Cobalt complexes carbonate complex, hydrolysis

Sargeson and his coworkers have developed an area of cobalt(III) coordination chemistry which has enabled the synthesis of complicated multidentate ligands directly around the metal. The basis for all of this chemistry is the high stability of cobalt(III) ammine complexes towards dissociation. Consequently, a coordinated ammonia molecule can be deprotonated with base to produce a coordinated amine anion (or amide anion) which functions as a powerful nucleophile. Such a species can attack carbonyl groups, either in intramolecular or intermolecular processes. Similar reactions can be performed by coordinated primary or secondary amines after deprotonation. The resulting imines coordinated to cobalt(III) show unusually high stability towards hydrolysis, but are reactive towards carbon nucleophiles. While the cobalt(III) ion produces some iminium character, it occupies the normal site of protonation and is attached to the nitrogen atom by a kinetically inert bond, and thus resists hydrolysis. 

Another way of bringing reactants into close proximity, which is encountered commonly in transition metal chemistry, is through metal ion complexation. The coordination of a reactant to a metal ion complex often activates its reactivity and can bring the reactant into close proximity with a second reactant or with a catalytic group. One example, shown in Fig. 6, is a zinc (11) complex of 1,5,9-triazacyclononane, as a model for the enzyme carbonic anhydrase, which contains a zinc (11) cofactor in its active site (4). In the aqua complex, the bound water molecule has a dramatically reduced pKa value of 7.3, which is similar to the pKa of the active site nucleophihc water. The corresponding cobalt (111) complex catalyzed ester hydrolysis at twice the rate because Co(lll) can coordinate both the hydroxide nucleophile and the ester carbonyl via a... 

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. 

Aromatic acid chlorides are converted into the corresponding anhydrides in high yields (>95%), when reacted with carbon monoxide under solid liquid basic catalysed conditions in the presence of a complexed cobalt or palladium salt [6]. In the absence of the quaternary ammonium salt, only hydrolysis to the carboxylic acid occurs. 

The kinetic effects of C02 in the base catalyzed hydrolysis of some carboxylato amine cobalt(III) complexes have been reported (80-82). In the base catalyzed hydrolysis of oxalatopentaammine-cobalt(III) (80), C02 retarded the reaction due to the formation of a virtually unreactive ion-pair, f (N H .) r, 2 2 COi ]. The equilibrium constant for formation of carbonate ion-pairs with (glycinato-O) (tetraethylene-pentamine)cobalt(III), (81) and (o-methoxybenzoato) (tetraethylenepentamine)cobalt(III) (82) were, however, much smaller than for the oxalatopentamminecobat(III) and a very weak rate retardation and virtually no effect was observed in the base catalyzed hydrolysis of the latter two complexes. 

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). 

A variety of N-O-chelated glycine amide and peptide complexes of the type [CoN4(GlyNR R2)]3+ have been prepared and their rates of base hydrolysis studied.169 The kinetics are consistent with Scheme 8. Attack of solvent hydroxide occurs at the carbonyl carbon of the chelated amide or peptide. Amide deprotonation gives an unreactive complex. Rate constants kOH are summarized in Table 16. Direct activation of the carbonyl group by cobalt(III) leads to rate accelerations of ca. 104-106-fold. More recent investigations160-161 have dealt with... 

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... 

The peptide bond that is cleaved is the bond between Leu-189 and Asp-190. There are two peptide bonds in close proximity to the iron chelate on Cys-212. The other peptide bond is between He-144 and Gly-145. The Cys-212 sulfur is 5.1 A from the carbonyl carbon of Gly-145, and 5.3 A from the carbonyl carbon of Leu-189. However, the main difference is that the peptide bond of Leu-189-Asp-190 is oriented parallel to Cys-212, while the peptide bond of lie-144-Gly-145 is oriented away for Cys-212. As was seen with cobalt(lll) hydrolysis of peptide bonds, the proximity and orientation of the carbonyl carbon is important for hydrolysis. This approach has been extended to the cleavage of multisubunit proteins. Palladium(n) and platinum(ll) complexes as synthetic peptidases have been reviewed elsewhere. ... 

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