Cobalt base hydrolysis reactions

We have Investigated the kinetics of base hydrolysis reactions of the cobalt acldate complexes In aqueous solution and In several mlcroemulslon solutions In which detergent concentrations are at least twice the respective cmc. The results are complicated by the onset of a slow secondary reaction which Is presumably formation of Insoluble, polymeric hydroxo, or hydrated hydroxo compounds. 

Hydroxide ion is different from other reagents with respect to its reactivity toward cobalt(iii) ammine complexes. It reacts very rapidly (as much as 10 times faster than H2O) with cobalt(iii) ammine complexes in a base hydrolysis reaction (33). In this reaction, a first-order dependence on the substituting... 

Another pieee of evidenee to support the 7d CB mechanism is that if there is no N-H hydrogen present in a cobalt(m) complex, the complex reacts slowly with OIT. This suggests that acid-base properties of the complex are more important to the rate of reaction than are the nucleophilic properties of OIT. This base hydrolysis reaction of cobalt(ni) ammine complexes illustrates the fact that kinetic data often can be interpreted in more than one way and that rather subtle experiments must be performed to eliminate one or more possible meehanisms. 

Physical techniques can be used to investigate first order reactions because the absolute concentrations of the reactants or products are not required. Dixon et. al [3] studied the base hydrolysis of cobalt complex, [Co(NH3)5L]3+, where L = (CH3)2SO, (NH2)2C = O, (CH3)03P = O in glycine buffers. 

The long-running dispute over the mechanism of base hydrolysis of cobalt(III)-ammine and -amine complexes, SVj2 versus SVjlCB (better termed Dcb), was several years ago resolved in favor of the latter (73). Recent activity on reactions of this type has concentrated on attempting to locate the precise site of deprotonation of the complex, an exercise successfully accomplished for the complexes syn,anti-[Co (cyclen)(NH3)2]3+ and syn,[Pg.80]

For the reaction of MOH(n 1)+ with propionic anhydride,200 the Bronsted plot of log kMOH versus the pKa of MOH2n+ follows a smooth curve if the values for HzO and OH- are included (Figure 4). However, if the line is drawn to exclude the fcHj0 value, a Bronsted /3 of ca, 0.25 is obtained. Although kMOH for [Co(NH3)5OH]2+ (3 M s 1) is some 103-fold less than k0H, this reaction will compete favourably at neutral pH with base hydrolysis. At pH 7 where the cobalt(III) complex exists almost completely as the MOH2+ species the observed first order rate constant for nucleophilic attack by OH would be ca. 10-4 s 1. AIM solution of [Co(NH3)5OH]2+ would give a value of kobs 2.5 s 1, a rate acceleration of > 104-fold. Since the effective concentration of a nucleophile in the intramolecular reaction could be ca. 102 M, rate accelerations of 10° are possible. The role of the metal ion in such reactions is to provide an effective concentration of an efficient nucleophile at low pH. 

A number of studies have also been made of the hydrolysis of nitriles in the coordination sphere of cobalt(III). Pinnell et al.3 4 found that benzonitrile and 3- and 4-cyanophenol coordinated to pentaamminecobalt(III) are hydrolyzed in basic solution to the corresponding N-bonded carboxamide (equation 22). The reaction is first order in hydroxide ion and first order in the complex with koH= 18.8M 1s 1 at 25.6 °C for the benzonitrile derivative. As fc0H for the base hydrolysis of benzonitrile is 8.2 x 10-6 M-1 s at 25.6 °C, the rate acceleration is ca. 2.3 x 106-fold. The product of hydrolysis is converted to [(NH3)5CoNH2COPh]3+ in acidic solution and the pJC of the protonated complex is 1.65 at 25 °C. Similar effects have been observed with aliphatic nitriles.315 Thus, base hydrolysis of acetonitrile to acetamide is promoted by a factor of 2 x 106 on coordination to [Co(NH3)5]3+. 

The reactions of the pentaamminecobalt(III) complex of urea (O-coordinated) have also been studied.506 Under basic conditions [Co(NH3)5OH]2+ is the only cobalt(III) product. The main reaction pathway (ca. 97%) is SN1CB displacement of coordinated urea (Scheme 40) with kOH = 15.3 M s-1 at 25 °C. A limiting rate was approached at high pH as the complex dissociated to its inactive conjugate base. Hydrolysis of coordinated urea was not observed. 

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). 

Any detailed description of the mechanism of an octahedral substitution must also account for the stereochemical changes that accompany reaction. Werner recognized this and made use of it in his discussions of the stereochemistry of reactions of cobalt(III) complexes. The available experimental results can be explained on the basis of possible molecular rearrangements and some cautious predictions can even be made. The base hydrolysis of cobalt III)ammines appears to be unique in that it often occurs with rearrangement it also affords the few known examples of optical inversion. These results can be explained by formation of a 5-coordinated species with a trigonal bipyramidal structure. Optically active metal complexes racemize by either an intramolecular or an in-termolecular process. Substitution reactions of platinum metal complexes often occur with retention of configuration. 

A consequence of the addition of coordinated OH to alkenes is that other nucleophilies, for example a coordinated aminate ion, should also be active. This type of reaction is seen with the chloropentaammine complex [Co(NH3)500CCH=CHC02Bu ] in aqueous base (Scheme 49). The reaction of the t-butyl maleate complex occurs to the extent of ca. 50% and is complicated by hydrolysis of the maleate ester and some decomposition of the cobalt(III) complexes. Reactions of this type have recently been exploited in the synthesis of )3-carboxyaspartic acid in the coordination sphere of cobalt(III). ... 

Complexes of trifluoromethanesulfonate anion with cobalt(III) are labile oward substitution under mild conditions, and they have proved to be useful synthetic precursors to a variety of aminecobalt(III) complexes. The pentaammine-(trifluoromethanesulfonato-O)rhodium(III) ion, which is readily prepared from [Rh(NH3)5Cl]Cl2 in hot CF3SO3H, is also versatile as a synthetic precursor. " Its synthesis and solvolysis to give essentially quantitative yields of the penta-ammineaqua- and hexaamminerhodium(III) ions are described below. The aqua complex has previously been prepared by the base hydrolysis or Ag -induced aquation of [Rh(NH3)5Cl]Cl2 in water, but the present method presents a cleaner and more rapid alternative. The methods for preparation of the [RhCNHj) ] ion have evolved from the procedure of J0rgensen. They involve prolonged reaction of [Rh(NH3)5Cl]Cl2 with ammonia in a pressure vessel at elevated temperature. The solvolysis of [Rh(NH3)5(0S02CF3)](CF3S0j)2 in liquid ammonia is a simple, high-yield, and rapid alternative. 

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