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-kappa N-2,O]-Delta/?-zinc(II) {Ar=C6H5 (ZnRL1 or ZnSL1), p-CH3OC6H4 (ZnRL2 or ZnSL2) and p-ClC6H4 (ZnRL3 or ZnSL3)} compounds was synthesized and investigated by multiple methods. They feature ?/Delta-chirality-at-metal induction along the pseudo-C-2 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 ? or Delta-configuration at the metal. On the other hand, ? and Delta-diastereomers co-exist in solution. The ?⇆Delta equilibrium is solvent- and temperature-dependent. Electronic circular dichroism (ECD) spectra confirm the existence of a diastereomeric excess of ?-ZnRL1-3 or Delta-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.