Complexation of trivalent americium and Lanthanides with terdentate 'N' donor ligands: the role of rigidity in the ligand structure

A systematic study on the Ln(3+) complexation behaviour with two terdentate 'N' donor ligands of varying structural rigidity, viz. 5,6-dimethyl-(1,2,4)-triazinylbipyridine (Me(2)TBipy) and 5,6-dimethyl-(1,2,4)-triazinylphenanthroline (Me(2)TPhen), 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(2)v 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 Ln(3+) ions. The solid state crystal structure of the La3+ complex of Me(2)TPhen 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 Ln(3+) ions studied, indicating the selectivity of these ligands for the trivalent actinides over the lanthanides.