Metal—ligand bonds catalyst supports

One approach to achieving more stable metal-ligand bonds is to employ multidentate ligands. Multiple bound ligands are particularly desirable for catalytic applications because they can liberate coordination sites on the metal without complete dissociation. The common degradation of a catalyst over time is very often a result of dissociation or side reactions of the supporting ligand. The stability of REM complexes particularly benefits from the combination of... 

Table IV presents the results of the determination of polyethylene radioactivity after the decomposition of the active bonds in one-component catalysts by methanol, labeled in different positions. In the case of TiCU (169) and the catalyst Cr -CjHsU/SiCU (8, 140) in the initial state the insertion of tritium of the alcohol hydroxyl group into the polymer corresponds to the expected polarization of the metal-carbon bond determined by the difference in electronegativity of these elements. The decomposition of active bonds in this case seems to follow the scheme (25) (see Section V). But in the case of the chromium oxide catalyst and the catalyst obtained by hydrogen reduction of the supported chromium ir-allyl complexes (ir-allyl ligands being removed from the active center) (140) C14 of the...

Insertion of COz into a metal-hydride bond normally requires the prior dissociation of an ancillary ligand to generate a coordinatively unsaturated complex, because C02 coordination to the metal usually precedes the formal insertion (Scheme 17.3, lower pathway). Ah initio calculations [59] support this mechanism for the insertion of C02 into the Ru-H bond of RuH2(PH3)4, a model for the catalyst RuH2(PMe3)4. However, it is theoretically possible for C02 insertion to take place without prior C02 coordination (Scheme 17.3, upper pathway) [60, 61]. The... 

Raman spectroscopy is useful for detecting metal-ligand and metal-metal bonds in molecular complexes, but it has not yet been successfully applied to polymer-supported catalysts. We believe that the low concentrations of the metal species and the fluorescence associated with the support are largely responsible for the lack of good spectra. Further work is expected to bring success with this method. 

Axial binding of N-oxide throughout the catalytic cycle implies that the adjacent coordination sites required for oxametallacycle formation are unavailable. The 7-CO ordinate intermediate 20 (Fig. 5b) would suffer from severe steric interactions between the oxametallacycle and the ligand, and seems unHkely given the success of hindered Mn(salen) catalysts such as 8. While dissociation of N-oxide during oxametallacycle formation cannot be categorically excluded, the body of evidence supports a simpler mechanism involving direct attack of the olefin substrate at the metal-oxo bond. 

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