Ytterbium cations

Attempts to synthesize the clathrochelate complexes of lanthanide ions via template condensation of the tripodal amine tren with formaldehyde bis-(dimethylamino)methane derivative on the rare-earth metal ion were successful only for ytterbium. The [Yb(metr)](CF3S03)3 AN clathrochelate was obtained in 3-5% yield [165]. With ytterbium cation, as well as with cerium, praseodymium, europium, yttrium, and lanthanum ions, the major reaction products proved to be mono- and dibridged semiclathrochelate complexes with ligands 1 and 2 (Scheme 71). 

Kobayashi et al. discovered that Yb(OTf)3 and other lanthanide triflates (l,ri(() lf)(, Ln=La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, and Er) are excellent catalysts of hydroxymefhylation of propiophenone TMS enolate with aqueous formaldehyde solution at room temperature (Scheme 10.22) [70, 71]. The Yb(OTf) j-catalyzed hydroxymefhylation of a variety of SEE, including sterically hindered compounds, proceeds regiospecifically in high yield. In addition, almost 100% of Yb(OHf). is quite easily recovered from fhe aqueous layer and can be reused. Yb(OTf)3 also has high catalytic activity in fhe aqueous aldol reaction of other aldehydes. Interestingly, the catalytic activity is rather low in the absence of water. In aqueous media water would coordinate to ytterbium to form active ytterbium cations. 

Very recently, Aleonard et al. (1982) solved the structure of CsYbsFio, which is very similar to the KYb2F7 structure (see section 4.4) and can be regarded as a superstructure of the latter. The ytterbium cations are again seven-coordinated, and the pentagonal bipyramids form a network of hexagonal cation layers with the sequence -AA- in the direction of the x-axis. The cesium cations are similar to the potassium ions in KYb2Fv because they also reside halfway between the ytterbium layers. ... 

A proposed mechanism for this reaction involves the following three steps to generate the nitronium ion. The trifluoromethanesulfonate (triflate) ions act as spectators. The ytterbium cation is believed to be hydrated by the water present in the aqueous nitric acid solution. Nitric acid binds strongly to the hydrated ytterbium cation, as shown in equation 1. A proton is generated, as shown in equation 2, by the strong polarizing effect of the metal. Nitronium ion is then formed by the process shown in equation 3. Although the nitronium ion may serve as the active electrophilic species, it is more likely that a nitronium carrier, such as the... 

A product of different type is formed when ytterbium reacts with dichloride Ph2SnCl2. In this case the process includes a deep transformation of the initial tin dichloride leading to the ionic complex with tetratin anions [(Ph3Sn)3Sn] and binuclear ytterbium cation [(DME)3Yb(n-Cl)]2 [31]. 

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. 

Enthalpies of solution of rare-earth trichlorides in dimethyl sulfoxide are markedly more negative than in water, though the difference decreases on going from lanthanum across to ytterbium. Less favorable solvation of the chloride ions by the dimethyl sulfoxide must be more than balanced by favorable solvation of the rare-earth 3+ cations (cf. Section VI,B). This dimethyl sulfoxide-versus-water comparison should be contrasted with the alcohol-versus-water comparisons discussed earlier. 

For further contributions on the dia-stereoselectivity in electropinacolizations, see Ref. [286-295]. Reduction in DMF at a Fig cathode can lead to improved yield and selectivity upon addition of catalytic amounts of tetraalkylammonium salts to the electrolyte. On the basis of preparative scale electrolyses and cyclic voltammetry for that behavior, a mechanism is proposed that involves an initial reduction of the tetraalkylammonium cation with the participation of the electrode material to form a catalyst that favors le reduction routes [296, 297]. Stoichiometric amounts of ytterbium(II), generated by reduction of Yb(III), support the stereospecific coupling of 1,3-dibenzoylpropane to cis-cyclopentane-l,2-diol. However, Yb(III) remains bounded to the pinacol and cannot be released to act as a catalyst. This leads to a loss of stereoselectivity in the course of the reaction [298]. Also, with the addition of a Ce( IV)-complex the stereochemical course of the reduction can be altered [299]. In a weakly acidic solution, the meso/rac ratio in the EHD (electrohy-drodimerization) of acetophenone could be influenced by ultrasonication [300]. Besides phenyl ketone compounds, examples with other aromatic groups have also been published [294, 295, 301, 302]. 

The rare earth (RE) ions most commonly used for applications as phosphors, lasers, and amplifiers are the so-called lanthanide ions. Lanthanide ions are formed by ionization of a nnmber of atoms located in periodic table after lanthanum from the cerium atom (atomic number 58), which has an onter electronic configuration 5s 5p 5d 4f 6s, to the ytterbium atom (atomic number 70), with an outer electronic configuration 5s 5p 4f " 6s. These atoms are nsnally incorporated in crystals as divalent or trivalent cations. In trivalent ions 5d, 6s, and some 4f electrons are removed and so (RE) + ions deal with transitions between electronic energy sublevels of the 4f" electroiuc configuration. Divalent lanthanide ions contain one more f electron (for instance, the Eu + ion has the same electronic configuration as the Gd + ion, the next element in the periodic table) but, at variance with trivalent ions, they tand use to show f d interconfigurational optical transitions. This aspect leads to quite different spectroscopic properties between divalent and trivalent ions, and so we will discuss them separately. 

Solutions of alkali metals in ammonia have been the best studied, but other metals and other solvents give similar results. The alkaline earth metals except- beryllium form similar solutions readily, but upon evaporation a solid ammoniste. M(NHJ)jr, is formed. Lanthanide elements with stable +2 oxidation states (europium, ytterbium) also form solutions. Cathodic reduction of solutions of aluminum iodide, beryllium chloride, and teUraalkybmmonium halides yields blue solutions, presumably containing AP+, 3e Be2, 2e and R4N, e respectively. Other solvents such as various amines, ethers, and hexameihytphosphoramide have been investigated and show some propensity to form this type of solution. Although none does so as readily as ammonia, stabilization of the cation by complexation results in typical blue solutions... 

Oxidative homo-coupling of enolates from acyl oxazolidinones to give the corresponding dimers can be achieved in the presence of oxidants. Titanium and ytterbium enolates of 252 were coupled in the presence of a chiral diol or chiral bisoxazoline in the presence of ferrocenium cation 254 (Scheme 63) [166]. The amount of the meso dimer varied with the chiral ligand with a maximum of 5 1. TADDOL 172 performed best providing a 76% ee for the meso product. Ytterbium enolate gave a low ee of 34% with the same ligand. 

Ytterbium luminescence has also been seen for a nonametallic cluster assembled from hexylsalicylate (hesa) and with overall formula H i o Yb<)(hcsa) i e (/x-0)i o (NO3). The cluster has two fused square-pyramidal pentametallic units assembled via an apical metal ion. The two square pyramids are twisted 45° and the cluster core is stabilized by hydroxide bridges. The luminescence decay of the Ybm cluster is bi-exponential, with lifetimes of 0.2 and 0.6 ps, reflecting the two types of cations (Manseki et al., 2006). 

The only complexes of lanthanum or cerium to be described are [La(terpy)3][C104]3 175) and Ce(terpy)Cl3 H20 411). The lanthanum compound is a 1 3 electrolyte in MeCN or MeN02, and is almost certainly a nine-coordinate mononuclear species the structure of the cerium compound is not known with any certainty. A number of workers have reported hydrated 1 1 complexes of terpy with praseodymium chloride 376,411,438), and the complex PrCl3(terpy)-8H20 has been structurally characterized 376). The metal is in nine-coordinate monocapped square-antiprismatic [Pr(terpy)Cl(H20)5] cations (Fig. 24). Complexes with a 1 1 stoichiometry have also been described for neodymium 33, 409, 411, 413, 417), samarium 33, 411, 412), europium 33, 316, 411, 414, 417), gadolinium 33, 411), terbium 316, 410, 414), dysprosium 33, 410, 412), holmium 33, 410), erbium 33, 410, 417), thulium 410, 412), and ytterbium 410). The 1 2 stoichiometry has only been observed with the later lanthanides, europium 33, 411, 414), gadolinium, dysprosium, and erbium 33). 

Some substitution of strontium (up to 14 mol.%), of lead (2 mol.% reported) but no barium has been reported in aragonite, although investigations at elevated temperatures and pressures show almost complete miscibility of these elements in the structure (Gaines et al., 1997, p. 442), and SrCOs (strontionite), BaCOs (witherite), and PbCOs (cerussite) are common minerals. A calculated plot (Figure 3(b)) for cations in ninefold coordination shows that this coordination theoretically allows trivalent rare earth elements and quadravalent and many other elements to be substituents in the structure. Ytterbium, europium, samarium, and radium carbonates with aragonite structure have been synthesized (Spear, 1983). 

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