Zirconium and Titanium Propylene Polymerization Precatalysts Supported by a Fluxional C₂-Symmetric Bis(anilide)pyridine Ligand

Titanium and zirconium complexes supported by a bis­(anilide)­pyridine ligand (NNN = pyridine-2,6-bis­(N-mesitylanilide)) have been synthesized and crystallographically characterized. C₂-symmetric bis­(dimethylamide) complexes were generated from aminolysis of M­(NMe₂)₄ with the neutral, diprotonated NNN ligand or by salt metathesis of the dipotassium salt of NNN with M­(NMe₂)₂Cl₂. In contrast to the case for previously reported pyridine bis­(phenoxide) complexes, the ligand geometry of these complexes appears to be dictated by chelate ring strain rather than metal–ligand π bonding. The crystal structures of the five-coordinate dihalide complexes (NNN)­MCl₂ (M = Ti, Zr) display a C₁-symmetric geometry with a stabilizing ipso interaction between the metal and the anilido ligand. Coordination of THF to (NNN)­ZrCl₂ generates a six-coordinate C₂-symmetric complex. Facile antipode interconversion of the C₂ complexes, possibly via flat C_2v intermediates, has been investigated by variable-temperature ¹H NMR spectroscopy for (NNN)­MX₂(THF)_n (M = Ti, Zr; X = NMe₂, Cl) and (NNN)­Zr­(CH₂Ph)₂. These complexes were tested as propylene polymerization precatalysts, with most complexes giving low to moderate activities (10²–10⁴ g/(mol h)) for the formation of stereoirregular polypropylene.