Cobalt mechanistic considerations

First mechanistic considerations by Hecht and Kroper involved the formation of a methyl cobalt intermediate via esterification of methanol with the strong acid hydrocobait tetracarbonyl [5. Tiic mcihyi cobalt intermediate was ihou t to react with and CO to give acetaldehyde as the primary reaction product, which then was hydrogenated to ethanol (cf. Hquations (8) <10)). 

Abstract This chapter focuses on well-defined metal complexes that serve as homogeneous catalysts for the production of polycarbonates from epoxides or oxetanes and carbon dioxide. Emphasis is placed on the use of salen metal complexes, mainly derived from the transition metals chromium and cobalt, in the presence of onium salts as catalysts for the coupling of carbon dioxide with these cyclic ethers. Special considerations are given to the mechanistic pathways involved in these processes for the production of these important polymeric materials. 

In recent years there has been considerable interest in the hydrolytic activity of cobalt(III) complexes of phosphate esters. This approach has been adopted in order to avoid some of the mechanistic complexities which can arise with kinetically labile metal centres. Some developments in this field have been recently discussed by Dixon and Sargeson.21... 

Carbonic anhydrase is a zinc(II) metalloenzyme which catalyzes the hydration and dehydration of carbon dioxide, C02+H20 H+ + HC03. 25 As a result there has been considerable interest in the metal ion-promoted hydration of carbonyl substrates as potential model systems for the enzyme. For example, Pocker and Meany519 studied the reversible hydration of 2- and 4-pyridinecarbaldehyde by carbonic anhydrase, zinc(II), cobalt(II), H20 and OH. The catalytic efficiency of bovine carbonic anhydrase is ca. 108 times greater than that of water for hydration of both 2- and 4-pyridinecarbaldehydes. Zinc(II) and cobalt(II) are ca. 107 times more effective than water for the hydration of 2-pyridinecarbaldehyde, but are much less effective with 4-pyridinecarbaldehyde. Presumably in the case of 2-pyridinecarbaldehyde complexes of type (166) are formed in solution. Polarization of the carbonyl group by the metal ion assists nucleophilic attack by water or hydroxide ion. Further studies of this reaction have been made,520,521 but the mechanistic details of the catalysis are unclear. Metal-bound nucleophiles (M—OH or M—OH2) could, for example, be involved in the catalysis. 

Considerable work has been expended on elucidating the mechanism of catalytic oxidation of hindered phenols. Reactions of this kind proved to be amenable to kinetic studies and structural work on isolated intermediates. Systems involving cobalt chelate complexes as catalysts have received much attention. In this section we discuss in some detail the most important mechanistic versions proposed thus far. 

A considerable amount of kinetic and mechanistic data has been reported for the base hydrolysis of cis- and trans-dihalobis(ethylenediamine)-cobalt(III) cations/ On the other hand, the only chromium(III) analogs which have been studied are cis- and trans-dichlorobis(ethylenediamine)-chromium(III) cations/ The present study was undertaken to gain additional base hydrolysis data for dihalobis(ethylenediamine)chromium(III) complexes, which will permit a more meaningful comparison with their cobalt(III) analogs. 

Much innovative and extensive research into transition-metal complex pre-catalysts in ethylene reactivity could not been included herein, but a few examples of complex models have been discussed in a limited manner in order to encourage more research and attract more consideration. The progress and achievements, indicated by the selected results, emphasize two characteristic species that can induce two different mechanistic polymerizations highly linear products (either oligomers or polyethylenes) formed by the catalytic systems of iron or cobalt complexes, and branched polyethylenes formed by the catalytic systems of nickel and palladium. 

There is considerable scope for much interesting kinetic and mechanistic work on cobalt(III) complexes of phosphate ligands. The early work of Lincoln and Stranks " on the hydrolysis of c -[Co(en)2P04] in acidic and basic solution has indicated some of the complexities to be expected in this area. 

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