Scandium and gadolinium complexes with aryldiimine NCN pincer ligands: Synthesis, characterization, and catalysis on isoprene and 1,5-hexadiene polymerization

Abstract

Several scandium and gadolinium complexes bearing tridentate aryldiimine NCN pincer ligands Ar[NCNH]ScCl2(THF)2 (Ar = Ph (1a); Ar = 2,6-Me2C6H3 (1b), Ar = 2,6-Et2C6H3 (1c), Ar = 2,6-iPr2C6H3 (1d), Ar = p-MeC6H4 (1e), Ar = 2,4,6-Me3C6H2 (1f)), Ar[NCNMe]MCl2(THF)2 (M = Sc, Ar = 2,6-Me2C6H3 (2b); M = Gd, Ar = 2,6-Me2C6H3 (3b), Ar = 2,6-iPr2C6H3 (3d)), and iPrPh[NCNMe]ScCl2 (2d) were synthesized via lithiation of the corresponding ligands with nBuLi and subsequent addition of ScCl3(THF)3 or GdCl3(THF)3 respectively. These complexes were well characterized by elemental analysis and the solid structures of 1c, 1d, 1f, and 2b were confirmed by X-ray diffraction analysis to be monomeric with N,C,N-tridentate ligands coordinating to the metal center in a typical meridional manner. In the presence of alkylaluminium and trityl borate, these well-defined complexes showed moderate to high activities in isoprene polymerization affording cis-1,4 enriched polymers (up to 98.3%). Moreover, the NCN pincer scandium complexes can also catalyze the 1,5-hexandiene polymerization giving methylene-1,3-cyclopentane enriched polymers (MCP content up to 99.3%).