Non-planar co-ordination of C_2-symmetric biaryl-bridged Schiff-base ligands: well expressed chiral ligand environments for zirconium

摘要

The C_2-symmetric Schiff-base proligand (H_2L~1) synthesized by condensation of 2, 2'-diamino-6, 6'-dimethylbiphenyl and 3, 5-di-tert-butylsalicylaldehyde reacted in its deprotonated form with [MCl_4(THF)_2] (M = Zr or Hf) to give [ML~1Cl_2]. 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 [ZrL~1(CH_2Ph)_2] was synthesized from H_2L~1 and Zr(CH_2Ph)_4. Amido complexes [ZrL~1(NR_2)_2] (R = Me or Et; R_2 = HBu~t) were synthesized by substitution reactions of [ZrL~1Cl_2] and by reaction of H_2L~1 with Zr(NR_2)_4. The reactions of [ZrL~1(NR_2)_2] (R = Me or Et) with SiH(OEt)_3 and SiMe_3Cl gave [ZrL~1(OEt)_2] and [ZrL~1Cl_2] respectively. The reaction of [ZrL~1Cl_2] with LiOPh gave [ZrL~1(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 [ZrL~1Cl_2] with LiAlH_4 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. [ZrL~2(py)_2]. This complex is also formed by reaction of H_4L~2 with Zr(NR_2)_4 in pyridine.