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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 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; 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 [(ZrLCl2)-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 arene interactions between the phenoxide ligands and the salicylidene units. The reaction of [(ZrLCl2)-Cl-1] 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 L-1, i.e. [ZrL2(py)(2)]. This complex is also formed by reaction of H4L2 with Zr(NR2)(4) in pyridine

Topics: QD
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