An equilibrium study on bivalent metal complexes with 1,5,9,13,17,21-hexaazacyclotetracosane ([24]aneN6) in aqueous solution

Abstract

The macrocycle 1,5,9,13,17,21-hexaazacyclotetracosane (L =[24]aneN₆) was synthesized as both its HNO₃ and HCl salts. The protonation and stability constants with Ni²⁺, Cu²⁺, Zn²⁺ and Cd²⁺ have been determined potentiometrically in 1.0 mol dm^–3 KNO₃ aqueous solution at 25 °C. For all these metal ions only mononuclear complexes are formed. In the case of the HNO₃ salt log K_ML obtained is 10.66 ± 0.02 for [NiL]²⁺, 14.87 ± 0.04 for [CuL]²⁺, 10.68 ± 0.01 for [ZnL]²⁺ and 8.27 ± 0.01 for [CdL]²⁺, whereas in the case of the HCl salt log K_ML is 16.47 ± 0.02 for [CuL]²⁺ in 1.0 mol dm^–3 KCl aqueous solution at 25 °C. The results for the metal complexes with [24]aneN₆ were compared with those for the M²⁺-[18]aneN₆(1,4,7,10,13,16-hexaazacyclooctadecane) system and discussed in terms of a ring effect. The smaller K_CuL in the nitrate system compared with that in the chloride system suggests an affinity of the macrocycle towards Cl^–.