Macrocyclic Schiff base lanthanide(III) complexes, [Ln(H(2)L(A))(NO3)(2)](NO3) (Ln = Y3+, La3+, Nd3+, Sm3+, Eu3+, Ho3+, Yb3+, Lu3+), have been prepared by cyclocondensation of 2,6-diformyl-4-chlorophenol and 1,5-diamino-3-azapentane, in the presence of the appropriate metal nitrate as templating agent. In these complexes the metal ion occupies only one coordination site of the compartmental ligand, as evidenced by the X-ray structure of [Y(H(2)L(A))(NO3)(2)](NO3). Crystals of this complex, grown from a dimethylformamide/diethyl ether solution, are monoclinic, space group C2/c [No. 15], with cell constants a = 23.717(8) Angstrom, b = 14.651(7) Angstrom, c = 19.019(7) Angstrom, and beta = 91.97(5)degrees for Z = 8. The yttrium atom is nine-coordinated, in a distorted tricapped trigonal prism environment formed by two bidentated nitrate groups and five donor atoms (two phenolic oxygens and three nitrogens) of the cyclic Schiff base H(2)L(A). The third nitrate group is ionic. Metal-ligand interatomic distances : Y-O (nitrate), 2.48 Angstrom (mean); Y-O (phenolic), 2.25 Angstrom (mean). Variable-temperature proton and carbon NMR spectra of the diamagnetic Y(III), La(III), and Lu(III) complexes provide clear evidence for a novel chemical exchange process consisting of the reversible formation and breaking of an imidazole ring formed by the intramolecular nucleophilic attack of a secondary amino group at the imino carbon of a neighboring azomethine group. This dynamic process may be envisaged as a racemization of the enantiomeric forms of the structure found in the solid state. The basicity of the solvent appears to have a pronounced effect on the rearrangement rate. A direct role of the solvent is suggested in the N-H bond activation. The proton MMR spectra of the paramagnetic complexes [Ln(H(2)L(A))(NO3)2] (NO3) (Ln = Nd3+, Sm3+, Eu3+, Ho3+, Yb3+) are consistent with the solution structure and dynamics found for the diamagnetic derivatives.