Characterization of Diastereomeric Equilibria of Pseudotetrahedral Bis[(R or S)-N-1-(Ar)Ethylsalicylaldiminato-κ2 N,O]zinc(II) with Λ/Δ-Chirality-At-Metal Induction

A family of bis[(R or S)-N-1-(Ar)ethylsalicylaldiminato-κ2 N,O]-Δ/Λ-zinc(II) {Ar=C6 H5 (ZnRL1 or ZnSL1 ), p-CH3 OC6 H4 (ZnRL2 or ZnSL2 ) and p-ClC6 H4 (ZnRL3 or ZnSL3 )} compounds was synthesized and investigated by multiple methods. They feature Λ/Δ-chirality-at-metal induction along the pseudo-C2 axis of the molecules. The chirality induction is quantitative in the solid state, explored by X-ray crystallography and powder X-ray diffraction (PXRD), where R or S-ligated complexes diastereoselectively yield Λ or Δ-configuration at the metal. On the other hand, Λ and Δ-diastereomers co-exist in solution. The Λ⇆Δ equilibrium is solvent- and temperature-dependent. Electronic circular dichroism (ECD) spectra confirm the existence of a diastereomeric excess of Λ-ZnRL1-3 or Δ-ZnSL1-3 in solution. DSC analysis reveals thermally induced irreversible phase transformation from a crystalline solid to an isotropic liquid phase. ECD spectra were reproduced by DFT geometry optimizations and time-dependent DFT (TD-DFT) calculations, providing ultimate proof of the dominant chirality atmetal in solution.

1-Methoxycarbonyl-2-(4-nitrophenyl)ethanaminium nitrate

In the title compound, C₁₀H₁₃O₄N₂⁺·NO₃⁻, the nitro group and the benzene ring are essentially coplanar. The dihedral angle between the benzene ring and the methyl­carboxyl­ate plane is 49.6 (3)°. The crystal structure is stabilized by cation–anion N—H⋯O and N—H⋯N hydrogen bonds, building sheets parallel to (001).

(S)-1-Methoxy-carbonyl-2-(4-nitro-phen-yl)ethanaminium chloride

The title compound, C₁₀H₁₃N₂O₄⁺·Cl⁻, comprises a Cl⁻ anion and a protonated aminium cation. The crystal packing is stabilized by cation–anion N—H⋯Cl hydrogen bonds and N—H⋯O hydrogen bonds, building an infinite two-dimensional network parallel to the (001) plane. The S absolute configuration at the chiral center was deduced from the synthetic pathway and confirmed by the X-ray analysis.

2-Ammonio-3-(4-nitro-phen-yl)propanoate monohydrate

The title compound, C(9)H(10)N(2)O(4)·H(2)O, exists as a zwitterion with a deprotonated carboxyl group and a protonated amine group. The crystal packing is stabilized by inter-molecular N-H⋯O and O-H⋯O hydrogen bonds, building sheets parallel to the (001) plane.

(S)-2-Ammonio-3-(4-nitro-phen-yl)propanoate monohydrate

The title compound, C₉H₁₀N₂O₄·H₂O, exists as a zwitterion with a deprotonated carboxyl group and a protonated amino group. The crystal packing is stabilized by N—H⋯O and O—H⋯O hydrogen bonds, building sheets parallel to the (001) plane. The absolute configuration was deduced from the synthetic pathway.