Chiral Complexes of a New Diazaallyl Ligand:  Group 4 Aminooxazolinates

A new biaryl-bridged bis(iminooxazolidine) proligand H₂L is prepared in good yield from 2,2-diamino-6,6‘-dimethylbiphenyl. The direct reaction of H₂L with [Ti(CH₂Ph)₄] leads via deprotonation of the ligand to the C₂-symmetric dibenzyl complex [LTi(CH₂Ph)₂] (85%) containing diazaallyl ligation. The analogous group 4 complexes [LZr(CH₂Ph)₂] (79%) and [LHf(CH₂Ph)₂] (91%) are similarly obtained. Molecular structures of these three compounds indicate C₂-symmetry in all cases and that the chirality of the backbone is well expressed in the coordination sphere. Reaction of H₂L with Ti(NMe₂)₄ gives the amide [LTi(NMe₂)₂] (90%), which on reaction with SiMe₃Cl gives the chloride [LTiCl₂] (78%). The dichloride [LZr(NMe₂H)Cl₂] is prepared via treatment of H₂L with Zr(NMe₂)₂Cl₂(THF)₂ (86%). The direct reaction of H₂L with TiCl₄(THF)₂ gives the adduct [(H₂L)TiCl₄] (83%), which is shown by X-ray crystallography to contain intramolecular NH···Cl hydrogen bond contacts. The complexes were tested as precatalysts for the polymerization of ethene and 1-hexene using a range of cocatalysts and were found to display low activity. Correspondingly, NMR studies on a presumed active species [LZr(CH₂Ph)][B(C₆F₅)₃(CH₂Ph)] were consistent with tight ion pairing on the NMR chemical shift time scale.