Complexation of trivalent americium and lanthanides with terdentate ‘N’ donor ligands: the role of rigidity in the ligand structure

Abstract

A systematic study on the Ln³⁺ complexation behaviour with two terdentate ‘N’ donor ligands of varying structural rigidity, viz. 5,6-dimethyl-(1,2,4)-triazinylbipyridine (Me₂TBipy) and 5,6-dimethyl-(1,2,4)-triazinylphenanthroline (Me₂TPhen), is performed in the present work by UV-Vis spectrophotometry, time resolved fluorescence spectroscopy (TRFS) and electrospray ionization mass spectrometric (ESI-MS) studies. These studies indicate the formation of a 1 : 1 complex of La³⁺, 1 : 2 complexes of Eu³⁺ and Er³⁺ with both the ligands. Density functional theoretical (DFT) study is carried out to determine the solution phase structure of the Eu³⁺ complex considering the species (from UV-Vis spectrophotometry) and C_2v site symmetry around the Eu³⁺ ion (from TRFS study). Me₂TPhen is found to be a stronger complexing ligand as compared to Me₂TBipy irrespective of the Ln³⁺ ions. The solid state crystal structure of the La³⁺ complex of Me₂TPhen is determined using the single crystal X-ray diffraction (SCXRD) technique. The complexation of the trivalent Am³⁺ ion is also studied with both these ligands using UV-Vis spectrophotometric titrations which show the formation of 1 : 2 complexes with higher complexation constant values as compared to all the Ln³⁺ ions studied, indicating the selectivity of these ligands for the trivalent actinides over the lanthanides.