Encircling of metal ions by tetra-aza macrocycles. Enthalpies of formation of high- and low-spin nickel(II) complexes of 14-membered ligands

Abstract

In aqueous solution, both the high- and low-spin forms of nickel(II) ions are encircled by the 14-membered marcrocycle 1,4,7,11-tetra-azacyclotetradecane (chelate-ring sequence 5,5,6,6) less exothermically than by the isomeric 1,4,8,11-tetra-aza ligand (chelate-ring sequence 5,6,5,6), due to a decrease of the in-plane Ni–N interaction. Insertion of a pyridine ring into the ligand framework makes complexation much less exothermic.