Experimental and computational studies on pseudotetrahedral nickel(ii)-(S or R)-dihalogen-salicylaldiminates with Δ- or Λ-chirality induction at-metal

Abstract

In this study, three pairs of pseudotetrahedral C₂-symmetrical bis[(S or R)-N-1-(phenyl)ethyl-(2,4-X¹,X²-salicylaldiminato-κ²N,O)]-Δ/Λ-nickel(II), Δ/Λ-NiSL^1–3 or Λ/Δ-NiRL^1–3 (X¹, X² = Cl, Cl; Br, Br; Cl, Br) with Δ- or Λ-chirality induction at-metal were synthesized, respectively. X-ray molecular structures showed the formation of a N₂O₂-chromophore around nickel(II) from two molecules of Schiff bases in a pseudotetrahedral geometry. The structural analysis further suggested the isolation of opposite configured Δ-Ni and Λ-Ni diastereomers for S- and R-HL^1–3 Schiff base ligands in an enantiopure crystal, resulting from diastereoselection and chirality induction at-metal. The gas-phase optimized structures for Δ-NiSL^1–3 or Λ-NiRL^1–3 are also slightly more stable by 0.10–1.70 kcal mol⁻¹. Electronic circular dichroism (ECD) spectra showed expected mirror-image relationships with opposite Cotton effects, confirming the enantiopurity or enantiomeric excess of NiRL^1–3 and NiSL^1–3 complexes in solutions. Combined analyses of experimental and simulated ECD spectra suggested diastereomeric excess of Δ-NiSL^1–3 or Λ-NiRL^1–3 in solutions, demonstrating the preservation of chirality induction at-metal alike in solid or gas phases. The analysis of supramolecular packing explored several kinds of intermolecular interactions with the strongest one for halogen⋯oxygen atoms and only halogen at the para-position of salicylaldehyde is involved in these interactions with Br⋯O > Cl⋯O. Hirshfeld surface analysis also supported such kind of interactions at a distance shorter than the sum of the van der Waals radii. The non-covalent interaction (NCI) indices explored the presence of weak attractive interactions resulting from the halogen atoms of the respective structures. The experimental and simulated PXRD patterns revealed a fair matching, confirming the phase purity of bulk crystal samples of these complexes.