A study of lanthanide complexes formed with the terdentate nitrogenligand 4-amino-bis(2,6-(2-pyridyl))-1,3,5-triazine. Relevance to the separation of actinides and lanthanides by solvent extraction

Abstract

The terdentate ligand 4-amino-bis(2,6-(2-pyridyl))-1,3,5-triazine L⁴ has been shown to have useful actinide(III)/lanthanide(III) separation properties relevant to nuclear reprocessing. The complexation of this ligand with the lanthanides has been investigated. Crystallographic analyses show the formation of five structural types. In the first part of the lanthanide series, (La–Sm), these structural types are (1) [LnL⁴(NO₃)₃(H₂O)₂], 11-coordinate, only for Ln = La; (2) [LnL⁴(NO₃)₃(H₂O)], 10-coordinate, for Ln = La, Pr, Nd, Sm; (3) [LnL⁴(NO₃)₂(H₂O)₃][NO₃] 10-coordinate for Ln = Nd, Sm. In the second part of the series from Eu to Lu the structures are all 9-coordinate but have two different stoichiometries; (4) [LnL⁴(NO₃)₂(H₂O)₂][NO₃] for Ln = Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; (5) [LnL⁴(NO₃)₃(H₂O)] unusually with one unidentate nitrate for Ln = Yb, and Y. With M = Sc the structure obtained is [ScL⁴(NO₃)₃] which is 8-coordinate with one unidentate nitrate. The structure types found for particular lanthanides can be correlated with the size of the ion, although several of the lanthanides form two structure types. Particularly noticeable is the alternative pairing of one bidentate nitrate ion or two water molecules in the coordination sphere with a concomitant free nitrate anion in the latter case. Type 5 includes the relatively rare sighting of a unidentate nitrate anion. Extensive intermolecular hydrogen bonding involving the free amine group is found in all the structures.