Selective Solid-Liquid and Liquid-Liquid Extraction of Lithium Chloride Using Strapped Calix[4]pyrroles.

LiCl is a classic "hard" ion salt that is present in lithium-rich brines and a key component in end-of-life materials (that is, used lithium-ion batteries). Its isolation and purification from like salts is a recognized challenge with potential strategic and economic implications. Herein, we describe two ditopic calix[4]pyrrole-based ion-pair receptors (2 and 3), that are capable of selectively capturing LiCl. Under solid-liquid extraction conditions, using 2 as the extractant, LiCl could be separated from a NaCl/KCl salt mixture containing as little as 1 % LiCl with circa 100 % selectivity, while receptor 3 achieved similar separations when the LiCl level was as low as 200 ppm. Under liquid-liquid extraction conditions using nitrobenzene as the non-aqueous phase, the extraction preference displayed by 2 is KCl>NaCl>LiCl. In contrast, 3 exhibits high selectivity towards LiCl over NaCl and KCl, with no appreciable extraction being observed for the latter two salts.

"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.

CO2 Capture from Ambient Air by Crystallization with a Guanidine Sorbent.

Carbon capture and storage is an important strategy for stabilizing the increasing concentration of atmospheric CO2 and the global temperature. A possible approach toward reversing this trend and decreasing the atmospheric CO2 concentration is to remove the CO2 directly from air (direct air capture). Herein we report a simple aqueous guanidine sorbent that captures CO2 from ambient air and binds it as a crystalline carbonate salt by guanidinium hydrogen bonding. The resulting solid has very low aqueous solubility (Ksp =1.0(4)×10-8 ), which facilitates its separation from solution by filtration. The bound CO2 can be released by relatively mild heating of the crystals at 80-120 °C, which regenerates the guanidine sorbent quantitatively. Thus, this crystallization-based approach to CO2 separation from air requires minimal energy and chemical input, and offers the prospect for low-cost direct air capture technologies.

Aqueous Sulfate Separation by Sequestration of [(SO4 )2 (H2 O)4 ]4- Clusters within Highly Insoluble Imine-Linked Bis-Guanidinium Crystals.

Selective crystallization of sulfate with a simple bis-guanidinium ligand, self-assembled in situ from terephthalaldehyde and aminoguanidinium chloride, was employed as an effective way to separate the highly hydrophilic sulfate anion from aqueous solutions. The resulting bis-iminoguanidinium sulfate salt has exceptionally low aqueous solubility (Ksp =2.4×10-10 ), comparable to that of BaSO4 . Single-crystal X-ray diffraction analysis showed the sulfate anions are sequestered as [(SO4 )2 (H2 O)4 ]4- clusters within the crystals. Variable-temperature solubility measurements indicated the sulfate crystallization is slightly endothermic (ΔHcryst =3.7 kJ mol-1 ), thus entropy driven. The real-world utility of this crystallization-based approach for sulfate separation was demonstrated by removing up to 99 % of sulfate from seawater in a single step.

Aqueous Sulfate Separation by Crystallization of Sulfate-Water Clusters.

An effective approach to sulfate separation from aqueous solutions is based on the crystallization of extended [SO4(H2O)5(2-)]n sulfate-water clusters with a bis(guanidinium) ligand. The ligand was generated in situ by hydrazone condensation in water, thereby bypassing the need for elaborate syntheses, tedious purifications, and organic solvents. Crystallization of sulfate-water clusters represents an alternative approach to the now established sulfate separation strategies that involve encapsulation of the "naked" anion.

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.

Bipyrrole-strapped calix[4]pyrroles: strong anion receptors that extract the sulfate anion.

Cage-type calix[4]pyrroles 2 and 3 bearing two additional pyrrole groups on the strap have been synthesized. Compared with the parent calix[4]pyrrole (1), 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 2 and 3 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 1). These two receptors also solubilize into chloroform the otherwise insoluble sulfate salt, (TMA)2SO4 (tetramethylammonium sulfate).

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 K(1)(DPP) for the Ln(III) ions has a peak at Ln(III) = Sm(III) in a plot of log K(1) versus 1/r(+) (r(+) = ionic radius for 8-coordination). For Ln(III) ions larger than Sm(III), there is a steady rise in log K(1) from La(III) to Sm(III), while for Ln(III) ions smaller than Sm(III), log K(1) decreases slightly to the smallest Ln(III) ion, Lu(III). This pattern of variation of log K(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)(H(2)O)(5)](3+) and [Ln(qpy)(H(2)O)(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)](ClO(4))(2) (triclinic, P1, R = 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)(H(2)O)(2)(ClO(4))(2)](ClO(4)) (triclinic, P1, R = 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)(H(2)O)(n)](3+) and [Ln(DPP)(H(2)O)(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)(H(2)O)(n)](3+) with Ln = La and n = 6, and Ln = Lu with n = 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)(H(2)O)(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.

Near Quantitative Removal of Selenate and Sulfate Anions from Wastewaters by Cocrystallization with Chelating Hydrogen-Bonding Guanidinium Ligands.

Selenium (Se) has become an environmental contaminant of aquatic ecosystems as a result of human activities, particularly mining, fossil fuel combustion, and agricultural activities. By leveraging the high sulfate concentrations relative to Se oxyanions (i.e., SeO n 2-, n = 3, 4) present in some wastewaters, we have developed an efficient approach to Se-oxyanion removal by cocrystallization with bisiminoguanidinium (BIG) ligands that form crystalline sulfate/selenate solid solutions. The crystallization of the sulfate, selenate and selenite, oxyanions and of sulfate/selenate mixtures with five candidate BIG ligands are reported along with the thermodynamics of crystallization and aqueous solubilities. Oxyanion removal experiments with the top two performing candidate ligands show a near quantitative removal (>99%) of sulfate or selenate from solution. When both sulfate and selenate are present, there is near quantitative removal (>99%) of selenate, down to sub-ppb Se levels, with no discrimination between the two oxyanions during cocrystallization. Reducing the selenate concentrations by 3 orders of magnitude or more relative to sulfate, as found in many wastewaters, led to no measurable loss in Se removal efficiencies. This work offers a simple and effective alternative to selective separation of trace amounts of highly toxic selenate oxyanions from wastewaters, to meet stringent regulatory discharge limits.

Metal ion complexing properties of dipyridoacridine, a highly preorganized tridentate homologue of 1,10-phenanthroline.

DPA (dipyrido[4,3-b;5,6-b]acridine) may be considered as a tridentate homologue of phen (1,10-phenanthroline). In this paper some of the metal ion complexing properties of DPA in aqueous solution are reported. Using UV-visible spectroscopy to follow the intense π-π* transitions of DPA as a function of pH gave protonation constants at ionic strength (µ) = 0 and 25 °C of pK(1) = 4.57(3) and pK(2) = 2.90(3). Titration of 10(-5) M solutions of DPA with a variety of metal ions gave log K(1) values as follows: Zn(II), 7.9(1); Cd(II), 8.1(1); Pb(II), 8.3(1); La(III), 5.23(7); Gd(III), 5.7(1); Ca(II), 3.68; all at 25 °C and µ = 0. Log K(1) values at µ = 0.1 were obtained for Mg(II), 0.7(1); Sr(II), 2.20(1); Ba(II), 1.5(1). The log K(1) values show that the high level of preorganization of DPA leads to complexes 3 log units more stable than the corresponding terpyridyl complexes for large metal ions such as La(III) or Ca(II), but that for small metal ions such as Mg(II) and Zn(II) such stabilization is minimal. Molecular mechanics calculations (MM) are used to show that the best-fit M-N length for coordination with DPA is 2.60 Å, accounting for the high stability of Ca(II) or La(III) complexes of DPA, which are found to have close to this M-N bond length in their phen complexes.

Control of metal ion size-based selectivity through chelate ring geometry. metal ion complexing properties of 2,2'-biimidazole.

Some metal ion complexing properties of 2,2'-biimidazole (BIM) are presented. The ligand BIM forms minimum steric strain complexes with hypothetical metal ions with M-N (metal-nitrogen) bond lengths of 4.2 A, in contrast to more usual ligands such as bipy (2,2'-bipyridyl) that prefer metal ions with M-N bond lengths of 2.51 A. This metal ion size-based preference of BIM suggests that ligands with such architecture could be used to produce selectivity (differences in log K(1)) for very large metal ions. To test this hypothesis, the crystal structure of [Pb(BIM)(2)(ClO(4))(2)](2) (1) was determined as the first example of a complex of BIM with a large metal ion. In addition, formation constants (log K(1)) for BIM with metal ions ranging from the very small Cu(II) to the very large Ba(II) ion were determined to examine the effect of the architecture of BIM on metal ion selectivity. The structure of 1 gave: Triclinic, P1, a = 8.314(2) A, b = 8.677(2) A, c = 14.181(3) (A), alpha = 91.143(3) degrees , beta = 104.066(2) degrees , gamma = 106.044(3) degrees , V = 949.5(4) A(3), Z = 1, R = 0.030. Pb(II) in 1 is eight-coordinate, with relatively short Pb-N bonds to the two BIM ligands ranging from 2.366(5) to 2.665(5) A, while the four Pb-O bonds are very long at 2.826(5) to 3.123(5) A. This is typical of the structure of Pb(II) complexes that have a stereochemically active lone pair of electrons, which is postulated to be situated in the vicinity of the long Pb-O bonds. The geometry of the chelate rings formed by BIM with Pb(II) in 1 is analyzed, and it is shown that these are closer in structure to the minimum-strain chelate ring formed by BIM with a very large metal ion than is the case for structures reported in the literature with smaller metal ions. The formation constants (log K(1)) determined for BIM at 25 degrees C in 0.1 M NaClO(4) by UV-visible spectroscopy are as follows: Cu(II), 6.35; Ni(II), 4.89; Zn(II), 3.42; Cd(II), 3.86; Ca(II), -0.2; Pb(II), 3.2; Ba(II), 0.2. The log K(1) values for BIM complexes show that, as expected from the geometry of the chelate ring formed by BIM, the complexes of BIM with small metal ions such as Cu(II) are considerably weaker than with ligands such as bipy, where the ligand architecture is more favorable for forming chelate rings with small metal ions. In contrast, for very large metal ions such as Pb(II) or Ba(II), the log K(1) values for BIM complexes are larger than for bipy. The use of ligand architecture in BIM-type ligands to engineer selectivity for very large metal ions is discussed. Some fluorescence results for BIM and its complexes are presented. BIM itself fluoresces very strongly, while all of its complexes except for Ca(II) show diminished fluorescence intensity, ranging from small shifts and decreases for Ba(II) to very large decreases for Cd(II), which may be due to the distortion of the ligand geometry in its complexes by metal ions that are too small for low-strain coordination with BIM.

Synthesis and reactivity of (benzoxazol-2-ylmethyl)phosphonic acid.

An efficient three step synthesis of (benzoxazol-2-ylmethyl)phosphonic acid (6-H(2)) is described along with IR, mass spectrometry (MS), and (1)H, (13)C, and (31)P NMR spectroscopic characterization data, and a single crystal X-ray diffraction structure determination. 6-H(2) is unstable in acidic aqueous solutions (pH < 4) undergoing ring-opening to give [(2-hydroxyphenylcarbamoyl)methyl] phosphonic acid (7-H(2)) that is characterized by IR, MS, and NMR methods. The protonation constants (pK(a)) for 7-H(2) have been measured, and crystal structure determinations for (NH(4))(7-H) and K(7-H)·DMF are described. Reactions of NaOH and KOH with 6-H(2) in MeOH/H(2)O solutions led to isolation and crystal structure determinations of the salts [Na(6-H)·H(2)O](2), K(6-H), Na(3)(6)(6-H)·H(2)O, and [K(2)(6)](2)·3H(2)O. The complexation reactions of 7-H(2) with La(III), Nd(III), and Gd(III), as a function of pH, were also examined by titrametric methods, and a model for the 1:1 anion binding with Ln(III) cations is proposed.

Dialing in Direct Air Capture of CO2 by Crystal Engineering of Bisiminoguanidines.

Direct air capture (DAC) technologies that extract carbon dioxide from the atmosphere via chemical processes have the potential to restore the atmospheric CO2 concentration to an optimal level. This study elucidates structure-property relationships in DAC by crystallization of bis(iminoguanidine) (BIG) carbonate salts. Their crystal structures are analyzed by X-ray and neutron diffraction to accurately measure key structural parameters including molecular conformations, hydrogen bonding, and π-stacking. Experimental measurements of key properties, such as aqueous solubilities and regeneration energies and temperatures, are complemented by first-principles calculations of lattice and hydration free energies, as well as free energies of reactions with CO2, and BIG regenerations. Minor structural modifications in the molecular structure of the BIGs are found to result in major changes in the crystal structures and the aqueous solubilities within the series, leading to enhanced DAC.

Possible steric control of the relative strength of chelation enhanced fluorescence for zinc(II) compared to cadmium(II): metal ion complexing properties of tris(2-quinolylmethyl)amine, a crystallographic, UV-visible, and fluorometric study.

The idea is examined that steric crowding in ligands can lead to diminution of the chelation enhanced fluorescence (CHEF) effect in complexes of the small Zn(II) ion as compared to the larger Cd(II) ion. Steric crowding is less severe for the larger ion and for the smaller Zn(II) ion leads to Zn-N bond length distortion, which allows some quenching of fluorescence by the photoinduced electron transfer (PET) mechanism. Some metal ion complexing properties of the ligand tris(2-quinolylmethyl)amine (TQA) are presented in support of the idea that more sterically efficient ligands, which lead to less M-N bond length distortion with the small Zn(II) ion, will lead to a greater CHEF effect with Zn(II) than Cd(II). The structures of [Zn(TQA)H(2)O](ClO(4))(2).1.5 H(2)O (1), ([Pb(TQA)(NO(3))(2)].C(2)H(5)OH) (2), ([Ag(TQA)(ClO(4))]) (3), and (TQA).C(2)H(5)OH (4) are reported. In 1, the Zn(II) is 5-coordinate, with four N-donors from the ligand and a water molecule making up the coordination sphere. The Zn-N bonds are all of normal length, showing that the level of steric crowding in 1 is not sufficient to cause significant Zn-N bond length distortion. This leads to the observation that, as expected, the CHEF effect in the Zn(II)/TQA complex is much stronger than that in the Cd(II)/TQA complex, in contrast to similar but more sterically crowded ligands, where the CHEF effect is stronger in the Cd(II) complex. The CHEF effect for TQA with the metal ions examined varies as Zn(II) >> Cd(II) >> Ni(II) > Pb(II) > Hg(II) > Cu(II). The structure of 2 shows an 8-coordinate Pb(II), with evidence of a stereochemically active lone pair, and normal Pb-N bond lengths. In 3, the Ag(I) is 5-coordinate, with four N-donors from the TQA and an oxygen from the perchlorate. The Ag(I) shows no distortion toward linear 2-coordinate geometry, and the Ag-N bonds fall slightly into the upper range for Ag-N bonds in 5-coordinate complexes. The structure of 4 shows the TQA ligand to be involved in pi-stacking between quinolyl groups from adjacent TQA molecules. Formation constants determined by UV-visible spectroscopy are reported in 0.1 M NaClO(4) at 25 degrees C for TQA with Zn(II), Cd(II), and Pb(II). When compared with other similar ligands, one sees that, as the level of steric crowding increases, the stability decreases most with the small Zn(II) ion and least with the large Pb(II) ion. This is in accordance with the idea that TQA has a moderate level of steric crowding and that steric crowding increases for TQA analogs tris(2-pyridylmethyl)amine (TPyA) < TQA < tris(6-methyl-2-pyridyl)amine (TMPyA).

Complexation of mercury(I) and mercury(II) by 18-crown-6: hydrothermal synthesis of the mercuric nitrite complex.

A dimercury(I) 18-crown-6 complex is isolated, and its possible role in the hydrothermal preparation of the mercuric nitrite complex is discussed. The reported structures are of [Hg(2)(18-crown-6)(2)(H(2)O)(2)](ClO(4))(2) (1), monoclinic, C2/c, a = 21.0345(9), b = 12.1565(5), c = 16.8010(7) A, beta = 113.2000(10) degrees , V = 3948.7(3) A(3), Z = 16, R = 0.0230; [Hg(18-crown-6)](NO(2))(2) (2), monoclinic, P2(1)/c, a = 8.027(5), b = 14.437(9), c = 7.827(5) A, beta = 95.165(11) degrees , V = 905.6(10) A(3), Z = 2, R = 0.0175. The complex cation in compound 1 consists of a mercurous dimer exhibiting a Hg-Hg bond length of 2.524(2) A. Non-bonding interactions between adjacent crown ether macrocycles across the Hg-Hg bond result in large variations in mercury to oxygen distances within equatorial coordination sites. At low pH compound 1 is proposed to be preferentially formed under hydrothermal conditions affording compound 2 upon disproportionation. Nitrite ions ligate via a unidentate nitrito (cis to metal) coordination mode as interpreted using vibrational (infrared) spectroscopy. The conformation adopted by 18-crown-6 in compounds 1 and 2 is closely related to a D(3d) conformation as evidenced by X-ray crystallography. Band splitting readily observed in vibrational spectra of the metal free crown ether, attributed to vibrational modes of oxyethylene fragments, is absent in spectra of 1 and 2 confirming a regular D(3d) macrocyclic orientation. Short Hg-O bonds observed for axially coordinated water molecules in 1 and coordinated nitrite ligands in 2, illustrate the prevalence of relativistic effects commonly observed in mercury complexes.

Complexation of metal ions of higher charge by the highly preorganized tetradentate ligand 2,9-bis(hydroxymethyl)-1,10-phenanthroline. A crystallographic and thermodynamic study.

The metal ion selectivity for M(III) (M = metal) ions exhibited by the highly preorganized ligand PDALC is investigated (PDALC = 2,9-bis(hydroxymethyl)-1,10-phenanthroline). The structures are reported of [Bi(PDALC)(H(2)O)(2)(ClO(4))(3)] x H(2)O (1), monoclinic, P2(1)/c, a = 12.8140(17), b = 19.242(3), c = 9.2917(12) A, beta = 91.763(2) degrees, V = 2289.9(5) A(3), Z = 4, R = 0.0428; [Th(PDALC)(NO(3))(4)] x 3 H(2)O (2), monoclinic, P2(1)/n, a = 7.876(3), b = 22.827(9), c = 12.324(5) A, beta = 94.651(6) degrees, V = 2208.4(15) A(3), Z = 4, R = 0.0669; [Cd(PDALC)(2)](ClO(4))(2) (3)), triclinic, P1, a = 7.5871(16), b = 13.884(3), c = 14.618(3) A, alpha = 74.081(2) degrees, beta = 88.422(2) degrees, gamma = 78.454(2) degrees, V = 1450.2(5) A(3), Z = 2, R = 0.0267. The Bi in 1 is best regarded as 9-coordinate, with four short bonds to the PDALC, and two short bonds to the coordinated water molecules, with three long bonds to perchlorate oxygens. The Bi-N bonds at 2.35 A are by a considerable margin the shortest Bi-N bonds to 1,10-phenanthroline (phen) type ligands, which is suggested to be due to the Bi adapting to the metal ion size requirements of PDALC. The Th(IV) in 2 is 12-coordinate, with four bonds to PDALC, and the four chelated nitrates, with close to normal bond lengths to the PDALC ligand. The Cd(II) in 3 is 8-coordinate, with Cd-N and Cd-O bonds that are similar to those found in other 8-coordinate Cd(II) complexes. The five known structures of PDALC complexes, including the three reported here, suggest that the M-N bonds to PDALC are quite easily varied in length in response to differing metal ion sizes, but that the M-O bonds are more constrained by the rigid ligand to be close to the ideal value of 2.50 A. The formation constants (log K(1)) for M(III) ions with PDALC show that for small metal ions such as Ga(III) and Fe(III), log K(1) is only slightly higher than for phen, suggesting that these metal ions are too small to coordinate to the alcoholic oxygen donors of PDALC. For larger metal ions such as Bi(III), Gd(III), Th(IV), and UO(2)(2+), log K(1) for PDALC is higher than log K(1) for phen by more than 5 log units, which stabilization is attributed to the fact that PDALC is preorganized for complexation with large metal ions with an ionic radius of about 1.0 A. The fluorescence of M(III) complexes of PDALC is discussed. PDALC free ligand gives fluorescence typical of phen ligands, with the protonated form giving a broad less intense band, and the non-protonated form of the ligand giving an intense structured set of bands. Large lanthanide ions without partially filled f-subshells, such as La(III), Lu(III), and also Y(III), give a fairly strong CHEF (chelation-enhanced fluorescence) effect, while those with partially filled f-subshells, such as Gd(III), Yb(III), and Tb(III), strongly quench the fluorescence of PDALC. A heavy element such as Bi(III) has strong spin-orbit coupling effects that act to quench the fluorescence of PDALC almost completely, which effect is enhanced by the covalence of the Bi-N bonds.

Strong metal ion size based selectivity of the highly preorganized ligand PDA (1,10-phenanthroline-2,9-dicarboxylic acid) with trivalent metal ions. A crystallographic, fluorometric, and thermodynamic study.

The selectivity of the rigid ligand PDA (1,10-phenanthroline-2,9-dicarboxylic acid) for some M(III) (M = metal) ions is presented. The structure of [Fe(PDA(H)(1/2))(H(2)O)(3)] (ClO(4))(2).3H(2)O.(1)/(2)H(5)O(2) (1) is reported: triclinic, P1, a = 7.9022(16) A, b = 12.389(3) A, c = 13.031(3) A, alpha = 82.55(3) degrees , beta = 88.41(3) degrees , gamma = 78.27(3) degrees , V = 1238.6(4) A(3), Z = 2, R = 0.0489. The coordination geometry around the Fe(III) is close to a regular pentagonal bipyramid, with Fe-N lengths averaging 2.20 A, which is normal for a 1,10-phenanthroline type of ligand coordinated to seven-coordinate Fe(III). The Fe-O bonds to the carboxylate oxygens average 2.157 A, which is rather long compared to the average Fe-O length of 2.035 A to carboxylates in seven-coordinate Fe(III) complexes. The structure of 1 supports the idea that the Fe(III) is too small for ideal coordination in the cleft of PDA, and the structure shows that the Fe(III) adapts to this by inducing numerous small distortions in the structure of the PDA ligand. The log K(1) values for PDA at 25 degrees C in 0.1 M NaClO(4) were determined by UV spectroscopy with Al(III) (log K(1) = 6.9), Ga(III) (log K(1) = 9.7), In(III) (log K(1) = 19.7), Fe(III) (log K(1) = 20.0), and Bi(III) (log K(1) = 26.2). The low values of log K(1) for PDA with Al(III) and Ga(III) are because these ions are too small for the cleft in PDA, which requires a large metal ion with an ionic radius (r(+)) of 1.0 A. In(III) and Fe(III) (r(+) = 0.86 and 0.72 A for a coordination number (CN) of 7) are somewhat too small for the cleft in PDA but may adapt by increasing the coordination number, which increases the metal ion size, and have high log K(1) values. Very large log K(1) values are found, as expected, for Bi(III) (r(+) = 1.17 A, CN = 8), which fits the cleft quite well. Fluorescence studies show that Y(III) produces the largest CHEF (chelation enhanced fluorescence) effects, followed by La(III) and Lu(III), in the PDA complexes. Metal ions with nonfilled d or f subshells produce very large quenching of the fluorescence, as do heavy-metal ions such as In(III) and Bi(III), which have large spin-orbit coupling effects. The Al(III)/PDA complex produced an intense broad band at longer wavelength than the pi*-pi emissions of the PDA ligand, which is at a maximum at pH 6, and the possibility that this might reflect an exciplex, where one PDA ligand in the Al(III) complex pi-stacks with the excited state of a second PDA ligand, is discussed.

Enhancing selectivity of cation exchange with anion receptors.

We introduce a new supramolecular strategy where an anion receptor modifies the selectivity ligands for cations. This is demonstrated by combining the classic anion receptor calix[4]pyrrole (C4P) and a phenolic ligand, which leads to remarkable enhancement in selectivity for Cs+ over Na+. Crystal structures and molecular simulations confirmed the persistent formation of ion-pair C4P-Cs+-phenolate complexes, while the smaller Na+ ion cannot efficiently interact.

Peroxide-treated metal-organic framework templated adsorbents for remediation of high level nuclear waste.

Remediation of legacy nuclear waste is one of the greatest challenges faced by the US Department of Energy, with projected cleanup efforts requiring over five decades and hundreds of billions of dollars. New materials are necessary to accelerate waste processing, achieving time and financial savings. Herein we report a peroxide treatment to a Ti metal-organic framework (MOF) and related MOF-templated adsorbents. The resulting materials displayed exceptional affinity for Am(III), achieving distribution coefficients in excess of 105 mL/g, and out-performing state-of-the-art benchmarks monosodium titanate (MST) and peroxo-treated modified MST (mMST) for removal of 85Sr(II) and 239, 240Pu(IV) from legacy nuclear waste simulant.