Chelate-ring sequence and enthalpy of formation of copper(II) complexes with tetra-aza macrocycles

Abstract

The direct calorimetric determination of the enthalpy of formation of copper(II) complexes with three tetra-aza macrocyclic ligands of different ring size has been made at 25 °C. The enthalpy of formation for complex of 1,4,7,10-tetra-azacyclotridecane is intermediate between those of 1,4,7,10-tetra-azacyclododecane and 1,4,8.11-tetra-azacyclotetradecane (cyclam). The calorimetric results show that the 14-membered macrocycle 1,4,7,11-tetra-azacyclotetradecane(isocyclam) incorporates Cu^II ion less exothermically than its more symmetric isomer cyclam and the proposed explanation is the formation of weaker in-plane Cu–N interactions. The introduction of a pyridine ring into the macrocyclic framework further reduces the heat of complexation.