Lanthanides salts

Originally, general methods of separation were based on small differences in the solubilities of their salts, for examples the nitrates, and a laborious series of fractional crystallisations had to be carried out to obtain the pure salts. In a few cases, individual lanthanides could be separated because they yielded oxidation states other than three. Thus the commonest lanthanide, cerium, exhibits oxidation states of h-3 and -t-4 hence oxidation of a mixture of lanthanide salts in alkaline solution with chlorine yields the soluble chlorates(I) of all the -1-3 lanthanides (which are not oxidised) but gives a precipitate of cerium(IV) hydroxide, Ce(OH)4, since this is too weak a base to form a chlorate(I). In some cases also, preferential reduction to the metal by sodium amalgam could be used to separate out individual lanthanides. 

Amongst the known examples of this arrangement are a number of [M(H20)9] + hydrates of lanthanide salts and [ReH9] . The latter is... 

If lanthanide salts (e.g., Yb(OAc)3 H20) are used as the metal compound, hexadecasub-stituted bis(phthalocyanines) can be prepared starting from 2,3,9,10,16.17,23,24-octa-substituted metal-free phthalocyanines.201... 

At this stage, the influence of several lanthanide salts was evaluated (Figure 35.2) (19). In the presence of lanthanide nitrates, the conversion and the selectivity towards 1,3-PDO decreased compared to the reference, i.e., the reaction run without any additives, except H2WO4. In contrast, the addition of lanthanide chlorides had a positive effect on the selectivity to 1,3-PDO. However, low conversions were observed in all cases. Under these conditions, the highest ratio l,3-PDO/l,2-PDO reached 0.9. 

Figure 35.2 Influence of lanthanide salts addition on the hydrogenolysis of glycerol.

The lanthanide salts are unique among Lewis acids in that they can be effective as catalysts in aqueous solution.61 Silyl enol ethers react with formaldehyde and benzaldehyde in water-THF mixtures using lanthanide triflates such as Yb(03SCF3)3. The catalysis reflects the strong affinity of lanthanides for carbonyl oxygen, even in aqueous solution. 

These reactions can be catalyzed by Lewis acids such as bw-alkoxytitanium dichlorides61 and lanthanide salts.62... 

Lanthanide salts, such as Sc(03SCF3)3, are also effective catalysts.89... 

Scheme 10.17 illustrates allylation by reaction of radical intermediates with allyl stannanes. The first entry uses a carbohydrate-derived xanthate as the radical source. The addition in this case is highly stereoselective because the shape of the bicyclic ring system provides a steric bias. In Entry 2, a primary phenylthiocar-bonate ester is used as the radical source. In Entry 3, the allyl group is introduced at a rather congested carbon. The reaction is completely stereoselective, presumably because of steric features of the tricyclic system. In Entry 4, a primary selenide serves as the radical source. Entry 5 involves a tandem alkylation-allylation with triethylboron generating the ethyl radical that initiates the reaction. This reaction was done in the presence of a Lewis acid, but lanthanide salts also give good results. 

Nitration can be catalyzed by lanthanide salts. For example, the nitration of benzene, toluene, and naphthalene by aqueous nitric acid proceeds in good yield in the presence of Yb(03SCF3)3.5 The catalysis presumably results from an oxyphilic interaction of nitrate ion with the cation, which generates or transfers the N02+ ion.6 This catalytic procedure uses a stoichiometric amount of nitric acid and avoids the excess strong acidity associated with conventional nitration conditions. 

The yttrium, lanthanum and other lanthanide salts exploded after dehydration during heating to above 300°C. 

It should be noted that the basic reactions used to prepare phthalocyanine derivatives today are fundamentally those developed by Linstead and coworkers in the 1930s [52-54]. Due to the large number of substituted phthalocyanines described in the literature, space limitations mean that a detailed review of synthetic aspects cannot be provided here. The following discussion is concerned with the synthesis of lanthanide phthalocyanines via (i) template tetramerization of phthalonitrile with lanthanide salts, (ii) direct metalation of the metal-free ligands by the salts or (iii) metal exchange of a labile metal ion or ions for a lanthanide. 

The template methods have also been used for the synthesis of a number of substituted Ln di(naphthalocyanine) complexes, LnNc2 [82-88]. Apart from thermal fusion by conventional heating processes, complexation has been initiated by microwave radiation, although only a few publications are devoted to the template synthesis of lanthanide bis(phthalocyanine) complexes by this method [89, 90]. The use of microwave radiation (MW) reduces the reaction time from several hours to several minutes. Unsubstituted complexes LnPc2 (Ln = Tb, Dy, Lu) were prepared [90] by irradiation (650-700 W) of a mixture of phthalonitrile with an appropriate lanthanide salt for 6-10 min (yields >70%). 

Although direct reaction of lanthanide mono-porphyrins with free phthalo-cyanine or its lithium derivatives is generally more efficient than the template synthesis, and gives rise to mixed-ligand complexes, the template strategy can also be applied for synthesis of phthalocyanine-porphyrin complexes, as in the case of unsymmetric bisphthalocyanine complexes (Scheme 8.2, B(b)) [106, 136, 145, 146]. Thus, metallation of free porphyrins with lanthanide salts in TCB or n-octanol leads to single-decker complexes, which then react with phthalonitriles under the action of DBU in alcoholic media to give the desired compounds. 

Shortly after the key mechanistic papers on rhodium-catalyzed hydroboration, Marks reported a hydroboration reaction catalyzed by lanthanide complexes that proceeds by a completely different mechanism.63 Simple lanthanide salts such as Sml3 were also shown to catalyze the hydroboration of a range of olefins.64 The mechanism for this reaction was found to be complex and unknown. As in other reactions catalyzed by lanthanides, it is proposed that the entire catalytic cycle takes place without any changes in oxidation state on the central metal. 

Dehydrating agents have commonly been employed in the preparation of lanthanide sulfoxide complexes from hydrated lanthanide salts. For example, dimethoxypropane has been used to prepare both (CH2)4SO (65) and rePr2SO (56) complexes of the lanthanide nitrates. An alternative dehydrating agent is ethyl orthoformate [Eq. (14)]. 

A number of complexes of urea and substituted ureas with various lanthanide salts have been isolated. The lanthanide acetates give both anhydrous and hydrated complexes with urea (67, 68). The hydrated complexes could be dehydrated by drying the complexes over CaCl2 or P4Oi0 (68). It is interesting to note that in the complexes of substituted ureas like EU (70) and CPU (71), the L M is independent of the anion. The anions in these complexes with a L M of 8 1 are apparently nonco-ordinated. Seminara et al. (72) have reported complexes of lanthanide chlorides with DMU and DEU which contain five and three molecules of the ligand respectively per... 

A copper-catalyzed reaction using a chiral diphosphine hgand, DuPHOS, with an added lanthanide salt, provides good levels of enantioselectivity (67-91% ee) in additions of the simple allylboronate 31 to both aromatic and aliphatic ketones that present a large difference of steric bulk on the two sides of the carbonyl group. One such example is shown in Eq. 81. On the basis of B NMR experiments and on the lack of diastereoselectivity in crotylation reactions, the... 

Lanthanide salts have been found to catalyze addition of oc-nitroesters, even in aqueous solution.105 106... 

Lanthanide salts have also been found to catalyze Diels-Alder reactions. For example, with 10 mol % Sc(03SCF3)3 added, isoprene and methyl vinyl ketone react to give the expected adduct in 91% yield after 13 h at 0°C.18... 

Lanthanide salts, such as Sc(03SCF3)3 are also effective catalysts.62 Conventional silylating reagents such as TMS—I and TMS triflate have only a modest catalytic effect, but a still more powerfid silylating reagent, (CH3)3SiB(03SCF3)2, does induce addition to aldehydes.63... 

No product was formed in this reaction in the absence of the soluble lanthanide salt or even in the presence of CeCL. Heteroaryl Grignard reagents react smoothly in the presence of LaCl3 2LiCl even with highly sterically hindered ketones like 211 (equation 137). 

In this section are discussed hydrates of lanthanide salts where the water molecules form a majority of the coordination sphere and where there is evidence concerning the number or... 

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