Ring-size effects of macrocyclic ligands on complex stabilities. Mercury(II) complexes of twelve- to fifteen-membered cyclic tetra-amines

Abstract

A polarographic method has been used to study the equilibria of complex formation of mercury(II) ion with linear and 12–15-membered cyclic tetra-amines: 1,4,7,10-tetra-azacyclododecane(L¹) 1,4,7,10-tetra-azacyclotride-cane(L²), 1,4,8,11-tetra-aracyclotetradecane(L³), and 1,4,8,12-tetra-azacyclopentadecane(L⁴). The stability constants are 10^25.5, 10^25.3, 10^23.0, and 10^23.7(at 25 °C. I= 0.20 mol dm^–3) far the 1 : 1 complexes, respectively, indicating no special cavity selectivities. The constants are a little greater than those for related open-chain tetraamines(10^24.5 for 3,6-diaraoctane-1,8-diamine and 10^22.1 for 3,7-diazanonane-1,9-diamine). Ligand cyclizationin general causes mercury(II) complexes to undergo enthalpy loss which outweighs the normal compensation of the entropy terms, thus preventing an occurrence of the large macrocyclic effect seen for smaller metal ions. However, a unique macrocyclic ring-sire effect towards Hg^II occurs with the 14-membered ring, where the large negative enthalpy contribution outweighs the unfavourable entropy effect, a trend opposite to those in other macrocyclic systems.