Proton NMR, luminescence and electrochemical study of 18-membered Schiff-base macrocyclic lanthanum (III) complexes

摘要

An NMR, electrochemical and photophysical study has been made of lanthanide (III) complexes with a 18-membered hexadentate Schiff-base N_4O_2 polyoxaazamacrocycle L~1 derived from the condensation of 2, 6-bis (2-aminophenoxy-methyl) pyridine and 2, 6-diformylpyridine, as well as their interaction with the chelating agents 1, 10-phenanthroline and 2, 2'-bipyridine. Separation of the contact and pseudocontact contributions to the observed ~1H NMR lanthanideinduced shift values point to an isostructural series of [Ln(L~1)][ClO_4]_3 (Ln=La-Eu, except Pm) in acetonitrile. The interaction of the lanthanum (III) complex with 1, 10-phenanthroline and 2, 2'-bipyridine was demonstrated by the use of different spectroscopic techniques and the stability constant for the 1, 10-phenanthroline adduct was calculated from ~1H NMR data. Upon excitation through the metal excited levels, the terbium(III) complexes displays the typical luminescence originating from the excited ~5D_4 level, while the adduct with 1, 10-phenanthroline presents emission upon excitation through the ligand bands. The reaction of 2, 6-bis(2-aminophenoxymethyl) pyridine and 2, 6-diformylpyridine and the perchlorate of Gd~(III) or Tb~(III) in ethanol solution yielded a reduced macrocycle L~2 instead of the expected complexes of L~1. Electrochemical studies in acetonitrile solution showed that the reduction of the imine groups is favoured when the ionic radius of the lanthanide (III) ion decreases.