RESUMEN
The reaction of 2-(6-methyl-2-pyridyl)-1,1-diphenyl-ethanol [HONCH3] with Ln(AlMe4)3 (Ln = La, Nd, Y) via a deprotonation/C-H-bond activation sequence gave complexes [ONCH2]Ln(AlMe3)2(AlMe4) (Ln = La, Nd, Y) bearing the dianionic tridentate ligand [ONCH2]. In contrast, the reactions involving the smaller rare-earth metals yttrium and lutetium resulted in double C-H-bond activation and formation of [ONCH]Ln(AlMe3)3 (Ln = Y, Lu) with the formally trianionic tridentate ligand [ONCH]. The solid-state structures of all complexes as obtained by X-ray structure analysis revealed an axial chirality which could be corroborated by low-temperature 1H NMR spectroscopy. All complexes displayed high activity in the polymerization of isoprene, upon activation with standard fluorinated borate/borane cocatalysts. The catalyst activity and cis-1,4-selectivity could be increased by using of two equivalents of cocatalyst instead of one. For example, when activated with two equivalents of [PhNMe2H][B(C6F5)4] complex [ONCH]Y(AlMe3)3 gave almost complete conversion after 15 minutes fabricating a polyisoprene with a cis-1,4-content of 83.5% (no trans-1,4-content detected).
RESUMEN
The hitherto unknown homoleptic tetramethylaluminate complex [Sc(AlMe4)3] could be obtained by reacting the ate complex [Li3ScMe6(thf)1.2] with AlMe3 in the cold. It cocrystallizes with AlMe3 as [Sc(AlMe4)3(Al2Me6)0.5] and decomposes at ambient temperature in n-pentane via multiple C-H bond activations to the mixed methyl/methylidene complex [Sc3(µ3-CH2)2(µ2-CH3)3(AlMe4)2(AlMe3)2]. Donor-induced methylaluminate cleavage of [Sc(AlMe4)3(Al2Me6)0.5] produced [ScMe3]n in good yield, which could be derivatized with trimethyltriazacyclononane (Me3TACN) to form the structurally characterizable [(Me3TACN)ScMe3]. Additionally, half-sandwich complex [Cp*Sc(AlMe4)2] and sandwich complex [Cp*2Sc(AlMe4)] were accessible by salt metathesis reactions from [Sc(AlMe4)3(Al2Me6)0.5] and KCp* (Cp* = C5Me5). 45Sc NMR spectroscopy was applied as a significant probe to evidence the existence of [ScMe3]n. Compounds [(Me3TACN)ScMe3] (+624.6 ppm) and [ScMe3(thf)x] (+601.7 ppm) gave large 45Sc NMR shifts, revealing the strong deshielding effect of the σ-bonded alkyl ligands on the scandium nuclei. Ultimately, cationized [Sc(AlMe4)3(Al2Me6)0.5] was employed in isoprene polymerization, leading to polymers in high yields (>95%) and with high (>90%) cis-1,4-polyisoprene content.