8 research outputs found
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>â-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-d<sub>4</sub> solutions with stepwise log <i>K</i><sub>1</sub> and log <i>K</i><sub>2</sub> 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>âČ,<i>N</i>âČ-tetraoctyl diglycolamide (TODGA) for adjacent lanthanides
in a liquidâliquid 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
calixÂ[4]Âpyrroles <b>2</b> and <b>3</b> bearing
two additional pyrrole groups on the strap have been synthesized.
Compared with the parent calix[4]Âpyrrole (<b>1</b>), they were
found to exhibit remarkably enhanced affinities for anions, including
the sulfate anion (TBA<sup>+</sup> salts), in organic media (CD<sub>2</sub>Cl<sub>2</sub>). 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 methyltrialkylÂ(C<sub>8â10</sub>)Âammonium (A336<sup>+</sup>) salt) from aqueous
media into a chloroform phase with significantly improved efficiency
(>10-fold relative to calix[4]Âpyrrole <b>1</b>). These two
receptors
also solubilize into chloroform the otherwise insoluble sulfate salt,
(TMA)<sub>2</sub>SO<sub>4</sub> (tetramethylammonium sulfate)
Bipyrrole-Strapped Calix[4]pyrroles: Strong Anion Receptors That Extract the Sulfate Anion
Cage-type
calixÂ[4]Âpyrroles <b>2</b> and <b>3</b> bearing
two additional pyrrole groups on the strap have been synthesized.
Compared with the parent calix[4]Âpyrrole (<b>1</b>), they were
found to exhibit remarkably enhanced affinities for anions, including
the sulfate anion (TBA<sup>+</sup> salts), in organic media (CD<sub>2</sub>Cl<sub>2</sub>). 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 methyltrialkylÂ(C<sub>8â10</sub>)Âammonium (A336<sup>+</sup>) salt) from aqueous
media into a chloroform phase with significantly improved efficiency
(>10-fold relative to calix[4]Âpyrrole <b>1</b>). These two
receptors
also solubilize into chloroform the otherwise insoluble sulfate salt,
(TMA)<sub>2</sub>SO<sub>4</sub> (tetramethylammonium sulfate)
Cyclo[6]pyridine[6]pyrrole: A Dynamic, Twisted Macrocycle with No Meso Bridges
A large porphyrin analogue, cyclo[6]Âpyridine[6]Âpyrrole,
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 BF<sub>3</sub> acts to lock the macrocycle into a figure-eight conformation.
The various forms of cyclo[6]Âpyridine[6]Âpyrrole 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 SRSâ15 (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-DiÂ(pyrid-2-yl)-1,10-phenanthroline)
are reported with a variety of LnÂ(III) (LanthanideÂ(III)) ions and
alkali earth metal ions, as well as the uranylÂ(VI) cation. The intense
ÏâÏ* transitions in the absorption spectra of aqueous
solutions of 10<sup>â5</sup> M DPP were monitored as a function
of pH and metal ion concentration to determine formation constants
of the alkali-earth metal ions and LnÂ(III) (Ln = lanthanide) ions.
It was found that log <i>K</i><sub>1</sub>(DPP) for the
LnÂ(III) ions has a peak at LnÂ(III) = SmÂ(III) in a plot of log <i>K</i><sub>1</sub> versus 1/<i>r</i><sup>+</sup> (<i>r</i><sup>+</sup> = ionic radius for 8-coordination). For LnÂ(III)
ions larger than SmÂ(III), there is a steady rise in log <i>K</i><sub>1</sub> from LaÂ(III) to SmÂ(III), while for LnÂ(III) ions smaller
than SmÂ(III), log <i>K</i><sub>1</sub> decreases slightly
to the smallest LnÂ(III) ion, LuÂ(III). This pattern of variation of
log <i>K</i><sub>1</sub> with varying size of LnÂ(III) ion
was analyzed using MM (molecular mechanics) and DFT (density functional
theory) calculations. Values of strain energy (âU) were calculated
for the [LnÂ(DPP)Â(H<sub>2</sub>O)<sub>5</sub>]<sup>3+</sup> and [LnÂ(qpy)Â(H<sub>2</sub>O)<sub>5</sub>]<sup>3+</sup> (qpy = quaterpyrdine) complexes
of all the LnÂ(III) ions. The ideal MâN bond lengths used for
the LnÂ(III) ions were the average of those found in the CSD (Cambridge
Structural Database) for the complexes of each of the LnÂ(III) ions
with polypyridyl ligands. Similarly, the ideal MâO bond lengths
were those for complexes of the LnÂ(III) ions with coordinated aqua
ligands in the CSD. The MM calculations suggested that in a plot of
âU versus ideal MâN length, a minimum in âU occurred
at PmÂ(III), adjacent in the series to SmÂ(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 AmÂ(III) and LnÂ(III) ions in the treatment
of nuclear waste, and (2) AmÂ(III) is very close in MâN bond
lengths to PmÂ(III), so that an important aspect of the selectivity
of polypyridyl type ligands for AmÂ(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 CaÂ(II), which
has the most appropriate MâN lengths. The structures of DPP
complexes of ZnÂ(II) and BiÂ(III), as representative of a small and
of a large metal ion respectively, are reported. [ZnÂ(DPP)<sub>2</sub>]Â(ClO<sub>4</sub>)<sub>2</sub> (triclinic, <i>P</i>1, <i>R</i> = 0.0507) has a six-coordinate ZnÂ(II), with each of the
two DPP ligands having one noncoordinated pyridyl group appearing
to be Ï-stacked on the central aromatic ring of the other DPP
ligand. [BiÂ(DPP)Â(H<sub>2</sub>O)<sub>2</sub>(ClO<sub>4</sub>)<sub>2</sub>]Â(ClO<sub>4</sub>) (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 BiÂ(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 [LnÂ(tpy)Â(H<sub>2</sub>O)<sub><i>n</i></sub>]<sup>3+</sup> and [LnÂ(DPP)Â(H<sub>2</sub>O)<sub>5</sub>]<sup>3+</sup> complexes
is reported. The structures predicted by DFT are shown to match very
well with the literature crystal structures for the [LnÂ(tpy)Â(H<sub>2</sub>O)<sub><i>n</i></sub>]<sup>3+</sup> 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 [LnÂ(DPP)Â(H<sub>2</sub>O)<sub>5</sub>]<sup>3+</sup> 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 LnÂ(III) ions. The
effect that such short O--O nonbonded distances has on thermodynamic
complex stability and coordination number is then discussed
âStrainingâ 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 liquidâliquid
extraction experiments with solution-phase X-ray absorption spectroscopy
(XAS) and density functional theory (DFT) simulations, spanning the
lanthanide series. The homoleptic [(DGA)<sub>3</sub>Ln]<sup>3+</sup> 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