Rare earth metals Ytterbium

Active catalyst species or catalysis intermediates can often be trapped by stoichiometric reactions of the precatalyst with the substrate. The following example describes the successful isolation of such an intermediate with participation of Ln-O cr-bonds. Reduction processes mediated by low oxidation states of the lanthanide elements are of special interest in organic synthesis [256]. One of the most intensively studied reactions is the stoichiometric reduction of arylketones by rare earth metals ytterbium and samarium [277]. Thus formed dianions possess high nucleophilic character and excess lanthanide metal can even accomplish complete cleavage of the C-O double bond (Scheme 36). 

The valences of the rare-earth metals are calculated from their magnetic properties, as reported by Klemm and Bommer.14 It is from the fine work of these investigators that the lattice constants of the rare-earth metals have in the main been taken. The metals lutecium and ytterbium have only a very small paramagnetism, indicating a completed 4/ subshell and hence the valences 3 and 2, respectively (with not over 3% of trivalent ytterbium present in the metal). The observed paramagnetism of cerium at room temperature corresponds to about 20% Ce4+ and 80% Ce3+, that of praseodymium and that of neodymium to about 10% of the quadripositive ion in each case, and that of samarium to about 20% of the bipositive ion in equilibrium with the tripositive ion. 

All the rare earth metals except samarium, europium, and ytterbium can be prepared in a pure form by reducing their trifluorides with calcium. Magnesium fluoride is less stable than the rare earth fluorides and so magnesium does not figure as a reductant. Lithium forms a fluoride which is stabler than some of the rare earth fluorides and thus finds some use as a reductant. 

On the other hand it may be noticed that some aspects of the chemistry and alloying behaviour of Ca, Sr and Ba could be conveniently compared with those of the divalent rare earth metals europium and ytterbium. 

Within the lanthanides the first ones from La to Eu are the so-called light lanthanides, the other are the heavy ones. Together with the heavy lanthanides it may be useful to consider also yttrium the atomic dimensions of this element and some general characteristics of its alloying behaviour are indeed very similar to those of typical heavy lanthanides, such as Dy or Ho. An important subdivision within the lanthanides, or more generally within the rare earth metals, is that between the divalent ones (europium and ytterbium which have been described together with other divalent metals in 5.4) and the trivalent ones (all the others, scandium and yttrium included). 

Yttrium (j Y) is often confused with another element of the lanthanide series of rare Earths— Ytterbium ( Yb). Also confusing is the fact that the rare-earth elements terbium and erbium were found in the same minerals in the same quarry in Sweden. Yttrium ranks second in abundance of all 16 rare-earth, and Ytterbium ranks 10th. Yttrium is a dark silvery-gray hghtweight metal that, in the form of powder or shavings, will ignite spontaneously. Therefore, it is considered a moderately active rare-earth metal. 

Recently, rare-earth metal complexes have attracted considerable attention as initiators for the preparation of PLA via ROP of lactides, and promising results were reported in most cases [94—100]. Group 3 members (e.g. scandium, yttrium) and lanthanides such as lutetium, ytterbium, and samarium have been frequently used to develop catalysts for the ROP of lactide. The principal objectives of applying rare-earth complexes as initiators for the preparation of PLAs were to investigate (1) how the spectator ligands would affect the polymerization dynamics (i.e., reaction kinetics, polymer composition, etc.), and (2) the relative catalytic efficiency of lanthanide(II) and (III) towards ROPs. 

Recovery of ytterbium from ores involves several processes that are mostly common to all lanthanide metals. These are discussed individually under each rare earth metal. Recovery involves three major steps (1) processing of ores, (2) separation of ytterbium from rare earth mixtures, and (3) preparation of the metal. 

The metal dissolves in dilute and concentrated mineral acids. Evaporation crystallizes salts. At ordinary temperatures, ytterbium, similar to other rare earth metals, is corroded slowly by caustic alkalies, ammonium hydroxide, and sodium nitrate solutions. The metal dissolves in liquid ammonia forming a deep blue solution. 

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). 

In order to prevent the use of an excess of the Lewis acid, TiC, for coupling of the electrophilic and nucleophilic flavanyl moieties, the rare earth metal Lewis acid, ytterbium(III)triflate [Yb(OTf)3] was recently employed to induce coupling between tetra-0-benzylcatechin (50) and tetra-0-benzylepicatechin (52), respectively, with the 4/3-methoxyepicatechin derivative (165). The appropriate deprotection steps gave access to procyanidins B-1 (90) and B-2 (5). Procyanidins B-3 (93) and B-4 (94) were similarly accessible via equimolar Yb(OTf)3-induced coupling of the catechin and epicatechin derivatives (50) and (52), respectively, with the 4/3-methoxycatechin derivative (143), followed by the appropriate deprotection steps. 

Europium amalgam is very easily prepared by this method. Simultaneously, some potassium amalgam is formed. After the electrolysis, the latter may be removed nearly completely by action of water, which scarcely attacks europium amalgam as long as any potassium amalgam remains. Ytterbium and samarium are the only other rare earth metals that yield amalgams by the electrolysis of their acetate-citrate solutions. The electrical efficiency is high for europium, considerably less for ytterbium, and small for samarium. 

The isolated compounds (table 16) show identical infrared spectra with a characteristic band at 1195 cm for a methyl group attached to a rare earth metal. The single crystal X-ray analysis of the yttrium (table 18, fig. 25) and of the ytterbium derivative (table 18) show both compounds to be isostructural with an approximately tetrahedral metal environment and a R(ju-CH3)2R unit like the trimethyl aluminum dimer. The and NMR spectra of the diamagnetic yttrium complex were invariant between — 40°C and +40°C with a triplet for the bridging methyl protons due to the coupling with the two equivalent yttrium atoms (tables 17, 19). 

Carboranyl derivatives of lanthanum, thulium and ytterbium are formed when the C-mercuro derivatives of methyl- and phenylcarboranes react with the rare earth metals in tetrahydrofuran at 20°C (Suleimanov et al., 1982a), or from the lithium derivatives of methyl- and phenylcarboranes with the rare earth trichlorides in benzene-ether at 20°C (Bregadze et al., 1983) as complexes with THF. A carboranyl derivative with a thulium-boron bond is also described. The reaction (eq. 62) may proceed via the formation of B-Tm-C derivatives, followed by disproportionation. 

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