Stereochemical Properties of Two Schiff-Base Transition Metal Complexes and Their Ligand by Using Multiple Chiroptical Spectroscopic Tools and DFT Calculations

Abstract

Two transition metal complexes were synthesized with Ni(II) and Cu(II) using a tetradentate Schiff-base ligand, (R,R) and (S,S)-N,N′-Bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine. The stereochemical properties of the ligand and the metal complexes were investigated using a combined experimental and theoretical approach. Multiple spectroscopic techniques, which include IR, vibrational circular dichroism (VCD), UV-Vis and electronic circular dichroism (ECD), as well as Raman and the newly discovered ECD-circularly polarized Raman (i.e., eCP-Raman) spectroscopies were utilized. The good agreement achieved between the experimental and simulated IR, VCD, UV-Vis and ECD spectra of the ligand allowed one to identify the presence of three main ligand conformers in solution, thanks, especially to the high VCD sensitivity to the conformations associated with the tertbutyl groups. The helicity of the metal complexes was identified to be M and P for those with the (R,R) and (S,S) ligands, respectively. Furthermore, eCP-Raman measurements were carried out for the two metal complexes under (near) resonance. Their induced solvent chiral Raman features were explained, and the potential application of eCP-Raman was discussed.

Conformational Distributions of Phenyl β-D-Glucopyranoside and Gastrodin in Solution by Vibrational Optical Activity and Theoretical Calculations

“…[ 57 ] to scale the simulated frequencies in the current study. This procedure was applied recently in a study of transition metal complexes [ 58 ] and was shown to facilitate better comparison with the experimental IR and VCD spectra.…”

Diastereoselective synthesis of cis‐1,2‐limonene oxide using dimeric Salen‐Mn (III) complexes as reusable catalysts

“…9,20,[22][23][24] One of the most used catalysts in the selective oxidation of prochiral olefins and monoterpenes from citrus fruit wastes, such as R-(+)-limonene, are complexes from the Salen (bis-[salicidene]ethylenediamine) ligand with chiral carbons on the imino bridge located between two aromatic groups. 25,26 These ligands are formed by a condensation reaction of salicylaldehyde (substituted or unsubstituted) with a chiral diamine. The most widely used amine is 1,2-cyclohexanediamine, while the simplest amine is 1,2-ethylendiamine because it is an achiral amine.…”