A LANTHANIDE LANTHOLOGY (.pdf) - Davidson Physics

The term "organo-metallic" here defines compounds in which the lanthanide atom is boundto carbon through single sigma-bonds or through multiple pi-bonding. The term excludes materialscontaining Ln's bonded to oxygens that are constituent parts of organic groupings such as theanions found in, for example, carboxylates or acetylacetonates.Only in the last two decades have a wide range of such derivatives been isolated andcharacterized[1]. For example, many cyclopentadienyl compounds such as Ln(C 5 H 5 ) 3 and morecomplex species[2], have been made. In some the bonding is considered to involve Ln(0) atomsand π-bonded arene ligands[3]. These materials, all very sensitive to oxygen and water, havebecome a fruitful field for research.The absence of a rigid stereo-chemical arrangement around the Ln atom coupled with aninherent high reactivity creates potential applications in catalysis and as organic synthesis reagents.Organo-lanthanide compounds are particularly effective in activating C-H bonds and some areknown to activate methane, to break the C-H bond. Many react readily with, and promotepolymerization of, olefines. The freedom of access to the Ln atom means that the stereochernistryaround the Ln is determined by the ligands themselves bound to the Ln. In turn the polymerizationcan be highly stereospecific with different specificity to that seen with other catalytic systems[4].The active lanthanide organo-metallic in catalysts can be generated in situ, e.g. from ananhydrous Ln halide plus an aluminum alkyl, rather than introduced as a previously isolatedcompound. In this way Ln analogues of the well recognized Ziegler Natta (metal-halidealuminum-alkyl) polymerization catalysts can be obtained [5]. As an alternative the lanthanide,initially present as an organo-metallic, can be activated by the Al alkyl[6].[1] Organo-Lanthanoid Compounds, H.Schumann, Angew.Chem.Int.Ed., 1984,23,474: OrganometallicCompounds of the Rare Earths, H.Schumann and W.Genthe, in Handbook Phys. Chem. Rare Earths, ed.K.A.Gschneidner and LEyring, 1984, Vol. 6, 445[2] Achieving new Lanthanide Chemistry within the Tetracyclopentadienyl Cavity formed by two (C 5Me 5)Moieties, W.H.Evans, J.Allcys Comp., 1993, 192, 205[3] Zero Oxidation State Compounds of Scandium, Yttrium and the Lanthanides, F.G.N.Cloke,Chem.Soc.Rev., 1993,17[4] Polymerization of Dienes on Lanthanide containing Catalytic Systems. Review, N.G.Marina et al., PolymerSci., 1984, 26(6), 1251 ; Lanthanide Compounds - Catalysts of Stereospecific Polymerization of DieneMonomers. Review, ibid, 1991, 33(3), 387[5] Titanium and Rare Earth Chloride Catalysts for Ethylene Polymerization, J.L.Martin, J.Polym.Sci.,Polym.Chem., 1984, 22, 3843 : Mechanism of Rare-Earth Catalysis in Coordination Polymerization, H.L.Hsiehand G.H.C.Yeh, Ind.Eng.Chem.Prod.Res.Dev., 1986, 25, 456[6] Polymerization of Styrene by Rare Earth Coordination Catalysts, M.Yang et al., Polym.J., 1990, 22(10),91916