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 O-17 NMR investigations on [Gd(DO3ASQ)(H2O)], C-13 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.