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. C 2 -symmetric bis(dimethylamide) complexes were generated from aminolysis of M(NMe 2 ) 4 with the neutral, diprotonated NNN ligand or by salt metathesis of the dipotassium salt of NNN with M(NMe 2 ) 2 Cl 2 . 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)MCl 2 (M = Ti, Zr) display a C 1 -symmetric geometry with a stabilizing ipso interaction between the metal and the anilido ligand. Coordination of THF to (NNN)ZrCl 2 generates a six-coordinate C 2 -symmetric complex. Facile antipode interconversion of the C 2 complexes, possibly via flat C 2 v intermediates, has been investigated by variable-temperature 1 H NMR spectroscopy for (NNN)MX 2 (THF) n (M = Ti, Zr; X = NMe 2, Cl) and (NNN)Zr(CH 2 Ph) 2 . These complexes were tested as propylene polymerization precatalysts, with most complexes giving low to moderate activities (10 2 –10 4 g/(mol h)) for the formation of stereoirregular polypropylene.