Alkyls lanthanide

Methyl derivatives, octahedral [EnMeg] " species, are known for most of the lanthanides. The first homoleptic, neutral lanthanide alkyl was obtained using the bulky alkyl CH(SiMe3)2 ... 

The highly syndiospecific-living polymerization of methyl methacrylate has been initiated by the neutral bis(pentamethylcyclopentadienyl)lanthanide-alkyl or -hydride complexes [215,216]. The plausible reaction mechanism is shown in Scheme XI. 

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]. 

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,... 

From the relative stabilities of the actinide homoalkyls or -allyls and the tris(cyclopentadienyl) actinide alkyls, it appears that a coordinatively saturated metal center is necessary for kinetic stability. In contrast to f-transition metal alkyls, the absence of hydrogens appears to be of minor importance. In the case of the lanthanide alkyls and the tetrabenzylthorium, where the formal coordination number is only four, the steric bulkiness of the Hgands must be responsible for their observed thermal stability. 

Lanthanide aryloxides have proved to be excellent precursors to homoleptic lanthanide alkyls (B, Eq. 13) [140], The reaction can be conducted in non-polar solvents because of the good solubility of the starting compounds. The formation of insoluble alkali metal aryloxides is the driving force (kinetic control). Complexes derived from aliphatic alcohols [141] and acetylacetonates [131] are... 

Furthermore the homoleptie Ln-anilides might open up a similar route to lanthanide alkyls as do lanthanide phenoxides [281]. 

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] ... 

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-... 

Dicyclopentadienyl lanthanide alkyl and aryl complexes are prepared by the reaction [57]... 

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... 

U CH(SiMe3)2 3] has a trigonal pyramidal structure, like the corresponding lanthanide alkyls (Section 6.2.1) and also like the isoelectronic amides [M N(SiMe3)2 3] (M = U,... 

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 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... 

Lanthanide alkyl and aryl complexes react with organoelement hydride compounds, such as hydrides of silicon, germanium, and tin, and so on, resulting in a hydride transfer to the lanthanide metal atom. Among the organoelement hydrides, organosilanes are the most popular source of the hydride. 

Half-sandwich lanthanide alkyl complexes and, subsequently oranolanthanide amides were found to be highly efficient catalysts for the cross-coupling reactions of carbodiimides with alkynes and amines, respectively [136, 137]. Although the half-sandwich lanthanide alkyl complexes can also catalyze the dimerization of alkynes, no homodimerization product is observed in the reaction of alkynes with carbodiimides. These reactions offer a wide scope for the substrates of terminal alkynes and amines, respectively [138]. 

In recent years, a large number of mono- and dicationic lanthanide alkyl complexes have been found to be efficient catalysts for ethylene polymerization, and in some cases, the dicationic lanthanide derivatives show higher activity and selectivity than their monocationic counterparts. Ionic radii of lanthanide metals also affect the catalytic behavior, and polymerization activity often increases with ionic radius [5, 76],... 

Arndt, S., Spaniol, T.P., and Okuda, J. (2002) The first structurally characterized cationic lanthanide - alkyl complexes. Chemical Communications, 896. 

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