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

Abstract

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].

Rare-earth-metal half-sandwich complexes incorporating methyl, methylidene, and hydrido ligands

“…5 The Cp* hydrogen atoms appeared as a singlet at 1.90 ppm, indicating an unhindered rotation about the yttrium–Cnt(Cp*) axes. 2 a ,6 a , b The quartet at −0.28 ppm is assigned to the μ 3 -bridging CH 2 group, and shows a similar chemical shift as related half-sandwich methylidene complexes, 6 but slightly shifted toward lower field. The μ 2 -bridging methyl ligands resonate at −1.03 ppm.…”

“…For comparison, the respective μ 2 -CH 3 and μ 3 -CH 2 signals of complex [Cp* 3 Y 3 (μ 2 -CH 3 ) 3 (μ 3 -CH 2 )(μ 3 -CH 3 )(thf) 2 ] ( I ) were reported at 24.7 and 95.6 ppm, respectively. 6…”

“…The XRD analysis further revealed that the μ 3 -bridging moieties cap the faces of a six-membered metallacycle with alternating μ 2 -bridging methyl groups and metal centers. By nature, compound 1 Y displays metrics similar to the respective methyl/methylidene-bridged complex [Cp* 3 Y 3 (μ 2 -CH 3 ) 3 (μ 3 -CH 2 )(μ 3 -CH 3 )(thf) 2 ] ( I ), 6 as evidenced by the Y1–C(Cp*) ( 1 Y : 2.700(2)–2.728(2) Å; I : 2.703(3)–2.711(3) Å) and Y1–C(μ 2 -Me) distances ( 1 Y : 2.572(2) Å; I : 2.591(4) and 2.548(4) Å). However, the Y1–C(μ 3 -CH 2 ) distances deviate notably between the two complexes.…”

“…The synthesis shown in Scheme 1 also applies for other mid-sized rare-earth metals, as demonstrated for paramagnetic dysprosium. Keeping the corresponding dysprosium polymethyl complex [Cp*Dy(μ 2 -CH 3 ) 2 ] 3 (ref. 6 b ) for several days in THF solution afforded the respective mixed methylidene/hydride complex [Cp* 3 Dy 3 (μ 2 -CH 3 ) 3 (μ 3 -CH 2 )(μ 3 -H)(thf) 3 ] ( 1 Dy , 74%).…”

Nanoscale Organolanthanum Clusters: Nuclearity‐Directing Role of Cyclopentadienyl and Halogenido Ligands

“…Viable synthesis protocols for the half‐sandwich coordination clusters include salt metathesis, hydrogenolysis, thermolysis/desolvation, and Ln redox transformations . In the realm of Ziegler–Natta catalysts and respective model compounds, we have embarked on alkylaluminato/halogenido ligand exchange and donor‐promoted alkylaluminate cleavage of discrete half‐sandwich complexes [Cp*Ln(AlMe 4 ) 2 ] and gained access to {La 6 } and {Gd 8 } entities of the type [Cp* 6 La 6 Cl 8 (AlMe 4 ) 4 ] and [(Cp* 8 Gd 8 Me 4 (AlMe 4 ) 4 (CH 2 O t Bu) 8 ] . Spurred by the ease and efficiency of the former partial alkylaluminato/halogenido ligand exchange and the dearth of data on higher‐nuclearity organolanthanide clusters, we now targeted new types of donor‐free CpLnX 2 derivatives via complete alkyl/halogenido exchange.…”