Issue 24, 2020

Solid-state to solution helicity inversion of pseudotetrahedral chiral copper(ii) complexes with 2,4-dihalo-salicylaldiminate ligands

Abstract

The enantiopure Schiff bases (R or S)-N-1-(phenyl)ethyl-2,4-X1,X2-salicylaldimine (X1, X2 = Cl, Br, I) coordinate to copper(II) and provide pseudotetrahedral bis[(S or R)-N-1-(phenyl)ethyl-(2,4-X1,X2-salicylaldiminato-κ2N,O)]-Δ/Λ-Cu(II) (Λ/Δ-Cu-R or Δ/Λ-Cu-S). An induced Λ and Δ-chirality at-metal centre has been launched along the C2-axis of the molecule. Steric constraints brought by halogen substituents on the coordinating salicylal ring provide diastereoselectively Λ-Cu-R or Δ-Cu-S as major and Δ-Cu-R or Λ-Cu-S as minor diastereomers at solid-state, as evidenced by X-ray crystal structures and PXRD analyses. These results reveal inversion of induced chirality at-metal in comparison to the similar complexes without halogen substituents on the salicylal ring. Electronic circular dichroism (ECD) spectra show mirror-image relationships, confirming enantiomeric excess of the R or S-ligated complexes in solution. Comparisons of experimental and simulated ECD spectra suggest diastereomeric excess of Δ-Cu-R or Λ-Cu-S in solution which correspond to an inversion from the found solid-state Λ-Cu-R or Δ-Cu-S as major diastereomers. In addition, the optimized gas-phase structures also reveal Δ-Cu-R or Λ-Cu-S as slightly more stable than Λ-Cu-R or Δ-Cu-S. Thus, solid-state versus solution (or gas-phase) studies also indicate an interconversion of induced chirality (helicity inversion) at-metal from Λ-Cu-R or Δ-Cu-S to Δ-Cu-R or Λ-Cu-S. Thermal stability increases with the molecular weight of the complexes following Cl < ClBr < Br < I substituents. Hirshfeld surface analyses explore the strongest halogen (ortho)–halogen (para) interactions between two molecules in Λ-Cu-R3 or Δ-Cu-S3 at a distance shorter than the sum of the van der Waals radii of the two iodine atoms, provide a brilliant red spot on the dnorm surfaces. EPR spectra along with simulation suggest an axial symmetry with gz > gx,y > 2.0 and values for gz/Az ≥ 135 cm indicate pseudotetrahedral geometry for the complexes.

Graphical abstract: Solid-state to solution helicity inversion of pseudotetrahedral chiral copper(ii) complexes with 2,4-dihalo-salicylaldiminate ligands

Supplementary files

Article information

Article type
Paper
Submitted
06 May 2020
Accepted
02 Jun 2020
First published
02 Jun 2020

Dalton Trans., 2020,49, 8247-8264

Solid-state to solution helicity inversion of pseudotetrahedral chiral copper(II) complexes with 2,4-dihalo-salicylaldiminate ligands

N. Kordestani, H. Amiri Rudbari, G. Bruno, S. Rosario, J. D. Braun, D. E. Herbert, O. Blacque, I. Correia, M. A. Zaman, M. M. Bindu, C. Janiak and M. Enamullah, Dalton Trans., 2020, 49, 8247 DOI: 10.1039/D0DT01649G

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