SOLUTION CHEMISTRY OF MACROCYCLES .2. ENTHALPIC AND ENTROPIC CONTRIBUTIONS TO PROTON BASICITY OF CYCLIC TETRA-AMINE LIGANDS - "1,4,8,11-TETRA-AZACYCLOTETRADECANE, "1,4,8,12-TETRA-AZACYCLOPENTADECANE, AND 1,4,8,11-TETRAMETHYL-1,4,8,11-TETRA-AZACYCLOTETRADECANE

The enthalpy changes of stepwise protonation of the cyclic tetra-amines: 1,4,8,11-tetra-azacyclotetradecane, [14]aneN4. 1,4,8,12-tetra- azacyclopentadecane, [15]aneN4, and 1,4,8,11-tetramethyl-1,4,8,11 -tetra-azacyclotetradecane, Me4[14]aneN4, have been determined at 25 °C in 0.5M-KNO3 by calorimetry using the continuous titration procedure. Through a combination of the values of the enthalpy changes with the corresponding free-energy changes, reported elsewhere, the entropies of protonation have been obtained. In order to explain these thermodynamic functions two factors, which are not involved in the case of open-chain polyamines, are to be taken into account: (i) the presence of internal hydrogen bonds of the type N-H ⋯ N and (ii) conformational rearrangements of the macrocyclic ring due to the nitrogen inversion, the lone pairs being directed either towards the inside or the outside of the ring.