C–H-Bond Activation and Isoprene Polymerization Studies Applying Pentamethylcyclopentadienyl-Supported Rare-Earth-Metal Bis(Tetramethylaluminate) and Dimethyl Complexes

As previously shown for lutetium and yttrium, 1,2,3,4,5-pentamethylcyclopentadienyl (C5Me5 = Cp*)-bearing rare-earth metal dimethyl half-sandwich complexes [Cp*LnMe2]3 are now also accessible for holmium, dysprosium, and terbium via tetramethylaluminato cleavage of [Cp*Ln(AlMe4)2] with diethyl ether (Ho, Dy) and tert-butyl methyl ether (TBME) (Tb). C–H-bond activation and ligand redistribution reactions are observed in case of terbium and are dominant for the next larger-sized gadolinium, as evidenced by the formation of mixed methyl/methylidene clusters [(Cp*Ln)5(CH2)(Me)8] and metallocene dimers [Cp*2Ln(AlMe4)]2 (Ln = Tb, Gd). Applying TBME as a “cleaving” reagent can result in both TBME deprotonation and ether cleavage, as shown for the formation of the 24-membered macrocycle [(Cp*Gd)2(Me)(CH2OtBu)2(AlMe4)]4 or monolanthanum complex [Cp*La(AlMe4){Me3Al(CH2)OtBu}] and monoyttrium complex [Cp*Y(AlMe4)(Me3AlOtBu)], respectively. Complexes [Cp*Ln(AlMe4)2] (Ln = Ho, Dy, Tb, Gd) and [Cp*LnMe2]3 (Ln = Ho, Dy) are applied in isoprene and 1,3-butadiene polymerization, upon activation with borates [Ph3C][B(C6F5)4] and [PhNHMe2][B(C6F5)4], as well as borane B(C6F5)3. The trans-directing effect of AlMe3 in the binary systems [Cp*Ln(AlMe4)2]/borate is revealed and further corroborated by the fabrication of high-cis-1,4 polybutadiene (97%) with “aluminum-free” [Cp*DyMe2]3/[Ph3C][B(C6F5)4]. The formation of multimetallic active species is supported by the polymerization activity of pre-isolated cluster [(Cp*Ho)3Me4(CH2)(thf)2].publishedVersio

1,3-Diene Polymerization Mediated by Homoleptic Tetramethylaluminates of the Rare-Earth Metals

During the past two decades homoleptic tetramethylaluminates of the trivalent rare-earth metals, Ln(AlMe4)3, have emerged as useful components for efficient catalyst design in the field of 1,3-diene polymerization. Previous work had focused on isoprene polymerization applying Ln(AlMe4)3 precatalysts with Ln = La, Ce, Pr, Nd, Gd and Y, in the presence of Et2AlCl as an activator. Polymerizations employing Ln(AlMe4)3 with Ln = La, Y and Nd along with borate/borane co-catalysts [Ph3C][B(C6F5)4], [PhNMe2H][B(C6F5)4] and [B(C6F5)3] were mainly investigated for reasons of comparison with ancillary ligand-supported systems (cf. half-sandwich complexes). The present study investigates into a total of eleven rare-earth elements, namely Ln = La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Y, Er and Lu. A full overview on the polymerization behavior of Ln(AlMe4)3 in the presence of perfluorinated borate/borane cocatalysts and R2AlCl-type activators (R = Me, Et) is provided, probing the monomers isoprene and 1,3-butadiene (and preliminary ethylene). Virtually complete cis-1,4-selectivities are obtained for several catalyst/cocatalyst combinations (e.g., Gd(AlMe4)3/Me2AlCl, >99.9%). Insights into the ‘black box’ of active species are obtained by indirect observations via screening of pre-reaction time and cocatalyst concentration. The microstructure of the polydienes is investigated by combined 1H/13C NMR and ATR-IR spectroscopies. Furthermore, the reaction of [LuMe6(Li(thf)x)3] with AlMe3 has been applied as a new strategy for the efficient synthesis of Lu(AlMe4)3. The solid-state structures of Gd(AlMe4)3 and Tb(AlMe4)3 are reported

Rare-Earth-Metal Allyl Complexes Supported by the [2‑(<i>N</i>,<i>N</i>‑Dimethylamino)ethyl]tetramethylcyclopentadienyl Ligand: Structural Characterization, Reactivity, and Isoprene Polymerization

Rare-earth-metal half-sandwich allyl complexes bearing an amino-functionalized cyclopentadienyl ligand (Cp^NMe2 = 1-[2-(<i>N</i>,<i>N</i>-dimethylamino)­ethyl]-2,3,4,5-tetramethylcyclopentadienyl) were synthesized in a two-step salt-metathesis reaction. Treatment of LnCl₃(THF)_<i>x</i> with LiCp^NMe2, followed by an in situ reaction with the Grignard reagent C₃H₅MgCl, generated the bis­(allyl) half-sandwich complexes Cp^NMe2Ln­(η³-C₃H₅)₂ only for the smaller rare-earth metals (Ln = Y, Ho, Lu) in good yields (82–88%). In case of the larger neodymium, the dimeric mono­(allyl) chlorido half-sandwich complex [Cp^NMe2Nd­(η³-C₃H₅)­(μ-Cl)]₂ was obtained in 68% yield. All complexes show moderate to high activity in isoprene polymerization upon cationization with organoborates [Ph₃C]­[B­(C₆F₅)₄] and [PhNMe₂H]­[B­(C₆F₅)₄], the yttrium, holmium, and neodymium metal centers yielding mainly 3,4-microstructures (maximum 79%). Addition of 10 equiv of AlMe₃ to the catalyst systems Cp^NMe2Ln­(η³-C₃H₅)₂ (Ln = Y, Ho)/[PhNMe₂H]­[B­(C₆F₅)₄] and [Cp^NMe2Nd­(η³-C₃H₅)­(μ-Cl)]₂/[PhNMe₂H]­[B­(C₆F₅)₄] switched the polyisoprene stereoregularity from 3,4-specific to trans-1,4-selective (maximum 85%). The use of Al<i>i</i>Bu₃ instead led to polymers with mainly cis-1,4-microstructure for the monomeric yttrium and holmium complexes (maximum 74%). Treatment of the bis­(allyl) complexes with Et₂AlCl (as cocatalyst) did not provide active species for isoprene polymerization but led to [allyl] → [Cl] exchange and isolation of the hexameric rare-earth-metal clusters [{(Cp^NMe2AlEt3)₂(Cp^NMe2)­Ln₃(μ₂-Cl)₃(μ₃-Cl)₂}­(μ₂-Cl)]₂ (Ln = Y, Ho). The complexes Cp^NMe2Ln­(η³-C₃H₅)₂ (Ln = Y, Ho, Lu), [Cp^NMe2Nd­(η³-C₃H₅)­(μ-Cl)]₂, and [{(Cp^NMe2AlEt3)₂(Cp^NMe2)­Ln₃(μ₂-Cl)₃(μ₃-Cl)₂}­(μ₂-Cl)]₂ (Ln = Y, Ho) were analyzed by X-ray crystallography

C-H-Bond activation and isoprene polymerization studies applying pentamethylcyclopentadienyl-supported rare-earth-metal Bis(tetramethylaluminate) and dimethyl complexes

As previously shown for lutetium and yttrium, 1,2,3,4,5-pentamethylcyclopentadienyl (C5Me5 = Cp*)-bearing rare-earth metal dimethyl half-sandwich complexes [Cp*LnMe2]3 are now also accessible for holmium, dysprosium, and terbium via tetramethylaluminato cleavage of [Cp*Ln(AlMe4)2] with diethyl ether (Ho, Dy) and tert-butyl methyl ether (TBME) (Tb). C–H-bond activation and ligand redistribution reactions are observed in case of terbium and are dominant for the next larger-sized gadolinium, as evidenced by the formation of mixed methyl/methylidene clusters [(Cp*Ln)5(CH2)(Me)8] and metallocene dimers [Cp*2Ln(AlMe4)]2 (Ln = Tb, Gd). Applying TBME as a “cleaving” reagent can result in both TBME deprotonation and ether cleavage, as shown for the formation of the 24-membered macrocycle [(Cp*Gd)2(Me)(CH2OtBu)2(AlMe4)]4 or monolanthanum complex [Cp*La(AlMe4){Me3Al(CH2)OtBu}] and monoyttrium complex [Cp*Y(AlMe4)(Me3AlOtBu)], respectively. Complexes [Cp*Ln(AlMe4)2] (Ln = Ho, Dy, Tb, Gd) and [Cp*LnMe2]3 (Ln = Ho, Dy) are applied in isoprene and 1,3-butadiene polymerization, upon activation with borates [Ph3C][B(C6F5)4] and [PhNHMe2][B(C6F5)4], as well as borane B(C6F5)3. The trans-directing effect of AlMe3 in the binary systems [Cp*Ln(AlMe4)2]/borate is revealed and further corroborated by the fabrication of high-cis-1,4 polybutadiene (97%) with “aluminum-free” [Cp*DyMe2]3/[Ph3C][B(C6F5)4]. The formation of multimetallic active species is supported by the polymerization activity of pre-isolated cluster [(Cp*Ho)3Me4(CH2)(thf)2]

Half-Sandwich Rare-Earth-Metal Alkylaluminate Complexes Bearing Peripheral Boryl Ligands

[(C₅Me₅)­LnMe₂]₃ (Ln = Y, Lu) dissolve readily in a <i>n</i>-hexane/toluene mixture upon addition of 3 equiv of the organoaluminum boryl compound [Me₂Al­{B­(NDippCH)₂}]₂ (Dipp = C₆H₃<i>i</i>Pr₂-2,6). The half-sandwich complexes (C₅Me₅)­Ln­[(AlMe₃)­{B­(NDippCH)₂}]₂ thus formed display unsymmetrical heteroaluminate coordination not only in the solid state but also at lower temperatures in solution, which is distinct from the behavior of the homoaluminate congeners (C₅Me₅)­Ln­(AlMe₄)₂. The effect of homo- versus heteroaluminate coordination is assessed in the coordinative polymerization of isoprene

Half-Sandwich Rare-Earth-Metal Alkylaluminate Complexes Bearing Peripheral Boryl Ligands

[(C₅Me₅)­LnMe₂]₃ (Ln = Y, Lu) dissolve readily in a <i>n</i>-hexane/toluene mixture upon addition of 3 equiv of the organoaluminum boryl compound [Me₂Al­{B­(NDippCH)₂}]₂ (Dipp = C₆H₃<i>i</i>Pr₂-2,6). The half-sandwich complexes (C₅Me₅)­Ln­[(AlMe₃)­{B­(NDippCH)₂}]₂ thus formed display unsymmetrical heteroaluminate coordination not only in the solid state but also at lower temperatures in solution, which is distinct from the behavior of the homoaluminate congeners (C₅Me₅)­Ln­(AlMe₄)₂. The effect of homo- versus heteroaluminate coordination is assessed in the coordinative polymerization of isoprene