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

Abstract

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).

C–H Bond Activation and Isoprene Polymerization by Lutetium Alkylaluminate/gallate Complexes Bearing a Peripheral Boryl and a Bulky Hydrotris(pyrazolyl)borate Ligand

“…Comparing the performance of 1 with its [AlMe 4 ] analogue [(Tp t Bu,Me )Lu(Me)(AlMe 4 )] ( I Lu ), (Table , runs 10 and 11) revealed that I Lu is only active, when activated by a Brønsted acid, that is, cocatalyst B under the given conditions. Binary system I Lu / B produced polymers with higher trans contents compared to the boryl‐implemented system 1 / B , which is in analogy the polymers fabricated by similar Cp*‐supported systems , . Furthermore, complex [Cp*Lu(AlMe 3 {B(NDippCH) 2 }) 2 ] ( II ) has been investigated previously by us (Table , runs 12–17) .…”

“…Given the importance of organoaluminum cocatalysts in rare‐earth‐metal‐based 1,3‐diene polymerization, we set out to develop a bis(hydrocarbylaluminate) postmetallocene library to better understand any potential Ln–Al bimetallic cooperativity as well as ancillary ligand effects . Initially, we and others introduced half‐sandwich bis(tetramethylaluminate) rare‐earth‐metal complexes of the general type [Cp R₅ Ln(AlMe 4 ) 2 ] [R 5 = HMe 4 , Me 5 , H 2 t Bu 3 , Me 4 SiMe 3 ,, H 3 (SiMe 3 ) 2 , Me 4 (quinolyl), Me 4 ( N , N ‐dimethylanilyl), Me 4 (C 2 H 4 NMe 2 )] as donor‐solvent free derivatives and established them as versatile precatalysts for the synthesis of highly stereoregular polyisoprene, when pretreated with borane/borate cocatalysts (Figure ). Varying the substitution pattern of the cyclopentadienyl (Cp) ligand allowed for a comprehensive assessment of the electronic and steric implications of the ancillary ligand for the catalytic performance ,.…”

Eine Seltenerdmetallvariante des Tebbe‐Reagens

“…[9, 10] Vorangegangene Untersuchungen unserer Arbeitsgruppe befassten sich mit Seltenerdmetall(III)-TetramethylaluminatKomplexen [L x Ln{Al(CH 3 ) 4 } y ] (y = 1, 2, 3; x + y = 3, L = einzähniger Hilfsligand, Ln = Seltenerdmetall und Sc, Y, La) als Polymerisationskatalysatoren [11,12] und führten zur Isolation von Ln III -Clustern mit Methylen-, [13,14] Methin- [15] und Carbidfunktionalitäten.[ …”

Precisely Controlled Polymerization of Styrene and Conjugated Dienes by Group 3 Single‐Site Catalysts

“…Then the group presented a more comprehensive account of the synthesis and structural chemistry of half‐sandwich complexes [Ln(AlMe 4 ) 2 (Cp′)] 89 – 91 containing various substituted Cp ancillary ligands. After systematically investigating the effect of metal size, the substituents on the Cp ligand, and the interaction with cocatalysts, the highest stereoselectivities were reported for the systems 90 /B(C 6 F 5 ) 3 ( trans ‐1,4‐: 95.6 %, M w / M n =1.26) and 88 /B(C 6 F 5 ) 3 ( trans ‐1,4‐: 99.5 %, M w / M n =1.18). in toluene at 40 °C.…”