Unusual redox stability of pentavalent uranium with hetero-bifunctional phosphonocarboxylate: insight into aqueous speciation.

The +5 state is an unusual oxidation state of uranium due to its instability in the aqueous phase. As a result, gaining information about its aqueous speciation is extremely difficult. The present work is an attempt in that direction and it provides insight into the existence of a new pentavalent species in the presence of hetero-bifunctional phosphonocarboxylate (PC) chelators, other than the carbonate ion, in the aqueous medium. The aqueous chemistry of pentavalent uranium species with three environmentally relevant PCs was probed using electrochemical and DFT methods to understand the redox energy and kinetics of conversion of the U(VI)/U(V) couple, stability, structure, stoichiometry, binding modes, etc. Interestingly, pentavalent uranium complexes with PCs are quite persistent over a wide range of pH starting from acidic to alkaline conditions. The PC chelators block the cation-cation interaction (CCI) of U(V) through strong hetero-bidentate chelation and intermolecular hydrogen bonding (IMHB) interactions which stabilize the pentavalent metal ion against disproportionation. For uranyl species in the presence of PCs, acting as chelators, CV plots were obtained at varying pH values from 2 to 8. The obtained results indicate an irreversible single redox peak involving U(VI) to U(V) conversion and association of a coupled chemical reaction with the electron transfer step. ESI-MS studies were performed to understand the speciation effect on the U(VI)/U(V) redox couple with varying pH. Speciation modelling of U(V) with the PC ligands was carried out, which indicated that the U(V) is redox stable in nearly 47% of the pH region in the presence of the PCs as compared to the carboxylate-based chelators. The free energy and reduction potential of the U(V) complexes and the reduction free energy and disproportionation free energy for the U(VI)/U(V) couple were determined by DFT computations in the presence of the PCs. In situ spectroelectrochemical spectra were recorded to provide evidence for the existence of U(V) species with PCs in the aqueous medium and to acquire its absorption spectra. The present study is highly significant for understanding the coordination chemistry of pentavalent uranium species, accurate modelling of uranium, and isolation of U(V).

Combined Thermodynamic, Theoretical, and Biological Study for Investigating N-(2-Acetamido)iminodiacetic Acid as a Potential Thorium Decorporation Agent.

The most effective approach to mitigate the toxic effects of internal exposure of radiometals to humans is metal-ligand (ML) chelation therapy. Thorium (Th)-induced carcinogenesis as well as other health hazards to humans as a result of chronic internal exposure necessitates the development of efficient Th-decorporating agents. In this regard, chemical and biological studies were carried out to evaluate N-(2-Acetamido)iminodiacetic acid (ADA), a comparatively cost-effective, readily available, and biologically safe complexing agent for Th decorporation. In the present work, detailed thermodynamic studies for complexation of ADA with Th(IV) have been carried out to understand Th-ADA interaction, using potentiometry, calorimetry, electrospray ionization mass spectrometry, and theoretical studies, followed by its biological assessment for Th decorporation. Thermodynamic studies revealed the formation of strong Th-ADA complexes, which are enthalpically as well as entropically favored. Interestingly, density functional theory calculations, to obtain a thermodynamically favored mode of coordination, showed the uncommon trend of lower denticity of ADA in ML than in ML2, which has been explained on the basis of stabilization of ML by hydrogen bonding. The same was also reflected in the unusual trend of enthalpy for Th-ADA complexes. Biological experiments using human erythrocytes, whole human blood, and lung cells showed good cytocompatibility and ability of ADA to significantly prevent Th-induced hemolysis. Th removal of ADA from erythrocytes, human blood, and normal lung cells was found to be comparable with that of diethylenetriamine pentaacetate (DTPA), an FDA approved decorporating agent. The present study contributed significant data about Th complexation chemistry of ADA and its Th decorporation efficacy from human erythrocytes, blood, and lung cells.

Chemical and Redox Speciation of Uranyl with Three Environmentally Relevant Bifunctional Chelates: Multi-Technique Approach Combined with Theoretical Estimations.

Carbon and phosphorous are two primary elements common to the bio-geosphere and are omnipresent in both biotic and abiotic arenas. Phosphonate and carboxylate are considered as building blocks of glyphosate and humic substances and constituents of the cellular wall of bacteria and are the driving functionalities for most of the chemical interactions involving these two elements. Phosphonocarboxylates, a combination of both the functionalities in one moiety, are ideal models to dig deep into for understanding the chemical interactions of the two functional groups with metal ions. Phosphorous and carbon majorly exist as inorganic/organic phosphate and carboxylate, respectively, in the bio-geosphere. Aquatic contamination is a major concern for uranium, and the presence of complexing agents would alter the uranium concentrations in aquifers. Determination of solution thermodynamic parameters, speciation plots, redox patterns, Eh-pH diagrams, coordination structures, and molecular-level understanding by density functional theory calculations was carried out to interpret the uranyl (UO22+) interaction with three environmentally relevant phosphonocarboxylates, namely, phosphono-formic acid (PFA), phosphono-acetic acid (PAA), and phosphono-propanoic acid (PPA). UO22+ forms 1:1 complexes with the three phosphonocarboxylates in the monoprotonated form, having nearly the same stability, and the complexes [UO2(PFAH)], [UO2(PAAH)], and [UO2(PPAH)] involve chelate formation of five, six, and seven membered rings, respectively, through the participation of an oxygen each from the carboxylate and phosphonate, strengthened by an intra-molecular hydrogen bonding through the proton of the phosphonate moiety with uranyl oxygen. The complex formations are favored both enthalpically and entropically, with the latter being more contributive to the overall free energy of formation. The redox speciation showed an aqueous soluble complex formation over a wide pH range of 1-8. Electrospray ionization mass spectrometry and extended X-ray absorption fine structure established the coordination modes, which are further corroborated by density functional calculations. The knowledge gained from the present studies provide potential inputs in framing the cleanup, sequestering, microbial, and bio-remediation strategies for uranyl from aquatic environments.

The aqueous interaction of neodymium with two omni existent biomoieties - a mechanistic understanding by experimental and theoretical studies.

Neodymium (Nd), a technologically important metal ion, has emerged as a major contaminant in aquatic systems in recent years owing to its surge in electrical and electronic applications as a permanent magnet. The chelating molecules present in hydro- and biospheres could substantially enhance its absorption and lead to transportation and migration of Nd from the source. The mechanistic understanding of the Nd interaction with naturally relevant biomoieties present in flora and fauna is of primitive importance to estimate the toxicological effects of the metal ion. The present studies aimed at understanding the aquatic interaction of Nd with two biomoieties namely pyrazine-2-carboxylic acid (P2C) and pyrazine-2,3-dicarboxylic acid (P23C) by multiple experimental determinations and theoretical estimations. Potentiometry and spectrophotometry were employed to determine the aquatic speciation and thermodynamic stability of the complexes. Both techniques supported the formation of MLi (i = 1-4) complexes by Nd(III) with P2C and MLi (i = 1-3) complexes with P23C. The Nd-P23C complexes are more stable than the Nd-P2C complexes for ML formation, while the opposite trend is observed for the ML2 and ML3 complexes. Titration calorimetry was used to determine the enthalpies of complexation which was found to be exothermic and majorly favored by entropy contributions. The formation of the Nd(III)-P2C complexes is more exothermic than that of the respective Nd(III)-P23C complexes. Density functional theory was employed for the geometry optimization of the predicted complexes and for the estimation of the bond distances and partial charges on the coordinating atoms in the optimized geometries. Experimental insights provide crucial inputs at the macro (thermodynamic) level and theoretical calculations help in understanding the complexation process at the molecular level.

Experimental and theoretical approach to probe the aquatic speciation of transuranic (neptunyl) ion in presence of two omnipresent organic moieties.

Pyrazines are omnipresent in nature and have their occurrence in plants, microbes, food supplies, marine arenas. The present studies aimed at aquatic speciation of the neptunyl ion (NpO2+) with two pyrazine compounds namely pyrazine monocarboxylic acid (PMC) and pyrazine dicarboxylic acid (PDC). Absorption spectrophotometry was used to probe the stability, speciation and spectral properties for the complexation process. NpO2+ forms a more stable complex with PMC than PDC for 1:1 (ML), while for 1:2 (ML2) the opposite trend is observed. The extent of shift in λmax, which is also an indicator for the strength of complexation, reflected similar trends for the complexation process. Isothermal titration calorimetry was employed to determine the enthalpies of complex formation, which is found to be endothermic. The complexation process is entropy driven. Linear free energy correlations were established to retrieve the coordination modes of the complexes. The variation in peak potentials (the cyclic voltammograms) with change in pH and metal to ligand ratio were explored to understand redox speciation, electron transfer kinetics and Eh-pH characteristics for the interaction of NpO2+ with pyrazine carboxylate ligands. Density functional theory calculations were employed to optimize the geometries and to calculate the bond distances and partial charges on key atoms of the optimized geometries. The theoretical calculations helped to reveal the contributions from two different configurations of the same geometry towards the optical absorption. The bond distances and partial charges estimated theoretically helped to understand the aqueous interactions at the molecular level.

Characterization of Thorium-Pyrazinoic acid complexation and its decorporation efficacy in human cells and blood.

Thorium (Th) exposure to the human beings is a radiochemical hazard and the chelation therapy by suitable drugs is the major prevention approach to deal with. The present studies aimed at usage of pyrazinoic acid (PCA), which is a prodrug to treat tuberculosis, for its usage as decorporating agent for thorium from human body. The present studies provide a comprehensive knowledge on the chemical interaction and biological efficacy of pyrazinoic acid (PCA) for decorporation of Thorium from the human body. The thermodynamic parameters for Th-PCA speciation are determined by both experiment and theory. The potentiometric data analysis and Electro-Spray Ionization Mass Spectrometry (ESI-MS) studies revealed the formation of MLi (i = 1-4) species with the decrease in stepwise stability constants. All the species formations are endothermic reactions and are predominantly entropy-driven. Biological experiments using human erythrocytes, whole blood and normal human lung cells showed cytocompatibility and decorporation ability of PCA for Thorium. Density functional calculations have been carried out to get insights on interaction process at molecular level. The experimental results and theoretical predictions found to be in line with each other. Present findings on complexation of Th by PCA and its evaluation in human cells and blood would further motivate determination of its safety levels and decorporation efficacy in animal models.

Accountancy for intrinsic colloids on thorium solubility: The fractionation of soluble species and the characterization of solubility limiting phase.

The extensive hydrolysis of tetravalent actinides leads to polynuclear formations through oxygen bridging facilitating the formation of colloids as end products. The pH, ionic strength has phenomenal effects on Thorium colloids formation. The quantitative estimation of colloids facilitates the fraction of soluble fraction into ionic, polymeric and colloidal forms of thorium. The colloids accountability and precipitate characterization explains the discrepancies in estimated solubility limits. The supernatants of long equilibrated (∼3 years) saturated thorium solution under various pH (5- 11) and ionic strengths (0-3 M NaClO4) were analysed by Inductively Coupled Plasma Mass Spectrometer (ICP-MS) and Ion Chromatography (IC) to determine total and ionic thorium respectively. Laser Induced Breakdown Detection (LIBD) was employed to determine the colloid size and concentrations. The precipitates were characterized by calorimetry and XRD to determine the solubility limiting phase. The results of pH, IC, ICP-MS, and LIBD measurements on the aged thorium samples are discussed with regard to the mechanism of the formation of thorium colloids. The results revealed the formation of colloids having particle size (10-40 nm) at concentrations (109-1011 particles/mL). The colloids accountancy resulted in estimated solubility products to 2-4 orders lower than their inclusion as soluble thorium. The soluble thorium was fractionated quantitatively into ionic, polymeric and colloidal forms of thorium. The precipitates formed are found to be semi amorphous.

Aquatic interaction of uranium with two naturally ubiquitous pyrazine compounds: Speciation studies by experiment and theory.

The present studies interpret the speciation of uranyl (UO22+) with the most ubiquitous class of natural species named pyrazines in terms of stability, speciation and its identification, thermodynamics, spectral properties determined by a range of experimental techniques and further evidenced by theoretical insights. UO22+ forms ML and ML2 kind of species with a qualitative detection of ML3 species, while the ESI-MS identified the formation of all the complexes including ML3. Both the ligands act as bidentate chelators with a difference in ring size and coordinating atoms in the complex formed. The ML3 complexes involve the third ligand participation as monodentate via carboxylate only due to the restricted coordination number and space around the UO22+ ion to accommodate three ligand molecules in its primary coordination sphere. All the complexes are found to be endothermic and purely entropy driven formations. The complex formations showed redshift in the absorption spectra and the shift was further enhanced from ML to ML2 formation. The UO22+ ion redox properties are used to explore the redox potential and heterogeneous electron-transfer kinetic parameters as a function of pH and concentration of UO22+ in presence of pyrazine carboxylates. Interestingly, the cyclic voltammograms identified the ligands also as redox sensitive. The theoretical calculation gave inputs to understand the complex formation at the molecular level with major emphasis on geometry optimization, energetics, bonding parameters, molecular orbital diagrams and bond critical point analyses. The experimental observations in combination with theoretical addendum provided detailed knowledge on the interaction of UO22+ with pyrazine-2-carboxylate and pyrazine-2,3-dicarboxylates.

Reduction in Coordination Number of Eu(III) on Complexation with Pyrazine Mono- and Di-Carboxylates in Aqueous Medium.

The denticity, flexibility, and steric hindrance of the ligand are key factors in deciding the mode and number of coordination around a metal ion on complex formation. The thermodynamic aspects of lanthanide complexation with various multidentate ligands provides a significant insight into understand the coordination chemistry of lanthanides in framing the relevant metal organic networks for the applications in biological, biochemical and medical aspects. The pyrazine carboxylic acids are known to form many structurally important complexes and further can form chelates with coordination number of eight for europium in which more water molecules can be knocked out from the primary coordination sphere than demanded by denticity of the ligand. The present studies aimed at ESI-MS characterization and determination of the thermodynamic parameters (log ß, ΔG, ΔH, and ΔS), luminescence properties of europium complexes with pyrazine-2-carboxylate and pyrazine-2,3-dicarboxylate in aqueous solutions by experiment as well as theory. Time resolved luminescence spectroscopy supported by DFT calculations are carried out to optimize the stable geometries of the complexes with various modes of binding and coordination. Furthermore, the thermodynamic parameters estimated theoretically have been used to trace the path of complex formation.

Complexation of U(VI) with Cucurbit[5]uril: Thermodynamic and Structural investigation in aqueous medium.

The assessment of cucurbituril (CBn) for selective removal of actinides from nuclear waste streams requires comprehensive understanding of binding parameters and coordination of these complexes. The present work is the first experimental report on complexation of actinide ion with Cucurbit[5]uril (CB5) in solution. The thermodynamic parameters (ΔG, ΔH and ΔS) for complexation of CB5 with U(VI) in formic acid water medium were determined using microcalorimetry and UV-Vis spectroscopy. The enthalpy and entropy of complexation revealed the partial binding of U(VI) to CB5 portal. The partial binding was confirmed by spectroscopic techniques viz. extended X absorption fine structure spectroscopy (EXAFS), 1H and 13C NMR. The EXAFS χ(r) versus r spectra for U-CB5 complex has been fitted from 1.4 to 3.5 Šwith two oxygen shells and a carbon shell. The presence of three carbon atom in secondary shell shows the involvement of only three carbonyl oxygens directly bonding to U(VI) which is in contrast to that calculated from gas phase DFT calculation of unhydrated system. The combined effect of hydration and formic acid encapsulation led to the enhanced stability of partially bound U(VI) to CB5. In the present work the binding of formic acid has also been studied by fluorescence spectroscopy. ESI-MS data shows the unusual stabilization of U(VI) by CB5 in gas phase.

Complexation of thorium with pyridine monocarboxylate-N-oxides: Thermodynamic and computational studies.

The feed wastes and waste water treatment plants are the major sources for the entry of N-oxides into the soils then to aquatic life. The complexation of actinides with potentially stable anthropogenic ligands facilitate the transportation and migration of the actinides from the source confinement. The present study describes the determination of thermodynamic parameters for the complexation of Th(IV) with the three isomeric pyridine monocarboxylates (PCNO) namely picolinic acid-N-oxide (PANO), nicotinic acid-N-oxide (NANO) and isonicotinic acid-N-oxide (IANO). The potentiometric and isothermal calorimetric titrations were carried out to determine the stability and enthalpy of the formations for all the Th(IV)-PCNO complexes. Th-PANO complexes are more stable than Th-NANO and Th-IANO complexes which can be attributed to chelate formation in the former complexes. Formation of all the Th-PCNO complexes are endothermic and are entropy driven. The geometries for all the predicted complexes are optimized the energies, bond distances and charges on individual atoms are obtained using TURBOMOLE software. The theoretical calculation corroborated the experimental determinations.

Structural, luminescence, thermodynamic and theoretical studies on mononuclear complexes of Eu(III) with pyridine monocarboxylate-N-oxides in aqueous solution.

The mononuclear complexes formed by Eu(III) with three isomeric pyridine monocarboxylate-N-oxides namely picolinic acid-N-oxide (PANO), nicotinic acid-N-oxide (NANO) and isonicotinic acid-N-oxide (IANO) in aqueous solutions were studied by potentiometry, luminescence spectroscopy and isothermal titration calorimetry (ITC) to determine the speciation, coordination, luminescence properties and thermodynamic parameters of the complexes formed during the course of the reaction. More stable six membered chelate complexes with stoichiometry (MLi, i=1-4) are formed by Eu(III) with PANO while non chelating ML and ML2 complexes are formed by NANO and IANO. The stability of Eu(III) complexes follow the order PANO>IANO>NANO. The ITC studies inferred an endothermic and innersphere complex formation of Eu(III)-PANO and Eu(III)-IANO whereas an exothermic and outer-sphere complex formation for Eu(III)-NANO. The luminescence life time data further supported the ITC results. Density functional theoretical calculations were carried out to optimize geometries of the complexes and to estimate the energies, structural parameters (bond distances, bond angles) and charges on individual atoms of the same. Theoretical approximations are found to be in good agreement with the experimental observations.

Selective Micellar Extraction of Ultratrace Levels of Uranium in Aqueous Samples by Task Specific Ionic Liquid Followed by Its Detection Employing Total Reflection X-ray Fluorescence Spectrometry.

A task specific ionic liquid (TSIL) bearing phosphoramidate group, viz., N-propyl(diphenylphosphoramidate)trimethylammonium bis(trifluoromethanesulfonyl)imide, was synthesized and characterized by 1H NMR, 13C NMR, 31P NMR, and IR spectroscopies, elemental (C H N S) analysis, and electrospray ionization mass spectrometry (ESI-MS). Using this TSIL a cloud point extraction (CPE) or micelle mediated extraction procedure was developed for preconcentration of uranium (U) in environmental aqueous samples. Total reflection X-ray fluorescence spectrometry was utilized to determine the concentration of U in the preconcentrated samples. In order to understand the mechanism of the CPE procedure, complexation study of the TSIL with U was carried out by isothermal calorimetric titration, liquid-liquid extraction, 31P NMR and IR spectroscopies, and ESI-MS. The developed analytical technique resulted in quantitative extraction efficiency of 99.0 ± 0.5% and a preconcentration factor of 99 for U. The linear dynamic range and method detection limit of the procedure were found to be 0.1-1000 ng mL-1 and 0.02 ng mL-1, respectively. The CPE procedure was found to tolerate a higher concentration of commonly available interfering cations and anions, especially the lanthanides. The developed analytical method was validated by determining the concentration of U in a certified reference material, viz., NIST SRM 1640a natural water, which was found to be in good agreement at a 95% confidence limit with the certified value. The method was successfully applied to the U determination in three natural water samples with ≤4% relative standard deviation (1σ).

Stability, speciation and spectral properties of NpO2+ complexes with pyridine monocarboxylates in aqueous solution.

Neptunyl ion as NpO2+ is the least reacting and most mobile radioactive species among all the actinides. The picolinic acid used for decontamination is co-disposed along with the radioactive waste. Thus, in long term storage of HLW, there is high possibility of interaction of actinides and long lived fission products with the picolinate and can cause migration. The complexation of NpO2+ with the three structural isomers of pyridine monocarboxylates provides an insight to explore the role of hetero atom (nitrogen) with respect to key binding moiety (carboxylate). In the present study, the log ß values, speciation and spectral properties of NpO2+ complexes with pyridine monocarboxylates viz. picolinate, nicotinate and isonicotinate, have been studied at 298K in 0.1M NaClO4 medium using spectrophotometry. The complexation reactions involving protonated ligands are always accompanied by protonation/deprotonation process; thus, the protonation constants of all the three pyridine monocarboxylates under same conditions were also determined by potentiometry. The spectrophotometric data analysis for complexation of NpO2+ with pyridine monocarboxylates indicated the presence of ML and ML2 complexes with log ß values of 2.96±0.04, 5.67±0.08 for picolinate, 1.34±0.09, 1.65±0.12 for nicotinate and 1.52±0.04, 2.39±0.06 for isonicotinate. The higher values of log ß for picolinate were attributed to chelation while in other two isomers, the binding is through carboxylate group only. Density Functional Theory (DFT) calculations were carried out to get optimized geometries and electrostatic charges on various atoms of the complexes and free pyridine monocarboxylates to support the experimental data. The higher stability of NpO2+ nicotinate and isonicotinate complexes compared to simple carboxylates and the difference in log ß between the two is due to the charge polarization from unbound nitrogen to the bound carboxylate oxygen atoms.

Complexation of Eu(III) with cucurbit[n]uril, n = 5 and 7: a thermodynamic and structural study.

Cucurbit[n]urils (CBn) are a new class of macrocyclic cage compounds capable of binding organic and inorganic species, owing to their unique pumpkin like structure comprising of both a hydrophobic cavity and a hydrophilic portal. The thermodynamics of the complexation of Eu(III) with CBn of a different cavity size viz. cucurbit[5]uril (CB5) and cucurbit[7]uril (CB7) has been studied by UV-Vis spectroscopy and calorimetry at 25 °C whereas the structure of the complexes was investigated using time resolved fluorescence spectroscopy (TRFS) and extended X-ray absorption fine structure spectroscopy (EXAFS) in a formic acid-water mixture (50 wt%). This is the first report on the structural investigation of Eu-CBn complexes in solution. The thermodynamic data (ΔG, ΔH and ΔS) for Eu(III) complexation with CBn reveal the formation of a 1 : 1 complex with CB5, while both 1 : 1 and 1 : 2 complexes are observed with CB7. The signatures of these species are observed in ESI-MS measurements, which corroborates with the species postulated in thermodynamic studies. The complexation reactions are found to be driven by ΔS as ΔH is either small negative or positive indicating the formation of inner sphere complexes, which is in line with TRFS and EXAFS results. These studies show that Eu(III) caps one of the CB5 portals by binding with all the carbonyl groups in the 1 : 1 Eu-CB5 complex, whereas in the 1 : 1 Eu-CB7 complex, Eu(III) interacts with only a few of the carbonyl groups of CB7. The computational studies (DFT calculations) on Eu-CB5 and Eu-CB7 complexes further support the experimental data.