Samarium hydride complex

The reactivity of the samarium hydride complex is evidenced [170] by the formation of [Cp2Sm(THF)]2[/4-72-(OSiMe20SiMe20)] a bridged samarium siloxide, from [Cp2Sm(/r-H)]2. The samarium hydride complex is also used in the polymerization of methyl methacrylate [171]. 

The dimeric yttrium and samarium hydride complexes are converted into monomers in the first step of the reaction. The preparation of block-co-polymers of 1-hexene or 1-pentene with MMA and e-caprolactone, respectively, has also been reported. The polymerization of ethylene with MMA, e-caprolactone and 2,2-dimethyltri-methylenecarbonate was studied in detail racemic Me2Si(CsH2-2-SiMe3-4-But)2Sm(THF)2 or meso-Me2Si(Me2SiOSiMe2)(CsH2-3-But)Sm(THF) were active in the ABA-type triblock-co-polymerization.986... 

A range of rare earth metal complexes were subsequently shown to catalyze ethylene polymerization and, on occasion, living characteristics have been reported.226-228 Dimeric hydrides such as (79)—(82) are extremely active with turnover numbers > 1800 s-1 recorded for (79) at room temperature. The samarium hydride (82) also effects the block copolymerization of methyl methacrylate (MMA) and ethylene 229 further discussion may be found in Section 9.1.4.4. 

The lutetium hydride complex [Cp 2Lu(/t-H)]2 efficiently cleaves the Si-C bond of PhSiH3 to produce benzene and cross-linked polysilanes (SiHJ (Scheme 290).604 Formation of Ph2SiH2 and SiH4 has also been observed during the samarium-catalyzed redistribution of PhSiHj.545... 

A recent exhaustive review [1] presents the known and well characterised lanthanide hydrides. Recently, a few additional original complexes have been synthesised. These include phospholyl [7] and indenyl derivatives [8], a bimetallic metallocenic structure (Scheme 1) [4], a samarium hydride supported by a calix-tetrapyrrolyl ring, with a triply bonded hydride ligand (Scheme 2) [9], metallocenes with chiral groups which has been used for the asymmetric hydrogenation [10], a tris(pyrazolyl)borate ytterbium(II) hydride [11], Cp amido-[12] and bis(amido)yttrium hydrides [13]. The latter was not isolated but obtained in situ from the methyl derivative (Scheme 2). 

It is noteworthy that while the electron rich Cp 2SmH is only moderately stable in solution, the analogous more strained dinuclear bridging complex is less stable and must be kept under hydrogen in order to record the NMR spectrum [2]. The synthesis of a samarium hydride supported by a calix-tetrapyrrolyl system was recently published [9] (Scheme 2), with the hydride ligand bonded to one samarium and to two lithium atoms. This cage affords an efficient protection against bimolecular collisions and the complex is described as thermally robust. 

Acetylide hydride complexes of samarium, erbium and ytterbium have been made by the cocondensation reactions of Sm, Er, and Yb metal vapor with 1-hexyne at 77K. Polymeric compounds containing [(BuC C)2SmH], [(BuC C)2ErH] and [(BuC=C)3Yb2H] units are isolated and shown to be active catalysts for hydrogenation reactions (W.J. Evans et al., 1981c). 

On the basis of these experimental results, a possible mechanism has been proposed for the reaction of 1-215 with Sml2 (Scheme 1.52). After formation of the syn-complex A, a rearrangement occurs to give the aldehyde B, which coordinates to the added aldehyde RCHO to afford complex C. Subsequent samarium-catalyzed nucleophilic attack of the secondary alcohol to the carbonyl of RCHO generates a hemiacetal, D. There follows an irreversible intramolecular 1,5-hydride transfer via... 

Arene(tricarbonyl)chromium complexes, 19 Nickel boride, 197 to trans-alkenes Chromium(II) sulfate, 84 of anhydrides to lactones Tetrachlorotris[bis(l,4-diphenyl-phosphine)butane]diruthenium, 288 of aromatic rings Palladium catalysts, 230 Raney nickel, 265 Sodium borohydride-1,3-Dicyano-benzene, 279 of aryl halides to arenes Palladium on carbon, 230 of benzyl ethers to alcohols Palladium catalysts, 230 of carboxylic acids to aldehydes Vilsmeier reagent, 341 of epoxides to alcohols Samarium(II) iodide, 270 Sodium hydride-Sodium /-amyloxide-Nickel(II) chloride, 281 Sodium hydride-Sodium /-amyloxide-Zinc chloride, 281 of esters to alcohols Sodium borohydride, 278 of imines and related compounds Arene(tricarbonyl)chromium complexes, 19... 

The catalysts are Tim hydrides which are generated in situ by reduction of enantio-merically pure L TiCl2 with butyllithium and PhSiH3 they are also highly effective in the asymmetric hydrogenation of imines and enamines.20 The lutetium ansa-metallocene complex (22-XVII) catalyzes the deuteration of 1-pentene (63% e.e.)21 Related samarium compounds hydrogenate imines.22... 

The above mentioned synthesis methods do not allow to obtain phosphide complexes for the first members of REM block, specifically those of samarium. The synthesis of a derivative of this metal has been successful [2] when the well known [10] hydride method of synthesis was used ... 

---