Heterogeneity and lability of Pb(II) complexation by humic substances: practical interpretation tools

The complexation of Pb(II) by natural organic matter (NOM) is better described by taking into account the dependence of the strength of binding on metal loading conditions. The utility of a linear differential equilibrium function for interpretation of metal ion binding data is demonstrated. This approach considers the binding intensity (log K*) as a function of metal ion loading (ı = bound metal/binding site concentration). Three methods for calculating this function are presented: – direct calculation from metal titration curves, – direct calculation from polarograms, and – compilation of data derived from interpretation of complexation in terms of one- or two- binding sites (e.g. Scatchard analysis), i.e. Cc (complexation capacity = effective site concentration)–K pairs. Heterogeneity also impacts on the apparent lability of complexes; complexes formed at the lowest metal loadings are the least labile. Received: 28 December 2000 / Revised: 23 February 2001 / Accepted: 28 February 2001     

Carbonate adsorption on goethite in competition with phosphate

Competitive interaction of carbonate and phosphate on goethite has been studied quantitatively. Both anions are omnipresent in soils, sediments, and other natural systems. The PO4-CO3 interaction has been studied in binary goethite systems containing 0-0.5 M (bi)carbonate, showing the change in the phosphate concentration as a function of pH, goethite concentration, and carbonate loading. In addition, single ion systems have been used to study carbonate adsorption as a function of pH and initial (H)CO3 concentration. The experimental data have been described with the charge distribution (CD) model. The charge distributions of the inner-sphere surface complexes of phosphate and carbonate have been calculated separately using the equilibrium geometries of the surface complexes, which have been optimized with molecular orbital calculations applying density functional theory (MO/DFT). In the CD modeling, we rely for phosphate on recent parameters from the literature. For carbonate, the surface speciation and affinity constants have been found by modeling the competitive effect of CO3 on the phosphate concentration in CO3-PO4 systems. The CO3 constants obtained can also predict the carbonate adsorption in the absence of phosphate very well. A combination of inner- and outer-sphere CO3 complexation is found. The carbonate adsorption is dominated by a bidentate inner-sphere complex, (FeO)2CO. This binuclear bidentate complex can be present in two different geometries that may have a different IR behavior. At a high PO(4) and CO3 loading and a high Na+ concentration, the inner-sphere carbonate complex interacts with a Na+ ion, probably in an outer-sphere fashion. The Na+ binding constant obtained is representative of Na-carbonate complexation in solution. Outer-sphere complex formation is found to be unimportant. The binding constant is comparable with the outer-sphere complexation constants of, e.g., SO(2-)4 and SeO(2-)4.     

Host-guest preorganization and complementarity: A molecular mechanics and molecular dynamics study of cation complexes of a cyclic urea-anisole spherand

Molecular mechanics and molecular dynamics calculations were carried out in vacuo for 1 and for the complexes of 1 with alkali metal cations and t-BuNH. The calculations identify perching and nesting conformations of the complexes not available from X-ray data. For the Li⁺ ? 1 complex, the MD simulations identify a new global minimum not found by the molecular mechanics calculation. In general, the net favorable ion-spherand complexation energy is due to the offset of the unfavorable reorganization energy of the spherand by the overwhelmingly favorable electrostatic component of the ion-spherand interaction energy. The host is least preorganized for the binding of Li⁺ and, even in its complexed conformation, presents the least steric complementarity to this ion. The complexation energy becomes significantly more favorable due to a large increase in the electrostatic complementarity of the ion binding site when the spherand adopts its complexed conformation. Correction of the calculated complexation energy by the experimental free energy of ion aqueous desolvation leads to results in line with the findings of Cram and co-workers that K⁺ is the most, and Li⁺ the least, favorably bound by 1 .     

Structural Criteria for the Rational Design of Selective Ligands. 2. Effect of Alkyl Substitution on Metal Ion Complex Stability with Ligands Bearing Ethylene-Bridged Ether Donors

A novel approach is presented for the application and interpretation of molecular mechanics calculations in ligand structural design. The methodology yields strain energies that (i) provide a yardstick for the measurement of ligand binding site organization for metal ion complexation and (ii) allow the comparison of any two ligands independent of either the number and type of donor atoms or the identity of the metal ion. Application of this methodology is demonstrated in a detailed examination of the influence of alkyl substitution on the structural organization of ethylene-bridged, bidentate, ether donor ligands for the alkali and alkaline earth cations. Nine cases are examined, including the unsubstituted ethylene bridge (dimethoxyethane), all possible arrangements of individual alkyl groups (monoalkylation, gem-dialkylation, meso-dialkylation,d,l-dialkylation, trialkylation, and tetraalkylation), and both cis and trans attachments of the cyclohexyl group. The calculated degree of binding site organization for metal ion complexation afforded by these connecting structures is shown to correlate with known changes in complex stability caused by alkyl substitution of crown ether macrocycles.     

Use of Two-Column Ion Chromatography for the Separation of Transition Metal Complexes of 1,2-Cyclohexanediaminetetraacetic Acid and Study of Complexation in Alkaline Solutions

The ion-chromatographic behavior of anionic transition metal complexes of 1,2-cyclohexanediaminetetraacetic acid (CHDTA) on polymeric anion exchangers was studied. It was demonstrated that two-column ion chromatography can be used for studying the complexation of CHDTA with transition metal ions in alkaline solutions. A method was proposed for the calculation of equilibrium constants of complexation reactions and stability constants from the data of ion-chromatographic measurements.     

Structure and properties of a series of 2-cinnamoyl-1,3-indandiones and their metal complexes

A series of seven 2-cinnamoyl-1,3-indandiones and their metal(II) complexes were synthesized and characterized by means of spectroscopic (IR, NMR, electron absorption and emission spectroscopy) and/or single-crystal X-ray diffraction methods. The optical spectra of the organic compounds show very strong absorption in the visible region and weak fluorescence with moderate to strong Stokes shift. The effect of concentration, water addition and metal ion complexation on the optical properties was also studied. In search of potential practical application, the complexation of 2-cinnamoyl-1,3-indandiones with metal(II) ions was investigated. A series of non-charged complexes with Cu(II), Cd(II), Zn(II), Co(II) and Ni(II) was isolated and analyzed by elemental analyses and IR. Most of the complexes show presence of water molecules, most probably coordinated to the metal ion, thus forming octahedral geometry. For the paramagnetic Cu(II) complexes a distorted, flattened tetrahedral structure is proposed, basing on the EPR data. The optical properties of the metal complexes, however, do not differ appreciably from those of the free ligands.     

Mechanism of host-guest complexation by cucurbituril

The factors affecting host-guest complexation between the molecular container compound cucurbit[6]uril (CB6) and various guests in aqueous solution are studied, and a detailed complexation mechanism in the presence of cations is derived. The formation of the supramolecular complex is studied in detail for cyclohexylmethylammonium ion as guest. The kinetics and thermodynamics of complexation is monitored by NMR as a function of temperature, salt concentration, and cation size. The binding constants and the ingression rate constants decrease with increasing salt concentration and cation-binding constant, in agreement with a competitive binding of the ammonium site of the guest and the metal cation with the ureido carbonyl portals of CB6. Studies as a function of guest size indicate that the effective container volume of the CB6 cavity is approximately 105 A(3). It is suggested that larger guests are excluded for two reasons: a high activation barrier for ingression imposed by the tight CB6 portals and a destabilization of the complex due to steric repulsion inside. For example, in the case of the nearly spherical azoalkane homologues 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH, volume ca. 96 A(3)) and 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO, volume ca. 110 A(3)), the former forms the CB6 complex promptly with a sizable binding constant (1300 M(-1)), while the latter does not form a complex even after several months at optimized complexation conditions. Molecular mechanics calculations are performed for several CB6/guest complexes. A qualitative agreement is found between experimental and calculated activation energies for ingression as a function of both guest size and state of protonation. The potential role of constrictive binding by CB6 is discussed.     

Immobilized tris(hydroxymethyl)aminomethane as a scaffold for ion-selective ligands: the auxiliary group effect on metal ion binding at the phosphate ligand

The metal ion affinities of a ligand in a polymer-supported reagent can be enhanced by the presence of a proximate group capable of hydrogen bonding. A new polymer-supported reagent has been synthesized by immobilizing tris(hydroxymethyl)aminomethane (Tris) onto cross-linked poly(vinylbenzyl chloride) and then phosphorylating the -OH moieties. The -NH- acts as the auxiliary group to increase the extent of complexation by the phosphate ligand. Additionally, Tris acts as a scaffold, wherein the phosphate ligands are in a known stereochemical arrangement. The Tris resin is mono-, di-, and triphosphorylated, depending on the concentration of the phosphorylating agent. The highest metal ion affinities are found with the resin having a phosphorus-to-nitrogen ratio of 2.36, consistent with one-third of the ligands being triphosphorylated and the remainder being diphosphorylated. The unphosphorylated Tris and phosphonate diester resins have no ionic affinities under the same conditions. Trivalent ions (Fe(III), Al(III), La(III), Eu(III), Lu(III)) are preferred over divalent ions (Pb(II), Cd(II), Cu(II), Zn(II)) from solutions at pH 2. The distribution coefficients of the divalent ions correlate with the Misono softness parameters, indicating that the polarizability of the phosphoryl oxygen is important to binding of the metal ions. The mechanism of complexation is probed with Fe(III) in 0.01-5 M HNO3 and HCl. The high affinities are ascribed to activation of the P=O ligand toward metal ion binding by the N-H moieties acting as auxiliary groups, coupled with intraligand cooperation among the phosphate moieties at a given site. FTIR spectra show that the P=O band at 1261 cm-1 shifts as a function of the extent of hydrogen bonding. Binding at the P=O requires a balance between activation by hydrogen bonding and availability of the lone pair electrons to the metal ions.     

Cation-π interactions of curved polycyclic systems: M (M=Li and Na) ion complexation with buckybowls

B3LYP/6-311+G** calculations on alkali metal ion (Li⁺ and Na⁺) complexation with corannulene and sumanene indicate stronger binding compared to [5]-radialene or benzene. The dependence of binding to the convex and concave site is marginal, albeit the preference was consistent for convex binding in the range of 1-4 kcal/mol. The bowl-to-bowl inversion barriers are only marginally affected, below 2 kcal/mol, by metal ion complexation.     

The 13C NMR of olefins complexed with a binuclear Ag(I)–Yb(III) chelate

The effect of silver ion complexation on the ¹³C NMR of several rigid olefin structures has been determined. The silver ion induced chemical shifts (AgIS) are not amenable to easy interpretation. Addition of Yb(fod)₃ forms binuclear Ag–Yb complexes with the olefin. Lanthanide induced shifts (LIS) fall off rationally with distance from the site of complexation and the averaged position of the lanthanide. The complexes may be used as probes of olefin stereochemistry.     

Determination of alkali metal ion binding selectivities of calixarenes by matrix-assisted laser desorption ionization and electrospray ionization in a quadrupole ion trap

Matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) and electrospray ionization mass spectrometry (ESI-MS) are used to evaluate the alkali metal ion binding selectivities of a series of calixarenes. Each calixarene of interest is mixed with one or more alkali metal salts (1:100 ratio of calixarene to metal), either in the ESI solution or on the MALDI probe surface, and the relative binding selectivities are directly determined from the intensities of the calixarene/metal complexes in the mass spectra. For t-butylcalix[4]arene-tetraacetic acid tetraethyl ester (calixarene 1), complexation of Na⁺ is favored over complexation of K⁺, in agreement with prior solution results obtained by conventional methods. For the three calixarenes that do not have t-butyl groups on the upper rims, the calixarenes preferentially bind K⁺ over Na⁺, thus demonstrating that size selective complexation can be probed with both the ESI and MALDI methods. Collision-activated dissociation results indicate that the phenyl oxygens, but not necessarily the ethoxy ethyl oxygens of the lower rims, are the primary binding sites for the alkali metal ions.     

Complexation of flavonoids with iron: structure and optical signatures

Flavonoids exhibit antioxidant behavior believed to be related to their metal ion chelation ability. We investigate the complexation mechanism of several flavonoids, quercetin, luteolin, galangin, kaempferol, and chrysin, with iron, the most abundant type of metal ions in the body, through first-principles electronic structure calculations based on density functional theory (DFT). We find that the most likely chelation site for Fe is the 3-hydroxyl-4-carbonyl group, followed by 4-carbonyl-5-hydroxyl group and the 3'-4' hydroxyl (if present) for all of the flavonoid molecules studied. Three quercetin molecules are required to saturate the bonds of a single Fe ion by forming six orthogonal Fe-O bonds, though the binding energy per molecule is highest for complexes consisting of two quercetin molecules and one Fe atom, in agreement with experiment. Optical absorption spectra calculated with time-dependent DFT serve as signatures to identify various complexes. For the iron-quercetin complexes, we find a redshift of the first absorbance peak upon complexation in good agreement with experiment; this behavior is explained by the narrowing of the optical gap of quercetin because of Fe(d)-O(p) orbital hybridization.     

Equilibrium ion exchange method: methodology at low ionic strength and copper(II) complexation by dissolved organic matter in a leaf litter extract

The equilibrium ion exchange method (EIM) is a powerful tool for the investigation of metal cation complexation by dissolved organic matter (DOM) in natural systems. Tests with different ion exchange resins demonstrated that under low ionic strength conditions (0.01 mol/kg) and in the presence of DOM, equilibration times of at least 24 h are required for experiments with Cu(II). The classical approach to the EIM was modified by using nonlinear reference adsorption isotherms in order to expand the method to a broader range of experimental conditions. For Cu(II) at low ionic strength (0.01 mol/kg), the reference isotherms between pH 4 and 6 were identical and were mathematically modeled in terms of Langmuir adsorption parameters. The EIM using nonlinear reference isotherms was validated between pH 4 and 6 by the correct determination of the stability constants for the complexes CuOxalate and Cu(Oxalate)(2). Then the method was used to quantitatively characterize the Cu(II) complexation behavior of DOM in an aqueous chestnut leaf litter extract between pH 4 and 6. In contrast to the classical approach to the EIM, data were analyzed by using plots [Cu](bound)/[Cu](free)vs. [Cu](bound). This allowed the determination of both, conditional stability constants and metal binding capacities for two different binding site classes. The logarithmic values of the stability constants were about 8 for the strong binding sites and 5.5-6 for the weak binding sites. The total Cu(II) binding capacity increased from 0.22 mol/(kg C) at pH 4 to 2.85 mol/(kg C) at pH 6.     

Geometry, charge distribution, and surface speciation of phosphate on goethite

The surface speciation of phosphate has been evaluated with surface complexation modeling using an interfacial charge distribution (CD) approach based on ion adsorption and ordering of interfacial water. In the CD model, the charge of adsorbed ions is distributed over two electrostatic potentials in the double-layer profile. The CD is related to the structure of the surface complex. A new approach is followed in which the CD values of the various surface complexes have been calculated theoretically from the geometries of the surface complexes. Molecular orbital calculations based on density functional theory (MO/DFT) have been used to optimize the structure of a series of hydrated surface complexes of phosphate. These theoretical CD values are corrected for dipole orientation effects. Data analysis of the PO4 adsorption, applying the independently derived CD coefficients, resolves the presence of two dominant surface species. A nonprotonated bidentate (B) complex is dominant over a broad range of pH values at low loading (< or =1.5 micromol/m(2)). For low pH and high loading, a strong contribution of a singly protonated monodentate (MH or MH-Na) complex is found, which differs from earlier interpretations. For the conditions studied, the doubly protonated bidentate (BH2) and monodentate (MH2) surface complexes and the nonprotonated monodentate (M) complex are not significant contributors. These findings are discussed qualitatively and quantitatively in relation to published experimental in-situ CIR-FTIR data and theoretical MO/DFT-IR information. The relative variation in the peak intensities as a function of pH and loading approximately agrees with the surface speciation calculated with the CD model. The model correctly predicts the proton co-adsorption of phosphate binding on goethite and the shift of the IEP at low phosphate loading (< or =1.5 micromol/m(2)). At higher loading, it deviates.     

Investigation of heptakis(2,6-di-O-methyl)-beta-cyclodextrin inclusion complexes with flavonoid glycosides by electrospray ionization mass spectrometry

The non-covalent complexes between three flavonoid glycosides (quercitrin, hyperoside and rutin) and heptakis(2,6-di-O-methyl)-beta-cyclodextrin (DM-beta-CD) were investigated by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS). The 1:1 complexation of each flavonoid glycoside (guest) to the DM-beta-CD (host) was monitored in the negative ion mode by mixing each guest with an up to 30-fold molar excess of the host. The binding constants for all complexes were calculated by a linear equation in the order: DM-beta-CD:quercitrin > DM-beta-CD:rutin > DM-beta-CD:hyperoside. A binding model for the complexes has also been proposed based on the binding constants and tandem mass spectrometric data of these complexes.     

Effect of phosphate on sorption of Eu(III) by montmorillonite

Eu(III) sorption by Na-montmorillonite, the principal component of bentonite, has been studied in absence and presence of phosphate under varying experimental conditions of pH, metal ion, phosphate and sorbent concentration. The sorption edge was found to shift to high pH with decreasing sorbent concentration indicating site heterogeneity on the clay. Eu(III) sorption by Na-montmorillonite was found to increase in presence of phosphate at lower sorbent concentration of 0.5 g/L while at higher sorbent loading no effect of phosphate was observed. ATR–FTIR spectroscopy has been used to understand transition from surface complexation to surface precipitation with decreasing sorbent concentration.     

The binding of strontium and europium by an aquatic fulvic acid--ion exchange distribution and ultrafiltration studies

The complexation of an aquatic fulvic acid, FA, with Sr(2+) and Eu(3+) was studied at 0.10 and O1.O1M NaClO(4) using trace levels of metal ([Sr(2+)] = 10(-9)M and [Eu(3+)] = 10(-11)M) and a constant FA concentration (0.12 g/l) by an ultrafiltration technique (UF) and an ion exchange distribution method (IEDS). The overall complex formation function, beta(OV) for the two metals was calculated and its dependence on pH, ionic strength and method was investigated. The absolute value of log beta(OV), the pH dependence and the influence of the ionic strength on the complexation differed depending on the metal ion and experimental technique employed. By considering the functional group heterogeneity of the FA molecule, it was possible to predict the most predominantly bound site (keto-enol) and resolve the complex formation function for this site and EU(3+) (IEDS: 9.43 +/- 0.29 l/eq at 0.10M and 10.58 +/- 0.72 l/eq at 0.01M; UF: 7.19 +/- 1.51 l/eq at 0.01M and 6.88 +/- 0.91 l/eq at 0.01M). The results are discussed in the light of possible intrinsic problems of the two experimental methods.     

Pt(II) uptake by dendrimer outer pockets: 1. Solventless ligand exchange reaction

Density functional theory is used to elucidate molecular-level details of the complexation of Pt(II) metal compounds with PAMAM dendrimers. Particular attention is given to the ligand exchange reaction (LER). Binding of Pt(II) complexes to one dendrimer atom site (monodentate binding) is found to be thermodynamically feasible. Tertiary amine nitrogen (N3) is found to be the most favorable binding site in agreement with previous experimental work. Comparing the binding of Pt(II) species to atom sites in simple molecules with those to similar sites in dendrimer outer pockets allowed us to assess the impact of dendrimer branches on the binding. The impact of branches is manifested in more complex reaction profiles for complexation of Pt(II) species, because of the numerous ways in which a single molecule could be hosted by an outer dendrimer pocket. It is found that branches slightly improve the binding strength to all sites, particularly to N3. However, they could also be responsible for the increase of the activation energy for direct LER of PtCl(4)(2-) and PtCl(3)(H(2)O)- at the N3 site. Considering the thermodynamics of both complexation steps, namely noncovalent binding (NCB) and LER, it is found that to have a PtCl(3)(-) moiety bound to N3, as a result of NCB + LER operating on PtCl(4)(2-), is more likely than to have any other ion hosted in the outer pockets. However, the activation energy for direct LER of PtCl(4)(2-) at the N3 site is found to be the largest among all Pt(II) metal complexes and even larger than the barrier to its own aquation yielding PtCl(3)(H(2)O)(-).     

A calix(4)arene pyridine derivative and its monomeric component: structural and thermodynamic aspects of their complexation with metal cations

The interaction of a calix(4)arene derivative, namely 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetra[2-(4-pyridyl)methoxy]calix(4)arene, 1a, and its monomeric component, p-tert-butylphenoxy-4-pyridine, 1b, with metal cations has been investigated in acetonitrile and methanol. (1)H NMR measurements carried out in CD(3)CN show the primary role played by the pyridyl nitrogens in their complexation with metal cations. Conductance measurements demonstrated that for all cations (except mercury) the composition of the metal ion complexes of 1a is 1:1 (ligand:metal cation). However, 1a hosts two mercury cations per unit of ligand. For the monomer 1b, complexes of 2:1 (ligand:metal cation) stoichiometries are formed with the exception of Pb(2+) (1:1 composition). The thermodynamics of complexation of these systems are reported in acetonitrile. Data in methanol are limited to stability constant values for mercury(II) and these ligands. This paper demonstrates for the first time that thermodynamic data for the complexation of the monomeric component of the ligand and metal cations contribute significantly to the interpretation of systems involving cation-calixarene interactions in solution.     

EFFECT OF THE DEGREE OF DVB CROSSLINKING ON THE METAL ION SPECIFICITY OF POLYACRYLAMIDE-SUPPORTED GLYCINES

Metal ion specificity studies of divinylbenzene (DVB)-crosslinked polyacrylamide-supported glycines in different structural environments were investigated. The effect of the degree of crosslinking on the specific rebinding of the desorbed metal ion was investigated towards Co(II), Ni(II), Cu(II), and Zn(II) ions. The metal ion-desorbed resins showed specificity for the desorbed metal ion and the specificity characteristics increases with an increasing degree of the crosslinking agent. The polymeric ligands and metal complexes were characterized by IR, UV-visible and EPR spectra, and by SEM analysis. The swelling and solvation characteristics of the crosslinked polymers, polymeric ligands and metal complexes, the effect of the pH dependence on metal ion binding and rebinding and the kinetics of metal ion binding and rebinding were also followed. The complexation resulted in the downfield shift of the carboxylate peak in the IR spectra. The EPR parameters are in agreement with a distorted tetragonal geometry. The Cu(II) ion-desorbed resins selectively rebinds Cu(II) ions from a mixture of Cu(II) and Co(II) and Cu(II) and Ni(II) ions. The resin could be regenerated several times without loss of capacity and effective for the specific and selective rebinding of Cu(II) ions.