Chirality and diastereoselection of Δ/Λ-configured tetrahedral zinc(II) complexes with enantiopure or racemic Schiff base ligands

• Diastereoselection of S or R-HL1 with Zn(II) gives major diastereomer Δ or Λ-Zn-S or R-L1.
• CD pattern corresponds to Δ-Zn-S-L1 and Λ-Zn-R-L1 as the major diastereomer.
• Racemic R/S-HL2 gives major diastereomers of enantiomeric pair Λ-Zn-S-L2, Δ-Zn-R-L2.
• The enantiomeric pair crystallizes in non-centrosymmetric polar space group Iba2.
• Diastereomerization of Λ/Δ-Zn-S/R-L2 to Δ/Λ-Zn-S/R-L2 shows a ratio of 54:46 over 36 h.

Diastereoselection of enantiopure (S or R)-2-{(E)-(2-hydroxy-1-phenylethylimino)methyl}phenol (S- or R-HL1) with zinc(II)acetate leads to Δ-bis[(S)-2-{(E)-(2-hydroxy-1-phenylethylimino)methyl}phenoxide-κ2N,O]zinc(II) (Δ-Zn-S-L1) or Λ-bis[(R)-2-{(E)-(2-hydroxy-1-phenylethylimino)methyl}phenoxide-κ2N,O]zinc(II) (Λ-Zn-R-L1) as major products while racemic (R/S)-2-{(E)-(2-hydroxy-2-phenylethylimino)methyl}phenol (R/S-HL2) yields racemic Δ/Λ-bis[(R/S)-2-{(E)-(2-hydroxy-2-phenylethylimino)methyl}phenoxide-κ2N,O]zinc(II) (Δ/Λ-Zn-R/S-L2), respectively. The complexes are characterized by IR, UV–Vis, 1H NMR, mass spectrometry, circular dichroism (CD), and polarimetry. The bidentate N,O-chelation of two chiral enantiopure or racemic Schiff base ligands to four-coordinated, non-planar Zn(N^O)2 complexes leads to the preferential formation of one diastereomer or enantiomeric pair, respectively, in the solid state. Freshly prepared (30 min) chloroform solutions from the solid product indicate a major diastereomeric component (ca. 67%) and a minor component (ca. 33%) by 1H NMR. The compound Zn-R/S-L2 crystallizes in the non-centrosymmetric polar space group Iba2 with formation of the enantiomeric pair Λ-Zn-S-L2 and Δ-Zn-R-L2 as a racemic mixture in the solid state. Time-dependent 1H NMR studies indicate that a single enantionmeric pair is still present in chloroform solution within 10 min. In solution diastereomerization takes place, and the other diastereomers re-form within 40 min (36 h) to yield a diastereomeric ratio of 67:33% (54:46%) with the enantiomeric pair Λ-Zn-S-L2 and Δ-Zn-R-L2 as major diastereomers and Δ-Zn-S-L2 and Λ-Zn-R-L2 as minor diastereomers. The non-unity ratio of the different-energy enantiomeric pairs Λ-S/Δ-R (54%) versus Δ-S/Λ-R (46%) can be the basis for observed preferential crystallization of Λ-Zn-S-L2 and Δ-Zn-R-L2 with concomitant slow enantiomerization in solution as one component is removed from the solution equilibrium.

Diastereoselection of enantiopure S- or R-HL1 with zinc(II)acetate leads to Δ-Zn-S-L1 or Λ-Zn-R-L1 and Λ-Zn-S-L1 or Δ-Zn-R-L1 as major and minor diastereomers, respectively, in chloroform. While racemic R/S-HL2 gives racemic Λ/Δ- and Δ/Λ-Zn-R/S-L2 which crystallizes in non-centrosymmetric polar space group Iba2 with formation of the enantiomeric pair Λ-Zn-S-L2 (pink) and Δ-Zn-R-L2 (green) in solid state.Figure optionsDownload as PowerPoint slide