Synthesis and characterization of enantiopure copper(ii) complexes using chiral bidentate ligands

Abstract

Chiral copper(II) complexes of the types [Cu^II(R/S-ppme)₃](ClO₄)₂ and [Cu^II(R/S-ppme)₂(SO₄)] have been synthesized by reactions of Cu(ClO₄)₂·6H₂O and CuSO₄·5H₂O with R/S-ppme in methanol under aerobic conditions (R/S-ppme = (R/S)-(±)-1-phenyl-N-(pyridine-2-ylmethylene)ethanamine). All complexes were isolated and characterized by X-ray crystallography and circular dichroism (CD): (Λ)-[Cu^II(R-ppme)₃](ClO₄)₂ (1), (Δ)-[Cu(S-ppme)₃](ClO₄)₂·H₂O (2), (Δ)-[Cu^II(R-ppme)₂(SO₄)]·4CH₃OH (3), and (Λ)-[Cu^II(S-ppme)₂(SO₄)]·4CH₃OH (4). 1 and 2 with ClO₄⁻ anions are formed as tris(R/S-ppme) copper(II) complexes, while 3 and 4 with SO₄²⁻ are formed as bis(R/S-ppme) copper(II) complexes. The structures of 1 with R-ppmes and 4 with S-ppmes show the same absolute configurations (Λ). In contrast, 2 with S-ppmes and 3 with R-ppmes exhibit Δ configurations. In the electron paramagnetic resonance (EPR) spectra of 1 and 3, both compounds demonstrate z-axis elongated structures. The CD spectra of 1 and 2 show Cotton effects in an enantiomeric pattern; those of 3 and 4 also display the same effects.