Lanthanide Ions In Aqueous Solution Research Articles

Longitudinal and Transverse 1H Nuclear Magnetic Resonance Relaxivities of Lanthanide Ions in Aqueous Solution up to 1.4 GHz/33 T.

The longitudinal and transverse nuclear magnetic resonance relaxivity dispersion (NMRD) of 1H in water induced by the paramagnetic relaxation enhancement (PRE) of dissolved lanthanide ions (Ln3+) can become very strong. Longitudinal and transverse 1H NMRD for Gd3+, Dy3+, Er3+ and Ho3+ were measured from 20 MHz/0.47 T to 1382 MHz/32.5 T, which extended previous studies by a factor of more than two in the frequency range. For the NMRD above 800 MHz, we used a resistive magnet, which exhibits reduced field homogeneity and stability in comparison to superconducting and permanent NMR magnets. These drawbacks were addressed by dedicated NMRD methods. In a comparison of NMRD measurements between 800 MHz and 950 MHz performed in both superconducting and resistive magnets, it was found that the longitudinal relaxivities were almost identical. However, the magnetic field fluctuations of the resistive magnet strongly perturbed the transverse relaxation. The longitudinal NMRDs are consistent with previous work up to 600 MHz. The transverse NMRD nearly scales with the longitudinal one with a factor close to one. The data can be interpreted within a PRE model that comprises the dipolar hyperfine interactions between the 1H and the paramagnetic ions, as well as a Curie spin contribution that is dominant at high magnetic fields for Dy3+, Er3+ and Ho3+. Our findings provide a solid methodological basis and valuable quantitative insights for future high-frequency NMRD studies, enhancing the measurement accuracy and applicability of PRE models for paramagnetic ions in aqueous solutions.

Magnetic Nanohydrometallurgy Applied to Lanthanide Separation

Lanthanides play an important role in modern technology because of their outstanding optical, electronic, and magnetic properties. Their current hydrometallurgical processing involves lixiviation, leading to concentrates of elements whose separation requires exhaustive procedures because of their similar chemical properties. In this sense, a new nanotechnological approach is here discussed, involving the use of iron oxide nanoparticles functionalized with complexing agents, such as diethylenetriaminepentaacetic acid (DTPA), for carrying out the magnetic extraction and separation of the lanthanide ions in aqueous solution. This strategy, also known as magnetic nanohydrometallurgy (MNHM), was first introduced in 2011 for dealing with transition metal recovery in the laboratory, and has been recently extended to the lanthanide series. This technology is based on lanthanide complexation and depends on the chemical equilibrium involved. It has been better described in terms of Langmuir isotherms, considering a uniform distribution of the metal ions over the nanoparticles surface, as evidenced by high angle annular dark field microscopy. The observed affinity parameters correlate with the lanthanide ion contraction series, and the process dynamics have been studied by monitoring the nanoparticles migration under an applied magnetic field (magnetophoresis). The elements can be reversibly captured and released from the magnetically confined nanoparticles, allowing their separation by a simple acid-base treatment. It can operate in a circular scheme, facilitated by the easy magnetic recovery of the extracting agents, without using organic solvents and ionic exchange columns. MNHM has been successfully tested for the separation of the lanthanide elements from monazite mineral, and seems a promising green nanotechnology, particularly suitable for urban mining.

Linear 2-Ethylhexyl Imidophosphoric Esters as Effective Rare-Earth Element Extractants

Imidophosphoric organic esters containing phosphoryl groups are potential polydentate ligands and promising extractants of rare-earth elements. For their preparation, a monophosphazene salt [PCl3=N−PCl3]+[PCl6]− and short phosphazene oligomers of the general formula [Cl–(PCl2=N)n–PCl3]+[PCl6]−, where n = 4–7, were synthesized via living cationic polymerization of Cl3P=NSiMe3 and used as starting compounds. All phosphazenes were reacted with 2-ethylhexanol to obtain the corresponding esters of imidophosphoric acids (EIPAs). The formation of imidophosphoric acids occurs due to the phosphazene-phosphazane rearrangement of –P(OR)2=N– or –P(OH)(OR)=N– units, where R = 2-ethylhexyl. The prepared EIPAs were characterized by 1H, 31P NMR, and MALDI-TOF analyses and their extractive capacity towards lanthanide ions in aqueous solutions of nitric acid was examined. The EIPAs are mixtures of mono-, di-, and trifunctional compounds of the type HxA, where x = 1–3, which can form chelate complexes of lanthanide ions [Ln(A)z], where z = 3–6, depending on the chain length. The longer chain EIPAs are more suitable for collective rare-earth elements extraction. A comparison of the extraction properties of the EIPAs with the industrially used polyalkylphosphonitrilic acid (PAPNA) was drawn.

Lanthanide ion processing from monazite based on magnetic nanohydrometallurgy

A systematic work has been performed for the separation of the lanthanide elements from monazite mineral using magnetic nanohydrometallurgy. The process was based on DTPA (diethylenetriaminepentaacetic acid) functionalized Fe3O4 superparamagnetic nanoparticles as the active species for carrying out the magnetic extraction of the lanthanide ions in aqueous solution, at pH 6. The process was monitored by means of X-ray fluorescence (EDXRF) techniques, and the corresponding Langmuir isotherms were carefully elaborated, allowing to evaluate the equilibrium (affinity) constants involved. The results obtained for La3+, Pr3+, Nd3+, Sm3+, Gd3+ and Dy3+ ions followed the trends dictated by the lanthanide ion contraction. In the process, the elements are reversibly captured and released from the magnetically confined nanoparticles after applying a mild acid treatment (pH 2). Their chemical separation based on their relative affinity constants, can be performed by applying successive magnetic extraction/releasing cycles in aqueous solution, at room temperature, using the same reactor, dispensing the use of organic solvents and more intensive chemical processing.

The role of colloid particles in the albumin-lanthanides interaction: The study of aggregation mechanisms

We studied the interaction between bovine serum albumin (BSA) and lanthanide ions in aqueous solution in the 4.0÷9.5pH range. A strong increase of the solution turbidity was observed at pH values exceeding 6, which corresponds to the formation of Ln(OH)3 nanoparticles, while no changes were observed near the isoelectric point of BSA (pH 4.7). The results of the dynamic light scattering and protein adsorption measurements clearly demonstrated that the observed turbidity enhancement was caused by albumin sorption on the surface of Ln(OH)3 and colloid particles bridging via adsorbed protein molecules. Upon pH increase from 4.5 to 6.5, albumin adsorption on lanthanide colloids was observed, while the following increase of pH from 6.5 to 9.5 led to protein desorption. The predominant role of the electrostatic interactions in the adsorption and desorption processes were revealed in the zeta-potential measurements. No reversibility was observed upon decreasing pH from 9.5 to 4.5 that was suggested to be due to the other interaction mechanisms present in the system. It was shown that while for all lanthanide ions the interaction mechanism with BSA was similar, its manifestation in the optical properties of the system was significantly different. This was interpreted as a consequence of the differences in lanthanides hydrolysis constants.

Hydroxy-bisphosphinic acids: synthesis and complexation properties with transition metals and lanthanide ions in aqueous solution

Treatment of acyl chlorides with a mixture of ammonium hypophosphite and hexamethyl disilazene gave 1-hydroxy-1,1-bisphosphinic acids. Acid dissociation constants (pKa) of 1-hydroxy-1,1-bisphosphinic acids were determined by the pH-potentiometric technique. Complexation properties of 1-hydroxy-1-phenylmethyl-1,1-bis(H-phosphinic acid) (HBPA 1) with Ca2+ as well as transition metals Fe2+, Ni2+, Cu2+, Cr3+ and lanthanide ions La3+, Gd3+ were studied in aqueous solution. The stabilizing ability of complexes of HBPA 1 with these ions in aqueous solutions was determined to show a significant difference in the complex stability due to the transition metals. The coordination of 1-hydroxy-1,1-bis(H-phosphinic acid) to metals.

Structural parameters of the nearest surrounding of lanthanide ions in aqueous solutions of their salts

Published data on the structural characteristics of the local surrounding of lanthanide ions in aqueous solutions of respective salts obtained by different research methods under standard conditions are reviewed. Structural parameters like the coordination number, interparticle distance, parameters of the second coordination sphere, and types of ionic association are discussed.

Lanthanide(III) Complexes with Ligands Derived from a Cyclen Framework Containing Pyridinecarboxylate Pendants. The Effect of Steric Hindrance on the Hydration Number

Two new macrocyclic ligands, 6,6′-((1,4,7,10-tetraazacyclododecane-1,7-diyl)bis(methylene))dipicolinic acid (H2DODPA) and 6,6′-((4,10-dimethyl-1,4,7,10-tetraazacyclododecane-1,7-diyl)bis(methylene))dipicolinic acid (H2Me-DODPA), designed for complexation of lanthanide ions in aqueous solution, have been synthesized and studied. The X-ray crystal structure of [Yb(DODPA)](PF6)·H2O shows that the metal ion is directly bound to the eight donor atoms of the ligand, which results in a square-antiprismatic coordination around the metal ion. The hydration numbers (q) obtained from luminescence lifetime measurements in aqueous solution of the Eu(III) and Tb(III) complexes indicate that the DODPA complexes contain one inner-sphere water molecule, while those of the methylated analogue H2Me-DODPA are q = 0. The structure of the complexes in solution has been investigated by 1H and 13C NMR spectroscopy, as well as by theoretical calculations performed at the density functional theory (DFT; mPWB95) level. The minimum energy conformation calculated for the Yb(III) complex [Λ(λλλλ)] is in good agreement with the experimental structure in solution, as demonstrated by the analysis of the Yb(III)-induced paramagnetic 1H shifts. The nuclear magnetic relaxation dispersion (NMRD) profiles recorded for [Gd(Me-DODPA)]+ are typical of a complex with q = 0, where the observed relaxivity can be accounted for by the outer-sphere mechanism. However, [Gd(DODPA)]+ shows NMRD profiles consistent with the presence of both inner- and outer-sphere contributions to relaxivity. A simultaneous fitting of the NMRD profiles and variable temperature 17O NMR chemical shifts and transversal relaxation rates provided the parameters governing the relaxivity in [Gd(DODPA)]+. The results show that this system is endowed with a relatively fast water exchange rate k(ex)(298) = 58 × 10(6) s(–1).

Synthesis and Complexation Properties of N,N-Bis(phosphinomethyl)amine as a New Class of 1-Aminophosphinic Acids with Transition Metals and Lanthanide Ions in Aqueous Solution

The treatment of aromatic aldehydes with ammonia and hypophosphorus acid gave novel C2-symmetric N,N-bis(phosphinomethyl)amines as a new class of 1-aminophosphinic acid compounds. Complexation properties of N,N-bis(phosphinomethyl)amines with transition metals such as Co2+, Ni2+, Zn2+, Cu2+, and Cd2+ and lanthanide ions La3+ and Gd3+ were studied in aqueous solution by the pH-potentiometric method. Dissociation constants (pK) of the compound were determined by the pH-potentiometric technique. The complexation studies with compound dl-2a showed the best fit of the titration curves were obtained when the ligand–metal stoichiometric ratio was 1:1 and 2:1.

13C NMR study of D-pantothenic acid complexes with heavy lanthanide ions in aqueous solution

The 13C NMR study of the interaction between the trivalent lanthanide ions (from Dy to Lu) and D-pantothenic acid in aqueous solution was presented. The unambiguous resonance assignments were made on the basis of pH dependence and the analysis of molecular geometry. 13C paramagnetic shifts were separated into the contact and dipolar components by Reilley's method. The complex stability constant is 12.0 L / mol for the 1:1 ytterbium complex. An analysis of the shift data shows that heavy lanthanide ions form isostructural complexes with the substrate and the hyperfine coupling constant is independent of the ions used in the study. D-pantothenic acid is coordinated to the metal via the two oxygens of the carboxyl group with 2.25Å of Ln—O distances. The electron spin density distribution and the structural details of the complex were elucidated from the shift data. It is stated that contact contributions, while small for nuclei of several bonds away from the metal, should not be dismissed a priori.

Macrocyclic Receptor Exhibiting Unprecedented Selectivity for Light Lanthanides

We report a new macrocyclic ligand, N,N'-bis[(6-carboxy-2-pyridil)methyl]-4,13-diaza-18-crown-6 (H(2)bp18c6), designed for complexation of lanthanide ions in aqueous solution. Potentiometric measurements evidence an unprecedented selectivity of bp18c6 for the large Ln(III) ions. Among the different Ln(III) ions, La(III) and Ce(III) show the highest log K(ML) values, with a dramatic drop of the stability observed from Ce(III) to Lu(III) as the ionic radius of the Ln(III) ions decreases (log K(CeL) - log K(LuL) = 6.9). The X-ray crystal structures of the Gd(III) and Yb(III) complexes show that the metal ion is directly bound to the 10 donor atoms of the bp18c6 ligand. The structure of the complexes in solution has been investigated by (1)H and (13)C NMR spectroscopy, as well as by theoretical calculations performed at the DFT (B3LYP) level. Our results indicate that a conformational change occurs around the middle of the lanthanide series: for the larger Ln(III) ions the most stable conformation is Delta(deltalambdadelta)(deltalambdadelta), while for the smallest Ln(III) ions (Gd-Lu) our calculations predict the Delta(lambdadeltalambda)(lambdadeltalambda) form being the most stable one. This structural change was confirmed by the analysis of the Ce(III)-, Pr(III)-, and Yb(III)-induced paramagnetic (1)H shifts. The selectivity that bp18c6 shows for the large Ln(III) ions can be attributed to a better fit between the light Ln(III) ions and the relatively large crown fragment of the ligand. Indeed, our DFT calculations indicate that the interaction between the Ln(III) ion and several donor atoms of the crown moiety is weakened as the ionic radius of the metal ion decreases.

Lanthanide Complexes Based on a 1,7-Diaza-12-crown-4 Platform Containing Picolinate Pendants: A New Structural Entry for the Design of Magnetic Resonance Imaging Contrast Agents

We have synthesized a new macrocyclic ligand, N,N'-Bis[(6-carboxy-2-pyridyl)methyl]-1,7-diaza-12-crown-4 (H 2bp12c4), designed for complexation of lanthanide ions in aqueous solution. The X-ray crystal structure of the Gd (III) complex shows that the metal ion is directly bound to the eight donor atoms of the bp12c4 ligand, the ninth coordination site being occupied by an oxygen atom of a carboxylate group of a neighboring [Gd(bp12c4)] (+) unit, while the structure of the Lu (III) analogue shows the metal ion being only eight-coordinate. The hydration numbers obtained from luminescence lifetime measurements in aqueous solution of the Eu (III) and Tb (III) complexes suggest an equilibrium in aqueous solution between a dihydrated ( q = 2), ten-coordinate and a monohydrated ( q = 1), nine-coordinate species. This has been confirmed by a variable temperature UV-vis spectrophotometric study on the Eu (III) complex. The structure of the complexes in solution has been investigated by (1)H and (13)C NMR spectroscopy, as well as by theoretical calculations performed at the DFT (B3LYP) level. The results indicate that the change in hydration number occurring around the middle of the lanthanide series is accompanied by a change in the conformation adopted by the complexes in solution [Delta(lambdalambdalambdalambda) for q = 2 and Lambda(deltalambdadeltalambda) for q = 1]. The structure calculated for the Yb (III) complex (Lambda(deltalambdadeltalambda)) is in good agreement with the experimental structure in solution, as demonstrated by the analysis of the Yb (III)-induced paramagnetic (1)H shifts.

Inversion of the relative probabilities of the f-f and f-d transitions in the Ln3+ lanthanide ions at their radio- and sonoexcitation as compared to photoexcitation

The results of measuring the efficiencies of the formation of electronically excited states of the Ln3+ lanthanide ions in aqueous solutions in the processes of radioluminescence and multibubble sonoluminescence are analyzed. In both cases, electronic excitation occurs due to inelastic collisions of Ln3+ ions with (for radioluminescence) charged ionizing particles in liquid and (for multibubble sonoluminescence) high-energy particles, primarily electrons, in the gas phase of cavitation bubbles. In both processes, the efficiencies of exciting ions whose luminescence states appear in the 4f-5d transitions (Ce3+ and Pr3+) are significantly lower (by an order of magnitude or larger) than the efficiencies of exciting ions whose luminescence states appear in the 4f-4f transitions (Gd3+ and Tb3+). Therefore, the probability of the f-d transitions is lower than the probability of the f-f transitions in lanthanide ions excited by collisions with the charged particles and the relative probabilities of these transitions are inverted in these processes as compared to photoexcitation.

Energy transfer between Ln(III) ions in aqueous solutions upon formation of labile binuclear fluoride and nitrate complexes of these ions

The influence of anions on nonradiative electronic energy transfer between lanthanide ions in aqueous solutions is studied. The rate constant k t of energy transfer was found to increase by three orders of magnitude upon addition of salts of hydrofluoric acid to the solution. This effect is caused by the formation of bridge labile binuclear complexes, an excited energy donor-fluorine anion-acceptor, which increase the encounter time between a donor and an acceptor, resulting in the increase in the probability of energy transfer. The independence of k t of the Forster overlap integral in the bridge complex containing F− was explained by the fact that energy transfer occurs in each event of the binuclear complex formation, the maximum rate constant of energy transfer being equal to the rate constant of the binuclear complex association. Bridge complexes formed via the NO 3 − anion are also considered. These binuclear complexes are unstable, and k t for them is proportional to the Forster overlap integral. In this case, energy transfer occurs not during each event of the complex formation. This allows us to estimate the lower limit of the dissociation constant of the binuclear complex and its stability constant. Thus, the study of influence of various anions on energy transfer represents a new efficient luminescent method for analysis of properties of labile binuclear complexes in solutions.

Energy transfer from the copolymer of styrene-maleic acid to lanthanide ions in aqueous solution

The luminescence properties of PSMA-Ln(III) (Ln = Sm, Dy, Tb, Gd, Yb, Eu) in aqueous solution were investigated. Upon excitation of the styrene subunit, energy migrates from the PSMA polymer to Ln 3+ ions, and luminescence enhancements of Tb 3+, Eu 3+, Sm 3+ and Dy 3+ were observed. In PSMA-Eu 3+, the effect of environment around the metal ion largely contributes to the luminescence enhancement. Fluorescence from PSMA in PSMA-Ln 3+ complexes was also measured. PSMA fluorescence intensity was enhanced upon coordination to Ln(III) (Ln = Sm, Dy, Tb, Gd), and λ em,max was blue-shifted due to Ln(III) association to the polymer chain and the reduced chain mobility of PSMA. In the presence of Eu 3+ and Yb 3+, however, PSMA luminescence intensity was decreased. Compared to Tb 3+, the unlikely low efficiency of energy transfer from styryl moieties in PSMA−Eu 3+ and diminished PSMA fluorescence intensities are attributed to the possibility of a deactivating charge transfer state due to the low reduction potentials of Eu 3+ and Yb 3+.

Aromatic polyaminocarboxylate ligands for energy transfer luminescence measurements of lanthanide ions in aqueous solutions

The promising ligand candidates for the energy transfer luminescence measurements of lanthanide (Ln) chelates on aqueous matrices are first proposed. The ligands are; 2[(2-amino-5-methyl-phenoxy)methyl]-6-methoxy-8-aminoquinoline- N,N,N′, N′-tetraacetate (Quin 2), 1,2-bis(2-amino-phenoxy)ethane- N,N,N′, N′-tetraacetate (BAPTA), and 1,2-bis(2-amino-5-fluoro-phenoxy)ethane- N,N,N′, N′-tetraacetate (F-BAPTA). The Ln-chelates of these aromatic polyaminocarboxylates show the sensitized emission which results from efficient ligand-centered light absorption, and the interesting selectivity is seen; BAPTA and F-BAPTA form the luminescent chelates only with Tb(III) and Dy(III) ions, whereas the emission from Sm(III) and Eu(III) ions is greatly sensitized with Quin 2. The sufficient emission intensity can be obtained even in slightly alkaline aqueous solutions without any addition of surfactants or organic solvents. These octadentate ligands are fairly capable of shielding central Ln ions from quenching by surrounding water molecules. The luminescence enhancement factors are 1600 for Tb(III) ion with BAPTA (em.544 nm) and 1380 for Eu(III) ion with Quin 2 (em. 615 nm), respectively, being relative to their aqueous chloride solutions.

Ultraviolet Spectroscopic Determination of Five Lanthanide Elements without Prior Separation

This investigation has resulted in an analytical method for the quantitative determination of total lanthanide concentration in aqueous solution by absorbance at 240 nm in the ultraviolet followed by quantitative determination of individual lanthanide ion concentrations by the use of concentration-responsive absorption peaks in the 190–235 nm region. The 240-nm peak is present and is proportional to concentration regardless of the ligand employed to complex the lanthanides (including H2O). The individual lanthanide/ligand peaks in the 190–235 nm region were selected on the basis of their separation from one another, their linearity of absorbance vs. concentration, and their statistical reliability based on replicate sample analyses. Lanthanides involved in this investigation were La+3, Nd+3, Eu+3, Ho+3, and Yb+3. Ligands ultimately selected for complexation were citrate for La+3, Nd+3, and Ho+3, and DTPA for Eu+3, Ho+3, and Yb+3. When large amounts of heavy metal ions were present, a modified method was developed with citrate as the only complexing ligand for all five lanthanides. The method here developed permits the analyses of lanthanide ions in aqueous solution without prior separation and involves the use of comparatively inexpensive instrumentation (UV absorption spectrophotometer).

Energy transfer from the excited uranyl ion to various lanthanide ions in aqueous solution

Energy transfer from the excited uranyl ion to dysprosium was observed by means of detailed analysis of nanosecond laser flash photolysis.

A study on hydrolytic polymerization of lanthanide ions

AbstractExperimental data of hydrolytic polymerization of lanthanide ions in aqueous solution were treated by graphic method, computer fitting and pq analysis, species present were ascertained and hydrolysis constants obtained. Ln (OH)2+ and Ln2 (OH)24+ predominated in the species of all hydrolysis products. When the first and the second hydrolysis constants were plotted against radii and effective nuclear charge of lanthanide ions, the curves obtained conformed with the “three‐division groups” rule. Correlation between hydrolysis constants of Ln2 (OH)24+ and other hydrolysis constants is linear.Using the above empirical correlations we calculated hydrolysis constants of all lanthanide ions and obtained satisfactory results which showed good regularity for hydrolysis of lanthanide ions and thus systematized all data of the reaction.

Direct determination of traces of lanthanide ions in aqueous solutions by laser-induced time-resolved spectrofluorimetry

The most convenient spectral regions for dye laser excitation are presented for seven lanthanide ions. Fluorescence lifetimes are given for 0.5 M sulphuric acid and 3 M potassium carbonate media. With the excimer-pumped dye laser, the detection limits are in the range 0.1 μg 1 −1 (Eu) to 50 μg 1 −1 (Gd). Examples of the resolution of mixtures are given.