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 C2v 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.