Ligand mixture effects in metal complex lability

The degree of lability of a given metal complex species is modified in the presence of a mixture of ligands. This modification is a consequence of the coupling of the association and dissociation processes of all of the complexes according to the competitive complexation reaction scheme. We show that, because of the mixture effect, the lability of a given complex usually increases when another more labile complex is added into the system, while it decreases upon addition of a less labile one. Typically, complexes tend to adapt to the global lability of the mixture. A quantitative evaluation of these effects for diffusion-limited conditions in a finite domain by rigorous numerical simulation in a system with two complexes indicates that the lability degree of a complex can change by more than 100% with respect to that in the single ligand system. The impact of the mixture effect on the metal flux depends at least on two main factors: the respective abundance of the metal species and the particular values of their lability degrees. Dominant complexes (i.e., those most abundant when these complexes have equal diffusion coefficients) undergo smaller changes in their own lability degree, but these changes have the greater impact on the overall metal flux. Partially labile complexes are more easily influenced by the mixture than labile or inert ones. Some mixture effects can be qualitatively predicted by an analytical expression for the lability index derived using the reaction layer approximation. For a mixture of many complexes, the change in the lability degree of a complex due to the mixture effect can be understood as a combination of the changes due to all of the complexes present.     

Lability criteria for successive metal complexes in steady-state planar diffusion

The lability of sequential metal complexes, ML, ML2, ML3, ... , up to a general 1:n metal/ligand stoichiometric ratio is considered for the case of metal ions (M) being accumulated at a surface (analytical sensor or organism). The analytical solution for the steady-state diffusion of M within a sequential complexation scheme allows quantification of the contribution from the dissociation of all of the complex species to the metal flux through the so-called lability degree, xi. A lability degree for each sequential complexation step is also defined which, due to the sequential character of the complexation scheme, depends not only on the proper kinetic constants of the given complexation step but also on the kinetics of the previous ones. When all contributions from the complexes are diffusion limited, the system is fully labile and xi=1. To provide simple lability criteria, the reaction layer approximation is extended to specifically deal with this sequential complexation scheme, so that a reaction layer thickness is defined when the existence of one particular rate-limiting step is assumed. Expressions for the classical lability parameter, L, are formulated using the reaction layer approximation. The change of the lability of the system as the diffusion layer thickness is modified is analyzed in detail. The contribution of the complex flux reflects the evolution of the system from labile to inert as the thickness of the sensor is appropriately decreased.     

Lability criteria in diffusive gradients in thin films

The penetration of metal complexes into the resin layer of DGT (diffusive gradients in thin films) devices greatly influences the measured metal accumulation, unless the complexes are either totally inert or perfectly labile. Lability criteria to predict the contribution of complexes in DGT measurements are reported. The key role of the resin thickness is highlighted. For complexes that are partially labile to the DGT measurement, their dissociation inside the resin domain is the main source of metal accumulation. This phenomenon explains the practical independence of the lability degree of a complex in a DGT device with respect to the ligand concentration. Transient DGT regimes, reflecting the times required to replenish the gel and resin domains up to the steady-state profile of the complex, are also examined. Low lability complexes (lability degree between 0.1 and 0.2) exhibit the longest transient regimes and therefore require longer deployment times to ensure accurate DGT measurements.     

Computing steady-state metal flux at microorganism and bioanalogical sensor interfaces in multiligand systems. A reaction layer approximation and its comparison with the rigorous solution

In complicated environmental or biological systems, the fluxes of chemical species at a consuming interface, like an organism or an analytical sensor, involve many coupled chemical and diffusion processes. Computation of such fluxes thus becomes difficult. The present paper describes an approximate approach, based on the so-called reaction layer concept, which enables one to obtain a simple analytical solution for the steady-state flux of a metal ion at a consuming interface, in the presence of many ligands, which are in excess with respect to the test metal ion. This model can be used for an unlimited number of ligands and complexes, without limit for the values of the association/dissociation rate constants or diffusion coefficients. This approximate solution is compared with a rigorous approach for the computation of the fluxes based on an extension of a previously published method (J. Galceran, J. Puy, J. Salvador, J. Cecília, F. Mas and J. L. Garcés, Phys. Chem. Chem. Phys., 2003, 5, 5091-5100). The comparison is performed for a very wide range of the key parameters: rate constants and diffusion coefficients, equilibrium constants and ligand concentrations. Their combined influence is studied in the whole domain of fully labile to non-labile complexes, via two combination parameters: the lability index, L, and the reaction layer thickness, mu. The results show that the approximate solution provides accurate results in most cases. However, for particular combinations of metal complexes with specific values of L or mu, significant differences between the approximate and rigorous solutions may occur. They are evaluated and discussed. These results are important for three reasons: (i) they enable the use of the approximate solution in a fully reliable manner, (ii) when present, the differences between approximate and rigorous solution are largely due to the coupling of chemical reactions, whose importance can thus be estimated, (iii) due to its simple mathematical expression, the individual contribution of each metal species to the overall flux can be computed.     

Determination of Dynamic Metal Complexes and their Diffusion Coefficients in the Presence of Different Humic Substances by Combining Two Analytical Techniques

The combined use of a competing ligand exchange (CLE) method and a diffusive gradient in thin films (DGT) technique in a quasi-labile system provides a better understanding of dynamic metal (Cu and Ni) complexes in the presence of humic substances of different origins. The CLE and DGT techniques provide total labile (dynamic) metal complexes (Cu and Ni) and their dissociation rate constants in environmental systems. DGT was found to estimate lower concentrations of labile metal complexes than CLE. These discrepancies were caused by diffusion controlled metal flux (towards the binding resin gel) in the diffusive gel of DGT. The interactions of Cu and Ni with humic acids are stronger than their interactions with fulvic acid and natural organic matter. Changes in the lability of Ni and Cu complexes (complexed with humic substances of different origins) with the changing analytical detection window indicate that the complexes of these metals were formed with different binding sites with diverse binding energies in the humic substances. The combination of these two techniques was found to be very useful in determining diffusion coefficients of labile metal-humate complexes in quasi-labile systems. The values of diffusion coefficients of labile Ni and Cu complexes determined in this study are in good agreement with limited results from the literature. This finding is novel and can be very useful in further improving our understanding of the metal-humate interactions in natural environments.     

Metal flux through consuming interfaces in ligand mixtures: boundary conditions do not influence the lability and relative contributions of metal species

In a mixture of metal ions and complexes, it is difficult to predict ecological risk without understanding the contribution of each metal species to biouptake. For microorganisms, the rate of uptake (internalization flux) has not only a major influence on the total metal flux but also on the bioavailability of the various metal species and their relative contributions to the total flux. In this paper, the microorganism is considered as a consuming interface, which interacts with the metal ion, M, via the Michaelis-Menten boundary conditions. The contribution of each metal complex to the overall metal flux, in relation to its lability, is examined for a number of important boundary parameters (the equilibrium constant K(a) of metal with transport sites, internalization rate constant k(int) and total transport sites concentration {R}(t)). Computations were performed for Cu(II) complexes, in a multicomponent culture medium for microoganisms. For a one-ligand system, results were acquired using rigorous mathematical expressions, whereas approximate expressions, based on the reaction layer approximation (RLA) and rigorous numerical computations (computer codes MHEDYN and FLUXY), were employed for ligand mixtures. Under the condition of ligand excess, as often found in the natural environment, the relative contribution of each metal species to the total flux is shown to be independent of the boundary conditions. This finding has important implications, including an improved basis for relating the analytical signals of dynamic metal speciation sensors to metal bioavailability.     

Copper and nickel speciation in mine effluents by combination of two independent techniques

To control potentially toxic metals in water resources it is necessary to know metal speciation and changes in the metal speciation that occur after aqueous effluents containing metals are discharged into freshwaters. This work explores the speciation of nickel and copper in metal-mining aqueous effluents. Diffusive gradients in thin films (DGT) technique and competing ligand exchange (CLE) method have been applied to determine the speciation of nickel and copper. The results of this investigation demonstrate that combination of two analytical techniques having complementary analytical capabilities can provide a better physicochemical picture of metal speciation than either one of the analytical technique can do alone. The combined use of these techniques revealed that copper formed labile complexes having slow diffusion coefficient along with the presence of small labile copper complexes. Nickel-dissolved organic complexes (DOC) complexes in the aqueous effluent have been found to have fast diffusion coefficient. The results are likely to have environmental significance for providing a link between the metal species in mine aqueous effluent and their bioavailability by determining the characteristics of copper and nickel complexes in metal-mine aqueous effluents. This knowledge is expected to promote a better understanding of the lability of DOC complexes of copper and nickel in mining effluents.     

Impact of ligand protonation on eigen-type metal complexation kinetics in aqueous systems

The impact of ligand protonation on metal speciation dynamics is quantitatively described. Starting from the usual situation for metal complex formation reactions in aqueous systems, i.e., exchange of water for the ligand in the inner coordination sphere as the rate-determining step (Eigen mechanism), expressions are derived for the lability of metal complexes with protonated and unprotonated ligand species being involved in formation of the precursor outer-sphere complex. A differentiated approach is developed whereby the contributions from all outer-sphere complexes are included in the rate of complex formation, to an extent weighted by their respective stabilities. The stability of the ion pair type outer-sphere complex is given particular attention, especially for the case of multidentate ligands containing several charged sites. It turns out that in such cases, the effective ligand charge can be considerably different from the formal charge. The lability of Cd(II) complexes with 1,2-diaminoethane-N,N'-diethanoic acid at a microelectrode is reasonably well predicted by the new approach.     

Application of permeation liquid membrane and scanned stripping chronopotentiometry to metal speciation analysis of colloidal complexes

The potential of permeation liquid membrane (PLM) to obtain dynamic metal speciation information for colloidal complexes is evaluated by measurements of lead(II) and copper(II) complexation by carboxyl modified latex nanospheres of different radii (15, 35, 40 and 65 nm). The results are compared with those obtained by a well characterized technique: stripping chronopotentiometry at scanned deposition potential (SSCP). Under the PLM conditions employed, and for large particles or macromolecular ligands, membrane diffusion is the rate-limiting step. That is, the flux is proportional to the free metal ion concentration with only a small contribution from labile complexes. In the absence of ligand aggregation in the PLM channels, good agreement was obtained between the stability constants determined by PLM and SSCP for both metals.     

Impact of ligand protonation on higher-order metal complexation kinetics in aqueous systems

The impact of ligand protonation on the complexation kinetics of higher-order complexes is quantitatively described. The theory is formulated on the basis of the usual situation for metal complex formation in aqueous systems in which the exchange of water for the ligand in the inner coordination sphere is rate-determining (Eigen mechanism). We derive expressions for the general case of lability of ML(n) species that account for the contributions from all outer-sphere complexes to the rate of complex formation. For dynamic complexes, dissociation of ML is usually the rate-determining step in the overall process ML(n) --> M. Under such conditions, it is the role of ligand protonation in the step ML --> M that is relevant for the kinetic flux. 1:2 complexes of Cd(II) with pyridine-2,6-dicarboxylic acid fall into this category, and their lability at a microelectrode is reasonably well predicted by the differentiated approach. For non-dynamic systems, the kinetic flux arising from dissociation of higher-order complexes contributes to the rate-determining step. In this case, the weighted contribution of protonated and unprotonated outer-sphere complexes in all contributing dissociation reactions must be taken into account. The kinetic flux arising from the dissociation of 1:2 complexes of Ni(II) with bicine at a conventional electrode was quite well described by this combined approach. The results establish the generic role of ligand protonation within the overall framework of metal complexation kinetics in which complexes may be dynamic to an extent that depends on the operational time scale of the measurement technique.     

Roles of dynamic metal speciation and membrane permeability in metal flux through lipophilic membranes: general theory and experimental validation with nonlabile complexes

The study of the role of dynamic metal speciation in lipophilic membrane permeability in aqueous solution requires accurate interpretation of experimental data. To meet this goal, a general theory is derived for describing 1:1 metal complex flux, under steady-state and ligand excess conditions, through a permeation liquid membrane (PLM). The theory is applicable to fluxes through any lipophilic membrane. From this theory, fluxes in the three rate-limiting conditions for metal transport are readily derived, corresponding, namely, to (i) diffusion in the source solution, (ii) diffusion in the membrane, and (iii) the chemical kinetics of formation/dissociation of the metal complex in the interfacial reaction layer. The theory enables discussion of the reaction layer concept in a more general frame and also provides unambiguous criteria for the definition of an inert metal complex. The theoretical flux equations for fully labile complexes were validated in a previous paper. The general theory for semi- or nonlabile complexes is validated here by studying the flux of Pb(II) through PLMs in contact with solutions of Pb(II)-NTA and Pb(II)-TMDTA at different pHs and flow rates.     

Gel Chromatographic Behavior of Labile Metal Complexes Trimeta- And Tetrametaphosphate Complexes with Bivalent Metal Cations

Abstract

The gel chromatographic behavior of metal ions in a labile complex formation system was expressed as a function of the ligand concentration in an eluent and the stability constants of the complexes. Trimeta- and tetrametaphosphate complexes with bivalent metal ions were used as examples. The retention volumes of the metal complexes were found to be always greater than those of the corresponding free ligands.     

Effects of various competing ligands on the kinetics of trace metal complexes of Laurentian Fulvic Acid in model solutions and natural waters

The objective of this work was to study the effects of the following Ligands: Chelex-100, Dowex MAC-3 and Dowex 50WX-8 using Competing Ligand Exchange Method. This objective was achieved by investigating complex dissociation kinetics of trace metals: Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Mn(II) and Pb(II) of a well-characterized Laurentian Fulvic Acid (LFA) in model solutions and in a natural waters of Lake Heva (Québec, Canada). The effects of variation in the competing ligands (including their quantities) on the complex dissociation kinetics were quantitatively characterized by their first-order dissociation rate coefficients. The kinetic lability of the metal complexes varied with the metal-to-LFA ratio, as expected from the theory of metal complexes of the chemically and physically heterogeneous complexants, LFA. The general trend in the metal-binding by the above competing ligands was: Dowex 50WX-8 > Chelex-100 > Dowex MAC-3. However, no difference was found between the Dowex 50WX-8 and Chelex-100 for Cd(II), Zn(II), and Co(II). The results revealed the importance of the quantity of Chelex-100 as a competing ligand in the metal(II)-LFA complexation, on the dissociation kinetics of these complexes in model solutions. By developing Competing Ligand Exchange Method as an analytical technique, for studying the relative affinities of the above competing ligands for metals complexation in natural waters this work has made a substantial contribution to analytical chemistry.     

In situ differentiation of labile/inert metal species in Brazilian tropical rivers by means of a time-controlled batch-procedure based on TEPHA resin

The present study deals with a new analytical procedure based on a cellulose diffusion membrane and immobilised tetraethylene-pentamine-hexaacetate chelator (DM-TEPHA) for an in situ differentiation of labile and inert metal species in aquatic systems. The DM-TEPHA system was prepared by placing TEPHA chelator in pre-purified cellulose bags and in situ applied immersing the system in two Brazilian rivers to study the relative lability of metal species (Cu, Pb, Fe, Mn and Ni) as a function of the time and the quantity of exchanger, respectively. The procedure is simple and enables a new perspective for understanding the complexation, transport, stability and lability of metal species in aquatic systems rich in organic matter.     

Ionic Pd/NHC Catalytic System Enables Recoverable Homogeneous Catalysis: Mechanistic Study and Application in the Mizoroki–Heck Reaction

N-Heterocyclic carbene (NHC) ligands are ubiquitously utilized in catalysis. A common catalyst design model assumes strong M–NHC binding in this metal–ligand framework. In contrast to this common assumption, we demonstrate here that lability and controlled cleavage of the M−NHC bond (rather than its stabilization) could be more important for high-performance catalysis at low catalyst concentrations. The present study reveals a dynamic stabilization mechanism with labile metal–NHC binding and [PdX₃]⁻[NHC-R]⁺ ion pair formation. Access to reactive anionic palladium intermediates formed by dissociation of the NHC ligands and plausible stabilization of the molecular catalyst in solution by interaction with the [NHC-R]⁺ azolium ion is of particular importance for an efficient and recyclable catalyst. These ionic Pd/NHC complexes allowed for the first time the recycling of the complex in a well-defined form with isolation at each cycle. Computational investigation of the reaction mechanism confirms a facile formation of NHC-free anionic Pd in polar media through either Ph–NHC coupling or reversible H–NHC coupling. The present study formulates novel ideas for M/NHC catalyst design.     

Unexpected influence of stereochemistry on the cytotoxicity of highly efficient Ti(IV) salan complexes: new mechanistic insights

The effect of stereochemistry on the cytotoxicity of highly active and hydrolytically stable N-methylated Ti(IV) salan complexes is reported. Four bis(isopropoxo) complexes incorporating N-methylated salan ligands with different aromatic substitution patterns have been prepared in racemic and optically active forms for the first time by ligand-to-metal chiral induction from trans-diaminocyclohexyl-based chiral ligands. The configuration of the metal center that derives from that of the ligand has an enormous influence on cytotoxicity, with the racemic mixture mostly being more active than the single enantiomers that are of either similar or different activity. This implies that the active species is a salan-bound heterochiral polynuclear compound, interacting with a chiral target. Four additional complexes of achiral salan and chiral labile sec-butoxo ligands, analyzed as racemic and as homochiral, revealed no influence of stereochemistry, supporting early dissociation of the labile ligands to give the polynuclear products.     

ASV determination of cadmium complexation in seawater. Methodology evaluation

The methodology for using DPASV to study cadmium complexation in seawater is evaluated using EDTA as a model ligand and by analysing natural samples. The results show that the methodology gives an accurate evaluation of metal complexation when inert complexes are studied, both as regards the ligand concentration and the conditional stability constant; the error for both the parameters is lower than 10% at a ligand concentration of about 10(-8) M and a conditional stability constant of 10(9) M-1. Cadmium complexes with ligands present in natural seawater show an evident kinetic lability that may lead to underestimation of the conditional stability constant when a working electrode characterised by a very thick diffusion layer is used. The conditional stability constant in one water sample of the Adriatic coast ranged between 0.14 and 1.4 l/nmol using a rotating disk electrode at rotation rates of 300 and 6000 rpm. The results of cadmium complexation obtained for samples collected in coastal seawater show that the ligands present low specificity for the metal.     

Metal speciation dynamics in colloidal ligand dispersions

In this work we propose a dynamic metal speciation theory for colloidal systems in which the complexing ligands are localized on the surface of the particles; i.e., there is spatial heterogeneity of binding sites within the sample volume. The differences between the complex formation and dissociation rate constants of complexes in colloidal dispersions and those in homogeneous solutions originate from the differences in kinetic and mass transport conditions. In colloidal systems, when the effective rate of dissociation of the surface complexes becomes fully diffusion controlled, its value is defined via the geometrical parameters of the particle. We assess the extent to which the conventional approach of assuming a homogeneously smeared-out ligand distribution overestimates the lability of surface complexes in colloidal ligand dispersions. The validity of the theory is illustrated by application to binding of lead and cadmium by carboxyl modified latex particles: our approach correctly predicts the formation/dissociation rate constants, which differ by several orders of magnitude from their homogeneous solution counterparts.     

New analytical procedure based on a cellulose bag and ionic exchanger with p-aminobenzoic acid groups for differentiation of labile and inert metal species in aquatic systems

A new procedure was developed for the in situ characterization of the lability of metal species in aquatic systems by using a system equipped with a diffusion membrane and cellulose organomodified with p-aminobenzoic acid groups (DM-Cell-PAB). To this end, the DM-Cell-PAB system was prepared by adding cellulose organomodified with p-aminobenzoic acid groups (Cell-PAB) to pre-purified cellulose bags. After the DM-Cell-PAB system was sealed, it was examined in the laboratory to evaluate the influence of complexation time, mass of exchanger, pH, metal ions (Cu, Cd, Fe, Mn, and Ni), and concentration of organic matter on the relative lability of metal species. It was found that the pH and kinetics strongly influence the process of metal complexation by the DM-Cell-PAB system. At all pH levels, Cd, Mn, and Ni showed lower complexation with Cell-PAB resin than Cu and Fe metals. Note that relative lability of metals complexed to aquatic humic substances (AHS) in the presence of Cell-PAB resin showed the following order: Cu≅Fe≫Ni>Mn=Cd. The results presented here also indicate that increasing the AHS concentration decreases the lability of metal species by shifting the equilibrium to AHS–metal complexes. Our results indicate that the system under study offers an interesting alternative that can be applied to in situ experiments for differentiation of labile and inert metal species in aquatic systems.     

Stereodynamics of Metal–Ligand Assembly: What Lies Beneath the “Simple” Spectral Signatures of C2‐Symmetric Chiral Chelates

A series of C₂‐symmetric chiral tetra‐dentate ligands were prepared by using [4,5]‐ or [5,6]‐pinene‐fused 2,2′‐bipyridyl units that are supported across a rigid arylene–ethynylene backbone. These conformationally pre‐organised chelates support stable 1:1 metal complexes, which were fully characterised by UV/Vis, fluorescence, circular dichroism (CD), and ¹H NMR spectroscopy. A careful inspection of the exciton‐coupled circular dichroism (ECCD) and ¹H NMR spectra of the reaction mixture in solution, however, revealed the evolution and decay of intermediate species en route to the final 1:1 metal–ligand adduct. Consistent with this model, mass spectrometric analysis revealed the presence of multiple metal complexes in solution at high ligand‐to‐metal ratios, which were essentially unobservable by UV/Vis or fluorescence spectroscopic techniques. Comparative studies with a bi‐dentate model system have fully established the functional role of the π‐conjugated ligand skeleton that dramatically enhances the thermodynamic stability of the 1:1 complex. In addition to serving as a useful spectroscopic handle to understand the otherwise “invisible” solution dynamics of this metal–ligand assembly process, temperature‐dependent changes in the proton resonances associated with the chiral ligands allowed us to determine the activation barrier (ΔG^≠) for the chirality switching between the thermodynamically stable but kinetically labile (P)‐ and (M)‐stereoisomers.