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

Chiral copper(II) complexes of the types [CuII(R/S-ppme)3](ClO4)2 and [CuII(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)-[CuII(R-ppme)3](ClO4)2 (1), (Δ)-[Cu(S-ppme)3](ClO4)2·H2O (2), (Δ)-[CuII(R-ppme)2(SO4)]·4CH3OH (3), and (Λ)-[CuII(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.