Functional Catalysts from Precursor Complexes

Many efforts have been undertaken to graft transition metal complexes onto various supports in order to retain the performance of the soluble catalyst precursors and to allow easy separation of the catalysts from the reaction products. Most studies have been concerned with polymers, particularly with functionalized styrene-divinylbenzene resins. This approach to immobilize homogeneous catalysts has been reviewed, with all the strategies to anchor metal complexes on organic or inorganic supports examined (57-59). 

In 1989, Gupta and coauthors showed that highly active catalysts can be prepared from less complex molecules [63]. In their work, they impregnated a carbon black with polyacrylonitril (PAN) and an iron or cobalt acetate. The precursors were heat-treated at different temperatures and the ORR activity was measured. It was found that one can generalize the preparation of Me-N-C catalysts Whenever a metal precursor is heat-treated with nitrogen and carbon sources at temperatures of >600°C (Co) or >700°C (Fe), an active catalyst can be obtained. A scheme of their preparation route and the achieved ORR activities (as a function of pyrolysis temperature) are given in Fig. 16.18. 

Modem cross coupling chemistry is heavily dominated by the use of palladium and nickel complexes as the catalysts, which show an impressively wide scope and an excellent compatibility with many functional groups.2 This favorable application profile usually overcompensates the disadvantages resulting from the high price of the palladium precursors, the concerns about the toxicity of nickel salts, the need for ancillary ligands to render the complexes sufficiently active and stable, and the extended reaction times that are necessary in certain cases. 

A new parameter space for the synthesis of silsesquioxane precursors was defined by six different trichlorosilanes (R=cyclohexyl, cyclopentyl, phenyl, methyl, ethyl and tert-butyl) and three highly polar solvents [dimethyl sulfoxide (DMSO), water and formamide]. This parameter space was screened as a function of the activity in the epoxidation of 1-octene with tert-butyl hydroperoxide (TBHP) [26] displayed by the catalysts obtained after coordination of Ti(OBu)4 to the silsesquioxane structures. Fig. 9.4 shows the relative activities of the titanium silsesquioxanes together with those of the titanium silsesquioxanes obtained from silsesquioxanes synthesised in acetonitrile. The values are normalised to the activity of the complex obtained by reacting Ti(OBu)4 with the pure cyclopentyl silsesquioxane o7b3 [(c-C5H9)7Si7012Ti0C4H9]. 

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