Complexation of a Schiff base ligand having two coordination sites (N2O2 and O2O2) with lanthanide ions (Ln = La, Pr): an NMR study

Abstract

With the help of spectroscopic data, we have characterized several isolated entities resulting from the reaction of lanthanide ions with a hexadentate Schiff base ligand obtained by reacting o-vanillin and 1,2-diaminoethane. This H2L ligand possesses two coordination sites, an N2O2 inner site and an O2O2 outer site. In the first isolated complex, formulated as (H2L)Pr(NO3)3·H2O 1Pr, the NMR data confirm that the Pr3+ ion is only linked to the ligand by its deprotonated phenol functions while the ligand does exist in a neutral zwitterionic phenolato-iminium form (O−, NH+) in agreement with a migration of the protons from the phenolic functions to the imine groups. Deprotonation of the ligand in 1Pr yields a new and original product, LPr(NO3)·H2O·CH3OH 2Pr for which spectroscopic results agree with a coordination of the Pr3+ ion at the N2O2 site. This result, which is surprising in view of the oxophilic character of the lanthanide ions, is also confirmed by comparison with the NMR data of the LZnPr(NO3)3 complex 3Pr that retains the Pr3+ ion in the O2O2 site. Another anionic complex 4Pr, L2PrCs, with two ligands linked to the lanthanide centre by their N2O2 sites is also isolated. Starting with the mononuclear cationic or anionic entities, 2Pr or 4Pr, we have not been able to isolate and characterize genuine homo- or hetero-dinuclear lanthanide complexes.