Europium properties

Ethylamine, 338 W-Ethyl acetamide, 338 Ethyl bromide, 328 reactions of, 330 Ethyl iodide, 336 Ethylene, 346 chemical reactivity, 296 double bond in, 296 Ethylene glycol, 325 Ethyl group, 329 Europium, properties, 412 Exothermic reaction, 40, 135 Experiment, 2... 

Although rare-earth ions are mosdy trivalent, lanthanides can exist in the divalent or tetravalent state when the electronic configuration is close to the stable empty, half-fUed, or completely fiUed sheUs. Thus samarium, europium, thuUum, and ytterbium can exist as divalent cations in certain environments. On the other hand, tetravalent cerium, praseodymium, and terbium are found, even as oxides where trivalent and tetravalent states often coexist. The stabili2ation of the different valence states for particular rare earths is sometimes used for separation from the other trivalent lanthanides. The chemicals properties of the di- and tetravalent ions are significantly different. 

Separation Processes. The product of ore digestion contains the rare earths in the same ratio as that in which they were originally present in the ore, with few exceptions, because of the similarity in chemical properties. The various processes for separating individual rare earth from naturally occurring rare-earth mixtures essentially utilize small differences in acidity resulting from the decrease in ionic radius from lanthanum to lutetium. The acidity differences influence the solubiUties of salts, the hydrolysis of cations, and the formation of complex species so as to allow separation by fractional crystallization, fractional precipitation, ion exchange, and solvent extraction. In addition, the existence of tetravalent and divalent species for cerium and europium, respectively, is useful because the chemical behavior of these ions is markedly different from that of the trivalent species. 

Selective Reduction. In aqueous solution, europium(III) [22541 -18-0] reduction to europium(II) [16910-54-6] is carried out by treatment with amalgams or zinc, or by continuous electrolytic reduction. Photochemical reduction has also been proposed. When reduced to the divalent state, europium exhibits chemical properties similar to the alkaline-earth elements and can be selectively precipitated as a sulfate, for example. This process is highly selective and allows production of high purity europium fromlow europium content solutions (see Calcium compounds Strontiumand strontium compounds). 

A closely related method does not require conversion of enantiomers to diastereomers but relies on the fact that (in principle, at least) enantiomers have different NMR spectra in a chiral solvent, or when mixed with a chiral molecule (in which case transient diastereomeric species may form). In such cases, the peaks may be separated enough to permit the proportions of enantiomers to be determined from their intensities. Another variation, which gives better results in many cases, is to use an achiral solvent but with the addition of a chiral lanthanide shift reagent such as tris[3-trifiuoroacetyl-Lanthanide shift reagents have the property of spreading NMR peaks of compounds with which they can form coordination compounds, for examples, alcohols, carbonyl compounds, amines, and so on. Chiral lanthanide shift reagents shift the peaks of the two enantiomers of many such compounds to different extents. 

Lanthanide-doped inverse photonic crystals have been reported.282 The lattices were prepared by infilling self-assembled polystyrene sphere templates with a mixture of zirconium alkoxide and europium at 450 °C, the polystyrene spheres were burnt out leaving hollow spheres of air, and the infilled material was converted to Zr02 Eu3+. The PL properties of the resulting photonic lattice were reported.282 The possibility of including phosphors into photonic lattices could lead to many... 

Until very recently, studies of the use of luminescent lanthanide complexes as biological probes concentrated on the use of terbium and europium complexes. These have emission lines in the visible region of the spectrum, and have long-lived (millisecond timescale) metal-centered emission. The first examples to be studied in detail were complexes of the Lehn cryptand (complexes (20) and (26) in Figure 7),48,50,88 whose luminescence properties have also been applied to bioassay (vide infra). In this case, the europium and terbium ions both have two water molecules... 

Organic fluorescent dyes with the appropriate spectral properties also can be paired with lanthanide chelates in FRET systems. For instance, many rhodamine dyes and the cyanine dye Cy5 have ideal excitation wavelengths for receiving energy from a nearby europium chelate. The LeadSeeker assay system from GE Healthcare incorporates various Cy5-labeled antibodies for developing specific analyte assays. In addition, if using a terbium chelate as the donor, then a Cy3 fluorescent dye can be used in assays as the acceptor. 

In addition to the coupled-signal method just described, phosphorylated carbon signals can be detected by use of praseodymium chloride, which displaces a- and /8-carbon resonances of a,/8-D-mannose 6-phosphate and a-D-mannosyl phosphate downfield, with little effect on other resonances. Europium chloride has analogous properties, except that the displacements are upfield. With certain polysaccharides, such as the O-phosphonomannan of Hansenula capsulata (29), the sig-... 

Europium and ytterbium di-valence. The oxidation state II for Eu and Yb has already been considered when discussing the properties of a number of divalent metals (Ca, Sr, Ba in 5.4). This topic was put forward again here in order to give a more complete presentation of the lanthanide properties. The sum of the first three ionization enthalpies is relatively small the lanthanide metals are highly electropositive elements. They generally and easily form in solid oxides, complexes, etc., Ln+3 ions. Different ions may be formed by a few lanthanides such as Ce+4, Sm+2, Eu+2, Yb+2. According to Cotton and Wilkinson (1988) the existence of different oxidation states should be interpreted by considering the ionization... 

A. R. Bugos, S. W. Allison, and M. R. Cates, Laser-induced fluorescent properties of europium-doped scandium orthophosphate phosphors for high-temperature sensing applications, Proc. of IEEE 1991 Southeast Conf, 1143-1147 (1991). 

After the discovery of plutoninm and before elements 95 and 96 were discovered, their existence and properties were predicted. Additionally, chemical and physical properties were predicted to be homologous (similar) to europium (gjEu) and gadolinium ( Gd), located in the rare-earth lanthanide series just above americium (gjAm) and curium ((,jCm) on the periodic table. Once discovered, it was determined that curium is a silvery-white, heavy metal that is chemically more reactive than americium with properties similar to uranium and plutonium. Its melting point is 1,345°C, its boihng point is 1,300°C, and its density is 13.51g/cm. ... 

Am3+ is the most stable oxidation state of the metal. In trivalent state, its properties are simdar to europium. Am3+ reacts with soluble fluoride, hydroxide, phosphate, oxalate, iodate and sulfate of many metals forming precipitates of these anions e.g., Am(OH)3, Am(103)3, etc. 

Trivalent europium is an excellent ionic probe for materials and its luminescence properties are extensively studied. Eu is one of the mostly informative elements in mineralogy, especially when the ratio Eu /Eu may be assessed. Both oxidation states are luminescent, but the hnes of Eu in minerals are usually very weak and concealed by other centers. By steady state liuninescence spectroscopy its luminescence has been confidently detected only in scheehte and anhydrite (Tarashchan 1978 Gorobets and Rogojine 2001). 

Of all the properties of the rare earths that contribute to their many and varied applications one that ranks of special interest is the extremely high thermal neutron capture cross-section associated with the elements gadolinium, samarium, europium and dysprosium, see Table IV. 

In 1901 Eugene-Anatole Demargay in Paris showed that the samples of samarium and gadolinium produced until that time harboured yet another rare-earth element, which he named generously after all of Europe europium. This element is in fact one of the most naturally abundant of the group the Earth s crust contains twice as much europium as tin. It is harvested today largely for a very special and useful property its emission of very pure red and blue light. 

The very negative Ln t + /Ln potentials are consistent with the electropositive nature of the lanthanide elements their Allred-Rochow electronegativity coefficients are all 1.1 except for europium, which has a value of 1.0. The lighter elements of Group 3, Sc and Y, both have electronegativity coefficients of 1.3. The nearest p-block element to the lanthanides in these properties is magnesium (Mg2+/Mg) = -2.37 V, and its electronegativity coefficient is 1.2. 

The unsymmetrical derivative, L2, in which one benzimidazole group is replaced by a 2-pyridyl moiety, and its complexes with europium and terbium have been further studied for their photophysical properties (37). The coordinated nitrate anions in the [ ( 2)( ) ]... 

For those complexes where the chromophore is not coordinated to the metal center directly, the orientation of the chromophore is important to ensure efficient energy transfer. The series of ligands L29-L32 were investigated for correlations between structural parameters found in the solid state (see, for example, Fig. 12) and solution (by NMR spectroscopy) and photophysical properties (69,70). It was found that both chromophore-metal separation and the angle of orientation have a direct influence on the quantum yield of the europium complexes. For example, the difference in quantum yield between [Eu(L29)]3+ and [Eu(L30)]3+ (0.06 and 0.02, respectively) cannot be attributed solely to the chromophore-metal separation, so may also depend on the better orientation of the chromophore in the L29 system as measured by the angle a between the metal center, the amide nitrogen atom, and the center of the phenyl ring. 

Photophysical studies have been conducted on a number of lanthanide complexes of calix[n]arenes, and a significant number of these are discussed in a recent review (79). The first europium and terbium calixarene complexes showed promising photophysical properties, with terbium luminescence lifetime of 1.5 ms and quantum yield of 0.20 in aqueous solution (80). 

Mono- and bimetallic lanthanide complexes of the tren-based macrobicyclic Schiff base ligand [L58]3- have been synthesized and structurally characterized (Fig. 15), and their photophysical properties studied (90,91). The bimetallic cryptates only form with the lanthanides from gadolinium to lutetium due to the lanthanide contraction. The triplet energy of the ligand (ca. 16,500 cm-1) is too low to populate the terbium excited state. The aqueous lifetime of the emission from the europium complex is less than 0.5 ms, due in part to the coordination of a solvent molecule in solution. A recent development is the study of d-f heterobimetallic complexes of this ligand (92) the Zn-Ln complexes show improved photophysical properties over the homobinuclear and mononuclear complexes, although only data in acetonitrile have been reported to date. 

Prodi, L. Maestri, M. Balzani, V. Lehn, J.-M. Roth, C. Luminescence properties of cryptate europium(III) complexes incorporating heterocyclic N-oxide groups. Chem. Phys. Lett. 1991,180, 45-50. 

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