Coordination equilibrium in an Ln(III) macrocyclic chelate modulated by a reversible interaction with a weakly donor substituent

The introduction of a squaric acid moiety at the secondary nitrogen center of DO3A [DO3A = 1,4,7‐tris(carboxymethyl)‐1,4,7,10‐tetraazacyclododecane] yields the DO3ASQ ligand endowed with a potential octadenticity towards Ln(III) ions. On the basis of 17O NMR investigations on [Gd(DO3ASQ)(H2O)], 13C NMR observations on [Lu(DO3ASQ)(H2O)] and UV spectra of [Eu(DO3ASQ)(H2O)], it is suggested that [Ln(DO3ASQ)(H2O)] complexes are present in solution as a mixture of two isomers differing in the coordination number of the Ln(III) ion, being 8 and 9, respectively. Only in the latter isomer is the squaric moiety coordinated to the Ln(III) ion. Interestingly, the two isomers display different exchange rates of the coordinated water molecule. Copyright © 2001 John Wiley & Sons, Ltd.