Synthesis of four novel pendant armed macrocyclic ligands and their interaction with lanthanide(III) cations

The ability of La3+ ions to form stable complexes with four novel pendant-armed NxOy-macrocycles derived from 2,6-bis(2-formylphenoxymethyl)pyridine, L1, L2, L3, and L4, has been studied. The corresponding (unsubstituted) parent ligands were prepared by the reaction between 2,6-bis(2-formylphenoxymethyl)pyridine and three different amines: 1,2-bis(2-aminophenoxy)propane (L1), diethylenetriamine (L2), and 3,6-dioxa-1,8-octanediamine (L3 and L4). This was followed for the parent ligands of L1, L3, and L4 by in situ reduction with sodium borohydride. The pendant-armed ligands were then synthesized by the alkylation of the free -NH groups with p-(L1 and L3) and o-nitrobenzyl bromide (L4), and 2-chloromethylpyridine chlorohydrate (L2). A series of Ln(III) complexes (Ln=La, Ce, Eu, Sm, Ho, Pr) were prepared for the four ligands by the direct synthesis between the corresponding macrocycle and Ln(III) hydrated nitrates and perchlorates. The number of complexes obtained from the pendant-armed macrocycles is lower than that of the (unsubstituted) parent ones, suggesting that the introduction of pendant arms in the macrocyclic skeletons increases the selectivity of the ligands. More complexes were synthesized when using nitrate as the counter ion, showing the important role of the counter ion in the complexation reaction