Quantifying the binding strength of U(VI) with phthalimidedioxime in comparison with glutarimidedioxime.

Studies of the complexation of U(vi) with amidoxime-related ligands help in the development of efficient sorbents for the extraction of uranium from seawater. In the present study, the stability constants of the U(vi) complexes with two phthalimidedioxime ligands were determined by potentiometry and spectrophotometry, and compared with glutarimidedioxime previously studied. Density functional theory calculations were performed to identify the most probable protonation sites of the ligand and to help interpret the trend in the binding strength of the ligands. The phthalimidedioxime complexes were found to be 2-3 orders of magnitude weaker than the corresponding glutarimidedioxime complexes, which was attributed to the difference between the ligands in the electronic and structural properties. The incorporation of the aromatic ring into phthalimidedioxime reduces the electron density on the donor atoms of the ligand and makes the imidedioxime moiety less complementary for binding UO2(2+)via its equatorial plane. Though weaker than glutarimidedioxime, phthalimidedioxime still forms fairly strong U(vi) complexes and can still effectively compete with carbonate for the complexation of U(vi) at seawater pH and carbonate concentration. Due to its higher chemical stability in acidic solutions than that of glutarimidedioxime, phthalimidedioxime is a valuable ligand that could have potential use in the extraction of U(vi) from seawater.

Synthesis of a hydrophilic naphthalimidedioxime.

Imidedioximes are formed in hydroxylamine-treated polyacrylonitrile adsorbents used in the extraction of uranium from seawater. Although known to be a good uranophile, the glutarimidedioxime model compound 1 is rapidly hydrolyzed under acidic conditions used to elute metals from the adsorbent. This work reports the synthesis of a hydrophilic naphthalimidedioxime derivative 14, which is stable under acidic elution conditions. The synthesis starts from simple acenaphthenequinone 7 and converts it to a functional group dense imidedioxime 14 in 7 steps.

A case for molecular recognition in nuclear separations: sulfate separation from nuclear wastes.

In this paper, we present the case for molecular-recognition approaches for sulfate removal from radioactive wastes via the use of anion-sequestering systems selective for sulfate, using either liquid-liquid extraction or crystallization. Potential benefits of removing sulfate from the waste include improved vitrification of the waste, reduced waste-form volume, and higher waste-form performance, all of which lead to potential cleanup schedule acceleration and cost savings. The need for sulfate removal from radioactive waste, especially legacy tank wastes stored at the Hanford site, is reviewed in detail and primarily relates to the low solubility of sulfate in borosilicate glass. Traditional methods applicable to the separation of sulfate from radioactive wastes are also reviewed, with the finding that currently no technology has been identified and successfully demonstrated to meet this need. Fundamental research in the authors' laboratories targeting sulfate as an important representative of the class of oxoanions is based on the hypothesis that designed receptors may provide the needed ability to recognize sulfate under highly competitive conditions, in particular where the nitrate anion concentration is high. Receptors that have been shown to have promising affinity for sulfate, either in extraction or in crystallization experiments, include hexaurea tripods, tetraamide macrocycles, cyclo[8]pyrroles, calixpyrroles, and self-assembled urea-lined cages. Good sulfate selectivity observed in the laboratory provides experimental support for the proposed molecular-recognition approach.

Influence of charge on anion receptivity in amide-based macrocycles.

Binding and structural aspects of anions with tetraamido/diquaternized diamino macrocyclic receptors containing m-xylyl, pyridine, and thiophene spacers are reported. (1)H NMR studies indicate that the quaternized receptors display higher affinities for anions compared to corresponding neutral macrocycles. The macrocycles containing pyridine spacers consistently display higher affinity for a given anion compared to those with either m-xylyl or thiophene spacers. The m-xylyl- and pyridine-containing receptors exhibit high selectivity for H(2)PO(4)(-) in DMSO-d(6) with association constants, K(a) = 1.09 × 10(4) and >10(5) M(-1), respectively, and moderate selectivity for Cl(-) with K(a) = 1.70 × 10(3) and 5.62 × 10(4) M(-1), respectively. Crystallographic studies for the Cl(-) and HSO(4)(-) complexes indicate that the m-xylyl-containing ligand is relatively elliptical in shape, with the two charges at ends of the major axis of the ellipse. The anions are hydrogen bonded with the macrocycle but are outside the ligand cavity. In the solid state, an unusual low-barrier hydrogen bond (LBHB) was discovered between two of the macrocycle's carbonyl oxygen atoms in the HSO(4)(-) complex. The pyridine-containing macrocycle folds so that the two pyridine units are face-to-face. The two I(-) ions are chelated to the two amides adjacent to a given pyridine. In the structure of the thiophene containing macrocycle with two BPh(4)(-) counterions, virtually no interaction was observed crystallographically between the macrocycle and the bulky anions.

How amidoximate binds the uranyl cation.

This study identifies how the amidoximate anion, AO, interacts with the uranyl cation, UO(2)(2+). Density functional theory calculations have been used to evaluate possible binding motifs in a series of [UO(2)(AO)(x)(OH(2))(y)](2-x) (x = 1-3) complexes. These motifs include monodentate binding to either the oxygen or the nitrogen atom of the oxime group, bidentate chelation involving the oxime oxygen atom and the amide nitrogen atom, and η(2) binding with the N-O bond. The theoretical results establish the η(2) motif to be the most stable form. This prediction is confirmed by single-crystal X-ray diffraction of UO(2)(2+) complexes with acetamidoxime and benzamidoxime anions.

Rapid determination of enantiomeric excess: a focus on optical approaches.

High-throughput screening (HTS) methods are becoming increasingly essential in discovering chiral catalysts or auxiliaries for asymmetric transformations due to the advent of parallel synthesis and combinatorial chemistry. Both parallel synthesis and combinatorial chemistry can lead to the exploration of a range of structural candidates and reaction conditions as a means to obtain the highest enantiomeric excess (ee) of a desired transformation. One current bottleneck in these approaches to asymmetric reactions is the determination of ee, which has led researchers to explore a wide range of HTS techniques. To be truly high-throughput, it has been proposed that a technique that can analyse a thousand or more samples per day is needed. Many of the current approaches to this goal are based on optical methods because they allow for a rapid determination of ee due to quick data collection and their parallel analysis capabilities. In this critical review these techniques are reviewed with a discussion of their respective advantages and drawbacks, and with a contrast to chromatographic methods (180 references).

Enthalpy vs Entropy Driven Complexation of Homoallylic Alcohols by Rh(I) Complexes.

The thermodynamics of binding between several homoallylic alcohols and simple olefinic Rh(I) compounds was examined with (1)H NMR spectroscopy and ITC. (1)H NMR titrations revealed moderate binding of these alcohols with [Rh(COD)(2)](+) (1) and [Rh(COD)(CH(3)CN)(2)](+) (3), but weaker binding with [Rh(NBD)(2)](+) (2). ITC indicated that the complexation with [Rh(COD)(2)](+) is mainly governed by enthalpy whereas binding with [Rh(COD)(CH(3)CN)(2)](+) is entirely driven by entropy. The thermodynamic parameters for the homoallylic alcohol binding of Rh(I) complexes 1-3 are consistent with crystallographic data.

Cryptand-like anion receptors.

The design of supramolecular hosts for anions began with simple diaza bicycles, named katapinands, and has evolved over the last 40 years to a number of elegantly designed receptors capable of binding many different anions. About the same time the term cryptand appeared in reference to another bicyclic compound that was selective for alkaline-earth ions. Since the first report these simple bicycles, a vast arena of hosts has appeared, including acyclic, monocyclic, and other multicyclic supramolecular receptors. Studies of these systems have revealed considerable information about anion coordination chemistry, including the fact that many of these complexes mimic their transition-metal corollaries in terms of coordination numbers. However, for anions interactions occur via H-bonding most often, rather than the coordinate covalent or dative bonds observed in transition-metal coordination. This critical review examines the design of enclosed, primarily bicyclic cryptands as hosts for anions, with a small scattering of higher polyhedra when deemed appropriate to the discussion. In order to show the development (evolution) of the field, key examples of early work will be noted and compared with more recent developments (136 references).

Tricyclic host for linear anions.

A tricyclic host for anions consisting of two tetraamide monocycles attached by two ethylene chains was designed and synthesized. Structural and binding results indicate that the receptor is selective for linear triatomic anions. Crystallographic data for two hydrated free bases, along with FHF(-), N(3)(-), and SO(4)(2-) complexes indicate that there are at least two preferred gross conformations for the host, one of which possesses pseudo-D(2) symmetry and the other pseudo-C(2h) symmetry. Both FHF(-) and N(3)(-) are encapsulated in the pseudo-D(2) symmetric complex, bridging the two tetraamido macrocyclic halves. The pseudo-C(2h) octahydrate structure shows an ice-like H-bonded (H(2)O)(6) array of water molecules embedded in the host cavity. The SO(4)(2-) structure has a nearly superimposable host conformation to the octahydrate but with the SO(4)(2-) anions lying outside the host. Binding studies in DMSO-d(6) indicate selectivity for FHF(-), with lesser affinity for other inorganic anions.

Tetra-butyl-ammonium N-benzoyl-6-nitro-1,3-benzothia-zol-2-aminide.

In the title salt, C(16)H(36)N(+)·C(14)H(8)N(3)O(3)S(-), the torsion angles within the cation reveal that one butyl group displays an anti conformation and the other three butyl groups show gauche conformations. The anion is almost planar, with a largest deviation of 0.166 (6) Šfrom the least-squares plane (r.m.s. deviation of fitted atoms = 0.052 Å). In the crystal structure, the component ions inter-act by means of weak inter-molecular C-H⋯O hydrogen bonds.

Fluoride: solution- and solid-state structural binding probe.

Solid-state and solution studies were performed to determine if F(-) is encapsulated by anion hosts in both media. X-ray crystal structure determinations were compared with both (1)H and (19)F solution NMR data. Three hosts were studied: (1) two polyamide hosts, one with isophthaloyl spacers and the other with pyridine spacers, and (2) a polythioamide host with pyridine spacers. Binding studies showed that the pyridine-containing amide cryptand shows the highest affinity (K(a) > 10(5) in DMSO-d(6)), with the other hosts at least a factor of 10 lower. All of the cryptands appear to encapsulate F(-) in solution, where a deuterium-exchange reaction with DMSO-d(6) can be monitored by (19)F NMR. Four crystal structures are reported and compared: two for the pyridine-containing free base hosts and two for encapsulated F(-) complexes of the two amide-based cryptands.

The influence of amine functionalities on anion binding in polyamide-containing macrocycles.

Mixed amide/amine macrocyclic anion hosts of varying sizes and with different amine substituents have been synthesized and characterized. Host 2, containing a 28-membered ring and secondary amines, has shown selective binding for HSO(4)(-) over other oxo anions and halides in DMSO-d(6) using NMR titrations. Crystal structures of SO(4)(2-), HPO(4)(2-), H(2)PO(4)(-), and H(2)P(2)O(7)(2-) with the 28-membered ring hosts indicate different macrocyclic conformations depending on the N-substituent. Anion affinities appear to be correlated with macrocycle conformation.

Cyclophane capsule motifs with side pockets.

Neutral and charged multitopic cyclophane-capped anion hosts connected by three or four diamide/monoamine chains and a decomposition product with two chains have been synthesized and characterized. The chains in the two former hosts fold together to form one or two binding pockets, respectively, and FHF(-) and several phosphate complexes have been obtained with the anions nestled in these pockets. The decomposition product also shows propensity for binding dicarboxylates, as evidenced by an isophthalate crystal structure.

Amide-based ligands for anion coordination.

Anion recognition is an active area of research in supramolecular chemistry. The rapidly increasing amount of structural data now allows anion coordination chemistry to be formalized in terms of coordination numbers and geometries based on hydrogen-bonding interactions between the host (ligand) and the guest (anion). This Minireview targets just one class of anion receptors, namely, amide-based ligands. The structural data for a series of five anion shapes are compiled according to coordination number, and distinct commonalities are observed within a given anion topology. The results also indicate a number of similarities between the coordination of anions and transition-metal ions.

Anion binding motifs: topicity and charge in amidocryptands.

An expanded amidocryptand with propyl linkages provides multitopic sites for binding anions and water molecules. Upon quaternization of the two bridgehead amines, the molecular shape changes from an inverted Y to that of a bowl, which is filled with water and topped by the anion.

Encapsulated sulfates: insight to binding propensities.

Two crystal structures of sulfate inclusion complexes in an aza- and amido-cryptand represent the first examples of encapsulated sulfate in synthetic cryptand receptors and indicate penta- and octa-coordination, respectively.

Fluoride-facilitated deuterium exchange from DMSO-d6 to polyamide-based cryptands.

Multiple deuterium exchange between DMSO-d6 and amide hydrogens in two hexaamido cryptand fluoride receptors has been verified by 19F and 2H NMR and FAB mass spectral studies. Structural results for one of the complexes indicate a tricapped trigonal prism hydrogen bond coordination geometry around an encapsulated fluoride, with hydrogen bonds from fluoride to six amide and three phenyl hydrogens.

Elite new anion ligands: polythioamide macrocycles.

Prototypes for a new class of polythioamide-based macrocycles have been synthesized and anion-binding capabilities assessed. Results indicate higher anion binding for H(2)PO(4)(-), HSO(4)(-), and F(-) for monocycles, but somewhat lessened binding capabilities for bicycles compared with amide corollaries.