Sc(iii) complexes with neutral N3- and SNS-donor ligands – a spectroscopic study of the activation of ethene polymerisation catalysts

Abstract

Scandium trichloride complexes with tridentate N₃- and S₂N-donor ligands (L₃) have been synthesised and characterised by IR, ¹H, ¹³C{¹H} and ⁴⁵Sc NMR spectroscopy, microanalysis, and solid state and solution XAFS spectroscopy. Catalytic testing of a subset of these complexes with ethene has been undertaken in chlorobenzene with MMAO-3A and PMAO-IP at 60 °C and 40 bar ethene, giving low activity ethene polymerisation. The reactions of these complexes with MeLi and AlMe₃ were studied by ¹H, ¹³C{¹H}, ²⁷Al and ⁴⁵Sc NMR spectroscopy and in situ via Sc K-edge XAFS spectroscopy. Three or four mol. equivalents of MeLi react with [ScCl₃(Me₃-tacn)] in THF solution to form [ScMe₃(Me₃-tacn)] cleanly, while complexes of type [ScCl₃(R-SNS)] {R-SNS = HN(CH₂CH₂SC₁₀H₂₁)₂} form two different species proposed to be [ScMe₃(R-SN(Li)S)] and [ScMe₂(R-SN⁻S)]. In contrast, in situ⁴⁵Sc NMR and Sc K-edge XAFS spectroscopic studies of the reaction of [ScCl₃(Me₃-tacn)] with 10 mol. equivalents of AlMe₃ strongly suggest that alkylation at the Sc(III) centre does not occur, instead retaining the Cl₃N₃ coordination environment and most likely forming Sc−Cl−AlMe₃ bridging interactions. Similar studies on [ScCl₃(decyl-SNS)] with 10 mol. equivalents of AlMe₃ are also consistent with this, indicating that alkylation at the Sc centre does not occur except in the presence of co-catalyst [Ph₃C][Al{OC(CF₃)₃}₄] and the α-alkene, hex-1-ene.