Base hydrolysis kinetics complexes

Structures have been determined for [Fe(gmi)3](BF4)2 (gmi = MeN=CHCF[=NMe), the iron(II) tris-diazabutadiene-cage complex of (79) generated from cyclohexanedione rather than from biacetyl, and [Fe(apmi)3][Fe(CN)5(N0)] 4F[20, where apmi is the Schiff base from 2-acetylpyridine and methylamine. Rate constants for mer fac isomerization of [Fe(apmi)3] " were estimated indirectly from base hydrolysis kinetics, studied for this and other Schiff base complexes in methanol-water mixtures. The attenuation by the —CH2— spacer of substituent effects on rate constants for base hydrolysis of complexes [Fe(sb)3] has been assessed for pairs of Schiff base complexes derived from substituted benzylamines and their aniline analogues. It is generally believed that iron(II) Schiff base complexes are formed by a template mechanism on the Fe " ", but isolation of a precursor in which two molecules of Schiff base and one molecule of 2-acetylpyridine are coordinated to Fe + suggests that Schiff base formation in the presence of this ion probably occurs by attack of the amine at coordinated, and thereby activated, ketone rather than by a true template reaction. ... 

In much of the early kinetic work on macrocyclic complexes the stereochemical complexities were largely ignored. Fortunately in basic solution the diastereoisomers will equilibrate (by base catalysed proton exchange) to give the most thermodynamically stable trans-lll diastereoisomer which has chair six-membered and gauche five-membered chelate rings. As a result the base hydrolysis kinetics reported for [14]aneN4 and its alkyl substituted derivatives probably relate to the trans-III diastereoisomer,(3.12)... 

Two reports have appeared of kinetic studies of the base hydrolysis of complex ions of the type [Co(NH3)5(OCOR)] +. Activation parameters are reported from rate constants extrapolated to zero ionic strength for R = CM3 CI (/i = 1,2, or 3). When RCO2" = salicylate ion a simple second-order rate law is not observed owing to the importance of the deprotonation of the phenolic group. " If K is the equilibrium constant for this deprotonation reaction, the pseudo-first-order rate constant (k) in the presence of an excess of OH" ion is given by k=(A iis [OH-] k2K[OH-] )IH J5 [OH"])... 

All the available kinetic parameters for base hydrolysis of complexes... 

The kinetic behavior of the base hydrolysis of TcCl2(acac)2 is described as an example [26], A plot of the logarithm of the concentration of the technetium complex in the organic phase against time gives a straight line. Thus, the reaction rate of the base hydrolysis of TcCl2(acac)2 is expressed as... 

Studies of the base-hydrolysis mechanism for hydrolysis of technetium complexes have further been expanded to an octahedral tris(acetylacetonato)techne-tium(III) [30], Although a large number of studies dealing with base hydrolysis of octahedral metal(III) complexes have been published [31], the mechanism of the tris(acetylacetonato)metal complex is still unclear. The second-order base hydrolysis of the cationic complex tris(acetylacetonato)silicon(IV) takes place by nucleophilic attack of hydroxide ion at carbonyl groups, followed by acetylacetone liberation, and finally silicon dioxide production [32], The kinetic runs were followed spectrophotometrically by the disappearance of the absorbance at 505 nm for Tc(acac)3. The rate law has the following equation ... 

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. 

A review of recent advances in chromium chemistry (82) supplements earlier comprehensive reviews of kinetics and mechanisms of substitution in chromium(III) complexes (83). This recent review tabulates kinetic parameters for base hydrolysis of some Cr(III) complexes, mentions mechanisms of formation of polynuclear Cr(III) species, and discusses current views on the question of the mechanism(s) of such reactions. It seems that both CB (conjugate base) and SVj2 mechanisms operate, depending on the situation. The important role played by ionpairing in base hydrolysis of macrocyclic complexes of chromium(III) has been stressed. This is evidenced by the observed order, greater... 

Reaction kinetics and mechanisms for oxidation of [Fe(diimine)2(CN)2], [Fe(diimine)(CN)4] (diimine = bipy or phen) (and indeed [Fe(CN)6] ) by peroxoanions such as (S20g, HSOs", P20g ) have been reviewed. Reactivity trends have been established, and initial state— transition state analyses carried out, for peroxodisulfate oxidation of [Fe(bipy)2(CN)2], [Fe(bipy)(CN)4] , and [Fe(Me2bsb)(CN)4] in DMSO—water mixtures. Whereas in base hydrolysis of iron(II)-diimine complexes reactivity trends in binary aqueous solvent mixtures are generally determined by hydroxide solvation, in these peroxodisulfate oxidations solvation changes for both partners affect the observed pattern. ... 

The concept of preassembly as a requirement for substitution may throw light upon the vexed question of the mechanism of the base hydrolysis reaction. It has long been known that complexes of the type, [Co en2 A X]+n can react rapidly with hydroxide in aqueous solution. The kinetic form is cleanly second-order even at high hydroxide concentrations, provided that the ionic strength is held constant. Hydroxide is unique in this respect for these complexes. Two mechanisms have been suggested. The first is a bimolecular process the second is a base-catalyzed dissociative solvolysis in which the base removes a proton from the nitrogen in preequilibrium to form a dissociatively labile amido species (5, 19, 30). 

The kinetics of aquation of a number of azidochromium(III) complexes have been investigated.303,655 Compared with other acidochromium(III) complexes, the chromium-azide bonds in these species seem remarkably stable to thermal substitution. Hence in the base hydrolysis of [CrN3(NH3)s]2+ a pathway involving initial loss of NH3 concurs with the usual base hydrolysis pathway involving loss of Nj. The aquation of azidochromium(III) complexes is H+-assisted with protonation of the azido ligand accounting for the enhanced reactivity. 

The mechanism provides (in equation 1) a pathway for proton exchange in amine complexes. (In fact, it was the observation that proton exchange in [Co(NH3)6]3+313 and the base hydrolysis of (Co(NH3)5Br]2+298 had the same kinetic form that led Garrick to propose this mechanism in 1937.314) The pathway for base catalyzed solvolysis is made up of (1) + (2) + (3) and the pathway for base catalyzed substitution is (1) + (2) + (4). It was the observation of base catalyzed ligand substitution that provided the first strong evidence for the dissociative nature of the process.315... 

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... 

Complexes such as (39) are likely intermediates in the reaction of [Co(dien)(OH)(OH2)2]2+ with dipeptides.177,181 Hay and Piplani182 have studied the kinetics of base hydrolysis of a variety of complexes of type (39) and have determined values of kOH for hydrolysis of the peptide bond and the N02 and Cl ligands (Table 17). The rate constant kon for peptide bond hydrolysis falls within the range 0.7 to 7 M-1 s I and is some 104 to 105 times greater than that for the uncomplexed peptide. 

Hay and Nolan407 have carried out a detailed kinetic study on the hydrolysis of N-2-pyridyl-methyleneaniline (117 = L) and its copper(II) complex (118). Very substantial rate accelerations were observed in this system. Base hydrolysis of [CuL(OH2)2]2+ (118) is some 105 times faster than base hydrolysis of L at 25 °C. The rate constants for this system are summarized in Table 26. 

A number of interesting studies of lactam hydrolysis have been published. The metal(II)-catalyzed hydrolysis of some penicillin and cephalosporin derivatives displays saturation kinetics.410"412 A 1 1 complex is formed between the metal ion and penicillin which undergoes base hydrolysis up to 10s times faster than the free ligand. The catalytic activity follows the order Cu,>ZnII>NiII = Co11. Coordination of penicillin to copper(II) is believed to occur via the /3-lactam nitrogen and the carboxylate group (121)4l0-4n but other sites have been proposed.413... 

Metal ion binding to (133) is quite strong and saturation kinetics occur at low metal ion concentrations presumably leading to complexes of type (134). Base hydrolysis of the Cu11 complex is observed, but attack by both H20 and OH occurs with the Ni11, Co11 and Zn11 complexes. Rate enhancements of > 102 occur for water attack. 

Buckingham and Engelhardt200 have studied the hydrolysis of propionic anhydride in the presence of kinetically inert complexes of the type [M(NH3)5OH]n+. These reactions occur by nucleophilic attack of coordinated hydroxide on the anhydride (Scheme 32). For reactions of M-OHl" l,+ with propionic anhydride, the Bronsted plot of log kMOH versus the p.Ka of M—OH2k+ is a smooth curve if values for reaction with HzO and OH- are included. Although Icmoh for [(NH3)5CoOH]2+ (3 M-1 s-1) is about 103-fold less than fcoH. its reaction will compete favourably at neutral pH with base hydrolysis. Such effects are considered in more detail in Section 61.4.2.2.3. 

For the kinetically inert low-spin Co(III) complexes the mechanism of exchange is certainly dissociative although kinetic studies can give results that are superficially misleading. For example, the base hydrolysis reaction... 

Basolo555 noted that reactions of Rh111 amine complexes were not dramatically accelerated by hydroxide ion, but did show that substitutions in base do follow the standard kobs = k, + k2[OH" ] format, with k, representing the first-order aquation observed in acidic solution, and k2 representing the second-order base-catalyzed path. Poe has studied the kinetics of the base hydrolysis of a variety of frans-[Rh(en)2XY]"+ complexes (Table 41). Studies on the base hydrolyses of trans-[Rh(en)2(OH)X]+ (X = Cl, Br, I) showed that the coordinated hydroxide has an intrinsic kinetic tram effect comparable to that of Cl" but that its position in a thermodynamic trans effect series is much higher.635 For oms-[Rh(en)2X2]+ (X = Cl, Br), virtually complete tram -+ cis isomerization occurs upon hydrolysis in base, and ca. 50% isomerization is observed when X = I.653 No such... 

Ligand substitution at frans-[Ir(X)2(en)2]+ occurs without stereochemical change, and with substitution rates independent of the concentration and nature of the incoming ligand and the ionic strength. The substitution mechanism involves an aqua intermediate, as illustrated in Scheme 10.231 Aquation kinetics of complexes such as fra s-[Ir(X)2(en)2]+ were not directly accessible, the equilibrium in Scheme 10 lying to the left. Table 2 shows various ligand substitution rates and activation parameters for /ranj-[Ir(X)(en)2(L)]+. The base hydrolysis of cis- and mms--[Ir(Cl)2(en)2]+ also proceeds with stereoretention. 

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