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

Abstract

An NMR, electrochemical and photophysical study has been made of lanthanide(III) complexes with a 18-membered hexadentate Schiff-base N₄O₂ polyoxaazamacrocycle L¹ derived from the condensation of 2,6-bis(2-aminophenoxymethyl)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 lanthanide-induced shift values point to an isostructural series of [Ln(L¹)][ClO₄]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) complex displays the typical luminescence originating from the excited ⁵D₄ 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 L² instead of the expected complexes of L¹. 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.