NMR EVIDENCE FOR INTERCONVERSION BETWEEN 2 ENANTIOMERIC FORMS OF MACROCYCLIC SCHIFF-BASE LANTHANIDE(III) COMPLEXES THROUGH REVERSIBLE RING CONTRACTION AND EXPANSION

Macrocyclic Schiff base lanthanide(III) complexes, [Ln(H(2)L(A))(NO3)( 2)](NO3) (Ln = Y3+, La3+, Nd3+, Sm3+, Eu3+, Ho3+, Yb3+, Lu3+), have be en prepared by cyclocondensation of 2,6-diformyl-4-chlorophenol and 1, 5-diamino-3-azapentane, in the presence of the appropriate metal nitra te as templating agent. In these complexes the metal ion occupies only one coordination site of the compartmental ligand, as evidenced by th e X-ray structure of [Y(H(2)L(A))(NO3)(2)](NO3). Crystals of this comp lex, grown from a dimethylformamide/diethyl ether solution, are monocl inic, space group C2/c [No. 15], with cell constants a = 23.717(8) Ang strom, b = 14.651(7) Angstrom, c = 19.019(7) Angstrom, and beta = 91.9 7(5)degrees for Z = 8. The yttrium atom is nine-coordinated, in a dist orted tricapped trigonal prism environment formed by two bidentated ni trate groups and five donor atoms (two phenolic oxygens and three nitr ogens) of the cyclic Schiff base H(2)L(A). The third nitrate group is ionic. Metal-ligand interatomic distances: Y-O (nitrate), 2.48 Angstro m (mean); Y-O (phenolic), 2.25 Angstrom (mean). Variable-temperature p roton and carbon NMR spectra of the diamagnetic Y(III), La(III), and L u(III) complexes provide clear evidence for a novel chemical exchange process consisting of the reversible formation and breaking of an imid azole ring formed by the intramolecular nucleophilic attack of a secon dary amino group at the imino carbon of a neighboring azomethine group . This dynamic process may be envisaged as a racemization of the enant iomeric forms of the structure found in the solid state. The basicity of the solvent appears to have a pronounced effect on the rearrangemen t rate. A direct role of the solvent is suggested in the N-H bond acti vation. The proton MMR spectra of the paramagnetic complexes [Ln(H(2)L (A))(NO3)2] (NO3) (Ln = Nd3+, Sm3+, Eu3+, Ho3+, Yb3+) are consistent w ith the solution structure and dynamics found for the diamagnetic deri vatives.