Base hydrolysis cobalt complexes

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. 

In a different approach three different structurally defined aza-crown ethers were treated with 10 different metal salts in a spatially addressable format in a 96-well microtiter plate, producing 40 catalysts, which were tested in the hydrolysis of /xnitrophenol esters.32 A plate reader was used to assess catalyst activity. A cobalt complex turned out to be the best catalyst. Higher diversity is potentially possible, but this would require an efficient synthetic strategy. This research was extended to include lanthanide-based catalysts in the hydrolysis of phospho-esters of DNA.33... 

Rate Constants for Aquation and for Base Hydrolysis of Selected penta-ammine-Cobalt(III) Complexes a... 

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]

The kinetics of base hydrolysis of several complexes of the type [Co(NH3)3L3] have been examined in order to see whether the mechanism for these uncharged complexes is the same as that operating for base hydrolysis of the standard cationic complexes (75). A comparison of kinetic parameters - a small selection is given in Table II (76,77) - suggests that all cobalt(III)-nitro-amine complexes, charged and uncharged, undergo base hydrolysis by the SnICB (Dch) mechanism. 

Competition experiments, in which the intermediate is scavenged by two reactants and the amount and nature of products examined, have played an important role in establishing the mechanism for base hydrolysis of cobalt(III) complexes. The intermediate produced in the base hydrolysis of CofNHjljX", = 3, 2 or 1 is Co(NHj)4(NH2) if the mechanism is correct (Sec. 4.3.4). Normally this intermediate is considered to react with H2O to produce Co(NH3)50H + but if another nucleophile Y-Z)" is also present and this can attack the intermediate also, then ColNHjljfY-Z) " and Co(NH3)5(Z-Y) " will also result (Scheme (2.99)). These are linkage isomers (Sec. 7.4) and are spectrally distinguishable... 

The second-order rate constants k for the base hydrolysis of a number of cobalt(lll) complexes were measured with a simple flow apparatus using conductivity as a monitoring device. Equal concentrations (Ag) of reactants were used. Show that a plot of R,/R — R, vs time is linear, having slope s, and that... 

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... 

Table 16 Rate Constants for the Base Hydrolysis of Ester, Amide and Peptide Bonds in Various Cobalt(III) Complexes (25 °C, / = 1.0 M)a...

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. 

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. 

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). 

Cobalt(III) hexakismethylamine is prone to hydrolysis in neutral or slightly basic aqueous solution 158). For the corresponding chloropen-taamine complexes 159), an increase of some 10 was observed in the rate of base hydrolysis of [Co(NH2Me)5Cl] compared to that of [Co(N-HalsCl]. The difference was attributed to steric effects and pointed to a dissociative type of mechanism for the hydrolysis, consistent with an SnI(CB) path. Similar arguments may be employed for the reactivity of the hexaamines. 

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. 

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