Rare-earth metal-promoted (double) C–H-bond activation of a lutidinyl-functionalized alkoxy ligand: formation of [ONC]–(2–) pincer-type ligands and implications for isoprene polymerization
The reaction of 2-(6-methyl-2-pyridyl)-1,1-diphenyl-ethanol 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 addition of two eqivalents of cocatalyst instead of one. For example, when actviated 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).