Scandium II Trichloride

Submitted by K. R. POEPPELMEIER and J. D. CORBETT Checked by GERD MEYERt 

Standard vacuum line and dry-box techniques must be used for manipulation and storage of the starting materials and the product both moisture and oxygen must be scrupulously excluded to avoid contamination by ScOCl. 

The CsCl ( 99.9% purity) is dried at 200° under high vacuum and then melted in a silica container. The latter step reduces the possibility of recontamination by surface moisture and makes weighing and complete sample transfer easier in the dry box. 

The next step in the preparation is to combine in the dry box the stoichiometric amounts (molar ratio 3 2 1) of CsCl, ScClj, and Sc (powder, if available otherwise small chunks or foil) respectively, the total quantities conveniently varying from one to several grams depending on the size of the tantalum tube and the precision of weighing possible. The tantalum container is crimped and sealed by arc welding and the welded tube next sealed under vacuum in a fused-silica tube. The sample is now heated to 700° for 2-3 

Either procedure when properly applied will give an essentially quantitative yield. The compound ScOCl, which has a light pink tinge in reducing systems, is the most likely impurity but can be avoided by careful work. The stronger lines in the ScOCl powder pattern (with intensities in parentheses) are about 8.2(vs), 3.56(vs), 2.63(vs), 2.023(ms), 1.878(s), 1.585(ms) A. It is estimated that ScOCl can be detected at 1 % visually and 2% by careful Guinier work. 

The compound CsScCl3 has the 2H (hexagonal perovskite) structure of CsNiClj, space group Pb lmmc, a = 7.350(2) A, c = 6.045(3) A, and represents the first example of a scandium(II) compound, although many more reduced metal-metal bonded phases are known. The material is black in color in bulk and blue when ground. The material is relatively stable to moisture for a reduced scandium compound but still readily reacts with water with the evolution of hydrogen gas. 

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Taking into account the competitive hydrolysis of the silyl enol ether, this reaction is remarkable. The method was shown to be general and was extended to a variety of aldehydes and several a,j9-unsaturated carbonyl compounds giving uniformly 1,4-addition with aldehydes and a mixture of 1,4- and 1,2-adducts in the case of ketones [187]. Later, this aqueous version of the Mukaiya-ma reaction was shown to give near quantitative yields in the presence of a water-tolerant Lewis acid such as ytterbium triflate [188]. Keeping with the same concept,copper(II) triflate [189],indium(III) trichloride [190],tris(pentafluoro-phenyl)boron [191] and scandium(III) triflate in the presence of a surfactant [192] have proved to be active catalysts. 

A recent improvement in the rate of the aqueous Diels-Alder reaction came with the use of Lewis acid in aqueous media. The first study deals with the Diels-Alder reaction between cydopentadiene and a bidentate dienophile. A large acceleration can be achieved by the combined use of copper(II) nitrate as a catalyst and water as a solvent [10, 34], Lanthanide and scandium triflates [9, 35] as well as indium trichloride [36] were found to catalyze the Diels-Alder reaction in water. 

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