Encircling of 3d-metal ions by polyazamacrocycles: thermodynamic aspects

Abstract

We report and discuss the enthalpy values for the formation of copper(II) and nickel(II) complexes with a series of tetraazamacrocycles, from {12]aneN4 to [16]aneN4, and three pentaazomacrocydes from [15]aneN5 to [l7JaneN5. Since their first appearance in the early sixties, tetra—aza macrocyclic complexes have attracted the attention of the inorganic chemists for some unusual properties. They show some properties, which have not been encountered with the corresponding complexes formed by the open-chain analogoues: an extreme kinetic stability, the possibility to attain unusual oxidation states of the encircled metal ion, an extremely high thermodynamic stability. The last aspect was pointed out by Cabbines and Margerum in a pioneering paper in which the equilibrium constant for the formation of the copper(II) complex with the saturated macrocycle, [141 aneN4, having six methyl substituents on the aliphatic backbone was reported and compared with that of the most stable complex among those with open chain tetramine, i.e •, 2,3,2-tet. (Ref. 1). The remarkable extra-stability of the complex with the closed tetramine (about 3-4 log units) was named macrocyclic effect, and is considered to be a sort of additional contribution to the well known chelate effect due to the cyclisation of the multidentate ligand. The interpretation of the thermodynamic nature of the macrocyclic effect was only tentative, in the absence of reliable, experimentally obtained, enthalpy and entropy data. The determination of the enthalpy change associated with the complexation of a macrocycle was crucial. As a matter of fact, heat changes obtained in different laboratories by the temperature dependence of equilibrium data led to conflicting results and interpretation (Ref. 2&3). The extreme inertness of macrocyclic complexes both towards the metal plus ligand reaction and towards destruction of the preformed complexes (for instance in strong acid solution) prevented the use of the calorimetric techniques that has been tested in the study of complexation with non-cyclic (and labile) systems. These difficulties were circumvented in our laboratory through: 1) the disign of appropriate experiments; 2) the use of special calorimetric apparatus, able to detect small amounts of heat, even if evolved in a comparatively long period (on the time scale of the calorimetric measurements). In the case of the copper(II) complexes we carried out the complexation reaction in IM NaOH, where the copper exists as hydroxo species and the strongly basic tetramine is not protonated (Ref. 4). In these conditions the incorporation of themetal into the macrocycle is instantaneous so that the associated heat effect can be determined. Due to the high dilution of the