Kinetics of aquation and base hydrolysis of the macrocyclic complex trans-Bromo(N-meso-5,12-dimethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene)nitro-cobalt(III) perchlorate

Abstract

The macrocyclic complex trans-[Co(dtcd)(NO₂)Br] [ClO₄] (dtcd = 5,12-dimethyl-l,4,8,1l-tetraaza-cyclotetradeca-4,ll-diene) has been prepared and its hydrolysis kinetics investigated. At 25ºC and 0.1 M HN0₃ the aquation occurs with k_aq = 6.2 x 10^-3 s^-1 to give the trans-Co(dtcd)(NO₂)- (OH₂)]²⁺ cation. The activation parameters at 298 K are ΔH? = 75.0 kJ mol^-1 and ΔS? = -35.6 J K^-1 mol^-1. Hydrolysis of the bromide in the pH range 7.5-8.8 follows the rate expression k_obs = k_aq + k_OH[OH-]. At 25ºC (I = 0.1 M, NaClO₄) k_OH = 1.21 x 10³ 1. mol^-1 s^-1 with the activation parameters for base hydrolysis being ΔH? = 74.2 kJ mol^-1 and ΔS? = +63.2 J K^-1 mol^-1 at 298 K. Aquation and base hydrolysis of the bromo complex at 25ºC occur at rates 14 and 5 times faster respectively than those previously reported for the analogous trans-[Co(dtcd)(NO₂)Cl]⁺ complex, the acceleration being due to a more favourable entropy of activation in each case.