Pseudotetrahedral copper()-complexes with enantiopure ( or )-2-(((aryl)ethylimino)ethyl)phenolate Schiff base ligands

• Published in: Dalton transactions : an international journal of inorganic chemistry 2021-07, Vol.5 (26), p.9236-9249
• Main Authors: Enamullah, Mohammed, Anwar Hossain, Mohammad, Islam, Mohammad Khairul, Woschko, Dennis, Janiak, Christoph
• Format: Article
• Language: English
• Subjects: Acetonitrile; Charge transfer; Chirality; Coordination compounds; Copper; Copper compounds; Crystallization; Crystallography; Dichroism; Diffraction patterns; Imines; Ligands; Microcrystals; Nitrogen atoms; Phenols; Sodium cyanide; Spectra; X ray powder diffraction
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d1dt01671g

Condensation of 2-hydroxy-benzophenone (HL′) with ( R or S )-(Ar)ethylamine yields the enantiopure Schiff bases ( S or R )-2-(( E )-1-(1-(Ar)ethylimino)ethyl)phenol {Ar = C 6 H 5 ( S - or R -HL 1 ), p -CH 3 OC 6 H 4 ( S - or R -HL 2 )}. These Schiff bases react with copper( ii ) acetate under reflux to give green microcrystals of bis[( R or S )-2-(( E )-1-(1-(Ar)ethylimino)ethyl)phenolato-κ 2 N,O]-Λ/Δ-copper( ii ), {Ar = C 6 H 5 (Λ/Δ-Cu- R - or S -L 1 ), p -CH 3 OC 6 H 4 (Λ/Δ-Cu- R - or S -L 2 )} with induction of Λ/Δ-chirality at-metal. The presence of Schiff base ligands in the paramagnetic green microcrystals is confirmed by decomplexation reaction with NaCN via reduction of Cu( ii ) to Cu( i ) in DMSO-d 6 solution. Crystallization attempts of the green microcrystalline Schiff-base Cu complexes provide deep-green block-shaped crystals of an about equal admixture of bis[2-oxo-benzophenonato-κ 2 O,O′]-copper( ii ), (CuL′ 2 ) and bis[2-(imino(phenyl)methyl)phenolato-κ 2 N,O]copper( ii ), (CuL′′ 2 ) via in situ hydrolysis of the coordinated Schiff base ligands back to 2-hydroxy-benzophenone (HL′) and to 2-(imino(phenyl)methyl)phenol (HL′′), which in-turn bind with the copper( ii ) ion. Powder X-ray diffraction (PXRD) patterns of R -HL 1 and Cu- R -L 1 allowed their structure determinations using the program Expo-2014 followed by Rietveld refinement. The Cu structures refined to four-coordinated Λ/Δ-copper( ii )-complexes by the two phenolate-oxygen and two imine-nitrogen atoms from two Schiff base ligands in a pseudotetrahedral geometry. DFT optimized structures (at gas-phase) reveal the Δ-Cu- S -L 1 or Λ-Cu- R -L 1 diastereomer as slightly more stable than the corresponding Λ-Cu- S -L 1 or Δ-Cu- R -L 1 by ca. 7.60 kcal mol −1 , resulting from diastereoselectively induced Λ vs. Δ-chirality at-metal. Electronic circular dichroism (ECD) spectra display mirror-image relationships and comparisons of experimental and simulated ECD spectra by TDDFT suggest an excess of the Δ-Cu- S -L 1 or Λ-Cu- R -L 1 diastereomer in solution. The cyclic voltammograms demonstrate two one electron charge transfer processes for Cu 2+ /Cu + and Cu + /Cu 0 couples in acetonitrile, respectively. For the diastereoselection of Λ/Δ-Cu- S - or R -L a combination of experimental ECD and computational procedures including structure optimizations and excited state properties by DFT/TDDFT is employed to assess the chirality induction at-metal.