Zirconium Complexes Containing Tetradentate O,P,P,O Ligands: Ethylene and Propylene Polymerization Studies

The synthesis of a series of zirconium complexes bearing a linear diphosphanyl-bisphenoxide O,P,P,O ligand is described. The diphosphanyl-bisphenol proligand was prepared from Ph(Cl)PC2H4P(Cl)Ph as a 1:1 mixture of rac and meso diastereomers, which could be separated as a result of their differing solubilities in methanol. A relatively low barrier to phosphine inversion (ΔG⧧373K = 28.02(3) kcal/mol) allows interconversion of the two isomers. The zirconium dichloride complex of each diastereomer was prepared via treatment of ZrCl4(THF)2 with the sodium (rac) or lithium (meso) salt of the respective bisphenol. Crystallographic and NMR analyses revealed a cis-α structure for the complex bearing the rac-O,P,P,O ligand and a seven-coordinate quasi-cis-β structure for the complex derived from the meso-O,P,P,O ligand. Dibenzyl complexes were synthesized by treatment of Zr(CH2Ph)4 with the rac and meso bisphenols; NMR evidence revealed cis-α and fluxional cis-β structures, respectively. Reactions of the dibenzyl complexes with [CPh3][B(C6F5)4] afforded cationic alkyl derivatives in which the pre-existing ligand geometries were maintained. Ethylene polymerization studies afforded productivities up to 5450 and 116 g/mmol·h·bar for the cis-α and cis-β complexes, respectively, when using [CPh3][B(C6F5)4]/Al(iBu)3 as the activator. Under a propylene atmosphere, oligomeric products were obtained with low to moderate productivities.