The preparation, aquation kinetics and activation volumes of trans-chloro-and trans-bromo-azido(meso-5,12-dimethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene)cobalt(III) perchlorate

Abstract

The macrocyclic complexes trans-[Co(dtcd)(N₃)X]ClO₄ (dtcd = meso-5,12- dimethyl-1,4,8,11-tetra-azacyclotetradeca-4,11-diene; X = Cl^- or Br^-) have been prepared and characterized. Halide aquation in 0.1 M HNO₃ at 25°C proceeds at rates of 4.6 × 10^-4 s^-1, ΔH^‡ 77(±2) kJ mol^-1, ΔS?-51(±6) J K^-1 mol^-1 (X = Cl^-) and 10.2x10^-4 s^-1, ΔH^‡ 80(±3) kJ mol^-1, ΔS^‡ -34(±10) J K^-1 mol^-1 (X = Br^-). The pressure dependence of the aquation rate at elevated pressures yields ΔV^‡ +8.3(±0.5) cm³ mol^-1 (X = Cl^-) and ΔV^‡ +5.3(±0.7) cm³ mol^-1 (X = Br^-). While the observed positive ΔV^‡ values are consistent with a dissociative mechanism they differ markedly from reported negative ΔV^‡ values for acid hydrolysis of [Co(NH₃)₅X]²⁺ ions.