Proton exchange and base-catalysed hydrolysis of the cations trans-dichloro[(RSSR)-1,4,8,11-tetra-azacyclotetradecane]cobalt(III) and cis-dichloro[RRRR(SSSS)-1,4,8,11-tetra-azacyclotetradecane]-cobalt(III)

Abstract

The ratio of the rate constants for the solvolysis (k₂) and the reprotonation (k_–1) of the amido-conjugate base of trans-[Co{RSSR-[N,N′,N″,N‴-²H₄]cyclam}Cl₂]⁺(cyclam = 1,4,8,11-tetra-azacyclotetradecane) has been determined from an analysis of the dependence of the amount of deuterium exchange in the recovered unreacted complex upon the extent of base hydrolysis in aqueous buffer solution : k₂/k_–1= 0.79 ± 0.15 at 0° C. Values for the deprotonation rate constants have been estimated. The rate constant (k_OH) for the base hydrolysis of the ¹H₄ species has been measured over a range of temperatures and the curvature of the Eyring plot confirmed. The rate of proton exchange in cis-[Co{RRRR(SSSS)cyclam}Cl₂]⁺ is considerably faster than base hydrolysis and the rate constant was determined directly using the ²H₄-labelled species in ¹H₂O buffer. The value of the ratio k₂/k_–1 was estimated to be 0.041 at 0 °C. The previously reported low enthalpy of activation for the base hydrolysis is not therefore a criterion for rate-limiting deprotonation.