Zirconium and Titanium Propylene Polymerization Precatalysts Supported by a Fluxional C_2-Symmetric Bis(anilide)pyridine Ligand

Abstract

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)_2Cl_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_(2v) intermediates, has been investigated by variable-temperature ^1H NMR spectroscopy for (NNN)MX_2(THF)_n (M = Ti, Zr; X = NMe_2, Cl) and (NNN)Zr(CH_2Ph)_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.

Files

Additional details