LANTHANIDE IONS titrations

Complexation of dendritic ligands 1 and 2 with lanthanide ions (Nd3+, Eu3+, Gd3+, Tb3+, Dy3+) [17f] leads to results qualitatively similar to those obtained upon Zn2+ complexation (see above) an increase in the monomer naphthalene emission band at 337 nm and a complete disappearance of the exciplex band at 480 nm. However, the complex stoichiometry is different. Emission data were best fitted considering the formation of 1 3 and 1 2 (metal/ligand) complexes (log f 1 2 = 14.1 and log [ivi = 20.0) in the case of 1 and a 1 3 (metal/ligand) complex (log / 1 3 = 20.3) for compound 2. Therefore, at low metal ion concentration, only the [M(2)3]3+ species is present, as also demonstrated by NMR titration. It is likely that in this complex... 

The detection of aromatic carboxylates via the formation of ternary complexes using lanthanide ion complexes of functionalised diaza-crown ethers 30 and 31 has been demonstrated [134]. Like the previous examples, these complexes contained vacant coordination sites but the use of carboxylic acid arms resulted in overall cationic 2+ or 1+ complexes. Furthermore, the formation of luminescent ternary complexes was possible with both Tb(III) and Eu(III). A number of antennae were tested including picolinate, phthalate benzoate and dibenzoylmethide. The formations of these ternary complexes were studied by both luminescence and mass spectroscopy. In the case of Eu-30 and Tb-30, the 1 1 ternary complexes were identified. When the Tb(III) and Eu(III) complexes of 30 were titrated with picolinic acid, luminescent enhancements of 250- and 170-fold, respectively, were recorded. The higher values obtained for Tb(III) was explained because there was a better match between the triplet energy of the antenna and a charge transfer deactivation pathway compared to the Eu(III) complex. 

The kinetics of the hydrolysis of di(2,4-dinitrophenyl) phosphate (DDNPP) were studied in basic solutions buffered with Bis-Tris propane (BTP) in the presence of La3+, Sm3+, Tb3+, and Er3+. Two equivalents of the 2,4-dinitrophenolate ion were liberated for each equivalent of DDNPP and the reaction showed first-order kinetics. Potentiometric titrations showed the formation of dinuclear complexes such as [Ln2(BTP)2(OH) ](6 " i, with values of n varying as a function of pH for all studied metals. Hence the catalytic effect depends on the formation of dinuclear lanthanide ion complexes with several hydroxo ligands.97... 

One other point of controversy concerns the Ae values obtained in UV difference titrations of lanthanide ions. These are significantly higher than the corresponding values for transition metal ions (7400-8700 cm compared with 5000-6600 cm ), leading... 

Titration curves of HS fluorescence quenching versus concentration of added metal quencher have been used to obtain the CC values of HS ligands and the stability constants of HS-metal complexes (Saar and Weber, 1980, 1982 Underdown et al., 1981 Ryan et al., 1983 Weber, 1983 Dobbs et al., 1989 Grimm et al., 1991 Hernandez et al., 2006 Plaza et al., 2005, 2006). Two fluorescence techniques, lanthanide ion probe spectroscopy (LIPS) and fluorescence quenching of HSs by Cu-+, have been used in conjunction with a continuous distribution model to study metal-HS complexation (Susetyo et al., 1991). In the LIPS technique, the HS samples are titrated by Eu-+ ions, and the titration plot of the ratio of the intensities of two emission lines of Eu + is used to estimate the amount of bound and free species of the probe ion. In the other technique, titration curves of fluorescence intensity quenched by Cu versus the logarithm of total added Cu2+ are used. 

Lanthanide ions are also excellent candidates to include in cyclam-cored dendrimers. These systems were investigated using ions such as Nd(III), Eu(III), Gd(ffl), Tb(III), or Dy(III) (Fig. 5.9) [57].1H NMR titration confirmed the presence of [M(dendrimers)2]2 + self-assembled macrostructures, and was accompanied, as... 

Titration of the ditopic pentadentate ligands with either a kinetically labile d-transition metal (M = Fe°, Co", Zn") or a lanthanide ion often leads to the observation of several different species in equilibrium. Qn the other hand, the RML3 helicate is always the most abimdant species when the titrant is a stoichiometric R"Vm" mixture, when the total ligand concentrations is >10 M, and in presence of poorly coordinating anions the other complexes are often not detected in NMR spectra under these conditions. The spectrophotometric data have been fitted to the following set of equations (A stands for nd (M) or for 4f (R) ion, charges are omitted for clarity) ... 

Lanthanide ions have been used as substitutes for Ca and Zn° in proteins to obtain information on the number of metallic sites (by simple titration) and on their composition. This may of course be extended to any molecule or materials. One very useful transition in this respect is the highly forbidden and faint Eu ( Do Fo) transition which is best detected in excitation mode by analyzing the emission of the hypersensitive transition Do — Fa since both the emitting and end states are ncm-degenerate, its number of components indicates the number of different metal-ion sites. Moreover, the energy of this transition depends on the nephelauxetic effect generated by coordinated atoms and ions at 298 K ... 

Lutetium, lanthanum, and, to some extent, yttrium are often chosen in these NMR experiments, as they are nonparamagnetic ions. The presence of one main species is attested by the presence of one series of peaks, and the more symmetrical the coordination sphere the less peaks present (ligands are then equivalents at the NMR time scale). A titration can also be conducted with increasing amoimts of the Ln , giving information on the formation of intermediate species. Alternatively, H-NMR spectra of europium species (paramagnetic ion) have been reported in the literature, giving access to spectra with extended chemical shift scale and broader peaks but for which information can still be obtained for the coordination sphere of the lanthanide ion. 

The competition titrations presented in the previous paragraph are interesting to establish the stability of a given complex compared to another. They are also useful tools to ensure the kinetic inertness of the complex over time. Any decrease of intensity would iudicate a modificatiou of the coordination sphere. On release of any ligand, the lanthanide ion would be exposed to water molecules in its vicinity, thus causing a decay of its luminescence intensity. Nevertheless, in snch long experiments, one has to ensure that the lamp is stable over time, that no evaporation of the solution or precipitation takes place over time, etc. 

The reaction of linear bissalamo 29 with Zn + quantitatively gives the trinuclear Zn + complex, which is converted to a variety of heterotrinuclear complexes in a synergistic fashion (Scheme 1.9). The formation of the Zn complex proceeds very cooperatively. The intermediary 1 1 and 1 2 complexes were not observed in the H NMR titration experiment. Only the central Zn was replaced by Ca " and lanthanide ions. 

At a sufficiently low pH the ions form unhydrolyzed aqua complexes if stronger complexing ligands are not present. Hydrolyzed species begin to form first around a pH of 4 to 6, depending on the basicity of the lanthanide ion and on the other ions present (Burkov et al., 1982). For instance, solutions of EuClj and Eu2(S04)j, when titrated with NaOH, gave initial precipitation at pH = 6 and Eu(N03)3 at pPI = 6.5 (Mironov and Polyashov, 1970). Due to its smaller ionic radius the hydrated Sc ", however, readily undergoes acid dissociation above pH = 3.0 (Komissarova, 1980). 

Hydroxides. Thorium (TV) is generally less resistant to hydrolysis than similarly sized lanthanides, and more resistant to hydrolysis than tetravalent ions of other early actinides, eg, U, Np, and Pu. Many of the thorium(IV) hydrolysis studies indicate stepwise hydrolysis to yield monomeric products of formula Th(OH) , where n is integral between 1 and 4, in addition to a number of polymeric species (40—43). More recent potentiometric titration studies indicate that only two of the monomeric species, Th(OH) " and thorium hydroxide [13825-36-0], Th(OH)4, are important in dilute (<10 M Th) solutions (43). However, in a Th02 [1314-20-1] solubiUty study, the best fit to the experimental data required inclusion of the species. Th(OH) 2 (44). In more concentrated (>10 Af) solutions, polynuclear species have been shown to exist. Eor example, a more recent model includes the dimers Th2(OH) " 2 the tetramers Th4(OH) " g and Th4(OH) 2 two hexamers, Th2(OH) " 4 and Th2(OH) " 2 (43). 

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