A thermodynamic study of large tetraaza ligands and their Cu (II) complexes

Abstract

Six macrocyclic (L1-L6) and ten non-cyclic (L7-L16) tetraaza ligands containing only secondary nitrogen atoms were synthesised and characterised. A further tetramine ligand (L17) with two primary and two secondary nitrogen atoms was obtained commercially. The stepwise protonation constants of all these ligands and the Cu(II) complex stability constants of all but three of the ligands (L10-L12) were determined by potentiometric titration. The Cu(II) complex formation enthalpies of L¬1¬-L9 and L13-L17 were determined using a batch microcalorimetric technique. The enthalpies of solution of L1-L17 were determined by isoperibolic calorimetry and the enthalpies of vaporisation of L1-L9, L16 and L17 determined using a custom built vapour pressure-weight loss apparatus. The data for the ligands L1-L17 and for a number of macrocyclic and noncyclic tetraaza and tetramine ligands with smaller macrocyclic and/or chelate ring sizes (L(I)-L(XVIII)) has been examined to determine the effect of increasing macrocyclic and/or chelate ring size on the thermodynamic properties described above, and to determine the thermodynamic origins of the macrocyclic effect, the observed increase in stability of a macrocyclic ligand relative to that of an analogous non-cyclic ligand, and to determine the effect of increasing ligand size on the macrocyclic effect. The enthalpy contribution to the macrocyclic effect has been shown to be equal to the difference between the hydration enthalpy of the free macrocyclic and non-cyclic ligands. Differences in complex hydration appear to be small. The entropy contribution appears to be due to the greater loss of conformational entropy of the non-cyclic ligand on copper complex formation. The magnitude of the macrocyclic effect appears to be independent of ligand size but is dependent on the non-cycllic ligand chosen as a model for the macrocyclic ligand.