Pseudotetrahedral Zn(ii)-(R or S)-dihalogen-salicylaldiminato complexes with Λ- or Δ-chirality induction at-metal

Abstract

Reactions of enantiopure (S or R)-N-1-(phenyl)ethyl-2,4-X¹,X²-salicylaldimine (S-H or R-H; X¹, X² = dihalogen) with Zn(II)-nitrate give bis[(S or R)-N-1-(phenyl)ethyl-2,4-X¹,X²-salicylaldiminato-κ²N,O]-zinc(II), (Δ-ZnS or Λ-ZnR) with Δ/Λ-chirality induction at-metal in the C₂-symmetric molecules. EI-mass spectra show parent ion peaks. X-ray structures indicate that two phenolate-oxygen and two imine-nitrogen atoms from two molecules of the Schiff bases coordinate to the Zn(II) ion in a pseudotetrahedral geometry. Structural analyses give evidence that the S- or R-ligand chirality gives only one diastereomer Δ-ZnS or Λ-ZnR in an enantiopure crystal. Gas-phase optimized structures suggest that the Δ-ZnS or Λ-ZnR diastereomers are slightly more stable than Λ-ZnS or Δ-ZnR by 1–2 kcal mol⁻¹. The intramolecular interactions were analyzed with the Independent Gradient Model (IGM) using the program Multiwfn on the optimized structures and also indicate the diastereomeric preference of Δ-ZnS1 over Λ-ZnS1 (or Λ-ZnR1 over Δ-ZnR1). Variable time and temperature ¹H NMR spectra support the presence of only one diastereomer Λ-ZnR or Δ-ZnS in the bulk samples, backed by the simulated spectra which exhibit no diastereomerization in solution. In contrast, the reported Zn(II)-(R or S)-salicylaldiminato/naphthaldiminato complexes show a diastereomeric mixture of both Δ- and Λ-forms and a Δ ⇄ Λ equilibrium in solution. Electronic circular dichroism (ECD) spectra in solution display expected mirror-image relationships for the (S or R)-Schiff base ligands and the (S or R)-ligated complexes. Combined analyses of experimental and simulated ECD spectra further support the notion of diastereomeric excess of Δ-ZnS or Λ-ZnR in solution. The overall results thus suggest the preservation of chirality at-zinc induced by S- or R-ligands in a solid or solution. Supramolecular packing analyses explore different kinds of intermolecular interactions with the strongest one for X⋯O. Only the halogen atom in the para position is involved in these interactions with Br⋯O > Cl⋯O. Hirshfeld surface analyses also support these interactions between two molecules at a distance shorter than the sum of the vdW radii. Comparison of the experimental and simulated PXRD patterns from the single-crystal X-ray structures shows a good matching and confirms the phase purity of the bulk samples.