Induced chirality-at-metal and diastereoselectivity at Δ/Λ-configured distorted square-planar copper complexes by enantiopure Schiff base ligands: combined circular dichroism, DFT and X-ray structural studies

Abstract

Bidentate enantiopure Schiff base ligands, (R or S)-N-1-(Ar)ethyl-2-oxo-1-naphthaldiminato-κ(2)N,O, diastereoselectively yield Δ/Λ-chiral four-coordinated, non-planar Cu(N^O)2 complexes [Ar = C6H5 R/S-L1, m-C6H4OMe R-L2, p-C6H4OMe R/S-L3, and p-C6H4Br R/S-L4]. Two N,O-chelate ligands coordinate to the copper(II) atom in distorted square-planar mode, and induce metal-centered Δ/Λ-chirality at the copper atom in the C2-symmetric complexes. In the solid state, the R-L1 (or R-L4) ligand chirality diastereoselectively induces a Λ-Cu configuration in Λ-Cu-R-L1 (or Λ-Cu-R-L4), the S-L1 ligand a Δ-Cu configuration in Δ-Cu-S-L1, forming enantiopure crystals upon crystallization. Conversely, the R-L2 ligand combines both Λ/Δ-Cu-R-L2 as a diastereomeric pair in the crystals. In solution, electronic circular dichroism (CD) spectra show full or partial diastereoselectivity towards Λ-Cu for R ligands and towards Δ-Cu for S ligands. The electronic CD spectra measured on all complexes obtained from R ligands (or S ligands), e.g. Cu-R-L1, Cu-R-L2, Cu-R-L3, and Cu-R-L4 (or Cu-S-L1, Cu-S-L3, and Cu-S-L4), show consistent spectral features. TDDFT calculations of the electronic CD spectra for the diastereomers Λ-Cu-R-L1 and Δ-Cu-R-L1 suggest that the CD spectra are largely dominated by the configuration at the metal center (Λ vs. Δ). The experimental CD spectrum of Cu-R-L1 agrees well with the one calculated for the Λ-Cu-R-L1 configuration. Cyclic voltammetry of Cu-R-L1 reveals a quasi-reversible redox wave corresponding to one-electron transfer for the [Cu(II)L2](0)/[Cu(I)L2](-1) couple in acetonitrile. DSC analyses for the complexes show an exothermic peak between 377 and 478 K (ΔH = -12 to -43 kJ mol(-1)), corresponding to a phase transformation from distorted square-planar/tetrahedral to regular tetrahedral geometry on heating.

Synthesis, X-ray, and Spectroscopic Study of Dissymmetric Tetrahedral Zinc(II) Complexes from Chiral Schiff Base Naphthaldiminate Ligands with Apparent Exception to the ECD Exciton Chirality

“…A consistent stereochemical induction is observed here: complexation with S-configured ligand 1S or 2S leads to Δ-configured zinc (that is, Δ-Zn-1S or Δ-Zn-2S), while the corresponding Rligand 1R or 2R affords opposite Λ-configured zinc (Λ-Zn-1R or Λ-Zn-2R) as in the analogous Cu/Ni(II) complexes with the similar naphthaldiminate ligands. 20,24 However, in the Cu(II) structure with 2R, the two symmetry-independent molecules in the asymmetric unit formed a diastereomeric pair, with Cu1 having Λ-and Cu2 having Δ-configuration. 20 All four Zn structures given here fulfill the criterion of a Flack parameter x close to zero, with standard deviation u less than 0.04 and |x|/u < 3 (vide infra).…”

“…With chelate ligands, α and β are greater than 109.5°; thus, τ 4 < 1, even if the dihedral planes are perfectly perpendicular. 20,24 Inorganic Chemistry ECD spectra were also recorded in the solid state 45 on microcrystalline samples in mixtures with an inert salt (KCl). 46a,b Solid-state spectra (Supporting Information, Figure S5) are comparable with solution ones in the accessible range (>250 nm).…”

Different Types of Nickel, Cobalt, and Manganese Complexes originating from 2‐Imidazolecarboxaldiimine of Triethylenetetraamine

“…It may be noted that different symmetries are generated due to flexibility present in macrocycles of Schiff base. [52,53] Intramolecular cyclization of H2imida led to rings in tetraimida. However, the flexibility to provide six coordination sites to embed a metal ion resulting in an octahedral environment similar to H2imida was lost in tetraimida.…”

Complex formation of pyridine‐azacrown ether amide macrocycles with proton and heavy metal ions in aqueous solution

“…The example of divalent transition metal complexes with chiral N,O chelate Schiff-bases and heterocyclic azine ligands were early described. [11][12][13][14] In spite of forming less stable complexes than those of amine macrocyclic compounds containing pyridine, [15,16] amide macrocyclic ligands demonstrate structural rigidity to crown ether due to the presence of the pyridine-2,6-dicarbamide fragment, resulting the molecule has an open cavity and faster kinetics of formation of metal complexes. Other advantages of some properties of amide macrocycles are water solubility, easy synthetic, and purification procedures.…”