Lanthanide complexes hydroxy acids

The use of aqueous chiral lanthanide complexes in the determination of the enantiomeric purity of chiral a-hydroxy acids has also been assessed by H NMR [21], Large lanthanide induced shifts, chemical shift non-equivalence and an apparent absence of kinetic resolution in complex formation is observed upon addition of racemic lactate to [Yb.3a]3+ (Figure 1). The lactate CH3 resonances are clearly resolved for the... 

More intense bands in the 4f-4f region were observed with lanthanide complexes of mandelic, salicylic, thiosalicylic, furoic, and thiophenic acids than the acetate and haloacetate analogues [235]. Both increase in intensity and nephelauxetic effect have been observed. It seems plausible that the aromatic groups in the hydroxy acids such as benzene, thiophene or furan contribute significantly to the intensity of the hypersensitive bands of the complexes. It appears that pH has an important role in determining the stoichiometry of the complex formed. Only ML species is formed up to pH 4 and both ML2+ and ML2 are formed at a pH of 6.0. 

Complexes of lactic acid with a wide variety of metal ions are known and their stability constants have been determined. A variety of techniques has been used in the study of the species formed in the solutions of this hydroxy acid and inorganic ions. Electrochemical and spectrophoto-metric methods have been used with the main aim of determining stability constants of the complex ions. CD and optical rotatory dispersion (ORD) have also proved to be powerful methods in studying, for instance, Mo, Mo , Cu" and Co", as well as lanthanide complexes. Very recently, Brittain et used circularly polarized luminescence (CPL) techniques in the study... 

Reported coordinating properties of aromatic hydroxy acids have been concerned mainly with salicylic acid (2) and substituted salicylic acids (12). Most published work refers to solution, where detailed characterization of the stereochemistry of the species formed is very rare, A series of recent publications by Lajunen ef a/. " on the coordination chemistry of substituted hydroxynaphthoic adds deals with the complexation of Fe ", Al ", Be" and lanthanides(III) by these ligands. The... 

The L1 complexes of the middle lanthanides Gd(III), Eu(III), and Tb(III) decompose less rapidly at pH 7.4, 37 °C than do the L1 complexes of La(III) or Lu(III) (14). The fit of the lanthanide ion into the macrocycle may be important here. Certainly, the macrocycle fit will vary for La3+ (116 pM) compared to Lu3+ (97.7 pM) (41). A recent study using luminescence measurements suggests a greater lability of the Eu(L1)3+ complex than previously reported (28). Detection of the Eu(DPTA)-complex produced upon addition of diethylenetriaminepentaacetic acid (DTPA) to Eu(L1)3+ indicates that the complex decomposes approximately 12% in 48 h at 37 °C, pH 7.4. It is noteworthy that solutions of Eu(L1)3+ contain two different species (28). One of them, possibly a hydroxy-bridged dimer, is present in greater amounts at high concentrations of Eu(L1)3+. 

Carboxylic and hydroxy carboxylic acids form complexes readily with lanthanides with high stability constants (Chapter 3) and they have been widely used in the ion exchange... 

Complex formation is useful for metal speciation and also for the separation of diverse metal ions. Among a variety of complexing reagents [20-22] cyanide is probably the most important. IPC separation of metal ions as metallocyanide complexes with a suitable cationic IPR is a reliable technique [23]. Complexation of trace level lanthanides with a-hydroxy isobutyric acid and separation under IPC condition shortened analysis time from days to minutes [24]. Flow injection was successfully coupled to IPC to simplify batch precomplexation detection limits were at sub-microgram per liter levels [2]. 

Since tropolones and 3-hydroxy-4-pyrones are taken in this chapter to be enols, we now cite their binding as enolato ligands to lanthanum and all the other trivalent lanthanides (save the radioactive promethium) " " . Likewise, we note such studies for complexes with enolato ligands derived from 3-acetyl-4-hydroxycoumarin, dehydroacetic acid and their oximes, and with the aromatic enediolates, squarate and croconate. Periodic trends in thermodynamic parameters were reported and analyzed in these studies. 

Based on earlier research on the separation of the lanthanides, ion exchange separations were also done using citric add as a complexant. Because the citrate eluent is plagued by relatively slow kinetics, other hydroxy-carboxylic adds were tested as elutriants for the higher actinide elements. Lactic add was among the first spedes tried, but better interactinide separations were observed using a-hydroxyisobutyric acid... 

The inorganic lanthanide triflate complexes Ln(OTf)3 (made in aqueous solution) have been shown by Kobayashi to be efficient Lewis-acid catalysts for hydroxy-methylation (using commercial aqueous formaldehyde solutions) of silicon enolates in aqueous medium (water -i- THF) or even in water alone in the presence of a surfactant. In these reactions, activation proceeds by coordination of the aldehyde oxygen atom by the Ln center that is a strong Lewis acid due to its hard character. Among the lanthanide triflates, ytterbium triflate was found to be the most active catalyst, but scandium triflate can sometimes also be efficiently used. Enantio-selective versions are also known in the presence of chiral macrocyclic ligands. The water-soluble catalyst is recovered in water after extraction of the organic products. 

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