Melanie Zimmermann

Homoleptic Rare-Earth Metal Complexes Containing Ln−C σ-Bonds†

ion using borate reagents [Ph3C][B(C6F5)4] and [PhNMe2H][B(C6F5)4], respectively, results in highly active polymerization catalysts (Table 4). Scandium bis(alkyl) complex 51Sc shows excellent activity for the syndiospecific styrene homopolymerization (activity, 1.36 × 104 (kg PS)/ (mol Sc h); Mw/Mn ) 1.37), and complexes 51, 53, and 54 proved suitable for the coand terpolymerization of a series of monomers.204,206,210-225 8.2.3. Constraint Geometry Complexes Incorporation of the cyclopentadienyl ancillary ligand into a chelate array of (pendant) donor functionalities gives access to prominent Cp derivatives. Since the original introduction by the Bercaw group, the linked amido-cyclopentadienyl (Cp) ligand has advanced to be one of the most versatile ligands for group 4 metal polymerization catalysts.235 Catalysts based on this type of ligand provide a constrained ligand environment but are anticipated to be more active toward sterically demanding monomers than metallocenes. Aminecyclopentadiene proligands react with Ln(CH2SiMe3)3(thf)x (Jthf) according to an alkane-elimination reaction in a similar manner as shown in Scheme 30. Because of the dianionic nature of the resulting ligand, only one [CH2SiMe3] ligand is retained in the resulting compounds, which allows for further derivatization (Chart 11). In the presence of Ph3SiH or H2, complexes 66-73 form dimeric hydrido complexes,176,177,204,236 showing high potential in the catalytic hydrosilylation of olefins.226,237,238 Catalytic activities and stereoselectivities are hereby influenced by the length of the linker between the cyclopentadienyl and the amido-functionality and the substituents at the amidonitrogen.237 Remarkable catalytic activity was observed for complexes 68cyclohexyl. Upon activation with equimolar amounts of [Ph3C][B(C6F5)4], such compounds polymerized ethylene and isoprene, regiospecifically yielding 3,4-polyisoprene with isotactic-rich stereo microstructures and relatively narrow molecular weight distribution (Mw/Mn ) 1.8).222 Complex 67Y was found to initiate the polymerization of the polar monomers tert-butyl acrylate and acrylonitrile, however, yielding atactic polymeric products (see Table 5).204 8.2.4. Complexes with Neutral Nitrogenand Oxygen-Based Ligands While early work in organorare-earth metal chemistry was dominated by complexes supported by cyclopentadienyl-type ligands of varying substitution and modification, the limitations inherent to these ligand sets triggered the development of alternative ancillary ligands. Particularly in the past 15 years, advanced ligand design gave access to a wide variety of rare-earth metal complexes supported by noncyclopentadienyl ligand environments. Because of the Lewis acidic nature of the rare-earth metal ions, ligands based on the hard donor elements oxygen and nitrogen are most commonly used, while some notable exceptions have been reported. To avoid ligand redistribution, multidentate ligands are generally favored. Since rare-earth metal cations are invariable in the +3 oxidation state (except Eu(II), Sm(II), Yb(II), and Ce(IV)), neutral, monoanionic, or dianionic ligand sets are the most desirable. Table 4. Further Applications of Half-Sandwich Complexes (Cp)Ln(CH2SiMe3)2(donor)x compound further application ref 50 [CH2SiMe3] exchange reactions 205-209 formation of mono(cations) alternating ethylene-norbornene copolymerization

A Rare‐Earth Metal Variant of the Tebbe Reagent

Tebbe reagent [Cp2Ti{(m-CH2)(m-Cl)Al(CH3)2}] (A ; Cp = cyclopentadienyl) belongs to the most enigmatic organometallic compounds. Its successful synthesis, resulting from the careful investigation of the reaction of [Cp2TiCl2] with two equivalents Al(CH3)3, was triggered by important discoveries in two fundamentally different areas of homogeneous catalysis. Indeed, the initial studies of methane (and methylidene) formation from [Cp2TiCl2]/Al(CH3)3 mixtures were conducted in the context of Ziegler–Natta polymerization catalysis, but the methylene unit was structurally characterized by X-ray crystallography for the first time in tantalum alkylidene complexes, such as [Cp2Ta(CH2)(CH3)], [3] and tungsten methylene compounds, related to proposed catalysts for olefin metathesis. Although catalytically active in olefin metathesis, the Tebbe reagent is currently used for efficient carbonyl methylenation reactions. In his initial studies, Tebbe also commented on the synthesis of the structurally similar all-methyl derivative [Cp2Ti{(m-CH2)(m-CH3)Al(CH3)2}] (B) from the labile [Cp2Ti(CH3)2] and Al(CH3)3, suggesting [Cp2Ti(CH3)2{Al(CH3)3}] (B ) as a stabilized intermediate. Although the structures of the bis(neopentyl) derivative [Cp2Ti{(m-CH2)(m-Cl)Al(CH2C(CH3)3)2}] [7] and a zirconium analogue [Cp2Zr{(m-CHCH2C(CH3)3)(m-Cl)Al(CH2CH(CH3)2)2}] [8] have been reported, there are no X-ray structures of the Tebbe reagent nor of discrete metallacycles of the type [M(m-CH2)(m-R)Al(CH3)2] (R = Cl, CH3). [9, 10] Previous studies from our laboratories on rare-earthmetal(III) tetramethylaluminate complexes [LxLn{Al(CH3)4}y] (y = 1, 2, 3; x + y = 3, L = monovalent ancillary ligand, Ln = lanthanides and Sc, Y, La) as polymerization catalysts 12] led to the isolation of Ln clusters with methylene, 14] methine, and carbide functionalities. We also found that complex [Cp*3Y3(m-Cl)3(m3-Cl)(m3-CH2)(thf)3] (Cp* = C5(CH3)5) displayed Tebbe-like reactivity. [13]

Half‐Sandwich Bis(tetramethylaluminate) Complexes of the Rare‐Earth Metals: Synthesis, Structural Chemistry, and Performance in Isoprene Polymerization

The protonolysis reaction of [Ln(AlMe(4))(3)] with various substituted cyclopentadienyl derivatives HCp(R) gives access to a series of half-sandwich complexes [Ln(AlMe(4))(2)(Cp(R))]. Whereas bis(tetramethylaluminate) complexes with [1,3-(Me(3)Si)(2)C(5)H(3)] and [C(5)Me(4)SiMe(3)] ancillary ligands form easily at ambient temperature for the entire Ln(III) cation size range (Ln=Lu, Y, Sm, Nd, La), exchange with the less reactive [1,2,4-(Me(3)C)(3)C(5)H(3)] was only obtained at elevated temperatures and for the larger metal centers Sm, Nd, and La. X-ray structure analyses of seven representative complexes of the type [Ln(AlMe(4))(2)(Cp(R))] reveal a similar distinct [AlMe(4)] coordination (one eta(2), one bent eta(2)). Treatment with Me(2)AlCl leads to [AlMe(4)] --> [Cl] exchange and, depending on the Al/Ln ratio and the Cp(R) ligand, varying amounts of partially and fully exchanged products [{Ln(AlMe(4))(mu-Cl)(Cp(R))}(2)] and [{Ln(mu-Cl)(2)(Cp(R))}(n)], respectively, have been identified. Complexes [{Y(AlMe(4))(mu-Cl)(C(5)Me(4)SiMe(3))}(2)] and [{Nd(AlMe(4))(mu-Cl){1,2,4-(Me(3)C)(3)C(5)H(2)}}(2)] have been characterized by X-ray structure analysis. All of the chlorinated half-sandwich complexes are inactive in isoprene polymerization. However, activation of the complexes [Ln(AlMe(4))(2)(Cp(R))] with boron-containing cocatalysts, such as [Ph(3)C][B(C(6)F(5))(4)], [PhNMe(2)H][B(C(6)F(5))(4)], or B(C(6)F(5))(3), produces initiators for the fabrication of trans-1,4-polyisoprene. The choice of rare-earth metal cation size, Cp(R) ancillary ligand, and type of boron cocatalyst crucially affects the polymerization performance, including activity, catalyst efficiency, living character, and polymer stereoregularity. The highest stereoselectivities were observed for the precatalyst/cocatalyst systems [La(AlMe(4))(2)(C(5)Me(4)SiMe(3))]/B(C(6)F(5))(3) (trans-1,4 content: 95.6 %, M(w)/M(n)=1.26) and [La(AlMe(4))(2)(C(5)Me(5))]/B(C(6)F(5))(3) (trans-1,4 content: 99.5 %, M(w)/M(n)=1.18).