Complexation of a Schiff baseligand 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.