Synthesis and complexing properties of polyazamacrocycles with pendant N-methylenephosphinic acid

Abstract

Two new polyazamacrocycles 1,4,7-triazacyclononane-1,4,7-triyltrimethylenetris(phosphinic acid)(H₃L¹) and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayltetramethylenetetrakis(phosphinic acid)(H₄L²) were synthesised and their acid–base and complexing properties with Mg²⁺, Ca²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ determined pH-metrically at 25 °C and at an ionic strength of 0.1 mol dm^–3(KNO₃). Both form 1:1 complexes over a wide pH range. The stability constants of H₃L¹ with transition metals lay in a narrow region and point to non-selectivity of the ligand; H₄L² forms more stable complexes with larger cations such as Cd²⁺ or Pb²⁺ than with the cations of the first transition row. This selectivity is much better than for analogous macrocycles bearing carboxymethyl or methylenephosphonic pendant groups.