Simple guanidinium motif for the selective binding and extraction of sulfate

<p>It is shown that a simple guanidinium molecule binds sulfate selectively in methanol/water solution, and a synthesized lipophilic analog permits the selective extraction of sulfate from aqueous sodium chloride into 1,2-dichloroethane. This receptor, <i>N</i>,<i>N</i>’-bis(2-pyridyl)guanidinium, features a rigid pseudo-bicyclic conformation in binding anions in the solid state. It selectively binds sulfate in 10% water/90% MeOD-d₄ solutions with stepwise log <i>K</i>₁ and log <i>K</i>₂ values of 3.78 ± 0.12 and 2.10 ± 0.23, respectively. Density functional theory calculations were performed to predict the conformational preferences of guanidinium receptors upon anion coordination in solution.</p

Outer-Sphere Water Clusters Tune the Lanthanide Selectivity of Diglycolamides

Fundamental understanding of the selective recognition and separation of <i>f</i>-block metal ions by chelating agents is of crucial importance for advancing sustainable energy systems. Current investigations in this area are mostly focused on the study of inner-sphere interactions between metal ions and donor groups of ligands, while the effects on the selectivity resulting from molecular interactions in the outer-sphere region have been largely overlooked. Herein, we explore the fundamental origins of the selectivity of the solvating extractant <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetraoctyl diglycolamide (TODGA) for adjacent lanthanides in a liquid–liquid extraction system, which is of relevance to nuclear fuel reprocessing and rare-earth refining technologies. Complementary investigations integrating distribution studies, quantum mechanical calculations, and classical molecular dynamics simulations establish a relationship between coextracted water and lanthanide extraction by TODGA across the series, pointing to the importance of the hydrogen-bonding interactions between outer-sphere nitrate ions and water clusters in a nonpolar environment. Our findings have significant implications for the design of novel efficient separation systems and processes, emphasizing the importance of tuning both inner- and outer-sphere interactions to obtain total control over selectivity in the biphasic extraction of lanthanides

Bipyrrole-Strapped Calix[4]pyrroles: Strong Anion Receptors That Extract the Sulfate Anion

Cage-type calix­[4]­pyrroles <b>2</b> and <b>3</b> bearing two additional pyrrole groups on the strap have been synthesized. Compared with the parent calix[4]­pyrrole (<b>1</b>), they were found to exhibit remarkably enhanced affinities for anions, including the sulfate anion (TBA⁺ salts), in organic media (CD₂Cl₂). This increase is ascribed to participation of the bipyrrole units in anion binding. Receptors <b>2</b> and <b>3</b> extract the hydrophilic sulfate anion (as the methyltrialkyl­(C_8–10)­ammonium (A336⁺) salt) from aqueous media into a chloroform phase with significantly improved efficiency (>10-fold relative to calix[4]­pyrrole <b>1</b>). These two receptors also solubilize into chloroform the otherwise insoluble sulfate salt, (TMA)₂SO₄ (tetramethylammonium sulfate)

Bipyrrole-Strapped Calix[4]pyrroles: Strong Anion Receptors That Extract the Sulfate Anion

Cage-type calix­[4]­pyrroles <b>2</b> and <b>3</b> bearing two additional pyrrole groups on the strap have been synthesized. Compared with the parent calix[4]­pyrrole (<b>1</b>), they were found to exhibit remarkably enhanced affinities for anions, including the sulfate anion (TBA⁺ salts), in organic media (CD₂Cl₂). This increase is ascribed to participation of the bipyrrole units in anion binding. Receptors <b>2</b> and <b>3</b> extract the hydrophilic sulfate anion (as the methyltrialkyl­(C_8–10)­ammonium (A336⁺) salt) from aqueous media into a chloroform phase with significantly improved efficiency (>10-fold relative to calix[4]­pyrrole <b>1</b>). These two receptors also solubilize into chloroform the otherwise insoluble sulfate salt, (TMA)₂SO₄ (tetramethylammonium sulfate)

Cyclo[6]pyridine[6]pyrrole: A Dynamic, Twisted Macrocycle with No Meso Bridges

A large porphyrin analogue, cyclo[6]­pyridine[6]­pyrrole, containing no meso bridging atoms, has been synthesized through Suzuki coupling. In its neutral form, this macrocycle exists as a mixture of two figure-eight conformers that undergo fast exchange in less polar solvents. Upon protonation, the dynamic twist can be transformed into species that adopt a ruffled planar structure or a figure-eight shape depending on the extent of protonation and counteranions. Conversion to a bisboron difluoride complex via deprotonation with NaH and treatment with BF₃ acts to lock the macrocycle into a figure-eight conformation. The various forms of cyclo[6]­pyridine[6]­pyrrole are characterized by distinct NMR, X-ray crystallographic, and spectroscopic features

Radiolytic Treatment of the Next-Generation Caustic-Side Solvent Extraction (NGS) Solvent and its Effect on the NGS Process

<div><p>It is shown in this work that the solvent used in the Next Generation Caustic-Side Solvent Extraction (NGS) process can withstand a radiation dose well in excess of the dose it would receive in multiple years of treating legacy salt waste at the US Department of Energy Savannah River Site. The solvent was subjected to a maximum of 50 kGy of gamma radiation while in dynamic contact with each of the aqueous phases of the current NGS process, namely SRS−15 (a highly caustic waste simulant), sodium hydroxide scrub solution (0.025 M), and boric acid strip solution (0.01 M). Bench-top testing of irradiated solvent confirmed that irradiation has inconsequential impact on the extraction, scrubbing, and stripping performance of the solvent up to 13 times the estimated 0.73 kGy/y annual absorbed dose. Stripping performance is the most sensitive step to radiation, deteriorating more due to buildup of p-sec-butylphenol (SBP) and possibly other proton-ionizable products than to degradation of the guanidine suppressor, as shown by chemical analyses. </p></div

Selectivity of the Highly Preorganized Tetradentate Ligand 2,9-Di(pyrid-2-yl)-1,10-phenanthroline for Metal Ions in Aqueous Solution, Including Lanthanide(III) Ions and the Uranyl(VI) Cation

Some metal ion complexing properties of DPP (2,9-Di­(pyrid-2-yl)-1,10-phenanthroline) are reported with a variety of Ln­(III) (Lanthanide­(III)) ions and alkali earth metal ions, as well as the uranyl­(VI) cation. The intense π–π* transitions in the absorption spectra of aqueous solutions of 10^–5 M DPP were monitored as a function of pH and metal ion concentration to determine formation constants of the alkali-earth metal ions and Ln­(III) (Ln = lanthanide) ions. It was found that log <i>K</i>₁(DPP) for the Ln­(III) ions has a peak at Ln­(III) = Sm­(III) in a plot of log <i>K</i>₁ versus 1/<i>r</i>⁺ (<i>r</i>⁺ = ionic radius for 8-coordination). For Ln­(III) ions larger than Sm­(III), there is a steady rise in log <i>K</i>₁ from La­(III) to Sm­(III), while for Ln­(III) ions smaller than Sm­(III), log <i>K</i>₁ decreases slightly to the smallest Ln­(III) ion, Lu­(III). This pattern of variation of log <i>K</i>₁ with varying size of Ln­(III) ion was analyzed using MM (molecular mechanics) and DFT (density functional theory) calculations. Values of strain energy (∑U) were calculated for the [Ln­(DPP)­(H₂O)₅]³⁺ and [Ln­(qpy)­(H₂O)₅]³⁺ (qpy = quaterpyrdine) complexes of all the Ln­(III) ions. The ideal M–N bond lengths used for the Ln­(III) ions were the average of those found in the CSD (Cambridge Structural Database) for the complexes of each of the Ln­(III) ions with polypyridyl ligands. Similarly, the ideal M–O bond lengths were those for complexes of the Ln­(III) ions with coordinated aqua ligands in the CSD. The MM calculations suggested that in a plot of ∑U versus ideal M–N length, a minimum in ∑U occurred at Pm­(III), adjacent in the series to Sm­(III). The significance of this result is that (1) MM calculations suggest that a similar metal ion size preference will occur for all polypyridyl-type ligands, including those containing triazine groups, that are being developed as solvent extractants in the separation of Am­(III) and Ln­(III) ions in the treatment of nuclear waste, and (2) Am­(III) is very close in M–N bond lengths to Pm­(III), so that an important aspect of the selectivity of polypyridyl type ligands for Am­(III) will depend on the above metal ion size-based selectivity. The selectivity patterns of DPP with the alkali-earth metal ions shows a similar preference for Ca­(II), which has the most appropriate M–N lengths. The structures of DPP complexes of Zn­(II) and Bi­(III), as representative of a small and of a large metal ion respectively, are reported. [Zn­(DPP)₂]­(ClO₄)₂ (triclinic, <i>P</i>1, <i>R</i> = 0.0507) has a six-coordinate Zn­(II), with each of the two DPP ligands having one noncoordinated pyridyl group appearing to be π-stacked on the central aromatic ring of the other DPP ligand. [Bi­(DPP)­(H₂O)₂(ClO₄)₂]­(ClO₄) (triclinic, <i>P</i>1, <i>R</i> = 0.0709) has an eight-coordinate Bi, with the coordination sphere composed of the four N donors of the DPP ligand, two coordinated water molecules, and the O donors of two unidentate perchlorates. As is usually the case with Bi­(III), there is a gap in the coordination sphere that appears to be the position of a lone pair of electrons on the other side of the Bi from the DPP ligand. The Bi-L bonds become relatively longer as one moves from the side of the Bi containg the DPP to the side where the lone pair is thought to be situated. A DFT analysis of [Ln­(tpy)­(H₂O)_<i>n</i>]³⁺ and [Ln­(DPP)­(H₂O)₅]³⁺ complexes is reported. The structures predicted by DFT are shown to match very well with the literature crystal structures for the [Ln­(tpy)­(H₂O)_<i>n</i>]³⁺ with Ln = La and <i>n</i> = 6, and Ln = Lu with <i>n</i> = 5. This then gives one confidence that the structures for the DPP complexes generated by DFT are accurate. The structures generated by DFT for the [Ln­(DPP)­(H₂O)₅]³⁺ complexes are shown to agree very well with those generated by MM, giving one confidence in the accuracy of the latter. An analysis of the DFT and MM structures shows the decreasing O--O nonbonded distances as one progresses from La to Lu, with these distances being much less than the sum of the van der Waals radii for the smaller Ln­(III) ions. The effect that such short O--O nonbonded distances has on thermodynamic complex stability and coordination number is then discussed

“Straining” to Separate the Rare Earths: How the Lanthanide Contraction Impacts Chelation by Diglycolamide Ligands

The subtle energetic differences underpinning adjacent lanthanide discrimination are explored with diglycolamide ligands. Our approach converges liquid–liquid extraction experiments with solution-phase X-ray absorption spectroscopy (XAS) and density functional theory (DFT) simulations, spanning the lanthanide series. The homoleptic [(DGA)₃Ln]³⁺ complex was confirmed in the organic extractive solution by XAS, and this was modeled using DFT. An interplay between steric strain and coordination energies apparently gives rise to a nonlinear trend in discriminatory lanthanide ion complexation across the series. Our results highlight the importance of optimizing chelate molecular geometry to account for both coordination interactions and strain energies when designing new ligands for efficient adjacent lanthanide separation for rare-earth refining