Solution chemistry of macrocycles. Part 2. Enthalpic and entropic contributions to the 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

Abstract

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.