NMR Evidence for Interconversion between Two Enantiomeric Forms of Macrocyclic Schiff Base Lanthanide(III) Complexes through Reversible Ring Contraction and Expansion

Macrocyclic Schiff base lanthanide(III) complexes, [Ln(H₂l_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(H₂L_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) Å, b = 14.651(7) Å, c = 19.019(7) Å, and β = 91.97(5)° 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 H₂L_a. The third nitrate group is ionic. Metal-ligand interatomic distances: Y-O (nitrate), 2.48 Å (mean); Y-O (phenolic), 2.25 Å (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 NMR spectra of the paramagnetic complexes [Ln(H₂L_A)(NO₃)₂](NO₃) (Ln = Nd³⁺, Sm³⁺, Eu³⁺, Ho³⁺, Yb³⁺) are consistent with the solution structure and dynamics found for the diamagnetic derivatives.