Ring-size effects of macrocyclic ligands on complex stabilities. Mercury(II) complexes of twelve- to fifteen-membered cyclic tetra-amines
- 1 January 1976
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in J. Chem. Soc., Dalton Trans.
- No. 22,p. 2335-2338
- https://doi.org/10.1039/dt9760002335
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 1025.5, 1025.3, 1023.0, and 1023.7(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(1024.5 for 3,6-diaraoctane-1,8-diamine and 1022.1 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 HgII 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.Keywords
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