Scandium appearance

The last major review of the coordination chemistry of scandium appeared in 1987 whilst other reviews have appeared concerned with scandium chemistry, with its structural chemistry, and with the role of scandium inorganic synthesis, this section is concerned with covering the area from the previous review, though for the sake of readability, there will be occasional reference to earlier work. 

The majority of the scandium intermetallic structure types can also be found among intermetallic compoimds of other rare earths as well as among intermetallics of the 4A elements (Villars and Calvert 1991). However, taking into consideration all 155 structure types of scandium intermetallics, from a crystal-chemical point of view, scandium appears to be closer to 4A elements, especially to Zr and Hf, rather than to the other rare earths. 

The second type is isostructural with CsjR(S04)3 (R = Tm, Yb, Lu) (Samartsev et al., 1978). Scandium appears to form monocUnic crystals (Komissarova et al., 1970b). 

Figure 10. Electron shell closure fails to coincide with the dosing of periods in the periodic table because the shells do not fill in strictly sequential order As shown here, die fourth shell begins to fill before the third shell has been completed. The resumption of third-shHl filling accounts for the appearance of the first transition-metal series, beginning with scandium and ending with zinc.

There appear to be no enthalpies of solution of rare-earth tribromides published in the available literature.2 However, Bommer and Hohmann reported a value of -230.5 kj mol-1 (at 20°C) for the enthalpy of solution of scandium tribromide in water (180). This value may be compared with -197.1 kj mol-1 for the chloride, and an estimate (from the published Lnl3 series, q.v.) of —240 to -250 kj mol-1 for the iodide. The markedly more negative values for the heavier ha-... 

The thenoyltrifluoroacetate (tta) complex, [Sc tta)3 OP(OBu)3)], has been prepared. Scandium also afforded, with l,2-dihydroxybenzene-3,5-disodium sulphonate (Tiron), a monoprotonated chelate below pH 2.5, but above pH 6.0 hydrolysis afforded a monohydroxo-Sc-Tiron complex. Ligand-exchange equilibria for the piperidium salts of [Y(ffac)4] and [Y(tfac)4] have been studied by n.m.r. spectroscopy. The fully fluorinated ligands appear to exchange faster than the partially fluorinated species, and the mixed compounds, [Y(Hffac) (tfac)4 ] , n = 0 — 4, were obtained. 

Analogous methyl compoimds are iU-characterized and appear to be polymeric [129). The air-sensitive phenyl derivatives when first obtained from THF are soluble in benzene, but when dried completely are no longer soluble — apparently due to pol5unerization. The homoaryl complexes of the smaller scandium, and yttrium ions form only the tris complexes Sc(CeH5)3 and Y(C6H5)3 [129) It is apparent that the structure and stabihty of the homoaryls are dependent on the metal ionic radii and the steric bulkiness of the phenyl group. 

In the Mukaiyama aldol additions of trimethyl-(l-phenyl-propenyloxy)-silane to give benzaldehyde and cinnamaldehyde catalyzed by 7 mol% supported scandium catalyst, a 1 1 mixture of diastereomers was obtained. Again, the dendritic catalyst could be recycled easily without any loss in performance. The scandium cross-linked dendritic material appeared to be an efficient catalyst for the Diels-Alder reaction between methyl vinyl ketone and cyclopentadiene. The Diels-Alder adduct was formed in dichloromethane at 0°C in 79% yield with an endo/exo ratio of 85 15. The material was also used as a Friedel-Crafts acylation catalyst (contain-ing7mol% scandium) for the formation of / -methoxyacetophenone (in a 73% yield) from anisole, acetic acid anhydride, and lithium perchlorate at 50°C in nitromethane. 

R)-(+)-2-Hydroxy-1,2,2-triphenylethyl acetate [(R)-HYTRA], To a mechanically stirred solution of (R)-(+)-1,1,2-triphenyl-1,2-ethanediol (35.0 g. 0.121 mol, Note 1) and acetic anhydride (17.1 mL, 0.181 mol, 1.5 eq, Note 2) in anhydrous acetonitrile (500 mL, Note 3) at room temperature under nitrogen is added a solution of scandium(lll) trifluoromethanesulfonate (1.23 g, 2.5 mmol, 2 mol%, Note 4) in anhydrous acetonitrile (125 mL) over approximately 35 min (Note 5). After about 8 min a white precipitate begins to appear, and the resulting mixture is stirred at room temperature under nitrogen for a total of 3 hr. The solid is filtered, washed with acetonitrile (2 x 25 mL), and dried under vacuum at 40°C overnight to afford (R)-(+)-2-hydroxy-1,2,2-triphenylethyl acetate (35.42 g, 0.107 mol, 88%) as a white solid (Note 6). 

To our knowledge dithio complexes with either scandium or yttrium still have not been prepared. Since 1968 two reports have appeared that deal with the lanthanide dithiocarbamate complexes. 

To date no triazole complexes appear to have been reported for the scandium, yttrium, and lanthanum group of metals. 

Scandium does not appear to be put to any use relevant to this chapter. The applications of the actinides relate to their radioactivity and the appropriate chapters of this volume should be consulted. 

Solid-supported reagents which have found utility include Nafion-scandium Lewis acid catalyst (allyl additions to aldehydes) [62], HOBt (medium-ring lactamization) [63], EDC (preparation of active esters) [64], and thiazolium hydrotribromide (brominations) [65], A review has also appeared describing the use of supported reagents in separation science, primarily for the selective sequestration of metal ions [66],... 

There appears to be no chemistry of tetrazenes or tetrazadienes with metals of the scandium, titanium, and vanadium groups. 

Carboxylic acids are conveniently utilized in the extraction of a number of metal ions. The dissolution of metal soaps in trichlorobenzene was first noticed by Biffen and Snell (12). The first example of the liquid-liquid extraction involving a metal carboxylate appears to be the extraction of scandium with benzoic acid prior to the colorimetric determination (54). 

It is a commonplace to say that there has been explosive growth in the use of lanthanides in organic chemistry. For many years, the use of cerium(iv) compounds as oxidants was widespread, but more recently a whole range of other compounds have made their appearance. Thus samarium(ii) compounds are now routinely used as one-electron reducing agents and the use of trifluoromethanesulfonate ( triflate ) salts of scandium and the lanthanides as water-soluble Lewis acid catalysts is widespread. Beta-diketonate complexes and alkoxides have also come into use there are even applications of mischmetal in organic synthesis. 

Figures 4 and 5 Illustrate the chemistry of iron and scandium atoms reacting with water. As noted earlier, both Fe and Fe2 water adducts are formed and labeled "a" and "b", respectively, in Figure A. It is interesting that Fe2...0H2 can be photolyzed without photolyzing the Fe...OH2 adduct. The "d" peaks which result from photolysis of Fe2...0H2 indicate that a species with terminally bonded H and OH groups is formed. The photolysis of Fe2...0H2 would appear to lead to formation of the HFeFeOH species.

Scandium mefallofullerenes are, in particular, interesting in terms of separation and purification because, as described in Section 2.1, scandium fullerenes appear as mono-, di-, fri-and even fetra-scandium fullerenes wifh several sfrucfural isomers which can be separated completely by HPLC. As an example, the HPLC separation of scandium fullerenes is briefly described in fhe following. 

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