Thermodynamic Study of Formation Reactions of Mercury(II) Complexes with Polyamino Polycarboxylic Macrocycles

We have investigated the equilibria of complex formation reactions of mercury(II) ions with polyamino polycarboxylic macrocycles: DOTA (H4dota, H4L), TETA (H4teta, H4L), PEPA (H5pepa, H5L), and HEHA (H6heha, H6L) by employing a polarographic method. Under the experimental conditions employed, tetra-, penta-, and hexa-amine macrocycles: TETA, PEPA, and HEHA, were found to form 1 : 1 ratio protonated complexes as well as normal complexes, while the other tetra-amine macrocycle, DOTA, forms solely a 1 : 1 ratio normal complex, HgL2−. Measurement of thermodynamic parameters, ΔH and ΔS, showed that the introduction of acetate groups into the polyaza frame gives rise to a greater increase in the entropy of reaction, but produces a less exothermic effect.