Lanthanoid salts

Fukuzawa, S., Fujinami, T., Yamauchi, S., Sakai, S. 1,2-Regioselective reduction of a,P-unsaturated carbonyl compounds with lithium aluminum hydride in the presence of lanthanoid salts. J. Chem. Soc., Perkin Trans. 11986, 1929-1932. 

Lanthanoid salts with non-coordinating anions carrying long perfluoroalkyl chains, Yb[C(SO2CgFi7)3[3 (19) and Sc[C(SO2CgFi7)3[3 (20), have been successfully used as Lewis catalysts for O-acylations, Friedel-Crafts, Diels-Alder, and Mu-kayama aldol reactions in fluorous biphasic media [21] (Scheme 3.6). In these reactions the fluorous medium avoids deactivation of the Lewis acid by solvent coordination. The catalyst can also be recycled and reused. 

The complexes between lanthanoid salts and crown ethers are usually isolated from nonaqueous solutions, the Ln(III)/polyether interaction being very small in water due to unfavorable energetics in removing water molecules from the inner coordination sphere of the metal ion. We report here the synthesis and the properties of complexes with 12-crown-4, 15-crown-5, and 18-crown-6 ethers, having different metal/crown ratios, namely 1 1, 1 2, and 4 3. The singlecrystal X-ray structures of four complexes have been solved. In the 1 1 complexes Eu(N03)3 (12-crown-4) and Eu(N03)3-(15-crown-5), which crystallize in a chiral space group, and Nd(N03)3-(18-crown-6), the metal ion displays coordination numbers of 10, 11, and 12, respectively. The first two complexes have similar structures the polyether sits to one side of the Eu(III) ions and the three bidentate nitrate groups are coordinated on the opposite side. The structure of [Nd(N03)3]4-(18-crown-6)3 revealed that this complex has to be formulated as [Nd(N03)2-( 18-crown-6)]3[Nd(N03)3]6. 

Except for Ln = Ce, the lanthanoid salts are prepared from the corresponding oxides (Glucydur, 99.99%) and concentrated acids (Merck). The solutions are filtered, evaporated to dryness, and the salts are dried for three days in a desiccator (KOH) and then for three days under vacuum (60°/2 x 10-2 torr). The salts contain one to five molecules of H20 per formula weight. 

For many purposes it is not particularly necessary to separate the metals, but if separation is required, the process is complex. Initially, the metals are extracted as salts from the ores by extraction with sulfuric acid (H2SO4), hydrochloric acid (HCl), and sodium hydroxide (NaOH). Modem purification techniques for these lanthanoid salt mixtures are ingenious and involve selective complexation techniques, solvent extractions, and ion exchange chromatography. 

Chemoselective reduction of aldehydes in the presence of ketones has been performed with lithium borohydride adsorbed on molecular sieve zeolites of types A and X. Although it is tempting to postulate that only aldehydes can penetrate into the pores containing the borohydride, preliminary evidence does not seem to support this idea. The more unusual reverse chemoselectivity, that is reduction of only the ketones in ketone-aldehyde mixtures, has been demonstrated for sodium borohydride in the presence of lanthanoid cations (Ln ) such as Ce (Scheme 7). Lanthanoid salts are known to catalyse the... 

These compounds are considered to have a role in the cathodic inhibition properties of lanthanoid salts.They are strong bases and absorb CO from the auto give carbonate species. The solubilities, and basicities of the rare earth hydroxides, as well as the pH values at which they precipitate, decrease with increasing atomic number, with Y(OH)3 being similar to Ho/Er(OH)3 because of similar ionic radii for the Ln ions. Similarly, CeO. xH O is precipitated by addition of base to Ce salts, for example sulfate or nitrate or double salts with ammonium sulfate or nitrate, and by oxidation of Ce(OH)3 precipitates by air (see Section 1.5.1). Single crystals of Ln(OH)3 have been grown by hydrothermal methods. For La(OH)3 to Er(OH)3, the compounds have hexagonal structures with 9-coordination for whereas Lu(OH)3 and Sc(OH)3 have cubic... 

The lanthanum or europium atoms had no interaction with the gold and silver centers, and the determined crystal structures exhibited hexagonal arrays of metal-metal interactions with diagonal interactions in the layer and the lanthanoids in the middle of the hexagonal prisms. In both examples, the lanthanum and the europium salts were isostructural (Fig. 5). 

The proposed mechanism for this catalytic asymmetric hydrophosphonylation is shown in Figure 35. The first step of this reaction is the deprotonation of dimethyl phosphite by LPB to generate potassium dimethyl phosphite. This potassium phosphite immediately coordinates to a lanthanoid to give I due to the strong oxophilicity of lanthanoid metals. The complex I then reacts (in the stereochemistry-determining step) with an imine to give the potassium salt of the a-aminophosphonate. A proton-exchange reaction affords the product... 

Alternatively, and this procedure is preferred for the lanthanoid complexes [135], the imidazolium salt can be deprotonated in situ and reacted with a suitable trialkyl lanthanide or reacted without deprotonation with an anionic tetraalkyl lanthanide complex (see Figure 4.63). 

Sugiura, M., Effect of quaternary ammonium salts on carrier mediated transport of lanthanoid ions through cellulose triacetate membranes. Sep. Sci. Tech., 1993, 28 1453-1463. 

Fujiwara and coworkers" have discovered that anhydrous lanthanoid trichloride salts are reusable Friedel-Crafts catalysts for the benzylation of benzene. All of the lanthanoid trichlorides offer fairly good yields of benzylated product (equation 15). The yields do not drop if the recovered catalyst is used for a second time, or in some cases even a third time, for the benzylation of benzene. 

To a limited extent, lanthanoids are separated from each other by tertiary amine extractants and by quaternary ammonium salts with long (Cg and Cio) alkyl groups. Tertiary amines in an organic phase preferentially extract the trivalent actinoids better than the lanthanoids by salting out (dehydrating) the cations from the aqueous phase with high LiCl concentration, e.g., the TRAMEX (tertiary amine extraction) process for Cm isolation . Recent developments in trivalent f-element separations, such as chelating and bifunctional extractants have been reviewed . 

A number of phosphonate and phosphinate derivatives where the phosphorus atom is directly bonded to non-aromatic cyclic systems have been reported. The synthesis and reactions of a number of compounds with the general structure 103 have been reported. Enantiomerically pure cyclopropanephosphonic acids which are constrained analogues of the GABA antagonist phaclophen, have been prepared by stereocontrolled Michael addition of a-anions derived from chiral chloromethylphosphonamides 104 to a,P-unsaturated esters followed by in situ cyclisation. Other asymmetric syntheses include those of (/ )- and (S)-piper-idin-2-ylphosphonic acid (105) via the addition to trialkyl phosphites to iminium salt equivalents and 4-thiazolidinylphosphonate 106 by catalytic asymmetric hydrophosphonylation of 3-thiazoline. In the latter case both titanium and lanthanoid (which give much better e.e. values) chiral catalysts are used. 

The hexafluorophosphate ion has proved to be very useful for studies with complexes, since the [PF6] anion has a very weak coordinating ability.1 Lan-thanoid hexafluorophosphates can be obtained in very concentrated aqueous solutions by reaction of a freshly prepared solutions of hexafluorophosphoric acid and a hydrated lanthanoid basic carbonate.2 The resulting solution, after filtration, is evaporated to near dryness. Attempts to isolate the hydrated salts are unsuccessful because of decomposition accompanied by hydrogen fluoride evolution. Nevertheless, the complexes containing diphenylphosphinic amide are isolable and are quite stable.3... 

In 1992, Shibasaki et al. reported for the time an application of chiral heterobimetallic lanthanoid complexes (LnLB) as chiral catalysts in asymmetric catalysis, namely the catalytic asymmetric nitroaldol reaction (Henry reaction), which is one of the most classical C-C bond forming processes [11]. Additionally, this work represents the first enantioselective synthesis of (3-nitroalcohol compounds by the way of enantioselective addition of nitroalkanes to aldehydes in the presence of a chiral catalyst. The chiral BINOL based catalyst was prepared starting from anhydrous LaCl3 and an equimolar amount of the dialkali metal salt of BINOL in the presence of a small amount of water [9]. 

The Ln ions are hard and show a preference for F and O-donor ligands, e.g. in complexes with [EDTA] Section 24.5), [Yb(OH)g] (equation 24.14) and in 3-diketonate complexes Box 24.4). In their aqua complexes, the Ln ions are typically 9-coordinate, and a tricapped trigonal prismatic structure has been confirmed in crystalline salts such as [Pr(H20)9][0S03Et]3 and [Ho(H20)g][0S03Et]3. High coordination numbers are the norm in complexes of Ln, with the highest exhibited by the early lanthanoids examples include ... 

Some difficulty was originally encountered in preparing homoleptic cr-bonded alkyl or aryl complexes of the actinoids, but (as for the lanthanoids. Section 24.8) use of the chelate TMED (Me2NCH2CH2NMe2) was the key to stabilizing the Li+ salt of [ThMey] (equation 24.42 and Figure 24.7a). Similarly, hexaalkyls of type Li2UR -7TMED have been isolated. 

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