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 kaq= 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 kobs.=kaq+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 (koba.= 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 kaq= 5.5 × 10–4 s–1. The additional methyl substituents in the macrocyclic ligand have little or no effect on the hydrolysis rates.