When activated with fluorinated borate cocatalysts the trimetallic complexes [Cp*LnMe2]3 (Ln=Y, Lu; Cp*=C5Me5) promote efficiently the formation of high‐cis polybutadiene. Respective polyisoprenes instead bear much less pronounced microstructures, but reveal crosslinked products at lower polymerization temperatures. Varying the amount of cocatalyst, the emerging active species were examined by NMR spectroscopic techniques (incl. 1H DOSY). The occurrence of donor‐solvent and thermally induced degradation products of the highly reactive precatalyst [Cp*YMe2]3 and derived catalyst species was revealed by the elucidation of methylidene clusters [Cp*3Y3Me4(CH2)(thf)2] and [Cp*6Y6Me4(CH2)4], as well as [(Cp*Y)2Me2(N(Me)2(C6H4)]n[B(C6F5)4]n, implying a multimetallic active species.

[(C5Me5)­LnMe2]3 (Ln = Y, Lu) dissolve readily in a n-hexane/toluene mixture upon addition of 3 equiv of the organoaluminum boryl compound [Me2Al­{B­(NDippCH)2}]2 (Dipp = C6H3 iPr2-2,6). 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.

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.

During the past two decades homoleptic tetramethylaluminates of the trivalent rare-earth metals, Ln(AlMe 4 ) 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

The bimetallic formamidinate complexes Ln­(Form)­(AlMe4)2 (Ln = Y, Form (ArNCHNAr) = EtForm (Ar = 2,6-Et2C6H3), MesForm (Ar = 2,4,6-Me3C6H2), DippForm (Ar = 2,6-iPr2C6H3), tBuForm (Ar = 2-tBuC6H4); Ln = La, Form = DippForm, tBuForm) were obtained in high yield by protonolysis reactions between formamidines (FormH) and homoleptic rare-earth-metal tetramethylaluminates Ln­(AlMe4)3. Y­(Form)­(AlMe4)2 (Form = EtForm, DippForm) were also prepared by treatment of Y­(Form)­[N­(SiHMe2)2]2(thf) with trimethylaluminum after the former were prepared by the protonolysis of Y­[N­(SiHMe2)2]3(thf)2 complexes with EtFormH or DippFormH. The monomeric six-coordinate complexes Ln­(Form)­(AlMe4)2 (Ln = Y, Form = EtForm, MesForm, DippForm, tBuForm; Ln = La, Form = DippForm, tBuForm) show similar molecular structures with distorted-octahedral geometry and bidentate (N,N′) Form and AlMe4 ligands. The complex [La­(EtFormAlMe3)­(AlMe4)2]­(C7H8)1.5 from a protonolysis reaction between La­(AlMe4)3 and EtFormH has the EtForm ligand adopting a configuration in which one nitrogen and one aryl substituent are coordinated to the eight-coordinate lanthanum center in an η1(N):η6(arene) manner. From the reaction of La­(AlMe4)3 with MesFormH, C–H bond activation of an o-methyl group of the mesityl moiety occurred, yielding [La­{η1(N):η6(Ar)-Me2CH2FormAlMe3}­(AlMe3)­(AlMe4)]­[La­(Me2CH2FormAlMe3)­(AlMe3)­(AlMe4)]­(C6H14)1.5 (Me2CH2Form = MesForm-H­(o-Me)), in which two linkage isomers of Me2CH2Form were observed. Investigations were carried out on the compounds [Ln­(Form)­(AlMe4)2] (Ln = Y, La; Form = EtForm, DippForm) as precatalysts activated by [Ph3C]­[B­(C6F5)4] or [PhNMe2H]­[B­(C6F5)4] in isoprene polymerization. While the lanthanum complexes showed narrower molecular weight distributions (PDI < 1.2), a stereodirecting role was evidenced for the cocatalysts (trityl borate, maximum 87% trans-1,4-selectivity; anilinium borate, maximum 82% cis-1,4-selectivity).

Rare-earth-metal half-sandwich allyl complexes bearing an amino-functionalized cyclopentadienyl ligand (Cp NMe2 = 1-[2-(N,N-dimethylamino)ethyl]-2,3,4,5-tetramethylcyclopentadienyl) were synthesized in a two-step saltmetathesis reaction. Treatment of LnCl 3 (THF) x with LiCp NMe2 , followed by an in situ reaction with the Grignard reagent C 3 H 5 MgCl, generated the bis(allyl) half-sandwich complexes Cp NMe2 Ln(η 3 -C 3 H 5 ) 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 NMe2

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Dialuminum formamidinate complexes [Me2Al(μ‐Form)(μ‐Me)AlMe2] [Form (ArNCHNAr) = EtForm (Ar = 2,6‐Et2C6H3) or DippForm (Ar = 2,6‐iPr2C6H3)] were obtained in good yields by treatment of formamidines (FormH) with trimethylaluminum in a 1:3 stoichiometry. The products contain both a bridging Form ligand and a bridging methyl group between the two aluminum centers. [M(Form)Me2] (M = Al, Form = DippForm, EtForm; M = Ga, Form = DippForm) were prepared in high yields by the protonolysis reactions of MMe3 (M = Al, Ga) with formamidines (FormH) in a 1:1 stoichiometry. [Al(DippForm)Me2] was also synthesized by other methods including concentration of the tetrahydrofuran/n‐hexane solution of [Me2Al(μ‐DippForm)(μ‐Me)AlMe2] to dryness and also by reaction of AlMe3 with [Ga(DippForm)Me2] in a 1:1 ratio. Partial dissociation of the dialuminum to monoaluminum complexes was detected in the 1H NMR spectra of [Me2Al(μ‐Form)(μ‐Me)AlMe2] complexes. All complexes exhibit a distorted tetrahedral stereochemistry of the Group 13 metal atoms.