Intramolecular Dynamics of Lanthanide(III) Tetraoxadiaza Macrocycle Complexes in Solution as Studied by NMR

¹H and ¹³C NMR and ¹H NMR relaxation spectroscopy (RS)measurements are reported for the CDCl₃ and CD₂Cl₂ solutions of La(NO₃)₃(diaza-18-crown-6)] ({bf I}), Pr(NO₃)₃(diaza-18-crown-6)] ({bf II}) and Nd(NO₃)₃(diaza-18-crown-6)] ({bf III}) complexes. Temperature dependencies of the ¹H NMR spectra of II have been analyzed using the dynamic NMR methods for multi-site exchange. Enantiomeric isomer interconversion in II is characterized by H = 21.5 ± 4 kJ mol^-1. Studies of the values of the lanthanide-induced shifts and the longitudinal relaxation rate enhancement revealed that the structure of complexes in solution is similar to that reported for the La(NO₃)₃(18-crown-6)] complex in the crystal state. Nevertheless, it appears that the principal values of the molar paramagnetic susceptibility tensor (_i) significantly differ in complexes II and III. The possible reasons for the different characteristics of these complexes are discussed.