Lanthanide-alkyl bond

At the first step, the insertion of MMA to the lanthanide-alkyl bond gave the enolate complex. The Michael addition of MMA to the enolate complex via the 8-membered transition state results in stereoselective C-C bond formation, giving a new chelating enolate complex with two MMA units one of them is enolate and the other is coordinated to Sm via its carbonyl group. The successive insertion of MMA afforded a syndiotactic polymer. The activity of the polymerization increased with an increase in the ionic radius of the metal (Sm > Y > Yb > Lu). Furthermore, these complexes become precursors for the block co-polymerization of ethylene with polar monomers such as MMA and lactones [215, 217]. 

The applicability of organolanthanide metallocenes as polymerisation catalysts can also be seen from the results of the block copolymerisation of ethylene and methyl methacrylate. The persistence of the lanthanide-alkyl bond has been utilised to prepare ethylene copolymers with polar poly(methyl methacrylate) blocks. For this purpose, ethylene is introduced as the first monomer into the polymerisation system with the samarocene catalyst, and then methyl methacrylate is polymerised, which leads to block copolymer formation [532-534] ... 

As described in Section IV,B, hydrogenolysis of lanthanide—alkyl bonds in (CsH4R)2LnR complexes provided a synthetic route to organolanthanide hydrides [Eqs. (10)-(12)]. The structure of the first ciystallographi-... 

Lanthanide alkyl compounds are important alkyl transfer reagents and initiate a variety of catalytic reactions. The transformation of lanthanide alkoxide bonds to lanthanide alkyl bonds seems to be an attractive alternative to the alkylation of lanthanide halides with alkali metal alkyl compounds. For example, the ary-loxide route affords homoleptic lanthanide alkyls in good yield [Eq. (22)] [135]. 

An approach other than steric hindrance has been used to overcome the previously mentioned instability of the actinide homoalkyls. It was found that the inclusion of jT-bonding ligands in the coordination sphere considerably enhanced the stability of the alkyl complex. Recently, the same line of reasoning has also yielded a new series of 7r-cyclopentadienyl lanthanide alkyls (C5H5)2LnR where Ln =Gd, Er, Yb and R = C=C, and CH3 120,121). The infrared data for these complexes are consistent with u-bonded structures and the room temperature magnetic susceptibilities are very close to the free ion values. The actinide complexes (75,... 

Surface organometallic chemistry with rare-earth metals (or lanthanides) on various inorganic oxides has advanced considerably during the past decade but is still in its infancy. A major drawback has been the difficult access and handling of pure (homoleptic) Ln alkyl precursors with reactive Ln-C(alkyl) bonds [101]... 

Labile Ln-C(alkyl) bonds are disrupted by most protic reagents. The transformation Ln-C(alkyl)/Ln-H is of considerable interest for the synthesis of homogeneous lanthanide precatalysts (C, Eq. 15) [143]. 

As a consequence monomeric complexes are obtained much more easily. Also, the tendency to bridge lanthanide centers is less distinct and, for example, a small cluster chemistry as it exists for alkoxides, e.g. OiPr [13] and OtBu derivatives [14], is not yet known. However, the Ln-N(amide) bond is less strong than the Ln-O(alkoxide) bond, and even comparable to Ln-C(alkyl) bonds, which has an effect in synthetic chemistry. This has been confirmed by the determination of absolute bond disruption enthalpies D by means of calorimetric titrations for the representative systems Cp Sm X (X = OrBu, D = 82.4 kcalmol-1 NMe2, 48.2 CH(SiMe3)2, 47.0) [15]. 

Lanthanide hydride derivatives are commonly synthesized by hydrogenol-ysis of lanthanide alkyl complexes [212], In order to further exploit the thermodynamic stability of the Al-N bond dizsobutylaluminum hydride (DIBAH), a common cocatalyst in diene polymerization mixtures and well-established reducing agent in organic synthesis, was used in the hydrogenol-... 

In contrast to Group 4 catalysts, lanthanide alkyl complexes tend to react with higher alkenes under tr-bond metathesis to give alkane and Ln-vinyl species, rather than insertion products.116... 

Lanthanide alkyl and aryl complexes can readily react with a range of substrates with acidic protons, such as alcohols, phenols, phenylacetylene, and amine, to be converted into the corresponding lanthanide derivatives. Lanthanide alkyl complexes react with H2 to form the corresponding hydride complex, which is the popular route to lanthanide hydride. Various unsaturated small molecules can insert into an Ln-alkyl bond to form the derivates containing Ln-heteroatom bonds. The reaction modes found are summarized in Figure 8.14 [59-62]. 

Some lanthanide alkyl complexes can induce intramolecular C-H bond activation via metalation of the ligand with elimination of CH4 or SiMc4 under suitable conditions [63-65]. A representative example of the sp -hybridized C-H activation is presented in Figure 8.15 [65]. 

The second approach is a popular route to cationic lanthanide alkyl complexes, which have proven to be the important intermediates for ethylene polymerization and the stereospecific polymerization of diene [5]. Various monocationic lanthanide monoalkyl complexes have been synthesized by the alkyl abstraction/elimination reaction of lanthanide dialkyl complexes. The reaction of a bisbenzyl scandium complex supported by P-diketiminate with B(C6Fs)3 affords the cationic complex with a contact ion pair structure, in which a weak bonding between the cation and the anion exists (Figure 8.21) [77]. The reaction of an amidinate... 

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... 

Whereas complexes of unsubstituted and substituted cyclopentadienyl ligands represent the vast majority of all published compounds in organolanthanide chemistry, examples of isolated and fully characterized (including X-ray structural analyses) compounds containing only cr-bonded alkyl and aryl ligands are still fairly rare. The first structurally characterized homoleptic lanthanide alkyls became available through the use of bulky mono-, bis-, and tris(trimethyl-silyl)-substituted methyl ligands. Simple unsolvated alkyls of the rare earth elements have not yet been synthesized. 

Synthesis and structural chemistry of non-cyclopentadienyl organolanthanide complexes have been reviewed by Edelmann and Schumann et /.13,13a 24 Marques et al. have published a review on the chemistry of the lanthanide using pyrazolylborate ligands.36 Theoretical investigations on lanthanide alkyls have been published by Eisenstein et al. under the title Lanthanide complexes electronic structure and H—H, C-H, and Si-H bond activation from a DFT perspective. ... 

Also the salt-free and solvent-free mono Cp lanthanide alkyl complex Cp La[CH-(SiMe3)2]2 has been synthesized and the X-ray structure together with that of its THF precursor show the stabilization of the unsaturated lanthanum center by unusual agostic Si-C bonds. The removal of THF produces a different conformation of the disylil ligands and pyramidalization of the Cp La [CH(SiMe3)2]2 moiety. 

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