Rare‐Earth‐Metal Alkylaluminates Supported by N‐Donor‐Functionalized Cyclopentadienyl Ligands: CH Bond Activation and Performance in Isoprene Polymerization

Abstract

Homoleptic tetramethylaluminate complexes [Ln(AlMe4)3] (Ln=La, Nd, Y) reacted with HCp(NMe2) (Cp(NMe2) =1-[2-(N,N-dimethylamino)-ethyl]-2,3,4,5-tetramethyl-cyclopentadienyl) in pentane at -35 °C to yield half-sandwich rare-earth-metal complexes, [{C5 Me4CH2CH2NMe2 (AlMe3)}Ln(AlMe4)2]. Removal of the N-donor-coordinated trimethylaluminum group through donor displacement by using an equimolar amount of Et2O at ambient temperature only generated the methylene-bridged complexes [{C5Me4CH2CH2NMe(μ-CH2)AlMe3}Ln(AlMe4)] with the larger rare-earth-metal ions lanthanum and neodymium. X-ray diffraction analysis revealed the formation of isostructural complexes and the C-H bond activation of one aminomethyl group. The formation of Ln(μ-CH2)Al moieties was further corroborated by (13)C and (1)H-(13)C HSQC NMR spectroscopy. In the case of the largest metal center, lanthanum, this C-H bond activation could be suppressed at -35 °C, thereby leading to the isolation of [(Cp(NMe2))La(AlMe4)2], which contains an intramolecularly coordinated amino group. The protonolysis reaction of [Ln(AlMe4)3] (Ln=La, Nd) with the anilinyl-substituted cyclopentadiene HCp(AMe2) (Cp(AMe2) =1-[1-(N,N-dimethylanilinyl)]-2,3,4,5-tetramethylcyclopentadienyl) at -35 °C generated the half-sandwich complexes [(Cp(AMe2))Ln(AlMe4)2]. Heating these complexes at 75 °C resulted in the C-H bond activation of one of the anilinium methyl groups and the formation of [{C5Me4C6H4NMe(μ-CH2)AlMe3}Ln(AlMe4)] through the elimination of methane. In contrast, the smaller yttrium metal center already gave the aminomethyl-activated complex at -35 °C, which is isostructural to those of lanthanum and neodymium. The performance of complexes [{C5Me4CH2CH2NMe(μ-CH2 )AlMe3}-Ln(AlMe4)], [(Cp(AMe2))Ln(AlMe4)2], and [{C5Me4C6H4NMe(μ-CH2)AlMe3}Ln(AlMe4)] in the polymerization of isoprene was investigated upon activation with [Ph3C][B(C6F5)4], [PhNMe2 H][B(C6F5)4], and B(C6F5)3. The highest stereoselectivities were observed with the lanthanum-based pre-catalysts, thereby producing polyisoprene with trans-1,4 contents of up to 95.6 %. Narrow molecular-weight distributions (Mw/Mn <1.1) and complete consumption of the monomer suggested a living-polymerization mechanism.

Heterobi‐ and ‐trimetallic Ion Pairs of Zirconocene‐Based Isoselective Olefin Polymerization Catalysts with AlMe3

“…surmised that, upon activation of 1 b with MAO, formation of the putative methylidene species [ rac ‐{Me 2 Si‐(2‐Me‐4‐Ph‐Ind) 2 }Zr(μ‐CH 2 )(μ‐Me)AlMe 2 ] + [“Me‐MAO”] − takes place from the corresponding parent { rac ‐[{Me 2 Si‐(2‐Me‐4‐Ph‐Ind) 2 }Zr(μ‐Me) 2 AlMe 2 ] + [“Me‐MAO”] − , accompanied by concomitant evolution of methane 7b. 16 This is also reminescent to Anwander’s work on the formation of Ln(μ‐CH 2 )Al species via CH activation of aminomethyl group in neutral lanthanoid alkylaluminate complexes 2c…”

Rare‐Earth‐Metal Pentadienyl Half‐Sandwich and Sandwich Tetramethylaluminates–Synthesis, Structure, Reactivity, and Performance in Isoprene Polymerization

“…However, an η 3 ‐coordination by one [AlMe 4 ] ligand likely compensates for enhanced steric unsaturation of the lanthanum centers in complexes 1 La (La⋅⋅⋅C10=3.083(4) Å; torsion angles: La‐C8‐Al1‐C9=50.62°; La‐C12‐Al2‐C13=8.96°) and 2 La (La⋅⋅⋅C14=3.106(7) Å; torsion angles: La‐C12‐Al1‐C13=51.59°; La‐C16‐Al2‐C17=9.41°). This potential interaction is only observed in the solid state . In complexes 3 Ln , the bulkier 2,4‐dtbp ligand apparently prevents such an η 3 ‐coordination, causing both [AlMe 4 ] groups to adopt an η 2 ‐coordination (torsion angles for 3 La : La‐C14‐Al1‐C15=6.32°; La‐C18‐Al2‐C19=14.44°).…”

Methyl Complexes of the Transition Metals

“…A large number of studies have been performed on neutral and anionic species, but cationic species have also been shown to be relevant intermediates in the context of α‐olefin polymerization catalysis and are related to the chemistry of Group 4 elements . Common ancillary ligands of Group 3 metal methyls include cyclopentadienyl derivatives, amidinates, and β‐diketiminates . Cationic methyl complexes usually feature crown ethers as stabilising ligands, but cyclic polynitrogen bases have also been successfully used (Scheme ) .…”