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

The heats of formation of high- and low-spin Ni(II) complexes with 1,4,7,11-tetraazacyclotetradecane (L; chelate ring sequence 5,5,6,6) were detd. calorimetrically in aq. soln. The chelation reactions with L are less exothermic than those with the isomeric 1,4,8,11-tetraaza 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 chelation much less exothermic.