Thermodynamics of the formation of metal complexes of 1,4,10-trioxa-7,13-diazacyclopentadecane-N,N′-diacetic acid and of 1,4,10,13-tetraoxa-7,16-diazacyclo-octadecane-N,N′-diacetic acid

Abstract

Thermodynamic functions for the formation of the alkaline-earth and some transition-metal complexes of the ligands L1(1,4,1O-trioxa-7,13-diazacyclopentadecane-N,N′-diacetic acid) and L2(1,4,10,13-tetraoxa-7,16-diazacyclo-octadecane-N,N′-diacetic acid) were determined by direct batch microcalorimetry. The ligand L1 gives more stable transition-metal complexes than L2 due to more favourable heats of reaction. The reverse situation is found for complexes of the alkaline-earth metals, although the differences in stability of the complexes of these metals with the two ligands are small due to less favourable entropy changes in the case of L2. The two macrocyclic ligands behave similarly to their non-cyclic analogues L3 and L4(3-oxapentane-1,5-diamine-and 3,6-dioxaoctane-1,8-diamine-N,N,N′,N′-tetra-acetic acid) but form less stable complexes due to less favourable heats of reaction. The transition-metal complexes of L1 do not follow the Irving-Williams order of stability, contrary to the situation with complexes of L2. The reason for the inversion of the stability order is discussed in terms of the changes in crystal field stabilization energies when the octahedrally co-ordinated aqueous ions form complexes of a different structure, either five-co-ordinated or tetrahedral.