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.