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