Scandium alkoxides

The acetylacetonates are stable in air and readily soluble in organic solvents. From this standpoint, they have the advantage over the alkyls and other alkoxides, which, with the exception of the iron alkoxides, are not as easily soluble. They can be readily synthesized in the laboratory. Many are used extensively as catalysts and are readily available. They are also used in CVD in the deposition of metals such as iridium, scandium and rhenium and of compounds, such as the yttrium-barium-copper oxide complexes, used as superconductors. 1 1 PI Commercially available acetyl-acetonates are shown in Table 4.2. 

Some well-defined scandium alkoxides have been synthesized and studied (together with lanthanide analogues). The compounds are [Sc(OR)3], Sc(OR)3(THF), Sc(OR)3(tppo) and (RO)2Sc(u-OR)2Na(THF), where R = 2,6-di-t-butyl-4-methylphenate. [Sc(OR)3] was prepared by reaction of Sc N(SiMe3)2 3 with ROH in hexane at 20°C followed by reflux, or by reaction of NaOR and ScCl3 in boiling THF, or by sublimation of Sc(OR)3(THF). The latter and... 

Synthetic strategies to alkoxide complexes have been covered in full by previous reviews [14]. The silylamide route proved to be an advantageous method of preparation, especially in the case of homoleptic derivatives [15]. The group (IIIA) elements - scandium, yttrium and lanthanum - are considered as lanthanides on the basis of their general chemical similarity to the true lanthanides. 

An intramolecular [2+2] photocycloaddition of allyl ethers with dioxinones followed by a base-induced fragmentation leads to substituted tetrahydropyran-4-ones <1997TL5579>. A one-pot scandium triflate catalyzed diastereoselec-tive cyclization between aldehydes and (3-hydroxy dioxinones 1046 followed by alkoxide addition to the resulting bicycles 1047 leads to 3-carboxy-substituted tetrahydropyran-4-ones 1048 with high levels of diastereoselectivity as a mixture of keto/enol tautomers (Scheme 268, Table 49) <20050L1113>. 

Few scandium alkoxides have been studied in detail, but monomeric 3-coordination exists in [(2,6-Bu2-4-MeC6H20)3Sc] where the bulky ligand enforces steric crowding. It will, however, expand its coordination sphere to form 4-coordinate adducts with PhsPO and THF - unlike the corresponding three-coordinate silylamide. Sc(OSiBu3)3.L (L = THF,... 

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. 

Scandium enolate chemistry is quite limited. The complex (nacnac)Sc(NHAr)(HBEt3) where nacnac = 21 and Ar = 2,6-(/-Pr2)2C6H3) reacts with THF to form the enolate (nacnac)Sc(NHAr)OCH=CH2 while diethyl ether forms the related alkoxide (nac-nac)Sc(NHAr)OEt. This suggests considerable stabilization due to the Sc—O bond. After all, ring opening of THE is energetically unfavorable (THE EtOVi A// > 40 kJ mor ). 

Electropositive metals M, for example Groups 1 (IA) (alkali), 2 (IIA) (alkaline earths), 3 (IIIB) (lanthanides, scandium, and yttrium), and 13 (IIIA) (aluminum), react directly with alcohols to yield (1, 2, 6) the corresponding metal alkoxides. 

The syntheses, physical properties, and molecular structures of alkoxides and aryloxides have been discussed in CCC (1987).161 The alkoxides of scandium and yttrium were reviewed in CCC (1987).1 There have been more recent developments in this area and the impetus for this chemistry has been the developments in materials research. Metal alkoxides and /3-diketonates can be used as precursors for oxide and nonoxide thin films.162 The stable M—O bond and the volatility of the metal alkoxides are important features of this area of chemistry. This has lead to more research in this area particularly in synthesis, NMR, and X-ray crystallography. 

Electrosynthesis by anodic dissolution of the metal in absolute alcohol appears for many elements to be a promising, inexpensive way to obtain large amounts of metal alkoxides [54]. The process goes smoothly and has good current yields for metals, such as scandium and lanthanides, and early transition metals (Ti, Zr, and Nb) using tetrabutylammonium bromide as an electrolyte. Oxo or hydroxo compounds are obtained for more electropositive metals (Mg and alkaline-earth metals), however, probably as a result of alcohol dehydration reactions. 

Scandium complexes, 1059-1116 alkoxides, 1065 amides, 1062,1066 amine oxides, 1065 amines, 1060 amino acids, 1066 aromatic amines, 1061 arsenic compounds, 1065 2,2 -bipyridyl, 1061... 

The method involving direct reaction of a metal with alcohol was extended by Mazdi-yasni et for the formation of scandium, yttrium, and lanthanide alkoxides using mercuric chloride (10 -10 " mol per mol of metal) as a catalyst ... 

To date no unfluorinated alkoxides of scandium, yttrium, and lanthanides in the common -1-3 oxidation state appear to have been prepared by the ammonia method. By contrast, yttrium and lanthanides (Ln) fluoroalkoxide derivatives of the types Ln OCH(CF3)2 3 and Ln OCH(CF3)2 3.2NH3 have been isolated by this route. 

The first crystal structure determined of a scandium alkoxide was that of the dimeric ferf-butoxide [Sc2(a<--OBu )2(OBu )4] containing four-coordinated Sc in edge-bridged distorted tetrahedra as in Fig. 4.1. With the less sterically demanding but more acidic... 

The formation of multiple metal-oxo bonds by penta or hexa valent transition metals (d°) is not surprising but the more recent discovery that scandium, yttrium, and the tervalent lanthanides form oxo-isopropoxides rather than triisopropoxides was unexpected. Considerable efforts have been devoted to ensuring that the formation of these pentanuclear oxo-alkoxides [M5(/r5-0)(/r,3-0Pr )4(/r-0Pr )4(0Pr )5] was not due to hydrolysis, and there is evidence that elimination of ether is involved. An alternative mode of decomposition would be alkene elimination with concomitant formation of alcohol (Eq. 5.5). 

Table 5.3 Scandium, yttrium, lanthanide, and actinide-containing oxo-alkoxides (including heterometallic compounds)...

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