Kinetics of aquation and base hydrolysis of the macrocyclic complex trans-chloro(5,12-dimethyl-1,4,8,11-tetra-azacyclotetradeca-4,11-diene)nitrocobalt(III) perchlorate

Abstract

The macrocyclic complex trans-[Co(dtcd)(NO₂) Cl][ClO₄](dtcd = 5,12-dimethyl- 1,4,8,11 -tetra-azacyclotetradeca-4,11 -diene) has been prepared and its hydrolysis kinetics investigated. At 25 °C and 0.1 M-HNO₃ the aquation occurs with k_aq= 4.4 × 10^–4 s^–1 to give the trans-[Co(dtcd)(NO₂)(OH₂)]²⁺ cation. The activation parameters are ΔH^‡= 77.4 kJ mol^–1 and ΔS^‡=–49.8 J K^–1 mol^–1 at 298 K. Hydrolysis of the chloride in the pH range 7.7–8.9 follows the rate expression k_obs.=k_aq+k₂[OH^–]. At 25 °C (I= 0.1 M Na[ClO₄])k₂= 2.5 × 10² l mol^–1 s^–1 and the activation parameters for base hydrolysis are ΔH^‡= 69 kJ mol^–1 and ΔS^‡=+24.5 J K^–1 mol^–1. Base hydrolysis of the nitro-group has been studied at 25 °C and 0.1 M–NaOH and k= 2.1 l mol^–1 s^–1. Slow decomposition of the complex occurs, possibly as a result of hydrolysis of the imine linkages; however, this decomposition (k_oba.= 2.2 × 10^–3 s^–1) is much slower than nitro-hydrolysis. The analogous complex trans-[Co(htcd)(NO₂)Cl][ClO₄](htcd = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetra-azacyclotetradeca-4,11-diene) exhibits similar kinetic behaviour for base hydrolysis of the chloride. At 25 °C (I= 0.1M), k₂= 2.2 × 10² l mol^–1 s^–1 and k_aq= 5.5 × 10^–4 s^–1. The additional methyl substituents in the macrocyclic ligand have little or no effect on the hydrolysis rates.