Scandium compounds, pure

When Mendeleev produced his original Periodic Table in 1869, he left a space for a metallic element of atomic mass 44 preceding yttrium. The first fairly pure scandium compounds were isolated by Cleve in 1879, but it was not until 1937 that the element itself was isolated. Although a relatively abundant element, it is fairly evenly distributed in the earth s crust and has no important ores, though it is the main component of the rare ore thortveitite (Sc2Si207), thus being relatively expensive. In fact, it is mainly obtained as a by-product from uranium extraction. 

Much work in the review period has concerned enantioselective substitution in five-membered heterocyclics. The enantioselective alkylation of some pyrroles by unsaturated 2-acylimidazoles catalysed by the bis(oxazolinyl)pyridine-scandium(in) triflate complex (31) has been reported.39 Compound (33) is formed in 98% yield and 94% ee from the 2-acylimidazole (32) and pyrrole at —40 °C. A series of enantiomer- ically pure aziridin-2-ylmethanols has been tested as catalysts in the alkylation of /V-mclhylpyrrolc and (V-methylindole by ,/l-unsalura(cd aldehydes.40 Enantiomeric excesses of up to 75% were observed for the alkylation of /V-mcthylpyrrole by ( >crotonaldehyde using (2.S ,3.S )-3-mclhylazirin-2-yl(diphenyl)methanol TFA salt as catalyst to form (34). 

The substance discovered by Nilson was not pure scandium metal, but a compound of scandium and oxygen—scandium oxide (SC2O3). It is quite difficult to produce pure scandium metal from scandium oxide. In fact, it was not until 1937 that the metal was isolated. Then, it was another 20 years before a large sample (weighing one pound) was produced. Today, companies that use scandium often buy the oxide rather than the pure metal. In 2008, scandium oxide of 99.9995 percent purity cost about 3,260 per kilogram. 

Compound 24, the corresponding IV,]V -dioxide of 23, presents enhanced fluorescence when complexed with Sc(OTf)3. 24 forms a strong Sc[( )-24]2 complex whose fluorescence disappears upon addition of amino alcohols. Optically pure scandium complex formed with optically pure 24, Sc[(+)-24]2 generates diastereomeric complexes with a racemic aminoalcohol such as alaninol. The fluorescence signal could be switched off enantioselectively and it was exploited for sensing purpose. The enantiomers of 24 were obtained by chromatography on (R,R)-Whelk-01 (08JOC4267). 

The methyl complexes dissolve in methylene chloride, toluene and benzene, but ethyl derivatives are sparingly soluble in alkanes. Reduced solubility of Sm and Gd complexes in toluene, in the authors opinion, shows some increase in the ionicity of such compounds from heavy to early lanthanoids [226]. Due to the low solubility in methylene chloride the attempts to isolate the samarium complex in analytically pure state have failed. The thermal stability of methyl complexes is rather high. Almost all compounds melt without decomposition in the temperature range 110-140°C, but the scandium and yttrium compounds may be sublimed in vacuum. The ethyl derivatives are less stable. They decompose with a notable rate even at room temperature. 

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