlled-pore glass or silica, and is composed of a hydrophilic liquid and a hydrophilic organometallic complex (see Figure 1). 

Several metalloporphyrins and phthalocyanines and numerous coordination and organometallic complexes of Rh, Ir, and Pd have also been immobilized directly onto micro- and mesoporous carbon materials, as well as onto CNTs. 

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