Solvent dependence of the stereochemistry of base-catalyzed solvolysis of trans-[Co(NH3)4(15NH3)X]3+/2+(X = Me2SO or Cl) ions

Abstract

The cis-trans product ratio for the base-catalyzed solvolysis of the 15NH3 labelled ~~~~S -[ C O ( N H , ) , ( ~~N H ~) C I ] ~+ion has been determined as a function of the solvent. The solvents used were H20-CH30H and H20-dmso (dimethyl sulphoxide) mixtures, as well as anhydrous CH30H and CH3NH2. The trans-[C0(NH,),(~5NH~)(drnso)]3+ ion was included also in the study. The base hydrolysis results do not show any solvent or leaving-group dependence, within experimental error (2%). All trans systems give (44 f 1)% rearrangement. Small, significant differences are shown by the penta-amminechloro-complex in CH3NH2 (50% rearrangement) and by the dmso complex (48% rearrangement in H20 and 46% in CH30H). The results are interpreted as indicating the adequacy of a mechanistic model involving a five-co-ordinate intermediate.Stereochemical change in base hydrolysis is still to a large extent an unsolved problem in the conjugate base (c.b.) mechanism. This mechanism generally consists of a fast proton transfer pre-equilibrium, followed by the rate-determining dissociative elimination of the leaving g ~o u p . ~J Early stereochemical theories assumed the five-co-ordinate intermediate, inherent in the dissociative activation, to have a fixed geometry. This idea was combined with that of a stereospecific entry of the incoming ligand, directed by the amido-group.4Vs Recent results on the ' red ' s and p (secondary and primary) isomers of the [CoL(NH3)XI3 + I 2 + ion [L = 2,2'-diamino-2"methylaminotriethylamine, X = C1, Br, N3, or NH3], bearing on the lifetime of the intermediate,'jn7 have been interpreted as indicating that the five-co-ordinate species do not relax towards a common trigonal-bipyramidal structure, although this process would require only a small change in bond angle. The resulting more dynamic picture is the immediate capture of a solvent molecule, before bond angles can equilibrate.If indeed the process of capture of the entering group by the five-co-ordinate species competes in rate with angle equilibration and solvent structure rea~rangement,~.~ relative rates and consequently product stereochemistry should be affected by the nature of the solvent. This conclusion does not seem consistent with the fact that experiments in our laboratory revealed a close agreement in product stereochemistry for the base-catalyzed reactions of [ c ~( e n ) ~X Y ] ~+ ' + (en = ethylenediamine; X = C1, Br, or dmso; Y = Cl, NCS, NO2, N3, or NH3) complexes in liquid ammonia and in aqueous solut i ~n . ~* ~ In this report we investigate the solvent dependence in the base-catalyzed solvolysis reaction of tran~-[Co(NH~)~-(15NH3)X]3+/2+ (X = dmso or C1) ions, a reaction for which a fixed five-co-ordinate intermediate seems to explain aqueous stereochemical results well.

Rate-limiting deprotonation in the base-catalyzed ammoniation of asymmetric trans-diacidobis(ethylenediamine)cobalt(III) perchlorates in liquid ammonia

“…This may be taken to imply that the existence of one or more discrete intermediates of reduced coordination number is a reasonable assumption. 21 Our second comment is the fact that the steric course does not depend greatly on the leaving group implies dissociative activation. The slight dependence on the nature of the leaving group can then be explained from the assumption, introduced by Sargeson and co-workers,23 that the solvent sheet of the substrate remains essentially intact during the activation and the formation of the pentacoordinate intermediate.…”