Selective Complexation Of Metal Ions Research Articles

Predicting stability constants of transition metals; Y3+, La3+, and UO2 2+ with organic ligands using the 3D-QSPR methodology

Stability constants prediction plays a critical role in the identification and optimization of ligand design for selective complexation of metal ions in solution. Thus, it is important to assess the potential of metal-binding ligand organic in the complex formation process. However, quantitative structure-activity/property relationships (QSAR/QSPR) provide a time-and cost-efficient approach to predict the stability constants of compounds. To this end, we applied a free alignment three-dimensional QSPR technique by generating GRid-INdependent Descriptors (GRINDs) to rationalize the underlying factors effecting on stability constants of transition metals; 105 (Y3+), 186 (La3+), and 66 (UO2 2+) with diverse organic ligands in aqueous solutions at 298 K and an ionic strength of 0.1 M. Kennard– Stone algorithm was employed to split data set to a training set of 75% molecules and a test set of 25% molecules. Fractional factorial design (FFD) and genetic algorithm (GA) applied to derive the most relevant and optimal 3 D molecular descriptors. The selected descriptors using various feature selection were correlated with stability constants by partial least squares (PLS). GA-PLS models were statistically validated ( = 0.96, q2 = 0.82 and R2 pred=0.81 for Y3+;  = 0.90, q2 = 0.73 and R2 pred=0.82 for La3+ and  = 0.95, q2 = 0.81 and R2 pred=0.88 for UO2 2+), and from the information derived from the graphical results confirmed that hydrogen bonding properties, shape, size, and steric effects are the main parameters influencing stability constant of metal complexation. The provided information in this research can predict the stability constant of the new organic ligand with the transition metals without experimental processes.

Conformational Stability Effect of Polymeric Iron Chelators.

SummaryThe design and synthesis of metal chelators with extraordinary metal affinities is a basic and challenging scientific problem of both fundamental and practical importance. Here, we demonstrate a “conformational stability effect” that can significantly enhance the metal affinity of ligands after conjugation to polymer chains with the ability to spontaneously adopt a specific conformation as an optimal “soft” scaffold to ensure maximum thermodynamic stability of the metal complexes. Using iron chelators as models, we show that simple conjugation of small molecule catechol ligands to a polyallylamine chain resulted in more than 8–9 orders of magnitude enhancement of the iron-binding affinity, which is comparable to that of enterobactin, the strongest iron chelator ever known. This study demonstrates that flexible polymer chelators may realize the highest possible metal affinities of the conjugated ligands owing to their ability to achieve an optimal conformation, which could advance the identification of strong metal chelators.

ChemInform Abstract: The Pyridyl Group in Ligand Design for Selective Metal Ion Complexation and Sensing

AbstractReview: 173 refs.

The pyridyl group in ligand design for selective metal ion complexation and sensing

Factors in polypyridyl ligands that control their thermodynamic metal ion selectivity in aqueous solution, and their use in selective fluorescent sensing, are examined. Preorganization of polypyridyl ligands ranging from bidentate to tetradentate by bridging benzo groups, as are present in 1,10-phenanthroline (phen) compared to 2,2'-bipyridyl (bpy), is discussed. The role of solvation is considered in relation to the relative affinity of ligands containing pyridyl groups for divalent and trivalent metal ions in aqueous solution. The effects of steric clashes between H atoms on polypyridyl ligands in decreasing complex stability are evaluated, as well as the effect of chelate ring size on metal ion selectivity. Phen ligands with other donor groups present at the 2 and 9 positions, such as alcohols, amides, carboxylates, and oximes are discussed. The design of pyridyl-based ligands for the separation of Am(III) from lanthanide(III) ions is considered, as well as ligands for the removal of metal ions such as Cu(II) or Zn(II) in neurological diseases such as Alzheimer's. The design of pyridyl-based fluorescent sensors for selective sensing of metal ions is examined in terms of the role of spin-orbit coupling constants (ζ), paramagnetism, and steric effects in the development of selective fluorescent sensors that operate via chelation enhanced fluorescence (CHEF). It is concluded that for lighter metal ions with smaller ζ values such as Zn(II) and Ca(II), and to a lesser extent Cd(II), that the CHEF effect can be achieved with pyridyl-containing fluorophores that coordinate directly to the metal ion. The way in which steric effects can be used to decrease the CHEF effect in Zn(II) relative to Cd(II) to enable selective sensing of the latter is analyzed. For heavier metal ions such as Hg(II) and Pb(II), because of their large ζ values which quench fluorescence, it is concluded that the fluorophore should be tethered to the metal-binding part of the sensor, and prevented from binding to the metal ion by steric and electronic factors. How Hg(II) can quench the CHEF effect by π-contact with fluorophores such as the anthracenyl group, which at first sight might not seem able to bond with metal ions, is examined.

Highly selective complexation of metal ions by the self-tuning tetraazacalixpyridine macrocycles

By means of UV–vis and 1H NMR titrations and X-ray crystallography, complexation of tetramethylazacalix[4]pyridine L1 and tetramethylazacalix[2]arene[2]pyridine L2 with metal ions was studied. While no interaction was observed with alkali and alkaline earth metal ions, both ligands have been found to act as powerful and selective macrocyclic hosts to complex a number of transition and heavy metal ions. Due to the intrinsic nature of the bridging nitrogen atoms that can adopt different electronic configurations and form varied degrees of conjugations with their adjacent pyridine rings, tetramethylazacalix[4]pyridine L1 regulated its conformation and cavity structure to best fit the guest metal ion species, yielding a 1:1 square planar L1–M n + complex with binding constants log K 1:1 ranging from 2.7(1) to 8.2(8).

New polymer-supported ion-complexing agents: Design, preparation and metal ion affinities of immobilized ligands

Polymer-supported reagents are comprised of crosslinked polymer networks that have been modified with ligands capable of selective metal ion complexation. Applications of these polymers are in environmental remediation, ion chromatography, sensor technology, and hydrometallurgy. Bifunctional polymers with diphosphonate/sulfonate ligands have a high selectivity for actinide ions. The distribution coefficient for the uranyl ion from 1 M nitric acid is 70,000, compared to 900 for the monophosphonate/sulfonate polymer and 200 for the sulfonic acid ion-exchange resin. A bifunctional trihexyl/triethylammonium polymer has a high affinity and selectivity for pertechnetate and perchlorate anions from groundwater. In one example, its distribution coefficient for perchlorate ions in the presence of competing anions is 3,300,000, compared to 203,180 for a commercially available anion-exchange resin. Polystyrene modified with N-methyl- d-glucamine ligands is capable of selectively complexing arsenate from groundwater. It complexes 99% of the arsenate present in a solution of 100 mg/L arsenate with 560 mg/L sulfate ions. Its selectivity is retained even in the presence of 400 mg/L phosphate. There is no affinity for arsenate above pH 9, allowing for the polymer to be regenerated with moderate alkali solution. In studies aimed at developing a Hg(II)-selective resin, simple amine resins were found to have a high Hg(II) affinity and that affinity is dependent upon the solution pH and the counterion.

Comparison of Copper Speciation in Estuarine Water Measured Using Analytical Voltammetry and Supported Liquid Membrane Techniques

The supported liquid membrane (SLM) is a promising separation and preconcentration technique that is well-suited for trace metal speciation in natural waters. The technique is based on the selective complexation of metal ions by a hydrophobic ligand (carrier) dissolved in a water-immiscible organic solvent immobilized in a porous, inert membrane. This membrane separates two aqueous solutions: the test (or donor) solution and the strip (or acceptor) solution. The metal carrier complex is transported by diffusion across the membrane from the source to the strip solution where metal ions are back-extracted. The technique offers great potential to tune the selectivity by incorporating different complexing ligands in the membrane. A SLM was used to analyze the dissolved (<0.45 microm) copper speciation from two sites in the San Francisco Bay estuary; Dumbarton Bridge, [Cu]total approximately 27 nM, and San Bruno Shoals, [Cu]total approximately 23 nM. The sites were also characterized independently by differential pulse anodic stripping voltammetry (DPASV) using a Nafion-coated thin mercury film electrode (NCTMFE). The SLM employed 10 mM lasalocid, a naturally occurring carboxylic polyether ionophore, in nitrophenyl octyl ether (NPOE) asthe membrane complexing ligand, supported by a microporous, polypropylene, hydrophobic membrane. This is the first study where SLM technique has been compared with an independent speciation technique in marine waters. Results of copper speciation measurements from Dumbarton Bridge, a site in South San Francisco Bay where copper speciation has been well-characterized in previous studies using various voltammetric techniques, indicated that only about 3% (0.9 nM) of the total dissolved copper was SLM labile. The corresponding DPASV labile copper fraction was <0.4% (<0.1 nM) of total dissolved copper. The concentration of total copper binding ligands measured by the membrane technique was 471 nM as compared to 354 nM measured by DPASV, more than 1 order of magnitude higher than the total dissolved copper concentration. The SLM measurements were consistent with earlier copper speciation measurements that were made in South San Francisco Bay using other voltammetric stripping techniques.

Some recent developments in ion-exchange resins

change have been published in recent years. Beauvais and Alexandratos gave an overview of polymer-supported reagents for the selective complexation of metal ions.3 Abrams and Millar published a history of the origin and development of macroporous ion-exchange resins.4 The literature indicates an increasing interest in the use of ion-exchange processes for applications in the environmental fields. The recently published Environmental Ion Exchange: Principles and Design gives a comprehensive overview of ion exchange’s application to treating water/wastewater.5 Also, a number of recent U.S. patents involve ion-exchange resins in the areas of waste treatment and minimization. Two patents were issued for the removal of heavy metal cations from organic solutions, while another was issued for the removal of anions from organic solutions.6 One utilizes a chelating resin for removal,7 while the other uses a mixture of a chelating resin and an anion-exchange resin.8 Two

Selective hydrolysis of polymer-bound ethoxycarbonylethyl phosphonate and e.p.r. studies of copper(II) complexes with the parent resin and its derivatives

New ion exchange/coordination resins for the selective complexation of metal ions have been obtained by selective hydrolysis of a polymer-bound ligand—ethoxycarbonylethyl phosphonate. Their properties such as water regain, phosphorus content, acid capacity and sorption properties towards copper(II) from nitric acid solutions were determined. Structures of ligand complexes with copper(II) were studied using electron paramagnetic resonance and compared to the structures of complexes formed by diphosphonate and dicarboxylate ligands.

Synthesis and characterization of a bifunctional ion exchange resin with polystyrene-immobilized diphosphonic acid ligands

A new ion exchange resin for the selective complexation of metal ions has been synthesized by functionalizing vinylbenzyl chloride-styrene-divinylbenzene copolymer beads with the sodium salt of tetra(isopropyl) methylene diphosphonate. The effects of bifunctionality, matrix rigidity, degree of functionalization, and macroporosity on final resin properties have been quantified. A sulfonic acid-diphosphonic acid bifunctional resin is highly selective with rapid complexation kinetics. A macroporous polymer matrix crosslinked with 10% divinylbenzene provides optimum results; for example, 99.7% Eu(III) is complexed from a 1M nitric acid solution with a 30-min contact time. The importance of physical crosslinking as well as chemical crosslinking in limiting access of substrates into polymer-supported reagents is discussed. © 1996 John Wiley & Sons, Inc.

Ligand design for selective complexation of metal ions in aqueous solution

Ligand design for selective complexation of metal ions in aqueous solution