Cobalt complexes acid hydrolysis

In certain instances, the coordinated oxygen atom may undergo reaction. The complete and elegant work carried out on carbonato-ammine complexes of cobalt (III) illustrates this situation. As Equation 34 reveals, the acid hydrolysis of [ (NH3)5CoC03]+ involves substitution at the carbon-oxygen bond, rather than the cobalt-oxygen bond (4). 

The kinetics of the acid hydrolysis of dihydroxo-bridged cobalt(III) complexes have been studied for both cationic and anionic species. The stoichiometry of the hydrolysis reaction for cationic complexes can be expressed by Eq. (56). The equilibrium lies completely to the right at low pH (typically less than 3) and the reverse process in Eq. (56) can normally be disregarded. For all the systems studied to date the observed rate laws can be interpreted in terms of Scheme 4. 

All of the carbonato cobalt(III) complexes reported here are reddish in color and extremely soluble in water. The rhodium complex is pale-yellow, whereas the iridium salt is virtually white they are both soluble in water. Treatment with dilute acid immediately gives the corresponding aqua complex with evolution of carbon dioxide. The characterization and the mechanistic details of acid hydrolysis of these complexes have been reported.3,4,11... 

Owing to the snccess of cobalt(III)-mediated hydrolysis of amino acid esters, the next step was to examine how these complexes reacted with peptides. If similar hydrolytic results could be obtained with peptides, then one of the potential uses of cobalt(III) complexes would be in the N-terminal determination and seqnential analysis of polypeptides. This area has been investigated by several gronps. Peptides... 

Hydrolysis of cobalt(III) amine complexes occurs by two routes. One route is pH-independent, which is usually measured in acidic conditions and is thus often termed acid hydrolysis or aquation. The second route, base hydrolysis, is usually first order in hydroxide ion and complex concentration, although under certain conditions the reaction may become independent of [OH ] or dependent on the general base (156). 

Co(in) complexes promote similar reactions. When four of the six octahedral positions are occupied by amine ligands and two cis positions are available for further reactions, it is possible to study not only the hydrolysis itself, but the steric preferences of the complexes. In general, these compounds catalyze the hydrolysis of N-terminal amino acids from peptides, and the amino acid that is removed remains as part of the complex. The reactions apparently proceed by coordination of the free amine to cobalt, followed either by coordination of the carbonyl to cobalt and subsequent reaction with OH or H2O from the solution (path A in Figure 12-15) or reaction of the carbonyl carbon with coordinated hydroxide (path B). As a result, the N-terminal amino acid is removed from the peptide and left as part of the cobalt complex in which the a-amino nitrogen and the carbonyl oxygen are bonded to the cobalt. Esters and amides are also hydrolyzed by the same mechanism, with the relative importance of the two pathways dependent on the specific compoimds used. 

Although aquation (or acid hydrolysis) reactions of cobalt(III) complexes have been studied more extensively than any other octahedral substitutions, it still is impossible to assign detailed mechanistic paths for them. An aquation reaction takes place at a pH less than 4 and is one in which a ligand is replaced by a molecule of water, such as that shown by Equation 1. 

Table I. Steric Course of the Acid Hydrolysis of Some Cobalt (III) Complexes...

Chromium, Manganese, Cobalt, Nickel, Ruthenium, Rhodium, Platinum, and Uranium Complexes Metalloporphsrrin Complexes Metal Ions and Nucleic Acid Hydrolysis... 

The cobalt(III)-promoted hydrolysis of amino acid esters and peptides and the application of cobalt(III) complexes to the synthesis of small peptides has been reviewed. The ability of a metal ion to cooperate with various inter- and intramolecular acids and bases and promote amide hydrolysis has been investigated. The cobalt complexes (5-10) were prepared as potential substrates for amide hydrolysis. Phenolic and carboxylic functional groups were placed within the vicinity of cobalt(III) chelated amides, to provide models for zinc-containing peptidases such as carboxypeplidase A. The incorporation of a phenol group as in (5) and (6) enhanced the rate of base hydrolysis of the amide function by a factor of 10 -fold above that due to the metal alone. Intramolecular catalysis by the carboxyl group in the complexes (5) and (8) was not observed. The results are interpreted in terms of a bifunctional mechanism for tetrahedral intermediate breakdown by phenol. 

The /i.-amido-/x-carbonato complex (11) has been prepared and characterized, and the kinetics of acid hydrolysis leading to the iit>amido-/Lt-hydroxo>bis(bis(eth> ylenediamine)cobalt(III)) ion studied in detail/ Values of kobs are independent of [H" ] over the acidity range [H ] = 0.9 M to 0.01 M. The kinetics of base hydrolysis were also studied over the range [OH ] = 0.7 M to 0.025 M The results are consistent with a kinetic scheme involving a rapid preequilibrium followed by a rate-determining step resulting in a /i-amido-dihydroxo complex. 

This reaction is sometimes called acid hydrolysis since reactions of this type are normally carried out in acidic solution to repress base hydrolysis. The published data on cobalt(III) complexes are consistent with an mechanism involving dissociative interchange. The bond between cobalt and the leaving group is considerably stretched, and the new bond between the metal and the entering group (water) is little formed (3.2). This view is consistent with the values of AS AV and the effects of sterically hindered ligands on the rates of reaction. 

Cyanocobalamin is widely distributed in living organisms it is found in bacteria, in algae and in animal tissues, but it does not appear to be present in the green leaves of plants. For man, it is an important vitamin, being one of the extrinsic factors of haemopoiesis. It was crystallized in 1948 the crystals are dark red, melt at 320 and their solution has well pronounced absorption bands at 278, 361 and 550 m/i. It contains cobalt and phosphorous and the molecular weight is around 1,500. On acid hydrolysis, cyanocobalamin yields 5,6-dimethylbenzimidazole, ribofuranose, phosphoric add, l-amino-2-propanol and a cobalt complex in which the metal... 

A discovery that extends the scope of asymmetric epoxidation reactions is the hydrolysis of a racemic mixture of epoxides, using the cobalt complex 8.36 as a chiral precatalyst. Under the catalytic conditions, in the presence of air and small amounts of acetic acid, 8.36 is converted to a Co -containing active catalytic intermediate with acetate and water present as additional ligands. 

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