Non-planar co-ordination of C2-symmetric biaryl-bridged Schiff-base ligands: well expressed chiral ligand environments for zirconium†

Abstract

The C₂-symmetric Schiff-base proligand (H₂L¹) synthesized by condensation of 2,2′-diamino-6,6′-dimethylbiphenyl and 3,5-di-tert-butylsalicylaldehyde reacted in its deprotonated form with [MCl₄(THF)₂] (M = Zr or Hf) to give [ML¹Cl₂]. The molecular structures of these complexes show that they belong to the hitherto unknown cis-cis class of quadridentate Schiff-base complex. Most importantly, the two chloride ligands occupy mutually cis co-ordination sites in a well expressed chiral ligand environment that is structurally analogous to that of the ansa-metallocenes. Alkylations of the zirconium complex were unsuccessful, but an unstable complex [ZrL¹(CH₂Ph)₂] was synthesized from H₂L¹ and Zr(CH₂Ph)₄. Amido complexes [ZrL¹(NR₂)₂] (R = Me or Et; R₂ = HBu^t) were synthesized by substitution reactions of [ZrL¹Cl₂] and by reaction of H₂L¹ with Zr(NR₂)₄. The reactions of [ZrL¹(NR₂)₂] (R = Me or Et) with SiH(OEt)₃ and SiMe₃Cl gave [ZrL¹(OEt)₂] and [ZrL¹Cl₂] respectively. The reaction of [ZrL¹Cl₂] with LiOPh gave [ZrL¹(OPh)₂]. The molecular structure of the latter confirms that it also has the cis-cis structure and also indicates the presence of edge–face arene interactions between the phenoxide ligands and the salicylidene units. The reaction of [ZrL¹Cl₂] with LiAlH₄ in pyridine (py) led to reduction in situ of the imine groups and elimination of both chloride ligands to give a complex containing the quadruply deprotonated reduced form of L¹, i.e. [ZrL²(py)₂]. This complex is also formed by reaction of H₄L² with Zr(NR₂)₄ in pyridine.