Aquation Solvolysis

Surfactants have also been of interest for their ability to support reactions in normally inhospitable environments. Reactions such as hydrolysis, aminolysis, solvolysis, and, in inorganic chemistry, of aquation of complex ions, may be retarded, accelerated, or differently sensitive to catalysts relative to the behavior in ordinary solutions (see Refs. 205 and 206 for reviews). The acid-base chemistry in micellar solutions has been investigated by Drummond and co-workers [207]. A useful model has been the pseudophase model [206-209] in which reactants are either in solution or solubilized in micelles and partition between the two as though two distinct phases were involved. In inverse micelles in nonpolar media, water is concentrated in the micellar core and reactions in the micelle may be greatly accelerated [206, 210]. The confining environment of a solubilized reactant may lead to stereochemical consequences as in photodimerization reactions in micelles [211] or vesicles [212] or in the generation of radical pairs [213]. 

In all of the systems so far investigated the aquation rate has been found to be pH independent in alkaline solution, at least up to 0.1MOH ", the largest concentration investigated. It may be noted that this behavior is entirely analogous to that of trityl chloride and other organic halides which undergo solvolysis by well established Svl mechsnisms. (S)... 

One of the first differences to be noted about the Rhm acido-amine complexes is that the chloro complexes are frequently much more stable with respect to solvolysis than their Co111 and Cr111 analogues and solvolytic equilibrium is reached when very little of the chloro complex has aquated, even in dilute solution and in the absence of added chloride ions. This is, to some extent, the consequence of the move away from class a character already mentioned above. As a result, the rate constants for aquation are obtained from ligand substitution reactions (including chloride exchange) which first have to be shown to be mediated by a rate determining aquation. More recently, data have been obtained from a study of the solvolysis in basic solution. This serves to... 

Other systems are ambiguous and require a careful consideration of the magnitudes of the derived rate constants before a conclusion can be drawn. An extreme case can be found in the pH dependence of the solvolysis of m-[Co(en)2(H20)Cl]2+.330 The rate is independent of pH in the range 7—9, where the complex is almost entirely in the form of ris-[Co(en)2(OH)Cl]+ and it is usually, and probably correctly, assumed that the pH independent rate constant is that for the uncatalyzed aquation of this species.180 However, consideration ought to be given to the possibility that the observed process is the base catalyzed hydrolysis of the aquo complex in which a primary amine proton is removed. Problems of this sort are discussed in ref. 301, p.84. 

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. 

Kinetic parameters have been established for solvolysis of the pentacyanofer-rate(III) derivative [Fe(CN)5(N02)] . For aquation, which is acid-catalyzed at pH <5, A//= = 43kJmol-, =-80 J K" mol", and A =+2 cm" mol". Intrinsic and solvational contributions are presumably closely balanced in the case of A Rate constants for solvolysis of [Fe(CN)5(N02)] in water, methanol, dimethyl sulfoxide, and dimethylformamide correspond with the electron-donating abilities of the respective solvents. Activation volumes for the nonaqueous solvents, between +20 and +27 cm" mor reflect the dissociative nature of these solvoly-ses. " Rate constants for dissociation of the [Fe(CN)5(2,6-Me2pyrazine)]" anion in binary aqueous solvents containing methanol, acetone, or acetonitrile correlate well with acceptor numbers for the respective media, though with a very different... 

Kinetic studies of aquation of [Fe(phen)3] and derivatives in binary aqueous media remain popular. A group additivity approach has been applied to aquation of [Fe(5N02phen)3] in aqueous alcohols (faster reaction) and formic and acetic acids (slower), to investigate its potential for mechanism diagnosis. Rate constants for dissociation of the parent complex increase tenfold on going from water to 100% dimethylformamide. Aquation rate constants and activation parameters have also been reported for the 5-nitro, 5-phenyl, and 4,7-diphenyl derivatives in water-dioxan mixtures. Both papers contain obscure discussions of solvolysis mechanisms in DMF-rich and dioxan-rich media. In the latter media it seems that ion pairs play a key role, as evidenced by activation entropies. The discussion of reactivities in terms of hydrophobicities of the complexes and their respective transition states represents a qualitative initial state-transition state analysis. An explicit analysis of this type has been published for the iron(II) complexes of the... 

To turn to classical substitution, the rate constant for aquation of [0s(NH3)5(03SCF3)] is 8-8 x 10" s at 25 °C in 1 M CF3SO3H. Rate constants for solvolysis in triethyl phosphate and in acetone are similar. All these solvolysis rates are relatively fast for osmium(III), since fluorosulfonate is a good leaving group. " Loss of triphenyl phosphine oxide from cis-[Os(bipy)2(OPPh3)2] is very slow, with a half-life of many hours in acetonitrile. This observation is a byproduct of a redox investigation, which also provided information on reaction... 

Activation volumes for solvolysis of [Rh(NH3)5(0S02CF3)] in methanol and in acetonitrile are -6.6 and -7.8cm mor These values, like that of -4.1cm mor for aquation, indicate associative activation, even for this easily lost leaving group. In comparisons with activation volumes for solvolysis of [Rh(NH3)5Cl] one has to bear in mind the partial molar volumes of the leaving groups as well as electrostriction effects. Base hydrolysis of [Rh(en)2(ox)] obeys a simple second-order rate law, with 2 = 4.6 x 10" dm mol s" at 60 Base hydrolyses of [Rh(LLLL)Cl2], where LLLL = optically active... 

---