The hitherto unknown homoleptic tetramethylaluminate complex [Sc(AlMe 4 ) 3 ] could be obtained by reacting the ate complex [Li 3 ScMe 6 (thf) 1.2 ] with AlMe 3 in the cold. It cocrystallizes with AlMe 3 as [Sc-(AlMe 4 ) 3 (Al 2 Me 6 ) 0.5 ] and decomposes at ambient temperature in n-pentane via multiple C−H bond activations to the mixed methyl/methylidene complex [Sc 3 (μ 3 -CH 2 ) 2 (μ 2 -CH 3 ) 3 (AlMe 4 ) 2 (AlMe 3 ) 2 ]. Donor-induced methylaluminate cleavage of [Sc(AlMe 4 ) 3 (Al 2 Me 6 ) 0.5 ] produced [ScMe 3 ] n in good yield, which could be derivatized with trimethyltriazacyclononane (Me 3 TACN) to form the structurally characterizable [(Me 3 TACN)ScMe 3 ]. Additionally, half-sandwich complex [Cp*Sc(AlMe 4 ) 2 ] and sandwich complex [Cp* 2 Sc(AlMe 4 )] were accessible by salt metathesis reactions from [Sc-(AlMe 4 ) 3 (Al 2 Me 6 ) 0.5 ] and KCp* (Cp* = C 5 Me 5 ). 45 Sc NMR spectroscopy was applied as a significant probe to evidence the existence of [ScMe 3 ] n . Compounds [(Me 3 TACN)ScMe 3 ] (+624.6 ppm) and [ScMe 3 (thf) x ] (+601.7 ppm) gave large 45 Sc NMR shifts, revealing the strong deshielding effect of the σ-bonded alkyl ligands on the scandium nuclei. Ultimately, cationized [Sc(AlMe 4 ) 3 (Al 2 Me 6 ) 0.5 ] was employed in isoprene polymerization, leading to polymers in high yields (>95%) and with high (>90%) cis-1,4-polyisoprene content.

Aiming at half-sandwich complexes of the type (FluR)­Ln­(AlMe4)2, homoleptic tetramethylaluminates Ln­(AlMe4)3 (Ln = Y, La, Nd, and Lu) were treated with equimolar amounts of R-substituted potassium fluorenyls. The salt metathesis reaction of La­(AlMe4)3 with K­(Flu) (Flu = fluorenyl = C13H9) and K­(Flu tBu) (Flu tBu = di­(tert-butyl) fluorenyl) selectively gave the desired half-sandwich complexes (FluR)­La­(AlMe4)2. The corresponding reactions of Y­(AlMe4)3 with K­(Flu)/K­(Flu tBu) and Lu­(AlMe4)3 with K­(Flu tBu) gave mixtures of half-sandwich and sandwich complexes, while treatment of Lu­(AlMe4)3 with K­(Flu) produced only the lutetocene complex (Flu)2Lu­(AlMe4). Sterically more demanding 1-trimethylsilyl fluorenyl (FluSi) allowed for the isolation of half-sandwich complexes for the entire Ln­(III) size range (Ln = La, Nd, Y, and Lu), in crystalline yields up to 94%. Upon activation with routinely employed borate or borane activators [Ph3C]­[B­(C6F5)4], [PhNMe2H]­[B­(C6F5)4], and B­(C6F5)3, highly active initiators for isoprene polymerization were obtained. The catalyst activity as well as molecular weight (distribution) and stereoregularity of the obtained polyisoprenes are governed by the rare-earth metal size, fluorenyl ligand, and cocatalyst: highest activity for La/FluSi/[Ph3C]­[B­(C6F5)4], lowest M w/M n = 1.11 for La/Flu/[PhNMe2H]­[B­(C6F5)4], maximum trans-1,4 selectivity = 85% for La/Flu tBu/[PhNMe2H]­[B­(C6F5)4], maximum cis-1,4 selectivity = 78% for Lu/FluSi/[Ph3C]­[B­(C6F5)4]. The formations of the active species were investigated by NMR spectroscopy revealing not only established cationization pathways but also fluorenyl abstraction in lanthanum complexes (Flu)­La­(AlMe4)2 and (Flu tBu)­La­(AlMe4)2 by trityl borate [Ph3C]­[B­(C6F5)4]. The reaction of half-sandwich complexes (FluR)­Ln­(AlMe4)2 with equimolar amounts of Me2AlCl did not give access to catalytically active species. Crystallization of binary mixtures (FluSi)­Y­(AlMe4)2/Me2AlCl in distinct molar ratios of 1:1 and 1:1.7 yielded complexes [(FluSi)­Y­(AlMe4)­(μ-Cl)]2 and (FluSi)6Y6Cl12, respectively.

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

Lanthanum indenyl half-sandwich complexes of the composition (IndR)­La­(AlMe4)2 were synthesized in high crystalline yields by a salt-metathesis protocol applying La­(AlMe4)3 and Li­(IndR). In the solid state, the parent indenyl (Ind) and 2-ethylindenyl (IndEt) complexes exhibit a dimeric structural motif with the methyl groups of the linearly aligned La­(μ-CH3)Al moieties being cis-positioned to the indenyl ligand. In contrast, 1-trimethylsilyl indenyl (IndSi) directs the η1-coordinated methyl group of the bridging aluminato ligand into a trans-position, while 2-tert-butyl indenyl afforded the monomeric half-sandwich complex (Ind tBu)­La­(AlMe4)2. The reactions of Lu­(AlMe4)3 with 1 or 2 equiv of Li­(IndR) gave predominantly bis­(indenyl) sandwich complexes (IndR)2­Lu­(AlMe4). All (half-)­sandwich complexes were characterized by X-ray structure analysis, 1H/13C­{1H} NMR and FTIR spectroscopy, and microanalysis. The performance of all half-sandwich complexes in isoprene polymerization was assessed upon activation with [Ph3C]­[B­(C6F5)4], [Ph­NMe2H]­[B­(C6F5)4], or B­(C6F5)3. The choice of indenyl ligand and cocatalyst had a major impact on the polymerization efficiency and stereospecificity. The highest selectivities could be achieved with the binary catalyst systems (IndEt)­La­(AlMe4)2/​[Ph3C]­[B­(C6F5)4] (cis/trans content 10.4/85.9) and (IndSi)­La­(AlMe4)2/​B­(C6F5)3 (cis/trans content 77.0/13.0).

η 6 -Arene(iodido-/bromido-aluminato)lanthanoid(III) complexes, [Ln(η 6 -C 6 H 5 Me)(AlI 4 ) 3 ] [Ln = La (1), Ce (2), Nd (3), (Gd) (4); C 6 H 5 Me = toluene], [Ln(η 6 -C 6 H 3 Me 3 -1,3,5)(AlI 4 ) 3 ] [Ln = La (5), Ce (6), Pr (7), Nd (8), Sm (9), Gd (10); C 6 H 3 Me 3 -1,3,5 = mesitylene], and [Ln(η 6 -C 6 H 5 Me)(AlBr 4 ) 3 ] [Ln = La (11), Nd (12), Sm (13)] were prepared by reactions of aluminium triiodide or aluminium tribromide with the corresponding lanthanoid metals and 1,2diiodoethane or 1,2-dibromoethane in an arene (toluene or mesitylene) solution (molar ratio : 6 : 2 : 3). The first X-ray crystal structures of arene(iodidoaluminato)lanthanoid(III) complexes are reported. The lanthanoid atom is coordinated by an η 6arene and three chelating k(I, I')-tetraiodidoaluminato ligands. The tetrabromidoaluminate complexes have similar structures. The precatalyst 3 was treated with AlR 3 (R = Me or iBu) to give [Nd(η 6 -C 6 H 5 Me)(AlI 3 R) 3 ] species in situ, which were then tested for catalytic activity towards isoprene polymerization. Although the resulting polyisoprene had a desirable high cis-1,4 content, the catalyst performance was well below known best performing systems and indicates that iodidoaluminates are the least favorable of the halogenidoaluminatolanthanoid(III) complexes. Results and DiscussionThe iodido-and bromido-aluminato(η 6 -arene)lanthanoid complexes have been synthesized by heating a mixture of six molar [a] Dr.