Rare-earth metal-promoted (double) C-H-bond activation of a lutidinyl-functionalized alkoxy ligand: formation of [ONC] pincer-type ligands and implications for isoprene polymerization

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 H-1 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).