Lanthanide hydride cyclopentadienyl complexe

The same group also showed that mono(cyclopentadienyl) mixed hydride/ aryloxide dimer complexes of several lanthanide elements (Y, Dy, Lu) could be synthesized easily by the acid-base reaction between the mixed hydride/alkyl complexes and an aryl alcohol [144]. These complexes reacted with C02 to generate mixed formate/carboxylate derivatives, which were moderately active initiators for the copolymerization of C02 and cyclohexene oxide, without requiring a co-catalyst. The lutetium derivative 21 was the most active (at 110°C, TOF = 9.4 h ), yet despite a good selectivity (99% carbonate linkages), the molecular weight distribution remained broad (6.15) (Table 6). 

Cleavage of Ln-C a-bonds of lanthanide alkyls and aryls by a hydrogen molecule at ambient pressure and room temperature is a popular method for the synthesis of neutral lanthanide hydride complexes (Equation 8.25). The first structurally characterized neutral lanthanide hydrides were prepared by hydrogenolysis of bi(cyclopentadienyl) lanthanide alkyl... 

Lanthanide monohydride complexes, such as bi(cyclopentadienyl) lanthanide hydrides, can be conveniently prepared by the reactions of lanthanide mono-alkyl or -aryl complexes with organosilanes under mild reaction conditions (Figure 8.27) [82]. 

With a few exceptions,most tris(cyclopentadienyl) complexes contain lanthanides (Ln) or actinides as the central metal. CpjLn have polymeric structures, with chains of (ri -CpljLn units linked by cyclopentadienides that are bound in an T) -fashion toward one or both of the metals they bridge. Cp Ln and Cp U are monomeric. The Sm(III) complex has been prepared from Sm(II) and Cp jPb (Equation 3.83) the U(III) complex was prepared from the U(III) hydride and tetramethylfulvene (Equation 3.84). ... 

The potential use of non-solvated lanthanide cyclopentadienyl hydride complexes as catalysts in alkene C-H bond activation, hydrogenation of alkynes led to synthesis of aluminum hydride organo lanthanide complexes. Examples of such complexes with polymeric structure and chain structure have been characterized [251]. 

It is nowadays well established that the hydrogenolysis of an alkyl complex involves the initial formation of the corresponding hydride. This may be an unstable transient species, especially when the alkyl precursor includes non-bulky ligands. A monomeric stable [Ln]R complex can lead to an unstable [Ln]H moiety, because the smaller size of the hydride ligand may not ensure the saturation of the co-ordination sphere about the lanthanide. The rearrangement into tris(cyclopentadienyl) derivative may also compete with both dimerisation or reduction (Scheme 4). 

The mechanism of the reaction was studied for cyclopentadienyl lanthanide complexes, [26, 29, 31]. A monomeric metal hydride is proposed to be the active species. The catalytic cycle turns via a fast and irreversible insertion of the olefin into the metal hydrogen bond to form an alkyl species which reacts with the silane in the rate determining step with regeneration of the hydride (Scheme 12). 

Any modification of a complex, in order to increase or decrease its reactivity towards a substrate for catalytic purposes, can be easily accomplished by modulating the steric effects, even though modification of the electronic properties of the reactive site may also be necessary to render the catalyst more efficient. The evaluation of such electronic influence requires a comparison of complexes bearing ligands of the same size (isosteric ligands) but different in their chemical nature. The organometallic chemistry of the hard lanthanide elements is dominated by the soft cyclopentadienyl type ligands, while the number of non cyclopentadienyl hydrides is really modest. Due to the paucity of the published syntheses of... 

In 1992 Yasuda et al. [236, 237] reported that organolanthanide complexes of the type Cp 2Sm-R (where Cp is pentamethyl cyclopentadienyl, and R is either an alkyl, alkylaluminum or a hydride) initiate highly syndiotactic, living polymerizations of methacrylates. It was also reported that lanthanide complexes such as Cp 2Yb(THF)i 3, Cp 2Sm(THF)2, and (indenyl)2Yb(THF)2 can also initiate polymerizations of methylmethacrylate [238]. Although very low initiator efficiencies were... 

Because of their larger ionic radii, lanthanide (hereafter taken to include Y and Sc) atoms are usually hoimd to heavily substituted cyclopentadienyl groups such as CslCHsls. Although ionization of the lanthanocene is unnecessary, preparation of the appropriate metal hydride or metal alkyl that can initiate polymerization is a matter of considerable art. Lanthanide alkyl complexes with -hydrogens tend to decompose, while hydride and chloride species readily dimerize or form -ate complexes with lithium salts. In fact, not only the Cp-ligands but also the metal-boimd alkyl substituent are often required to be large (such as —CH(Si(CH3)3)2) (96), or the complex is further coordinated by a Lewis base molecule such as THF. 

The second review is due to Pepper and Bursten (1991). This review focussed on the electronic structure of actinide-containing molecules. Note that the present chapter complements this in that our chapter is mostly on lanthanide-containing species. Consequently, the reader is referred to the excellent review by Pepper and Bursten (1991) for a comprehensive summary of the electronic structure of actinide-containing species. The review by Pepper and Bursten (1991) contains the details of calculations on actinide hydrides, actinide halides, actinide oxides, cyclopentadienyl-actinide complexes, aetinocenes, metal-metal bonding in actinide systems and miscellaneous other actinide systems. This review also consists of descriptions of theoretical techniques employed to study the actinide-containing molecules. The reader is directed to this review for further details on such calculations on actinide-containing molecules. 

Qian, C. Zhu, D. Studies on organolanthanide complexes. Part 55. Synthesis of fiiran-bridged bis(cyclopentadienyl)lanthanide and yttrium chlorides, and hgand and metal tuning of reactivity of organolanthanide hydrides (in situ). J. Chem. Soc., Dalton Trans. 1994, 1599-1603. 

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