Non-planar co-ordination of C-2-symmetric biaryl-bridged Schiff-base ligands: well expressed chiral ligand environments for zirconium

The C-2-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 [ (MLCl2)-Cl-1]. The molecular structures of these complexes show that they b elong to the hitherto unknown cis-cis class of quadridentate Schiff-base co mplex. Most importantly, the two chloride ligands occupy mutually cis co-or dination sites in a well expressed chiral ligand environment that is struct urally analogous to that of the ansa-metallocenes. Alkylations of the zirco nium complex were unsuccessful, but an unstable complex [ZrL1(CH2Ph)(2)] wa s synthesized from H2L1 and Zr(CH2Ph)(4). Amido complexes [ZrL1(NR2)(2)] (R =Me or Et; R-2=HBut) were synthesized by substitution reactions of [(ZrLCl2 )-Cl-1] 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 [(Z rLCl2)-Cl-1] respectively. The reaction of [(ZrLCl2)-Cl-1] 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 are ne interactions between the phenoxide ligands and the salicylidene units. T he reaction of [(ZrLCl2)-Cl-1] with LiAlH4 in pyridine (py) led to reductio n in situ of the imine groups and elimination of both chloride ligands to g ive a complex containing the quadruply deprotonated reduced form of L-1, i. e. [ZrL2(py)(2)]. This complex is also formed by reaction of H4L2 with Zr(N R2)(4) in pyridine.