Co-ordinating tendencies of a 14-membered tetra-aza macrocycle which forms a seven-membered chelate ring

Abstract

The basicity constant of 1,4,7,10-tetra-azacyclotetradecane (L⁵) and the stability constants of the complexes [CuL⁵]²⁺ and [Cu(HL⁵)]³⁺ have been determined by potentiometry at 25 °C in 0.5 mol dm^–3 K[NO₃]. The enthalpies of formation of the copper(II) and nickel(II) complexes of L⁵ have been determined by flow and batch micro-calorimetry respectively. The co-ordination of L⁵ to copper(II) and nickel(II) to form a system with a (5,5,5,7) chelate ring sequence is remarkably less exothermic than that of 1,4,8,11-tetra-azacyclotetradecane (L³) and 1,4,7,11-tetra-azacyclotetradecane (L⁴) which form (5,6,5,6) and (5,5,6,6) chelate ring sequences respectively. The introduction of a hindered seven-membered chelate ring overrules completely the enthalpic advantage of the 14-membered cavity. The macrocyclic complex [CuL⁵]²⁺ is more stable than the corresponding complex with a non-cyclic polyamine [Cu(trien)]²⁺(trien = triethylenetetra-amine). This extra stability is due to a favourable entropy contribution, the enthalpy being slightly unfavourable. Finally, the stability of the monoprotonated complex [Cu](HL⁵)³⁺ is compared with similar protonated complexes formed by other tetra-aza macrocycles.