Syntheses, spectroscopy, optical properties, and diastereoselectivity of copper(II)-complexes with chiral aminoalcohol based Schiff bases

Abstract

Chiral aminoalcohol based Schiff bases (R or S)-2-{(E)-(2-hydroxy-1-phenylethylimino)methyl}phenol and (R/S)-2-{(E)-(2-hydroxy-2-phenylethylimino)methyl}phenol coordinate to copper(II)acetate to give enantiopure Λ/Δ- or Δ/Λ-bis[(R or S)-2-{(E)-(2-hydroxy-1-phenylethylimino)methyl}phenoxide-κ2N,O]copper(II), {Λ/Δ-Cu(R-L1)2 (1) or Δ/Λ-Cu(S-L1)2 (2)}, and racemic Δ/Λ- and Λ/Δ-bis[(R/S)-2-{(E)-(2-hydroxy-2-phenylethylimino)methyl}phenoxide-κ2N,O]copper(II), {Δ/Λ- and Λ/Δ-Cu(R/S-L2)2 (3)}, respectively. The complexes are characterized by elemental analyzes, IR, UV–Vis, polarimetry, circular dichroism (CD), differential scanning calorimeter (DSC), and mass spectroscopy. Polarimetry shows the rotation to the left at −113.6° (1) and to the right at +106.4o (2). CD spectra show the expected mirror-image relationship with opposite sign of ellipticity maxima (Δεmax = +0.43 for 1 and −0.42 M−1 dm3 cm−1 for 2 at 638 nm) due to the d-d transitions of the metal ion. CD spectral analyzes further reveal a diastereoselectivity or diastereomeric excess towards Λ-Cu(R-L1)2 or Δ-Cu(S-L1)2 configuration for 1 or 2 in solution. Similarly, the enantiomeric pair of Δ-Cu(R-L2)2 and Λ-Cu(S-L2)2 configurations (CD inactive) for 3 will be preferred in solution. Electronic spectra in different solvents reveal a negative solvatochromism by shifting absorption maxima of the MLCT band to higher energies in solvents of increasing polarity as well as acceptor number. DSC analyzes show an endothermic peak at 525.5 (1) or 528.7 K (2), corresponding to a thermally induced structural phase transformation from distorted square-planar to regular tetrahedral.