Understanding Molecular Factors That Determine Performance in the Rare Earth (TriNOx) Separations System

Rare
earth metal complexes of the proligand H3TriNOx
([(2-tBuNOH)­C6H3CH2]3N) have been shown to afford separations
of simple mixtures of rare earth metal salts. In particular, separations
systems were developed for applications to technologically relevant
mixtures, e.g., Nd/Dy and Eu/Y for targeted, rare earths recycling
chemistry. More recently, it was demonstrated that an electron-donating
derivative of the proligand H3TriNOxR (([(2-tBuNOH)­C6H3RCH2]3N; R = 5-OMe) influenced electronic and physical properties
to effect improved separations. To further probe substituent effects,
in the current work, derivatives with electron-donating and -withdrawing
groups along the aryl-backbone were synthesized (R = 4-tBu, 5-Ph, 4-CF3). The new proligands were
coordinated to rare earths (RE) through protonolysis reactions, and
the resulting complexes (RE = Nd, Dy) were characterized. Dimerization
equilibrium constants and molar solubility were determined where applicable.
Overall, the studies indicated that increased electron-donation of
the aryl-substituents resulted in an increased driving force for the
dimerization of the Nd complexes. This dimerization equilibrium and
resultant solubility differences were used to separate mixtures of
neodymium/dysprosium as well as mixtures of europium/yttrium. These
findings demonstrate the tunability of the TriNOx3– framework to achieve tailored RE separations.