Lanthanoid metals

Ytterbium triflate [Yb(OTf)3] combined with TMSG1 or TMSOTf are excellent reagents for the conversion of a-methyl styrene and tosyl-imines into homoallylic amides 32 (Equation (19)) (TMS = trimethylsilyl).29 These conditions produce the first examples of intermolecular imino-ene reactions with less reactive imines. Typically, glyoxalate imines are necessary. A comprehensive examination of the lanthanoid metal triflates was done and the activity was shown to directly correlate with the oxophilicity scale. The first report used preformed imines, and subsequently it was found that a three-component coupling reaction could be effected, bypassing the isolation of the intermediate imine.30 Particularly noteworthy was the successful participation of aliphatic aldehydes to yield homoallylic amines. 

As exemplified by zirconium and lanthanoid metals, various types of metal complexes have recently been developed as soluble catalysts for the polymerization of ethylene and propylene. In this Section these topics are briefly surveyed in connection with the synthesis of living polyolefins. 

Shen et al.120,121) found that the compounds of lanthanoid metals (from La to Lu) were active for the stereospecific polymerization of butadiene in the presence of alkylaluminum. Recently, Ouyangetal.122) reported that a NdCl3/C2H5OH/Al(C2Hs)3 catalyst exhibited a living character for the polymerization of diene and ethylene at temperatures below —30 °C. Diblock or triblock copolymers of diene and ethylene were obtained upon further addition of a diene monomer to a living polydiene or polyethylene. 

In both catalytic, asymmetric Michael reactions and nitroaldol reactions, enones and/or aldehydes appear to coordinate to the lanthanoid metal. Why, then, is LSB more effective for catalytic, asymmetric Michael reactions, whereas LLB is more effective for catalytic, asymmetric nitroaldol reactions This disparity might arise from slight differences in bond lengths in the chelated intermediate, as well as slight differences in bite angle for the BINOL moiety caused by varying the alkali metal. 

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... 

Cross-coupling reactions of ArCOAr. Reaction of Yb(0) with diaryl ketones changes the reactivity of the carbonyl group from electrophilic to nucleophilic. Thus in the presence of this lanthanoid metal, diaryl ketones couple with other ketones, nitriles, and epoxides to give pinacols, a-hydroxy ketones, and 1,3-diols, respectively, via the intermediate a. 

Bonding of ligands to alkali, alkaline earth and lanthanoid metal ions is mainly electrostatic. Consequently, these bonds can be described as a combination of electrostatic and van der Waals terms (see Chapter 15). Similar approaches have also been used for metallocene compounds (see Chapter 14). 

Using a highly hindered phosphino-alkoxide ligand, lanthanoid metal complexes [Ln(OCBu2CH2PMe2)3l (Ln = Y or Nd) have been obtained [30], These are the first monomeric homoleptic examples of lanthanide alkoxide complexes. The bulky substituents... 

Figure 6.10 Crystal structure of [Ln(r -OCBu2CH2PMe2)3] (Ln = Y, Nd) [30]. (Redrawn from P.B. Hitchcock, M.F. Lappert and A.I. MacKimion, Use of a highly hindered phosphinoafkoxide ligand in the formation of monomeric homoleptic lanthanoid metal complexes X-ray structmes of [Ln(r -OCBu2CH2PMe2)3] (Ln = Y or Nd), Journal of the Chemical Society, Chemical Communications, 1557-1558, 1988.)...

In the presence of [Cp2Mo2(CO)4], 3 > [Ni2(COD)2( JL-ri2-RC R)],223 = and [Ni4(RNC)4( X3-V-RC=CR)3], alkynes are converted to ds-alkenes. The alkenes once formed no longer bind to the complex and hydrogenation does not proceed further to give alkanes or even isomerized alkenes. Cocondensation of lanthanoid metal atoms with internal alkynes generates lanthanoid complexes of alkynes, which are potential catalysts for hydrogenation. [ Sm(l-hexyne) n] or [ Er(3-hexyne) n] catalyzes hydrogenation of hex-3-yne to ds-hex-3-ene (97% cis) at room temperature and atmospheric pressure of H2. ... 

Some inner transition metals are prepared from oxides on the research scale. Most of the lanthanoid metal are prepared from oxides via halides (see Section 9.2.2.3). A powerful reductant however, is required to produce Sm, Eu, Tm, and Yb, because of the stability of the difluorides of these metals. Since Sm, Eu, Tm, and Yb also have relatively high vapor pressures, they are best prepared by reduction of their oxides ... 

The free energy of reaction (a) is favorable, and the starting materials are easily prepared. However, high purity is not achieved because fluorides often retain water or oxygen that remains in the product, and because slags such as Cap2 are admixed with the product and must be removed mechanically. In research-scale reductions, high purity fluoride [from oxide treated with HF(g)] and Ca can achieve > 99.9% pure lanthanoid metals, if the metals are further purified by vacuum fusion or distillation -. ... 

Monomeric tris-(r/2-pyrazolate)-lanthanoid complexes with the bulky 3,5-di(/-butyl)pyrazolate ligand, of general formula [M(r-Bu2pz)3(thf )2] (M = La, Nd, Gd. or Er), were obtained by reaction of Hg(C6F3)2 and 3.5-/-BupzH with an excess of lanthanoid metal in THF at room temperature. The X-ray crystal structure of the neodymium derivative was also reported (191). 

Kumaii S, Singh AK, Rao TR (2009) Mesogenic lanthanoid metal complexes of a non-mesogenic Schiff-base, N, N -di-(4-hexadecyloxysahcyhdene)-l, 8 -diamino-3, 6 -dioxaoctane. Mater Sci Eng C 29 2454-2458... 

Figure2. The structure of (R)-LnMB. (M=alkali metal, Ln=lanthanoid metal)...

This last observation is due to the so-called lanthanoid contraction (the steady decrease in size along the 14 lanthanoid metals between La and Hf see Section 24.3). 

In the electronic spectra of lanthanoid metal ions, absorptions due to f-f transitions are sharp, but bands due to Af-5d transitions are broad. 

Comment on why the spin-only formula is not appropriate for estimating values of Meff for lanthanoid metal ions. 

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