Proton exchange in the base hydrolysis of deuteriated trans-dichloro-bis(ethylenediamine)cobalt(III) and trans-dichloro[(RS)-1,9-diamino-3,7-diazanonane]cobalt(III) cations

Abstract

The extent of replacement of the labelled amine protons in the complexes trans-[Co(en)₂Cl₂]⁺(en = ethylenediamine) and trans-[Co(ddn)Cl₂]⁺[ddn =(RS)-2,3,2-tet =(RS)-1,9-diamino-3,7-diazanonane] during base hydrolysis has been measured in the substrate and product as a function of the extent of reaction. The results are consistent with a dissociative conjugate-base mechanism and, in the en complex, the ratio of the rate constants for the reprotonation of the conjugate base (k_–1) and its dissociation (k₂) lies within the range 10 > k_–1: k₂ > 4. The active conjugate base from the ddn complex has lost a secondary amine proton and 5 > k_–1: k₂ > 0·5. The anomalously fast base hydrolysis of these complexes arises mainly from the unusually high lability of their conjugate bases. Rate constants for base hydrolysis of the N-deuteriated complexes are compared with those of the untreated complexes