Comparison of Two Tetrapodal N,O Ligands: Impact of the Softness of the Heterocyclic N-Donors Pyridine and Pyrazine on the Selectivity for Am(III) over Eu(III)

Abstract

To quantify the impact of the N-donor softness on the coordination of f elements in aqueous solution, and in particular on the selectivity for Am(III) over Eu(III), we have designed the two tetrapodal hexadentate ligands N,N-bis(2-pyridylmethyl)ethylenediamine-N′,N′-diacetic acid (L py ) and N,N-bis(2-pyrazylmethyl)ethylenediamine-N′,N′diacetic acid (L pz ). These ligands bear two hard acetate groups to provide stability to the An(III) and Ln(III) complexes and two N-heterocyclic soft groups to provide Am(III) versus Eu(III) selectivity. They only differ in their N-donor moieties, pyridine or pyrazine. The proton NMR and potentiometric analyses performed on the lanthanide complexes of the two ligands indicate that a unique metallic complex, LnL, is formed and that LnL py+ and LnL pz+ have the same structure in water. Furthermore, the hydration numbers of the europium and terbium ions in these complexes, measured by luminescence decay, have the same value (q ) 3), indicating that the two ligands act as hexadentate donors in both systems. As expected, the softer pyrazine-based ligand gives less stable complexes than the pyridinebased ligand with the hard Ln(III) cations. The fragment N(CH 2 pz) 2 containing two pyrazine functions has a very low contribution to the stability of the lanthanide complexes, even though the pyrazine groups are coordinated to the cation in water. The stabilities of the americium(III) complexes were determined by potentiometry and are greater than those found for the isoelectronic europium complexes. The selectivity for Am(III) over Eu(III) increases from 60 to 500 when the pyridine-containing fragment N(CH 2 py) 2 is substituted by the pyrazine-containing fragment N(CH 2 pz) 2 , which demonstrates that the selectivity for Am(III) over Eu(III) is significantly enhanced when the softness of the N-heterocycle increases from pyridine to pyrazine. These new hydrophilic ligands present attractive selectivities for Am(III) over Eu(III) that could make them good candidates for the selective back extraction of Am(III) from organic solutions containing 4f and 5f elements.

A Gadolinium‐Binding Cyclodecapeptide with a Large High‐Field Relaxivity Involving Second‐Sphere Water

“…For instance, the 1 H NMR spectrum of the paramagnetic Yb 3 + complex shows only a few very broad resonances with small paramagnetic shifts. As previously discussed for Ln 3 + complexes with tetrapodal ligands, this broadening can be attributed to exchange dynamics between several conformations of the complex, [37] the spectral coalescence time of which is close to the experimental temperature 298 K. In contrast, the average conformation is detected for the La 3 + complex because the spectral coalescence temperature is lower for this diamagnetic ion (smaller differences of the proton chemical shifts between the various conformations).…”

Lanthanide(III) Complexes with Two Hexapeptides Incorporating Unnatural Chelating Amino Acids: Secondary Structure and Stability

“…The NMR spectra of Eu 3 + and Yb 3 + complexes show very broad proton resonances because of intramolecular dynamics, as already observed for tetrapodal ligands with soft donor nitrogen groups. [45] More details on the structures of the complexes were obtained from the NMR data of the diamagnetic lanthanum complexes.…”

Synthesis and evaluation of novel water-soluble ligands for the complexation of metals during the partitioning of actinides

“…For the synthesis of ligand 17, with a malonamide backbone, first bis(dichlorophosphino)methane (15) All these ligands showed infinite solubility in water.…”

“…Some water-soluble diglycolamides (tetraalkyl diglycolamide) have been studied by Sasaki et al, 13,14 however, their water solubility is limited. Water-soluble N-donor ligands, containing pyridine and pyrazine rings described by Heitzmann et al, 15,16 demonstrate a correlation of the selectivity for trivalent actinides (e.g. Am) over Ln with the N-donor groups coordinated to the cation (softness of the molecule).…”