Tethered complexes on supported metals

No other detailed studies on the mechanisms of hydrogenation of aromatics have been reported to date and are clearly desirable. Although this area of research has remained relatively dormant, in comparison with other homogeneous hydrogenation reactions, the current urgent need for better processes aimed at reducing the aromatic 

Some examples of such systems for the hydrogenation of arenes are already available. A particularly appealing possibility for practical applications in the case of deep desulfurization would be the development of a two stage process incorporating a highly active novel coordination catalyst (liquid biphasic or supported metal complex) for the hydrogenation of one or both of the benzene rings in e.g. 4,6-dimethyldibenzothiophene, followed by a mild HDS of the saturated product which is 

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Fig. 16.4 Some examples of tethered complexes on supported metals (as described by Angelici).

Silica-supported metal (e.g., Pd/Si02) catalysts also have surface silanol groups that can react with the alkoxysilane groups of the complexes. These combination catalysts consist of a tethered complex on a supported metal. A Rh complex was tethered to the surface of a Pd/Si02 catalyst, and the tethered catalyst was more active for the hydrogenation of aromatic compounds than the free complex or the supported catalyst separately.33 It is possible that the H2 is activated on the supported metal and the hydrogen atoms migrate to the silica, where they react with the reactant molecules coordinated by the tethered complex. 

The area of catalyst immobilization has received considerable attention as can be judged from the available literature reviews.[1 30] Immobilization of oxidation catalysts shows intrinsic advantages over other catalysts as the tendency for selfoxidation will decrease. Moreover, complexes with generally low solubility, such as heme-type transition metal complexes, can be dispersed molecularly on supports. It is the aim of the present work to overview the state of knowledge on the immobilization of transition metal complexes using microporous supports, such as zeolites and laminar supports like clays. The wealth of information available for complexes immobilized on LDHs or tethered to the mesopore walls in hierarchically organized oxides will not be dealt with. 

Organometallic Complexes Tethered on Silica-Supported Metal Catalysts Although... 

FIGURE 13.1 Illustration of a tethered homogeneous [RhdLVL j complex catalyst on a silica-supported metal (M) heterogeneous catalyst. 

El. Gao, 1999, Rhodium-amine complexes tethered on silica-supported metal catalysts. Highly active catalysts for the hydrogenation of arenes. New J. of Chem. 23, 6,633-640 F.R Hartley, 1985, Supported Metal Complexes, D. Reidel Publ. Co. Dordrecht... 

Surface science results have thrown new light on modified (chiral) metal surfaces, highlighting new phenomena such as complex adsorption phases, two-dimensional organization and the creation of extended chiral surfaces [307]. New strategies to immobilize an asymmetric catalyst onto a support (adsorption, encapsulation, tethering using a covalent bond and electrostatic interaction) also result in an... 

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