Co-ordination tendency of [3k]aneNk polyazacycloalkanes. Thermodynamic study of solution equilibria

Abstract

Equilibria between polyazacycloalkanes of the series [3k]aneN_k(k= 3–11) and the bivalent metal ions of Mn through Zn have been studied. The stability constants (log K) for the complexes of Mn²⁺, Cu²⁺ and Fe²⁺ with 1,4,7,10,13,16-hexaazacyclooctadecane (L⁴), 1,4,7,10,13,16,19-heptaazacyclo-henicosane (L⁵) and 1,4,7,10,13,16,19,22-octaazacyclotetracosane (L⁶) have been determined potentiometrically in 0.15 mol dm^–3 NaClO₄ at 25°C: [MnL⁴]²⁺, 10.50; [MnL⁶]²⁺, 6.27; [CuL⁴]²⁺, 24.40; [CuL⁵]²⁺, 19.48; [Cu₂L⁵]⁴⁺, 30.49; [Cu₂L⁶]⁴⁺, 35.25; and [FeL⁵]²⁺ 12.09. The enthalpy changes (–ΔH°/kcal mol^–1) for the formation of [CuL⁴]²⁺(23.9), [CuL⁵]²⁺(19.7) and [NiL⁵]²⁺(11.7) have been measured by means of microcalorimetric techniques. The results are discussed in terms of the increasing number of donor atoms and size of the macrocycles. As the size increases the co-ordination features of the metal compleses largely depend on the metal ion.